During a geomagnetic storm, the strong asymmetric ring current is formed in the night sector of the magnetosphere16. It mainly consists of ions with energies of 10–100 KeV. The RIT is developed as a result of hot ions precipitation from the magnetospheric ring current during its decay at the recovery phase of the storm3. The precipitation occurs when hot ions interact with the cold particles of the plasmasphere. During the recovery phase, the plasmasphere fills up and expands gradually. Therefore, the decay process is most intense in the outer part of the plasmasphere, i.e., in relatively narrow latitude belt. The precipitating ions heat the thermosphere, its temperature rises, the recombination increases, and an ionization trough is formed.
During the great storm (Dst < -300 nT), the equatorward edge of the ring current reaches L~1.7, i.e. the geomagnetic latitude of ~40°17. This is extremely low latitude for the RIT at the maximum of the main phase or the beginning of the recovery phase of the storm. As the recovery phase develops, the plasmasphere expands, and the equatorward edge of the ring current, due to its decay, also moves to the larger L-shells. At the late stage of the recovery phase, the ions precipitation from the region of the residual ring current is observed for a long time at L~2.7‒4.0 (52‒60°)18. The RIT is most often observed at latitudes of 54‒56°. Thus, the region of the RIT existence is located in the latitude belt from 40° to 60° (L~1.7‒4.0).
The magnetospheric ring current is located in the region of quasi-trapped particle population; the lower the height of the mirror point in the atmosphere, the more intense the ions precipitation is. The height of the mirror point depends on the geomagnetic field magnitude. The magnitude, in its turn, strongly depends on longitude. Thus, we can assume that the RIT behavior should also depend on longitude. In Fig. 1 in the Southern hemisphere, bold lines indicate longitudes of 270‒360‒45° with low geomagnetic field magnitudes. It can be seen, that the RIT is actually formed mainly at longitudes where the geomagnetic field is weak. It appeared just at the latitude of ~40° and was observed for a long time at the latitude of ~54° at the late stage of the storm recovery phase. In the Northern hemisphere, the longitudes with reduced geomagnetic field magnitudes are almost the same as in the Southern hemisphere. However, in absolute values, they are much larger than the ones in the Southern hemisphere. Therefore, being oscillate along the magnetic field line and drift around the Earth, all the particles should be precipitated in the Southern hemisphere at longitudes with low geomagnetic field magnitudes19. However, as seen in Fig. 1, the RIT in the Northern hemisphere was most frequently recorded in the period of 47–55 UT, just at the longitudes marked by a bold line. In this regard, it should be noted that in the same work Berg found that the intensity of proton fluxes in the Northern hemisphere is not directly related to local longitude of observation. In addition, longitudinal effect decreased sharply when the geomagnetic activity increased.
The latitudes from 34° to 45° (L~1.45‒2.00), where the LLP is observed, refer to the region of the inner radiation belt (IRB). It is usually populated by trapped protons with energies of 20‒500 MeV and occupies the region just up to L~2, with a maximum at L=1.5. During a magnetic storm, the fluxes of energetic protons and electrons increase sharply (by 1-3 orders of magnitude), apparently due to radial diffusion from higher L-shells20,21,22,23. These increases are also observed during the storm recovery phase. The latitudinal cross-section of the energetic particles fluxes during a storm has the complex structure, it consists of several peaks. During various storms, the KORONAS, SERVICE-1, ACTIVE satellites and MIR station recorded peaks in proton and electron fluxes at L ~ 1.1, 1.4, 1.5, 1.7, 1.8, 1.9, 2.1, 2.215,22,24,25. These peaks are quasi-stationary time-wise, but change their position in latitude. The fluxes of the trapped particles are associated with very intense precipitation of energetic particles during great geomagnetic storms26,27,28.
The quasi-trapped energetic particles precipitate into the ionosphere as a result of pitch-angular diffusion. The diffusion is caused by recharge on neutral hydrogen, scattering on magnetic field irregularities, and ion-cyclotron waves23,29. In addition to the direct ionization effect, the enhanced particle precipitation causes an increase in the ionospheric conductivity at the heights of E layer, growth of the conductivity gradients and, subsequently, a generation of strong local electric fields, that are capable of producing strong upward/downward drifts of highly ionized plasma21,30,31. In case of sudden changes in the Bz IMF, rapid penetration of the interplanetary electric field is also observed at low latitudes32.
Depending on the direction of the electric field, the drift is either upwards or downwards. This can lead to either increase or decrease in the ionospheric plasma density. Ion density enhancements in the topside low-latitude ionosphere during the Bastille storm on 15–16 July 2000 and Halloween storms on 29–31 October 2003 were recorded using data from ROCSAT-1/IPEI experiment21. Prominent ion density enhancements demonstrate similar temporal dynamics both in the sunlit and in the night-side hemispheres. The ion density increases dramatically (up to two orders of magnitude) during the geomagnetic storms. The density enhancements are mostly localized in the region of the South Atlantic Anomaly (SAA), which is characterized by very intense fluxes of energetic particles. These fluxes were investigated using SAMPEX/LEICA data on >0.6 MeV electrons and >0.8 MeV protons at around 600 km altitude. During the magnetic storms, the energetic particle fluxes in the SAA region and in its vicinity increase more than by three orders of magnitude.
An analysis of the ion temperature and drift velocity in the SAA region shows that the ion enhancements are accompanied by the enhanced temperature33. Plasma heating leads to an increase in recombination at the heights of the F layer, and to density depletion. Thus, the processes occurring at the IRB latitudes can lead to the formation of ionization peaks and the development of troughs. Since in study21 sharp increases in the ion density were found and attributed to the electron precipitation, the Fig.1 was carefully analyzed to determine whether the structures on the latitudinal fp profiles are peaks or troughs. And yet, the clearly defined structures on passes 18 and 20 in the Northern hemisphere and 7, 23, 25 on April 12 and 21 on April 13 in the Southern hemisphere speak in favor of ionization troughs.
The mirror points for quasi-trapped particles of the inner radiation belt, as well as for the ring current, depend on geomagnetic field magnitude. They fall most low in the SAA region. Accordingly, when particles drift around the Earth, the L-shell is rapidly emptied at the longitudes and latitudes of this anomaly. Therefore, the majority of the phenomena associated with precipitation of energetic particles from IRB: an increase in plasma density, in ion temperature, atmospheric glows, etc. are most often observed in the SAA region. However, particle precipitation from radiation belts and the related phenomena are recorded at other longitudes, too24,25,33,34. Fig. 1 illustrates that the most pronounced LLT were observed at longitudes with high magnetic field magnitudes, for example, at passes 19–25 in the Southern hemisphere. Thus, the dependence of the appearance of RIT and LLT on longitude is rather statistical. This issue will be conveyed in the next paper.