LAUSR

The FORMOSAT‐3/COSMIC (F3/C) satellites are used to study the climatology of equatorial plasma bubbles (EPBs) during the low to moderate solar flux years (2008–2013). We use the F3/C total electron content to identify the presence of EPBs and investigate the background conditions for the initiation of EPBs. The results reveal that the EPB activities have strong solar dependence. The longitudinal and seasonal trends of EPBs are highly correlated to the angle between the dusk solar terminator and magnetic field lines near the magnetic equator. Asymmetries of EPBs between solstices and equinoxes exist and could be due partly to the asymmetry of equatorial ionization anomaly structures, which result in longitudinal differences as well. EPBs extend to higher altitudes and latitudes during the ascending phase of Solar Cycle 24 (2011–2013) due mainly to the increase of background electron density. However, an altitudinal asymmetry of EPBs occurs in moderate solar flux years, which is likely due to the suppression or lower growth and occurrence rates of EPBs. In addition to vertical drift, tidal forcing also contributes to the longitudinal and seasonal distributions of EPBs. Upwellings and precursor waves preceding the EPBs are observed climatologically, which likely play a vital role in initiating the EPBs. This study also reveals a vertical connection between the equatorial ionospheric irregularities and atmospheric forcing on a climatological basis. Plain Language Summary Equatorial plasma bubbles (EPBs) are an important space weather phenomenon that are often generated over the magnetic equator around twilight. They are ionospheric irregularities that can cause plasma depletions along the geomagnetic field lines, displaying geomagnetic conjugacy in both hemispheres. Since the ionospheric plasma is one of the significant error sources of the Global Navigation Satellite System (GNSS), EPBs are considered to be a primary disrupter of GNSS communication and navigation. Postsunset rise (PSSR) of the equatorial ionosphere due to vertical ion drift is often used to explain the occurrence of EPBs; however, it cannot fully explain the day‐to‐day variability of EPBs and the features of EPBs in patches and clusters. Mysteries abound around the EPBs and the underlying physics that initiates the EPBs remain unsolved. Here we utilize the FORMOSAT‐3/COSMIC to investigate the underlying background conditions for the generation of EPBs. We find that, in addition to PSSR, symmetric equatorial ionization anomaly, as well as tidal forcing, appears to provide conditions favorable for the initiation of EPBs. Atmospheric tidal waves contribute to the longitudinal and seasonal distribution of EPBs as well. Bottomside ionospheric undulations due to atmospheric forcing are likely playing a vital role in initiating the EPBs.