Belén Yu Irureta-Goyena
Coronal mass ejections (CMEs) are large-scale eruptions from the Sun that can have significant impacts on the Earth and the near-Earth environment. In order to prepare for the consequences of such solar activity, we must better understand how these eruptions are initiated and the ways they evolve as they propagate through the solar system. Previous studies by other authors have examined the structures of two interplanetary CMEs that occurred in November 2011 and attempted to identify their solar origins with contradictory results. In this work, we extend the previous studies by analysing remote-sensing images of the lower corona to uncover the pre-eruptive structures of the CMEs. We further analyse the properties of the evolving CMEs between 0.4 and 1 AU by studying magnetic field and plasma signatures at the MESSENGER, Venus Express, and STEREO-B spacecraft, and applying a new flux rope modelling tool called AMUN-SA. We determine that the pre-eruptive magnetic structure of a CME that occurred late on 3 November 2011 had right-handed twist and is likely to be the source of an interplanetary CME detected by Venus Express. A second eruption from early on 4 November 2011 was found to have a left-handed magnetic structure, and we connect this with interplanetary CMEs seen at MESSENGER, Venus Express, and STEREO-B. Our analysis has revealed a possible solution to the contradictory results of previous investigations. In addition, we find it likely that there was some form of interaction between the interplanetary CMEs. Future work will study how the CMEs evolved, from their formations in the solar atmosphere to their detections in the solar wind at different radial distances. Finally, this study can also be regarded as preliminary work for CME tracking in the inner solar system with the Solar Orbiter mission, which enters its nominal mission phase in 2021.
On the 5th and 6th November 2011, ICME signatures were detected by Messenger (orbiting Mercury at 0.44 AU), Venus Express, (orbiting Venus at 0.73 AU) and STEREO-B (at 1.09 AU). The longitudinal angular separation between Venus Express and the Messenger-STEREO-B line was less than 30º.
Remote Sensing observations reveal that the ICME signatures detected in-situ can be linked to two eruptions: CME A (from a small active region near the north-eastern limb seen by STEREO-B) and CME B (from a larger active region west of the CME A source). Viewed along the Earth-Sun line, CME A was first seen in the field of view of the SOHO LASCO C2 at 23:12 UT on 3rd November 2011, and CME B was first seen at 01:25 UT on 4th November 2011. According to the SOHO LASCO CME catalogue, CME A had a plane-of-sky speed of 991 km/s, whereas CME B had a speed of Ionospheric and Solar Terrestrial km/s. Using observational proxies, we inferred that the magnetic flux ropes that erupted in CMEs A and B had right-handed and left-handed magnetic twist, respectively.
We compared the measured arrival times of the ICME detected at Messenger, two ICMEs detected at Venus Express and the ICME detected at STEREO-B to the predicted arrival times inferred from the onset times and speeds of the CMEs, noting that the speeds interpolated from the observations were similar to those found in the catalogue. We examined ICME signatures (e.g., associated with the presence of flux ropes, shock fronts and sheath regions) in both the magnetic-field data (Messenger, Venus Express and STEREO-B) and the plasma data (Venus Express and STEREO-B).
Since the handedness of a CME flux rope remains invariant throughout its whole interplanetary journey, we used a catalogue to verify the handedness of the ICMEs at Messenger (left-handed) and Venus Express (right-handed and left-handed, respectively). For STEREO-B, we fitted the Hidalgo flux-rope model to the magnetic-field data to determine the handedness of the ICME (left-handed). Our findings suggest that CME A was fast and detected at Venus Express early on the 5th of November. Whereas CME B was slower and detected at Messenger late on the 4th, Venus Express early on the 6th, and STEREO-B late on the 6th.
In the ongoing study, we will use the proton data of Venus Express to derive solar wind speed, temperature and alpha-particle/proton ratio in order to examine the plasma signatures of the flux roped detected by Venus Express (and compare them to the ICME observed by STEREO-B). We will also investigate the possible interaction between CME A and B, as well as the effect this interaction may have had on the solar energetic particle populations associated with the observations.
Similar studies of CME evolution in the inner heliosphere are planned from 2021 onwards with the Solar Orbiter spacecraft. Solar Orbiter has a comprehensive in-situ and Remote Sensing instrument suite and will experience several advantageous spacecraft conjunctions during the current mission plan.
We acknowledge the instrument teams of the missions MESSENGER, Venus Express, STEREO, SOHO and SDO for the data and their open policy. We acknowledge the financial support by the Spanish MINECO-FPI-2016 predoctoral grant with FSE and research fellowships from the European Space Agency.