Auroral activities and long-period geomagnetic pulsations: 1. Pc5 pulsations and concurrent auroras in the dawn sector.

Abstract

It is found that the occurrence and luminosity of diffuse auroras, including pulsating auroras, in the dawn sector show space-time variations synchronized with geomagnetic Pc5 pulsations. The activation region of aurora (not auroral structure) quasi-periodically propagates towards the northwest. The observed propagation velocities at the ionospheric level are about 1km/s northward and about 15km/s westward, respectively. These velocities are in good agreement with the phase velocities of Pc5 pulsations previously measured by radars and by ground networks of geomagnetic field observations. Oscillations in drifts of auroral patches generally in the east-west direction are found to be synchronized with the Pc5 pulsations. The observations suggest that the ground Pc5 pulsations do not necessarily reflect the HM wave structures in the magnetosphere, but the wave nature could be inferred from a close examination of the relationships between the ground magnetic pulsations and concurrent auroral dynamics.

Study of a global auroral Pc5 pulsation event with concurrent ULF waves

“…The intensity of diffuse auroras can also oscillate in phase with concurrent geomagnetic Pc5 pulsations [ Oguti , ]. Such events occur predominantly in the dawn sector [ Yamamoto et al ., ]. Modulations of riometer signals, associated with 10–100 keV electron precipitations, with Pc4 and Pc5 ULF micropulsations have also been reported [e.g., Olson et al ., , and references therein].…”

Auroral activities and long-period geomagnetic pulsations: 2. Ps5 pulsations following auroral breakup in the premidnight hours.

“…Our observations show that the leading edge of the drifting auroral forms has a discrete auroral (vortex-chain) feature. The auroral dynamics associated with Pc5 pulsations, described in the companion paper (YAMAMOTO et al, 1988), does not show any vortex-chain behavior, while Ps5's examined here are evidently related to the vortex-chain activity of aurora . The difference between them strongly suggests a difference in generation mechanisms of the two kinds of pulsations although the periods are in the same Pc5 range.…”

“…However, we are now sure that various sources must be taken into consideration for explaining magnetic pulsations even when the periods are in the same range. For example, PiC pulsations associated with pulsating auroras are mainly due to conductivity fluctuations in the ionosphere (OGUTI,1986), some Pc5 pulsations are mostly due to wave fields of shear Alfven mode coming from the deep magnetosphere , wave fields sometimes are greatly modified by conductivity heterogeneities (YAMAMOTO et al, 1988), and the fields could be associated with auroral particle precipitations of vortex-chain type as shown in this study. Since immediately after an auroral breakup the magnetosphere is in a transient state, inductive electric fields may also contribute to the magnetic field fluctuations on the ground.…”

“…The difference between them strongly suggests a difference in generation mechanisms of the two kinds of pulsations although the periods are in the same Pc5 range. The aurora-associated Pc5 pulsations observed in the morning sector (YAMAMOTO et al, 1988) have dominant wave field component originated from the magnetosphere, although the modification due to conductivity is taken into consideration. The Ps5 examined in this study is more likely due to the electric fields associated with the leading front of quasi-periodically injected plasma during the course of a substorm.…”

Ground-Space Observations of Pc5 Poleward Moving Auroral Arc Pulsations and Field-Line Resonances in the Post-Midnight Sector: THEMIS Observations

“…These physical processes are not well understood yet. On the other hand, the long period modulation of a diffuse patch aurora, including pulsating aurora is thought to be caused by the electron pitch-angle scattering with compressional mode Pc5 pulsations (Oguti et al, 1987;Yamamoto, 1988;Saka et al, 2014).…”