Frequency‐Drifting Plasmaspheric Hiss Events Triggered by Three Consecutive Substorms in the Earth's Magnetosphere

Abstract Whistler‐mode waves are frequently observed electromagnetic emissions in planetary space. In the Earth's magnetosphere, the generation processes of whistler‐mode waves within the high‐density plasmasphere, referred to as plasmaspheric hiss, remain an open question. Here, based on the Van Allen Probes data, we report plasmaspheric hiss events triggered by three consecutive substorms within an interval of 10 hr. These plasmaspheric hiss waves span a wide spatial region from post‐midnight to prenoon (MLT), with varying intensity, frequency coverage, and propagation characteristics, yet share a similar frequency‐drifting feature characterized by an increasing lower cutoff frequency over timescales exceeding 1 hr. Our analyses indicate that energy‐dispersive injected electrons associated with the three substorms excite frequency‐drifting whistler‐mode waves in the outer plasmasphere around dawn, which subsequently propagate through the plasmasphere with the frequency‐drifting feature retained. These events elucidate the direct link between substorm injection and plasmaspheric hiss, providing observational evidence for the excitation of hiss waves inside the plasmasphere by substorm injections.