Pulsating aurora: Local and global morphology

Abstract

Pulsating describes a low‐intensity aurora that undergoes rapid alternating increases and decreases in luminosity. Extensive new data available from ground‐based low‐light‐level television cameras and satellite scanners have allowed a detailed study of the pulsating aurora phenomenon. Intensity variations in pulsating auroras may be repetitive, quasi‐periodic, or occasionally periodic with a time scale ranging from less than 1 s to several tens of seconds. The maximum intensity does not exceed approximately 10 kR in N2+ 1 NG (first negative group) 4278 Å, and a background luminosity is often observed in association with the pulsating forms. Pulsations occur in auroral arcs, arc segments, and patches of fixed and variable area. The temporal and spatial characteristics are highly variable over a broad and continuous spectrum; rapid changes from one set of characteristics to another frequently occur, as do reversible changes from pulsating to nonpulsating auroras. Diffuse, slowly pulsating arcs occur in the evening sector immediately before and after the passage of the westward traveling surge. In the midnight sector, both arc segments and patches occupy a broad region behind the westward traveling surge. Poleward stretching torchlike structures containing pulsating patches and arc segments are often observed at the poleward boundary of the diffuse auroral oval in the midnight sector. A narrow region of pulsating arcs and eastward drifting patches occupies the morning sector, the arcs generally being located at the poleward boundary. A 3 ± 1 Hz modulation appears in more than 50% of all pulsating auroras in the midnight and morning sectors, the amplitude of modulation ranging up to 20%.

A morphological study of black vortex streets

“…Its wavelength was also 1.6 km, but it drifted eastward at 0.6 km/s. The first black curl system lasted for 8 s, and the second one lasted 5 s. These are the same two black curl systems reported by Royrvik [1976].…”

“…Black aurora has been defined as the lack of auroral emissions in well-defined regions within diffuse aurora [Royrvik, 1976;Davis, 1978] …”

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

“…A few cropped frames of the FSMI ASI at around ∼1135, 1140, and 1145 UT are given in Figure 7. For the PsA patch under investigation one can clearly see a quasiperiodic "brightening-dimming" cycle over a period of 12-18 s. The patch size is in general 40-80 km in latitudinal dimension while 100-150 km in longitudinal, typical of the PsA [e.g., Royrvik and Davis, 1977]. The patch exhibits an overall eastward bulk motion.…”

“…[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].…”

“…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]. 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.…”