12/2/2020 AGU - iPosterSessions.com Tracking a CME and SIR to Earth and Mars during the deep minimum of Solar Cycle 24 E. Palmerio (1,2), C. O. Lee (1), I. G. Richardson (3,4), N. V. Nitta (5), M. L. Mays (4), J. S. Halekas (6), C. Zeitlin (7), J. G. Luhmann (1), S. Xu (1) (1) Space Sciences Laboratory, University of California–Berkeley, (2) CPAESS, University Corporation for Atmospheric Research, (3) Department of Astronomy, University of Maryland, (4) Heliospheric Physics Division, NASA Goddard Space Flight Center, (5) Lockheed Martin Solar and Astrophysics Laboratory, (6) Department of Physics and Astronomy, University of Iowa, (7) Leidos Innovations Corporation https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 1/14 12/2/2020 AGU - iPosterSessions.com PRESENTED AT: https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 2/14 12/2/2020 AGU - iPosterSessions.com BACKGROUND Interplanetary coronal mass ejections (ICMEs) and stream interactions regions (SIRs) are large-scale solar wind structures responsible for space weather disturbances at Earth and the other planets (e.g., Zhang et al. 2007). Their occurrence usually depends on the phase of the solar cycle, with ICMEs being more frequent during solar maximum and SIRs dominating during solar minimum. Thus, periods of solar minimum are usually regarded as particularly suitable for following solar transients through the heliosphere, since they are often characterised by single-ICME events that can more or less easily be tracked back to their solar source. In this study, we follow a coronal mass ejection (CME) and solar wind high-speed stream (HSS) that occurred in August 2018, i.e. during the minimum of Solar Cycle 24. In particular, we analyse their in-situ signatures at Earth and Mars, which were nearly radially aligned. The configuration of the inner heliosphere during the period under study is shown in Figure 1. Figure 1. Positions and orbits of the inner planets and location of the STEREO-A spacecraft on 20 August 2018. Earth and Mars were separated by 2° in latitude and 8° in longitude. https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 3/14 12/2/2020 AGU - iPosterSessions.com SOLAR OBSERVATIONS The events that we focus on in this study commenced on 20 August 2018. A CME erupted around 08:00 UT from a quiet-Sun region in the northern hemisphere, located close to the central meridian from Earth’s perspective. The eruption involved the disappearance of a filament and the presence of weak dimmings and post-eruption arcades. This event has been analysed in several previous studies (e.g., Chen et al. 2019; Mishra and Srivastava 2019; Abunin et al. 2020; Piersanti et al. 2020). An overview of the various solar observations on 20 August is shown in Figure 2. Figure 2. Overview of solar observations on 20 August 2018. (a) SDO/AIA 193 Å image just before the CME eruption, showing the filament (F) involved in the eruption and the two coronal holes (CH1 and CH2) on either side. (b) SOHO/LASCO/C2 difference image, showing the 20 August CME. (d) Same as (b), but from the STEREO/SECCHI/COR2-A perspective. (c) and (e) Same as (b) and (d), but with the reconstructed GCS shell overlaid. Figure 2(a) shows an SDO image of the solar disc a couple hours before the CME onset. The filament involved in the eruption (marked as 'F') is rooted between two coronal holes (CHs), one to the southwest (marked as 'CH1') and one to the northeast (marked as 'CH2'). Figure 2(b)–(e) shows the 20 August 2018 CME in the solar corona observed in white light by the SOHO and STEREO-A spacecraft. In two panels, coronagraph data are overlaid with the reconstructed CME wireframe obtained using the Graduated Cylindrical Shell (GCS; Thernisien 2011) model. According to GCS results, the CME was relatively narrow (~45°), slow (<400 km/s), and directed 10° west and 6° north of the Sun–Earth line, suggesting possible impacts at both Earth and Mars (see Figure 1). https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 4/14 12/2/2020 AGU - iPosterSessions.com MEASUREMENTS AT EARTH The first location where we evaluate an impact of the 20 August 2018 CME is Earth. We combine solar wind measurements from Earth's Lagrange L1 point with ground-based observations and data taken from lunar orbit. In-situ magnetic field, plasma, and particle measurements at Earth, together with geomagnetic indices, are shown in Figure 3. https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 5/14 12/2/2020 AGU - iPosterSessions.com Figure 3. In-situ measurements at Earth. The grey vertical line indicates the passage of the leading edge of the sheath region of the 20 August CME, followed by its magnetic cloud ejecta (shaded in light grey). The CME–HSS interaction region is shaded in dark grey. The pink vertical line indicates the interplanetary shock driven by a preceding, small CME, followed by its ejecta (shaded in pink). Data are taken from the Wind and ACE spacecraft at Earth's Lagrange L1 point, the LRO spacecraft orbiting the Moon, and the NMDB, WDC Kyoto, and NGDC facilities on ground. Observations at Earth indicate that a small ICME (marked in pink in Figure 3) preceded the ICME related to the 20 August eruption (marked in grey in Figure 3). The "main" ICME arrived on 25 August 2018 and was associated with clear magnetic cloud signatures (see Burlaga et al. 1981), relatively slow speeds (~400 km/s) and a weak Forbush decrease (panels l–m, ~2% variation both measured in space by LRO and by high-latitude neutron monitors on Earth). Nevertheless, its strong, sustained southward BZ component resulted in the third most intense geomagnetic storm of Solar Cycle 24 in terms of the Dst index (Dstmin = –174 nT). Furthermore, the ICME magnetic-cloud ejecta is followed by an interaction region (marked in dark grey in Figure 3) with the following HSS. This indicates that the 20 August CME was compressed and "pushed" by the HSS emanating from the CH in the northern hemisphere ('CH2' in Figure 2), which in turn enhanced its geoeffectiveness. https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 6/14 12/2/2020 AGU - iPosterSessions.com MEASUREMENTS AT MARS The second location where we evaluate an impact of the 20 August 2018 CME is Mars. Mars does not have spacecraft that monitor the upstream solar wind continuously; nevertheless, we use data from MAVEN and Mars Express, which orbit Mars and spend part of their orbits in the solar wind. We complement these measurements with data taken by the Curiosity rover on the Martian surface. In-situ magnetic field, plasma, and particle measurements at Mars are shown in Figure 4. https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 7/14 12/2/2020 AGU - iPosterSessions.com Figure 4. In-situ measurements at Mars. The grey vertical line indicates the estimated arrival of an interplanetary shock. Data are taken from the MAVEN and MEX spacecraft orbiting Mars and the MSL/Curiosity rover on ground. Measurements at Mars reveal the clear passage of an interplanetary disturbance between 27–28 August 2018, marked by enhanced magnetic fields (panel a) and solar wind densities (panel f) but relatively slow solar wind (panel e), ahead of a following HSS. However, it is unclear whether this disturbance contains material from the 20 August CME or if the whole structure corresponds to a SIR. After the passage of a likely interplanetary shock in the early hours of 27 August, the density profile looks strikingly similar to that inside the magnetic cloud at Earth, but other ICME ejecta signatures are missing; e.g., bidirectional electrons are lacking and the plasma beta is not lower than in the ambient solar wind. The disturbance is also associated with a weak Forbush decrease (panels l–m, ~2% variation both in space and on ground, like at Earth), but such decrease could be equally driven by a CME or a SIR (see, e.g., Guo et al. 2018). Hence, it is difficult to conclude whether the 20 August CME merged with the SIR before reaching 1.4 AU, or whether it missed Mars completely (either because of its limited longitudinal extent or because it was deflected towards the west between the orbits of Earth and Mars). https://agu2020fallmeeting-agu.ipostersessions.com/Default.aspx?s=2E-1B-8E-DA-65-11-86-90-4E-E3-4A-CC-C9-B6-68-C6&pdfprint=true&guestview=true 8/14 12/2/2020 AGU - iPosterSessions.com HELIOSPHERIC CONTEXT In order to analyse in-situ measurements at Earth and Mars within the larger heliospheric context, we run a 3D MHD simulation using the WSA–Enlil+Cone model (Odstrcil 2003; Arge et al. 2004). The CME parameters that we use as input for the model are entirely derived from GCS reconstructions (see Figure 2) and propagated to 21.5 R⨀ (i.e.