Coronal polar plumes

Abstract

The purpose of this investigation was to determine what connection exists between coronal plumes and polar surface features. To this end the properties of plumes were re-examined by making a detailed statistical analysis of photographs of three eclipses (1962, 1963, and 1965) of the last sunspot minimum. It is found that a ‘typical’ plume has a core density ≈ 10⁸ cm⁻³, a half width ≈ 3.3 × 10⁴km, and a density profile with distance r from its axis characterized by

$$N \approx {\text{10}}^{\text{8}} \left( {1 - \frac{r}{{3.9 \times 10^4 }}} \right)^{1.6} $$

There is some (although only weak) regularity in the projected spacing of plumes with a mean separation of ≈ 7 × 10⁴ km.

The relation between plumes and various surface features is examined. Although little direct evidence can be assembled, we conclude that a direct connection exists between plumes and photospheric faculae, bright K₃ faculae, and the small-scale magnetic structure present in the chromospheric network. It is hypothesized that plumes originate at the bright cores of the rosettes which lie along the chromospheric network. The distribution of magnetic field in the corona above a surface covered with idealized chromospheric network cells is calculated. The fact that the shape and size of the magnetic flux tubes originating from the rosette agrees with that of observed plumes supports the hypothesis.