INTRODUCTION

Sun–Earth connection is one of the fundamental concepts of space physics, which describes the mechanisms of the influence of solar activity on the near-Earth space. The importance of this research area is associated with the development of practical space activity: in order to ensure continuous operation of the onboard electronic equipment, it is necessary to anticipate possible changes in physical conditions (space weather) on spacecraft (SC) orbits. After the launch of the first SC, it became clear that the space environment has a number of undesirable, and often dangerous, effects on the equipment and crews of space missions (e.g., [1–4]). The main damaging factors are energetic solar particles and particles of the Earth’s radiation belts and space plasma. Reliable information on charged particle fluxes of these populations can prevent dangerous effects of space weather on SC electronics. The most effective means to evaluate the degree of radiation danger, as well as to forecast space weather, is the monitoring of space factors on artificial Earth satellites. Scientific equipment was installed to study charged particles already on the first satellites in the Soviet Union and the United States. As a result, the radiation belts of the Earth were discovered [5–7]. Of particular interest is the outer electron radiation belt first detected in the experiment on the Sputnik-2 SC conducted by the team of the Skobeltsyn Institute of Nuclear Physics of Moscow State University (SINP MSU) under the leadership of S.N. Vernov in November 1957 [8]. In 1964, four Soviet Electron series satellites were launched. The purpose of the series was to study the Earth’s radiation belts and related physical phenomena. The orbits were highly elliptical, so that the satellites could study the radiation belts throughout their length. The studies on the Electron satellites showed the high variability of the electron radiation belt and the relative stability of the proton belt (e.g., [9]). Based on the obtained data, the first models of space radiation were created, which made it possible to assess the radiation hazard during SC flights and to develop radiation protection measures. The models were included in the first and subsequent editions of the collection Model of Space published at the SINP MSU (e.g., [10]). The data from the Electron satellites served as an important experimental confirmation of the model of radial diffusion of particles of the Earth’s radiation belts under the action of magnetic field fluctuations [11]. In the 1970s, the Molniya series satellites provided new opportunities for studying the structure and dynamics of radiation belts not only of electrons and protons but also of heavier ions [12]. The experiments on the Molniya series satellites were the beginning of the creation of a system of global radiation monitoring of the near-Earth space at the SINP MSU, which was later developed using other satellites as well: GLONASS, Kosmos, Gorizont, etc. (e.g., [13–15]). The Institute of Applied Geophysics (IAG) was established in 1956 to create a system for monitoring the contamination of geophysical environments with radioactive products. The IAG provides continuous radiation support for space missions and extensive information on space weather (http://ipg.geospace.ru).

The created model representations on the particle fluxes of the Earth’s radiation belts do not always make it possible to adequately predict the level of radiation exposure, especially under geomagnetically disturbed conditions [16–19]. The dynamics of the outer electron radiation belt is not a completely solved problem of magnetospheric physics.

An important factor that significantly increases the radiation risk of space flights is the fluxes of solar energetic particles accelerated during solar flares and coronal mass ejections. Measurements of solar particle fluxes have been provided for all SCs since the first satellites up to the present day. Catalogs and models play a major role in systematizing knowledge of solar proton events, which, at the same time, are important tools for planning and securing space missions (e.g., [20, 21]).

In connection with the development of the geostationary orbit, the problem of SC electrization resulting from SC exposure to hot magnetospheric plasma (e.g., [1]) arose. Electrization models based on experimental data are widely used at the SC design phase [4].

Real-time monitoring and prediction of parameters characterizing the state of the near-Earth space (NES) are necessary to prevent potentially dangerous consequences of changes in the radiation environment [22, 23]. SCs in the NES (e.g., Meteor, POES, Elektro, GOES, and Arktika series) make it possible to obtain information on charged particle fluxes in near-real time mode. The Space Monitoring Data Center (SPDC) was created at the SINP MSU to provide access to real-time data of space experiments and models for online forecasting of space weather phenomena (http://swx.sinp.msu.ru/).

The purpose of this work is to discuss the opportunities and benefits of using the results of measurements of charged particle fluxes using the scientific instruments developed at the SINP MSU and IAG and installed on SC orbiting at a wide range of altitudes: high-apogee elliptical Molniya orbit (Arktika-M no. 1), low-altitude polar orbit (Meteor-M no. 2), and geostationary orbit (Elektro-L no. 2), for monitoring and forecasting the radiation state of the near-Earth space. Particular attention will be paid to the Arktika-M no. 1 satellite, the launch of which opened new opportunities for the program of space monitoring of the radiation state in the NES.

1 CHARACTERISTICS OF THE ORBIT AND SCIENTIFIC INSTRUMENTATION

Arktika-M no. 1 is a hydrometeorological satellite launched into orbit by a Soyuz-2.1b launch vehicle on February 28, 2021. The purpose of creating satellite systems in high-elliptical orbits is to provide information support for the solution of problems in real-time meteorology, hydrology, agrometeorology, and climate and environmental monitoring in the Arctic region. Two Arktika-M SCs should provide round-the-clock all-weather monitoring of the Earth’s surface and the seas of the Arctic Ocean, as well as constant and reliable communication. One of the problems of the experiments onboard the Arktika-M no. 1 SC is to study the dynamics of radiation conditions in the high-latitude magnetosphere.

Arktika-M no. 1 is the first of two SCs that will operate in a highly elliptical Molniya orbit with the following parameters.

— An SC orbital period of 43 565 s (∼12.1 h).

— A perigee altitude from 600 to 3000 km.

— An inclination of 63.3°.

— An apogee altitude from 37 400 to 39 800 km.

(https://www.laspace.ru/press/news/information_brochure_arctic.pdf)

In the course of a day, the Arktika-M no. 1 SC makes two turns around the Earth. On each turn of the highly elliptical orbit, the SC has working areas of about 6.5 h in the range from –3.5 to +3.5 h from apogee, and outside the working area the Arktika-M no. 1 SC enters the region of the Earth’s radiation belts twice.

The scientific instrumentation on the satellite is almost identical to that on the Elektro-L series SCs. The scientific instrumentation includes the following.

— A multispectral scanning device for hydrometeorological support (MSU-HS) designed for obtaining multispectral images of clouds and the Earth’s surface in the visible and infrared ranges within the visible disk of the Earth in the whole range of observation conditions and providing digital information to the onboard data acquisition system.

— A heliogeophysical instrumentation complex (GGAK-HE), the purpose of which is the continuous acquisition of heliogeophysical data at orbital altitude in order to

— monitor and forecast solar flare activity;

— control and predict radiation situation in the NES and the state of the geomagnetic field, and

— carry out diagnostics and control of the state of the natural and modified magnetosphere, ionosphere, and upper atmosphere (https://www.laspace.ru/projects/ information-systems/arctica-m/).

The GGAK-HE heliogeophysical instrumentation complex includes SKIF-HE instrumentation (developed at the SINP MSU) and GALS-HE spectrometer (developed at the IAG of Roshydromet).

1.1 SKIF-VE Instrumentation

The SKIF-HE instrumentation includes the SKIF-HE/V and SKIF-HE/H measuring units (hereinafter, referred to as V and H units) designed for monitoring the radiation environment in the near-Earth space and providing detection of fluxes of charged particles (electrons and protons) in the Arktika-M no. 1 SC orbit in a wide energy range from 0.05 keV to over 160 MeV. Units V and H are installed mutually perpendicular and duplicate each other. Each of the V and H units of the SKIF-HE instrumentation contains an energetic radiation spectrometer, low-energy electron and proton spectrometers, and gas-discharge Geiger counters.

1.1.1 The Energetic Radiation Spectrometer (ERS) Instrument

The ERS, a telescopic spectrometer, contains an assembly of four D1–D4 detectors on the same lengthwise axis. Three of these, D1, D2, and D4, are semiconductor (silicon) detectors 40, 500, and 1000 µm thick, respectively, while D3 is a 10-mm-thick CsJ scintillator. The ERS logic system, operating on principle of coincidence and anticoincidence of electric pulses from detectors with magnitude proportional to the energy released in them by a passing particle, forms several energy intervals (channels) of particles of each sort. The logic includes the necessary condition: an incoming particle should go through detector D2 with the release of energy not lower than some threshold. Only in this case the way for further operation of the logic opens, and this condition cuts off the detection of particles entering outside the entrance window of the spectrometer. The angular sizes of the detection field of incoming particles are formed by the input collimator and are 35°. Information of all channels on the number of detections is given every second.

Numerical modeling of the metrological characteristics of the detectors was performed using the GEANT4 software [24]. The ranges of energies of electrons and protons measured by the ERS: Ee = 0.15–10 MeV, six channels, P1–P6; Ep = 2–160+ MeV, 12 channels, P7–P18. The energy ranges of the ERS electron and proton measurement channels are presented in Tables 1 and 2.

Table 1. Energy intervals of the ERS electron measuring channels
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Table 2. Energy ranges of the ERS proton measuring channels
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1.1.2 Low-Energy Electron and Proton Spectrometers–Electrostatic Analyzers (ESAs)–SGMTD-E and SGMTD-P Units

In the SGMTD-E and -P units, the particles under study enter the guide channel of the analyzer formed by parts (segments) of two spherical surfaces, between which a voltage is applied that creates an electric field in the channel (the first segmentoid). Particles passing through the first segmentoid enter the field of the second segmentoid of the same type but with the electric-field vector in the opposite direction and, then, enter the detector input window, the VEU-7 secondary electron multiplier, in which they are detected.

The second segmentoid is necessary in order to improve the units’ protection against detection of scattered ultraviolet radiation of the Sun, as well as the fluxes of charged particles reflected from the inner walls of the channel.

The value of the electric-field strength in the analyzer channels varies programmatically, which ensures the passage of particles of different energies and thus the detection of the energy spectrum. The ranges of measured energies are

SGMTD-E (Ee = 0.15–20 keV, 12 steps–spectrum intervals, e1–e12); and

SGMTD-P (Ep = 0.15–20 keV, 12 steps–spectrum intervals, p1–p12).

The whole energy range of 0.15–20 keV is divided into 12 intervals with the detection time of each interval of 1 s; i.e., the spectrum is detected in 12 s (Table 3). Angular sizes of the particle detection field are 20°.

Table 3. Energy ranges of the steps of electron and proton spectra of SGMTD-E, P
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1.1.3 Gas-Discharge Geiger Counters (MIP 1–3)

MIP 1 is designed for detecting integral fluxes of protons and electrons with energies greater than 800 and 50 keV, respectively, of the CBT-9 end-window counter.

MIP 2 and MIP 3 are constructed for the detection of integral proton and electron fluxes with energies more than 15 MeV and 800 keV, respectively, of CBM‑20 and CI3-BG omnidirectional counters. The effective areas of the CBM-20 and CI3-BG counters differ by two orders of magnitude, which significant expands the dynamic range of particle detection.

The counter information on the number of detected particles is provided once per second.

1.1.4 GALS-VE Spectrometer

The GALS-HE spectrometer consists of two units: GALS-HE/-CH with a Cherenkov counter and GALS-HE/-G with gas-discharge Geiger counters (Cg-1–Cg-4).

The Cherenkov counter is designed to detect proton fluxes with energies greater than 600 MeV in three integral energy ranges. The calculated values of the lower limits of the ranges are 600, 800, and 1100 MeV. The detection angle is 4π, while the calculated effective area is ~40 cm2.

Four Geiger counters with screens of different thicknesses detect total electron and proton fluxes in four integral energy ranges.

The main characteristics of the Geiger counters are shown in Table 4.

Table 4. Characteristics of Geiger counters
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The SKIF-HE and GALS-HE instruments in the orbit of the Arktika-M no. 1 SC operate continuously.

2 DATA CENTER OF REAL-TIME SPACE MONITORING OF THE SINP MSU

The Space Monitoring Data Center (SMDC) of the SINP MSU provides access to real-time data from space experiments and models for real-time forecasting of space weather phenomena. Interactive SMDC applications run on the Space Weather website (https://swx.sinp.msu.ru/) and provide access to data and models in a PostgreSQL database. The Space Weather website collects the data needed to assess and analyze radiation conditions not only in the NES, but also in the interplanetary medium, as well as the level of solar activity. The data come from satellites, ground stations, and data-processing centers, and the results of model calculations are also uploaded to the system. The special Satellite Data Downloading System (SDDS) software is used to download and process the raw data. SDDS services convert the primary data coming from different sources into a single format and upload the data into different database tables (e.g., solar wind parameters, geomagnetic data, and electron fluxes of the outer radiation belt). One advantage of the SMDC data system is the ability to extract data from different sources as a single array of time series with different frequencies. When presented together, these databases can be used to intercalibrate measurements, diagnose errors in data from different satellites, and provide consistent analysis of data from multiple satellites.

As a space weather resource, the online service uses data from various active space missions such as Meteor-M no. 2, Elektro-L no. 2, Arktika-M no. 1, GOES, ACE, SDO, and DSCOVR. SMDC data services also make it possible to retrieve and analyze h-istorical data collected in the SMDC database. Advanced graphical applications make it possible to conduct a comparative analysis of both experimental data and simulation results. Most of the figures presented in this article are from the instruments represented on the SMDC website (https://swx.sinp.msu. ru/constructor2/).

Real-time applications on the Space Weather website (https://swx.sinp.msu.ru/) make it possible to forecast and construct a prehistory of the radiation environment in the NES [25]. These applications use data from the aforementioned SC collected in a single database. They provide predictions of the quasi-stationary solar wind speed based on the analysis of solar images by the SDO/AIA space observatory and access to several models of physical conditions in the near-Earth space, including predictions of geomagnetic indices, solar wind plasma and the interplanetary magnetic field in the Earth orbit, the magnetopause position at the subsolar point, the structure of the Earth’s magnetosphere field lines, and predictions of relativistic electron fluxes in the geostationary orbit.

3 RESULTS

For a number of years, the SINP MSU in cooperation with the IAG of Roshydromet has been monitoring energetic particle fluxes and magnetospheric plasma in the near-Earth space. Measurements are taken on orbits in a wide range of altitudes: polar low-orbit Meteor series satellites (~850 km) and geostationary Elektro series satellites (~36 000 km). SC data are used for the near-Earth space radiation condition diagnostics by Space Weather Centers at the IAG and SINP MSU. A new step in the development of the radiation monitoring system was the launch of the Arktika-M no. 1 satellite into a highly apogee elliptical orbit.

Experimental information from the Arktika-M no. 1 SC is located in the Space Monitoring Data Center (SMDC) at the SINP MSU and IAG. Data with the orbit coordinates of the Arktika-M no. 1 SC are available on the SMDC website (https://swx.sinp.msu.ru/ tools/davisat.php?gcm=1). Experimental data on particle fluxes are available upon request (e-mail smdc@sinp.msu.ru).

3.1 Earth’s Radiation Belts

The energetic particle fluxes of the Earth’s radiation belts are a major threat to the near-Earth space missions. Therefore, monitoring and predicting particle fluxes is a prerequisite for their radiation safety. The advantage of the Arktika-M no. 1 SC orbit lies in the possibility of studying the Earth’s radiation belts (ERBs) and the passage through the center of the radiation belts in detail.

3.1.1 Results of Comparative Analysis of Experimental and Model Data on Particle Fluxes of the Earth’s Radiation Belts

Comparison of the radiation observation data from the ERS instruments of the Arktika-M no. 1 SC with the AE8/AP8 model of the Earth’s radiation belts [16, 17] is a good way to check the adequacy of the new instrumentation, to reveal its features and to specify parameters and calibrations, as well as to become familiar with the nature of magnetospheric phenomena typical of the Molniya orbit. The lack of adjustment of the attitude of the Arktika-M no. 1 SC to the direction of the surrounding magnetic field, which is important for radiation observations, is unusual for a research SC; it was thought that two separated units with radiation sensors would help to cover different pitch angles.

Figure 1 shows the data from two ERS instruments installed in units V and H of the SKIF-HE of the Arktik-a-M no. 1 SC when the SC passed the perigee segment of the orbit, as well as the results of the simulation of radiation near the SC during such a passage according to the AE8/AP8 model [26, 27]. The ERS data are shown in panels 2 and 3 (six channels each for electrons, units V and H, respectively) as well as 6 and 7 (also six channels each for protons, in units V and H). The energy range for electrons is 0.15–10 MeV; the plots for lower energies tend to be higher (higher flux intensity). For protons, the energy range is 2–160 MeV, but the spectrum in flux rises in the radiation belts is not always falling. The ERS data are shown as the product of the count rate measured in the channel by the geometric factor of the instrument but not as the flux intensity, because there is still a need to refine some details, and for this stage it is important to make sure that the measurements and model are generally similar. Panels 4 and 5 show the results of model calculations: along the orbit, the energy differential of particle-flux intensities from the total solid angle (4π) is calculated. Pitch-angle anisotropy of particle fluxes was not taken into account, which probably led to some difference between the results of measurements and calculations. Panel 8 shows magnetic local time MLT and the Lm parameter calculated from the IGRF + OPq field model. Panel 1 represents the angle in degrees between the ERS input window axis and the direction back to the geomagnetic field vector at the SC location, for both ERSs in units V and H. The field is also calculated by IGRF + OPq. If the angle is zero and the SC is near perigee, the ERS looks along the field line toward the Earth following the precipitating particles. Model data for electrons (panel 4) show a strongly averaged increase near the Earth: for minimum L, the inner radiation belt, and, for L > 3, another increase, the outer radiation belt. In the proton belt model (panel 5), there is only one increase. In both cases, the closer the maximum of the flux of trapped particles to the Earth, the higher their energy.

Fig. 1.
figure 1

Differential energy measurements of electron (panels 2, 3) and proton (panels 6, 7) fluxes in the Earth radiation belts by the ERS instrument of the Arktika-M no. 1 SC in comparison to the statistical models AE8min and AP8min (panels 4, 5) for the period 14:40–22:40 UT on Apr. 17, 2021. In panel 8, Lm and MLT are given; in panel 1, the ERS axis angle (for each of two units V and H) with respect to the local geomagnetic field is given. Zero is the direction opposite to the field vector, i.e., near the southern polar cap, and a zero angle means the ERS is directed downward, toward the Earth.

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Located above the electron belt model, the electron data from the two ERSs, as well as the model, contain a double-humped structure. There is no distortion in this structure on the left side of panel 3; there is little distortion in the right side of panel 2 and in panel 3, and the distortion is greatest in the left side of panel 2. It should be noted that the largest distortions coincide in time (passage–abscissa axis; at 18:10 UT) with reaching the zero angle of the ERS-V radiation pattern (main distortions can be seen in the V unit). The nature of the distortions is through, which corresponds to the precipitations, i.e., the returns of particles that have flown to the Earth do not return. The data on electrons from the ERS are explicable, i.e., adequate.

The right side of the proton belt in panel 6 (unit V) looks most similar to the model (panel 5). The difference in the right side of panel 7 (unit H) looks like a not-so-intense entrainment of the region below the minimum (19:05 UT) of the ERS-H radiation pattern from panel 1. We can speculate that it is the high-intensity entrainment that significantly distorted the left side of the radiation belt in panel 6 (unit V) due to the ERS-V turn exactly into the loss cone at 18:10 UT. There is also a large difference from the model in the left side of panel 7, with a peak at 18:00 UT, i.e., with ERS-H directed at a 180° angle, i.e., along the field. Other effects can also be involved. The ERS proton data are also explicable and, in this regard, adequate.

The data indicate very significant pitch-angle anisotropy of the particle fluxes in the example above (the Dst index at this time was kept moderately low, on the order of –30 nT, and varied; not shown). It is important that not only the considered understandable effects are observed, but also smaller and different ones, in different energies, which is probably connected with changes of the field structure and paths of charged particles, with possible manifestation of particle-acceleration mechanisms and their losses. A high information saturation of the observed data contrasts with the coarsened anisotropic averaging in the used ERB model. In general, the comparison of the measured data with the model showed the adequacy of the new instrument and the presence of new possibilities.

3.1.2 Magnetic Storm of January 15, 2022

A magnetic storm can significantly change the state of the outer radiation belt. During the first year of operation of the Arktika-M no. 1 satellite, numerous variations of energetic electron fluxes associated with the development of geomagnetic disturbances were detected. One of the events took place in January 2022. The arrival of a flux of high-speed solar wind under the southern orientation of the interplanetary magnetic field caused a geomagnetic storm, which began on January 14, 2022, and peaked on January 15, 2022 (|Dst|max ∼ 90 nT, Fig. 2). It can be seen that, in the recovery phase of the magnetic storm, the electron flux in the geostationary orbit according to the Elektro-L no. 2 SC (Fig. 2c) increased as compared to the prestorm level.

Fig. 2.
figure 2

Time profiles of the (a) solar wind velocity and (b) Bz-component of the IMF in the Earth’s orbit, (c) electron flux with E > 0.6 MeV from the data of the Elektro-L no. 2 SC, and (d) Dst-variation on January 14–18, 2022.

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Experimental data from the Meteor-M no. 2 and Arktika-M no. 1 SCs can give insight into the dynamics of particle fluxes in the entire radiation belt region: data from the Meteor-M no. 2 SC do so only at high latitudes, while data from the Arktika-M no. 1 SC do so from the center of radiation belts at middle latitudes (Fig. 3). The Meteor-M no. 2 SC passes through the region of the outer ERB in ∼5 min. For a 10-s data averaging, it can be seen that the geomagnetic storm of January 14–15, 2022, resulted in a significant increase in the electron flux in the ERB (Fig. 3a). The Arktika-M no. 1 SC passes through the region of the outer ERB in the middle part of the orbit in about an hour, which makes it possible to study its fine structure (Fig. 3b). Almost in the main phase of the storm, the electron fluxes with E = 4–8 MeV appear, which were almost absent before the storm, are observed in the outer radiation belt (Figs. 3b, 3c).

Fig. 3.
figure 3

Time profiles of (a) electron fluxes with E > 2 MeV from the Meteor-M no. 2 SC data of January (a1) 14 and (a2) 18, 2022, during the passage through the ERB (the thin line is the L parameter); (b) electron fluxes with E = 4–8 MeV according to the Arktika-M no. 1 SC (the thin line is the L parameter); and (c) the Dst-variation from 06:00 of January 14, 2022 to 16:00 of January 15, 2022.

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3.2 Solar Proton Event of October 28, 2021

In addition to the geomagnetic disturbances resulting in variations of electron fluxes in the outer radiation belt, solar proton events accompanied by increased fluxes of energetic solar protons in the high-latitude magnetosphere pose a danger to SCs. A solar proton event (SPE) was observed on October 28, 2021; it was the first ground level enhancement (GLE) event of solar cycle 25 and the 73rd GLE event since the beginning of observations (http://gle.oulu.fi/#/). The SPE of October 28, 2021, is associated with a solar flare which occurred at 15:17 UT on October 28, 2021. The X-ray and optical levels of the flare are X1.0/2N. The heliocoordinates of the flare are S26W05, and the active region is 12887. The maximum proton flux with E > 10 MeV was ∼25 (cm2 s sr)–1 according to the GOES-16 SC. The time profile of the solar proton fluxes has a fairly fast rising phase and a long falling phase of the flux (Figs. 4, 5). The Meteor-M no. 2 SC’s instruments measure solar proton fluxes above the Earth’s polar caps. Based on the results of a comparative analysis of the fluxes above the northern and southern polar caps, one can judge about the anisotropy of the particle flux. It can be seen that, in the SPE of October 28, 2021, the proton fluxes were almost isotropic during the entire event (Fig. 4a).

Fig. 4.
figure 4

Time profiles of solar proton fluxes (a) from the Meteor-M no. 2 SC with E = 3–10 MeV above the northern and southern polar caps and (b) from the Elektro-L no. 2 SC with E = 9–20 and 20–40 MeV; (c) solar wind speed and (d) the absolute value of the interplanetary magnetic field on October 27–November 5, 2021.

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Fig. 5.
figure 5

Time profiles of solar proton fluxes obtained from the (a) Meteor-M no. 2 and (b) Arktika-M no. 1 SCs from October 27 to November 2, 2021 (see text).

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The solar proton flux propagated in a fairly quiet interplanetary medium: a relatively slow solar wind (Fig. 4c) and a relatively weak interplanetary magnetic field (Fig. 4d). However, the time profiles of solar protons, both above the polar caps on open geomagnetic field lines (data from the Meteor-M no. 2 SC, Fig. 4a) and the ones penetrating into the magnetosphere in the geostationary orbit region (data from the Elektro no. 2 SC, Fig. 4b), have a feature on November 3–4, 2021, when a new structure of the interplanetary medium (coronal mass ejection) with a shock wave, a high-speed solar wind, and a strong magnetic field came to Earth (Figs. 4c, 4d). Data from the Meteor-M no. 2 SC clearly show an increased concentration of proton flux before the shock wave (Fig. 4a), while data from the Elektro-L no. 2 SC show the geomagnetic field response, expressed in particle variations, to the shock wave’s impact on the Earth’s magnetosphere.

The SPE of October 28, 2021, is a GLE event, and, hence, the flux of high-energy solar particles came to the NES, which was also detected in the polar caps on the Meteor-M no. 2 SC (Fig. 5a) and inside the magnetosphere on the Arktika-M no. 1 SC (Fig. 5b). The curves in Fig. 5 correspond to the solar proton fluxes for the indicated energies: from E > 1 to E > 800 MeV.

Experimental data on solar energetic particle fluxes from scientific instruments installed on the Meteor-M no. 2 SC and on the Arktika-M no. 1 SC make it possible to study the SPE in a very wide energy range and to construct a detailed energy spectrum of the solar proton fluxes. For the SPE of October 28, 2021, an energy spectrum of maximum solar proton fluxes was constructed using experimental data of all available SCs (Fig. 6). The spectrum was approximated in the whole range of energies by the following function:

$$J\left( E \right) = {{J}_{0}}{{E}^{{ - \gamma }}}\exp \left( { - \frac{E}{{{{E}_{0}}}}} \right).$$
Fig. 6.
figure 6

Energy spectrum of solar proton flux maxima on October 28, 2021.

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Parameters γ and E0 were determined by the least squares method: γ = 0.511, and E0 = 228 keV. The concept of a proton energy spectrum in the event maximum was introduced historically when almost instantaneous injection of solar particles into the interplanetary space and their further propagation by diffusion were assumed. In this case, the spectrum in the maximum reflects the spectrum of particles accelerated on the Sun. The development of ideas about solar particle acceleration and propagation led to scenarios that consider prolonged and/or repeated acceleration and a complex pattern of particle propagation in the solar corona and in the interplanetary medium taking into account the geometry of the magnetic field. Diffusion, convection, adiabatic cooling, and particle drift are taken into account. Obviously, all these factors can affect the particle flux in the event maximum and often lead to the presence of a flat maximum or two or three maxima. Nevertheless, the maximum spectrum is still one of the main characteristics of the event, because it gives an idea of the first, often the most powerful, stage of the energy release on the Sun. Figure 6 also shows the tenfold reduced galactic cosmic-ray (GCR) fluxes for the solar maximum period according to GOST (State Standard) 25645.150-90. The intersection of the SPE and GCR particle spectra is the quasi-maximal energy, the first approximation to the real maximum energy of the observed proton fluxes in the given solar event [28].

3.3 Space Plasma Research

Studies of the effects of the space environment on materials and components of SC equipment have begun at the SINP MSU almost since the beginning of the space age (e.g., [4]) and of the space plasma, since the launch of the first geostationary satellites (e.g., [29]). The low-energy electron and proton spectrometers installed on the Arktika-M no. 1 SC, as well as on the Meteor-M no. 2 and Elektro-L no. 2 SCs, have provided a large amount of information, but this is material for further studies and for another publication.

4 DISCUSSION

The main problems of the monitoring are the evaluation and forecasting of the radiation conditions in the NES, testing and refinement of the existing and development of new dynamic models of radiation fields, and obtaining of experimental data for solving key problems of space physics. Due to its orbit features, the Arktika-M no. 1 SC is exposed to a strong influence of external environment factors, because it passes through all the basic structural regions of the inner magnetosphere—the inner and outer electron radiation belts, auroral zone, and polar caps—and, therefore, the experimental data obtained from the Arktika-M no. 1 SC can provide new and interesting information on the radiation fields in these regions. The example of the SPE of October 28, 2021, shows that the fluxes of solar energetic particles can increase by several orders of magnitude in a short time. The fluxes of electrons in the outer radiation belt can also experience rapid and strong variations. The time dependence of the radial distribution of electron fluxes in the outer radiation belt (Fig. 7a) shows there to be changes in the belt structure and intensity as a result of influence of the interplanetary medium on the magnetosphere (Figs. 7b, 7c) and development of geomagnetic disturbances (Fig. 7d) on October 20–December 16, 2021.

Fig. 7.
figure 7

October 20–December 16, 2021: (a) time dependence of the radial distribution of electron fluxes with E > 0.1 MeV in the ERB according to the Meteor-M no. 2 SC, (b) time profiles of the solar wind speed, (c) Bz-components of the interplanetary magnetic field, and (d) Dst-variation.

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An interesting, and infrequent, structure of electron radiation belts with three maxima that appeared after a weak geomagnetic storm (|Dst|max ∼ 40 nT) on November 30–December 2, 2021, can be seen from the Arktika-M no. 1 SC data (Fig. 8a). The numbers in the figures are the time (UT) of the SC’s passage through L = 4, the region in which the maximum of the outer radiation belt is most often located. Three-maximum radiation belt structures were found in the experiment on two Van Allen Probes SCs, the elliptical orbit of which was near the equatorial plane (e.g., [30]). The orbit of the Arktika-M no. 1 SC also makes it possible to conduct such studies. The event of November 30–December 2, 2021, is interesting because the storm was weak, but the geomagnetic disturbance was prolonged. The effect of the third radiation belt arose as a result of the combined effect of the main factors of the interplanetary medium on the Earth’s magnetosphere: the speed (Fig. 8b) and density (Fig. 8c) of the solar wind and the Bz-component of the interplanetary magnetic field (Fig. 8d). The response of the magnetosphere to the influence of the interplanetary medium can be seen in the Dst-variation (Fig. 8d). A detailed analysis of this event will be the subject of further work.

Fig. 8.
figure 8

(a) Radial profiles of electron fluxes with E > 0.7 MeV from the data of the Arktika-M2 no. 1 SC, November 30, December 1, and December 2, 2021. Time profiles from November 30–December 2, 2021: (b) speed and (c) density of the solar wind, (d) the Bz-component of the interplanetary magnetic field, and (e) the Dst-variation.

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CONCLUSIONS

Real-time monitoring of radiation conditions at different orbits in the near-Earth space is the most important and, probably, the only tool for ensuring radiation safety of space missions and creating space weather forecasts. The dynamics of ionizing radiation fluxes both in the interplanetary space and in the magnetosphere are such that the flux intensities can change by several orders of magnitude within a short time; therefore, averaged empirical models of radiation belts cannot always be useful for assessing radiation conditions in specific orbits. It should also be noted that there are currently no methods for long- or medium-term forecasts of flares and solar proton events. The only way to prevent their dangerous effects is to report increases in charged particle fluxes in time, which requires continuous measurement.

Obtaining a large volume of regular monitoring data on the radiation situation in near-Earth space on the basis of multilayer measurements on different orbits makes it possible to study its short- and medium-term dynamics, as well as its relationship with solar activity, and is a prerequisite for the reconstruction of the 3D dynamic pattern of radiation in the magnetosphere. There are good prerequisites for the implementation of space radiation monitoring by domestic SCs of the Elektro, Meteor, and Arktika series, as well as the planned missions of Ionosfera and Cubsaty (Moscow State University project Universat-Socrat).