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The correlation length of ULF waves around Venus: VEX observations

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Fränz,  Markus
Department Planets and Comets, Max Planck Institute for Solar System Research, Max Planck Society;

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Citation

Franco, A., Fränz, M., Echer, E., Bolzan, M., & Zhang, T. (2020). The correlation length of ULF waves around Venus: VEX observations. Planetary and Space Science, 180: 104761. doi:10.1016/j.pss.2019.104761.


Cite as: http://hdl.handle.net/21.11116/0000-0005-869E-2
Abstract
A study of the correlation length of ultra-low frequency (ULF) waves around Venus was developed using electron density and magnetic field data obtained from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) and magnetometer (MAG), respectively, on board of the mission Venus Express (VEX). The analysis was conducted using the whole interval of the mission (2006–2014). The correlation scales have been calculated by the correlation length parameter that is a characteristic distance over which fluctuations in a variable are correlated. We limited the study to the frequency range from 8 to 50 mHz because previous studies have shown that ULF waves produced in the foreshock have the highest power in this range. In this study the correlation length was calculated by an exponential fit employed to the auto-correlation curve. The auto-correlation function was calculated lagged by a time between 0 and 60 s and sliding a window with 120s across the data. This analysis has been also extended to correlation length determinations in spatial scale. In order to obtain the correlation length in a spatial domain, the temporal correlation length must be multiplied by the solar wind velocity. Here, the ASPERA-4/IMA (Ion Mass Analyzer) velocity data was used. It was found that the dominant correlation length in temporal scale varies from 9 to 14 s in electron density and between 7.5 and 11 s in the magnetic field. In spatial scale, correlation length varies between 2.8×103 and 5×103 km in electron density data, and between 1.7×103-4X103km for the total magnetic field data value in this frequency window. Fluctuations in the magnetosheath and in the MPB may be correlated with fluctuations at the ionosphere, since correlation lengths in those regions are larger than the size of these regions, indicating that local resonant effect of wave trains at the ionopause may enhance the atmospheric ion escape at Venus. Our results also show that pickup heavy ions can interact with discontinuities in the magnetosheath of Venus and can destroy ULF wave trains during periods of low solar wind pressure. The results obtained here are compared with a similar previous analysis in the Mars environment.