Magnetosperic boundary

Magnetopause structure

The magnetopause is the layer that shields the Earth environment from the solar wind.The magnetopause surface has a rather complex shape; the magnetosheath plasma (shocked solar wind) flows around the magnetopause.

The equilibrium magnetopause can be locally a tangential discontinuity (TD)  or a rotational discontinuity (RD). Since we have developed a kinetic model of TDs, our main objective has been a TD description of the magnetopause.



Impulsive penetration

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 (BIRA-IASB) 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).


Energy transport by MHD waves

Another aspect of the solar wind - magnetosphere interaction is related to diffusion. In the case of a collisionless plasma, the diffusion is not due to collisions but due to the ever-present turbulence in the electromagnetic fields. The magnetopause, in particular, has been observed to be a region characterized by an increased fluctuation level of  electromagnetic waves of ultra-low frequency. 

We have examined the propagation of waves at the magnetopause using a linear magnetohydrodynamic (MHD) description. Even though the mass flux across the magnetopause turns out to be zero, there may very well be an energy flux into the magnetosphere.

Energy transport by MHD waves

The figure shows the wave amplitudes (maximum and minimum modulus of the magnetic field, left frames) and the time-averaged energy flux (right frames) as a function of the distance to the center of the magnetopause (in km). The magnetosphere is to the left, the magnetosheath to the right of the magnetopause.

  • Class 1: a non-vanishing perturbation intensity in the magnetosphere indicates transmission, while the energy flux remains constant as the incident flux equals the transmitted flux plus the reflected one. In the reference frame adopted here, a negative flux indicates energy transfer from the magnetosheath to the magnetosphere.
  • Class 2: the wave does not penetrate into the magnetosphere and is  completely reflected, resulting in a net zero energy flux.
  • Class 3: similar to class 2 but now resonant amplification (infinite wave amplitude) occurs at 1 or 2 points inside the magnetopause; resonant absorption implies a jump in the energy flux profile.
  • Class 4: reflection, transmission and resonant amplification occur simultaneously; again, the energy flux is discontinuous at the resonant points.



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