Tracking a CME and SIR to Earth and Mars During the Deep Minimum of Solar Cycle 24

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

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PRESENTED AT:

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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.

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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).

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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.

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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.

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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.

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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).

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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. the inner boundary of the Enlil heliospheric domain, where CMEs are injected). Movie 1 shows the propagation of the 20 August 2018 CME in the simulation.

Movie 1. WSA–Enlil+Cone simulation of the 20 August 2018 CME. The quantities shown are (left) solar wind speed and (right) solar wind number density, both on the constant Earth-latitude plane.

WSA–Enlil+Cone results show that the CME under study clearly impacts Earth, but just grazes Mars. Given the uncertainties related to GCS reconstructions (often of the order of a few degrees), it is not possible to draw strong conclusions on the impact of the 20 August 2018 CME at Mars based on this simulation run alone.

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DISCUSSION

The events of late August 2018 are a clear example that the analysis of solar transients from in-situ measurements is not a trivial task even during solar minimum. This was especially true for observations at Mars, where the arrival of the 20 August CME that clearly encountered Earth could not be determined with certainty, even though the two planets were nearly radially aligned. Such observations demonstrate the value of continuous solar wind monitoring at Mars, either from a monitor at L1 or multiple orbiting spacecraft that together provide a continuous presence in the solar wind.

One important aspect to consider, in fact, is that Mars does not currently have a solar wind monitor at its Lagrange L1 point, as Earth does. It is possible that, had such measurements been available at Mars, interpretation of in-situ data would have been more straightforward. Nevertheless, we anticipate that further analysis of in-situ measurements at Mars and additional WSA–Enlil simulation runs will shed more light on the relationship between the solar wind structures observed at the two planets.

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DISCLOSURES

This research is supported by the NASA Living With a Star Jack Eddy Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award no. NNX16AK22G.

I. G. Richardson acknowledges support from NASA LWS project NNH19ZDA001N-LWS.

WSA–Enlil+Cone simulation results have been provided by the Community Coordinated Modeling Center (CCMC) at NASA Goddard Space Flight Center through their public Runs on Request system (http://ccmc.gsfc.nasa.gov).

We thank the teams of all the spacecraft and ground facilities whose data were employed in this study.

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AUTHOR INFORMATION

Contact: [email protected]

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ABSTRACT

Interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs) are large-scale solar wind structures that can often be tracked at widely-separated locations in the heliosphere. Their occurrence is strongly dependent on the phase of the solar cycle, with ICMEs being more frequent during solar maximum and SIRs being dominant during solar minimum. Nevertheless, a few ICMEs are present during solar minimum, and coordinated studies of these ICMEs, from their origin throughout their evolution in the heliosphere, are facilitated by the simpler solar wind structure, a near absence of other transients, and low levels of solar activity.

We present a detailed analysis of an ICME and SIR that were observed during the solar minimum between Solar Cycles 24 and 25. To achieve this, we combine remote-sensing solar data with in-situ measurements at Earth and Mars, which were separated by ~10° in longitude at the time of the events under analysis. The aim of this study is to determine how the two structures evolve and interact as they travel away from the Sun, as well as to evaluate and compare the space weather response from the same chain of events on two different planets.

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