Polar Plume Anatomy: Results of a Coordinated Observation

Abstract

On 7 and 8 March 1996, the SOHO spacecraft and several other space- and ground-based observatories cooperated in the most comprehensive observation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the morphology of the plumes observed over the south pole of the Sun during the SOHO observing campaign. Individual plumes have been characterized from the photosphere to approximately 15 R⊙ yielding a coherent portrait of the features for more quantitative future studies. The observed plumes arise from small (∼ 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrations, on chromospheric network cell boundaries. They are denser and cooler than the surrounding coronal hole through which they extend, and are seen clearly in both Feix and Fexii emission lines, indicating an ionization temperature between 1.0–1.5 x 10⁶ K. The plumes initially expand rapidly with altitude, to a diameter of 20–30 Mm about 30 Mm off the surface. Above 1.2 R⊙ plumes are observed in white light (as ‘coronal rays’) and extend to above 12 R⊙. They grow superradially throughout their observed height, increasing their subtended solid angle (relative to disk center) by a factor of ∼10 between 1.05 R⊙ and 4–5 R⊙ and by a total factor of 20–40 between 1.05 R⊙ and 12 R⊙. On spatial scales larger than 10 arc sec, plume structure in the lower corona (R < 1.3 R⊙) is observed to be steady-state for periods of at least 24 hours; however, on spatial scales smaller than 10 arc sec, plume XUV intensities vary by 10–20% (after background subtraction) on a time scale of a few minutes.