Sofija Durward
Due to the absence of a continuous upstream solar wind monitor at Mars, little is known about the variability of the interplanetary magnetic field as it reaches the planet. As such it is often assumed that the IMF is slow changing, although it is likely that this is a poor assumption. It is expected that corotating interaction regions (CIRs) will be more prevalent at Mars than at Earth, and interplanetary coronal mass ejections (ICMEs) may have evolved so that their signatures are different from terrestrial observations.
This study uses pristine interplanetary magnetic field measurements upstream of Mars to quantify the autocorrelation of the measurements at later times. Using historic measurements from both the Mars Global Surveyor and the current MAVEN mission, we provide an insight into the solar cycle dependencies of the steadiness of the interplanetary magnetic field at 1.5AU and allow for more accurate estimation of IMF properties hours after the previous measurement.
To investigate the variability, we calculated the correlation coefficient of each component of the IMF with itself at a range of time lags and formed autocorrelation functions. These functions showed that the IMF was much steadier in strength and cone angle during the MAVEN measurements than the MGS measurements. This suggests that the IMF was more variable in the rising phase of the solar cycle (covered by MGS) than the declining phase (covered by MAVEN). Additionally, this study found that the cone angle was the most important component when forecasting the variability of the IMF. Periods where the IMF was near parallel to the ecliptic plane were notably more variable than periods where the IMF lied near the ecliptic plane.