Abstract
We use a newly developed global Hall MHD code to investigate how
reconnection drives magnetotail asymmetries in small magnetospheres.
Here, we consider a scaled-down, Earth-like magnetosphere where we have
artificially inflated the ion inertial length
($\delta_i$) to one Earth radius (the real Earth’s
$\delta_i\approx 1/15-1/20 R_E
\approx 300-400\unit{km}$ in the
magnetotail). This results in a magnetotail width on the order of $30
\delta_i$, slightly smaller than Mercury’s tail and
much smaller than Earth’s. At this small size, we find that the Hall
effect has significant impact on the global flow pattern, changing from
a symmetric, Dungey-like convection under resistive MHD to an asymmetric
pattern similar to that found in previous Hall MHD simulations of
Ganymede’s subsonic magnetosphere as well as other simulations of
Mercury’s using multi-fluid or embedded kinetic physics. We demonstrate
that the Hall effect is sufficient to induce a dawnward asymmetry in
observed dipolarization front locations and find quasi-periodic global
scale dipolarizations under steady, southward solar wind conditions. On
average, we find a thinner current sheet dawnward; however, the measured
thickness oscillates with the dipolarization cycle. During the
flux-pileup stage, the dawnward current sheet can be thicker than the
duskward sheet. This could be an explanation for recent observations
that suggest Mercury’s current sheet is actually thicker on the
duskside: a sampling bias due to a longer-lasting “thick’ state in the
sheet.