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Abstract We report on the 2024 September 9 sustained gamma-ray emission (SGRE) event observed by the Large Area Telescope (LAT) on board the Fermi satellite. The hevent was associated with a backside solar eruption observed by multiple spacecraft such as the Solar and Heliospheric Observatory (SOHO), Solar Terrestrial Relations Observatory (STEREO), Parker Solar Probe (PSP), Solar Orbiter (SolO), Solar Dynamics Observatory (SDO), Wind, and GOES, and by ground-based radio telescopes. Fermi/LAT observed the SGRE after the EUV wave from the backside eruption crossed the limb to the frontside of the Sun. SolO’s Spectrometer Telescope for Imaging X-rays (STIX) imaged an intense (X3.3) flare, which occurred ≈ 41° behind the east limb, from heliographic coordinates S13E131. Forward modeling of the coronal mass ejection (CME) flux rope revealed that it impulsively accelerated (3.54 km s⁻²) to attain a peak speed of 2162 km s⁻¹. SolO’s energetic particle detectors (EPD) observed protons up to ≈ 1 GeV from the extended shock and electrons that produced a complex type II burst and possibly type III bursts. The durations of SGRE and type II burst are consistent with the linear relation between these quantities obtained from longer duration (> 3 hours) SGRE events. All these observations are consistent with an extended shock surrounding the CME flux rope, which is the likely source of high-energy protons required for the SGRE event. We compare this event with six other behind-the-limb (BTL) SGRE eruptions and find that they are all consistent with energetic shock-driving CMEs. We also find a significant east-west asymmetry (3:1) in the BTL source locations. 

Abstract The sustained gamma-ray emission (SGRE) from the Sun is a prolonged enhancement of >100 MeV gamma-ray emission that extends beyond the flare impulsive phase. The origin of the >300 MeV protons resulting in SGRE is debated, with both flares and shocks driven by coronal mass ejections (CMEs) being the suggested sites of proton acceleration. We compared the near-Sun acceleration and space speed of CMEs with “Prompt” and “Delayed” (SGRE) gamma-ray components. We found that “Delayed”-component-associated CMEs have higher initial accelerations and space speeds than “Prompt Only”-component-associated CMEs. We selected halo CMEs (HCMEs) associated with type II radio bursts (shock-driving HCMEs) and compared the average acceleration and space speed between HCME populations with or without SGRE events, major solar energetic particle (SEP) events, metric, or decameter-hectometric (DH) type II radio bursts. We found that the SGRE-producing HCMEs associated with a DH type II radio burst and/or a major SEP event have higher space speeds and especially initial accelerations than those without an SGRE event. We estimated the radial distances and speeds of the CME-driven shocks at the end time of the 2012 January 23 and March 7 SGRE events using white-light images of STEREO Heliospheric Imagers and radio dynamic spectra of Wind WAVES. The shocks were at the radial distances of 0.6–0.8 au and their speeds were high enough (≈975 km s⁻¹and ≈750 km s⁻¹, respectively) for high-energy particle acceleration. Therefore, we conclude that our findings support the CME-driven shock as the source of >300 MeV protons. 

Abstract It is known that the weak state of the heliosphere due to diminished solar activity in cycle 24 backreacted on coronal mass ejections (CMEs) to make them appear wider for a given speed. One of the consequences of the weak state of the heliosphere is that more CMEs appear as halo CMEs (HCMEs), and halos are formed at shorter heliocentric distances. Current predictions for the strength of solar cycle (SC) 25 range from half to twice the strength of SC 24. We compare the HCME occurrence rate and other properties during the rise phase of cycles 23, 24, and 25 to weigh in on the strength of SC 25. We find that HCME and solar wind properties in SC 25 are intermediate between SCs 23 and 24, but closer to SC 24. The HCME occurrence rate, normalized to the sunspot number, is higher in SCs 24 and 25 than in SC 23. The solar wind total pressure in SC 25 is ∼35% smaller than that in SC 23. Furthermore, the occurrence rates of high-energy solar energetic particle events and intense geomagnetic storms are well below the corresponding values in SC 23, but similar to those in SC 24. We conclude that cycle 25 is likely to be similar to or slightly stronger than cycle 24, in agreement with polar-field precursor methods for cycle 25 prediction.