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

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 R . 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 R , 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 R 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 R in the visible, remains very stable.

Low polarised emission from the core of coronal mass ejections

“…Low polarisation in coronagraph images can have several explanations: 1) F-corona emission (e.g., Morgan & Habbal 2007), 2) Thomson scattering from enhanced plasma density far away from the POS (e.g., Billings 1966), and 3) Hα emission (e.g., Poland & Munro 1976). F-corona contributions can be ruled out because it forms a diffuse background and does not vary rapidly in time.…”

Automatically Detecting and Tracking Coronal Mass Ejections. I. Separation of Dynamic and Quiescent Components in Coronagraph Images

“…The errors involved are far less than those introduced by using simple time differencing to reveal CME signal, for example. In fact, the errors inherent in the radiometric calibration of coronagraphs and other uncertainties related to F-corona and stray light subtraction (see Morgan & Habbal 2007c for example) will in the case of most bright CMEs outweigh any systematic errors introduced by the separation method.…”

Mapping the Structure of the Corona Using Fourier Backprojection Tomography

“…In the context of LASCO observations, upB has been described previously by Llebaria et al (1999). It has also been used by Morgan & Habbal (2007c) as a proxy for the F corona, and by Morgan et al (2006) as a background subtraction suitable for image processing. The set of upB images covering ∼ half a solar rotation are then combined into an average upB image, or an upB image.…”

“…The set of upB images covering ∼ half a solar rotation are then combined into an average upB image, or an upB image. upB images calculated for LASCO C2 are remarkably stable over the solar cycle at heights above ∼2.6 R (Morgan & Habbal 2007c). The upB image is subtracted from the set of ∼200 B t images.…”