Correlation length around Mars:A statistical study with MEX and MAVEN observations

Correlation lengths of ultra-low-frequency(ULF)waves around Mars were computed for the first time,using data from MEX(electron density from 2004 to 2015)and MAVEN(electron density and magnetic field from 2014 to 2016).Analysis of the MEX data found that,for the frequency range 8 to 50 mHz,correlation length in electron density varied between 13 and 17 seconds(temporal scale)and between 5.5×10^3 km and 6.8×10^3 km(spatial scale).For the MAVEN time interval,correlation length was found to vary between 11 and 16 seconds(temporal scale)and 2×10^3–4.5×10^3 km in spatial scale.In the magnetic field data,correlation lengths are observed to be between 8–15 seconds(temporal scale)and between 1×10^3 and 5×10^3 km(spatial scale)over the same frequency range.We observe that the cross sections of the plasma regions at the dayside of Mars are smaller than these correlation lengths in these regions in both analyses,where the correlation length derived from the MEX electron density data was between 5 and 25 times the size of the magnetosheath and the magnetic pile-up region(MPR),respectively.For MAVEN these ratios are about 4(magnetosheath)and 11(MPR)in electron density and between 1.5 and 5.5 for magnetic field data,respectively.These results indicate that waves at the magnetosheath/MPR can be related to oscillations in the upper ionosphere.In a local region,wave trains may cause resonance effects at the planetary ionopause,which consequently contributes to the enhanced ion escape from the atmosphere.