[P-EM11] Space Weather and Space Climate

[PEM11-P02] Multipoint spacecraft measurements of the ICME structure in October 2021

*Yumi Bamba¹, Takuya Hara², Ryuho Kataoka³, Katsuhide Marubashi¹, Kazumasa Iwai⁴, Go Murakami⁵, Beatriz Sanchez-Cano⁶, Daniel Heyner⁷, Ingo Richter⁷, Hans-Ulrich Auster⁷ (1.National Institute of Information and Communications Technology, 2.University of California, Berkeley, 3.National Institute of Polar Research, 4.Institute for Space-Earth Environmental Research, 5.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 6.School of Physics and Astronomy, University of Leicester, UK, 7.Institute for Geophysics and extraterrestrial Physics, Technical University of Braunschweig)

Keywords:Space Weather, Coronal Mass Ejection, Planar Magnetic Structure

Coronal Mass Ejection (CME) sometimes makes a great impact on our social infrastructures, such as, global positioning satellites and electric power supply system. It is thus important from a perspective of space weather forecast to predict CME occurrence, propagation, and arrival to Earth. There are many previous studies of CMEs, some of which studied that the apex of magnetic flux rope hits Earth. However, more frequently, only the flank of a flux rope glances off Earth, or interplanetary shocks without any flux ropes arrive at Earth. Some of them are still geo-effective. Here we investigated a halo-CME that occurred on October 9, 2021, which was extensively observed in situ by multiple spacecraft including BepiColombo (0.3AU, W02), Solar Orbiter (0.68AU, E15), Parker Solar Probe (0.76AU, E48), STEREO-A (0.96AU, E39), and the solar wind monitor at Earth (e.g., ACE). The coronal material was ejected toward the north-west direction from the Sun, whereas the five spacecraft were situated to the east from the Sun-Earth line. Although this ICME was also a "non-direct" event, it caused a geomagnetic storm at Earth.

We found a dip-like structure in the normal component of the magnetic field in the sheath of the interplanetary shock at the four spacecraft except for Parker Solar Probe. Interestingly, a smooth rotation of the magnetic field was observed by all of the four spacecraft, despite that they were located at different radial distances and longitudes in the inner heliosphere. We initially considered a possibility that the magnetic feature of interest is a kind of small-scale flux rope (cf. Ruohotie etal 2022) embedded in a CME-driven sheath. We thus attempted to reproduce flux rope geometry at each spacecraft location by applying a model fitting technique (Marubashi & Lepping 2007), under the assumption that the shape is either a cylinder or torus. However, our first attempt was not very much successful in providing reliable and consistent parameter settings among multiple spacecraft. It was also questionable to consider the observed dip-like structure as that typically found in the sheath, because the observed magnetic field variations are too smooth. We another considered the possibility that this structure corresponds to a planar magnetic structure (PMS). PMS is a characteristic magnetic field structure in which the magnetic field is oriented approximately along a single plane. One of the specific regions where PMSs are usually detected in the CME-driven sheath. Previous studies (Palmerio+2016, Ruan+2023) suggested that PMS can be identified with the minimum variance analysis. We thus applied the minimum variance analysis to the magnetic field data during the time interval including the dip-like structure. The result shows that the magnetic field observed by all the five spacecraft is well aligned to the plane perpendicular to the minimum variance direction. This is consistent with an interpretation that a PMS passed by the five spacecraft, as proposed by a previous study (Ruan+2023). We further examined the angle between the shock normal direction and the minimum variance direction at each spacecraft location in order to discuss the distribution of the PMS relative to the global CME structure. The result shows that the PMS tends to be formed along the shock surface. The in-situ spacecraft measurements indicate that the PMSs might be also geo-effective when the magnetic field points southward for a certain time interval, such as ICME ejecta. Our results suggest that we should be also careful of CMEs glancing off Earth, because such a PMS might be embedded in a CME-driven sheath.