Differential rotation of the solar corona: A new data-adaptive multiwavelength approach

Abstract

Context. The characterization of the differential rotation of the extended corona is still lacking conclusive results about the actual rotation rate profiles, and it is also expected to vary along the solar cycle. While some studies supported the quasi-rigidity of coronal rotation, others have found evidence of differential rotation to occur. Aims. For the purpose of investigating the differential rotation of the solar corona, we analyzed ultraviolet (UV) spectral line observations acquired on both the east and west limbs at 1.7 R⊙ by the Ultraviolet Coronagraph Spectrometer (UVCS) on-board the Solar and Heliospheric Observatory during the solar minimum preceding solar cycle 23. To obtain a reliable and statistically robust picture of the rotational profile, we used a set of simultaneous 400 day long spectral line intensities of five different spectral lines: O VI 1032 Å, O VI 1037 Å, Si XII 499 Å, Si XII 521 Å, and H I Lyα 1216 Å, which are routinely observed by UVCS. Methods. The data were analyzed by means of two different techniques: the generalized Lomb-Scargle periodogram and a multivariate data-adaptive technique called multichannel singular spectrum analysis. Among many other positive outcomes, this latter method is unique in its ability to recognize common oscillatory modes between the five time series observed at both limbs. Results. The latitudinal rotation profile obtained in this work emphasizes that the low-latitude region of the UV corona (about ±20° from the solar equator) exhibits differential rotation, while the higher-latitude structures do rotate quasi-rigidly. Moreover, in contrast to previous results obtained using only O VI 1032 Å data over a 365 day time interval during solar minimum activity, the alleged north-south rotational asymmetry of the UV corona, if existent, is much less pronounced. Conclusions. The differential rotation rate of the solar corona as evinced at low-latitudes is consistent with the rotational profile of the near-surface convective zone of the Sun, suggesting that the rotation of the corona at 1.7 R⊙ is linked to intermediate-scale magnetic bipole structures anchored near 0.99 R⊙. The quasi-rigid rotation rate found at mid and high latitudes is instead attributed to the influence of large-scale coronal structures linked to the rigidly rotating coronal holes. We further suggest that the methodology presented in this paper could represent a milestone for future investigations on differential rotation rates when dealing with simultaneous multiwavelength data.