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The activity of the Sun alternates between a solar minimum and a solar maximum, the former corresponding to a period of "quieter" status of the heliosphere. During solar minimum, it is in principle more straightforward to follow eruptive events and solar wind structures from their birth at the Sun throughout their interplanetary journey. In this paper, we report analysis of the origin, evolution, and heliospheric impact of a series of solar transient events that took place during the second half of August 2018, i.e. in the midst of the late declining phase of Solar Cycle 24. In particular, we focus on two successive coronal mass ejections (CMEs) and a following high-speed stream (HSS) on their way towards Earth and Mars. We find that the first CME impacted both planets, whilst the second caused a strong magnetic storm at Earth and went on to miss Mars, which nevertheless experienced space weather effects from the stream interacting region (SIR) preceding the HSS. Analysis of remote-sensing and in-situ data supported by heliospheric modelling suggests that CME--HSS interaction resulted in the second CME rotating and deflecting in interplanetary space, highlighting that accurately reproducing the ambient solar wind is crucial even during "simpler" solar minimum periods. Lastly, we discuss the upstream solar wind conditions and transient structures responsible for driving space weather effects at Earth and Mars., 27 pages, 7 figures, 1 table, accepted for publication in Space Weather

The Martian crustal magnetic anomalies create a varied, asymmetric obstacle for the draped interplanetary magnetic field (IMF) to interact with. One possible result of this interaction is magnetic reconnection, a process by which anti-parallel magnetic field lines connect and reconfigure, transferring energy into the surrounding environment and mixing previously separated plasma populations. Here, we present an analysis to determine the draped IMF conditions that favor reconnection with the underlying crustal anomalies at Mars. First, we plot a map of the crustal anomalies’ strength and orientation compiled from magnetic field data taken throughout the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Second, we create “shear maps” which calculate and plot the angle of shear between the transverse component of the anomalies and a chosen overlaid draping direction. Third, we define a “shear index” which quantifies the susceptibility of a particular region to undergo reconnection based on a given draped IMF orientation and the resulting shear map for that region. We then compare the shear index for a variety of draped field orientations within different regions of the Martian magnetosphere. Our results suggest eastward/westward (horizontal) draped fields present regions that are more likely for anti-parallel magnetic reconnection to occur with the crustal anomalies than northward/southward (vertical) draped fields, with one notable exception being the strongest crustal anomalies located in the southern hemisphere ~180° longitude. An east/west draped field roughly corresponds to a +/- By IMF direction on the dayside, implying the rate of magnetic reconnection on the dayside of Mars may be enhanced for IMF field lines pointing in the +/- YMSO direction compared to that of IMF field lines pointing in the +/- ZMSO direction, with MSO referring to the Mars Solar Orbital coordinate system. Understanding the interplay between Mars’s crustal magnetic fields and the IMF is crucial to answer outstanding science questions regarding nightside magnetospheric activity at Mars, namely how IMF orientation affects the twisting of the magnetotail, open magnetic topology observations on the nightside, and discrete aurora observations in the southern hemisphere.

The Martian magnetotail exhibits a highly twisted configuration, shifting in response to changes in polarity of the interplanetary magnetic field's (IMF) dawn-dusk (BY) component. Here, we analyze ∼6000 MAVEN orbits to quantify the degree of magnetotail twisting (θTwist) and assess variations as a function of (a) strong planetary crustal field location, (b) Mars season, and (c) downtail distance. The results demonstrate that θTwist is larger for a duskward (+BY) IMF orientation a majority of the time. This preference is likely due to the local orientation of crustal magnetic fields across the surface of Mars, where a +BY IMF orientation presents ideal conditions for magnetic reconnection to occur. Additionally, we observe an increase in θTwist with downtail distance, similar to Earth's magnetotail. These findings suggest that coupling between the IMF and moderate-to-weak crustal field regions may play a major role in determining the magnetospheric structure at Mars.

This is a two-spacecraft study, in which we investigate the effects of the upstream solar wind conditions on the Martian induced magnetosphere and upper ionosphere. We use Mars Express (MEX) magnetic field magnitude data together with interplanetary magnetic field (IMF), solar wind density, and velocity measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, from November 2014 to November 2018. We compare simultaneous observations of the magnetic field magnitude in the induced magnetosphere of Mars (|B|(IM)) with the IMF magnitude (|B|(IMF)), and we examine variations in the ratio |B|(IM)/|B|(IMF) with solar wind dynamic pressure, speed and density. We find that the |B|(IM)/|B|(IMF) ratio in the induced magnetosphere generally decreases with increased dynamic pressure and that a more structured interaction is seen when comparing induced fields to the instantaneous IMF, where reductions in the relative fields at the magnetic pile up boundary (MPB) are more evident than in the field strength itself, along with enhancements in the immediate vicinity of the optical shadow of Mars. We interpret these results as evidence that while the induced magnetosphere is indeed compressed and induced field strengths are higher during periods of high dynamic pressure, a relatively larger amount of magnetic flux threads the region compared to that available from the unperturbed IMF during low dynamic pressure intervals.