INTRODUCTION

It is well known that the parameters of the ionospheric plasma are primarily controlled by the solar activity. Their sporadic perturbations usually appear as a response to perturbations in the solar wind and magnetosphere. The problem of interaction between the solar wind, magnetosphere and ionosphere is one of the key problems in Solar-Terrestrial Physics, and the wide scientific community works in special programs dedicated to it. Data base used for such studies includes data from ground based magnetic stations observations, ionospheric sounding and the of SuperDARN radars measurements of the magnetospheric convection phenomena and direct satellite measurements. The goal of these research programs is to develop the methods of ionospheric-magnetospheric monitoring and to be able to predict parameter variations.

The modeling of ionospheric-magnetospheric coupling is a very difficult task. First of all the system is connected to external fluxes of particles, energy and pulses. Second, the interaction process is rather complicated. The process of magnetospheric convection controlled by the solar wind is intrinsically unstable due to the presence of the conductive ionospheric layer. The ionosphere is not a passive “target” that absorbs wave and particle flows penetrating from the magnetosphere or “resistance” that short-circuits magnetospheric currents. It provides an active feedback impact on the perturbations of magnetosphere, and moreover can itself be the source of the unstable perturbations that in their turn can penetrate and disturb the magnetosphere. This interaction takes place by means of field aligned currents in the magnetosphere that are closed by ionospheric plasma through the transverse (Pedersen’s and Hall’s) conductivity. The variations of this conductivity (that can be caused by different factors such as, for instance, the ion drag process by the atmospheric winds or by inhomogeneous heating of the neutral gas or by some other factors) can modify current distribution and associated global structure of electric fields in the magnetosphere.

Even though there were a large number of satellite investigations of these phenomena (Bulgaria 1300, S33, GEOTAIL, FAST, INTERBALL, etc.), still there are many unanswered questions in the problem of ionosphere-magnetosphere coupling. Namely, one can mention the following :

	electric current distribution at a day side of auroral ionosphere, its dependence on the interplanetary magnetic field orientation;
	electrodynamics of the polar cusp where plasma is strongly disturbed by interaction with solar wind;
	localization and structure of plasma convection cells of different spatial scales in the ionosphere (also dependent on interplanetary magnetic field parameters);
	statistical properties and analysis of the role of field aligned current structures in ionospheric-magnetospheric coupling.

Solar activity is the main factor that affects the ionosphere; another is the particles and energy flows penetrating up the ionosphere, from the Earth. Recent observations reveal that the ionosphere locally reflects tropospheric phenomena (storms, upward shooting lightning, large cyclones and atmospheric fronts), anthropogenic factors (rocket launches, especially powerful ones, descent of spacecrafts, explosions, radio transmitter operations, etc.) and lithospheric processes (seismic activity, earthquakes, active fractures). The possibility to use these data for the prediction of earthquakes and monitoring of the anthropogenic and natural hazards is a new area of current research.

The observed seismo-activity influence on the ionosphere is rather weak and is usually masked by the solar activity effects. The seismogenic disturbances are typically detected in the ionosphere under low solar activity conditions (Kp<4). Nevertheless, making use of modern signal processing methods there is a hope to extract them if the structural peculiarities of seismogenic “signals” will be reliably determined. This approach, however, requires a huge amount of experimental data as well as appropriate theoretical modeling.

The major goal of the VARIANT project is to carry out direct measurements of current density in the ionosphere combined with the measurements of the magnetic and electric field fluctuations that can serve as new elements in the data set of signatures of the solar wind-magnetosphere-ionosphere coupling. The secondary goal of VARIANT experiment is the statistical study of minor seismogenic effects and their comparison with the “background noise statistical data set”. 

SCIENTIFIC GOALS OF VARIANT PROJECT

Investigation of the Global Distribution of Field Aligned Currents, Large Scale Electric Fields and Ionospheric Plasma Convection

Combined investigation of the high latitude ionosphere reveals the mechanisms of energy transfer through the chain ‘solar wind ® magnetosphere ®   ionosphere’. Charged particle beams ejected from the magnetotail to ionosphere precipitate in aurorar oval zones. Solar wind particles can directly penetrate into ionosphere through the polar cusp causing optical emissions and additional plasma ionisation. Due to the high transverse conductivity longitudinal currents circuit by ionosphere leads to the modification of electric field distribution in the magnetosphere. Finally, the global distribution of electric currents reflects the structure of magnetospheric fields as well as the ionospheric plasma state (inhomogeneity of the transverse ionospheric conductivity, peculiarities in neutral atmosphere motion, etc.). Experimental investigation of these phenomena requires the measurement of currents and electric and magnetic fields in the high latitude ionosphere what is the major goal of the VARIANT mission.

PAYLOAD INSTRUMENTATION

Following the major project goal - the study of the ionospheric plasma response on the influence of the magnetosphere and solar wind from the top side, and the atmosphere and the Earth, from the botom side, - a set of electromagnetic experiments onboard the Ukrainian remote sensing satellite SICH-1M is planned. The satellite will be launched in 2000 in a circular orbit with the altitude of 670±30 km and inclination angle of about 83° . It should cross the main morphological structures of the ionosphere: middle latitude throat, polar cusps, auroral oval.

Onboard satellite instrumentation includes three instruments for the registration of electric current density as the main signature of ionosphere-magnetosphere coupling. These particular instruments should allow in future the separation of the spatial and temporal variations onboard single satellite. This possibility is based on the idea to carry out the direct measurements of the current layers by crossing them. This gives rise to the possibility to determine the current density of the plane wave, that really allows the independent evaluation of curl B. If the magnetic field fluctuations are measured simultaneously one can get the distribution of the k-vectors, and then calculate the real wave frequency. The scientific equipment devoted to this task includes split Langmuir probe (WZ), Rogovsky coil (ZF) and Faraday cup (FC). The simultaneous measurements of electric and magnetic field fluctuations in the ELF/VLF frequency band will be performed making use of the electric and magnetic fields sensors (instruments EZ and WZ). The waves in this frequency range are supposed to be associated with different types of the ionospheric disturbances. They are also assumed to be the most typical signatures of the seismogenic effects. Taking all this into account, the proposed payload includes 5 types of sensors (table 1).

 Table 1. Scientific Payload Proposed for SICH-1M Mission

The spectral noise density of all sensors is given for the frequency 100 Hz. The spatial density of electric current will be simultaneously measured by three independent instruments: split Langmuir probe (included into the wave probe unit WZ), Rogovsky coil (ZF) and Faraday cup (FC). These measurements represent a special and very important task of the experiment which procures the unique possibility to justify, validate and compare three existing methodologies of the ionospheric current registration in space experiment as well as to perform the inter-calibration of the instruments. The proposed scientific payload also includes a VLF analyser and allows fulfilment particular objectives of SICH-1M experiments.

It is necessary to foresee during the active phase of the experiment the possibility of changing the on-board software both for the satellite subsystems and scientific payload, in order to develop the procedure of telecommands and telemetric data acquisition. Taking into account the originality of the project and the importance of obtained information for further interpretation, data validation and archiving procedure has to be developed. Collaboration with the “Centre Francais des Donnees de la Physique de Plasmas” (CDPP) is planned. A special program will be arranged to determine the data format with the requirements of CDPP.