The Relationship Between Photoelectron Boundary and Steep Electron Density Gradient on Mars: MAVEN Observations

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. Plain Language Summary The upper boundary of the Martian ionosphere can be identified either by a steep electron density gradient or the photoelectron boundary (PEB). Previous results suggest that the PEB may be located higher than the steep electron density gradient in general, but there was a lack of in situ observations for the steep electron density gradient. Thanks to the Mars Atmosphere and Volatile Evolution Mission (MAVEN), for the first time, we can measure cold electrons and photoelectrons simultaneously in the Martian ionosphere. Based on 2.7 years of measurements, we compare the location of the steep electron density gradient and the PEB. Our results suggest that the two boundaries are collocated when they are observed simultaneously (in 15% of the available dataset); otherwise, the PEB is higher than the steep electron density gradient. Results may suggest a strong dependency of the Martian ionopause on magnetic field configuration and a more complicated coupling between the solar wind and the Martian ionosphere. Key Points The steep electron density gradient appears in 20% of all electron density profiles, while the PEB appears in 77% of all electron spectra The steep electron density gradient and PEB are collocated as long as they both appear The PEB without steep electron density gradient is averagely higher than