Positive density gradients at the magnetopause: interpretation in the framework of the impulsive penetration mechanism

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In situ observations indicate the presence of external/magnetosheath-like plasma inside the magnetosphere. Two-point measurements show that a positive density gradient develops inside these plasma irregularities such that the density increases from the front edge (closer to the Earth) to the trailing edge. It is shown that this effect can be explained by the expansion of the plasma blobs or plasmoids along magnetic field lines. Impulsively injected magnetosheath density irregularities may be stopped before they fully penetrate inside the magnetosphere and become isolated plasma blobs/islands. Furthermore, it is argued that non-detachment of these intruding plasma islands from the magnetopause does not rule out the impulsive penetration (IP) mechanism as a candidate for explaining the presence of magnetosheath plasma inside the magnetosphere. The main features of the kinetic description of the IP mechanism are briefly recalled first.