Solar-terrestrial physics is a complex of branches of knowledge that study the relationship of processes  occurring on the Earth with solar activity. Processes on the Earth are reflection of  the state of complicated dynamic system depending on both internal causes and external effects. The closest star, the Sun, greatly affects on the Earth. This effect manifests itself through the following two channels : electromagnetic radiation and corpuscular flows. The first channel is considered  to be decisive, because the main share of energy comes from the Sun through it. In this case, the main radiation flux which falls on the visible and infrared ranges, practically does not change, as a result of which it is called the “solar constant”. At the same time, radiation at the ultraviolet edge of the spectrum and X-ray radiation reflecting active processes in the Sun, have a significant dynamics. Although from the energy point of view, these radiations are significantly weaker and, at the same time, are strongly absorbed by the atmosphere, nevertheless, they play the important role in the dynamics of “space weather”. “Space weather” is considered to mean the state of near-Earth space, namely, the magnetic-plasma envelope of the Earth. The urgent necessity to study space weather is defined by growing understanding of the role that it can play in meteorological processes in the lower and middle atmosphere, as well as it can affect the reliability of functioning of space systems  both at the Earth’s surface and in space, and also  in the condition and health of people.Сorrespondingly, the solar-terrestrial physics studies the physical processes occurring on the Sun, in the interplanetary environment, and in near-Earth space, namely, in  the magnetosphere, ionosphere, and atmosphere.

The important factor defining the state of space weather is the solar corpuscular flows, the low-energy part of which is the solar wind, and the high-energy part is solar cosmic rays. The Sun constantly loses its substance in the form of plasma, which fills the whole circumsolar space, called the heliosphere. The plasma flux i.e. the “solar wind”, has an average speed of about 400 km / s. During periods of activation of solar processes, there occur the increased plasma ejections, the speed of which on the Earth’s orbit can reach values of 800-900 km/s. Such plasma ejections reach the planet for 2-3 days and magnetic storms rage on the Earth.

Fig. 1. The Sun — the solar wind — the Earth’s magnetosphere

Fig. 1 schematically shows the effect of the solar wind on the indicated magnetic-plasma envelope of the Earth, namely, the magnetosphere. The Earth’s magnetic field makes the solar wind particles to flow round the magnetosphere, although a part of magnetic field lines are nevertheless carried away by the solar wind towards the antisolar direction.

But the solar wind particles due to diffusion processes at the boundary of the magnetosphere can be gradually accumulated in it  i.e. in the so-called “geomagnetic tail”, and creating (enhancing) the Earth’s radiation belts. The arrival of regular solar disturbance to the Earth, more often with a preliminary shock wave, usually triggers an explosive development process of complex of interrelated phenomena in the magnetosphere.

Fig. 2. Structure of the Earth’s magnetosphere

The structure of the magnetic field, as well as the currents flowing in the magnetosphere and in the ionosphere, changes in a complex way, the injection of particles into inner regions of the magnetosphere occurs, the precipitation of particles from the magnetosphere into the atmosphere is observed, various wave processes leading to the generation of electromagnetic radiation and waves develop. The areas of particle precipitation and longitudinal currents are marked in Fig.2. The whole set of processes is usually united by the concepts of the phenomena of geomagnetic and auroral storms.

Since magnetospheric disturbances are characterized by global scales, then  spatially distributed networks of observation points are required to study them from the Earth’s surface. One of such networks was created at SHICRA SB RAS.  The base of this network is the observation complex in the form of  meridional chain of stations equipped with a variety of measuring equipment (Fig.3).

The experimental base of the Institute includes the following polygons and stations:

 Yakutsk cosmic ray spectrograph

 network of magnetometric stations on the territory of Yakutia

network of riometric stations on the territory of Yakutia

 network of ionospheric stations on the territory of Yakutia

“Maimaga” optical polygon

  “Oibenkyol” radiophysical polygon

 complex geophysical station in Zhigansk

 observational  complex at Polar Geocosmophysical Observatory in Tixie

  station of satellite information reception in Yakutsk.

Fig. 3. Location of the Institute’s experimental facilities on the territory of  Republic of Sakha (Yakutia)

The colors mean:

 light-blue -cosmic ray stations,

  red – magnetometers,

  yellow -riometers,

  purple -VLF recorders,

  blue –optical devices,

  white-  the extensive air shower array

 SHICRA developed and created a set of experimental facilities for research of high-energy cosmic rays   including  a large ionization chamber, neutron monitors, and a system of muon telescopes. Continuous measurements carried out for about 50 years made it possible to understand the nature of processes of interaction of cosmic rays with the interplanetary environment, to establish the reasons for the observed variations of cosmic rays intensity.

Spectral instruments of high resolution, sensitivity and reliability under winter conditions of the Far North were developed, created and put into practice for experimental research of the thermal regime and large-scale circulation of the upper atmosphere. Based on  the long-term observations, the main features of the large-scale circulation of the auroral and subauroral thermosphere over the territory of Eastern Siberia were determined.

To solve various problems, the created experimental hardware complex for multicomponent measurements of natural radio emission in the very low frequency (VLF) range is used. Complex VLF measurements on the meridional chain confirmed the significant role of VLF radiation in the dynamics of energetic particles of the Earth’s radiation belts, and long-term spatially spaced measurements helped to establish the nature of regular noise background of VLF radiation, the main contribution to the amplitude of which is made by global thunderstorms. The peculiarity of the spaced complex of observation points is its location at the meridian with the maximum difference in the location of the geographical and geomagnetic poles, which is manifested in a brighter expression of magnetospheric processes. The research carried out on the geomagnetic chain makes it possible to obtain a number of priority results. In particular, at the first stages of studying the features of geographical distribution and space-time variations of aurorae, we revealed the so-called coastal effect i.e. the bending of aurora bands along coasts of the mainland and islands, as well as the “spotting” effect, which manifests itself in the nonhomogeneity in the geographical distribution of aurorae.

SHICRA’s  observation complexes give the opportunity to solve problems in the following areas of research on solar-terrestrial physics:

research of mechanisms of  the generation and propagation of cosmic rays in the heliosphere

research of mechanisms of  the interaction between the solar plasma and Earth’s magnetosphere

complete research of the wave activity during periods of magnetospheric disturbances by data of the global geomagnetic observations, optical and radiophysical measurements  on the Yakut network of stations

                                                                                                                                                                                                                                                                                                                                                                             research  of the thermal regime, large-scale circulation and wave disturbances in the high-latitude thermosphere and mesosphere,

research  of atmospheric electrodynamic processes, their connection with ionosphere-magnetosphere processes, and solar activity.

Fundamental research at SHICRA is also supplemented by applied one. The study of manifestations of “space weather” is conducted in a close cooperation with Russian and foreign research groups. Experimental data on measurements of cosmic rays, magnetic variations, lightning discharges, ionospheric and optical stations are transmitted to international centers for the exchange of scientific information.