Structure and properties of the subsolar magnetopause for northward IMF: ISEE observations

Abstract

Detailed magnetopause structure and properties for the magnetic field, electric field and plasma are examined for an ISEE 1 magnetopause crossing which occurred near the subsolar point when the interplanetary magnetic field (IMF) was strongly northward. Because the crossing is slow, the spatial variations in the plasma are clearly resolved. This example illustrates the nature of the steady state interface of two magnetized thermal plasma populations with parallel fields and can serve as a guide to theoretical modeling and simulations.We have found that the magnetopause is composed of three layers, a sheath transition layer, an outer boundary layer and an inner boundary layer. In the sheath transition layer, there is a gradual density decrease without a change in temperature.The transition layer occurs totally within the magnetosheath plasma.The outer boundary layer and the inner boundary layer are dominated by magnetosheath and magnetospheric particles, respectively.In each of the boundary layers, the plasma can be interpreted as simple mixtures of the magnetosheath and magnetospheric populations.No significant heating or cooling is seen across the magnetopause during this crossing.The plasma within each of these layers is quite uniform and their boundaries are sharp, suggesting that there is very little diffusion present. The sharp boundaries between the transition layer, the boundary layers and the magnetosphere are all thinner than an ion gyroradius.Transverse waves with right hand or linear polarization near the ion gyrofrequency are observed in the transition layer.These appear to be generated in the transition layer and to be a common feature of this layer when the IMF is northward.

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

“…Although previous studies have demonstrated that enhanced amounts of plasma enter to form a relatively thick, lowlatitude boundary layer 2,12,13 and a denser plasma sheet 13,14 when the IMF is northward even during quiet times, the dominant mechanism of solar wind entry into the magnetosphere (and entry location on the magnetopause) under these conditions is less clear and still controversial. It is still not very clearly understood whether the entering plasma is a result of the highlatitude MR 12,[15][16][17][18][19][20][21][22] , impulsive penetration 23,24 , or from the low latitudes through instabilities 25,26 or gradient drift 27 .…”

“…T he Earth is one of the six solar system planets that have global magnetic fields 1 , which are sufficient to block and deflect most solar wind charged particles from the Sun's atmosphere, and prevent them from directly interacting with the atmosphere of the planet 2,3 . This shielding is thought to be one of the most important factors protecting life on the ground and spacecraft in geospace from being damaged 4,5 .…”

Initial observations of fine plasma structures at the flank magnetopause with the complex plasma analyzer SCA-1 onboard the Interball Tail Probe

“…Gary and Eastman (1979) proposed lower hybrid drift (kinetic) instability as a drive diffusive process for LLBL. However, the transition from magnetosheath to the boundary layer is often thin (less than an ion's gyroradius) and the plasma within the boundary layer is uniform, suggesting that diffusion is not important Song et al, 1990;Le et al, 1994). Another local entry mechanism, impulsive penetration, was proposed by Lemaire and Roth (1978), Lemaire (1985) and Ross (1992).…”

On the incidence of Kelvin-Helmholtz instability for mass exchange process at the Earth’s magnetopause

“…The magnetic topology of the LLBL is still unclear. Song et al (1993) found that under northward IMF, the inner (earthward) part of the LLBL is on closed magnetic field lines, whereas the outer part could be composed of open and closed field lines. Fuselier et al (1995) suggested that the LLBL could be on open field lines.…”

“…Pashmann et al, 1993;Song et al, 1993). Absence of magnetic flux pile-up is thus also interpreted as a consequence of the open structure of the MP (e.g.…”