Solar cycle changes in the interplanetary magnetic field variability at Mars

As Mars has no intrinsic magnetic field, it is inherently strongly influenced by the solar wind. However, little is known about the scale of variability of the solar wind at this distance from the Sun. The lack of an upstream solar wind monitor at Mars sometimes necessitates assumptions about the variability of solar wind and interplanetary magnetic field (IMF) conditions at Mars, and without a clear understanding of how the solar wind varies at Mars it is difficult to know whether these assumptions are appropriate. We present a study which quantifies the variation of the IMF at Mars during periods near solar maximum and minimum, by comparing magnetometer measurements of the pristine IMF from the Mars Global Surveyor (MGS) mission (between 1997 and 1999) and the MArs Volatile EvolutioN (MAVEN) mission (between 2014 and 2016). 

This study finds the IMF to be notably steadier in field strength and cone angle direction during the MAVEN mission than the MGS mission (shown in Figures 1 and 2), with little difference in the clock angle variation between the two missions. Additionally, the variability of both the field strength and the clock angle is found to be dependant on the cone angle of the measurements, with periods of IMF oriented close to the orbital plane of Mars being considerably steadier than periods of IMF oriented approximately perpendicular to it. Results from this study suggest that the IMF at Mars is steadier at times near solar maximum than at times near solar minimum, and quantify the error in estimates of IMF parameters hours after measurement. 

However, using solar 10.7cm flux as a proxy for solar activity, we show that the time for the IMF to decorrelate does not directly depend on the solar activity. Figure 3 shows the decorrelation time and the solar activity for both the MGS and MAVEN datasets. No clear correlation is visible in the graphs, suggesting that the reason for the differences in the IMF variability between the two missions is still unclear.