Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

Abstract We investigated the occurrence rate of the sustained gamma ray emission (SGRE) events from the Sun using data obtained by the Large Area Telescope (LAT) on board the Fermi satellite since its launch in 2008. The data cover the whole of solar cycle (SC) 24 and the rising and maximum phases of SC 25. One of the challenges was to estimate the number of SGRE events in SC 25 because of a solar array drive assembly’s malfunction starting in March 2018 that resulted in a large reduction in solar coverage by LAT. This is likely the reason for the small number (15) of SGRE events observed during the first 61 months of SC 25, whereas 25 events were observed during the weaker SC 24 over the corresponding epoch. Over the first 61 months, the average sunspot number (SSN) increased from 56.9 in SC 24 to 79.0 in SC 25. Other energetic events closely related to SGREs, viz., fast and wide (FW) coronal mass ejections (CMEs) and decameter-hectometric (DH) type II bursts also increased significantly in SC 25 by 29% and 33%, respectively when normalized to SSN. Therefore, the increase in solar activity should result in a higher number of SGREs in SC 25. We estimated the number of SGREs in SC 25 using three methods. (i) If the SGRE number varies commensurate FW CMEs and DH type II bursts, SC 25 should have 45–47 SGRE events (25×1.29 and 25×1.33). (ii) In SC 24, ~ 17% of FW CMEs and 25% of DH type II bursts were associated with SGRE events. If the same association rate prevails in SC 25, we should have 46 SGRE events in this cycle. (iii) Since SGRE events are invariably associated with > 100 keV hard X-ray (HXR) bursts, we identified DH type II bursts associated with > 100 keV HXR bursts from Fermi’s gamma ray burst monitor (GBM) during LAT data gaps. Based on our finding that SGRE events in SCs 24 and 25 were all associated with HXR bursts of duration > ~ 5 min, we found only 27 of the 79 LAT-gap type II bursts had > 100 keV HXR bursts with duration > ~ 5 min. These DH type II bursts are likely to indicate SGRE, bringing the total number of SGRE events to 42 (15 observed + 27 inferred). Thus, the three methods provide similar estimates of the number of SGRE events in SC 25. We, therefore, conclude that that SC 25 is stronger than SC 24 based on the estimated number SGRE events. Other energetic phenomena such as halo CMEs, ground level enhancement (GLE) events, and intense geomagnetic storms are also consistent with a stronger SC 25.