Structure of the magnetopause

Abstract

This paper reviews present understanding of the internal structure of the thin boundary layer, termed the magnetopause, that separates the distorted geomagnetic field in the magnetosphere from the flow of solar plasma in the magnetosheath. The fundamental theoretical concepts of the subject are introduced by considering the structure of the boundary layer that exists when a cold, unmagnetized stream of ions and electrons impinges normally on a vacuum magnetic field. This idealized model indicates how the earth's magnetic field is confined by the impact pressure of the solar wind, the geomagnetic field being terminated by induced shielding currents flowing in the magnetopause. The various idealizations and approximations in this model, such as the assumption of a cold solar plasma and the neglect of the interplanetary magnetic field, are examined critically. The presence of thermal plasma in the magnetosphere modifies the structure of the boundary layer so that the ions penetrate substantially deeper (∼100 km) than the electrons into the geomagnetic field. Complications then arise if the solar plasma has a component of velocity parallel to the confined field, as occurs in the downstream magnetopause that bounds the geomagnetic tail. This additional component of velocity generates electric currents parallel to the magnetic field that may destroy the small‐scale equilibrium of the magnetopause and result in a tangential drag on the geomagnetic cavity. The available experimental information on the magnetopause structure is summarized and related to the theoretical investigations. Although the observations confirm some of the theoretical predictions, much of the detailed theory remains speculative at the present time.

Structure and Dynamics of the Magnetopause and Its Boundary Layers

“…Observed in the MSBL are magnetosheath particles convected toward the magnetopause, and those of magnetosheath origin reflected off the magnetopause, and/or those of magnetosphere origin leaked out from the magnetosphere. (Although it is not fully understood exactly how particles are reflected at the magnetopause, a polarization electric field generated by ions with a larger Larmor radii than electrons (Willis, 1971) or a Hall electric field (e.g., Sonnerup, 1979), both of which can, on average, point toward the magnetosheath, may act to reflect the incoming ions.) On the other hand, observed in the LLBL (of open magnetosphere models) are magnetospheric particles convected toward the magnetopause, and those of magnetosphere origin reflected off the magnetopause, and/or those of magnetosheath origin that crossed the magnetopause.…”

Waves in the Vicinity of the Magnetopause

“…Kinetic theory investigates the incidence of plasma on a uniform field representing the magnetosphere (see review by Willis, 1971). Many theories in this class are deficient in that they neglect the magnetosheath field.…”

Currents at the subsolar low shear magnetopause

“…Calculations of Roth et al [1993] show that the two different electron populations (magnetosheath and electrostatic field is reduced still further. Willis [1971] mentioned that if the polarization electrostatic field at the magnetopause is almost completely neutralized, then the thickness of the boundary layer is comparable with the proton cyclotron radius (m 100 km) rather than the plasma skin depth (m 1 km). This theoretical deduction is…”

“…This electric field opposes the charge separation so that it remains small [Willis, 1971]. Sigov and Tverskoy [1963] noted that if the electron thermal energy is small compared with the directed kinetic energy of the ions, then the polarization electric field at the magnetopause (on its sunward side) can be short-circuited in the adjacent magnetosheath.…”

“…Ions penetrate substantially deeper than electrons into space plasma boundaries [e.g., Parker, 1967aParker, , 1967bWillis, 1970Willis, , 1971 ]. Calculations of Roth et al [1993] show that the two different electron populations (magnetosheath and electrostatic field is reduced still further.…”