Simultaneous remote-sensing and in situ observations of plasmaspheric drainage plumes

Plasmaspheric drainage plumes are regions of dense plasma that extend outward from the plasmasphere into the outer magnetosphere. We present observations of plumes for two events, 2 June 2001 and 26–27 June 2000. Our observations come from two sources. A global perspective is provided by the IMAGE extreme ultraviolet (EUV) imager, which routinely obtains images of the helium portion of the plasmasphere above total densities of 30–50 e−cm−3. Simultaneous in situ observations of plasmaspheric plumes are obtained by the Magnetospheric Plasma Analyzer (MPA) instruments onboard the Los Alamos National Laboratory (LANL) geosynchronous satellites. The in situ measurements of LANL MPA and the remote-sensing images of IMAGE EUV are complementary data sets that together provide a more complete picture than either alone. The MPA instruments measure density far below the EUV effective density threshold with greater spatial resolution, and often see plasma outside the EUV field of view. Flow speeds are also measurable by MPA. EUV images place the single-point measurements in a global dynamical context and allow separation of spatial and temporal effects. For the 2 June 2001 and 26–27 June 2000 events, both local and global measurements showed the same location, shape and temporal development of the plume(s), and a density distribution obtained from the EUV image at 3:05 UT on 2 June agrees with the LANL MPA density recorded at that time. Analysis of MPA flow data verifies that plume plasma moves sunward, as expected. Sunward flow speeds weaken with decreasing disturbance level, and duskside flow speeds may be increased by the sub-auroral polarization stream. The fine-scale density variations within plumes may be caused by a highly-structured inner magnetospheric E-field, and/or may be existing plasma structure that is carried sunward. The good agreement between the local and global measurements also validates the EUV image mapping method and promises to help quantify EUV images in terms of number density. 1. Plasmaspheric Drainage Plumes The innermost magnetosphere is occupied by the torus of cold dense plasma known as the plasmasphere. From the earliest plasmasphere observations (e.g., Carpenter [1967]; Chappell et al. [1970a]) and decades of subsequent study, it is known that the plasmapause radial location moves inward during geomagnetic disturbances, a process that came to be known as ‘plasmasphere erosion’ because the inward plasmapause motion was attributed in part to a stripping away of the outer layers of the plasmasphere. Although the details of the plasmasphere erosion process are not en1 Now at: Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX 78228 USA 2 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 USA 3 Space and Atmospheric Sciences Group, Los Alamos National Laboratory, Los Alamos, NM 87545 USA 4 STAR Lab, Electrical Engineering Department, Stanford University, Stanford, CA 94305 USA 5 Department of Physics and Astronomy, Rice University, Houston, TX 77005 USA Copyright by the American Geophysical Union. Paper number . 0148-0227/03/$9.00 tirely understood [Carpenter and Lemaire, 1997], one of the known byproducts of erosion is the plasmaspheric drainage plume. Plumes (also called ‘tails’) are regions of (presumably eroded) plasmaspheric plasma, connected to the main body of the plasmasphere, that extend into the more tenuous outer magnetosphere. Plumes were predicted by theoretical models of the disturbance-time inner magnetosphere [Grebowsky , 1970; Chen and Wolf , 1972; Spiro et al., 1981;Weiss et al., 1997; Lambour et al., 1997] and can be inferred from numerous in situ and ground-based observations [Chappell et al., 1970b; Horwitz et al., 1990; Carpenter and Anderson, 1992; Elphic et al., 1996; Su et al., 2001]. Global plasmasphere imaging by the IMAGE satellite extreme ultraviolet (EUV) imager [Burch, 2000] has proved the existence of plasmaspheric plumes [Sandel et al., 2001]. The EUV instrument works by detecting 30.4-nm ultraviolet light that is resonantly scattered by the He ions in the plasmasphere. The existing EUV database contains many examples of plumes during and/or after geomagnetic disturbance times [Burch et al., 2001a, b; Foster et al., 2002; Goldstein et al., 2002, 2003a, c, b; Sandel et al., 2003; Moldwin et al., 2003; Spasojević et al., 2003b, a]. IMAGE EUV provides global snapshots of the plasmasphere and plume with spatial and temporal resolution of about 0.1RE and ≈10 minutes, respectively. The EUV instrument’s effective lower density threshold (i.e., the lowest detectable total plasma density, at which the signal-to-noise ratio is unity) depends