Absolute velocity measurements in the solar transition region and corona

Abstract

Abstract We present an experimental technique to measure absolute velocities of minor ions formed in the solar transition region and corona. A sounding rocket experiment (27 July 1987) obtained high resolution EUV spectra along a solar diameter with spatial resolution of 20 × 20 arcsec. The wavelengths of the λ1533 Si II, λ1548 C IV, and λ770 Ne VIII emission lines were directly compared with wavelengths of known platinum lines generated by an inflight calibration lamp. On the assumption that horizontal motions cancel statistically so that the line-of-sight velocity approaches zero at the limb, we find a net radial downflow of approximately 7.5 ± 1.0 km/s for C IV and upper limits on the radial flow for Si II and Ne VIII. We then test this assumption by direct comparison to our on-board wavelength reference using recently published laboratory rest wavelengths of the solar emission lines. We find agreement within the published uncertainties of the laboratory wavelengths, ± 2 km/s in the case of C IV. Therefore, we suggest that improved laboratory wavelength measurements (± 1 km/s) in conjunction with inflight wavelength calibration would improve constraints on models of transition region and coronal dynamics.