A Method for Characterizing Rotation Rates in the Soft X-Ray Corona

Differential rotation rates of soft X-ray features in the solar corona are quantified by a method of harmonic filtering using the Lomb-Scargle periodogram. This approach leads reasonably to a quantitative discrimination between uncertainty estimates and spectral leakage of the fundamental rotation frequency due to the presence of multiple rotating tracers. Mean rotation rates as a function of latitude and year are calculated for the years 1992–1997 (roughly the declining phase of the last solar activity cycle). The corona is found to have a small but measurable latitudinal gradient in rotation rate. The presence of multiple features places a lower bound of 1–2% on the relative uncertainties with which a `mean' rotation rate can be measured. The results are compared with autocorrelation estimates and found to agree within 1σ.