Ion Temperatures in the Low Solar Corona: Polar Coronal Holes at Solar Minimum

TL;DR

This study measures ion temperatures in the low solar corona's polar coronal holes, revealing non-monotonic charge-to-mass ratio dependence and comparing empirical data with theoretical wave-based heating models.

Contribution

It provides new empirical ion temperature measurements and compares them with models of ion heating, highlighting discrepancies at high charge-to-mass ratios.

Findings

Ion temperatures are higher than electron temperatures.

Ion temperature dependence on charge-to-mass ratio is non-monotonic.

Models agree for charge-to-mass ratios below 0.25, but not above.

Abstract

In the present work we use a deep-exposure spectrum taken by the SUMER spectrometer in a polar coronal hole in 1996 to measure the ion temperatures of a large number of ions at many different heights above the limb between 0.03 and 0.17 solar radii. We find that the measured ion temperatures are almost always larger than the electron temperatures and exhibit a non-monotonic dependence on the charge-to-mass ratio. We use these measurements to provide empirical constraints to a theoretical model of ion heating and acceleration based on gradually replenished ion-cyclotron waves. We compare the wave power required to heat the ions to the observed levels to a prediction based on a model of anisotropic magnetohydrodynamic turbulence. We find that the empirical heating model and the turbulent cascade model agree with one another, and explain the measured ion temperatures, for charge-to-mass ratios smaller than about 0.25. However, ions with charge-to-mass ratios exceeding 0.25 disagree with the model; the wave power they require to be heated to the measured ion temperatures shows an increase with charge-to-mass ratio (i.e., with increasing frequency) that cannot be explained by a traditional cascade model. We discuss possible additional processes that might be responsible for the inferred surplus of wave power.