Some problems of the earth's magnetosphere physics

Conclusions

The study of ways of the solar wind energy penetration into the magnetosphere is one of the most important problems of the magnetosphere physics. The solution of such problems as the radiation belts origin, the determination of the magnetic storms and auroral nature is connected with this problem.

The magnetic measurements on the satellite OGO-A [157] probably indicate the possibility of the magnetosheath particles penetration into the tail through its side surface. The laboratory simulation of the solar wind flow around the magnetosphere [44–46] also indicate the comparatively rapid penetration of the incident plasma flow particles into the tail. On the other side the aurorae oval location [139–141] and the similarity of electrons spectrum in the aurorae and in the magnetic tail (according to the data of [138]) suggest that the aurorae on the earth nightside are caused by the penetration of particles from the tail plasmasheet. The dayside aurorae can probably appear as a result of the direct penetration of the solar wind particles (thermalized in the magnetosheath) in the vicinity of neutral points, as the geomagnetic field lines passing in this region intersect the earth's surface in the auroral Zone. It is reasonable to carry out the complex investigations of these regions by means of the special system of satellites. Possible experiments on the satellite passing in the region of the neutral point and requirement to the equipment onboard the satellite have been discussed in detail in [152].

In Figure 8 the sampling satellite orbits are shown passing through the regions of dayside neutral point and of neutral plane of the tail. The orbits with an apogee of about 30 R _E allow us to compare the different characteristics of the geomagnetic field, trapped and auroral radiation inside the magnetosphere with the solar wind parameters (such as a direction of frozen-in magnetic field, temperature and particles concentration, etc.) and with the magnetosheath parameters. However it is difficult technically to launch a satellite into the region of the south neutral point. Besides, with the prolonged satellite existence outside the magnetosphere boundary we lose the part of the measurements inside the magnetosphere. That is why it is reasonable instead of the orbit SII in Figure 8 to launch a satellite into the north neutral point region but with a less apogee, of about 15 R _E (orbit SII in Figure 9). A system of three satellites shown in Figure 9 (it is more profitable technically than the one shown in Figure 8) allows us to perform different measurements simultaneously in the solar wind, in the magnetosheath, in the vicinity of the neutral points, in the tail plasma sheet (including a neutral plane) and in the zone of auroral and trapped radiation.

By means of such a system of satellites it is possible, for example, to make a comparison of particles composition simultaneously in the denoted regions of the earth environment. This is necessary for solving many magnetosphere physics problems, for example, the mystery of aurorae. There are indications [144] that in the aurora the ratio of alpha-particles flux to proton flux (I _α/I_p) is approximately the same as in the solar wind. However it is unknown in which way these particles penetrate into the aurora region; no simultaneous measurements of the ratio I _α/I_p in the tail were made.

The measurements of the particles composition are also of interest in studies of different mechanisms of the particles transfer inside the magnetosphere. For example, the experiments [153–154] indicate that the ratio I _α/I_p in the inner magnetosphere is approximately by two orders of magnitude less than in the solar wind, and by one order less than predicted by the theory of particles diffusion under the influence of sudden pulses.

The other experimental problems connected with the questions discussed are listed below:

• 1. The determination of the magnetopause shape above the poles and the magnetic tail shape at large distances from the earth.

• 2. The investigation of the magnetosphere asymmetry in the north-south direction, as indicated by Explorer 12 experiments [155] (such type of asymmetry is not explained by the existing theories).

• 3. The continuous magnetic measurements on the synchronous satellite (L ∼ 6.6) allow us to restore the significant parameters of the magnetosphere, if a satisfactory quantitative model is available (see for more details [51]).

• 4. A detailed study of the magnetic field distribution on the magnetosphere boundary and in its vicinity.

  In the Hones-Taylor model the lines of force located nearly to the magnetosphere forward end, according to [55], have highlatitude minima of intensity B, topologically connected with the equatorial minima B on the nightside. As a result the particles which were equatorial on the nightside, drifting to the dayside, can be put into the higher latitudes. In the Mead-Williams model drift orbit branchings do not occur [51] due to the lack of a similar topological connection. Thus, these magnetic field measurements allow us to choose between different theoretical models and along with the direct particles measurements give a possibility to determine the boundaries of regions of trapped and quasi-trapped radiation. Besides, a detailed study of magnetopause location can answer the question if the magnetosphere boundary is the surface enveloping a family of the magnetic field lines (i.e. if the magnetic field normal component vanishes on the surface of discontinuity) that is of interest for the problem of the solar wind flow around the magnetosphere.

• 5. Detailed measurements of the magnetic field intensity vector in different regions of the magnetosphere are significant for an estimation of the quantitative models suitability and provide a distribution of the current density j = c/4π rot B; in particular, an experimental test of the idea about two ring currents [92] is of interest.

• 6. A construction of the model of the electric field in the magnetosphere is significant for the study of behaviour of particles with the energy of several tens keV and below. For this purpose in addition to the direct measurements of the electric field the Brice method [66] can be evidently used which permits one to restore the pattern of a large-scaled electric field from the position and shape of the plasmapause. Plasma ‘running away’ along the open lines of force (a polar wind [78]) must lead to plasmapause shape distortion on these field lines. An experimental test of this effect is necessary, i.e. the measurements of the plasmasphere boundary at different latitudes.

• 7. Some convection models [65] suggest that in the vicinity of the magnetopause the directions of plasma movement inside the magnetosphere and in the magneto-sheath can be opposite. This conclusion is not in contradiction with the conditions at the tangential discontinuity: V _n1 = V _n2, V _t1 ≠ V _n1. It is interesting to determine if such plasma flows really occur, that is necessary for the construction of an adequate picture of the electric field.

In conclusion let us enumerate the outstanding significant problems of the physics of the magnetosphere:

• 1. In which way is the magnetosphere boundary (including the polar regions and magnetic tail) formed as a result of an interaction between solar wind and geomagnetic field?

  To what extent is magnetohydrodynamics applicable to a description of these phenomena?

• 2. How does the energy (and/or particles) of the solar wind penetrate into the magnetosphere?

  In which way is the momentum transferred to the convective motions of plasma in the magnetosphere?

  Do the solar wind particles penetrate directly into some regions of the magnetosphere? Are the energy and momentum transferred into the magnetosphere with a constant speed or periodically?

• 3. Does the energy accumulation in a magnetic tail exist and in what way does it occur?

  What is the nature of a ‘trigger mechanism’ which releases suddenly the accumulated energy?

  What changes in the solar wind cause the transfer from quiet conditions to perturbed ones?

  What is the nature of a mechanism which causes the appearance of the magnetic substorms and how are they associated with the accumulation of the energy in the tail and with its release?

• 4. If the tail plasmasheet is the source of particles that causes the aurorae? In which way are these particles accelerated to appropriate energies? Especially important are the measurements in the plasmasheet ‘horns’ plunging into the upper atmosphere in the auroral zone.

• 5. Which are the sources of radiation belts particles and the mechanisms of the particles losses?

  In which way is the particles transport across geomagnetic drift shells accomplished? What is the nature of the ring current?

• 6. To what instabilities is plasma exposed in the magnetosphere? What is the role of instabilities in the particles acceleration?

  To what extent are the instabilities responsible for the particles transport across the magnetic field and for their losses?