Solar Wind - Magnetosphere Interactions via Neutral Atom Imaging

Moore, T. E., M. R. Collier, M.-C. Fok, S. A. Fuselier, D. G. Simpson, 
G. R. Wilson, M. O. Chandler

The development of the low energy neutral atom (LENA) imager was motivated
mainly by the need to remotely sense plasma heating and outflow from the
topside ionosphere, with the goal of enhanced temporal resolution of global
scope for this phenomenon. Earlier observations of ionospheric outflow have
been hampered by their in situ nature, which obscures their relationship to
potential drivers in the solar wind or magnetosphere. Fortuitously, the LENA
imager was found during ground testing to respond to neutral atoms with
energies well above its nominal range (10-300 eV), up to well above 1 keV,
owing to sputtering interactions with its conversion surface. On orbit, LENA
has been found to respond to a continuously-present neutral atom component of
the solar wind, as well as the neutral atoms formed by magnetosheath
interactions with the geocorona during periods of enhanced solar wind
pressure. Variations of the solar wind neutral component have been found on
time scales extending from the spacecraft spin period of 2 minutes, to CME
time scales of a few days, up to seasonal variations that have been related
to the distribution of neutral gas in the inner solar system. For CME time
scales, we have been able to validate earlier statistical inferences that
ionospheric heating responds promptly to solar wind dynamic pressure
variations. We determined that the response is at least as fast as
hydromagnetic wave propagation speeds, and that the heating source must lie
lower than 1000 km altitude, at least for the larger flux events. We
quantified the neutral atom content of the solar wind, showed it to have a
strong annual variation, and related this to the distribution of interstellar
neutral gas and dust in the inner solar system. Using simulations of the
magnetosheath interaction with the geocorona, we find that neutral atom
emissions reveal structures in the magnetosheath that are related to the
cusps. LENA imaging has thus emerged as a promising new tool for studying the
interplanetary medium and its interaction with the magnetosphere, from inside
the magnetosphere.

_______________
To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.