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| Titel |
Spacecraft Observations of Quasi-Electrostatic Chorus Waves: Effective Length of a Receiving Antenna |
| VerfasserIn |
Evgenii Shirokov, Andrei Demekhov, Yury Chugunov, Alexey Larchenko |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138509
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| Publikation (Nr.) |
 EGU/EGU2017-1547.pdf |
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| Zusammenfassung |
| Whistler-mode chorus is a typical example of very low frequency electromagnetic
emission in the Earth’s and planetary magnetospheres. Chorus emissions typically
propagate quasi-parallel to the ambient magnetic field in their source region but can
also propagate in the quasi-electrostatic mode close to the lower oblique resonance
cone.
However, the quasi-electrostatic wave field measurements by electric receiving antennas
in a magnetoplasma are nontrivial. Indeed, the incident wave electric field amplitude E
should be obtained from voltage U = Eleff induced on the receiving antenna, where leff is
the so-called antenna effective length. The value of leff can be greater and even much greater
than the antenna geometric length lgeom in case of the short (as compared to the
electromagnetic wavelength in a plasma) antennas. This is because such antennas effectively
re-radiate quasi-electrostatic waves in plasmas. Length leff should be calculated
considering the plasma and wave characteristics, and, in general, this is quite a difficult
problem.
In this work, we propose a method of calculating the effective length of a short receiving
antenna for the case of spacecraft observations of quasi-electrostatic chorus emissions, and an
analytical expression for this parameter is obtained. This method is based on the reciprocity
theorem for quasi-electrostatic waves, which is also correct in gyrotropic and dispersive
media such as magnetoplasmas, and requires an appropriate choice of the radiation source
model. Such a choice is possible on the basis of the measured emission parameters if a
quasi-monochromatic wave packet with wave normal angles near the resonance cone is
detected.
We calculate the receiver effective length for some measurements of chorus wave
quasi-electrostatic fields onboard THEMIS spacecraft using the proposed method. The
emission parameters such as the wave normal angle have been found using the singular value
decomposition method applied to the magnetic field waveforms. The electric field waveforms
have not been used for this purpose because the effective length is unknown a priori. The
effective length dependency on the dipole orientation has also been studied. The calculation
results have shown that ratio leff∕lgeom is typically larger than one and can go up to 30
which means the effective re-radiation of the incident waves. In the case, where the incident
wave electric field is quasi-orthogonal to the dipole, leff∕lgeom ≲ 1. This is because the
electric current is induced inefficiently on the receiver and, hence, the wave re-radiation is
weak.
Our results show that care should be taken when interpreting spacecraft data on wave
electric fields if the waves propagate in the quasi-electrostatic mode. Since leff∕lgeom > 1 in
many cases, the electric field value can actually be less and even much less than value
U∕lgeom, which is conventionally used as the measured electric field in the satellite data
analysis. In particular, this can be important for the estimates of electron energization by
quasi-electrostatic chorus waves. |
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