Varvara A. Andreeva

A pair of relatively mild geomagnetic storms of February 3–4, 2022, resulted in the loss of 38 Starlink satellites. In this work we reconstruct a sequence of magnetospheric configurations during that event, based on the largest ever archive of historical data, a new magnetic field model, and the nearest‐neighbor method of mining the solar wind, magnetosphere, and ground activity data. The model is constructed as a combination of main modular framework, explicitly representing magnetic effects of principal current systems, and a high‐resolution superstructure, based on expanding the field of equatorial currents into a sum of quasi‐orthogonal harmonics, whose purpose is to correct residual inaccuracies of the modular component. In addition, the variable penetration of the interplanetary magnetic field (IMF) into the magnetosphere is taken into account in the form of a potential field, parameterized by the observed IMF components. Despite relatively modest intensity of the event in terms of Sym‐H peak values, surprisingly strong transient distortions of the global magnetic field are found. Whereas under quiet pre‐storm conditions the midnight segment of synchronous orbit and the dayside cusps mapped, respectively, to ∼67° and ∼77° of corrected latitude, at the peak of the storm their footpoints shifted equatorward to as low as ∼63° and ∼67°–68°, with formation of entangled flux‐rope‐like structures and magnetic neutral lines at distances R ∼ 12–15 R_E on the nightside. At nearly the same time, a deep magnetic depression and, possibly, an island of reversed field polarity forms even closer, in the post‐dusk sector of geosynchronous orbit.

Quantitative empirical models combined with historical data archives serve as one of the main tools to reconstruct the geospace magnetic field dynamics during space weather events. The essence of the approach is to develop a flexible mathematical framework, aimed to derive the magnetosphere's response to varying interplanetary conditions using multi-year pools of spacecraft observations. There exist two different ways to formulate an empirical model: (i) the so-called "modular" approach, in which the total field is associated with a few current systems having a priori defined geometry, implied by previous experimental and theoretical knowledge. Alternatively, (ii) the magnetic field can be expanded into series of formal basis functions with a minimum of predetermined assumptions. Both techniques have their own pluses and minuses. In the present paper, an efficient method is described to maximize the advantages and minimize the drawbacks by combining the modular part of the model field, on the one hand, and the basis function expansions, on the other, into a synthetic "hybrid" approximation. In such a setting, the modular part renders the main features of the magnetospheric structure, while the basis function expansions take on the role of a second-order correction that removes the remaining inaccuracies. Our primary goal here is to reconstruct the magnetosphere dynamics during individual events rather than develop a universal magnetic field description. For that purpose, the hybrid model is combined with the "nearest-neighbor" data mining in the space of magnetospheric state variables, which allows one to create sequential data subsets and reconstruct the magnetic field for different time moments during a specific storm or substorm. The importance of proper choice of the state variables is discussed, including the ground activity indices and the solar wind/IMF parameters.

The evolution of the low‐latitude magnetosphere over the substorm cycle is reconstructed based on a new high‐resolution 3D representation of the magnetic field and nearest‐neighbor data mining. The study covers radial distances 2.5–25 and employs a record‐large pool of spacecraft data taken during 1995–2019. The magnetospheric state is quantified by four indices, representing the ground geomagnetic activity and its temporal trends in the entire range of geomagnetic latitude: the SuperMAG SMR, the midlatitude positive bay MPB, the auroral SML, and the polar cap PC index. The developed technique has been tested on specific substorm events, with the results presented in the form of 5‐min cadence diagrams and animations of the magnetic field line configurations and electric current distributions. In all the analyzed events, the initial intensification and radial expansion of the inner tail current is accompanied by a gradual stretching of the magnetic field, followed by its sudden collapse, dramatic depletion of the current beyond , and a large‐scale dipolarization of the field around the time of MPB peak, after which the system recovers and tends to its pre‐substorm state.

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.

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 R_E 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.

To study further the factors and mechanisms controlling 10–150 keV particle fluxes in the inner magnetosphere, we investigate empirically their behavior in the nightside transition region (6–14 Re) depending on solar wind parameters taken at different time lags. We aim to establish the hierarchy of predictors (V, N, Pd, Ekl = VByz sin²(θ/2), etc.) and the optimal range of their time delays, both depending on the distance and local time. We use THEMIS 5‐min averaged observations of energetic proton and electron fluxes in 2007–2018 near the plasma sheet midplane and build regression models exploring the combination of predictors, taken at time delays up to 24 h. The model obtained shows that protons and electrons are controlled differently by solar wind parameters: electrons are influenced equally by Vsw and Ekl, whereas protons are controlled mostly by Vsw and Pd and less by Ekl. We found that a wide range of time delays is involved depending on distance and particle energy. Specifically, the Ekl affects the energetic fluxes with time delays up to 24 h (or more), exhibiting the long delays in the innermost regions. As regards the mechanism of Vsw influence, the Vsw‐related flux changes are large and, to a large extent, established on the route of the energy flow from solar wind to the plasma sheet and, eventually, the inner magnetosphere. We also identified a new parameter, NBL = VByz cos²(θ/2), which helps to reveal the loss processes in the plasma sheet transition region.

First results are presented of reconstructing the evolution of magnetospheric configurations through the full cycle of isolated substorms. The modeling covers the low- and mid-latitude magnetosphere in the range of radial distances from 2 to 20 Re and is based on a synthesis of (1) a high-resolution representation of the magnetic field by cylindrical basis functions, (2) the ever largest pool of magnetospheric and interplanetary data spanning the last quarter century (1995-2019), (3) an archive of concurrent ground-based indices and their temporal trends, quantifying the geomagnetic activity over the full range of latitudes, including the low-latitude ring current SMR-index, the midlatitude positive bay MPB-index, the auroral SML-index, and the polar cap PC-index, (4) the data-mining nearest-neighbour (NN) technique of the data selection and weighting in the geometric and parametric spaces. The obtained successive diagrams of magnetic depression/compression, electric current, and field line maps demonstrate all the typical features of the substorm cycle: the initial relatively slow stretching of the nightside tail during the growth phase, followed by its sudden collapse associated with a dramatic disruption of the tail current at R~11-16 Re, and finally a gradual recovery of the configuration after the expansion phase is over.

First results are presented of the modeling of magnetospheric storm events, based on: (i) a new method to represent the magnetic field by means of the so‐called cylindrical basis functions, (ii) the data mining approach by Sitnov et al. (2008); https://doi.org/10.1029/2007ja013003, and (iii) upgraded and extended pool of multi‐mission data taken in 1995–2019. The study is focused on the low‐latitude magnetospheric domain in the distance range 3–18R_E bounded by field line shells with footpoint latitudes ±70°. The magnetic configurations are reconstructed from data subsets, selected from the grand database by the nearest‐neighbor method, using both interplanetary data and the ground disturbance indices. A strong storm of May 27–29, 2017, has been studied in relation to its effect on the reconfiguration of the low‐latitude magnetosphere. The modeling reproduces the main features of the magnetosphere dynamics in terms of the geomagnetic field depression/compression and extremely variable field line stretching. The initial contraction of the magnetosphere during the storm sudden commencement results in a local transient surge of the inner tail current and a dramatic antisunward discharge of the magnetic flux. As the storm progresses, the ring current buildup results in a strongly depressed magnetic field in the inner magnetosphere, which expels the magnetic flux to larger distances and increases the field line connection across the more distant tail plasma sheet. At the same time, a strong dawn‐dusk asymmetry is developed due to the formation of the duskside partial ring current, in agreement with previous independent results.

Omega bands are curved aurora forms, which appear as rows of inverted Greek letter Ω drifting eastward and may result in substantial magnetic field variations on the ground. Since they were reported for the first time more than 50 years ago, their ionospheric signatures were thoroughly studied to the present moment. In contrast, magnetospheric processes resulting in the omega-bands generation are poorly understood, mostly due to a small number of conjugated spacecraft observations. Therefore, the only possibility to statistically study magnetospheric features of the omega bands is to use different models.The goal of the present work is to find a characteristic magnetic field configuration corresponding to this type of aurora. We used the list of omega bands (Partamies et al., 2017), observed in the Fennoscandian Lapland in the period 1997-2007, the MIRACLE all-sky camera data, and new empirical magnetic field model (Tsyganenko and Andreeva, 2016) to identify the magnetospheric equatorial location of the observed omega structures. This work presents the most extensive statistical study of the omega bands projections; in previous papers only a case-study mapping based on few events was described. We found that for 90% of the omega bands aurora its possible source is located on the radial distances from the Earth 6-13 Re in the morning sector (2-4 h MLT), with the average position at R=8 Re and 3 MLT. We also estimated a minimal life-time of the omega bands source in the magnetosphere. This study has been funded by the Russian Science Foundation Grant 19-77-10016.Partamies, N., Weygand, J. M., and Juusola, L.: Statistical study of auroral omega bands, Ann. Geophys., 35, 1069–1083, https://doi.org/10.5194/angeo-35-1069-2017, 2017.Tsyganenko, N. A., and V. A. Andreeva (2016), An empirical RBF model of the magnetosphere parameterized by interplanetary and ground-based drivers, J. Geophys. Res. Space Physics, 121, doi:10.1002/2016JA023217.