Observations of a Solar Energetic Particle Event From Inside and Outside the Coma of Comet 67P

Abstract

We analyze observations of an SEP event at Rosetta's target comet 67P/Churyumov-Gerasimenko during March 6th-10th 2015. The comet was 2.15AU from the Sun, with the Rosetta spacecraft approximately 70km from the nucleus placing it deep inside the comet's coma and allowing us to study its response. The Eastern flank of an ICME also encountered Rosetta on March 6th and 7th. Rosetta's RPC data indicate increases in ionization rates, and cometary water group pickup ions exceeding 1keV. Increased charge exchange reactions between solar wind ions and cometary neutrals also indicate increased upstream neutral populations consistent with enhanced SEP induced surface activity.In addition, the most intense parts of the event coincide with observations interpreted as an infant cometary bow shock, indicating that the SEPs may have enhanced the formation and/or intensified the observations. These solar transient events may also have pushed the cometopause closer to the nucleus.We track and discuss characteristics of the SEP event using remote observations by SOHO, WIND and GOES at the Sun, in-situ measurements at STEREO A, Mars and Rosetta, and ENLIL modeling. Based on its relatively prolonged duration, gradual and anisotropic nature and broad angular spread in the heliosphere, we determine the main particle acceleration source to be a distant ICME which emerged from the Sun on March 6th 2015 and was detected locally in the Martian ionosphere but was never encountered by 67P directly. The ICME's shock produced SEPs for several days which traveled to the in-situ observation sites via magnetic field line connections.

Improved modelling of SEP event onset within the WSA–Enlil–SEPMOD framework

“…Nowadays, this dichotomy has become more blurred, as both flares and CMEs are able to contribute to SEP acceleration within a single event (e.g., Cane et al, 2010;Trottet et al, 2015). Significant progress in understanding SEP acceleration and transport has been achieved owing to multi-point measurements, which have allowed the study of, e.g., the broad longitudinal distribution of so-called widespread events (e.g., Xie et al, 2017;Kollhoff et al, 2021;Dresing et al, 2023), how the local heliospheric conditions may facilitate or hinder the transport of particles up to a given location (e.g., Richardson & Cane, 1996;Lario & Karelitz, 2014;Bain et al, 2016;Palmerio et al, 2021), as well as the longitudinal and/or radial variation of SEP profiles and properties (e.g., Rouillard et al, 2012;Cohen et al, 2014;Lario et al, 2016;Wellbrock et al, 2022). In addition to providing a wealth of data for the development and refinement of theories of particle acceleration and transport, multi-point observations of SEPs are also crucial for validating modelling efforts, since they conveniently provide results at multiple locations that can test the predictions of a single simulation run.…”