Coronal equatorial rotation during solar cycle 23: radial variation and connections with helioseismology

Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (To), Italy
e-mail: mancuso@oato.inaf.it

Received: 10 August 2011
Accepted: 21 December 2011

Abstract

Time-series observations of the O vi 1032 Å spectral line intensity provided by the UltraViolet Coronagraph Spectrometer (UVCS) telescope aboard the Solar and Heliospheric Observatory (SOHO) spacecraft have been analyzed to establish the rotational characteristics of the equatorial solar corona as a function of height and time during solar cycle 23. Overall, the coronal rotation period is observed to vary considerably from 1996 to 2006, with episodes of sudden acceleration and deceleration. On average, the rotation period in the equatorial corona tends to increase radially by  ~0.2 days/R_⊙ from 1.6 to 3.0 R_⊙. An anticorrelation throughout the solar cycle is observed between the radial gradients in the inner corona (≲2.2 R_⊙), where the magnetic pressure dominates and the plasma is more rigidly connected, and the outer corona (≳2.4 R_⊙), where the field lines open up. Around the equator, the extended corona is found to rotate faster than the underlying photosphere, but its rotation rate is comparable to that estimated within the subphotospheric layers in the outer 5% of the Sun. Moreover, a striking significant positive correlation (r = 0.629 at the 0.99 R_⊙ level) has been discovered between the variations in the residual rotation rates of the coronal and subphotospheric equatorial plasma, at least down to 0.95 R_⊙. This correlation suggests that the observed variations in the coronal rotation rate reflect the dynamic changes inferred within the near-surface shear layer, where the tracer structures responsible for the observed coronal emission are thus most probably anchored. These results raise the possibility that the plasma in the upper layers of the solar convection zone, at least around the equator, may be tightly connected to the plasma in the extended corona and that the deeper layers in the Sun might thus directly influence the dynamic evolution of the solar wind.