Imaging the near-Earth energetic ion environment: IMAGE/HENA results after
three years in orbit

Donald G. Mitchell
Applied Physics Laboratory/Johns Hopkins University


The intensity of the energetic ion population between two and six Earth
radii increases by several orders of magnitude during geomagnetic storms,
which depend on solar wind conditions.  This population, dominated by ions
with energies between about 10 keV and 200 keV, is known as the Ring Current
for it's characteristic spatial distribution in a ring about the Earth,
trapped in the Earth's magnetic field.  These ions are injected into the
near Earth region from the plasma sheet, a temporarily trapped ion
population in the Earth's magnetotail, during episodes of strong E X B
convection driven by southward interplanetary magnetic field and enhanced by
high solar wind velocities.  As they are transported Earthward, the ions
gain energy through a variety of processes, the most basic and important of
which is simply adiabatic acceleration, a combination of Fermi and betatron
acceleration as the ions move onto shorter, higher field strength magnetic
field lines.  The current in the ring current is driven by pressure
gradients in the ion distributions, so its distribution depends on the
details of the ion transport during a geomagnetic storm.  Before the launch
of the IMAGE spacecraft, our understanding of the evolution of the ring
current during a storm was based on statistical studies compiled from many
years of in-situ ion measurements, as well as ground-based magnetometer
measurements (which respond to the global distribution of electric current,
including the ring current), and increasingly sophisticated models and
simulations.  The IMAGE spacecraft carried the first suite of instruments
designed for high altitude, global imaging of the ring current.  This talk
will highlight recent results based on images from the High Energy Neutral
Atom imager (HENA), an instrument that produces global images of the
emission of energetic neutral atoms produced as ions in the ring current
charge exchange with the cold exopheric hydrogen gas (the geocorona) which
shares the same space.  These images have revealed new insights into the
global development of the ring current during a storm (its peak tends to lie
near local midnight, not local dusk as magnetometer data had suggested), and
into the role of ionospheric oxygen in the transport of mass and energy
through the magnetosphere during a storm.

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Seminar
Department of Physics and the Institute for Physical Science and Technology
University of Maryland
Computer & Space Sciences Building, Rm 2400
4:30 PM Monday, April 21, 2003