Shocks in tandem: Solar Orbiter observes a fully formed forward-reverse shock pair in the inner heliosphere

(Solar Orbiter Nugget #47 by D. Trotta (1,2), A. P. Dimmock (3), X. Blanco-Cano (4), R. J. Forsyth (2), H. Hietala (5), N. Fargette (2), A. Larosa (6), N. Lugaz (7), E. Palmerio (8), S. W. Good (9), J. E. Soljento (9), E. K. J. Kilpua (9), E. Yordanova (3), O. Pezzi (6) , G. Nicolaou (10) , T. S. Horbury (2),  R. Vainio (11), N. Dresing (11), C. J. Owen (10), and R. F. Wimmer-Schweingruber (12))

1. Introduction

The Sun is an active star, responsible for creating a highly dynamic and complex environment, namely the heliosphere. Solar eruptive phenomena are key consequences of such activity, which recently reached the peak of its 11-years cycle, and their study is of paramount importance to understand many unsolved mysteries of how energy is converted in space and astrophysical plasmas [1], as well as to advance our understanding of space weather, for which they are major drivers [2]. Further, novel spacecraft missions, such as Solar Orbiter [3], are opening a novel observational window into such phenomena with revolutionary measurements in the poorly explored inner heliospheric regions close to the Sun.
Solar eruptive phenomena can drive shock waves in the heliosphere (i.e., interplanetary IP shocks), which crucially can be detected in-situ, thus representing the missing link to remote observations of astrophysical systems. Sometimes IP shocks are observed in forward-reverse pairs, propagating away and towards the Sun in the local plasma frame. Forward-reverse shock pairs typically bound compressed plasma regions at solar wind Stream Interaction Regions (SIRs) between slow and fast wind originating from coronal holes [4]. Early observational evidence shows that fully formed forward-reverse shock pairs are very rare in the inner heliosphere, and more commonly observed beyond 1 AU [5]. Conversely, Coronal Mass Ejections (CMEs), the largest eruptive events from the Sun, are routinely found driving forward shocks able to accelerate particles to high energies [6]. Further, interaction between multiple CMEs has been shown as a promising pathway for fast energy conversion in the heliosphere, with a complex range of phenomena being observed in such interaction.

2. Solar Orbiter observations of a forward-reverse shock pair at 0.5 AU

In our study [7], exploiting the in-situ Solar Orbiter instrument payload, we identified a fully formed forward-reverse shock pair at the unusually short heliocentric distance of 0.5 AU. The observation is shown in Figure 1. We found that such shock pair was not originating from a solar wind SIR, but rather from the interaction between a fast CME interacting with a preceding, slow CME, thereby creating a compression region driving the shock pair due its expansion. This is readily seen by the presence of several clear indicators of CME material both before and after the interaction region, including the smooth magnetic field rotations upstream/downstream of the interaction, the enhanced O7+/O6+ ratios, and the bi-directional pitch angle distributions of suprathermal electrons (Figure1 c, h, i). 
We characterized the forward and reverse shocks with a full parameter estimation. The forward shock is oblique, with a shock normal angle of 60 degrees, and has small fast magnetosonic and Alfvenic Mach numbers (1.2, 1.1, respectively). The reverse shock appears to be more perpendicular and stronger, compatible with previous studies of solar wind SIR shocks [8]. This study enabled us to study IP shocks in highly unusual parameter regimes (for example the forward shock propagating in CME material). 

Figure 1. Summary of Solar Orbiter observations. a--b): Energetic ions differential fluxes (in as measured by EPD's Sun-directed Electron Proton Telescope (EPT, a) and Supra Thermal Electron Proton sensor (STEP, b). c) MAG normal mode magnetic field magnitude and components in spacecraft-centred Radial--Tangential--Normal (RTN) coordinates. d-e) Proton bulk flow speed, proton density and temperature as measured by SWA-PAS (Proton Alpha Sensor). f) Plasma total pressure. g) One-dimensional energy flux measured by PAS. h) Element abundance ratios measured by the SWA Heavy Ion Sensor (HIS). i) Integrated pitch angle distributions for electrons with energies larger than 100 eV as measured by SWA-Electron Analyzer Sensor (EAS). The continuous, dashed--dotted and dashed lines show the times at which Solar Orbiter crosses the CME1 wave, the forward and reverse shock, respectively.

We then focus on the interaction region properties (see zoom in Figure 2), and find that the trailing part of the interaction region is characterised by higher plasma densities, lower temperatures, and higher elemental abundances than the leading portion. Further, we found that the interaction region shows sub-structuring, with irregular behavior in many measured quantities (Figure 2 left). We suggest that this is due to the spacecraft probing, in rapid succession, the material at the end of CME1 and material in the front of the CME2 event. We also searched and found magnetic reconnection signatures, crucial for mixing plasmas efficiently and for energy conversion [e.g., 9]. The orange shaded regions in Figure 2 (left) correspond to reconnection exhaust crossings and highlight how the interaction region undergoes strong reconnection activity, very long-lasting around 18:00~UT, corresponding to the previously identified CME-CME interface and corroborating the interpretation of complex mixing of CMEs. Such interface is readily seen also in the three-dimensional plot in Figure 2i, showing magnetic field vectors throughout the event.

Figure 2. a-g: Zoom on the interaction region as in Figure 1without the energetic particles spectrogram. h: Simplified sketch representing the event,assuming head-on interaction, with the identified areas within the interaction and the Solar Orbiter trajectory (spacecraft model. i: Three-dimensional plot of magnetic field vectors in RTN for the event. Yellow, red and blue arrows are measurements taken in CME1, interaction and CME2 regions, respectively. The magenta/orange planes represent the forward-reverse shock pair.

3. A unique orbital configuration: remote sensing and direct observations at 1 AU

The spacecraft orbital configuration during the event makes it possible to get unique insights about the evolution of this novel interaction structure. By combining solar disk, coronagraph, and heliospheric imagery from STEREO-A and near-Earth spacecraft, we identified two candidate eruptions from the Sun, possibly the progenitors of the observed interaction event. An overview of our findings is provided in Figure 3. CME1 appears as a faint event in STEREO-A imagery and is not visible in SOHO data, while CME2 can be observed in both views (Figure 3 a, b). By performing reconstructions of both events using the Graduated Cylindrical Shell model [10], we find that  the propagation direction and speeds of the two events are compatible with later mixing and arrival at Solar Orbiter, as also shown by time--elongation maps that employ STEREO/SECCHI data (Figure 3c). There, the two CME tracks are seen to converge (possible indication of merging) beyond Solar Orbiter's heliocentric distance.

Figure 3. Overview of some available remote-sensing observations of the 2022 March CME--CME interaction event. (a--b) Coronagraph observations of (a) CME1 and (b) CME2 from the (top) SOHO and (bottom) STEREO-A viewpoints. The rightmost panels show the GCS wireframe projected onto each plane-of-sky view. (c) Time--elongation map built using data from the COR2, HI1, and HI2 cameras on board STEREO-A. The tracks of CME1 and CME2 are indicated with teal and magenta arrows, respectively, and the combined track after interaction is shown by orange arrows. The time-dependent elongation angles of Solar Orbiter and Earth are highlighted via light blue and green dashed lines, respectively. (d) Orbit plot showing the relative positions of Solar Orbiter, Earth, and STEREO-A during March 7, 2022. The propagation directions of the two CMEs according to the GCS results are shown with arrows, while the dashed lines indicate the fields of view of the SoloHI (blue), HI1-A (red), and HI2-A (orange) heliospheric imagers.

Finally, we could investigate the fate of this event by means of direct observations of well radially aligned Wind at 1 AU. Such observations, summarized in Figure 4, show that the structure dissipated at 1 AU, in stark contrast with what expected for SIRs. Only a fast-forward shock is observed at Wind ahead of the whole structure, crossing the spacecraft on March 10 at 16:11:32 UT. The shock has a complex magnetic structure in both the upstream and downstream regions, which probably dominates its small-scale evolution features. The event at Wind is compatible with the complex ejecta resulting from the interaction of multiple CMEs, as reported in [11], where characteristics of the individual "parent" eruptions can no longer be discerned. Therefore, these joint  observations highlight the transient nature of this novel interaction. Further, as it can be seen in the 7-days overview plot in Figure 4, where the interaction appears as a very moderate event, quite common during solar maximum.

Figure 4. Wind observations of the event at 1 au (shaded area). From top to bottom are displayed: magnetic field magnitude and components in RTN (a), proton bulk flow speed and tangential flows (b, c), protons density and temperature (d). The magenta line marks the forward shock crossing.

4. Conclusions

We reported direct observations of a fully-formed reverse-forward shock pair at the very low heliocentric distance of 0.5 AU. While such a shock pair is typically associated with an SIR, it was found to be originated from the interaction between a fast and a slow CME. To our knowledge, this is the first time that such an observation is reported at such small heliocentric distances.
The CME-CME interaction drives a complex compression region, where the interface separates plasma from two different sources and is characterised by a high level of magnetic reconnection activity and several irregularities in the measured plasma conditions. Such characterisation underlines the role of this structure in creating favourable conditions efficient energy dissipation.
This study exploited the unique orbital configuration during the event, with two remote CME candidates identified using STEREO-A and near-Earth observers. These are compatible with CME1 being a faint eruption, then interacting with CME2, which is more energetic. Despite the large uncertainties involved, GCS fits yield CME propagation speeds compatible with this scenario. 
These observations highlighted the importance of connecting remote and direct observations, particularly due to CME1 being particularly faint and slow, yet giving rise to such an interesting event. We also investigated the evolution of this structure at 1 AU using the Wind spacecraft, revealing a merged structure without forward-reverse shock pair and  mixed features between a CME and SIR event. At 1~au, the structure became a moderate event, common around solar maximum, underlining that without an inner heliosphere upstream observer we would have little knowledge of its origins and evolution.

For further details, see Trotta et al., ApJL, 971, L35 (2024) doi: 10.3847/2041-8213/ad68fa
 

Affiliations

(1)European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
(2) The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, UK
(3) Swedish Institute of Space Physics, 751 21 Uppsala, Sweden
(4) Departamento de Ciencias Espaciales, Instituto de Geofísica, Universidad Nacional autónoma de México, Ciudad Universitaria, 04150 Ciudad de México, Mexico
(5) Department of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
(6) Istituto per la Scienza e Tecnologia dei Plasmi (ISTP), Consiglio Nazionale delle Ricerche, I-70126 Bari, Italy
(7) Space Science Center, University of New Hampshire, Durham, NH 03824, USA
(8) Predictive Science Inc., San Diego, CA 92121, USA
(9) Department of Physics, University of Helsinki, FI-00014 Helsinki, Finland
(10) Department of Space and Climate Physics, Mullard Space Science Laboratory, University College London, Dorking RH5 6NT, UK
(11) Institute of Experimental and Applied Physics, Kiel University, D-24118 Kiel, Germany
 

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