High-latitude Dayside Research Articles

Solar wind energy transfer regions inside the dayside magnetopause—I. Evidence for magnetosheath plasma penetration

PROGNOZ-7 high temporal resolution measurements of the ion composition and hot plasma distribution in the dayside high latitude boundary layer near noon have revealed that magnetosheath plasma may penetrate the dayside magnetopause and form high density, high β, magnetosheath-like regions inside the magnetopause. We will from these measurements demonstrate that the magnetosheath injection regions most probably play an important role in transferring solar wind energy into the magnetosphere. The transfer regions are characterized by a strong perpendicular flow towards dawn or dusk (depending on local time) but are also observed to expand rapidly along the boundary layer field lines. This increased flow component transverse to the local magnetic field corresponds to a predominantly radial electric field of up to several mV m −1, which indicates that the injected magnetosheath plasma causes an enhanced polarization of the boundary layer. Polarization of the boundary layer can therefore be considered a result of a local MHD-process where magnetosheath plasma excess momentum is converted into electromagnetic energy (electric field), i.e. we have primarily an MHD-generator there. We state primarily because we also observe acceleration of “cold” ions inside the magnetopause as a result of this radial electric field. A few cases of polarity reversals suggest that the polarization is sometimes quite localized. The perhaps most significant finding is that the boundary layer is observed to be charged up to tens of kilovolts, a potential which may be highly variable depending on e.g. the presence of a momentum exchange by the energy transfer regions.

Solar wind energy transfer regions inside the dayside magnetopause—II. Evidence for an MHD generator process

In this paper a quantitative analysis of magnetosheath injection regions observed by PROGNOZ-7 in the dayside high latitude boundary layer is performed. Particular emphasis is laid on describing the consequences of the observed excess transverse momentum of solar wind ions (H + and He 2+) as compared to the magnetospheric ions (e.g. He + and O +) in the magnetosheath injection regions, hereafter referred to as energy transfer regions. An important result of this study is that the observed excess drift velocity of the solar wind ions as compared to the magnetospheric ions can be interpreted as a negative inertia current being present in the boundary layer. This means that the inertia current goes against the local electric field and that particle kinetic energy is converted into electric energy there. The dayside high-latitude boundary layer therefore constitutes a voltage generator (at least with respect to the injected magnetosheath plasma). The MHD-theory predicts a strong coupling of the energy transfer process in the boundary layer and the ionosphere, both regions being connected by field aligned currents. The rate of decay of the inertia current in the injected plasma element is in the range of a few minutes, a value which is directly proportional to the ionospheric resistance. By taking into account both the Hall and the Pedersen conductivities in the ionosphere, the theory also predicts a strong coupling between ionospheric East/West and North/South currents. A considerable part of the inertia current may actually flow in the tangential (East/West) direction due to this coupling. Thus, a consequence of the boundary layer energy transfer process is that it may generate currents, powering other magnetospheric plasma processes, down to ionospheric heights.

The magnetospheric boundary layer: Site of plasma, momentum and energy transfer from the magnetosheath into the magnetosphere

Observations with the Los Alamos Scientific Laboratory (LASL) plasma probe and the Goddard Space Flight Center (GSFC) magnetometer on the IMP 6 satellite show that the magnetospheric boundary layer, first identified along the flanks of the magnetosphere, is also present at the magnetosphere's sunward surface. The magnetic field lines in this sunward sector of the boundary layer are closed, and the plasma flow has a component transverse to the field. These observations suggest that the boundary layer is a site of continual transfer of plasma, momentum and energy from the magnetosheath to the magnetosphere. These transfer processes supply plasma and magnetic field to the magnetotail. Also, they produce, indirectly, the dawn‐to‐dusk electric field across the polar cap, the field‐aligned currents that border the dayside polar cap, and the soft particle fluxes that characterize the cleft precipitation, including recently reported dawn‐dusk asymmetries of these fluxes. Magnetosheath plasma directly enters the outer few hundred to few thousand kilometers of the magnetosphere's surface to form the boundary layer. There it is enabled to flow across the magnetic field (and approximately parallel to the magnetosphere's surface) by becoming electrically polarized. Leakage of the polarization charge along magnetic field lines to the earth produces the dayside high latitude effects mentioned above. The polarizing current flowing across the boundary layer interacts with the magnetic field to oppose the boundary layer plasma flow, taking up its momentum. In this way the magnetic field lines are pulled downstream. The process described here is independent of the interplanetary magnetic field (IMF) and thus may constitute the principal transfer mechanism during prolonged periods of northward IMF when the magnetosphere is very quiet. It is not clear how the effects of southward IMF are superposed on this process.

Simultaneous particle and field observations of field-aligned currents

Simultaneous measurements of low energy precipitating electrons and magnetic fluctuations from the low altitude polar orbiting satellite OGO-4 have been compared. Analysis of the two sets of experimental data for isolated events led to the classification of high latitude field-aligned currents as purely temporal or purely spatial variations. Magnetic field disturbances calculated using these simple current models and the measured particle fluxes were in good agreement with measured field values. While fluxes of greater than 1 keV electrons are detected primarily on the nightside, magnetometer disturbances indicative of field-aligned currents were seen at all local times, both in the visual auroral regions and dayside polar cusp. Thus electrons with energies less than approximately 1 keV are the prime charge carriers in high latitude dayside field-aligned currents. The satellite measurements are in good agreement with previously measured field-aligned current values and with values predicted from several models involving magnetospheric field-aligned currents.