2010 THEMIS SCIENCE NUGGETS

Small and Meso-Scale Properties of a Substorm Onset Auroral Arc

by Harald U. Frey

Introduction

A fortunate coincidence of simultaneous observations of a substorm onset arc by the low-altitude Japanese spacecraft Reimei together with the THEMIS satellites and the THEMIS Ground-Based Observatory (GBO) system allowed for a detailed investigation of the optical and particle properties of the arc. The in-situ measurements by the THEMIS spacecraft provided data about the source region for the auroral arc and enabled the reconstruction of the field-aligned current system. The optical observations allowed for a detailed analysis of the precipitating energy flux even in regions where the particle detector on the Reimei spacecraft was saturated. The measured and deduced arc properties were compared to theoretical predictions.

Observations

The Japanese Reimei spacecraft is equipped with three monochromatic auroral cameras and electron and ion detectors. It flies in a sun-synchronous orbit at 640 km altitude and the cameras are pointed along the local magnetic field to observe the night side aurora. On January 28, 2008 the THEMIS-GBO all-sky cameras at Inuvik and Fort Yukon stations observed the slow equatorward motion of a growth phase auroral arc which broke up for the substorm onset right at the moment when the Reimei spacecraft observed the arc from space (Figure 1).

Figure 1. Optical observations by the THEMIS GBO
all-sky cameras at Inuvik, Fort Yukon, and Fort
Simpson together with the observations by the
Reimei spacecraft. The footprints of two of the
THEMIS spacecraft are given together with a
blow-up of the Reimei images taken along the
satellite track.
Click here to enlarge the image.

THD and THE observed earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The magnetic field and flow disturbances during the flow bursts were used to identify the possible structure of the magnetic field and flow shear and thereby to infer the possible FAC direction and to check whether it was consistent with the disturbances observed in the ionosphere. Figure 2 illustrates the flow disturbances perpendicular to the magnetic field in the Y-Z plane during the bursts. Flow vectors show values relative to the average flow during the interval of the disturbance. The change in the flow pattern and possible direction of the FAC are illustrated at the bottom of the figure. The general sense of the flow disturbances was similar between TH-E and TH-D, indicating that there was the same shear flow pattern existing at ~1 RE scale in the azimuthal direction (distance between TH-E and TH-D), and suggesting a downward FAC (Figure 6, bottom left panel). The first negative then positive Vz disturbance suggests a motion of this pattern (dashed arrow), which is consistent with an eastward expansion of the azimuthally-aligned arc. The disturbance was first encountered by TH-E and then TH-D, carried downward FAC, and represented the dawnward part of a large earthward flow. Most likely there was an upward FAC further to the west that connected to the auroral activity seen on the ground.

Figure 2. Flow disturbance perpendicular
to the magnetic field at TH-D and TH-E in
the GSM Y-Z plane. Time proceeds in the
downward direction. The time interval
with colored background is the
dipolarization interval. The possible
field-aligned current configuration
as determined from velocity disturbances
is illustrated at the bottom.
Click here to enlarge the image.

Using the method of optical flow analysis the speed and direction of motion of small and meso-scale features inside the auroral arcs were determined. As the particle detectors on Reimei were saturated during a portion of the arc over flight a theoretical model was used to determine the average energy and energy flux of the energetic electrons precipitating into the arc. The measured and deduced arc properties were then used to compare to theoretical predictions about the source region, acceleration potential, field-aligned current density, and arc width and conductivities. A good agreement between observed and theoretically predicted properties of the poleward arc during substorms was achieved.

Conclusion

The lack of magnetic field measurements from the low altitude spacecraft made it difficult to determine the distribution of field-aligned currents. However it is most likely that the downward current that connects to the upward current of the newly developed onset arc was located poleward of it. Such a current distribution would fit the view of the development of the feedback instability in the downward current region [Streltsov and Lotko, 2003; 2004] together with Alfv&eaccute;n wave resonances and the formation of small-scale disturbances at the interface between the downward and upward currents as seen in the particle and optical data at the poleward edge of the large-scale auroral arc. The advection of current structuring generated through the ionospheric feedback instability could contribute to the fast motion of small-scale structures on the poleward boundary of the arc. Alternatively the structuring observed may have been a consequence of the action of current sheet instabilities through the acceleration region [Seyler, 1990; Seyler and Wu, 2001; Wu and Seyler, 2003] where it seems likely that the action of the tearing instability was responsible for the rope-like appearance of the arc.

Source

Frey, H.U. et al. (2010), Small and Meso-Scale Properties of a Substorm Onset Auroral Arc, J. Geophys. Res., 115, A10209, 2010JA015537.

Biographical Note

Harald Frey is a research physicist with the Space Sciences Laboratory at the University of California, Berkeley. His research interests are optical phenomena in the ionosphere, especially the aurora and transient luminous events like sprites and elves.


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