The solar wind interacts with the Earth's magnetic field. The geomagnetic field shields the immediate terrestrial environment from the solar wind particles. Mass and energy transfer to the Earth remain possible, however, by a variety of mechanisms.

The Belgian Institute for Space Aeronomy has developed the model of impulsive penetration of magnetosheath plasma elements through the magnetopause. This mechanism was first proposed in 1976 at an EGS meeting on "the magnetopause regions" in Amsterdam (Lemaire, J., and M. Roth, Penetration of solar wind plasma elements into the magnetosphere, J. Atmos. Terr. Phys., 40, 331, 1978). The summary below does not cover material dating back before 1990.

Non-steady-state solar-wind-magnetosphere interaction
J. Lemaire and M. Roth
Space Sci. Rev., 57, 59, 1991

Most of the theories proposed to explain the interaction between the solar wind and the geomagnetic field are stationary descriptions based on ideal MHD. In this review an alternative, non-stationary description is discussed. According to this description, most of the plasma-field irregularities, i.e., plasmoids, detected in the solar wind can penetrate inside the geomagnetic field beyond what is considered to be the mean position of the magnetopause. It is the patchy solar wind plasma impinging on the geomagnetic field which imposes rapidly changing and non-uniform boundary conditions over the whole outer magnetospheric surface. This contrasts with the general belief that the observed field variations or "events" arise sporadically near the magnetopause as the result of some plasma instability.
A brief historical review is given to illustrate the evolution of the theoretical models proposed to explain the interaction of the solar wind with the magnetosphere. The emergence of the idea of "impulsive penetration" of solar wind plasma irregularities into the magnetosphere is emphasized especially.
A kinetic model of the unperturbed magnetopause is described. This model corresponds to a closed magnetosphere whose surface is a tangential discontinuity. This transition layer can sustain plasma jettings and can be traversed by impulsive penetrating plasmoids. This is against the general belief which considers tangential discontinuities as the worse case with respect to impulsive penetration and plasma jettings.
The mean features of the theory of impulsive penetration are presented. Gusty penetration of solar wind plasmoids depends on their excess momentum density and on the orientation of the IMF. The motion of plasmoids across non-uniform magnetic field configurations (tangential discontinuities) is discussed theoretically. When the dielectric constant of the streaming plasma is large enough for collective polarization effects to become important, an electric field develops which permits cross-B motions of all charged particles as a whole plasma entity. It is re-emphasized that the value of the integrated Pedersen conductivity is a determining factor in cross-B plasma motion. On the other hand, interconnection of interplanetary magnetic field lines and geomagnetic field lines results from collective diamagnetic effects produced by magnetized plasmoids injected into the magnetosphere.
Several consequences of this penetration mechanism are discussed. These are: the escape of energetic particles out of the magnetosphere, the eastward deflection of penetrating plasmoids, the magnetospheric and ionospheric convection patterns, the erosion of plasmoids, and the mass/momentum loading effects.
Some significant experimental geophysical observations supporting the impulsive penetration model are also discussed.

On impulsive penetration of solar wind plasmoids into the geomagnetic field
M. Roth
Planet. Space Sci., 40, 193, 1992

The idea that solar wind plasma-field irregularities, i.e. plasmoids with an excess momentum density penetrate deeper into the geomagnetic field was introduced in 1976 by Lemaire and Roth at an EGS meeting. It was based on the observation that the solar wind is most of the time patchy over distances smaller than the diameter of the magnetosphere. In this early paper about "impulsive penetration", the authors did not attend to give a detailed physical description of the underlying mechanism. When Lemaire was more informed about some relevant laboratory plasma experiments carried out by Bostick, Baker and Hammel or Demidenko et al., he published in 1985 (Lemaire, J., Plasma Phys., 33, 425, 1985) a physical description of the mechanism, based on a theory first proposed by Schmidt in 1960 (Schmidt, G., Phys. Fluids, 3, 961, 1961).
Transient and impulsive interaction processes between the solar wind and the magnetosphere have now become an important and highly debated topic. In particular, Heikkila's argument claiming that the effects of induced electric fields are the primary cause for impulsive penetration has been shown by Owen and Cowley to be erroneous. Although the conclusions reached by Owen and Cowley (Owen, C.J. and Cowley, S.W.H., J. Geophys. Res., 96, 5565, 1991) are correct, at least within the framework contrived by Heikkila (i.e. that of ideal MHD) (Heikkila, W.J., Geophys. Res. Lett., 9, 159, 1982) they do not demonstrate that real plasmoids can not penetrate impulsively onto closed geomagnetic field lines. Indeed, non-ideal MHD processes, like collective polarization effects, formation of electrostatic potential barriers, adiabatic and non-adiabatic brakings or collective diamagnetic effects, have to be taken into account in the "real world".
Account of the theory of "impulsive penetration" both for weakly and strongly diamagnetic plasmoids is given, emphasizing in which respect the entry mechanism differs from ideal entry mechanisms like those proposed by Schindler (Schindler, J. Geophys. Res., 84, 7257, 1979) and by Heikkila in 1982.

Impulsive transport of solar wind into the magnetosphere
M. Roth
in P. Song, B.U. Sonnerup, and M.F. Thomsen, ed(s), Physics of the magnetopause, 
Geophys. Monogr. Ser., vol. 90, AGU, Washington, D.C., 343, 1995

According to the theory of "impulsive penetration" proposed by Lemaire and Roth, magnetosheath plasma irregularities with an excess momentum density enter the geomagnetic field by means of an E cross B drift resulting from their self electric polarization. Collective polarization thermo-electric charge separation, and non-adiabatic braking are important non-ideal MHD processes. The dipole-dipole interaction force between the Earth's dipole field and the current system of a penetrating 3-dimensional diamagnetic plasmoid can increase or decrease the entry velocity, depending on the orientation of the IMF. A large number of laboratory experiments as well as significant geophysical observations are consistent with this impulsive penetration model.
 

Author: J. De Keyser and M. Roth   Curator: J. De Keyser   Johan.DeKeyser@oma.be