A STUDY OF ULF WAVE FORESHOCK MORPHOLOGY .1. ULF FORESHOCK BOUNDARY

We have used ISEE 1 and 2 magnetometer data to determine the forward boundary of the upstream ULF wave foreshock. We first examine the presence and absence of ULF waves immediately upstream from the bow shock. The statistical results show that the ULF foreshock starts at theta(Bn) approximately 50-degrees on the bow shock, and fills the entire upstream region connected to the bow shock at smaller theta(Bn). Then we examine the spacecraft crossing the ULF foreshock boundary when the interplanetary magnetic field is steady. The upstream boundary of the ULF foreshock is less sensitive to small changes of the IMF direction at larger cone angles and is well defined by the spacecraft position along the foreshock boundary for cone angles greater-than-or-equal-to 45-degrees. This boundary corresponds to the guiding center trajectory of backstreaming ions with a velocity of approximately 1.4v(sw) along the IMF, or a net guiding center velocity of approximately 1.74v(sw) in the Earth's frame. However the boundary is not well defined when the IMF cone angle is small.