We discuss the reasons for the extreme compression of the magnetosphere during the storm on February 27, 2023, when the magnetopause crossed the geostationary orbit. At the same time, aurora was observed at middle latitudes. The global parameters of magnetospheric current systems were calculated from data on the parameters of the interplanetary medium and geomagnetic indices characterizing the evolution of the ring current and the westward auroral electrojet, using a paraboloid model of the magnetosphere. We have calculated the contributions of various current systems to the observed value of the D_st index. The contribution of the tail current sheet is comparable with the contribution of the ring current for this storm. The calculated modelled field is compared with the data of the GOES-16, 18 magnetometers; the results are in good agreement with observations.

В работе обсуждаются причины экстремального сжатия магнитосферы во время бури 27.II.2023, когда магнитопауза пересекала геостационарную орбиту. При этом полярные сияния наблюдались на средних широтах. Глобальные параметры магнитосферных токовых систем были рассчитаны по данным параметров межпланетной среды и геомагнитных индексов, характеризующих эволюцию кольцевого тока и западного аврорального электроджета, с использованием параболоидной модели магнитосферы. Был вычислен вклад различных токовых систем в наблюдаемое значение Dst-индекса. Вклад токового слоя хвоста сравним с вкладом кольцевого тока для данной бури. Рассчитанное модельное поле сопоставлено с данными магнитометров космических аппаратов GOES-16, 18; результаты достаточно хорошо согласуются с наблюдениями.

The results of a study of the dependence of the dynamics of relativistic electron fluxes of the Earth’s outer radiation belt on the geomagnetic storm intensity are presented. A total of 22 geomagnetic storms with |Dst|max of ∼50 to ∼200 nT are considered. The studies are based on experimental data on fluxes of electrons with an energy of ~2 MeV received from the Van Allen Probe spacecraft in the core of radiation belts and from the GOES satellite in geostationary orbit. It is shown that the dominant influence on the dynamics of relativistic electrons in the core of the Earth’s outer radiation belt during strong magnetic storms with |Dst|max ~ ≥120 nT is exerted by global changes of the magnetospheric magnetic field, which lead to adiabatic variations of fluxes of relativistic electrons.

The paper presents the results of a comparative analysis of the dynamics of relativistic electron fluxes of the Earth’s outer radiation belt and correspondent ring current and magnetospheric magnetic field variations for February 15–22, 2014, based on experimental data obtained in the radiation belt core aboard Van Allen Probes satellites and in geostationary orbit aboard the GOES-15 satellite. The A2000 paraboloid model of the magnetosphere is used to calculate the contributions of main magnetospheric current systems to the Dst variation. The influence of external factors—solar wind and interplanetary magnetic field—is considered. Comparison of the dynamics of electron fluxes in different orbits and the geomagnetic field variations for February 15‒22, 2014 indicate that the main mechanisms of the evolution of outer electron radiation belt are global processes: (i) outward and inward shifts of the trapped electron population inside the Earth’s magnetosphere due to large-scale variations of the magnetospheric magnetic field during geomagnetic disturbances and (ii) ExB drift of electrons from the magnetotail under the influence of electric and magnetic fields. Local particle acceleration is auxiliary process in the dynamics of radiation belts. The southward orientation of the interplanetary magnetic field is the necessary condition for an increase in the intensity of electron fluxes of the Earth’s outer radiation belt, while the function (–BzVsw) is the key external factor.

The results of studies of proton flux dynamics of the ring current and magnetic field in the middle of radiation belts during two strong magnetic storms in 2015 (March 17–18, 2015 and June 22–23, 2015) with a close amplitude of Dst -variations (∣ Dst max ∣ ~ 200 nT) are presented. An analysis of the experimental data obtained on board two space vehicles named Van Allen Probes (earlier: Radiation Belt Storm Probes, RBSP ), located at an orbit close to equatorial is performed. Modeling of the ring current and magnetic field in the frames of the paraboloid model of Earth’s magnetosphere А2000 is conducted. Confirmation of the existence of a mechanism for ring current development and, respectively, geomagnetic storm under the action of a strong pulse of solar-wind pressure is obtained as a result of comparative analysis of ring current dynamics and the magnetic field in the middle of radiation belts during the main phases of two storms in 2015. It is found that during the magnetic storm of June 22–23, 2015, the development of the ring current during the main phase continued despite the northward turn of the interplanetary magnetic field (IMF) and the following period with positive B z in the solar wind. The effect observed during June 22–23, 2015 is explained by the intensification of storm ring current due to a long and powerful pulse of the solar wind pressure that led to a nonadiabatic transport of the ring current particles to lower L shells.

The structure of the magnetic field in the magnetospheric during the storm of February 14, 2009 is studied. The model parameters that characterize the magnetospheric magnetic field are calculated every hour on the basis of solar wind data and the evolution of the magnetic field during the storm is reproduced using the A2000 model of the Earth’s magnetosphere. It is shown that extremely quiet geomagnetic conditions in 2009 promoted the expansion of the magnetosphere and were favorable for the formation of magnetic-island-like structures (plasmoids) in the geomagnetic tail. It is ascertained that negative variations in the B z component could occur in the nightside magnetosphere in situations where the magnetic flux through the tail lobes exceeded certain thresholds, which depend on the parameters of the magnetospheric current systems. It is shown that the formation of magnetic islands decreases the magnetic flux through the tail lobes and prevents excessively strong development of the magnetic field in the tail.

The structure of the magnetic field near the inner edge of the magnetospheric tail during the preliminary phase of the magnetic storm on Feb. 14, 2009 was studied based on the data of THEMIS satellites. The magnetic field of the currents that were flowing in the magnetospheric tail was obtained by subtracting the effect of the geomagnetic dipole and the magnetic field of magnetospheric currents other than the tail currents (mainly magnetopause currents) from the magnetic field measured on board the satellites. The paraboloid model of the magnetosphere (A2000) was used for calculating the magnetic field of magnetospheric currents. The comparison of calculations and measurements at five satellites in geomagnetically quiet periods allows one to obtain typical radial profiles of magnetic-field components along the magnetospheric tail. It was demonstrated that the nonstandard structure of the magnetic field near the inner edge of the tail-current sheet before the onset of the magnetic storm on February 14, 2009 is connected with the formation of the transient current system in the magnetosphere. A circular current in the equatorial plane was formed before the onset of the storm in the course of adiabatic compression of the magnetosphere by the solar wind to maintain the magnetic flux through the tail lobes. The spatial position and intensity of the current were determined.

A comparative analysis has been performed for the dynamics of the ring current and ion fluxes at low altitudes during the February 27, 2014 geomagnetic storm. We use concurrent experimental data on ion fluxes with an energy of ~30 to 250 keV in the near-equatorial magnetospheric region at altitudes up to 30000 km from the Van Allen Probes satellite and at a polar orbit up to 1000 km from the Polar Orbiting Environmental Satellite (POES). The main phase of the storm was characterized by increased ion fluxes with E < 100 keV and decreased fluxes with E > 100 keV, both near the ring current and in the low-orbit equatorial region, reflecting the fact that the particle spectrum of the ring current in the main phase of the storm becomes softed. The observed phenomenon may originate from the precipitation of ring-current particles registered by POES below the isotropization boundary. It was shown that the variations in ion fluxes at low orbit during the geomagnetic storm generally reflect the ring current dynamics, although each region under consideration has its own specific features complementing the general pattern of the Earth’s magnetosphere dynamics.

The magnetic field structure and the spatial characteristics of the large-scale currents in the magnetospheric tail were studied during quiet and moderately disturbed geomagnetic conditions in 2009. The magnetic field of the currents other than the tail current was calculated in terms of a paraboloid model of the Earth's magnetosphere, A2000, and was subtracted from measurements. It was found on the base of obtained tail current magnetic field radial distribution that the inner edge of the tail current sheet is located in the night side magnetosphere, at distances of about 10 R-E and of about 7 R-E during quiet and disturbed periods respectively. During the disturbance of February 14, 2009 (Dst(min) similar to -35 nT), the B-x and the B-z component of the tail current magnetic field near its inner edge were about 60 nT, and -60 nT that means that strong cross-tail current have been developed. The tail current parameters at different time moments during February 14, 2009 have been estimated. Solar wind conditions during this event were consistent with those during moderate magnetic storms with minimum Dst of about 100 nT. However, the magnetospheric current systems (magnetopause and cross-tail currents) were located at larger geocentric distances than typical during the 2009 extremely quiet epoch and did not provide the expected Dst magnitude. Very small disturbance on the Earth's surface was detected consistent with an "inflated" magnetosphere. (C) 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.