Geomagnetism and Aeronomy

The paper analyzes precipitation of relativistic (>800 keV) electrons on the basis of data from the NOAA/POES low-orbit satellite for the period of October 08-18 2017, which included a moderate geomagnetic storm. Along with precipitation into the local loss cone caused by the pitch-angle scattering, we provide the first detailed analysis of precipitation into the drift loss cone usually recorded in the zone of weakened magnetic field on the periphery of the South Atlantic Magnetic Anomaly, where the outer radiation belt projects. A common property of relativistic electron precipitations at local scattering is a small latitudinal size of the precipitation region (~1°), a high intensity (of the precipitating electron fluxes 10³–10⁵ (cm² s ster)^-1), a fairly high isotropy of the fluxes, and registration in both hemispheres. In contrast, the relativistic electrons precipitating into the drift loss cone can be recorded over a wide latitude range (up to ~8°), have low intensity (precipitating electrons fluxes are 10³–10⁴ (cm² s ster)^-1), and the fluxes of trapped particles exceed the fluxes of precipitating particles by more than an order of magnitude. The dynamics of precipitation of relativistic electron fluxes caused by various mechanisms at different stages of a geomagnetic storm is analyzed. Precipitations caused by interaction with electromagnetic ion cyclotron waves dominate during the main phase of the storm. Precipitations due to the large curvature of geomagnetic field lines and to interaction with whistler waves correlate with the substorm activity.

Characteristics of the auroral precipitation and auroral luminosity under extremely low and extremely high magnetospheric activity were examined using DMSP spacecraft data and the algorithms for calculating the ion pressure and the integral intensity of the emission of atomic oxygen (OI) 557.7 nm. Averaged radial pressure distributions were determined during magnetically quiet periods and during large magnetic storms. A regression equation is presented that relates the corrected geomagnetic latitude of the equatorial boundary of isotropic precipitation to the level of magnetic activity and a regression equation that allows the ion pressure to be estimated at this boundary depending on latitude. The equations can be used for periods with very high levels of magnetic activity, at least for Dst > -600 nT and AL > -2000 nT. The obtained patterns can be used to determine the position of the equatorial boundary of geo-induced currents that occur in conductive technological systems, primarily in extended power transmission lines during magnetic disturbances.

Long-term changes (trends) in the critical frequency and height of the E and F2 ionospheric layers are compared. The parameter Δ is analyzed, which characterizes changes in the frequency (height) of the layer in a given year relative to its value under the same solar activity in the reference period, when no trends were observed. The time changes in this parameter are considered for each characteristic of the E and F2 layers. The correlation coefficient R is used to quantitatively estimate the results of this comparison. Observations by the vertical sounding method at the Juliusruh, Moscow, and Sverdlovsk stations before 2024 are analyzed. It is established that both parameters of the F2 layer change synchronously throughout the entire analyzed period. The relation between the characteristics of the E layer and the height of the F2 layer is more complicated. No stable relation between these characteristics and ΔhmF2 was detected during the entire period under consideration. However, there are two periods (before 2005 and after 2014), when, at large R, this relation is either positive or negative depending on local time. At the same time, a decrease in Δh'E (i.e., a lowering of the E layer) is seen in both periods. Some suggestions are made concerning the aeronomical processes that may be responsible for the observed effects.

A model study of the ionosphere response to the geomagnetic storm of May 10-11, 2024 was carried out, and the causes of the abnormally strong negative change in the total electron content (TEC) were analyzed. The results of calculations using the Global Self consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP) were compared with the data from the total electron content maps. The comparison showed a satisfactory qualitative agreement in the space-time structure of the disturbances. However, the scale of negative TEC deviations in the models was lower, and their spatial coverage was smaller than in the experimental data. A tidal analysis showed that extreme TEC disturbances were caused mainly by changes in the zonal-mean thermospheric parameters rather than wave processes. Satellite observations indicate that discrepancies between the model and the TEC maps are associated with a decrease of the O/N₂ ratio in the GSM TIP. One of the possible reasons for the decrease in O/N₂, along with the heating of neutral gas, could be the intensification of turbulent processes enhanced by the increased energy deposition in the lower thermosphere during an intense storm.

The STEVE and Picket Fence events are studied using observation data from the Maimaga (58° GMLAT, 202° GMLONG) and Zhigansk (62° GMLAT, 195.5° GMLONG) stations for the period from 2015 to 2023. The following results are obtained: the spatial location of the STEVE and Picket Fence events in magnetic longitude at the Zhigansk and Maimaga stations was in the range of 19–01 MLT, and their averaged starting positions differed by ~1 hour MLT (more precisely, 13.5° of magnetic longitude). The STEVE and Picket Fence events occurred at the end of a prolonged (up to ~1 hour) expansion phase of magnetospheric substorm, when the SME index reached its peak. At Maimaga, these events were observed under stronger magnetic disturbance (the SME index value of 1200 nT) than at Zhigansk (600 nT). The occurrence of subauroral airglow was analyzed depending on parameters of the interplanetary medium, and the results of simultaneous observations of STEVE/Picket Fence and ELF-VLF disturbances recorded at the Oybenkel radiophysical station (56.8° GMLAT, 202° GMLONG) were described. The ELF-VLF noise disturbances at the nearby Oybenkel station were accompanied by the STEVE and Picket Fence at Maimaga recorded in 11 events with the magnetic index Kp ≥ 4-, which is explained by a shift of the plasmapause during the substorm expansion phase to subauroral latitudes up to Maimaga station.

Ground-based magnetometers were used to record specific geomagnetic field variations caused by disturbances in lower ionospheric current systems that followed a strong volcanic eruption in Kamchatka, Russia, on March 15, 2019 (Bezymyanny Volcano). The analysis of the measurements taken during a series of explosions revealed that the lower ionosphere is affected by both the seismic Rayleigh waves (which are the source of acoustic waves propagating into the ionosphere) and the atmospheric internal gravity waves generated by explosions. At distances from the source up to a thousand kilometers, two stations revealed a repeatable pattern of ionospheric disturbances.