Following space weather events to Earth and Mars during solar minimum

The Sun’s activity is characterized by a 11-year periodicity, known as the solar cycle, during which a “solar minimum” alternates with a “solar maximum”. At solar minimum, the Sun and its environment are in their simplest configuration, and eruptions are rarer. Because of these aspects, solar minima are excellent periods for attempting to track solar events from their origin throughout their interplanetary journey. Additionally, increasing interest in martian space weather makes studying the effects of solar activity at other planets than Earth a particularly timely effort.

Following these premises, we analyzed a series of solar events that took place during the second half of August 2018, i.e. during the deep minimum of Solar Cycle 24, and followed them to Earth and then Mars. At that time, the two planets were separated by ~8° in longitude and ~2° in latitude, hence they were very close to radial alignment. We observed two rather slow and weak coronal mass ejections (CMEs) that erupted in rapid succession from a region on the Sun between two coronal holes, i.e. sources of solar wind high-speed streams (HSSs). We modeled the two eruptions as well as the background wind using the WSA–Enlil model (see Figure 1), according to which the interplanetary CMEs (ICMEs) traveled between the two HSSs and started to interact around the time they reached Earth, arriving at Mars as a merged structure. 

Figure 1. Overview of the WSA–Enlil simulation results at Earth and Mars. Top: Snapshots of the simulation results for the solar wind radial speed on the ecliptic plane around the CME arrival times at (left) Earth and (right) Mars. Bottom: Modeled solar wind speed at both planets. WSA–Enlil simulation performed at NASA’s CCMC (https://ccmc.gsfc.nasa.gov).

 

The WSA–Enlil simulation results served to provide the overall heliospheric context and to aid interpretation of the in-situ measurements at Earth and Mars (shown in Figure 2). The sequence of events observed at Earth agrees with modeling results: The interplanetary shock driven by the second CME was traveling through the magnetic ejecta of the first CME as it impacted Earth, indicating that measurements at 1 AU captured the initial stages of CME–CME interaction. As expected, the second CME was followed by a stream interaction region (SIR) and a HSS. While the first ICME was not associated with significant space weather disturbances, the second one delivered the third strongest geomagnetic storm in terms of the Dst index, likely due to its prolonged negative Bz component. At Mars, on the other hand, the first ICME was observed with a similar structure to that at Earth—and a possible (due to data gaps) shock driven by the second CME was identified—but the clear flux rope signatures of the second ICME ejecta were missing. Instead, an extended SIR (which may have contained material from the second CME, nevertheless) was associated with moderate arieffectiveness (i.e., the level of disturbance to the martian system, from the Greek name for Mars, ′Aρης or Áris).

Figure 2. In-situ measurements at (left) Earth and (right) Mars, showing (a) magnetic field magnitude, (b) magnetic field components, (c) θ and (d) φ components of the magnetic field, solar wind (e) speed, (f) density, (g) dynamic pressure, and (h) temperature, (i) plasma beta, (j) electron pitch angle distribution, (k) ion intensities, galactic cosmic ray variation (l) in space and (m) on ground, and (n) indices to quantify geoeffectiveness for Earth, arieffectiveness for Mars.

 

We speculated that interaction of the second CME with the following HSS led to CME rotation and deflection toward western heliolongitudes in interplanetary space, resulting in a lack of clear CME flux rope signatures at Mars. This study highlights the importance of accurately modeling the ambient solar wind even during “simpler” solar minimum periods to improve space weather forecasts, as well as the need for continuous solar wind monitoring at Mars.

 

See the full journal publication at: Palmerio, E., Lee, C. O., Richardson, I. G., Nieves-Chinchilla, T., Dos Santos, L. F. G., Gruesbeck, J. R., Nitta, N. V., Mays, M. L., Halekas, J. S., Zeitlin, C., Xu, S., Holmström, M., Futaana, Y., Mulligan, T., Lynch, B. J., and Luhmann, J. G.: CME evolution in the structured heliosphere and effects at Earth and Mars during solar minimum, Space Weather, 20, e2022SW003215, doi:10.1029/2022SW003215, 2022

Author: Erika Palmerio (Predictive Science Inc.) .