Combined optical, EISCAT and magnetic observations of the omega bands/Ps6 pulsations and an auroral torch in the late morning hours: a case study

Abstract

Abstract. We present here the results of multi-instrument observations of auroral torch and Ps6 magnetic pulsations, which are assumed to be the magnetic signature of the spatially periodic optical auroras known as omega bands. Data from TV and ASC cameras in Barentsburg and NyÅlesund, EISCAT radars in Longyearbyen and Tromsø, as well as IMAGE network were used in this study. The auroral phenomenon which was considered differed from that previously discussed, as it occurred both in an unusual place (high latitudes) and at an unusual time (late morning hours). We show that this might occur due to specific conditions in the interplanetary medium, causing the appropriate deformation of the magnetosphere. In such a case, the IMF turned out to be an additional factor in driving the regime of Ps6/omega bands, namely, only by acting together could a substorm onset in the night sector and B z variations result in their generation. Since the presumable source of Ps6/omega bands does not co-locate with convection reversal boundaries, we suggest the interpretation of the phenomena in the frame of the interchange instability instead of the Kelvin-Helmholtz instability that is widely discussed in the literature in connection with omega auroras. Some numerical characteristics of the auroral torch were obtained. We also emphasize to the dark hole in the background luminosity and the shortlived azimuthally-restricted auroral arc, since their appearance could initiate the auroral torch development.

Midnight sector observations of auroral omega bands

“… Lühr and Schlegel [1994] described omega bands as “a luminous band from which tongue‐like protrusions extend toward the north” with the bright tongues shaped like a Greek Ω and the dark area separating adjacent tongues shaped like an inverted Ω. In recent research, the term omega band has been used to described all of the above variants on what is assumed to be the same basic auroral structure [ Syrjäsuo and Donovan , 2004; Safargaleev et al , 2005; Vanhamäki et al , 2009].…”

“…For example, whereas Akasofu and Kimball's omega bands were distorted arcs, Lyons and Walterscheid [1985] presented observations of omega bands with a dark, inverted W shape formed by bright torches extending poleward from the auroral oval, and Opgenoorth et al [1994] reported "streets" of multiple omega band structures in which undulations on the poleward boundary gave rise to alternating bright humps and dark bays. Lühr and Schlegel [1994] described omega bands as "a luminous band from which tongue-like protrusions extend toward the north" with the bright tongues shaped like a Greek W and the dark area separating adjacent tongues shaped like an inverted W. In recent research, the term variants on what is assumed to be the same basic auroral structure [Syrjäsuo and Donovan, 2004;Safargaleev et al, 2005;Vanhamäki et al, 2009].…”

A numerical simulation for the omega band formation

“…[3] The close connection between eastward drifting omega bands and Ps6 pulsations with periods of 5-40 min has been nearly established by the efforts of a number of authors [e.g., Saito, 1978;Kawasaki and Rostoker, 1979;Baumjohann, 1979;Gustafsson et al, 1981;André and Baumjohann, 1982;Opgenoorth et al, 1983;Wild et al, 2000;Safargaleev et al, 2005;Vanhamäki et al, 2009]. The major source of magnetic pulsations on the ground is thought to be a "meandering" ionospheric Hall current in the presence of a sequence of east-west oriented pairs of upward and downward field-aligned currents (FACs); the former flows out of the bright tongue of the omega band and the latter flows into the dark hole.…”

Non-triggered auroral substorms and long-period (1–4 mHz) geomagnetic and auroral luminosity pulsations in the polar cap

“…Although there are numerous papers describing aurora modulation by Pc5 pulsations (see, e.g., Xu et al, 1993;Safargaleev et al, 2005;Roldugin and Roldugin, 2008), amplitudes of geomagnetic and auroral luminosity pulsations are controlled by different physical factors. Thus, geomagnetic ULF power at high latitudes is essentially controlled by SW or IMF parameters, while the fluctuations of auroral luminosity are more local and their amplitudes depend on the energy distribution of particles at a particular L shell.…”