MIST

By Domenico Trotta (Imperial College London)

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.

In our study, 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 enabled us to study IP shocks in highly unusual parameter regimes (for example the forward shock propagating in CME material). Further, in the interaction region between the two CMEs, a large range of interesting phenomena of energy conversion, such as enhanced rate of magnetic reconnection, has been identified.

We then used remote observations from STEREO-A to identify the two CMEs as they were ejected from the Sun, revealing that the first, slow CME was an extremely faint event. Finally, we used well the radially aligned Wind spacecraft to investigate the fate of such interesting structure, and found that it dissipated at 1 AU, where only a weak forward shock is observed, in stark contrast with SIR-driven shock pairs expected to become stronger with heliocentric distances. Thus, we highlighted how without Solar Orbiter at inner heliocentric distance, such structure would not have been possible to observe and investigate, once again underlining how exploiting multiple heliospheric vantage points is invaluable to advance our understanding of both the Sun-Earth system and remote astrophysical environments.

 
Solar Orbiter direct observation of the forward-reverse shock pair and the interacting CMEs
(full details in the publication).

References:

[1] Rice et al., JGR: Space Physics, 108, 1369 (2003)
[2] Temmer, Liv. Rev. in Sol. Phys., 18, 4 (2021)
[3] Muller et al., A&A, 642, A1 (2020)
[4] Belcher, ApJ, 168, 509 (1971)
[5] Jian et al., Sol. Physics, 239, 337 (2006)
[6] Chen, Liv. Rev. in Sol. Phys., 8, 1 (2011)

See publication for details:
Domenico Trotta et al 2024 ApJL 971 L35
DOI 10.3847/2041-8213/ad68fa