Surface magnetic fields during the solar activity cycle

Abstract

We examine magnetic field measurements from Mount Wilson that cover the solar surface over a 13 89 year interval, from 1967 to mid-1980. Seen in long-term averages, the sunspot latitudes are characterized by fields of preceding polarity, while the polar fields are built up by a few discrete flows of following polarity fields. These drift speeds average about 10 m s 1 in latitude -slower early in the cycle and faster later in the cycle-and result from a large-scale poleward displacement of field lines, not diffusion. Weak field plots show essentially the same pattern as the stronger fields, and both data indicate that the large-scale field patterns result only from fields emerging at active region latitudes. The total magnetic flux over the solar surface varies only by a factor of about 3 from minimum to a very strong maximum (1979). Magnetic flux is highly concentrated toward the solar equator; only about 1% of the flux is at the poles. Magnetic flux appears at the solar surface at a rate which is sufficient to create all the flux that is seen at the solar surface within a period of only 10 days. Flux can spread relatively rapidly over the solar surface from outbreaks of activity. This is presumably caused by diffusion. In general, magnetic field lines at the photospheric level are nearly radial.

Evolution and rotation of large-scale photospheric magnetic fields of the Sun during cycles 21–23

“…Besides, the 1.3 yr period has also been seen in geomagnetic and auroral phenomena (Silverman & Shapiro 1983). The 1.3 yr period in the unsigned (and signed) magnetic flux occurs at latitudes where large-scale magnetic surges towards the poles (Howard & Labonte 1981;Wang et al 1989) seem to emanate. This can be nicely seen in the butterfly diagram of the signed flux in Fig.…”

Unusual Polar Conditions in Solar Cycle 24 and Their Implications for Cycle 25

“…Table 1 also shows that the times of reversal completion are close to the cessation times of HL PEs and several months after the end of B ∼ 0 conditions. RTTP and the polarity reversal can be episodic (Howard & Labonte 1981;Topka et al 1982;Makarov & Sivaraman 1989;Krivova & Solanki 2002;Gopalswamy et al 2003a;Ulrich & Tran 2013;Petrie 2015;Sun et al 2015;Mordvinov et al 2016), directly related to the series of poleward surges of alternating polarity. Surface flux transport simulations show that the emergence of a number of large active regions that violate Joy's law leads to the weak polar field during the Cycle 23/24 minimum (Jiang et al 2015).…”

Reconstruction of magnetic field surges to the poles from sunspot impulses

“…Leighton (1964) proposed that the polar magnetic field reversals are the result of trailing polarity transport by supergranular diffusion. On the contrary, Howard & LaBonte (1981) suggested that the transport of magnetic field poleward does not occur by diffusion, but by directed flow.…”

“…A high diffusion itself (without meridional flow) is able to reverse the polar fields (Leighton 1964;Baumann et al 2004). Howard & LaBonte (1981) noticed that the polar field formation is not continuous but episodic by the movement of the magnetic field from the sunspot latitudes to the poles. Further, Wang et al (1989) determined the magnetic field surges as polewardmoving streams (waves) of either polarity.…”