Using ULF waves as diagnostic probes of the magnetospheric density

Fred W. Menk, Mark A. Clilverd, Bill R. Sandel, Don L. Carpentar, 
Ian R. Mann


Ultra-low frequency (ULF; f~1-100 mHz) plasma waves distribute
energy of solar wind origin throughout the Earth«s magnetosphere
and down to the ground, where they are recorded as pulsations of
the geomagnetic field. The waves propagate through the
magnetosphere in the fast magnetosonic mode and may couple to
shear transverse mode field line eigenoscillations. The frequency
of these field line resonances (FLRs) is determined by the field
geometry and the plasma mass density near the equatorial plane.
The FLRs occur during the local daytime and can often be observed
for several hours. Ground-based measurements of the resonant
frequency can therefore provide information on the plasma mass
density near the field line apex in the dayside magnetosphere. An
array of latitudinally-separated ground magnetometers allows the
plasma density profile to be monitored throughout the
magnetosphere. This may be compared with ground-based VLF
electron density measurements, and satellite-borne particle and
imager observations. The average properties of the magnetospheric
plasma are well established. However, case studies demonstrate
that the magnetosphere exhibits density features that have only
been partially described and which are far from understood.
Regions of particular interest are in the vicinity of the
plasmapause, where complex structures and temporal variations are
frequently observed, and the outer plasmasphere, where small-
scale density perturbations may occur. These regions may be
examined by ULF FLR techniques on a time scale of 30- 60 min and
a spatial resolution in the range 0.15-0.4 RE. We describe case
studies of the magnetospheric plasma using FLRs, and compare the
observed mass densities with spacecraft data and models. In
particular, we present illustrative examples of the density
profile for the following cases: (1). Quiet conditions, when only
a vestigial plasmapause is present; (2). Several days after a
moderate magnetic disturbance; (3) Disturbed conditions; and (4).
Cases when there are sudden changes in solar wind pressure. (5).
A magnetic storm cycle. In addition to observing the erosion and
refilling of the plasmasphere, we can detect small localized
depletions and enhancements, and monitor the effect of heavy ion
contributions. We also outline experiments that can provide multi-
instrument studies of the same flux tube. These use (a) closely-
spaced ground magnetometers to record the field line resonance
frequency and hence provide mass density information, (b) co-
located VLF receivers that simultaneously monitor the electron
density, (c) RPI and EUV data from the IMAGE spacecraft,
providing in situ electron and helium density information, and
(d) mathematical modelling of the constituent densities and
temperatures for this flux tube. These measurements provide the
first intercalibration of different ground-based and in situ
techniques and lend new insight on plasma dynamics within the
plasmasphere.

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Presented at the IUGG/IAGA meeting, Sapporo, Japan 
30 June - 11 July 2003