Statistical description of the bulge‐type auroral substorm in the far ultraviolet

Abstract

[1] Using global auroral images at ultraviolet wavelengths during 116 substorms, we have obtained quantitative measures of key features of the bulge aurora and oval aurora: their temporal variations, their locations, rates, and characteristics of gross expansion and decay, and the variability of these parameters. The expansion period identified solely from images varied primarily from 10 to 40 minutes, with an average of 30.9 minutes. To avoid mixing expansion data with recovery data, we normalized the time of each substorm to one unit from onset to maximum expansion. The average onset location was 22.6 magnetic local time (MLT) and 66.8°invariant latitude (ILat), in good agreement with previous analyses. We found that the bulge aurora rapidly expanded out of the onset location approximately equally to the west (surge) and to the east, so that the average center of the bulge remained close to the onset MLT. This is also the case for average location of the maximum expansion in latitude of the bulge. Thus the bulge is offset about 1 1/2 hours west of midnight. By half the expansion period the bulge has usually expanded poleward sufficiently to reveal a brightened portion of the original auroral oval. This brightening expands less than 1 hour MLT to the west, but rapidly to the east, farther than the east end of the bulge. Thus the two auroras are offset in MLT. The bulge expansion is fastest initially but slows for the second half of the expansion period. The ends of the bulge continue a small expansion poleward during early recovery when the center of the bulge slowly retreats. The large spreads of substorm expansion times, the onset locations and in the locations in ÁMLT and ÁILat of the key features of these auroras, argue strongly for the need to normalize the time of expansion and location of key features of the substorm for any kind of superposed epoch analysis to be meaningful.

Investigations of the auroral luminosity distribution and the dynamics of discrete auroral forms in a historical retrospective

“…Disturbances were classified as substorms, if they were accompanied by a poleward bulge expansion from the auroral oval near midnight and by the development of an auroral surge in the evening sector to the west of the bulge. Gjerloev et al (2007) found that the Akasofu type substorm is the predominant type of auroral events for isolated substorms. In 116 cases of all the selected events, the auroral disturbances developed according to the classical scheme of Akasofu (1964a), and only 20 further cases did not display the classical characteristics.…”

“…21 proves to be the most complete generalisation about the conception of a single substorm development in the auroras and its description by Feldstein et al (2010). Gjerloev et al (2007) determined how often auroral manifestations of polar substorms occur according to the scheme shown in Fig. 20 (Akasofu, 1964a), but in a somewhat simplified manner.…”

“…The selection of the events was based both on their optical characteristics and the AL index pattern around the time of events. Detailed descriptions of the criteria used for the selection of events during the winter seasons of the years 1997-2011 is given by Gjerloev et al (2007) and Gjerloev et al (2008). Disturbances were classified as substorms, if they were accompanied by a poleward bulge expansion from the auroral oval near midnight and by the development of an auroral surge in the evening sector to the west of the bulge.…”

A superposed epoch analysis of auroral evolution during substorm growth, onset and recovery: open magnetic flux control of substorm intensity

“…In this paper we investigate the evolution of the aurora during the growth, expansion, and recovery phases of substorms in a quantitative, statistical fashion. Previous workers have undertaken similar studies, for instance Mende et al (2003), Coumans et al (2007) and Gjerloev et al (2007), providing much insight into the average characteristics of the auroral evolution during substorms. However, our primary focus in this paper is to examine the role of the substorm in solar windmagnetosphere coupling, specifically in the closure of open flux previously accumulated through magnetic reconnection at the dayside magnetopause.…”

Estimating the location of the open-closed magnetic field line boundary from auroral images

“…These techniques have included visual inspection (e.g., Elphinstone et al, 1990;Gjerloev et al, 2007), identifying a return to a threshold of low auroral luminosity poleward of the main emission (e.g., Brittnacher et al, 1999;Baker et al, 2000), identifying a return to a fraction of the peak intensity on the poleward edge of the main emission (e.g., Kauristie et al, 1999;Baker et al, 2000;Carbary et al, 2003;Boakes et al, 2008), and identifying a return to an intensity threshold relative to background intensity levels (e.g., Mende et al, 2003;Gjerloev et al, 2008). Carbary et al (2003) estimated over 23 000 poleward auroral luminosity boundaries (PALB) from images recorded by the ultraviolet imager (UVI) on the Polar satellite, by modelling latitudinal profiles of auroral luminosity intensity as a Gaussian function superimposed on a quadratic background.…”