Letter to the Editor

The long-term stability of the visible F corona at heights of 3–6 R_\odot

Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA e-mail: hmorgan@ifa.hawaii.edu

Accepted: 3 July 2007

Abstract

Context.CMEs can effect the distribution of dust grains in the corona. The brightness of the visible F corona is expected therefore to change as the frequency of CMEs varies with solar cycle.

Aims.We search for a variation in the F corona by comparing LASCO C2 observations from solar minimum and maximum.

Methods.An established inversion method is used to calculate the visible F corona brightness from LASCO C2 solar minimum observations made during 1996/10. Good agreement is found with the F corona brightness calculated from Skylab observations during 1973/05–1974/02 for heights of 3–6 . The unpolarized brightness, which is dominated by the unpolarized F corona brightness at these heights, is obtained by subtracting many pairs of polarized brightness images from total brightness images and averaging over a solar rotation. We calculate the unpolarized brightness for both solar activity minimum and maximum.

Results.The unpolarized brightness, and therefore the F corona, remain virtually unchanged between solar minimum and maximum at heights above 2.6 , despite the large change in the shape and activity of the corona. Using a simple density model, it is shown that the small variation in unpolarized brightness seen below 2.6  can arise from differences in the distribution of electron density, and therefore cannot be attributed to a variation in the F corona.

Conclusions.Despite the large rise in frequency of CMEs from solar minimum to maximum, the F coronal brightness, at heights of 3–6  in the visible, remains very stable.