DENSITY CAVITIES INSIDE THE PLASMASPHERE 23 , 325 Orbit 145 UHR AKR TYPE-Ill BURST September

Deep density troughs inside the plasmasphere in which electron density was a factor of from -2 to 10 below nearby plasmasphere l vels 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 ofplasmasphere rosion 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 i 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 raditionally have been associated with plasmapause radii n 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 œ ranged from Aœ 0.5 to 2, and various forms of evidence, including earlier studies, uggest an extent of more than 20 øin longitude. Such evidence includes plasma waves propagating i afree space mode within the inner trough while extending in frequency well above the upper limit of trapped continuum radiation detected beyond the plasmasphere. W suggest, as have previous authors, that the troughs are translated vestiges of plasma configurations established during preceding periods of plasmasphere rosion. Insome such cases, dense plasma features lying beyond the troughs were probably connected tothe 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 fordetaching plasma from an originally arger outer plasmasphere, such as by shear flows in the premidnight sector associated with subauroral ion drifts.

[1]  H. Matsumoto,et al.  Kilometric continuum detected by Geotail , 1999 .

[2]  F. Mozer,et al.  Experimental evidence on the role of the large spatial scale electric field in creating the ring current , 1998 .

[3]  J. Lemaire,et al.  Erosion and Recovery of the Plasmasphere in the Plasmapause Region , 1997 .

[4]  M. Moldwin.  Outer Plasmaspheric Plasma Properties: What We Know from Satellite Data , 1997 .

[5]  R. Elphic,et al.  Study of plasmaspheric evolution using geosynchronous observations and global modeling , 1997 .

[6]  D. Ober,et al.  Formation of density troughs embedded in the outer plasmasphere by subauroral ion drift events , 1997 .

[7]  R. Elphic,et al.  Evolution of plasmaspheric ions at geosynchronous orbit during times of high geomagnetic activity , 1996 .

[8]  T. Moore,et al.  On the azimuthal variation of core plasma in the equatorial magnetosphere , 1995 .

[9]  D. M. Haines,et al.  The feasibility of radio sounding in the magnetosphere , 1995 .

[10]  D. Mccomas,et al.  The fine‐scale structure of the outer plasmasphere , 1995 .

[11]  D. Gurnett,et al.  Electron number density fluctuations near the plasmapause observed by the CRRES Spacecraft , 1994 .

[12]  D. Mccomas,et al.  An examination of the structure and dynamics of the outer plasmasphere using multiple geosynchronous satellites , 1994 .

[13]  Barham W. Smith,et al.  Imagers for the magnetosphere, aurora, and plasmasphere , 1994 .

[14]  B. Barraclough,et al.  Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit , 1993 .

[15]  W. Hughes,et al.  Banded electrostatic emissions observed by the CRRES plasma wave experiment , 1992 .

[16]  E. Roelof,et al.  Global magnetospheric imaging , 1992 .

[17]  R. Anderson,et al.  An ISEE/Whistler model of equatorial electron density in the magnetosphere , 1992 .

[18]  P. Anderson,et al.  The ionospheric signatures of rapid subauroral ion drifts , 1991 .

[19]  H. Oya.  Studies on Plasma and Plasma Waves in the Plasmasphere and Auroral Particle Acceleration Region, by PWS on board the EXOS-D (Akebono) Satellite , 1991 .

[20]  J. Horwitz,et al.  A statistical characterization of plasmasphere density structure and boundary locations , 1990 .

[21]  Koons.  Observations of large-amplitude, whistler-mode wave ducts in the outer plasmasphere. Technical report , 1989 .

[22]  M. Kelley.  Intense sheared flow as the origin of large‐scale undulations of the edge of the diffuse aurora , 1986 .

[23]  B. Higel,et al.  Electron density and plasmapause characteristics at 6.6 RE : A statistical study of the GEOS 2 relaxation sounder data , 1984 .

[24]  D. Gurnett,et al.  Escaping nonthermal continuum radiation , 1981 .

[25]  R. Benson,et al.  Isis 1 observations at the source of auroral kilometric radiation , 1979 .

[26]  D. Gurnett,et al.  Initial results from the ISEE-1 and -2 plasma wave investigation , 1979 .

[27]  T. Birmingham,et al.  Electrostatic emissions between electron gyroharmonics in the outer magnetosphere , 1978 .

[28]  D. Carpenter,et al.  Outlying plasmasphere structure detected by whistlers , 1976 .

[29]  D. Gurnett,et al.  Electrostatic noise bands associated with the electron gyrofrequency and plasma frequency in the outer magnetosphere , 1975 .

[30]  D. Gurnett.  The Earth as a radio source: The nonthermal continuum , 1974 .

[31]  C. Chappell.  Detached plasma regions in the magnetosphere , 1974 .

[32]  J. Lemaire.  The ‘Roche-Limit’ of ionospheric plasma and the formation of the plasmapause , 1974 .

[33]  V. Ponomarev,et al.  Plasma convection in polar ionosphere , 1974 .

[34]  D. Gurnett,et al.  Electromagnetic radiation trapped in the magnetosphere above the plasma frequency , 1973 .

[35]  C. Park.  Whistler observations of the depletion of the plasmasphere during a magnetospheric substorm , 1973 .

[36]  R. Wolf,et al.  EFFECTS ON THE PLASMASPHERE OF A TIME-VARYING CONVECTION ELECTRIC FIELD. , 1972 .

[37]  J. Grebowsky,et al.  Structured variations of the plasmapause: Evidence of a corotating plasma tail , 1971 .

[38]  Chung G. Park,et al.  Westward electric fields as the cause of nighttime enhancements in electron concentrations in midlatitude F region , 1971 .

[39]  J. H. Mcgehee,et al.  VLF electric field observations in the magnetosphere , 1970 .

[40]  J. Grebowsky.  Model study of plasmapause motion , 1970 .

[41]  J. Grebowsky,et al.  STUDY OF THE THERMAL PLASMA ON CLOSED FIELD LINES OUTSIDE THE PLASMASPHERE. , 1970 .

[42]  D. L. Carpenter.  Whistler evidence of the dynamic behavior of the duskside bulge in the plasmasphere , 1970 .

[43]  D. L. Carpenter,et al.  Whistler evidence of large‐scale electron‐density irregularities in the plasmasphere , 1970 .

[44]  J. Grebowsky,et al.  Comparison of coincident OGO 3 and OGO 4 hydrogen ion composition measurements , 1970 .

[45]  D. L. Carpenter,et al.  Ion depletion in the high-latitude exosphere; simultaneous OGO 2 observations of the light ion trough and the VLF cutoff , 1969 .

[46]  W. Clark,et al.  Radio wave reflections from large horizontal gradients in the topside ionosphere , 1969 .

[47]  D. L. Carpenter.  Relations between the dawn minimum in the equatorial radius of the plasmapause and Dst, Kp , and local K at Byrd Station , 1967 .

[48]  A. Nishida.  Formation of plasmapause, or magnetospheric plasma knee, by the combined action of magnetospheric convection and plasma escape from the tail , 1966 .

[49]  R. Gendrin.  Le guidage des whistlers par le champ magnetique , 1961 .