Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves

Abstract

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.

Variations in energy, flux, and brightness of pulsating aurora measured at high time resolution

“…The dominant source of electron precipitation causing diffuse aurora has been shown to be chorus waves Ni et al, 2016). Miyoshi et al (2015) showed with observations of energy spectra and simulations that while lower band chorus waves can lie behind the main modulation of the electron precipitation, causing pulsating aurora, upper band chorus waves can produce a simultaneous, constant precipitation at about 1 keV. They furthermore argued that this is the source of the precipitation during the OFF period of pulsations.…”

Substructures with luminosity modulation and horizontal oscillation in pulsating patch: Principal component analysis application to pulsating aurora

“…Both FFT analysis and PCA demonstrate a new finding, that is, the PsA consisted of a few substructures in which more rapid variations than on‐off pulsations were strongly excited. Simulations and observations demonstrated that rising tone elements of whistler mode chorus waves drive a few Hz modulations superimposed on PsA variations [ Saito et al , ; Miyoshi et al , ]. Therefore, these substructures, in which the internal modulations are seen coherently in FFT analysis, are associated with wave packets of rising tone chorus elements.…”

“…More recently, Miyoshi et al [] demonstrated one‐to‐one correspondence between the time variations of PsA and chorus waves by comparative study between Reimei observations and computer simulation. Lower band chorus (LBC) bursts cause the on‐off main modulation, and rising tone chorus elements embedded in LBC bursts cause the internal modulations, while upper band chorus (UBC) produce stable precipitation around 1 keV that has been identified as a steady background precipitation [ Evans et al , ].…”

Observation of pulsating aurora signatures in cosmic noise absorption data

“…It is commonly agreed that the scattering of electrons into the loss cone by very low frequency waves in the near‐equatorial region of the magnetosphere is involved [ Jaynes et al , ]. Likely candidates for the scattering waves are lower‐band chorus waves [ Nishimura et al , ; Miyoshi et al , , ], but the cause of the observed precipitating flux modulations is still debated. The possible mechanisms of periodic modulations have been debated, while theories such as nonlinear relaxation oscillator [ Davidson , ] and flow cyclotron maser [ Demekhov and Trakhtengerts , ] have been suggested, among others.…”

“…Nevertheless, Miyoshi et al [] revealed a possible contribution of electrons with energies reaching up to 200 keV, which was confirmed by Turunen et al []. Miyoshi et al [, ] have proposed that chorus waves propagating along the field line can cause wide‐energy electron precipitation because the resonant energy depends on the ratio of ambient plasma frequency and electron gyrofrequency. Therefore, it is expected that electrons across a wide energy range simultaneously precipitate into the atmosphere in association with pulsating aurora.…”