Volume 63, Nº 4 (2023)

In this paper, we study the amplitude and phase characteristics of VLF signals of an anthropogenic
origin during solar proton events using the methods of a computational experiment. We consider the events of
October 30, 2003 and January 23, 2012. Electron density profiles are plotted using data from the VHF EISCAT
incoherent scatter radar located in Tromsø, Norway. Based on the processed data of computational experiments,
that under the conditions of solar proton events, mainly amplitude distortions of VLF signals were
shown to be observed while there is a frequency dependence of the magnitude of distortions of the signals of
the RSDN-20 long-range navigation radio system. The signal phases of the RSDN-20 system are less affected
by weak solar proton events. The effect of the lower boundary of the Earth-ionosphere waveguide in the cases
of propagation of signals from the RSDN-20 system over the surface of land and ocean during a solar proton
event was studied.

The study of diurnal variations of the statistical characteristics of the day-to-day variability of the
NmF2 electron number density of the F2 ionosphere layer for each month (M) of the year in geomagnetically
quiet conditions at low solar activity according to hourly measurements of the critical frequency of the F2 ionosphere
layer was carried out in Huancayo and Jicamarca. We calculated the NmF2E mathematical expectation,
the NmF2A arithmetic mean, the most probable NmF2MP, the NmF2MED arithmetic mean monthly
median, standard deviations σE, σA, σMP, and σMED and coefficients of variations CVE, CVA, CVMP, and
CVMED of the NmF2 value from NmF2E, NmF2A, NmF2MP, and NmF2MED, respectively. It has been found
that the difference of NmF2MED(UT,M) from NmF2E(UT,M) does not exceed 46%, NmF2MP(UT,M) from
NmF2E(UT,M): 102% and NmF2MP(UT,M) from NmF2MED(UT,M): 85%, where UT is Universal Time.
The calculated statistical parameters σE, σA, σMP, σMED, CVE, CVA, CVMP, and CVMED are the characteristics
of the NmF2 variability from one day to another day at fixed M and UT values over low-latitude Huancayo
and Jicamarca ionosondes in geomagnetically quiet conditions at low solar activity. The calculations showed
that the CVE, CVMED, and CVMP coefficients vary between 18%–82%, 19%–107%, and 18%–288%, respectively,
and in the majority of cases, CVE(UT,M) is less than CVMED (UT,M) and CVMP(UT,M). It was shown
that minimizing the standard deviation and the variation coefficient of NmF2 using the mathematical expectation
of NmF2 provides the best description of the set of NmF2 measurements with one single NmF2 statistical
parameter under the considered conditions. The lowest CVE varies from 18% (April) to 29% (September)
and the highest CVE ranges from 63% (November) to 73% (January). The average CVE value (average relative
day-to-day variability of NmF2) is highest in September (40%) and lowest in April (33%).

During the “Terminator” space experiment on board the International Space Station, a layered
optical structure was discovered in the lower thermosphere. The observations were carried out in the visible
(540 nm) and near infrared (700 and 830 nm) spectral ranges. A brief description of the scientific equipment
is given. This paper presents the results of processing the acquired images, which allowed interpreting them
as images of an aerosol layer that has a planetary scale. Possible variants of the origin of this aerosol formation
are considered.

The frequency dependence of transmitted information qualitative indicators is analyzed on the
example of two meridional radio links: single-hop (~2600 km) and dominant two-hop (~5100 km) for basic
modes of radio wave propagation in the ionosphere. It is shown that the presence of highly efficient receivingtransmitting
antennas in a radio communication system leads to the need to take the existence of a priori energetically
extremely weak modes into account in the problem of radio path specification statement. In this
case, we consider those formed exclusively by the mechanism of radiation transfer along the upper-angles ray
trajectories in the ionospheric propagation of radio waves. If the angles of departure and arrival for such
modes and the directions of the main lobes of the antenna patterns at the end points of the radio path coincide,
the signal-to-noise ratio for the wave field can reach the required threshold value and ensure the successful
operation of the radio communication system. This may expand the upper frequency limit for the passage
of radio waves in the transition regions of jump propagation of radio waves in the ionosphere; it should
be taken into account in forecasting the operation of ionospheric radio links.

We use data from the Atmospheric Imaging Assembly (AIA) telescope on the Solar Dynamics
Observatory (SDO) spacecraft in the 171 Å channel to investigate the spatial distribution of low-energy
flares (nanoflashes). We have studied two periods: 05/20/2019 from 1200 UT to 1300 UT and 05/10/2020
from 1200 UT to 1300 UT. In total, we found 87974 nanoflares with an average formation rate of 6.0 ×
10‒21 cm–2 s–1. For solar latitudes from 0° up to 50° the nanoflare formation rate is approximately uniform
with a standard deviation of about 25%. We have found an asymmetry in the rate of formation of nanoflares
in the southern and northern hemispheres of the Sun: the rate of formation of nanoflares in the southern
hemisphere was 34–42% higher than in the northern one. During this period, the same asymmetry was
also observed for ordinary f lares. We also found a weak dependence of the nanof lare formation rate on the
solar cycle: the number of nanof lares increases with higher solar activity.

Based on the catalogs of solar proton events for the 23rd and 24th solar cycles, events were chosen
that lacked a reliable source of particles but were accompanied by interplanetary and geomagnetic disturbances.
As a rule, these events involve small proton fluxes that are detected near the Earth. All of the selected
events occurred during the arrival of shock waves to the Earth, suggesting that they were likely caused by the
arrival of energetic storm particles. It has been shown that flares accompanied by coronal mass ejections,
which occurred tens of hours before the onset of the increase in particle fluxes in Earth’s orbit, could be the
source of these events. The selected events exhibited several specific features. Only one of them was accompanied
by a single shock front, while the others had two or three shock waves. The time profile of the events
resembled a structure bounded by two shock fronts, suggesting that the shock waves likely accelerated and
confined the particles within a limited region of space.

In this paper, we statistically analyzed substorm activity at auroral latitudes for 2007–2020 and its
relationship with magnetic disturbances at middle latitudes using the INTERMAGNET, SuperMAG, and
IMAGE magnetometer data. The appearance and development of magnetic disturbances at auroral latitudes
was monitored by the IL index (similar to the AL index, but calculated according to IMAGE data). For the
2007–2020 period, events that were observed near the meridian of the IMAGE network, in the night sector
(2103 MLT), were selected. Two samples of events were used: (1) IL < –200 nT for at least 10 min, with an
additional criterion for the presence or absence of positive bays at the Panagyurishte station in Bulgaria, and
(2) isolated substorms observed on the IMAGE meridian according to the list of Ohtani and Gjerloev (2020).
The distributions of the IL index, as well as the empirical and theoretical cumulative distribution functions,
are obtained, and the of the occurrence of extreme events are also estimated. It is shown that, in general, the
IL distributions are described well by exponential functions, and out of all events, events accompanied by
mid-latitude positive bays were observed in ~65% of cases while their fraction increased with increasing disturbance
intensity. Events with positive bays at midlatitudes of MPB and isolated substorms were better
described by the Weibull distribution for extreme events. From both distributions, annual and semi-annual
variations were identified: annual variations have a summer minimum and a winter maximum, and semiannual
variations have maxima near the equinoxes, which is most likely due to the Russell-McPherron effect.
The semi-annual variation is also shown to be more pronounced for events with accompanying mid-latitudinal
positive bays.