Abstract. Using data on three superstorms, we study new features of the saturation of the polar cap area when the solar wind (SW) increases. The polar cap saturation is shown to occur when the SW dynamic pressure and southward vertical (IMF) component rise. The saturation is realized not only during the passage of interplanetary magnetic clouds, but also at significant enhancement of SW density when the SW thermal pressure is comparable with the pressure of the interplanetary magnetic field. We assume that under such conditions the saturation is caused not only by a decrease in the efficiency of reconnection at the dayside magnetopause, but mainly by finite magnetosphere compressibility -stopping the magnetopause compression due to a rapid earthward growth of the geomagnetic field, i.e. the inner magnetospheric structure of the geomagnetic field. We have found signs of saturation depending on the northward IMF component. We assume that the IMF-dependent saturation exists for both signs of its vertical component due to an increase in the total pressure in the magnetosheath. Moreover, when penetrating into the magnetosphere, the southward IMF component reduces the geomagnetic field and thereby causes additional compression of the magnetopause and, accordingly, an increase in the saturation level of the polar cap area.

The magnetogram inversion technique (MIT), developed at ISTP SB RAS more than forty years ago, has been used until recently only in the Northern Hemisphere. In recent years, MIT has been improved and extended to make instantaneous calculations of 2D distributions of electric fields, horizontal and field-aligned currents in two polar ionospheres. The calculations were carried out based on one-minute ground-based geomagnetic measurements from the worldwide network of stations in both hemispheres (SuperMAG). In this paper, this extended technique is used in the approximation of uniform ionospheric conductance and is applied for the first time to calculations of equivalent and field-aligned currents in two hemispheres through the example of the August 17, 2001 geomagnetic storm. We have obtained the main and essential result: the advanced MIT-ISTP can calculate large-scale distributions of ionospheric convection and FACs in the Northern (N) and Southern (S) polar ionospheres with a high degree of expected hemispheric similarity between these distributions. Using the said event as an example, we have established that the equivalent and field-aligned currents obtained with the advanced technique exhibit the expected dynamics of auroral electrojets and polar caps in two hemispheres. Hall current intensities in polar caps and auroral electrojets, calculated from the equivalent current function, change fairly synchronously in the N and S hemispheres throughout the magnetic storm. Both (westward and eastward) electrojets of the N hemisphere are markedly more intense than respective electrojets of the S hemisphere, and the Hall current in the north polar cap is almost twice as intense as that in the south one. This interhemispheric asymmetry is likely to be due to seasonal conductance variations, which is implicitly contained in the current function. From FAC distributions we determine auroral oval boundaries and calculate magnetic fluxes through the polar caps in the N and S hemispheres. These magnetic fluxes coincide with an accuracy of about 5 % and change almost synchronously during the magnetic storm. In the N hemisphere, the magnetic flux in the dawn polar cap is more intense that that in the dusk one, and vice versa in the S hemisphere. These asymmetries (dawn–dusk and interhemispheric) in the polar caps are consistent with the theory of reconnection for IMF By>0 and with satellite images of auroral ovals; both of these asymmetries decrease during the substorm expansion phase.

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.

Аннотация. Исследуются особенности насыще-ния роста площади полярной шапки при усилении солнечного ветра (СВ) по данным трех супербурь. Показано, что насыщение роста площади полярной шапки наблюдается при увеличении как динамиче-ского давления СВ, так и южной компоненты меж-планетного магнитного поля (ММП). Насыщение реализуется не только во время прохождения меж-планетного магнитного облака, но и при значитель-ном усилении плотности СВ, когда тепловое давле-ние сравнимо с давлением ММП. Мы предполагаем, что при таких внешних условиях насыщение обу-словлено в первую очередь не процессами во внеш-ней магнитосфере (уменьшением эффективности пересоединения на дневной магнитопаузе), а конеч-ной сжимаемостью магнитосферы -торможением сжатия магнитопаузы вследствие быстрого роста гео-магнитного поля при ее приближении к Земле, т. е. внутримагнитосферной структурой геомагнитного поля. Определены признаки насыщения в зависимости от северной компоненты ММП. Мы предполагаем, что насыщение роста площади полярной шапки в зависи-мости от ММП существует при обоих знаках его вер-тикальной компоненты за счет увеличения полного давления в магнитослое. Южная компонента ММП, кроме того, проникая в магнитосферу и уменьшая гео-магнитное поле, тем самым вызывает дополнительное сжатие магнитопаузы и, соответственно, рост уровня насыщения площади полярной шапки.Ключевые слова: магнитосфера, солнечный ве-тер, полярная шапка, магнитный поток.Abstract. Using data on three superstorms, we study new features of the saturation of the polar cap area growth when the solar wind (SW) increases. The saturation of the polar cap is shown to occur when the SW dynamic pressure and the southward vertical (IMF) component rise. The saturation is realized not only during the passage of interplanetary magnetic clouds, but also at significant enhancement of SW density when the SW thermal pressure is comparable with the pressure of the interplanetary magnetic field. We assume that under such conditions the saturation is caused not only by a decrease in the efficiency of reconnection at the dayside magnetopause, but mainly by a finite magnetosphere compressibility -stopping the magnetopause compression due to a rapid earthward growth of the geomagnetic field, i.e. the inner magnetospheric structure of the geomagnetic field. We have found signs of saturation depending on the northward IMF component. We assume that the IMF-dependent saturation exists for both signs of its vertical component due to an increase in the total pressure in the magnetosheath. Moreover, when penetrating into the magnetosphere and reducing the geomagnetic field, the southward IMF component causes additional compression of the magnetopause and, accordingly, an increase in the saturation level of the polar cap area.

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.