Localization of wave fields in lower hybrid cavities
We investigate lower hybrid wave trapping in cylindrically symmetric density depletions in the electrostatic approximation. Our investigation is inspired by previous observations of such trapping by spacecraft in the auroral region at altitudes up to about 2000km, and the recent discovery of th...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2004-09-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/22/2951/2004/angeo-22-2951-2004.pdf |
| Summary: | We investigate lower hybrid wave trapping in cylindrically
symmetric density depletions in the electrostatic approximation.
Our investigation is inspired by previous observations of such trapping
by spacecraft in the auroral region at altitudes up to about
2000km, and the
recent discovery of this phenomenon at altitudes above
20000km
in the
inner magnetosphere.
No particular shape is assumed for the density depletion,
which need not be strictly zero outside some value of the
radial coordinate <i>r</i>.
Important previously known properties
concerning parabolic density depletions extending to finite
<i>r</i> are shown to hold also for arbitrary shapes and infinite
extent: for a given parallel wave number <i>k<sub>z</sub></i>,
modes below the ambient lower hybrid
frequency <i>f</i><sub>LH</sub> are trapped in the density depletion (in the
sense that they are evanescent outside the cavity),
have a discrete spectrum and rotate
in a left-handed sense, while there is a continuous spectrum
of freely propagating right-handed rotating modes above <i>f</i><sub>LH</sub>.
New results are such that even though the density depletion may
go to zero slowly with increasing <i>r</i>, and thus be essentially
infinite in extent, there is a maximum distance within which
a trapped mode with given <i>k<sub>z</sub></i> and azimuthal mode
number <i>m</i> may propagate.
Furthermore, we find that for any
monotonic density cavity and given <i>k<sub>z</sub></i>, there is a local
relation between plasma density gradient and the lowest possible
frequency that can be trapped.
We combine our theoretical results with spacecraft observations to
find an upper bound on <i>k<sub>z</sub></i>. Our examples indicate that the length of
the cavities is larger than the width by a factor of at least 100. |
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| ISSN: | 0992-7689 1432-0576 |