Solar Cycle Dependencies of the Interplanetary Magnetic Field Variability at Mars

Abstract:

The dynamics of the interplanetary magnetic field (IMF), e.g. the scale and frequencies over which it varies, have not been rigorously characterised at distances larger than 1 AU. The lack of a continuous upstream solar wind monitor at Mars sometimes necessitates assumptions about the variability of solar wind and interplanetary magnetic field conditions at Mars, but 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 (i.e. negligibly impacted by the solar wind interaction with Mars) IMF from the Mars Global Surveyor (MGS) mission (between 1997 and 1999) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission (between 2014 and 2016). We find that the most important measurement in determining the variability of the IMF in the near future is the cone angle, with IMF that lies close to the Sun-Mars line being significantly less variable than other configurations. Additionally, this study finds the IMF to be notably steadier in field strength and cone angle direction during the MAVEN mission than the MGS mission, with little difference in the clock angle variation between the two missions. Results from this study suggest that the IMF at Mars is steadier at times near solar maximum than at times near solar minimum. However, we show that the time for the IMF to decorrelate does not directly depend on the solar activity.