Are there optical differences between storm-time substorms and isolated substorms?

Abstract

Abstract.We have performed an extensive analysis of auroral optical events (substorms) that occurred during the development of the main phase of magnetic storms. Using images from the Earth Camera on the Polar spacecraft (Frank et al., 1995), we compared the optical emission features of substorms occurring during 16 expansion phases of magnetic storms with the features of isolated substorms occurring during non-storm times. The comparison used two techniques, visual inspection and statistical comparisons. The comparisons were based on the common characteristics seen in isolated substorms that were initially identified by Akasofu (1964) and quantified by Gjerloev et al. (2008). We find that when auroral activity does occur during main phase development the characteristics of the aurora are very dissimilar to those of the classical isolated substorm. The primary differences include the lack of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion phases, and greater intensities.Since a surge/bulge and bifurcation of the aurora are characteristics of the existence of a substorm current wedge, a key component of the magnetosphere-ionosphere current system during substorms, the lack of this component would indicate that the classical substorm model does not apply to the storm time magnetosphere-ionosphere current system. Rather several of the analyses suggest that the storm-time substorms are associated more closely with the auroral oval, at least spatially, and, therefore, probably with the plasma sheet dynamics during the main phase development. These results then must call into question the widely held assumption that there is no intrinsic difference between storm-time substorms and classical isolated substorms.

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

“…The most significant differences of the two substorm types are the lack of surges/bulges and bifurcations in multi-cycle substorms. The absence of these substorm characteristics is interpreted by Hoffman et al (2010) as a direct evidence for a changed 3-D current system during magnetic storm intervals that connects the magnetosphere and ionosphere. This comprises, in particular, the existence of a substorm current wedge that forms the ionospheric part of the FAC system.…”

“…the variations in shapes and sizes are much larger for multi-cycle substorms in contrast to isolated substorms. Hoffman et al (2010) relate the specified differences between the two substorm types to the intensifications of the large-scale current system and the electric field of magnetospheric convection as well as to the restructuring of the plasma domains in the nightside magnetosphere during magnetic storms, to the more intense electron precipitations, and to the enhanced integrated conductivity of the ionosphere. The most significant differences of the two substorm types are the lack of surges/bulges and bifurcations in multi-cycle substorms.…”

“…Structured forms that belong to different substorm phases appear in the television camera field of view. Hoffman et al (2010) compared the optical signatures of substorms that occurred during the development of the main phase of magnetic storms (disturbed or multi-cycle substorms) with the optical signatures of isolated substorms as defined by Akasofu (1964a). For the selection of substorm intervals they used only optical observations of the Polar spacecraft in the far ultraviolet (124-149 nm), supplemented by 557.7 nm visible images.…”

Annual fractions of high‐speed streams from principal component analysis of local geomagnetic activity

“…This is because the relative effect of CMEs is largest at these latitudes, reflecting the different behavior of auroral electrojets during CME-and HSS-related substorms. It is well known that under the influence of an intense solar wind electric field (especially in storm times) the polar cap and the auroral oval expand toward the equator and, consequently, substorm onsets (largest disturbances during a substorm) occur at lower latitudes than under a weaker electric field [Hoffman et al, 2010]. Since the average solar wind electric field during CMEs is stronger than during HSSs, it is expected that CME substorms occur at lower latitudes than HSS substorms, thus leading to the fact that the relative contribution of CMEs is larger at subauroral stations than at other stations.…”

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

“…When a double oval configuration exists, the poleward arc maps close to the OCB, at least in the nightside ionosphere (Elphinstone et al, 1995a). Accurate modelling of the auroral luminosity intensity profile during a double oval configuration is required for the best estimation of the OCB location, particularly during substorm recovery when this configuration is prevalent (Elphinstone et al, 1995b), although substorms occurring during ongoing geomagnetic activity do not always exhibit the classical evolution of isolated substorms with auroral bifurcation (Hoffman et al, 2010).…”