Observations of lower hybrid cavities in the inner magnetosphere by the Cluster and Viking satellites
Observations by the Viking and Cluster satellites at altitudes up to 35000km show that Lower Hybrid Cavities (LHCs) are common in the inner magnetosphere. LHCs are density depletions filled with waves in the lower hybrid frequency range. The LHCs have, until recently, only been found at altitu...
| Main Authors: | , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2004-09-01
|
| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/22/2961/2004/angeo-22-2961-2004.pdf |
| Summary: | Observations by the Viking and Cluster satellites at altitudes up to
35000km show that Lower Hybrid Cavities (LHCs) are common in the
inner magnetosphere.
LHCs are density depletions filled with waves in the lower hybrid
frequency range. The LHCs have, until recently, only been found at
altitudes up to 2000km.
Statistics of the locations and general shape of the LHCs is
performed to obtain an overview of some of their properties.
In total, we have observed 166 LHCs on Viking during 27h of data, and
535 LHCs on Cluster during 87h of data.
These LHCs are found at invariant latitudes from the auroral region to
the plasmapause. A comparison with lower altitude observations shows
that the LHC occurrence frequency does not scale with the flux tube
radius, so that the LHCs are moderately rarer at high altitudes.
This indicates that the individual LHCs do not reach from the
ionosphere to 35000km altitude, which gives an upper bound
for their length.
The width of the LHCs perpendicular
to the geomagnetic field at high altitudes is a few times the ion
gyroradius, consistent with observations at low altitudes.
The estimated depth of the density depletions vary with
altitude, being larger at altitudes of 20000-35000km
(Cluster, 10-20%),
smaller around 1500-13000km (Viking and previous Freja results, a
few percent)
and again larger around 1000km (previous sounding rocket
observations, 10-20%).
The LHCs in the inner magnetosphere are situated in regions
with background electrostatic hiss in the lower hybrid frequency
range, consistent with investigations at low altitudes.
Individual LHCs observed at high altitudes are stable at least on time
scales of 0.2s (about the ion gyro period),
which is consistent with previous results at lower altitudes,
and observations by the four Cluster
satellites show that
the occurrence of LHCs in a region in space is a stable phenomenon, at
least on time scales of an hour. |
|---|---|
| ISSN: | 0992-7689 1432-0576 |