Seasonal and hemispheric asymmetries in the cold ion outflow source
region: Swarm and CHAMP observations of F-region polar cap plasma
density
Abstract
One of the primary mechanisms of loss of Earth’s atmosphere is the
persistent “cold” ( ≲ 20 eV) ion outflow that has been observed in the
magnetospheric lobes over large volumes with dimensions of order several
Earth radii (R). As the main source of this cold ion outflow, the polar
cap -region ionosphere and conditions within it have a disproportionate
influence on these magnetospheric regions. Using 15 years of
measurements of plasma density N made by the Swarm spacecraft
constellation and the CHAMP spacecraft within the region of the polar
cap above 80{degree sign} Apex magnetic latitude, we report evidence
of several types of seasonal asymmetries in polar cap N. Among these, we
find that the transition between “winter-like” and “summer-like”
polar cap N occurs one week prior to local spring equinox in the
Northern Hemisphere (NH), and one week after local spring equinox in the
Southern Hemisphere (SH). Thus SH polar cap N lags NH polar cap N by
approximately two weeks with respect to local spring and fall equinox in
each hemisphere. We show that this lag cannot be explained by
differences in solar illumination alone. We also find that overall
variation of N in the SH polar cap is greater than overall variation of
N in the NH polar cap, except for an approximately two-month period
centered on June solstice, and that the greater degree of variability of
N in the SH polar cap is partly attributable to differences in
illumination of the SH polar cap.