Penetration of solar wind plasma elements into the magnetosphere

Abstract

Assuming that the solar wind is unsteady and non-uniform, it is suggested that field aligned plasma elements dent the magnetopause surface. This indentation makes the magnetopause boundary convex and therefore locally unstable with respect to flute instabilities. The intruding element is slowed down and stopped within 1 or 2 Earth radii from the magnetopause. Its excess convection kinetic energy is dissipated in the lower polar cusp ionosphere after ~ 50–500 s, depending on the value of the integrated Pedersen conductivity. Once the plasma element has been engulfed, keeping its identity, the warm plasma content is dissipated by precipitation and by drifting. The magnetosheath particles with large pitch angles are mirrored, and feed the plasma mantle flow. Several consequences of this penetration mechanism are pointed out: Ionospheric heating beneath the polar cusp; Birkeland currents on the eastward and westward edges of the plasma element; Diamagnetic field fluctuations within 1–2 R E from the magnetopause (multiple magnetopause crossings); Oscillation of the magnetopause surface after a new element has penetrated; Exit of energetic particles out of the magnetosphere, and entry of energetic solar wind particles into the magnetosphere along the magnetic field lines of the intruding element; Subtraction of magnetic flux from the dayside magnetosphere and its addition to the geomagnetic tail when the magnetic field of the element has a southward component.