Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also subrelativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler mode wave-particle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Tromsø VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to Tromsø and the satellite observed rising tone emissions of the lower band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave-particle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of subrelativistic electrons and the pulsating aurora.

This paper presents fundamental properties of the storm‐time ring current on the basis of the numerical simulations. This simulation model solves spatial and temporal evolution of the ion distribution in the magnetosphere by tracing the bounce‐averaged drift trajectories. The tracing is performed under the dipole magnetic field and the time‐dependent Volland‐Stern‐type convection field. After tracing particles, we calculate the differential flux, the plasma pressure, and the current density. The magnetic disturbance induced by the ring current is directly calculated from the Biot‐Savart integral over the whole three‐dimensional distribution of the calculated current density. We examined following subjects during the magnetic storms; the causes of the ring current buildup, the electric current distribution, the causes of the ring current decay, the energy composition of the plasma pressure, the response time of the plasma sheet density to the solar wind density, and the diamagnetic effect. This simulation suggests the following results: (1) The major variation of corrected Dst is mainly due to the changes in both the convection electric field and the plasma sheet density. (2) The Dessler‐Parker‐Sckopke relation overestimates the corrected Dst by a factor of 2.5–4. (3) The storm‐time ring current buildup is insensitive to the plasma sheet temperature for the temperature above 3 keV. (4) The ions with energies around 15–40 keV at L ∼4–6 in the dusk region mostly contribute to the perpendicular pressure. (5) The equatorial magnetic fields are dramatically distorted by the diamagnetic effect. The grad‐B drift trajectories under the distorted equatorial magnetic field can be classified into four patterns.

We perform test particle simulations of energetic electrons interacting with whistler mode chorus emissions. We compute trajectories of a large number of electrons forming a delta function with the same energy and equatorial pitch angle. The electrons are launched at different locations along the magnetic field line and different timings with respect to a pair of chorus emissions generated at the magnetic equator. We follow the evolution of the delta function and obtain a distribution function in energy and equatorial pitch angle, which is a numerical Green's function for one cycle of chorus wave‐particle interaction. We obtain the Green's functions for the energy range 10 keV–6 MeV and all pitch angles greater than the loss cone angle. By taking the convolution integral of the Green's functions with the distribution function of the injected electrons repeatedly, we follow a long‐time evolution of the distribution function. We find that the energetic electrons are accelerated effectively by relativistic turning acceleration and ultrarelativistic acceleration through nonlinear trapping by chorus emissions. Further, these processes result in the rapid formation of a dumbbell distribution of highly relativistic electrons within a few minutes after the onset of the continuous injection of 10–30 keV electrons.

Abstract. Particle drift simulation of low-energy ions (less than a few keV) with a simple geomagnetic field (dipole) and the Volland-Stem type convection electric field is found to be capable of investigating dispersed sub-keV ion events deep inside the dayside ring current region observed by Viking. Here we show three types of such events and simulation results.

We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low-altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified abovẽ 3 keV. Moreover, stable precipitations at~1 keV are found for the PsA. A "precipitation gap" is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave-particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on-off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.