World net Polar Faculae Observers

World net Polar Faculae Observers

Dear Solar Observer,

As you are probably aware of, the solar activity rises and falls with a period of approximately 11 years. The Solar Dynamo Model is one of the models trying to explain this behaviourism. Babcock (1961) states that in the beginning, the magnetic field of a new cycle is poloidal (i.e. parallel to the solar rotational axis). Because of the differential rotation of the sun (the sun rotates faster at its equator than at its poles), these magnetic lines are gradually winded up to a more toroidal field (parallel to the solar equator). As these magnetic field lines draw closer and closer to each other, the magnetic field strength increases, eventually giving rise to sunspots. This causes the field lines to simplify and, after about 11 years, result again in a poloidal field with reversed magnetic polarity. Though there are some physical difficulties in explaining some of the model’s characteristics, it offers an elegant explanation to peculiar solar cycle features such as the Butterfly-diagram (Spoerer′s law) or Hale′s cycle (22 year magnetic cycle)
The model might also imply that a strong poloidal field indicates a strong new solar cycle at hand. Schatten e.a. measures the strength of the magnetic field at the solar poles to determine the amplitude of the solar cycle to come. Schatten’s predictions for cycle 22 indicated an active cycle compared to other methods expecting a more moderate one, which proved to be correct. Also for the ongoing cycle 23, Schatten seems to do very well by expecting a moderate amplitude (Rmax = 138), while quite some other methods announced a high to very high amplitude (Rmax > 155). Currently, solar cycle 23 seems to be heading for a maximum smoothed Wolfnumber Rmax of about 110 (+/- 20). 
The problem with measuring the magnetic fields at the poles is that at such high solar latitudes, a lot of uncertainties arises in the measurements of the field strengths. Also, it assumes an overall magnetic field strength at the poles, which does not necessarily need to be the case (e.g. compare to high magnetic field strength in the sunspots, yet the "undisturbed" photospheric background field remains hundreds of times smaller). Amongst other parameters, Schatten and his collaborators are also looking into the number of faculaes that appear at the solar poles. Though polar faculae do definitely not belong to the easiest observable solar features, systematic monitoring is very much needed and might lead to interesting results related to the solar dynamo model, solar cycle forecasting and physical understanding of this phenomenon. 
Polar faculae appear as pointlike, bright photospheric spots near the solar limb at latitudes of 55 degrees or more (average of 65 degrees). Polar faculae tend to occur at lower latitudes (as low as 45 degrees) during the years in which there are only few observable. They can be distinguished from main zone faculae by their essentially pointlike and solitary appearance, in contrast to the more area- and grouplike appearance of the main zone faculae (55 degrees or lower). Polar faculae have also a much shorter lifetime (minutes to hours). The brightest can last for a couple of days, and can be traced farther from the solar limb too. Finally, polar faculae are most numerous at times of minimum solar activity, which in turn might be an additional hint for its relation with the upcoming new solar cycle.
Observing polar faculae is not an easy job. Moreover, years of experience have shown that in order to obtain reliable and comparable observations, a couple of conditions need to be met: Observing should occur with a telescope having a diameter of 4 inch (10 cm) or more. It is highly recommended to use a glass objectif filter, or alternatively for the refractors, a solar/penta prism or Herschel wedge. Solar screens tend to diffuse the light, whereas the projection method is heavily affected by straylight from external sources. Therefore, both methods are significantly poor in contrast, and thus quite unreliable for polar faculae observation. Needless to say that an ocular filter in combination with these high-diameter telescopes constitutes a permanent threat to eye-safety.
Observing conditions should reach at least 3 (preferably 4 or 5) on the SIDC-seeing scale, which means significant detail should be visible in even small sunspotgroups, and one should also have a good view on the solar granulation. The eventual aim is to have a direct view of the solar surface as crisp as possible.
In view of the short lifetime of most of the polar faculae, observing a couple of minutes per solar hemisphere is sufficient. Also, a couple of observations evenly spread through the month will do. When looking for polar faculae throughout the year, please be aware that an observer on Earth has a better view on the southern solar hemisphere during the February-May timeframe, and on the northern solar hemisphere during the August-November timeframe. This will not only affect the number of polar faculae one counts, it will seem as if polar faculae tend to occur at significantly lower latitudes. This optical effect can only be solved by photographic position determination, or by comparing with the location of main zone faculae if present. In general, one should look for polar faculae in an area between the polar solar limb and the outer quarter of the solar radius.
The main purpose of the polar faculae observation program is to count the number of polar faculae visible on the solar hemispheres. Persistent observers might try to determine the lifetime of polar faculae. Photographic efforts can be used for position determination. These data can be used to compile differential rotation of the polar zones, or zone wandering of the polar faculae throughout a solar cycle.
You’re kindly invited to send your data to the Section Leader Franky Dubois
Franky or myself Jan Janssens will gladly assist you if any questions or additional info required. Thank you for your collaboration, and good hunting!

Jan Janssens

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