Two mechanisms for solar wind plasma transfer across Earth's magnetopause are
expected on theoretical grounds: magnetic reconnection and particle diffusion.
Of these, reconnection has been confirmed observationally beyond any doubt,
while diffusive entry has never been demonstrated altogether convincingly. The
occurrence of reconnection naturally depends strongly on the relative
orientation between the interplanetary and terrestrial magnetic fields at the
magnetopause, and there is evidence that the process can be patchy and
time-varying. Diffusion, on the other hand, is often assumed to be constantly
operating, producing a boundary layer that does not depend on solar wind
conditions. If diffusion were indeed occurring over the entire frontside
magnetopause, a boundary layer would result that gets progressively thicker the
further one moves away from the subsolar point. An ideal vantage point is
therefore the flanks of the magnetopause where the accumulative effect of
diffusion should be most easily observable. We have analyzed AMPTE-IRM
crossings in these region and have found cases where no boundary layer of solar
wind plasma could be observed at all. And this in spite of the fact that the
magnetopause current layer itself lasted approximately 60 s and thus was
well-resolved in the measurements. In one particular case we could infer from
the measured plasma flow speed along the magnetopause normal direction that the
magnetopause was moving at less than 20 km/s. The magnetopause was therefore
no more than 1200 km thick. The time resolution of our measurements was
4.4 s. Taking 5 s as an upper limit for its true duration, we conclude that
the boundary layer could not have been thicker than 100 km in these cases.
Taking the measured plasma density and bulk speed values at the inner edge of
the magnetopause as representative of the boundary layer, we can compute the
average boundary layer particle flux per unit height. From the known distance
from the subsolar point, the magnetopause area across which this flux must have
entered can be computed, and this fixes the average diffusive particle flux
across the magnetopause. This flux should equal _{} , where D is the
diffusion coefficient and is the density change across a diffusion
layer of thickness h. Taking the measured densities and observing that
h = 1200 km in this case, we obtain an upper limit for the diffusion coefficient
of 5·10^{7}m^{2}s^{-1}. This is a factor of 20 lower than the canonical
value of 10^{9}m^{2}s^{-1} that is often used in the literature when
estimating the contribution of diffusive entry. It is interesting to note that
in the case discussed no signatures of reconnection were observed either. Thus
it seems entirely possible that at times very little solar wind plasma is
traversing the magnetopause.

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