Thin Ionospheric Layers Retrieved from Galileo Radio Occultations: Forcing by Gravity Waves?

Details Thin Ionospheric Layers Retrieved from Galileo Radio Occultations: Forcing by Gravity Waves? Thin Ionospheric Layers Retrieved from Galileo Radio Occultations: Forcing by Gravity Waves?

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.2113f&link_type=abstract

American Astronomical Society, DPS meeting #28, #21.13; Bulletin of the American Astronomical Society, Vol. 28, p.1136

Astronomy and Astrophysics

Astronomy

Scientific paper

Measurements of Jupiter's ionosphere from the first Galileo radio-occultation have indicated the presence of thin electron layers below the main broad ``F-type'' peak. At ingress (24(deg) S) the SNR was sufficently large to retrieve two layers just below the 10-nbar level ~ 75 km apart having widths ~ 40 km. We suggest that these layers are driven by vertically propagating gravity waves and are analogous to the sporadic E layers in Earth's ionosphere. At the altitude of the thin layers observed on Jupiter, the ion gyrofrequency is large compared to the ion-neutral collision frequency. With the passage of a gravity wave, the ions and electrons follow the transverse parcel motions of the neutral gas (which is the dominant constituent of the ionosphere here) but projected along the magnetic field, which in general lies at an angle relative to the transverse motions of the neutrals. The wave motion thereby produces layers of compression and rarefaction, even though the motions of the neutral consituents themselves are non-compressional. By this reasoning, the vertical separation of the layers indicates the vertical wavelength of the gravity waves at these altitudes. For horizontal wavelengths ~ several thousand kilometers, the implied zonal phase speed, relative to the background flow, is ~ 100 m s(-1) , and the doppler-shifted frequency is ~ 10(-3) s(-1) . The thin layers are below the level (several nbar) at which gravity waves of 75-km vertical wavelength are dissipated by molecular viscosity in a single oscillation, but they are at altitudes at which molecular viscosity can damp the wave-amplitude growth associated with the conservation of the vertical flux of zonal momentum; hence the layers are situated at altitudes where the driving waves deposit momentum and energy into the mean flow. Knowing the dip angle of the magnetic field and the recombination times of the ions, the observed electron (and ion) densities can be related to the amplitudes of the neutral gravity waves, and we present results from such calculations.

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