HR: 17:45h
AN: P14A-08 INVITED     [Abstracts]
TI: The First Radar Sounding of the Ionosphere of Mars
AU: * Gurnett, D A
EM: donald-gurnett@uiowa.edu
AF: University of Iowa, Dept. of Physics & Astronomy, Iowa City, IA 52242 United States
AU: Kirchner, D L
EM: donald-kirchner@uiowa.edu
AF: University of Iowa, Dept. of Physics & Astronomy, Iowa City, IA 52242 United States
AU: Huff, R L
EM: richard-huff@uiowa.edu
AF: University of Iowa, Dept. of Physics & Astronomy, Iowa City, IA 52242 United States
AU: Picardi, G
EM: picar@infocom.uniroma1.it
AF: University of Rome, Infocom Dept., Rome, 00184 Italy
AU: Plaut, J J
EM: jeffrey.j.plaut@jpl.nasa.gov
AF: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109 United States
AU: Safaeinili, A
EM: ali.safaeinili@jpl.nasa.gov
AF: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109 United States
AU: Nielsen, E
EM: nielsen@mps.mpg.de
AF: Max Planck Institute, Solar System Research, Katlenburg-Lindau, 37191 Germany
AB: In this paper we report the first radar soundings of the ionosphere of Mars. These soundings were obtained using the ionospheric sounding mode of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on board the Mars Express spacecraft. This mode of operation provides radar soundings of the ionosphere in 160 frequency steps, from 0.1 to 5.4 MHz. In the normal mode of operation the range resolution is about 15 km, the sweep time is 1.23 s, and the time between sweeps is 7.54 s. A typical ionospheric sounding pass starts at an inbound altitude of 1200 km, extends down to a periapsis altitude of about 300 km, and ends at an outbound altitude of 1200 km. Strong ionospheric echos are usually obtained through the entire pass, except on the nightside where the ionospheric plasma density is very low. When ionospheric echos are present nearly continuous measurements can be made of the maximum electron density in the ionosphere from the maximum frequency of the ionospheric echo. These measurements show the expected variation with solar zenith angle, with the maximum electron density varying from about 10² cm^-3, or less, on the nightside, to a maximum of about (2 to 3) x 10⁵ cm^-3 on the dayside. Surface reflections are also commonly observed above the maximum plasma frequency in the ionosphere. The intensities of these reflections vary over a large range, 30 db or more, apparently due to variations in atmospheric absorption caused by solar events. The dispersion of the surface reflection is used to compute the columnar electron density through the ionosphere. This information is important for analyzing sub-surface soundings, since the ionospheric dispersion must be removed from the chirp pulse before the subsurface signal can be optimally recovered. Several types of topside ionospheric echoes are observed, ranging from single echoes caused by vertical reflection from the ionosphere, to a wide variety of oblique and diffuse echoes. Many of the oblique echoes are believed to arise from compact plasma density structures associated with the crustal magnetic fields discovered by the Global-Surveyor spacecraft. A variety of local plasma resonances are also observed. These include excitation of oscillations at the electron plasma frequency and at the electron cyclotron frequency. The oscillations at the electron cyclotron frequency can be used to compute local magnetic field strengths. Crustal magnetic field strengths of up to 250 nT have been measured near periapsis using this technique.
DE: 2481 Topside ionosphere
DE: 2494 Instruments and techniques
DE: 5435 Ionospheres (2459)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005