Solar coronal-hole plasma densities and temperatures

Abstract

Polar plumes extending from the Sun into the solar corona have long been seen during eclipses, and can now be studied without this restriction with telescopes and spectrometers on board of spacecraft. Despite the large amount of observational data available on this prominent phenomenon, it is not clear whether plumes contribute substantially to the fast solar-wind streams emanating from coronal holes. An understanding of the processes leading to the formation of bright plumes and the surrounding darker inter-plume regions in coronal holes requires a good knowledge of the physical conditions in plumes and their environment. This investigation aims at measuring the electron densities and temperatures in these regions with the help of radiance ratios of ultraviolet emission lines obtained by SUMER on SOHO. It finds densities of about 7 × 10 7 cm −3 in bright plumes and 1.3 × 10 7 cm −3 in inter-plume lanes at ≈45 Mm above the limb. At this height, the total plume cross-section relative to the size of the coronal hole was found to be less than 8%. The densities drop by a factor of roughly two over the next 80 Mm in height, in lanes a little less than seen in plumes. In this height range, the electron temperatures in plumes are ≈7.5 × 10 5 K and ≈1.13 × 10 6 K in inter-plume regions. The effective ion temperatures, deduced from the line widths, are higher and nearly independent of the altitude in plumes, whereas they increase in inter-plume regions, starting from an even higher level. No systematic dependence of the line-of-sight bulk velocities on the brightness could be found in the coronal-hole plasma.

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…Such an increase was not seen in plumes. Other spectroscopic studies also indicated an increase of the electron temperature with height in coronal holes (Doschek et al 2001) and, specifically, in inter-plume regions (Wilhelm 2006). In Sect.…”

“…Using CDS (GIS) observations of plasma structures on the disk, Del Zanna et al (2003) obtained a density of n e 1.2 × 10 9 cm −3 for a plume, about twice the value of the surrounding coronal hole plasma. Wilhelm (2006) studied a south-polar coronal hole in May 2005 and found a density ratio of ≈5 between plumes and inter-plume regions, with plume material occupying less than 8 % of the intervening space along the LOS.…”

Solar Coronal Plumes

“…The interested reader can find a thorough description of the line ratio techniques in Mason and Fossi (1994): as the methods described earlier, these techniques are affected by density or temperature inhomogeneities along the LOS. Typically, ratios of the Si viii 1446 and 1440 and Si ix 342 and 345Å line intensities have been used to evaluate densities, while temperatures have been inferred from the ratios of the O vi 1730 and 1032 and of the Mg ix 706 and 750Å line intensities (e.g., Del Zanna and Bromage, 1999;Mohan et al, 2000;Wilhelm, 2006;Banerjee et al, 2009).…”

Propagating intensity disturbances in polar corona as seen from AIA/SDO

“…Plumes and interplumes are characterized by slightly different properties. Plumes are denser and cooler than interplume regions (e.g., Wilhelm 2006). Interplume regions exhibit broader line profiles than plumes (Banerjee et al 2000b).…”