High-beta plasma blobs in the morningside plasma sheet

Equator-S frequently encountered, i.e. on 30% of the orbits between 1 March and 17 April 1998, strong variations of the magnetic field strength of typ ically 5-15-min duration outside about 9R(E) during the late-night/early-mo rning hours. Very high-plasma beta values were found, varying between 1 and 10 or more. Close conjunctions between Equator-S and Geotail revealed the spatial structure of these "plasma blobs" and their lifetime. They are typi cally 5-10 degrees wide in longitude and have an antisymmetric plasma or ma gnetic pressure distribution with respect to the equator, while being altog ether low-latitude phenomena (less than or equal to 15 degrees). They drift slowly sunward, exchange plasma across the equator and have a lifetime of at least 15-30 min. While their spatial structure may be due to some sort o f mirror instability, little is known about the origin of the high-beta pla sma. It is speculated that the morningside boundary layer somewhat further tailward may be the source of this plasma. This would be consistent with th e preference of the plasma blobs to occur during quiet conditions, although they are also found during substorm periods. The relation to auroral pheno mena in the morningside oval is uncertain. The energy deposition may be mos tly too weak to generate a visible signature. However, patchy aurora remain s a candidate for more disturbed periods.