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Abstract

The Martian ionopause is generally identified as a steep electron density gradient and the peak of photoelectrons near 27 eV. However, it is not clear whether these two criteria identify the same position in the Martian ionopause region. The Mars Atmosphere and Volatile Evolution Mission (MAVEN) provides a good chance to compare both criteria for the first time with electron density data taken from the Langmuir probe and electron energy spectra from the Solar Wind Electron Analyzer. We identified 1,121 steep electron density gradients and 4,275 photoelectron boundary crossings within the first 2.7 years of MAVEN data. Observations show that two boundaries are nearly collocated when being observed simultaneously (about 15% of the available dataset). But when averaged over all observations, the photoelectron boundary locates higher than the steep electron density gradient. This may suggest a strong dependency of the Martian ionopause on magnetic field configuration in the Martian ionosphere. Results are generally consistent with previous observations made by Mars Express (MEX) and improve our understanding of the Martian ionosphere.