A study of the background corona near solar minimum

Abstract

The white light coronagraph data from Skylab is used to investigate the equatorial and polar K and F coronal components during the declining phase of the solar cycle near solar minimum. Measurements of coronal brightness and polarization brightness product between 2.5 and 5.5R.) during the period of observation (May 1973 to February 1974 lead to the conclusions that: (1) the equatorial corona is dominated by either streamers or coronal holes seen in projections on the limb approximately 50% and 30% of the time, respectively; (2) despite the domination by streamers and holes, two periods of time were found which were free from the influences of streamers or holes (neither streamers nor holes were within 30 ~ in longitude of the limb); (3) the derived equatorial background density model is less than 15% below the minimum equatorial models of Newkirk (1967) and Saito (1970); (4) a spherically symmetric density model for equatorial coronal holes yields densities one half those of the background density model; and (5) the inferred brightness of the F-corona is constant to within + 10% and +5% for the equatorial and polar values, respectively, over the observation period. While the F-corona is symmetric at 2Ro it begins to show increasing asymmetry beyond this radius such that at 5Ro the equatorial F-coronal brightness is 25% greater than the polar brightness.

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

“…The radial profiles of total and polarized brightness are shown in Figs. 2a and b respectively, along with values found by Saito et al (1977) for Skylab observations made during 1973/05-1974/02. The profiles found by LASCO in these figures show, as expected, a much higher brightness at the equators compared to the poles.…”

“…-in studying the dynamic K corona, B F is useful since it may be subtracted from total brightness observations to isolate B K (e.g., Saito et al 1977); -B F gives information on the composition and spatial distribution of interplanetary dust, and the influence of the Sun on the dust (e.g., Mann 1992). A comprehensive review of interplanetary dust and the F corona can be found in Gruen et al (2001), and references within.…”

“…This study is greatly facilitated by the high quality observations made by LASCO C2 over a solar cycle. An established inversion method to isolate the F coronal brightness (Saito et al 1977), using LASCO C2 data collected near the minimum of solar activity, is presented in Sect. 2.…”

VLA Measurements of Faraday Rotation through Coronal Mass Ejections

“…Several models employ two or three power law terms for the plasma density (e.g., see Pätzold et al 1987) or employ different model parameters depending on the region of the corona that is sampled (e.g., see Saito et al 1977). Similarly, the magnetic field is often represented by a dual power law in r, such as the sum of a dipole (∝ r −3 ) and interplanetary magnetic field term (∝ r −2 ) (Pätzold et al 1987;Mancuso & Spangler 2000;Kooi et al 2014).…”

The association of aJ-burst with a solar jet

“…We used the radial electron density model of Saito et al (1977) to estimate the height at the emission at 33 MHz occurs (the frequency where F2 suddenly stops). We chose this density model as the J-burst does not to originate from an active region with high densities but from an area of bipolar plage with lower densities.…”