2016 THEMIS SCIENCE NUGGETS
Statistical properties of substorm auroral onset beads/rays
by Toshi Nishimura
University of California Los Angeles and Boston University
Introduction
Substorm auroral onset is characterized by initial brightening near the equatorward boundary of the auroral oval, often along a pre-existing growth-phase arc. Substorm onset is associated with auroral initial brightening and waves along the arc, suggesting an instability process at substorm onset. Although a number of studies showed existence of optical waves during substorm initial brightening, wave properties vary event-by-event. So far, none of the existing studies have addressed how often onset waves occur. To evaluate the importance of onset waves on initial brightening, we should examine if all substorms are associated with waves during initial brightening or if some substorms occur without waves (hence may be triggered by a different process). Another key question is how properties of onset waves in each wave propagation direction relate to plasma sheet conditions. Wave properties and their geomagnetic activity dependence can provide a clue for understanding substorm onset instability but require a large number of events by distinguishing different propagation direction types.
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| Figure 1. Three types of onset wave propagation. Panels shown are east-west keograms of detrended intensity along the onset arc for (left) westward, (middle) bi-directional and (right) eastward propagation events. |
In the present study, we select 112 isolated substorm events and statistically determine occurrence properties of onset waves. We characterize wave properties, their geomagnetic activity dependence and their relation to arc orientation. We also discuss possible connections to kinetic ballooning interchange instability, and plasma sheet structures.
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| Figure 2. Scatter plots between (a) onset MLAT and IMF Bz before onset, (b) onset MLAT and ΔAL, and (c) onset MLT and IMF By (median By within 30 min before onset). Red, green and blue dots indicate westward, bi-directional and eastward propagating waves. |
Results
Figure 1 shows three representative events of onset waves with different propagation directions (left: westward, middle: bi-directional, and right: eastward propagation). The first vertical line in each event marks the initial rise of auroral intensity, i.e., initial brightening, and it coincides with intensification of optical waves. The westward-propagating wave case had pre-existing waves propagating from high to low magnetic longitudes (MLON) along the growth-phase arc without changing wave intensity, and then started to intensify without changing the wavelength. The entire east-west extent of the waves (except a small gap between the two imagers) was covered by the available imagers. The bi-directional wave case did not have notable waves prior to the onset but waves emerged at the initial brightening. Waves on the eastern side propagated eastward, and waves on the western side propagated westward. In the eastward-propagating wave event, the waves emerged initially in a ~5° longitude range and then propagated eastward, while the wave occurrence region also spread westward. The wave propagation was slower than in the westward-propagation event. The waves continued to grow at least until the initiation of the poleward expansion (second vertical lines). The wave signatures can still be found during the early phase of poleward expansion, but the wave intensity, wavelength and propagation speed change and become irregular. While the wave occurrence region spread azimuthally, the wavelength and propagation speed stayed almost the same until the initiation of the poleward expansion. The latitudinal extent of the initial brightening arc was essentially the same as the growth phase arc until initiation of the poleward expansion.
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| Figure 3. Selected plots of correlations among onset wave parameters. The black line in panel a) is a fit by γ=8/Δt curve. The black solid, grey and dashed lines in Figures 6c and d indicate kinetic model predictions by PC2010, P2010 and L1991, respectively. |
We have examined wave existence and wave propagation direction of 112 isolate substorm events detected in the THEMIS ASIs. Nearly half the events (42.9%) show eastward propagation. The second biggest category is bi-directional, and the rest of the events are westward-propagating. Within our dataset, all events showed onset waves, indicating that optical waves are common features of substorm onset. Figure 2 shows dependence of wave propagation directions on geomagnetic conditions and onset locations. The onset latitude shifts equatorward with stronger solar wind driving. Most events occurring below 65° MLAT are eastward-propagating events, and each wave propagation direction at typical onset latitudes (~67° MLAT) occurs almost equally. Similarly, most events under large southward IMF show eastward propagation. This tendency indicates that plasma sheet conditions related to the solar wind energy input during the growth phase are important for determining wave propagation directions.
Figure 3 shows scatter plots of selected combinations of wave properties. The highest correlation was found between the wave growth rate and wave duration. The growth rate is negatively correlated with the wave duration, meaning that waves with larger growth rates evolve more quickly to poleward expansion, while waves with smaller growth rates last longer. The data points approximately follow the gamma*delta_t=3 curve, and thus waves evolve to poleward expansion at a common level of exponential growth after initial brightening regardless of the growth rate. Similarly, the duration of the wave growth is longer for slower propagating waves. The wavelength is roughly proportional to the wave velocity, and the growth rate is larger for shorter periods. Comparisons to kinetic simulations show that the kinetic ballooning instability gives the closest estimate, while the other models give larger growth rate and propagation speed.
Conclusion
Auroral substorms are often associated with optical ray or bead structures during initial brightening (substorm auroral onset waves). Occurrence probabilities and properties of substorm onset waves have been characterized using 112 substorm events identified in THEMIS all-sky imager data and compared to RCM-E and kinetic instability properties. All substorm onsets were found to be associated with optical waves, and thus, optical waves are a common feature of substorm onset. Eastward propagating wave events are more frequent than westward propagating wave events and tend to occur during lower-latitude substorms (stronger solar wind driving). The wave propagation directions are organized by orientation of initial brightening arcs. We also identified notable differences in wave propagation speed, wavelength (wave number), period, and duration between westward and eastward propagating waves. In contrast, the wave growth rate does not depend on the propagation direction or substorm strength but is inversely proportional to the wave duration.
This suggests that the waves evolve to poleward expansion at a certain intensity threshold and that the wave properties do not directly relate to substorm strengths. However, waves are still important for mediating the transition between the substorm growth phase and poleward expansion. The relation to arc orientation can be explained by magnetotail structures in the RCM-E, indicating that substorm onset location relative to the pressure peak determines the wave propagation direction. The measured wave properties agree well with kinetic ballooning interchange instability, while cross-field current instability and electromagnetic ion cyclotron instability give much larger propagation speed and smaller wave period.
Reference
Nishimura, Y., J. Yang, P. L. Pritchett, F. V. Coroniti, E. F. Donovan, L. R. Lyons, R. A. Wolf, V. Angelopoulos, and S. B. Mende (2016), Statistical properties of substorm auroral onset beads/rays, J. Geophys. Res. Space Physics, 121, 8661–8676, doi:10.1002/2016JA022801.Biographical Note
Toshi Nishimura is an Associate Researcher at University of California, Los Angeles and Research Associate Professor at Boston University. His primary research interests are magnetosphere-ionosphere coupling at high-latitudes.
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