The quiet corona: Temperature and temperature gradient

Abstract

A study of the lower corona thermal properties was made using the best examples of solar wind heavy ion spectra obtained with Vela 5 and 6 plasma analyzers at times of quiet solar wind (low speed, low temperature). The multiple Si and Fe ion species peaks in the spectra were fit with solutions of the ionization equilibrium equations to determine ‘freezing in’ temperatures for the various species over a range of heliocentric distances r. Assuming a power law electron temperature model, T = =t ⊙ (R ⊙/r) α , spherical symmetry, and mass conservation, the following results for the quiet corona were obtained: (1) The average freezing in temperatures ranged near 1.5 × 106 K at r ∼- 2.4 to 3.9 R ⊙. (2) Values for T ⊙ ranged between 1.7 × 106 K and 2.5 × 106 K with an average of 1.84 ± 0.13 × 106 K. (3) The temperature gradient parameter α lay between 0.20 and 0.41 with an average value of 0.29 ± ±0.06. This is consistent with the predicted value α = 2/7 derived from conduction dominated spherically symmetric models of the corona. (4) The 0 and N lines which freeze in at a distance of r ∼-1.5 R ⊙ indicate temperatures of ∼ 2.1 × 106 K. Temperatures higher at 1.5 R ⊙ than at 3 R ⊙, in agreement with extrapolations of the power law model, suggest that coronal heating in regions of open field geometry is not important beyond r ∼- 1.5 R ⊙.