In situ and remote characterization of the external field temporal variations at Mars

Since there are currently no magnetic field measurements made at Mars' surface, it is not possible to directly monitor the temporal variability of the external field as it is done on Earth's ground magnetic observatories. In this paper we examine two indirect ways of quantifying this external field. First, we directly use Mars Global Surveyor magnetic field measurements to quantify the level of variability of the external field. We sort the measurements on a fine spatial mesh, 0.8° × 0.8° at the equator. We then subtract for each bin and measurement the internal, crustal (static) field without any a priori modeling. We finally compute daily averages of the individual residuals to obtain a time series of an in situ proxy. Second, we use the Advanced Composition Explorer mission which measures the solar wind about 1 h upstream of the Earth at the L1 Lagrange point. These measurements are extrapolated to Mars' position taking into account the orbital configurations of the Mars‐Earth system and the average velocity of particles carrying the interplanetary magnetic field to obtain a remote proxy time series. We compare these time series and demonstrate that they are complementary. When Mars and the Earth are close to the same Parker spiral arm, in situ and remote series have correlation coefficients close to 0.5. We show how these series, or proxies, can help to select magnetic field measurements on Mars. This dual approach and these proxies will especially be useful for upcoming magnetic field measurements made around or at the surface of Mars.