Temperature and density in a polar plume – measurements from CDS/SOHO

A detailed analysis of a particularly intense polar plume observed on the 25th of October, 1996, by the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) is presented. Above the limb, emission measure distributions derived for both the plume and a section of coronal hole background are found to be sharply peaked at approximately 1.0–1.1 million degrees in both regions. The temperature rises with height in the background, but no evidence is found for a rising temperature in the plume. The density of the background is approximately 108 electrons/cm3 and falls with height. In the plume the density is between 3.8 and 9.5×108 electrons/cm3, and exhibits no decrease with height up to 70 000 km. The plume base is visible on the solar surface and shows a strong brightening lying directly below the main body of the plume. This brightening has a temperature of 2 000 000 K, and a density of 2.5–5.6 × 109 electrons/cm3. Images from lines formed at different temperatures suggest that the morphology of the base is consistent with an emerged bipole in a region of unipolar magnetic flux. A measurement of the Mg/Ne relative abundance is made at two transition region brightenings at the base of the plume. An enhancement of only 1.5 is found over the photospheric value. Considerations of the geometry of both the high temperature brightening at the base of the plume and the off-limb section give filling factors of 0.5 and 1.0, respectively.