A theoretical study of the seasonal and solar cycle variations of stable aurora red arcs

Abstract

SAR arc statistics provide information on the seasonal and solar cycle variations in the subauroral region electron temperature peak and associated magnetospheric energy source. There are two sources of long‐term (solar cycle and seasonal) variability in the magnitude of the subauroral region electron temperature peak and associated SAR arc emission intensity: (1) the neutral atmosphere and ionosphere and (2) the magnetospheric energy source. The results of this study indicate that the observed seasonal variation in SAR arc intensities can be explained reasonably well by seasonal variations in the neutral atmosphere and ionosphere. True solstice effects are unlikely to result from differences in a near‐equatorial magnetospheric heat source since the same heat source supplies both the summer and the winter hemispheres at opposite ends of a common flux tube. Observed solar cycle variations in SAR arc intensity for a fixed ring current strength (as represented by the Dst index) are not consistent with variations predicted solely on the basis of a solar cycle changes in the neutral atmosphere and ionosphere. A reduction of the magnetospheric heat flux by a factor of between 5 and 20 from solar maximum to solar minimum conditions is necessary to bring model electron temperatures and 6300Å emission intensity into agreement with observational results for moderately disturbed conditions (Dst ∼ −80γ). The required reduction in the magnetospheric energy source with decreasing solar cycle is attributed to compositional changes in the magnetospheric plasma.

Correlations between SAR arc intensity and solar and geomagnetic activity

“…It is immediately obvious that for the same heat¯ow, the electron temperature is hotter in the presence of the low electron density in agreement with the conclusions of Kozyra et al (1990) and Fok et al (1991a, b). However, changes in the electron density do not signi®cantly eect the electron temperature (in comparison with those obtained by Kozyra et al (1997) and Fok et al (1991a, b) resulting from the given heat ux of electrons as demonstrated in Fig.…”

“…4, where I and I/J are plotted against the number of days in the year. The reason for these variations may be due to dierences in the high-altitude heat source and to seasonal changes in the atmosphere and ionosphere (Kozyra et al, 1990).…”

Perturbations of the upper atmosphere in the cleft region

“…A quantitative analysis of this interplay has been performed by Kozyra et al (1990). These authors demonstrate that it is the cooling rate of electrons to ions, not the heating rate of electrons by precipitating particles and subsequent downward heat conduction, which dominates solar cycle variations in F-region electron temperatures and solar cycle SAR arc intensities.…”

Steve: The Optical Signature of Intense Subauroral Ion Drifts

“…Pedersen et al (2007) reported feint optical structures as far as 10 • equatorward of the traditional auroral oval. At these temperatures there are sufficient high-energy electrons among the thermal population to excite SAR arcs (Cole, 1965;Kozyra et al, 1990). Stable auroral red (SAR) arcs are likely the subauroral optical phenomena that have been most thoroughly studied.…”