THE ROTATION OF THE SOLAR PHOTOSPHERIC MAGNETIC FIELD

The rotational characteristics of the solar photospheric magnetic field at four flux ranges are investigated together with the total flux of active regions (MF_ar) and quiet regions (MF_qr). The first four ranges (MF_1–4) are (1.5–2.9) × 10¹⁸, (2.9–32.0) × 10¹⁸, (3.20–4.27) × 10¹⁹, and (4.27–38.01) × 10¹⁹, respectively (the unit is Mx per element). Daily values of the flux data are extracted from magnetograms of the Michelson Doppler Imager on board the Solar and Heliospheric Observatory . Lomb–Scargle periodograms show that only MF₂, MF₄, MF_qr, and MF_ar exhibit rotational periods. The periods of the first three types of flux are very similar, i.e., 26.20, 26.23, and 26.24 days, respectively, while that of MF_ar is longer, 26.66 days. This indicates that active regions rotate more slowly than quiet regions on average, and strong magnetic fields tend to repress the surface rotation. Sinusoidal function fittings and cross-correlation analyses reveal that MF_ar leads MF₂ and MF₄ by 5 and 1 days, respectively. This is speculated to be related with the decaying of active regions. MF₂ and MF_ar are negatively correlated, while both MF₄ and MF_qr are positively correlated with MF_ar. At the timescale of the solar activity cycle, MF_ar leads (negatively) MF₂ by around one year (350 days), and leads MF₄ by about 3 rotation periods (82 days). The relation between MF₂ and MF_ar may be explained by the possibility that the former mainly comes from a higher latitude, or emerges from the subsurface shear layer. We conjecture that MF₄ may partly come from the magnetic flux of active regions; this verifies previous results that were obtained with indirect solar magnetic indices.