Observation of the Solar Wind and Magnetosphere Interaction Near the Subsolar 
Point During Active Geomagnetic Periods on October 24 and 31, 2003

G. B. Vieira (1), M. E. Hill (1), D. C. Hamilton (1), J. L. Green (2),
T. E. Moore (2), D. G. Mitchell (3), and R. M. Skoug (4)

(1) Department of Physics, University of Maryland, College Park, MD 
20742-4111 USA
(2) NASA Goddard Spaceflight Center, Greenbelt, MD  20771 USA
(3) Johns Hopkins University Applied Physics Laboratory, Laurel, MD  20723
USA
(4) Los Alamos National Laboratory, Los Alamos, NM  87545 USA

In October of 2003 the major axis of the IMAGE spacecraftÕs orbit was
roughly aligned with the Sun-Earth line, with a dayside apogee extending
approximately eight Earth radii from the Earth.  This fortuitous orientation
provided the opportunity to make measurements near the nose of the 
magnetosphere.  Starting at ~1530 UT on October 24 and again at ~0530 UT on 
October 31 - both periods of elevated geomagnetic activity - the LENA telescope 
on IMAGE recorded high counting rates of nominal ~keV neutral hydrogen 
distributed in an unusually wide angular distribution centered on the Sun.  
These LENA signals are unique to these two periods and concurrent plasma and 
neutral atom measurements from the RPI and HENA instruments on IMAGE also 
reveal unusual, difficult to explain features.  We use solar wind particle 
measurements from the SWEPAM instrument on the ACE spacecraft to compare the 
solar wind flux and thermal spread with the LENA data and as an input into 
magnetopause and bow shock models.  These models show magnetospheric 
contractions that are well aligned with variations in the IMAGE observations, 
suggesting that the spacecraft may have entered the supersonic solar wind.  
Although this interpretation is quite possibly correct, we continue to 
investigate remaining features that may not fit this interpretation.  We also 
study the possibility of a connection between these unusual observations and a 
similar, commonly observed diffuse signal thought to arise from solar wind 
charge exchange in the magnetosheath.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004