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6. Polar Energetic Particles

Finally we briefly mention the IMF effect on the magnetic field and particle populations at the site of POLAR. The instrument called CEPPAD/IPS on board POLAR [ Blake et al., 1995] observes ions in the energy range from 12 to 1500 keV. From 0000 to 0800 UT on March 28, 1996, this instrument observed almost nothing, except for the interval from 0333 to 0357 UT when the instrument recorded 60 keV ions (not shown). These ions were rich in He and high-charge state (>+6) Oxygen (not shown). Thus they were of solar wind origin. We note that IMF was negative during the interval from 0315 to 0420 UT (see Figure 1). Although IMF was negative throughout the interval, reconnection at the lobe magnetopause is possible due to the negative IMF , which could explain the observed solar wind-origin particles. at POLAR was generally increasing during this interval (see Figure 2), indicative of continuing dayside merging. However, there was a small dip in around 0339 UT, which could be a diamagnetic effect caused by the particle population. We also note transiently decreased at the same time, which indicates that the magnetic field tilt briefly recovered toward the pre-substorm value. Lobe reconnection at this time is consistent with this change in the magnetic field tilt angle.

Kawano Hideaki
Sat Jan 4 18:21:35 JST 1997