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| Titel |
Formation of wedge-like pattern on VLF spectrograms observed by DEMETER |
| VerfasserIn |
David Shklyar, Michel Parrot, Jaroslav Chum, Ondrej Santolik, Elena Titova |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032965
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| Zusammenfassung |
| The DEMETER satellite has almost circular polar orbit, with the altitude ~ 700 km. At
middle latitudes, DEMETER typically stays in the region where the height-dependent
variation of the lower hybrid resonance (LHR) frequency profile forms a trough, i.e. inside
the so-called LHR waveguide. In this region, LHR phenomena reveal themselves most
distinctly. A striking example of such phenomena is provided by wedge-like events (WLE)
registered sometimes on overview VLF spectrograms (time duration ~ 2 minutes, frequency
range 0 - 20 kHz) during thunderstorm activity. A characteristic feature of these
spectrograms is the presence of unusual upper and lower cutoff frequencies. The upper cutoff
frequency varies rapidly, approximately in proportion to L-3, where L is McIlwain
parameter on the satellite orbit. On the contrary, the lower cutoff frequency is almost
constant, so that the cutoffs cross at larger L. Between these cutoffs, which thus form a
wedge, intense whistlers are observed, whereas only 0+ whistlers and, probably,
ducted whistlers are found outside the cutoffs. We present numerous examples of
such spectrograms, and explain the formation of wedge-like structures by the wave
propagation features in the inner magnetosphere, and specific position of the satellite with
respect to the LHR maximum. In general terms, this explanation is as follows. WLE
consists of whistler mode waves originating from lightnings and, thus, is related to
thunderstorm activity. The wedge as such is formed by quasi-resonance whistler waves
that cannot propagate in the region where the wave frequency is below local LHR
frequency. Then, the lower frequency cutoff is determined by the LHR maximum, as
quasi-resonant waves with lower frequencies originating in opposite hemisphere do not reach
the satellite due to LHR reflection above it. The appearance of an upper cutoff
frequency is due to another feature of unducted VLF wave propagation, which
consists in trajectories merging into a limiting trajectory for waves with the same
frequency, but starting from different latitudes in the opposite hemisphere. As the further
increase of the initial latitude does not lead to an increase of the final L-shell in the
opposite hemisphere, there appears a maximum L-shell on which the waves with
the given frequency can be observed. This L-shell decreases with the increase of
wave frequency due to a more pronounced bending towards lower L-shells for
higher frequency waves. As the result, the accessible domain for quasi-resonance
whistler-mode waves on the (L-f)-plane takes the wedge-like shape. The observed
features of spectral intensity are also explained consistently by the above suggested
model. |
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