Autocorrelation Study of the Undisturbed Solar Wind at Mars

One of the goals of the Mars Atmospheric and Volatile EvolutioN mission (MAVEN) is to measure rates of atmospheric loss on Mars. Many of these processes are driven or influenced by the the solar wind. In order to understand atmospheric loss on Mars, we need to understand its interaction with the solar wind, but because MAVEN is only one spacecraft, it cannot measure both the solar wind in isolation as well as Mars' interaction with it simultaneously. For this reason, we perform an autocorrelation study on the undisturbed solar wind at Mars to characterize its temporal variability. Similar studies have been conducted at 1 AU at Earth by Gosling and Bame [1972] and Borovsky [2012]; we complement these while putting more focus on timescales relevant to MAVEN, specifically the duration of one MAVEN orbit. We use data taken by MAVEN's Solar Wind Ion Analyzer (SWIA) and magnetometer instruments to look at solar wind speed and density, as well as the strength of the interplanetary magnetic field. We utilize a modified version of the autocorrelation function in order to account for the irregular temporal sampling MAVEN's orbit introduces into the dataset. Ultimately we find that the after one MAVEN orbit, roughly 4.5 hours, the solar wind is still fairly well correlated, with coefficients for different parameters ranging from 0.7 to 0.97. Most parameters are correlated up to timescales on order of one day. We also find that there is a recurrence period in certain solar wind parameters over roughly one solar rotation, around 26 days, with autocorrelation coefficients reaching as high as 0.5, but we do not see significant evidence of repetition of this recurrence at two or more solar rotations. This allows others who perform calculations based on the solar wind to estimate the degree to which the solar wind has changed since it was last measured, even when MAVEN has not entered it for some time.