Variations over time in latitudinal distribution of the large-scale magnetic fields in the solar atmosphere at heights from the photosphere to the source surface

Abstract

Calculations of magnetic field in the solar atmosphere and the “potential field-source surface” model have been used to study time variations in several parameters of the large-scale magnetic field at various heights during the last four solar cycles. At ten heights from the solar surface (R=Ro) to the source surface (R=2.5Ro), we have constructed synoptic charts (SC) of the radial component Br of the estimated magnetic field. For these SC, we have identified 10-degree latitudinal zones. Within these zones, we found values of Sp (positive Br values averaged within the latitudinal zone over latitude and longitude), Sm (averaged modulus of negative Br values) and S+fields (a part of the latitudinal zone area (in %) occupied by positive Br values). At lower latitudes, cyclic variations in the Sp+Sm parameter are demonstrated to be similar (but not in detail) to time variations in Wolf numbers. Latitudes of 55° and higher exhibited virtually no cyclic peculiarities of time variations in this parameter. The authors believe that this indicates the diverse nature of the large-scale magnetic field in the near-equatorial and polar regions of the solar atmosphere. At R=2.5Ro, Sp+Sm cyclic variations are almost invisible at all latitudes and only slightly apparent near the equator. The analysis of S+fields variations revealed that at low latitudes at R=2.5Ro during solar cycles 21, 22 and ascending phase of cycle 23 there were almost no mixed-polarity periods. However, beginning from the maximum of cycle 23, in the near-equatorial region the mixed polarity was observed until the end of the long solar activity minimum. An assumption has been made that this might have been one of the forerunners and manifestations of the prolonged minimum between cycles 23 and 24. It has been found that during solar activity minima poleward there appears motion of magnetic fields with polarity opposite to that of the field at the pole. We have estimated the velocity of such a latitudinal movement of fields. The magnetic field at R=R⊙ during the maximum of cycle 24 is shown to be significantly less than that during the maximum of cycle 23; in a range 55°–75° this difference is as much as one order of magnitude. We have verified the conclusion drawn in our previous studies that in solar cycles 21–23 the polar-field reversal started and ended earlier at large heights than on the solar surface. Peculiarities of the magnetic-field reversal in solar cycle 24 have been revealed.