Thickness of an Auroral Curtain

As far as I am aware, I was the first to determine accurately the thickness of auroral curtains. The corona-type aurora is observed when an auroral curtain is located near the magnetic zenith. In some of the aurora photographs I took, the bottom edge of the curtain was clearly captured.

Using a star constellation map, I could measure the thickness to be about 500 m. Chapman encouraged me to publish the result, and I wrote a short paper that appeared in the Journal of Atmospheric and Terrestrial Physics. It should be noted that the reason for the thin curtain-like form of the aurora is still one of the long-standing unsolved problems. The question remains why the field-aligned currents occur in the form of thin sheets (Chapter 4).

John Wagner also simulated the auroral potential structure, which may be responsible for accelerating auroral electrons and for generating the radiation. However, there is still no agreed-upon mechanism for the acceleration process of auroral electrons. There have been a number of efforts to learn about the individual curtain-like structure of the aurora and precipitating electrons (Figure 2.26). The acceleration of charged particles in the magnetosphere and the solar atmosphere, perhaps even in galaxies, is one of the most fundamental issues in cosmic electrodynamics.

Figure 2.26. The Fast Auroral Snapshot Small Explorer (FAST) satellite passing through multiple auroral arcs at 0920:14 UT, February 6, 1997. The 110-km conjugate to the satellite is shown in the all-sky image at 10s intervals as FAST passed across from left (south) to right (north). The center panel is a "normal" format of the electron energy spectrum (integrated overall pitch angles) and shows a number of inverted-V structures. The bottom panel is the precipitated energy flux on a linear scale. The individual auroral arcs are clearly displayed here. The auroras, in particular the two arcs to the right (north), did change over the 4 minutes of the pass, so a detailed comparison between all-sky image and the particle data could not be attempted. Source: Kimball, C. Chaston, J. McFadden, G. Delory, M. Temerin, and C.W. Carlson, Geophys. Res. Lett., 25 , 2073, 1998

Figure 2.26. The Fast Auroral Snapshot Small Explorer (FAST) satellite passing through multiple auroral arcs at 0920:14 UT, February 6, 1997. The 110-km conjugate to the satellite is shown in the all-sky image at 10s intervals as FAST passed across from left (south) to right (north). The center panel is a "normal" format of the electron energy spectrum (integrated overall pitch angles) and shows a number of inverted-V structures. The bottom panel is the precipitated energy flux on a linear scale. The individual auroral arcs are clearly displayed here. The auroras, in particular the two arcs to the right (north), did change over the 4 minutes of the pass, so a detailed comparison between all-sky image and the particle data could not be attempted. Source: Kimball, C. Chaston, J. McFadden, G. Delory, M. Temerin, and C.W. Carlson, Geophys. Res. Lett., 25 , 2073, 1998

I salute Duncan Bryant's devotion to this subject. He has pursued the processes associated with the acceleration of auroral electrons for more than 20 years, in spite of the fact that his idea was not widely accepted. Bryant's efforts are well described in his recent book Electron Acceleration in the Aurora and Beyond (1999).

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