Halo coronal mass ejections, solar energetic particles, and sustained gamma-ray emission
Abstract
Halo coronal mass ejections (CMEs) are fast and wide and hence are very energetic. CMEs that produce space weather events such as intense geomagnetic storms and large solar energetic particle (SEP) events have high proportion of halo CMEs. One CME population has 100 percent halos: the CMEs associated with sustained gamma-ray emission (SGRE) from the Sun that last for at least 3 hours. CMEs associated with SGRE are ultrafast (average speed ~2000 km/s), very similar to CMEs that produce ground level enhancement (GLE) events. The SGRE - halo CME connection supports the idea that high-energy protons accelerated at the CME shock precipitate back to the solar surface, interact with ambient protons, and produce pion decay continuum observed as SGRE. In order to further clarify the relationship, we start with all ultrafast halo CMEs (sky-plane speed at least 1800 km/s) observed in solar cycles 24 that had simultaneous gamma-ray observations from the Sun. We identified 20 such CMEs that have an average sky-plane speed of ~2142 km/s, fourteen of which were frontsided. The soft X-ray flare sizes ranged from M3.7 to X8.2. We determined the CME kinematics using the graduated cylindrical shell model applied to SOHO and STEREO coronagraph data. The three-dimensional speed from the graduated cylindrical shell (GCS) model peaks at ≥2000 km/s (average 2698 km/s). The initial acceleration of the CMEs is >1 km s-2 (average 3.5 km s-2). These speeds and accelerations are typical of GLE events, indicating strong shocks close to the Sun accelerating highest energy particles. When we examined the Fermi Large Area Telescope (Fermi/LAT) >100 MeV gamma-ray data, we found that all but one of the 14 frontsided halo CMEs are associated with an SGRE event. The lone halo without SGRE had a Fermi/LAT data gap, so we cannot rule out the possibility of SGRE association. Among the 6 backside halo CMEs, one was associated with an SGRE, which is the famous 2014 September 1 event originating about 40 degrees behind the limb. Two events had backside location similar to that of the 2014 September 1 event, but the flux rope orientation is north-south indicating a smaller longitudinal extent of the source region, so no SGRE is observed on the frontside. The remaining three events were too far behind the limb. We present additional information on the properties of the SEP events and interplanetary type II radio bursts that further support the CME-SGRE connection.
- Publication:
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44th COSPAR Scientific Assembly. Held 16-24 July
- Pub Date:
- July 2022
- Bibcode:
- 2022cosp...44.1167G