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Measurements of Anisotropic Ion Temperatures, Non-Thermal Velocities, and Doppler Shifts in a Coronal Hole

Hahn, Michael; Savin, Daniel Wolf

We present a new diagnostic allowing one to measure the anisotropy of ion temperatures and non-thermal velocities, as well as Doppler shifts with respect to the ambient magnetic field. This method provides new results, as well as an independent test for previous measurements obtained with other techniques. Our spectral data come from observations of a low-latitude, on-disk coronal hole. A potential field source surface model was used to calculate the angle between the magnetic field lines and the line of sight for each spatial bin of the observation. A fit was performed to determine the line widths and Doppler shifts parallel and perpendicular to the magnetic field. For each line width component we derived ion temperatures T i, ⊥ and T i, ∥ and non-thermal velocities v nt, ⊥ and v nt, ∥. T i, ⊥ was cooler than off-limb polar coronal hole measurements, suggesting increasing collisional cooling with decreasing height. T i, ∥ is consistent with a uniform temperature of (1.8 ± 0.2) × 106 K for each ion. Since parallel ion heating is expected to be weak, this ion temperature should reflect the proton temperature. A comparison between our results and others implies a large proton temperature gradient around 1.02 R ☉. The non-thermal velocities are thought to be proportional to the amplitudes of various waves. Our results for v nt, ⊥ agree with Alfvén wave amplitudes inferred from off-limb polar coronal hole line width measurements. Our v nt, ∥ results are consistent with slow magnetosonic wave amplitudes inferred from Fourier analysis of time-varying intensity fluctuations. Doppler shift measurements yield outflows of ≈5 km s–1 for ions formed over a broad temperature range. This differs from other studies that found a strong Doppler shift dependence on formation temperature.


Also Published In

The Astrophysical Journal

More About This Work

Academic Units
Astronomy and Astrophysics
Published Here
March 22, 2013