Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

We investigated the occurrence rate of the sustained gamma ray emission (SGRE) events from the Sun using data obtained by Fermi Large Area Telescope (LAT) since its launch in 2008. Only 16 SGRE events were observed during the first 61 months of solar cycle (SC) 25, likely due to the solar array drive assembly's malfunction in 2018; 27 SGRE events were observed in SC 24 over the corresponding epoch. The average sunspot number (SSN) increased from 56.9 in SC 24 to 79.0 in SC 25. Fast and wide (FW) CMEs and decameter-hectometric (DH) type II bursts increased significantly in SC 25 by 29% and 33%, respectively when normalized to SSN. Therefore, we expect 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 with SSN, we should have 38 SGRE events in SC 25. However, FW CMEs and DH type II bursts in SC 25 were overabundant by 29% and 33%, so the number SGRE events should be 48 or 50. (ii) In SC 24, ~18% of FW CMEs and 27% of DH type II bursts were associated with SGRE events. At this rate SC 25 should have 48 and 49 SGRE events. (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. Almost all SGRE events in SCs 24 and 25, and 27 of the 79 LAT-gap type IIs were associated with HXR bursts of duration > ~5 min. These DH type II bursts are indicative of SGRE, bringing the total number of SGRE events to 43 (16 + 27). Thus, the three methods provide similar estimates of the number of SGRE events in SC 25. We, therefore, conclude that SC 25 is stronger than SC 24 based on the estimated number SGRE events.