Features of Nightside ULF Wave Activity in the Ionosphere

We investigate the features of nightside ultralow frequency (ULF) waves in the ionosphere using electric field data in the DC/ULF range observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite over a ~5.5‐year period from May 2005 to November 2010. The ULF wave events are recognized by an automatic detection algorithm, and a superposed epoch analysis is performed to study the characteristics of ULF waves during geomagnetic storms. The results show that (1) ULF waves of both nonmagnetospheric and magnetospheric origins occur in the ionosphere; (2) nonmagnetospheric ULF waves present seasonal variations but have no clear response to storms; (3) magnetospheric ULF waves show clear association with the recovery phase of storms but have no obvious seasonal variations; and (4) an interhemispheric asymmetry with higher magnetospheric ULF wave occurrence rate in the Southern Hemisphere particularly around storms can be partly explained by the fact that more of selected isolated storms occurred during summer in the Northern Hemisphere (NH), hence higher conductivity in the NH. However, an analysis of few storms during equinoxes also shows a minor asymmetry. These results still indicate that the ionospheric conductivity in the Southern Hemisphere would be lower than in the NH at nighttime. Plain Language Summary Past knowledge of ultralow frequency (ULF) waves are mainly based on single‐point or multipoint observations from high‐altitude satellites or ground‐based magnetometers. Satellites at low altitudes offer the capability of a global survey of ULF waves that help us to deeply understand ULF waves in the ionosphere. The origins of ULF fluctuations observed by low‐altitude satellites are still controversial to date. In this work we statistically explore the characteristics of ULF waves in the ionosphere originated from different sources (e.g., magnetosphere, ionosphere, and atmosphere). Higher occurrence rate of ULF waves from the magnetosphere is in the Southern Hemisphere, indicating lower conductivity there allowing waves more easily propagating into the ionosphere. We also find that ULF waves from different sources have different behaviors with respect to seasonal variations and the response to the strong external perturbations. This study will promote our understanding of ULF waves and their effects on dynamics of the ionosphere. Key Points Features of nonmagnetospheric and magnetospheric nightside ULF w