2008 THEMIS SCIENCE NUGGETS

INTENSIFICATION OF PREEXISTING AURORAL ARC AT SUBSTORM EXPANSION PHASE ONSET:
Wave-like Disruption During the First Tens of Seconds

by Jun Liang

Introduction

The onset of substorm expansion phase (EP) typically manifests itself in the ionosphere as a sudden brightening (or "breakup") of a preexisting arc. Many believe that the secret of substorm physics is locked in the tens of seconds surrounding the onset. To date, our attempts to answer the 10-second question have been hampered by the lack of global auroral observations of sufficiently high temporal resolution. The THEMIS mission contains a continent-wide all-sky imager (ASI) array which provides an unprecedented combination of mesoscale imaging at 3-sec cadence. The high-resolution THEMIS ASI observations allow us to establish the onset morphology and evolution in great detail, as well as much-needed observational constraints against which theoretical proposals are to be tested. In this paper, we report first results from a survey of THEMIS ASI data with an emphasis on a few tens of seconds around the onset.

THEMIS ASI Observations

Our event database is selected from the THEMIS ASI observations based on good viewing conditions and tractability of auroral brightening and expansions. In all presentations of the images we assume an emission height of 110 km, and use the altitude-adjusted corrected geomagnetic coordinate to define magnetic latitude (MLAT) and longitude (MLON).

The first event occurred on February 22, 2006. Figure 1 shows a sequence of the images of the event. Prior to onset, the auroras are stationary and dark inside the field of view of the ASI, with a predominantly azimuthally aligned quiet arc above 66? MLAT. Starting from 06:28:06 UT we see a few "spurs" sprouting from the original arc to lower latitude, marking the first signature of the disruption of the growth phase arc. The "spur" grew in the next couple of frames into an equatorward displaced arc segment in which the initial signature of the auroral intensification developed. The luminosity enhancement occurred both on the original arc and the new lower-latitude arc, whose formation over an extended azimuthal range becomes quite visible after 06:28:24. The overall auroral intensity was strongly enhanced in the following frames, marking substorm auroral breakup. A key feature constituting our core interest is that the auroral intensification developed in distinct forms of azimuthally-spaced structures. For example, immediately following the onset, a number of discrete spots were quite conspicuous and spanned ~1h of longitude in the 06:28:30 frame, while the subsequent brightening appeared to be the clear wave-like auroral intensification within the same longitudinal extent. After 06:28:39 noticeable poleward auroral expansion merged the original and new arcs into an overall latitudinally broad structure.

THEMIS ASI images from February 22, 2006.
Figure 1. A sequence of THEMIS ASI images showing
a substorm auroral breakup on Feburary 22, 2006.
Click here to enlarge the image.

The second event occurred on January 27, 2006. Figure 2 shows a sequence of auroral images observed by the Fort Yukon ASI. The auroral breakup does not began on any identifiable stable growth phase arc but instead on an arc which formed and slowly intensified ~1 min prior to the onset. Accompanying its gradual intensification are growing "ripples" in the new arc. Such wave-like modulation along the arc was quite noticeable by 10:02:18. The 10:02:24 frame is identified as the onset frame, from which we see near-simultaneous brightening over a ~10? longitudinal range. The intensification structures are embedded in the preexisting arc and also exhibited wave-like features as seen in 10:02:30 and 10:02:36 frames. From 10:02:36 to 10:02:42 UT we see that a part of the arc elevated in latitudes and became a tilted structure. Subsequently, the substorm auroras evolved into an undulation style featuring both latitudinally folded structures and longitudinally quasi-periodic structures.

THEMIS ASI images from January 27, 2006.
Figure 2. A sequence of THEMIS ASI images showing
a substorm auroral breakup on January 27, 2006.
Click here to enlarge the image.

As a summary of the above two events, we have shown that the onset appears to occur over a longitudinally extended (10-15?) arc segment and characterized by distinct wave-like disruptions. Although the two events described here are different in morphology, they both ultimately grow to have three luminosity peaks separated by 2-2.5? MLON and an overall bright arc spanning about ~1 h MLT. This behavior suggests that the underlying instability is similar for two events. The e-folding growth time Tg of the total substorm auroral brightness is found to be 9s for event 1 and 12s for event 2. Examination of other THEMIS ASI onset events suggests that azimuthal wave-like auroral activation is a quite common feature. We estimated wavelength and the growth timescale for all 8 events. The wavelength ranges between 1-3.4? MLON in the ionosphere, or roughly 800-2700 km at the NGPS, comparable to the plasma sheet ion gyroradius. The growth timescale is between 10-30 s.

Conclusion

In this study we focused on a few tens of seconds around the substorm EP onset and found two important details of arc intensification: (a) the pattern of intensification is neither uniform nor random, but features a wave-like action with a high longitudinal wave number between 100 and 300; (b) the auroral breakup spreads rapidly to a ~1h longitudinal extent within ~10s of the onset while the subsequent expansion is much slower or stalls. The two pieces of observations hint that the breakup is due to simultaneous destabilization of a wave mode over a finite azimuthal extent of the current sheet. The underlying instability is likely kinetic in nature, or a MHD instability significantly modified by the finite Larmor-radius effect.

Biographical Note

Jun Liang is a Visiting Fellow at Canadian Space Agency. His main research interest is a multi-instrument study of substorm process.



Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu