Y. Kasaba, H. Matsumoto, and R. R. Anderson.
Adv. Space Res., 20, 699-702, 1997.
GEOTAIL is now on the near-tail orbit with an apogee of ~30 Re and a perigee of ~10 Re, and we frequently observe 2fp emissions around the terrestrial electron foreshock by Plasma Wave Instrument (PWI). Since geometric features of the source region should reflect distribution functions of electron beams in the foreshock region, we try to determine geometry of source region of 2fp emission by three kinds of statistical remote sensing analysis: mapping of 2fp flux, timing analysis of bifurcation phenomena associated with solar wind density discontinuities in the solar wind, and propagation direction determined by spin modulation. We confirm three points:
(1) The 2fp source region is on the tangential field line to the bow shock with strong Langmuir waves. This is one piece of direct evidence that 2fp emission is generated from intense Langmuir waves.
(2) Both Langmuir waves and 2fp emissions are not strong around contact point of the tangential field line to the bow shock where acceleration of electrons is expected. This suggests that sharp electron beams are not formed enough to generate intense Langmuir and 2fp waves, because flight time in the region close to the contact point is too short to develop electron beams through the velocity filtering.
(3) Distance of the central position of the source region from the Earth is generally up to 40 Re. This suggests typical flight length of free energy consumption in the electron beam is enough to excite 2fp emissions.
These results are also consistent with triangular analysis of simultaneous GEOTAIL/PWI and WIND/WAVES observations.
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