Abstract.We report results of a systematic analysis of a large number of observations of equatorial noise between the local proton cyclotron frequency and the local lower hybrid frequency. The analysis is based on the data collected by the STAFF-SA instruments on board the four Cluster spacecraft. The data set covers their first two years of measurement in the equatorial magnetosphere at radial distances between 3.9 and 5 Earth radii. Inspection of 781 perigee passages shows that the occurrence rate of equatorial noise is approximately 60%. We identify equatorial noise by selecting data with nearly linearly polarized magnetic field fluctuations. These waves are found within 10 • of the geomagnetic equator, consistent with the published past observations. Our results show that equatorial noise has the most intense magnetic field fluctuations among all the natural emissions in the given interval of frequencies and latitudes. Electric field fluctuations of equatorial noise are also more intense compared to the average of all detected waves. Equatorial noise thus can play a non-negligible role in the dynamics of the internal magnetosphere.

[1] We present results of a systematic study of electron densities in the dayside Martian ionosphere measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board the Mars Express spacecraft. There are two distinct regions controlled by different physical mechanisms. The first region is located at altitudes up to about 5 neutral scale heights above the altitude of peak electron density. Electron densities in this region are well described by the basic Chapman theory. The observed small deviations can be most probably explained by the neutral scale height and electron temperature increasing with altitude rather than being constant. The second region is located at altitudes higher than about 10 neutral scale heights above the altitude of peak electron density. It is controlled primarily by diffusion, and the observed electron densities decrease exponentially with increasing altitude. The corresponding diffusion scale height increases with increasing solar zenith angle, which can be probably explained by nearly horizontal magnetic fields in the ionosphere induced by interaction with the solar wind. The obtained dependencies can be used as a simple empirical model of the dayside Martian ionosphere.

[1] We present results of a survey of the nightside ionosphere of Mars as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. The occurrence rate of the nightside ionosphere is studied as a function of solar zenith angle (SZA), magnetic field magnitude, and magnetic field inclination. It is shown that at locations with weak crustal magnetic fields the occurrence rate of the nightside ionosphere decreases with increasing SZA up to about 125°, suggesting that plasma transport from the dayside plays a crucial role in its formation. However, at locations with strong crustal magnetic fields, the dependence on SZA is no longer apparent and the inclination of magnetic field becomes a crucial parameter: the occurrence rate of the nightside ionosphere is more than 4 times larger at locations with nearly vertical magnetic fields as compared to the locations with nearly horizontal magnetic fields. This indicates that impact ionization by precipitating electrons is the main ionization source at these locations. Observed peak electron densities are less than 2 × 10 4 cm −3 in the vast majority of cases. Lower estimates of altitudes of peak electron densities are mostly between 100 and 150 km.

We present a survey of quasiperiodic (QP) ELF/VLF emissions detected onboard the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite (altitude of about 700 km, nearly Sun-synchronous orbit at 10:30/22:30 LT). Six years of data have been visually inspected for the presence of QP emissions with modulation periods higher than 10 s and with frequency bandwidths higher than 200 Hz. It is found that these QP events occur in about 5% of daytime half orbits, while they are basically absent during the night. The events occur predominantly during quiet geomagnetic conditions following the periods of enhanced geomagnetic activity. Their occurrence and properties are systematically analyzed. QP emissions occur most often at frequencies from about 750 Hz to 2 kHz, but they may be observed at frequencies as low as 500 Hz and as high as 8 kHz. Modulation periods of QP events may range from about 10 to 100 s, with typical values of 20 s. Frequency drifts of the identified events are generally positive, but they are lower for events with larger modulation periods. The events are usually limited to higher L values (L > 2). The upper L shell boundary of their occurrence could not be identified using the DEMETER data, but they are found to extend up to at least L~6. The occurrence rate of the events is significantly lower at the longitudes of the South Atlantic anomaly (by a factor of more than 2).