Far ultraviolet remote sensing of the isotropy boundary and magnetotail stretching 

C. Blockx, J.-C. Gerard, M. Meurant, B. Hubert, and V. Coumans 


Several studies have attempted to identify the isotropy boundary (IB) defining
the limit between the adiabatic and nonadiabatic trajectories of the trapped
protons along closed magnetic field lines. This boundary is an indicator of the
amount of magnetic field line stretching in the magnetotail. Previous studies
were based on in situ measurements, resulting in spatially and temporally
restricted samples. To overcome these limitations, we propose to use global data
obtained with the FUV spectrographic proton auroral imager (SI12) on board the
IMAGE satellite. We determine at each magnetic local time the position of an
optical boundary related to the IB and thereby to the stretching of the magnetic
field lines. We show that the correspondence between the latitude of the maximum
proton precipitation observed by SI12 and the IB measured by Defense
Meteorological Satellite Program satellites is statistically established and
depends on the magnetic local time. The relation between the position of the
maximum proton precipitation as well as the intensity of this maximum and the
magnetic field distortion is determined by comparison with GOES 8 data. We
suggest that SI12 images can be used as a tool to globally determine the
isotropy boundary and to monitor the level of stretching in the magnetotail.

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Journal of Geophysical Research, 110, A11215, doi:10.1029/2005JA011103, 2005