On the small-scale fluctuations in the peak electron density of Martian ionosphere observed by MEX/MARSIS

摘要

We reported the small-scale fluctuations in the peak electron density of dayside Martian ionosphere observed by MEX/MARSIS. MARSIS is an ionospheric sounder that can measure the vertical electron density profile of the topside ionosphere. In the experiment the ionospheric critical frequency can be determined within the accuracy of 5 kHz, corresponding to a typical accuracy in electron density of 0.02%. As a space-borne remote sensing instrument, MARSIS can observe the ionosphere with a time resolution of 8 s, enabling one to study the horizontal variation of the ionosphere. Besides the normal variations of the peak electron density with respect to the solar zenith angles, small fluctuations in peak electron density is detected within single orbit of observation. The appearance of these fluctuations is relatively smooth and wavy, distinct from measurement errors which are random and noisy. The fluctuations do not exhibit a prevailing wavelength either in space or in time. The peak-to-valley amplitude of the fluctuation can reach 200 kHz in frequency, corresponding to a 10% variation in peak electron density. Over 38,000 peak values of electron density profiles were recorded, organized along each orbit. A mean variation of ～ 6% in the peak electron density is found. Fourier analysis of the data was performed with respect to the horizontal distance the spacecraft passes in one orbit. Resulting spectra generally follow a -5/3 power law in a double logarithm plot, suggesting a turbulence-dominated nature. Perturbations propagating from upper ionosphere and fluctuations in the neutral density are discussed as potential causes for this observation. This observation reveals a sensitive proxy to monitor the dynamics of neutral Martian atmosphere at ～130 km altitude.