by Jonathan Rae
Substorm expansion phase onset marks the initiation of an explosive energy release in the magnetotail, converting stored magnetic energy to particle energy, that ultimately manifests itself as a sudden dramatic auroral display to the observer on the ground. Looking in detail at auroral behaviour during substorm onsets from the ground is critical to solving the time sequence of events that leads to this explosive energy release in space.
This paper details the spatial variations of the aurora at onset and during the first few minutes of the substorm expansion phase in the ionosphere, and uses these observations to predict the counterpart magnetospheric observations to search for in space probe data. Using data from the THEMIS All-Sky Imaging network, we demonstrate that the optical fluctuations along the substorm onset arc prior to auroral break-up have a periodic signature that is consistent with unstable waves developing in a well-defined region of the magnetosphere.
|Figure 1. (a) ASI data from the GILL and SNKQ THEMIS ASIs in geographic coordinates, projected to 110km altitudes at 05:52:30 UT. Overplotted are ULF wave onset contours of long-period Pi1/short-period Pi2 ULF wave activity co-located in time and space with the onset of periodic auroral fluctuations in the SNKQ THEMIS ASI from the 7th March 2007 event detailed in Rae et al. (2009). The bright region in the bottom right of each ASI shows the signature of the moon (not removed to maintain the integrity of the image) and the ground footprint of the GOES-12 spacecraft is shown by the G12 annotated symbol; (b) A zoom of the auroral fluctuations evident in Figure 1b in corrected geomagnetic coordinates and assuming an emission height of 110km. Horizontal lines denote latitudinal slices taken through this field-of-view and are discussed in the text.(c) Auroral intensity as a function of longitude and time through the central (dot-dashed) latitudinal slice in (c) for the ~3 minute interval containing auroral fluctuations.|
Click each image to enlarge.
A substorm onset occurred in the field-of-view of the SNKQ THEMIS All-Sky Imager on the 7th March 2007. Figure 1 shows the onset of auroral fluctuations along the most equatorward auroral arc at the same time and in the same location as the onset of ultra-low-frequency (ULF) fluctuations in the 30-90s period range. Also shown in Figure 1 is a close-up snapshot of the periodic auroral features, and a demonstration of how the auroral intensity at a constant latitude changes with time as the substorm expansion phase begins.
Analysis of the east-west variations as a function of time reveals that the auroral brightness of these structures grows exponentially over a few wave periods, suggesting that the auroral display is the optical manifestation in the ionosphere of the growth of a magnetospheric instability. We make the reasonable assumption that these instabilities scale along the field into the equatorial magnetosphere. We are confident of this mapping due to remarkable similarities between magnetic ULF waves observed with the same frequency as the auroral features which are observed simultaneously at geosynchronous orbit and on the ground.
We find that the azimuthal structuring of the magnetospheric counterpart to the aurora has a wavelength of ~0.34 RE, and the radial extent is ~0.7 RE in the magnetosphere. We compare these predictions and the growth rates of the east-west auroral structures to those predicted by theory for a number of magnetospheric instabilities.
|Figure 2. Estimated source location of the auroral undulations in the equatorial plane (ZGSM =0) of the magnetosphere (red). Black lines indicate contours of constant magnetic field strength in the T96 magnetic field model. Inset shows an enlarged view of the region.|
Click each image to enlarge.
We find that the near-Earth signature of substorm expansion phase onset can be constrained by studying the evolution of auroral displays using the THEMIS All-Sky Imager Array. Auroral features suggest that there exists a region of unstable wave growth in magnetosphere which is associated with periodic bands of precipitating auroral electrons. Future work will concentrate on identifying the instability from space-based measurements, and determining whether any other physical mechanism initiates this near-Earth instability, or whether the instability develops as a result of a more local process.
Rae, I. J., C. E. J. Watt, I. R. Mann, K. R. Murphy, J. C. Samson, K. Kabin, and V. Angelopoulos (2010), Optical characterization of the growth and spatial structure of a substorm onset arc, J. Geophys. Res., 115, A10222, doi:10.1029/2010JA015376.
Dr Jonathan Rae is a Research Associate at the University of Alberta, Canada.