We present a search for a light sterile neutrino using three years of atmospheric neutrino data from the DeepCore detector in the energy range of approximately 10-60 GeV. DeepCore is the low-energy subarray of the IceCube Neutrino Observatory. The standard three-neutrino paradigm can be probed by adding an additional light (Delta m(41)(2) similar to 1 eV(2)) sterile neutrino. Sterile neutrinos do not interact through the standard weak interaction and, therefore, cannot be directly detected. However, their mixing with the three active neutrino states leaves an imprint on the standard atmospheric neutrino oscillations for energies below 100 GeV. A search for such mixing via muon neutrino disappearance is presented here. The data are found to be consistent with the standard three-neutrino hypothesis. Therefore, we derive limits on the mixing matrix elements at the level of vertical bar U mu(4)vertical bar(2) < 0.11 and vertical bar U-tau 4 vertical bar(2) < 0.15 (90% C. L.) for the sterile neutrino mass splitting Delta m(41)(2) = 1.0 eV(2).

Auroral omega bands have been observed since the 1960s and their ionospheric electrodynamic properties are well established. At the same time magnetospheric source is poorly investigated leaving a room for few competing hypotheses on how the omega band forms. Here we present the statistical study of the projections of about 400 omega bands from the ionosphere to magnetosphere to investigate the location and properties of a possible omega source region in the magnetosphere. We used the Magnetometers?Ionospheric Radars?All-sky Cameras Large Experiment all-sky images on the list of individual omega structures (Partamies et al., 2017, http://doi.org/10.5194/angeo-35-1069-2017) which were observed in the Fennoscandian Lapland in the period of 1997?2007, and a new empirical magnetic field model (Tsyganenko & Andreeva, 2016, https://doi.org/10.1002/2016ja023217) to identify the magnetospheric equatorial projection of the observed omegas. We found that 90% of the auroral omega structures map to the radial distances of 6?14 RE from the Earth, coinciding with the bursty bulk flow braking region. An average magnetic field configuration in the vicinity of magnetospheric omega projections corresponds approximately to the transition region between tail- and dipole-like magnetic configuration. Velocity estimates for omega projections reveal the dawnward as well as the radial propagation in the magnetosphere with a typical speed of up to few tens of km/s. It is shown that the source of omega structures propagates earthward in most of the events.

Erroneous GNSS positioning, failures in spacecraft operations and power outages due to geomagnetically induced currents are severe threats originating from space weather. Knowing the potential impacts on modern society in advance is key for many end-user applications. This covers not only the timing of severe geomagnetic storms but also predictions of substorm onsets at polar latitudes. In this study, we aim at contributing to the timing problem of space weather impacts and propose a new method to predict the solar wind propagation delay between Lagrangian point L1 and the Earth based on machine learning, specifically decision tree models. The propagation delay is measured from the identification of interplanetary discontinuities detected by the advanced composition explorer (ACE) and their subsequent sudden commencements in the magnetosphere recorded by ground-based magnetometers. A database of the propagation delay has been constructed on this principle including 380 interplanetary shocks with data ranging from 1998 to 2018. The feature set of the machine learning approach consists of six features, namely the three components of each the solar wind speed and position of ACE around L1. The performance assessment of the machine learning model is examined based on of 10-fold cross-validation. The machine learning results are compared to physics-based models, i.e., the flat propagation delay and the more sophisticated method based on the normal vector of solar wind discontinuities (vector delay). After hyperparameter optimization, the trained gradient boosting (GB) model is the best machine learning model among the tested ones. The GB model achieves an RMSE of 4.5 min concerning the measured solar wind propagation delay and also outperforms the physical flat and vector delay models by 50% and 15% respectively. To increase the confidence in the predictions of the trained GB model, we perform a performance validation, provide drop-column feature importance and analyze the feature impact on the model output with Shapley values. The major advantage of the machine learning approach is its simplicity when it comes to its application. After training, values for the solar wind speed and spacecraft position from only one datapoint have to be fed into the algorithm for a good prediction.

We present a brief review of experimental investigations concerning the perturbation of the high-latitude upper ionosphere (F region) by powerful HF radio waves (pump waves), carried out by the team from the Arctic and Antarctic Research Institute (AARI) during a 25-year collaboration with the European Incoherent Scatter (EISCAT) Scientific Association. Three types of HF pumping experiments were carried out by the Russian team at EISCAT. The first type of experiments was related to the modification of the ionosphere-magnetosphere coupling by HF pumping into the nightside ionosphere auroral E and F regions. The second type of experiments was concerned with investigations of geophysical phenomena by using the EISCAT HF heating facility and multi-instrument diagnostics. Most of the experiments were closely related to radio science and nonlinear plasma experiments (a third type of experiment). The main attention was paid to the investigation of artificial ionospheric turbulence in the upper ionosphere (F region) induced by the extraordinary (X-mode) HF pump waves. A great number of X-mode experiments, carried out by the Russian team at EISCAT, clearly demonstrated for the first time that an X-polarized HF pump wave was able to produce various HF-induced phenomena in the high-latitude ionospheric F region, which could be even much stronger than the O-mode effects. We found for the first time that the powerful X-mode HF radio waves, injected into the high-latitude ionospheric F2 layer towards the magnetic zenith, were capable of generating artificial field-aligned irregularities, radio-induced optical emissions in the red (630 nm) and green (557.7 nm) lines, strong electron acceleration leading to Ne enhancements, ion-acoustic and Langmuir electrostatic waves, and spectral components in the narrowband stimulated electromagnetic emission observed at a large distance from the HF heater. The features and behavior of the X-mode effects were analyzed and compared with the O-mode effects.

The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena.

This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena.

ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult.

Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.

The high-latitude ionosphere-thermosphere system is strongly affected by the magnetospheric energy input during magnetospheric substorms. In this study, we investigate the response of the upper thermospheric winds to four substorm events by using the Fabry-Perot interferometer at Troms?, Norway, the International Monitor for Auroral Geomagnetic Effects magnetometers, the EISCAT radar, and an all-sky camera. The upper thermospheric winds had distinct responses to substorm phases. During the growth phase, westward acceleration of the wind was observed in the premidnight sector within the eastward electrojet region. We suggest that the westward acceleration of the neutral wind is caused by the ion drag force associated with the large-scale westward plasma convection within the eastward electrojet. During the expansion phase, the zonal wind had a prompt response to the intensification of the westward electrojet (WEJ) overhead Troms?. The zonal wind was accelerated eastward, which is likely to be associated with the eastward plasma convection within the substorm current wedge. During the expansion and recovery phases, the meridional wind was frequently accelerated to the southward direction, when the majority of the substorm WEJ current was located on the poleward side of Troms?. We suggest that this meridional wind acceleration is related to a pressure gradient produced by Joule heating within the substorm WEJ region. In addition, strong atmospheric gravity waves during the expansion and the recovery phases were observed.

In recent years, aurora observation networks using high-sensitivity cameras have been developed in the polar regions. These networks allow dimmer auroras, such as pulsating auroras (PsAs), to be observed with a high signal-to-noise ratio. We reconstructed the horizontal distribution of precipitating electrons using computed tomography with monochromatic PsA images obtained from three observation points. The three-dimensional distribution of the volume emission rate (VER) of the PsA was also reconstructed. The characteristic energy of the reconstructed precipitating electron flux ranged from 6 to 23 keV, and the peak altitude of the reconstructed VER ranged from 90 to 104 km. We evaluated the results using a model aurora and compared the model's electron density with the observed one. The electron density was reconstructed correctly to some extent, even after a decrease in PsA intensity. These results suggest that the horizontal distribution of precipitating electrons associated with PsAs can be effectively reconstructed from ground-based optical observations.

Pulsating auroras (PsAs) appear over a wide area within the aurora oval in the midnight–morning–noon sector. In previous studies, observations by magnetometers on board satellites have reported the presence of field-aligned currents (FACs) near the edges and interiors of pulsating aurora patches. PsAs are thus a key research target for understanding the magnetosphere–ionosphere coupling process. However, the three-dimensional (3-D) structure of the electric currents has yet to be clarified, since each satellite observation is limited to a single dimension along its orbit. This study's aim was a reconstruction of the 3-D structure of ionospheric conductivity, which is necessary to elucidate the 3-D ionospheric current. Tomographic analysis was used to estimate the 3-D ionospheric conductivity for rapidly changing auroral phenomena such as PsAs. The reconstructed Hall conductivity reached its maximum value of 1.4 × 10−3 S m−1 at 94 km altitude, while the Pedersen conductivity reached its maximum value of 2.6 × 10−4 S m−1 at 116 km altitude. A secondary peak in the Pedersen conductivity, due to electron motion, at 9.9 × 10−5 S m−1 appears at 86 km altitude. The electron Pedersen conductivity maximum value in the D region was approximately 38 % of the ion Pedersen conductivity maximum value in the E region. The FAC, derived under the assumption of a uniform ionospheric electric field, was approximately 70 µA m−2 near the edge of the PsA patch. This FAC value was approximately 10 times that observed by satellites in previous studies. If the conductivity around the patch is underestimated or the assumption of a uniform field distribution is incorrect, the FAC could be overestimated. By contrast, due to sharper boundary structures, the FAC could actually have had such a large FAC.

High energy protons accelerated during solar proton events (SPEs) can access the Earth's middle atmosphere at high and middle latitudes causing large-scale ionization and chemical changes. In this study, we have compared the performance of two cutoff latitude models that predict the limit of the SPE impact area in the atmosphere during 73 SPEs from 1997 to 2010. We use observations from 13 riometer stations and the D Region Absorption Prediction (DRAP) model to test the performance of the two cutoff latitude models by Dmitriev et al. (2010, https://doi.org/10.1029/2010JA015380) and Nesse Tyss?y and Stadsnes (2015, https://doi.org/10.1002/2014JA020508). We find similar performance from the two cutoff latitude models with respect to observations, but the Dmitriev et al. (2010, https://doi.org/10.1029/2010JA015380) model performs slightly better when observations are contrasted with the DRAP model results. The better performing model is also continuous with magnetic local time and particle energy, making it more suited for future use in climate model proton forcing. SPE forcing is currently included in climate models with a single static cutoff latitude limit at 60° geomagnetic latitude. In reality, the area that the solar protons can access is not static but varies with particle rigidity and geomagnetic conditions. We estimate that the SPE impact area is overestimated 90% of the time by this single static cutoff limit and the average overestimation of the impact area is about 15?25% for protons with energies <32?MeV.

Solar proton events (SPEs) cause large‐scale ionization in the middle atmosphere leading to ozone loss and changes in the energy budget of the middle atmosphere. The accurate implementation of SPEs and other particle ionization sources in climate models is necessary to understand the role of energetic particle precipitation in climate variability. We use riometer observations from 16 riometer stations and the Whole Atmosphere Community Climate Model with added D region ion chemistry (WACCM‐D) to study the spatial and temporal extent of cosmic noise absorption (CNA) during 62 SPEs from 2000 to 2005. We also present a correction method for the nonlinear response of observed CNA during intense absorption events. We find that WACCM‐D can reproduce the observed CNA well with some need for future improvement and testing of the used energetic particle precipitation forcing. The average absolute difference between the model and the observations is found to be less than 0.5 dB poleward of about 66° geomagnetic latitude, and increasing with decreasing latitude to about 1 dB equatorward of about 66° geomagnetic latitude. The differences are largest during twilight conditions where the modeled changes in CNA are more abrupt compared to observations. An overestimation of about 1° to 3° geomagnetic latitude in the extent of the CNA is observed due to the fixed proton cutoff latitude in the model. An unexplained underestimation of CNA by the model during sunlit conditions is observed at stations within the polar cap during 18 of the studied events.

The behaviors of atmospheric 129I and the global 129I cycle remain incompletely understood because the spatiotemporal resolution of monitoring is insufficient, and few measurement-based models have been reported. This study aims to quantitatively understand the global 129I budget. When quantifying, we conducted global atmospheric 129I dispersion simulations covering from the period 2007 to 2010. To achieve this goal, the present study newly incorporates the iodine chemical processes of two gas-phase chemical reactions, six photolysis reactions, and two heterogeneous reactions into an existing atmospheric 129I transport model (GEARN–FDM). In addition to the aerial release of 129I from nuclear fuel reprocessing facilities, the model includes the volatilization processes of 129I compounds from the Earth’s surface. The net 129I exchange fluxes from the atmosphere to the Earth’s oceans and land were estimated to be 5.3 GBq/y and 18.0 GBq/y, respectively. The global 129I emission from the oceans was estimated as 7.2 GBq/y, and nearly half of the emission totals are emitted from the English Channel (3.2 GBq/y). In addition, the global 129I emissions from land are estimated to be 1.7 GBq/y. The remarkable 129I emission levels from land mainly appear in Europe, Russia, and North America, and the emission distribution is impacted by the activities of past and ongoing nuclear fuel reprocessing facilities. The total 129I emissions from the oceans and land are lower than the 129I emissions from the model-included nuclear fuel reprocessing facilities (23.3 GBq/y), and show that the aerial releases from the nuclear fuel reprocessing facilities in operation are still important 129I sources.

Auroras can be regarded as the most fascinating manifestation of space weather and they are continuously observed by ground-based and, nowadays more and more, also by space-based measurements. Investigations of auroras and geospace comprise the main research goals of the Suomi 100 nanosatellite, the first Finnish space research satellite, which has been measuring the Earth's ionosphere since its launch on 3 December 2018. In this work, we present a case study where the satellite's camera observations of an aurora over Northern Europe are combined with ground-based observations of the same event. The analyzed image is, to the authors' best knowledge, the first auroral image ever taken by a CubeSat. Our data analysis shows that a satellite vantage point provides complementary, novel information of such phenomena. The 3D auroral location reconstruction of the analyzed auroral event demonstrates how information from a 2D image can be used to provide location information of auroras under study. The location modeling also suggests that the Earth's limb direction, which was the case in the analyzed image, is an ideal direction to observe faint auroras. Although imaging on a small satellite has some large disadvantages compared with ground-based imaging (the camera cannot be repaired, a fast moving spinning satellite), the data analysis and modeling demonstrate how even a small 1-Unit (size: 10 × 10 × 10 cm) CubeSat and its camera, build using cheap commercial off-the-shelf components, can open new possibilities for auroral research, especially, when its measurements are combined with ground-based observations.

The high-latitude magnetic substorms observed at geomagnetic latitudes higher 70° MLAT under the substorm absence at the lower latitudes are known as “substorms on the contracted oval” or “polar” substorms. Such substorms appear during quiet or weakly disturbed space weather conditions. The study of 254 “polar” substorms, recorded at the Scandinavian IMAGE magnetometer chain during the winter seasons of 2010–2020, confirmed a tendency to occur in the late evening (∼19–23 MLT), that is a bit earlier than the “normal” substorms (22–24 MLT). It was found that before the onset of “polar” substorms, like “normal” substorms, there is an increase in the PC-index indicating an energy input into the magnetosphere. We established that “polar” substorms, like “normal” substorms, are accompanied by positive mid-latitude magnetic bays, demonstrating a substorm current wedge (SCW) development. Several “polar” substorms are examined in detail. The ionospheric electrojets and field-aligned currents (FAC) distribution was studied basing on the AMPERE satellites measurements. We found that “polar” substorm onsets are associated with an enhancement of FACs in a localized evening area. Thus, “polar” substorms exhibited the properties, typical for “normal” substorms, so, they could be referred as a specific type of substorms developing under rather quiet space weather conditions.

Polar substorms include substorms observed at geomagnetic latitudes above 70° MLAT in the absence of simultaneous negative magnetic bays at lower latitudes, that is, substorms on the compressed contracted auroral oval. The general morphological features of polar substorms are considered based on the example of individual events registered on Svalbard arch. It is shown that polar substorms, like “classical” substorms, are characterized by the formation of a substorm current wedge and a steplike movement to the pole after the onset of a substorm, generation of Pi2 geomagnetic pulsations, and an increase of the PC-index of the polar cap before the onset of the substorm. At the same time, there are certain differences between polar substorms and “classical” substorms; namely, they start on more distant L-shells, develop in the region of a contracted auroral oval, occur at earlier pre-midnight hours, and generate only at low solar wind speeds and weakly disturbed geomagnetic conditions. It has been suggested that polar substorms may be a specific type of “classical” substorms that develop in the evening sector under magnetically quiet or weakly disturbed conditions when the auroral oval is concracted. The source of polar substorms may also be a local intensification of previously existing substorms in the post-midnight sector.

We have studied the geomagnetic conditions during the occurrence of auroral-hiss VLF emissions on the Earth’s surface in the form of noise bursts at frequencies above 5–6 kHz. The study is based on an analysis of auroral-hiss observations at auroral latitudes in northern Finland at Kannuslehto station (KAN, L ~ 5.5) during the winter campaigns of 2013–2018. It is shown that auroral-hiss bursts are most often observed in the interval of 20–01 MLT under low geomagnetic activity (Kp < 3). It has been found that the bursts are typical for the growth phase of a magnetospheric substorm; the bursts cease abruptly at the onset of the substorm (the break-up of auroras), which is apparently due to a drastic increase in the absorption of VLF waves in the ionosphere. The bursts are often accompanied by the generation of geomagnetic Pi2 pulsations. VLF observations have shown that auroral-hiss bursts are not observed on the Earth’s surface in the main phase of magnetic storms; however, they are typical of the recovery phase. It has been found from model data that, during the occurrence of auroral hiss, KAN station is usually projected to the near-equatorial region of the auroral oval or to the zone of diffuse precipitation of more energetic electrons, i.e., to lower latitudes than the typical position of visible auroras during that time.

We investigated statistically the orientation of about 10,000 auroral arcs observed during 2016?2021 in Abisko (68.36°N, 18.81°E, Sweden) in the equatorward part of the nightside auroral oval. The observations were made between 19 and 06 magnetic local time (MLT). On average, the orientation of the arcs, that is, the angle between an arc and the West-East direction, linearly changed with MLT at the rate 2.2°/hr. In most cases the mean orientation of the auroral arcs near midnight was parallel to the geomagnetic latitude of Abisko, except a few late winter or spring months. These anomalies cannot be explained by geomagnetic disturbances or interplanetary conditions. They do, however, coincide with spring transitions in the atmospheric circulations from winter-to summer-type. The spring transitions were manifested in the data of a meteor radar in the region of observations as reversals of the zonal wind at altitudes 90?100 km from eastward to westward. We propose that these spring transition changes in the global atmospheric circulation couple also the overlying thermosphere at 100?150 km where neutral winds may affect the ionosphere and subsequently the whole ionosphere-magnetosphere system. Then, changes in the coupled ionosphere-magnetosphere system are manifested in changes of the orientation of auroral arcs.

Abstract Multi-instrument observations by a meteor radar (MR), auroral cameras, ionosondes, and ground magnetometers were made in Northern Europe at auroral latitudes (between 64° and 72° corrected geomagnetic latitude) at 22?24 magnetic local time in the substorm growth phase. The southward drifting growth phase auroral arc was associated with enhanced electron density up to 2?1012 m-3 (corresponding to a plasma frequency, foEs of about 13 MHz) at about 110-km altitude. Such an enhanced E layer electron density caused bending toward the ground of the MR radio waves transmitted at a frequency, fr, of 36.9 MHz and at low elevation (el. < 25°), such that the radar received ground echoes characterized by a near-zero Doppler shift. The amplitude of the echoes was modulated at a frequency of a few hertz, and a similar modulation was found in the auroral luminosity at 427.8 nm near the location of the bending of MR radio waves. The modulation was due to irregular (random) fluctuations of auroral precipitation. Although such a few-hertz variation of the auroral precipitation cannot produce more than 1% modulation of the ionospheric electron density, even such a small modulation can lead to 50% modulation of the MR ground scatter provided foEs ≈ fr sin (el.). The ionosonde and MR data provide evidence that this condition was satisfied in the present case. Due to a high-frequency (>2 Hz) amplitude modulation of the ground scatter, the MR erroneously accepts such signals as echoes from meteor trails.

We present 3-D excitation rate estimates of artificial aurora in the ionospheric F layer, induced by high-frequency radio waves from the European Incoherent Scatter heating facility. Simultaneous imaging of the artificial aurora was done with four separate Auroral Large Imaging System stations, permitting tomography-like 3-D auroral reconstruction of the enhanced atomic oxygen emissions at 6,300, 5,577, and 8,446 Å. Inspection of the 3-D reconstructions suggests that the distribution of energized electrons is less extended in altitude than predicted by transport calculations of electrons accelerated to 2?100 eV. A possible reason for this discrepancy is that high-frequency pumping might induce an anisotropic distribution of energized electrons.

The International Magnetospheric Study (IMS) took place from 1977 to 1979. An objective was to study the magnetosphere at different heights on the Earth and in space simultaneously. The Scandinavian Magnetometer Array (SMA), where a dense magnetometer array was placed in the northern part of Scandinavia, was part of the IMS. This array extended 1,570 km in north-south and 1,290 km in east-west directions. The SMA-magnetometers contained a camera with 35 mm film and three wire-suspended magnets. These instruments recorded the movement of the magnets optically on the film. The usability of the SMA data has been limited by time-consuming digitization by hand. Thus, most of the recordings has been left nondigitized and unstudied. We have developed a method, named DigiMAG, to digitize the SMA recordings by using a custom-built device. This article presents the high-latitude dynamics of the strongest magnetic storm in 1977 between October 26 and 30. We analyze newly digitized data from Rostadalen (ROS), Evenes (EVE), Ritsemjokk (RIJ), and Kiruna (KIR) stations for the three storm-time substorms on October 26?29, 1977. The results show that in the vicinity of the 65° latitude, the storm-time differences of horizontal magnetic components can exceed 500 nT. During the recovery phase of a substorm, 2.8 nT km?1 difference arises over a distance of 167 km between the H-component time derivatives of ROS and RIJ stations in 3 minutes.

The EISCAT (European Incoherent Scatter Scientific Association) Heating facility was used to transmit powerful high-frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by several stations of ALIS (Auroral Large Imaging System) in northern Sweden, and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from  for the HF pump frequency f0=6.200 MHz ≈4.6fe to  when f0=5.423 MHz ≲4fe, where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0=5.423 MHz ≲ 4 fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of metre-scale wavelengths that can be excited with f0=5.423 MHz ≲ 4 fe.

The potential use of the wavelet-decomposition method developed by the authors for the analysis of geomagnetic data and cosmic ray variations is studied. With the use of adaptive threshold functions, the method allows the isolation of nonstationary, short-period (from 1 × 10–3 Hz and greater) variations in the data and the estimation of their parameters. Data from a network of ground-based magnetometers (www.inrtermagnet.org) and neutron monitors (http://cosray.unibe.ch/) were used in the work. The advantages of the method are shown on the example of magnetic storms on July 9 and September 27, 2017; the effectiveness of the method for the detection of low-amplitude anomalous variations in heterogeneous data has been proven experimentally. The dynamics of variations in the geomagnetic field at meridionally located stations and in the auroral zone is considered in detail; patterns of the occurrence and propagation of geomagnetic disturbances preceding and accompanying the intervals of magnetic storms are obtained. The application of the method made it possible to detect clearly and estimate weak short-term increases in geomagnetic activity observed against the background of increased cosmic ray intensity preceding the onset of magnetic storms. It is noted that the identified geomagnetic disturbances occurred synchronously at stations from high latitudes to the equator and correlated with the periods of southward turns of the IMF Bz component and increased auroral activity. During these intervals, cosmic rays exhibited low-amplitude Forbush effects, which were detected with this method.

Very low frequency (VLF) auroral hiss at Kannuslehto (KAN), Finland, was analyzed with reference to the progress of 98 isolated magnetic substorms measured during the winter months of 2015?2018. Of these, 91 were accompanied by auroral hiss during the substorm growth phase. No auroral hiss was recorded during the expansion and recovery phases. We found that auroral hiss was observed under rising polar cap (PC) index, showing an increased solar wind energy input into the magnetosphere during the substorm growth phase. We also found that in 58 of the 65 events studied, KAN was located in the vicinity of enhanced field-aligned currents (FACs) during auroral hiss occurrence. For the first time, it was established that auroral VLF hiss generation in the equatorial part of the auroral oval is a typical signature of a substorm growth phase.

Ground-based technological systems, such as power grids, can be affected by geomagnetically induced currents (GIC) during geomagnetic storms and magnetospheric substorms. This motivates the necessity to numerically simulate and, ultimately, forecast GIC. The prerequisite for the GIC modeling in the region of interest is the simulation of the ground geoelectric field (GEF) in the same region. The modeling of the GEF in its turn requires spatiotemporal specification of the source which generates the GEF, as well as an adequate regional model of the Earth?s electrical conductivity. In this paper, we compare results of the GEF (and ground magnetic field) simulations using three different source models. Two models represent the source as a laterally varying sheet current flowing above the Earth. The first model is constructed using the results of a physics-based 3-D magnetohydrodynamic (MHD) simulation of near-Earth space, the second one uses ground-based magnetometers? data and the Spherical Elementary Current Systems (SECS) method. The third model is based on a ?plane wave? approximation which assumes that the source is locally laterally uniform. Fennoscandia is chosen as a study region and the simulations are performed for the September 7?8, 2017 geomagnetic storm. We conclude that ground magnetic field perturbations are reproduced more accurately using the source constructed via the SECS method compared to the source obtained on the basis of MHD simulation outputs. We also show that the difference between the GEF modeled using laterally nonuniform source and plane wave approximation is substantial in Fennoscandia.

In this study, we perform three-dimensional (3-D) ground electric field (GEF) modeling in Fennoscandia for three days of the Halloween geomagnetic storm (29–31 October 2003) using magnetic field data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and a 3-D conductivity model of the region. To explore the influence of the inducing source model on 3-D GEF simulations, we consider three different approaches to source approximation. Within the first two approaches, the source varies laterally, whereas in the third method, the GEF is calculated by implementing the time-domain realization of the magnetotelluric intersite impedance method. We then compare GEF-based geomagnetically induced current (GIC) with observations at the Mäntsälä natural gas pipeline recording point. We conclude that a high correlation between modeled and recorded GIC is observed for all considered approaches. The highest correlation is achieved when performing a 3-D GEF simulation using a “conductivity-based” laterally nonuniform inducing source. Our results also highlight the strong dependence of the GEF on the earth's conductivity distribution.

The development of a magnetospheric substorm may be classified into three phases: growth, expansion, and recovery. The growth phase is important as it includes processes that lead to the expansion. In a recent growth-phase study, a type of fast discrete auroral transient phenomena?referred to as Lumikot?were observed. The Lumikot are several kilometers across and move in the high-energy precipitation region, parallel to the main growth-phase arc, with both east-west and west-east directions of travel during the same event. Their apparent transverse movement and quasi-stable intensity make them distinct from cooccurring optical pulsating aurorae. Comparison to other studies show that they occur in the cosmic noise absorption region and it is likely that the Lumikot are colocated with high-energy particle populations on the boundary between the outer radiation belt and the plasmasheet.

Ground-based neutron counters are a standard tool for detecting atmospheric showers from GeV range primary cosmic rays of either solar or galactic origin. Bare neutron counters, a type of lead-free neutron monitor, function much like standard neutron monitors but have different yield functions primarily because they are more sensitive to neutrons of lower energy. When operated together with standard monitors, the different yield functions allow estimates to be made of the energy spectrum of galactic or solar particles. In 2010 a new array of 12 bare neutron detectors was installed at the South Pole to operate together with the neutron monitor there. Prior to installation, two of the detectors were operated on a ship that traveled from Sweden to Antarctica and back from November 2009 to April 2010. The purpose of this latitude survey was to use Earth's magnetic field as a spectrometer, blocking cosmic rays below the local cutoff rigidity (momentum per unit charge), from which we determined the response function versus rigidity of these bare counters. By comparing that measured response function to direct measurements of the cosmic ray spectrum taken by the PAMELA spacecraft, we were able to make a direct determination of the yield function for these detectors.

Eleven proxy records of Northern Fennoscandia and North Atlantic climate variability were analyzed. Correlation of climatic records with (a) a quasi 11-year solar cycle of Schwabe, (b) a quasi 22-year solar cycle of Heil, (c) a quasi 20-year planetary-tidal cycle, related to wobbling of the Sun around the baricenter of the solar system, has been studied. A weak but stable and statistically significant correlation between the climatic proxies of Northern Fennoscandia and a double solar cycle was found to be present through the AD 1700–2000. No evidence of a connection between climatic records and both solar Schwabe cycle and quasi 20-year astronomic cycle were found. Possible physical mechanisms behind the revealed effect are discussed.

Eight proxy records of Northern Fennoscandian summer temperature variability were analyzed for the CE 1700–2000 period. Stable and statistically significant correlation between the summer temperature reconstructions and a quasi 22-year Hale solar cycle was found to be present through the entire study period. The revealed solar–climatic link is a result of the effect of a weak solar cycle signal on a climatic system having internal bi-decadal variability. Precise physical mechanisms to explain this link are far from clear but galactic cosmic ray flux appears a probable physical agent to mediate the solar effect to the lower troposphere. No evidence of a link between Northern Fennoscandian temperature and quasi 20-year planetary-tidal cycle was found.

The present study investigates the dependence of the local auroral electrojet (AEJ) intensity on solar illumination by statistically examining northward geomagnetic disturbances in the auroral zone in terms of the solar zenith angle χ. It is found that on the dayside, both westward and eastward electrojets (WEJ and EEJ) are more intense for smaller χ, suggesting that the solar extreme ultraviolet‐induced conductance is the dominant factor for the AEJ intensity. On the nightside, in contrast, the χ dependence of the AEJ intensity, if sorted solely by the magnetic local time, apparently depends on the station longitude and hemisphere. However, if additionally sorted by the dipole tilt angle ψ, a consistent pattern emerges. That is, although χ and ψ are correlated, the solar zenith angle and dipole tilt angle have physically different effects on the AEJ intensity. The nightside AEJ, especially the WEJ, tends to be more intense for smaller |ψ|. Moreover, whereas the WEJ is statistically more intense when the ionosphere is dark, the EEJ is more intense when it is sunlit. The preference of the WEJ for the dark ionosphere prevails widely in magnetic local time from premidnight to dawn, and therefore, it cannot be attributed to the previously proposed processes of the preferred monoenergetic or broadband auroral precipitation in the dark ionosphere. Instead, it may be explained, at least morphologically, in terms of the conductance enhancement due to the diffuse auroral precipitation, which is also prevalent from premidnight to dawn and is more intense in the dark hemisphere.

The present study investigates the cause of a sharp horizontal (H) magnetic depression observed on the dayside during the 2003 Halloween storm, and discusses if the same process could cause the 1,600 nT H depression observed at Colaba during the 1859 Carrington storm. For the Halloween H depression, it is found that (a) it developed in correlation with southward interplanetary magnetic field (IMF) BZ in the sheath region of a coronal mass ejection; (b) its magnitude decreased significantly with decreasing magnetic latitude; (c) it was highly correlated with westward and eastward sub-auroral zone magnetic deflections at earlier and later local times, respectively; and (d) the westward auroral electrojet (AEJ) enhanced simultaneously in the entire nightside auroral zone, whereas the enhancement of the dayside eastward AEJ was unclear. These features suggest that the dayside R1-sense wedge current system, which was driven by dayside magnetic reconnection, was the cause of the H depression, and the associated field-aligned currents closed primarily with the westward AEJ through the nightside. The Colaba H depression also took place on the dayside and lasted for ?2 hr. Furthermore, it took place within a few hours after the storm commencement, and the westward AEJ enhanced simultaneously in the dawn sector. These similarities suggest that the dayside wedge current system was also the cause of this historical event. The sharp recovery of the Colaba H depression, which has been a challenge to explain, may be attributed to the decay of this current system due to a northward IMF turning.

Various subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45?60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high-speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71?72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already-known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae.

The thermospheric wind response to a sudden westward turning of the ion velocity at a high latitude was studied by analyzing data obtained with a Fabry–Perot interferometer (FPI; 630 nm), Dynasonde, and Swarm A & C satellites during a conjunction event. The event occurred during a geomagnetically quiet period (Kp = 0 +) through the night, but some auroral activity occurred in the north. The collocated FPI and Dynasonde measured the thermospheric wind (U) and ionospheric plasma velocity (V), respectively, in the F region at the equatorward trough edge. A notable scientific message from this study is the possible role of thermospheric wind in the energy dissipation process at F-region altitude. The FPI thermospheric wind did not instantly follow a sudden V change due to thermospheric inertia in the F region. At a pseudo-breakup during the event, V suddenly changed direction from eastward to westward within 10 min. U was concurrently accelerated westward, but its development was more gradual than that of V, with U remaining eastward for a while after the pseudo-breakup. The delay of U is attributed to the thermospheric inertia. During this transition interval, U∙V was negative, which would result in more efficient generation of frictional heating than the positive U∙V case. The sign of U∙V, which is related to the relative directions of the neutral wind and plasma drift, is important because of its direct impact on ion-neutral energy exchange during collisions. This becomes especially important during substorm events, where rapid plasma velocity changes are common. The sign of U∙V may be used as an indicator to find the times and locations where thermospheric inertia plays a role in the energy dissipation process.

We report the results of a study on the movement of the solar wind diamagnetic structure (DS), which is a sequence of smaller-scale microDS being part of the May 18, 2013 coronal mass ejection, from a source on the Sun to Earth’s surface. DS determined from the high negative correlation coefficient (r=–0.9) between the IMF modulus (B) and the SW density (N) on the ACE and Wind satellites at the L1 point, on the THB and THC satellites (r=–0.9) in near-Earth orbit, and on the THA satellite inside the magnetosphere is carried by the solar wind from the Sun to Earth’s orbit, while maintaining its fine internal structure. Having a large size in the radial direction (≈763 Rᴇ, where Rᴇ is the Earth radius), DS flows around the magnetosphere. At the same time, microDS of size ≤13 Rᴇ passes through the bow shock and magnetopause as a magnetized plasmoid in which the ion concentration increases from 10 cm⁻³ to 90 cm⁻³, and the velocity decreases as it moves toward the magnetotail. When a microDS passes through the magnetopause, a pulsed electric field of ~400 mV/m is generated with subsequent oscillations with a period of T~200 s and an amplitude of ~50 mV/m. The electric field accelerates charged particles of the radiation belt and produces modulated fluxes of protons in an energy range 95–575 keV on the day side and electrons in 40–475 keV and protons in 95–575 keV on the night side. In the duskside magnetosphere (19–23 MLT), the substorm activation is observed in geomagnetic pulsations and auroras, but without a magnetic negative bay. In the post-midnight sector (01–05 MLT), a sawtooth substorm occurs without the growth phase and breakup with deep modulation of the ionospheric current and auroral absorption. The duration of all phenomena in the magnetosphere and on Earth is determined by the period of interaction between DS and the magnetosphere (~4 hrs). To interpret the regularities of the magnetospheric response to the interaction with DS, we consider alternative models of the impulsive passage of DS from SW to the magnetosphere and the classical model of reconnection of IMF and the geomagnetic field.

A set of 24 isolated, 46 compound, and 36 multi-night substorm events from the years 2008–2013 have been analysed in this study. Isolated substorm events are defined as single expansion–recovery phase pairs, compound substorms consist of multiple phase pairs, and multi-night substorm events refer to recurring substorm activity on consecutive nights. Approximately 200 nights of substorm activity observed over Fennoscandian Lapland have been analysed for their magnetic disturbance magnitude and the level of cosmic radio noise absorption. Substorm events were automatically detected from the local electrojet index data and visually categorized.

We show that isolated substorms have limited lifetimes and spatial extents as compared to the other substorm types. The average intensity (both in absorption and ground-magnetic deflection) of compound and multi-night substorm events is similar. For multi-night substorm events, the first night is rarely associated with the strongest absorption. Instead, the high-energy electron population needed to cause the strongest absorption builds up over 1–2 additional nights of substorm activity. The non-linear relationship between the absorption and the magnetic deflection at high- and low-activity conditions is also discussed. We further collect in situ particle spectra for expansion and recovery phases to construct median precipitation fluxes at energies from 30 eV up to about 800 keV. In the expansion phases the bulk of the spectra show a local maximum flux in the range of a few keV to 10 keV, while in the recovery phases higher fluxes are seen in the range of tens of keV to hundreds of keV. These findings are discussed in the light of earlier observations of substorm precipitation and their atmospheric effects.

We present an innovative method to reconstruct the characteristics of precipitated electrons in auroral regions from optical measurements. This method is based on an optimization implemented between numerical simulations of the Transsolo code and tomographic maps made from the Auroral Large Imaging System (ALIS) network. We focus on the Volume Emission Rate (VER) of the blue line   1NG 427.8 nm, which is the most representative line of the energy deposition by electrons. The optimization is tested with the ALIS measurements carried out on March 05, 2008, at 18:41:30 UT and 18:42:40 UT. The reconstruction is performed by extracting the energy flux and the mean energy of the precipitating particles. Both Maxwellian and quasi-monoenergetic energy distributions are considered. Calculations performed with a Maxwellian energy distribution yielded a mean energy ranging from 1.8 to 5.2 keV with energy flux from 0.1 to 44.3 erg·cm−2·s−1 for 18:41:30 UT, and a mean energy from 2.2 to 9.5 keV with energy flux from 2.1 to 136.7 erg·cm−2·s−1 for 18:42:40 UT. Assuming a quasi-monoenergetic energy distribution, we find a mean energy ranging from 4.2 to 11.8 keV with energy flux ranging from 0.1 to 45 erg·cm−2·s−1 for 18:41:30 UT, and 8 to 17.1 keV with energy flux ranging from 2.2 to 110.1 erg·cm−2·s−1 for 18:42:40 UT. Moreover, we show this method allows us to reconstruct the energy characteristic of the precipitating electrons on a large region covering approximately 150 km × 150 km. This study also shows that some VER profiles of the maps are better fitted by quasi mono-energetic distributions while some others correspond to broadband distributions. It appears clearly that the energy flux is linked to the column integrated intensity, the mean energy is linked with the peak altitude of the emission, and the width of the energy distribution with the altitude thickness of the emissions.

During the long main phase of the St Patrick’s Day storm on March 17, 2015, we found three separate enhancements of the westward electrojet. These enhancements are observed in the ionospheric equivalent currents computed using geomagnetic data over Fennoscandia. Using data from the IMAGE magnetometer network, we identified localised field-aligned current (FAC) systems superimposed on the pre-existing ionospheric current system. We suggest that these localised current systems are wedgelets and that they can potentially contribute to a larger-scale structure of a substorm current wedge (SCW). Each wedgelet is associated with a negative BX spike. Each spike is recorded at a higher latitude than the former one and all three are very localised over Fennoscandia. The first spike occurred at 17:34 UT and was observed at Lycksele, Rørvik and Nurmijärvi, the second spike was recorded at 17:41 UT and located at Lycksele and Rørvik, whereas the last spike occurred at 17:47 UT and was observed at Kevo and Abisko. Simultaneous optical auroral data and electron injections at the geosynchronous orbit indicate that one or more substorms took place in the polar ionosphere at the time of the wedgelets. This study demonstrates the occurrence of small and short-lived structures such as wedgelets at different locations over a short time scale, 15 min in this case.

In addition to existing empirical models describing the average distributions of energetic electron precipitation into the auroral ionosphere at different activity levels, we develop and test a semiempirical approach to construct dynamical models describing the recurrent features of spatiotemporal development of auroral absorption in the ionosphere during individual substorms. Its key ingredients are (a) usage of linear prediction filter technique to extract from riometer data the response function to the injection of unit magnitude and (b) characterization of injection parameters by midlatitude magnetic variations caused by the substorm current wedge. Using global riometer network we test the method performance for stations in the middle of auroral zone (at corrected geomagnetic latitudes of 65?67°) where generally the absorption amplitude is largest. In this paper we use the midlatitude positive bay index, recently developed by X. Chu and R. McPherron, to drive the model. We evaluate the model performance, discuss the dynamical properties of energetic electron precipitation as revealed by the linear prediction filter response function analyses, and finally, we discuss possible future improvements of this method intended for both science and applications.

During a typical Akasofu-type of substorm, the southward component of IMF Bz is necessary prior to the onset. However, a sudden compression of solar wind, if intense enough, can also sometimes trigger a substorm, and is independent of the IMF orientation. The Akasofu-type substorm and the Impulse-induced substorm may differ in their occurrence mechanism and ground-based observations. This is shown using the initial four substorm events discussed in this paper having distinctly different IMF and sudden impulse conditions. A question then arises is how will these signatures vary when both sudden impulse and a southward component of IMF Bz are present prior to the onset. To account for the same, we analyze two substorm events of 05th April 2010 and 22nd June 2015. The substorm onsets on these days not just coincided with the sudden impulse but also a southward component of IMF Bz was present prior to the onsets. The present study accounts for the similarities and differences among isolated IMF induced substorms, isolated impulse-induced substorms and when both sudden impulse and a southward component of IMF Bz are present. We examined the relative dominance between the two factors in triggering a substorm using ground-based and satellite-based observations. If IMF Bz is near zero, a strong pressure pulse and/or large IMF By can lead to particle precipitation away from the usual midnight. To further ensure whether a pressure pulse or IMF By predominantly influences the substorm onset location, a statistical analysis of isolated substorms will be needed.

The effects of solar flares in the D-region ionosphere at two high-latitude stations: Maitri (70.75°S, 11.75°E) and Abisko (68.4°N, 18.9°E), located in different hemispheres are studied. We analyzed 37 M-class flares and 6 X-class flares of the year 2014, which occurred when either or both stations were in the sun-lit side of the Earth. Cosmic Noise Absorption (CNA) curves are obtained using the datasets of riometers located at the two stations and are analyzed for all the 43 events under study. This paper discusses: 1) relationship between CNA and flare magnitude, 2) relationship between CNA and solar zenith angle (SZA), 3) hemispheric asymmetry in the observed solar flare associate CNA (or SCNA), and 4) the effect of background ionospheric condition in the SCNA magnitude at the two high latitude stations. It is observed that the solar flare effect in SCNA strongly depends on the SZA and flare intensity. Our analysis reveals that the flare response in SCNA for the year 2014 was stronger at Abisko than at Maitri. There is an observed hemispheric asymmetry in the solar flare ionization at D-region ionosphere for the given latitude. This asymmetry can be attributed to the previously enhanced background ionospheric ionization during particle precipitation processes. This study shows the need to establish an empirical relationship between the observed CNA vs. flare intensity, SZA and latitudinal position; especially when we go higher in latitudes.

We examine how Sudden Commencements (SCs) and Storm Sudden Commencements (SSCs) influence the occurrence of high rates of change of the magnetic field (R) as a function of geomagnetic latitude. These rapid, high amplitude variations in the ground-level geomagnetic field pose a significant risk to ground infrastructure, such as power networks, as the drivers of geomagnetically induced currents. We find that rates of change of ?30 nT min?1 at near-equatorial stations are up to 700 times more likely in an SC than in any random interval. This factor decreases with geomagnetic latitude such that rates of change around 30 nT min?1 are only up to 10 times more likely by 65°. At equatorial latitudes we find that 25% of all R in excess of 50 nT min?1 occurs during SCs. This percentage also decreases with geomagnetic latitude, reaching ≤1% by 55°. However, the time period from the SC to 3 days afterward accounts for ≥90% of geomagnetic field fluctuations over 50 nT min?1, up to ?60° latitude. Above 60°, other phenomena such as isolated substorms account for the majority of large R. Furthermore, the elevated rates of change observed during and after SCs are solely due to those classified as SSCs. These results show that SSCs are the predominant risk events for large R at mid and low latitudes, but that the risk from the SC itself decreases with latitude.

The response of seismic noise in the minute range of periods to the strong magnetic storms is studied. The noise is analyzed from the records by IRIS broadband seismic stations located in the different regions of the world. The minute variations in the X-, Y-, Z-components of the magnetic field at different observatories were obtained through the INTERMAGNET world data system. Seismic pulses that emerged at rapid variations in the rate of change of the magnetic field components dX, dY, dZ are detected. The pulses have the amplitudes of ~2 μm and duration of a few min. The high variability in the ratio of the amplitude of seismic pulses to the magnitude of changes in dHx, dHy, dHz indicates a nonlinear process. The amplitudes of seismic pulses are approximately identical at the stations located in the seismically active or quiet regions. The properties of the pulses also do not depend of weather conditions. The pulses are revealed in the records by all seismic stations located in the continents. In the records from the identical stations located on the volcanic islands in the deep part of the Pacific, such pulses are not detected. It is hypothesized that abrupt changes in the electromagnetic field during a storm act as a trigger for the release of energy accumulated in the Earth.

The aim of the present contribution is to investigate the timing relations between enhancements in the Polar Cap (PC) indices and power grid disturbances related to geomagnetically induced currents (GIC). The polar cap indices, PCN (North) and PCS (South), are based on measurements of geomagnetic variations in the central polar caps. These variations are strongly related to the transpolar convection of plasma and magnetic fields driven by the solar wind. During cases of enhanced merging processes at the front of the magnetosphere and subsequent tailward convection of plasma and embedded magnetic fields, the magnetospheric tail configuration may accumulate excess energy, which upon release may cause violent substorm activity. Earlier reports have disclosed remarkably lengthy intervals, ranging up to several hours, of elevated PC index values preceding GIC-related power grid disruptions. The present investigation has shown that the delays of typically 3–4 h between increases in the PC indices and GIC-related power grid disturbances are related to displacements of the substorm processes responsible for strong GIC events to subauroral latitudes where vulnerable power grids reside. The results have shown that PC index values remaining above an “alert level” of 10 mV/m through more than 1 h indicate a high risk for violent GIC events that may threaten power grids and other vulnerable technical systems. These results support the application of real-time PC indices in space weather monitoring and forecast services.

Pulsating aurora (PsA) is a diffuse type of aurora with different structures switching on and off with a period of a few seconds. It is often associated with energetic electron precipitation (>10 keV) resulting in the interaction between magnetospheric electrons and electromagnetic waves in the magnetosphere. Recent studies categorize pulsating aurora into three different types – amorphous pulsating aurora (APA), patchy pulsating aurora (PPA), and patchy aurora (PA) – based on the spatial extent of pulsations and structural stability. Differences in precipitation energies of electrons associated with these types of pulsating aurora have been suggested. In this study, we further examine these three types of pulsating aurora using electron density measurements from the European Incoherent Scatter (EISCAT) VHF/UHF radar experiments and Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) cosmic noise absorption (CNA) measurements. Based on ground-based all-sky camera images over the Fennoscandian region, we identified a total of 92 PsA events in the years between 2010 and 2020 with simultaneous EISCAT experiments. Among these events, 39, 35, and 18 were APA, PPA, and PA types with a collective duration of 58, 43, and 21 h, respectively. We found that, below 100 km, electron density enhancements during PPAs and PAs are significantly higher than during APA. However, there are no appreciable electron density differences between PPA and APA above 100 km, while PA showed weaker ionization. The altitude of the maximum electron density also showed considerable differences among the three types, centered around 110, 105, and 105 km for APA, PPA, and PA, respectively. The KAIRA CNA values also showed higher values on average during PPA (0.33 dB) compared to PA (0.23 dB) and especially APA (0.17 dB). In general, this suggests that the precipitating electrons responsible for APA have a lower energy range compared to PPA and PA types. Among the three categories, the magnitude of the maximum electron density shows higher values at lower altitudes and in the late magnetic local time (MLT) sector (after 5 MLT) during PPA than during PA or APA. We also found significant ionization down to 70 km during PPA and PA, which corresponds to ∼200 keV of precipitating electrons.

Pulsating auroras (PsAs) are low-intensity diffuse aurora, which switch on and off with a quasiperiodic oscillation period from a few seconds to ?10 s. They are predominantly observed after magnetic midnight, during the recovery phase of substorms and at the equatorward boundary of the auroral oval. PsAs are caused by precipitating energetic electrons, which span a wide range of energies between tens and hundreds of keV. Such energetic PsA electrons will deposit their energy at mesospheric altitudes and induce atmospheric chemical changes. To examine the effects of energetic PsA electrons on the atmosphere, we first collect electron flux and energy measurements from low-latitude spacecraft to construct a typical energy spectrum of precipitating electrons during PsA. Among the 840 PsA events identified using ground-based auroral all-sky camera (ASC) network over the Fennoscandian region, 253 events were observed by DMSP, POES, and FAST spacecraft over the common field of view of five ASCs. The combined measurements from these spacecraft enable us to obtain an energy spectrum consisting of nonrelativistic and relativistic (30 eV to 1,000 keV) electrons during PsA. The median spectrum was found to be in good agreement with earlier estimates of the PsA spectra. We then use the Sodankylä Ion-neutral Chemistry (SIC) model to assess the chemical effect of PsA electrons. The observed extreme and median spectra of PsA produce a significant depletion in the mesospheric odd oxygen concentration up to 78%.

During disturbed geomagnetic conditions, the energetic particles in the inner magnetosphere are known to undergo precipitation loss due to interaction with various plasma waves. This study, investigates the energetic particle precipitation events statistically using coordinate observations from the ground riometer network and the inner-magnetospheric satellite mission, Arase. We have compared cosmic noise absorption (CNA) data obtained from the Finnish ground riometer network located in the auroral/sub-auroral latitudes with the comprehensive data set of omnidirectional electron/proton flux and plasma waves in ELF/VLF frequency range from the Arase satellite during the overpass intervals. The study period includes one and a half years of data between March 2017 and September 2018 covering Arase conjunctions with the riometer stations from all magnetic local time sectors. The relation between the plasma flux/waves observed at the satellite with the riometer absorptions are investigated statistically for CNA (absorption >0.5 dB) and non-CNA (absorption <0.5 dB) cases separately. During CNA events, Arase observed elevated electron flux in the medium energy range (2–100 keV), and plasma wave activity in the whistler-mode frequency range (0.5–3 kHz) of the spectra. Our study provides an estimate of the statistical dependence of the electron flux and plasma wave observations at Arase with the ground reality of actual precipitation.

Using the list of the omega structures based on the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment network (Partamies et al., 2017, https://doi.org/10.5194/angeo-35-1069-2017), we obtained a number of important statistical characteristics describing the surface magnetic field. Based on 438 events, typical magnetic variations associated with the passage of the single omega were obtained. The typical variation, obtained using superposed epoch analysis, is associated with a local bending of the westward electrojet and statistically confirms the distribution of equivalent ionospheric currents obtained in earlier observations of single omegas. It was found that during low and moderate geomagnetic activity, the appearance of the omega structures in the dark morning magnetic local time (MLT) sector results in two times higher than average dB/dt on the ground surface. Also, the velocity, direction of movement, and area of omega structures were calculated. It is shown that faster and bigger omegas produce larger time derivatives of the ground magnetic field. Furthermore, we demonstrate that in the 03?08 MLT sector, superposed magnetic variations for the arbitrary events of very high time derivatives |dB/dt| > 10 nT/s, reveal magnetic signatures similar to omegas. Our findings, together with the results described by Apatenkov et al. (2020, https://doi.org/10.1029/2019gl086677), emphasize the important role of omega structures in the formation of large geomagnetically induced currents.

Atmospheric aerosols have an impact on the global radiation budget, and thus on climate, they reduce the air quality and visibility, and have multiple harmful health effects. The climatic significance of aerosols result from their ability to scatter and absorb solar radiation, and, if being large enough, mediate the cloud albedo and lifetime by acting as cloud condensation nuclei (CCN). The climatic effect, however, has a notable uncertainty. Particles can be either directly emitted to atmosphere or they can form there from precursor vapors. The latter is called new particle formation (NPF). Globally, NPF has been estimated to be responsible for even half of CCN sized tropospheric particles. The understanding of the NPF mechanisms and the spatial and temporal variation of NPF in many scales is necessary to correctly represent aerosols in climate models. In this work, we quantified the importance of biogenic organic vapours and anthropogenic sulfuric emissions in the NPF in northern boreal environment. Aerosol number size distribution data from three measurement sites were used to calculate the average continuous increase in aerosol particle diameter and number concentration when air masses travelled over land. A 14-year-long time series of aerosol and gas measurements were used to determine the effect of reduced Kola Peninsula SO2 emissions on aerosol population at Eastern Finnish Lapland. Secondly, this thesis describes in-situ aerosol measurements conducted with a light aircraft within the lowest 4 km of the troposphere. The data were used to determine the vertical and horizontal extent and variability of the NPF events in the surroundings of the Hyytiälä SMEAR II station. The airborne and ground level measurements were compared to find out the representativeness of the on ground measurements in the lowest parts of the atmosphere, in the planetary boundary layer. The results showed that the Aitken mode particles grew, on average, at the apparent rate of around 1 nm h−1 when they travelled over the northern boreal environment during the growing season. The average calculated growth rates during the NPF events were 3 6 times higher than this apparent average growth rate. The result implied that the condensation has a significant role in the particle growth even when NPF is not explicit. Also, the NPF events inside the planetary boundary layer were found to occur in area over a hundred kilometers. However, within this area, a notable variation in nucleation mode particles was observed.

We present the implementation of an improved technique to coherently model the high-latitude ionospheric equivalent current. Using a fixed selection of 20 ground magnetometers in Fennoscandia, we present a method based on Spherical Elementary Current Systems (SECS) to model the currents coherently during 2000?2020. Due to the north-south extent of the magnetometers, we focus on the model output along the 105° magnetic meridian. Our improvements involve fixed data locations and SECS analysis grid and using a priori knowledge of the large-scale currents improving the robustness of the inverse problem solution. We account for contributions from ground induced currents assuming so-called mirror currents. This study produces a new data set of divergence-free (DF) currents and magnetic field perturbations along the 105° magnetic meridian with 1-min resolution. By comparing averages of the data set with an empirical model of the ionosphere we demonstrate the validity of the data set. We show how our data set, in particular its temporal nature, is distinct from empirical models and other studies. Not only can the temporal evolution of the DF currents and magnetic field perturbations be investigated, but the time derivative of said quantities can be analyzed. For application in ground induced currents, we present the statistical properties of where (in magnetic latitude and local time) and at what rate (?Br/?t) the radial magnetic field component fluctuates, a temporal derivative that has received very little attention. We show that ?Br/?t is dependent on latitude, local time, and solar cycle. We present other applications such as Ultra Low Frequency Waves monitoring.

Three new methods for estimating a ratio of the ultralow frequency (ULF; 1?100 mHz) wave equatorial electric field amplitude in the Earth's magnetosphere to ground magnetic field amplitudes for field line resonances (FLR) are described. These methods use ratios of the time series extrema, ratios of the envelope waveform and the ratio of the spectral amplitude at the FLR frequency. These methods were applied to four ULF resonance intervals; three detected by the Van Allen Probe A spacecraft and one detected by the POLAR spacecraft. The intervals were conjoined with the CARISMA and IMAGE ground magnetometer arrays. The spectral ratio results for the Van Allen Probe intervals were approximately twice to three times the ratios estimated from the two time series based methods. The POLAR interval showed similar values across all three methods. The differences are attributed to broadband frequency signals that modify the time series amplitudes, while the spectral method avoids these off-resonant frequencies. Based on the results of this study, a spectral based method for calculating the ratio at the FLR frequency is best.

High temporal and high spatial resolution geoelectric field models of two Mäntsälä, Finnish pipeline geomagnetically induced current (GIC) intervals that occurred within the 7?8 September 2017 geomagnetic storm have been made. The geomagnetic measurements with 10 s sampling rate of 28 IMAGE ground magnetometers distributed over the north Europe (from 52.07° to 69.76° latitude) are the bases for the study. A GeoElectric Dynamic Mapping (GEDMap) code was developed for this task. GEDMap considers 4 different methods of interpolation and allows a grid of 0.05° (lat.) ? 0.2° (lon.) spatial scale resolution. The geoelectric field dynamic mapping output gives both spatial and temporal variations of the magnitude and direction of fields. The GEDMap results show very rapid and strong variability of geoelectric field and the extremely localized peak enhancements. The magnitude of geoelectric fields over Mäntsälä at the time of the two GIC peaks were 279.7 and 336.9 mV/km. The comparison of the GIC measurements in Mäntsälä and our modeling results show very good agreement with a correlation coefficient higher than 0.8. It is found that the auroral electrojet geoelectric field has very rapid changes in both magnitude and orientation causing the GICs. It is also shown that the electrojet is not simply oriented in the east-west direction. It is possible that even higher time resolution base magnetometer data of 1 s will yield even more structure, so this will be our next effort.