CRRES observations of density cavities inside the plasmasphere

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Deep density troughs inside the plasmasphere in which electron density was a factor of from ∼2 to 10 below nearby plasmasphere levels were found in ∼13% of 1764 near‐equatorial electron density profiles derived from the sweep frequency receiver data acquired in 1990–1991 by the CRRES satellite. These “inner troughs” appeared in the aftermath of plasmasphere erosion episodes and are interpreted as the near‐equatorial manifestations of geomagnetic‐field‐aligned cavities. Inner troughs were found at all local times but were most common in the 1800–2400 magnetic local time (MLT) sector and least common between 0600 and 1200 MLT. Their inner boundaries, plasmapause‐like in form, were mostly at L < 3.5 but in ∼30% of the cases were at L < 2.5 under geomagnetic conditions that traditionally have been associated with plasmapause radii in the L = 3–3.5 range or beyond. The trough outer walls were exceptionally steep, in several cases exhibiting a factor of 4 or more density change within less than 100 km along the near‐equatorial satellite orbit. The extent of the troughs in L ranged from ΔL ∼ 0.5 to 2, and various forms of evidence, including earlier studies, suggest an extent of more than 20° in longitude. Such evidence includes plasma waves propagating in a free space mode within the inner trough while extending in frequency well above the upper limit of trapped continuum radiation detected beyond the plasmasphere. We suggest, as have previous authors, that the troughs are translated vestiges of plasma configurations established during preceding periods of plasmasphere erosion. In some such cases, dense plasma features lying beyond the troughs were probably connected to the main plasmasphere in a local time sector to the east of the observing longitude. However, in some of the cases of troughs with steep outer walls the dense plasma feature beyond that wall may have been shaped by a mechanism for detaching plasma from an originally larger outer plasmasphere, such as by shear flows in the premidnight sector associated with subauroral ion drifts.