The Sun's polar field reversals

Each solar cycle, during the period of maximum activity, the magnetic field at the solar poles disappears and gets replaced by a magnetic field of opposite polarity. These reversals are a key feature of the solar dynamo: the physical process that generates the Sun's magnetic field necessary for the creation of sunspots and other magnetic phenomena. The plot underneath was produced by the Mount Wilson Observatory (MWO) and shows the evolution of the magnetic field over the solar surface since 1975 (red is negative polarity, blue positive). Polar reversals during previous cycles have been indicated with green ellipses.

This switch of the polar magnetic fields does not take place overnight. Indeed, it may take even several months before they are permanently established. The old polar field is replaced by opposite polarity field from the magnetic remnants of active regions, being transported towards the poles by the Sun's magnetic conveyor belt, as tree leaves floating on a rippling creek. This is a slow process as can be seen in this figure from the Marshall Space Flight Center (MSFC), annotated by green arrows indicating the movement from the active region belt towards the solar poles (yellow is positive polarity, blue negative).

Also, observers here on Earth have an oblique view on the Sun's poles. As the Sun's axis is tilted, they have a better view on the Sun's south pole during the spring months, and on the north pole during autumn. This changing view complicates measurements and delays firm determination of the timing of the reversal, as can be seen in the undulating evolution of the polar field strength (see figure from the Wilcox Solar Observatory (WSO) below).

The figures above readily show that there can be several months to over a year in difference between the timings of the reversal at the respective poles. For example, during the previous solar cycle, the north pole switched about a year prior to the south pole. So, for a few months around 2001, there were actually 2 "south" poles on the Sun!

For the current solar cycle 24 (SC24), the reversal on the Sun's north pole has been going on now for about a year or two, with positive and negative polarities alternating each other for the above mentioned reasons. It seems that only right now, the reversal on the north pole has become permanent. This still has to be confirmed in the months ahead. The reversal at the south pole seems to have only just begun (mid-2013). Therefore, a complete reversal still seems quite a few months away.

Once these reversals have been completed on both hemispheres, SC24 is expected to be about halfway through its cycle. However, this does not necessarily mean that solar cycle maximum is behind us. In particular low solar cycles have extended maximum periods showing short-lived increases in solar activity which may boost the sunspot number (SSN) to a new and higher maximum, which may then occur relatively late in the solar cycle. Thus, SC24 may see its maximum late 2013 or even 2014, about 5 years since its beginning instead of the average 4 years.

The reversals also do not affect the geomagnetic field in a significant way. Indeed, though the Sun's polar field reversals affect the entire solar system, the Earth's magnetic field disturbances are mainly driven by coronal mass ejections from solar eruptions and the high speed streams from coronal holes. These are not influenced by the polar field switches. Hence, the magnetic reversals will not spark an increase in powerful solar storms or other events that could have a damaging effect on Earth and its technology.

Once the polar field reversals have been completed, scientists will continue to monitor the evolution of the strength of the fields. Indeed, there are theories that use the maximum strength of the polar fields as an indicator of the strength of the next solar cycle. That's how some scientists predicted that SC24 would be a low activity solar cycle, as the maximum polar field strength was the lowest since they started measuring nearly 40 years ago. Each time, the polar field has been weaker, and each time, the next cycle was a bit weaker too (sketch above). Scientists would like the next maximum polar field strength to be a bit higher, followed by a cycle which would be higher as well. That would give a boost to the validity of these predictions and the theory behind it.

Credits and further reading - MSFC, WSO, Mount Wilson, Science at NASA, STCE Newsletter 156, Photobucket, PenguinGeek.



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