Whistler‐mode hiss waves play an important role in the radiation belt electron depletion. Whether the hiss waves with significant differences in amplitude and propagation direction within the plasmaspheric core and plume are related to each other remains unclear. We here show that the plasmaspheric plume facilitates the energy conversion from energetic electrons to hiss waves and then guides hiss waves into the plasmaspheric core. Three ground and space missions captured the initial formation and subsequent rotation of the plasmaspheric plume in the noon‐dusk‐midnight sector following a strong substorm. The observed hiss waves in the nightside plasmaspheric plume and core propagated oppositely but highly correlated with each other at a time lag of 4–10 s. The linear instability of energetic electrons in the plasmaspheric plume qualitatively explains the frequency‐dependence of hiss waves, and the ray‐tracing modeling reproduces the propagation direction and timing of hiss waves.

Evolution of large‐scale and fine‐scale plasmaspheric plume density structures was examined using space‐ground coordinated observations of a plume during the 7–8 September 2015 storm. The large‐scale plasmaspheric plume density at Van Allen Probes A was roughly proportional to the total electron content (TEC) along the satellite footprint, indicating that TEC distribution represents the large‐scale plume density distribution in the magnetosphere. The plasmaspheric plume contained fine‐scale density structures and subauroral polarization streams (SAPS) velocity fluctuations. High‐resolution TEC data support the interpretation that the fine‐scale plume structures were blobs with ∼300 km size and ∼500–800 m/s in the ionosphere (∼3,000 km size and ∼5–8 km/s speed in the magnetosphere), emerging at the plume base and drifting to the plume. The short‐baseline Global Navigation Satellite System receivers detected smaller‐scale (∼10 km in the ionosphere, ∼100 km in the magnetosphere) TEC gradients and their sunward drift. Fine‐scale density structures were associated with enhanced phase scintillation index. Velocity fluctuations were found to be spatial structures of fine‐scale SAPS flows that drifted sunward with density irregularities down to ∼10 s of meter‐scale. Fine‐scale density structures followed a power law with a slope of ∼−5/3, and smaller‐scale density structures developed slower than the larger‐scale structures. We suggest that turbulent SAPS flows created fine‐scale density structures and their cascading to smaller scales. We also found that the plume fine‐scale density structures were associated with whistler‐mode intensity modulation, and localized electron precipitation in the plume. Structured precipitation in the plume may contribute to ionospheric heating, SAPS velocity reduction, and conductance enhancements.

The plasmasphere is a region in the near-Earth space, extending up to ∼7 Earth radii (R E ), filled with cold (0.1-10 eV) and dense (up to 10 4 cm −3 ) plasma. It is populated mostly by the outflow of ions and electrons from the ionosphere and typically has an irregular toroidal shape around the Earth (Carpenter & Anderson, 1992) with an exponentially decreasing density profile as the radial distance increases. Previous studies have suggested that the shape of the plasmasphere is correlated with geomagnetic activity (Carpenter & Seely, 1976;Goldstein, 2007), and the global distribution of cold plasma can become complex during geomagnetically disturbed periods

Properties of plasma pitch angle distributions (PADs) have been widely investigated in the inner magnetosphere of the Earth. In general, there are four types of plasma PADs in the magnetosphere: (a) isotropic, (b) pancake, (c) butterfly, and (d) field-aligned distributions, defined as follows (e.g., Chen et al., 2014;Zhao et al., 2014Zhao et al., , 2018. The isotropic distribution has similar fluxes at all pitch angles, whereas the pancake (peak at 90°), the butterfly (depletion 0°, 90°, and 180°), and the field-aligned (peaks at 0° and 180°) have distinct shapes. Their characteristics have been investigated to understand plasma dynamics such as the source of the plasma around the Earth and their heating/loss mechanisms. Energetic electron PADs have been used to understand the radiation belts (e.g.,