On the Evolution of the Solar Photospheric and Coronal Magnetic Field

Abstract

The Extreme Ultraviolet Imaging Telescope (EIT) and the Michelsen Doppler Imager (MDI) instruments on the Solar and Heliospheric Observatory (SOHO) ran a coordinated observing campaign over the dates 1997 August 10-14. MDI generated 12 resolution magnetograms (06 pixels) of the solar photosphere at a nominal cadence of one minute while EIT observed Fe XII (195 A) (5'' resolution; 25 pixels) in the MDI high-resolution field of view at a cadence of ~17 minutes. We investigate the relationship between the quiet-Sun photospheric magnetic field and the quiet solar corona by first removing instrumental effects from the EIT data, time-averaging the MDI data for improved statistics, and coaligning the two data sets and applying solar rotation tracking. At the time of the observation, this was the longest continuous run of its kind. We conduct a detailed investigation of the processes of evolution in the photospheric magnetic field and look for sympathetic effects in the solar corona. We measure the lifetimes, dimensions, and orientations of small-scale coronal brightenings and reconcile these processes against the evolution of the underlying photospheric magnetic field. Using statistics collected from this study, we find that emerging bipoles in quiet Sun reach a typical length of ≈14 Mm before fading or reconnecting to other flux concentrations in a time period of ≈5-12 hr, and the quiet solar corona completely decorrelates in approximately 15 hr. We find that the majority of large coronal loops in quiet Sun are the products of numerous smaller magnetic flux concentrations coalescing in the photosphere rather than the product of a single large emerging bipole. This continuous process of emergence and coalescence leads to the observation that there is no preferred orientation to small-scale coronal brightenings in the solar corona.