Generation region of pulsating aurora obtained simultaneously by the FAST satellite and a Syowa‐Iceland conjugate pair of observatories

Abstract

[1] We have carried out a direct comparison of pulsating auroras observed from the ground at Syowa Station in Antarctica and on board the FAST satellite ($3100 km altitude), with reference to simultaneous data obtained by a Syowa-Iceland conjugate pair of observatories. The aurora at Syowa appeared as east-west aligned bands consisting of two different types: a poleward moving oscillation and a standing oscillation, each with a period of $6 s. Spatial and temporal variations of the downgoing high-energy (>5 keV) electron flux obtained by FAST showed a one-to-one correspondence with the optical pulsating aurora. The occurrence regions of the two different types of pulsating aurora were separated by a narrow gap ($7-10 km in width at 100 km altitude) in the inverted-V structure, and the gaps were colocated with the small-scale upward field-aligned currents. The time-varying magnetic fields (upward field-aligned current) observed by FAST were almost correlated (in-phase) with the downgoing electron flux (>5 keV) modulations. Both the optical emission intensity at Syowa and the downgoing high-energy electron flux (>7 keV) on board FAST showed $3 Hz modulation. The $3 Hz fine structure constituted the main body of the $6 s pulsating aurora. VLF wave activities were not observed by FAST in the region of pulsating aurora. The source regions of the generation or modulation of the energetic particles are estimated to be at a higher altitude than FAST, in the region of $2 Re to 6 Re from the satellite. This suggests that the source region is not located in the equatorial plane of the magnetosphere but is located earthward, far from the equatorial plane. The conjugate pair observations on the ground revealed that the aurora, though pulsating in both hemispheres, was not conjugate in shape, appearing as an east-west aligned band in the Southern Hemisphere but as a torch structure (omega band) in the Northern Hemisphere. The evidence presented in this study suggests that ionosphere-magnetosphere coupling processes are important in producing the pulsating aurora.

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

“…For example, rapid ∼3 Hz modulations were often found to be superimposed on the slower variations (i.e., the repetition period) of the PsA [Lepine et al, 1980;Yamamoto, 1988;Sato et al, 2004]. On the other hand, the analysis of the pulsation pattern has led to a distinction between "on time" and "off time" [Scourfield and Parsons, 1971;Yamamoto, 1988].…”

“…(3) In this event we do not have the satellite/rocket measurements in low-or medium-Earth orbit, above the ionosphere, to infer the electron precipitation spectra and their dynamic variations leading to the PsA. On the other hand, some observational evidences from the nonconjugacy of the PsAs in two hemispheres [Sato et al, 1998, 2004, Watanabe et al, 2007, and from the energytime dispersion of precipitation electrons observed by FAST satellite [Sato et al, 2002[Sato et al, , 2004, suggest that the source regions of the generation and/or modulation of the PsAs may not be located in the equatorial magnetosphere but somewhere closer to the Earth, presumably in a magnetosphere-ionosphere interface region. Satellite observations in such a region are thus very useful since they may not only provide the information on the energy spectrum of precipitating electrons responsible for the PsA, but also help to distinguish the source regions of the generation/modulation of the precipitations.…”

“…In Figure 8 we mark two occurrences of wave-burst mode measurements whose data are shown in Figures 5b and 5c, respectively; fluctuations of wave intensity at different time scales are evident. It is important to note that PsA observations from hightemporal-resolution ASIs often reveal prominent multitime scale feature and subsecond variations as well [e.g., Yamamoto, 1988;Sato et al, 2004]. We shall briefly discuss this multi-time scale aspect of the PsA and the limitation imposed by the temporal resolution of the THEMIS ASI in section 4.…”

“…Classical theories usually designate an equatorial magnetosphere origin of the PsA generation; the processes typically involve [e.g., Davidson, 1990;Demekhov and Trakhtengerts, 1994] (1) fast-drifting energetic electrons that provide the energy and/or particle source of the PsA, (2) flux tubes containing localized concentrations of low-energy plasma that control the dimension and bulk motion of the PsA patches, and (3) nonlinear interactions between one (or both) of the above noted ELF/ VLF waves and the electron loss cone distribution during the passing of the energetic electrons through the lowenergy plasma blobs. However, some observation evidences, in particular from the nonconjugacy of the PsAs in two hemispheres [Sato et al, 1998[Sato et al, , 2004Watanabe et al, 2007] suggest an independent modulation source for each hemisphere that is located away from the equatorial plane [Sato et al, 2004].…”

“…[3] Observationally, PsA is a repetitive modulation of the auroral luminosity with typical frequency range 0.05-1 Hz, while a ∼3 Hz modulation are often found to be superimposed on the pulsation structures [e.g., Royrvik and Davis, 1977;Lepine et al, 1980;Yamamoto, 1988;Sato et al, 2004]. The PsA usually occurs in the postmidnight sector during the late expansion and recovery phase of a substorm, and manifests in forms of geostable or slowly convecting luminous patches with scale sizes 10-200 km [Royrvik and Davis, 1977;Johnstone, 1983].…”