Cyclic variations in the differential rotation of the solar corona
Authors:
O.G. Badalyan, V.N. Obridko, J. Sykora
Image & caption:
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Image caption::
(Above) Latitude-time distribution of the coronal rotation periods. The regions of slow coronal rotation with the synodic periods >29 days are blue coloured. (Below) Monthly averaged sunspot numbers for more than five investigated recent solar activity cycles.
Description:
Rotation of the solar corona is analyzed using our original database on the brightness of the Fe XIV 530.3 nm coronal green line, covering six recent activity cycles. The synodic period of rotation is near 27 days at low helio-latitudes. Above +/-15°, the rotational period increases and gradually reaches values slightly exceeding 29 days. Then, the rate of rotation remains more or less constant from about 45° up to the poles. The rate of coronal differential rotation is clearly the cycle phase and the heliolatitude dependent (see figure). The total rate of coronal rotation can be expressed by a superposition of two modes, the fast (27 days, differential) and the slow (31 days, quasi-rigid) ones. A method to extract these modes is proposed. Contribution of each mode to the total rotational rate depends on both the cycle phase and the heliolatitude. At low latitudes the coronal rotation is almost entirely governed by the fast mode which also dominates at the descending phases of the solar activity cycles. The slow mode is completely prevailing at high latitudes during the ascending cycle phases of activity. The results on the time-latitude dependence of coronal rotation rate and on the hypothetical existence of two modes of coronal rotation are compared with the latest findings of helioseismology, also indicating the very structure of the velocity field in the convective zone being time-latitude dependent, as well.
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