Offset Power-law Dependence of the Sun's Radial Electron Density Profile: Evidence and Implications

The radial electron density profile ne(r) of the Sun's corona and solar wind contains information on the sources, heating, and acceleration of the coronal and solar wind plasma. Currently, several empirically derived density models are used to describe the corona, with varying degrees of success and little physical justification or predictive power. The offset power-law (OPL) profile , with radial offset r0 and power-law index , models radial outflow from r0 that conserves total electron number and may be accelerated and heated (affecting ), thus having physical significance and predictive power. We fit the OPL model to multiple sets of published radial density profiles obtained from spectroscopic, white light, and radio data from different regions on the Sun and during different periods of solar activity. The spectroscopic and white light data yield r0 = (1.02 0.06) RS, where the uncertainties are standard errors of the mean, and , consistent with plasma originating near the chromosphere and acceleration similar to the nominal Parker solar wind model. Comparisons with time-lapse coronagraph and spectroscopic observations are favorable and show evidence for significant variations with position and time. These are expected given the corona's well-known asymmetries, three-dimensional structures, and time variability. Radio burst data yield flatter profiles < 2, suggesting that pre-flare activity alters the density profile by increasing the coronal density at large heights. We discuss the possible interpretations and implications for coronal physics and solar radio bursts.