Latitude movements of solar prominences, 2010–’17


Most prominences at  the  Sun’s limb  have  longitudinal  axes  which  extend  onto  the  disk  as  dark  filaments (Figure 1), although they often lack contrast and are not always easy to see in small  H-alpha  telescopes. These,  like  all  filaments,  form  at  the  boundaries  between  magnetic  fields  of  opposite  polarity.  Lower latitude or ‘equatorial’  prominences occur within and between sunspot active regions, and are closely associated  with  sunspots  in  latitude,  position  and  activity. ‘Polar’  prominences are much less closely related to sunspots since they are located outside the customary sunspot zone,  i.e.  above latitude 40°.

Two types of apparent prominence  movement  can be discerned:  a  relatively  slow  drift  of mean  positions  towards the poles or the  equator, and a more  rapid  day-to-day  shift in the latitude  of  individual  prominences.  Although  filaments  are  dynamic  structures,  all  but  the  smallest  are  relatively  stable  in  shape and position over 24 hours. Day-to-day prominence  shifts can therefore  be  used  to  map  the  orientation  of  filaments,  and  their corresponding  magnetic  field  boundaries,  with  respect  to  the  solar poles and equator.


A  Coronado H-alpha  PST  (Personal Solar  Telescope) was used, fitted  with  a  rotating  reticle  eyepiece,  at  a  magnification  of  ×32. The Sun’s parallactic angle was determined by the ‘drift’  method, and the position angles of prominences around the Sun’s disk measured and recorded.  The base of a prominence (or central point) defined its position: measurements were reproducible to ±1°. As a check on accuracy, sunspot active region coordinates were also measured and compared with published values. Positions were plotted onto a 150mm outline of the Sun’s disk, converted to true heliographic latitudes using published values of P  and B0 and  an  appropriate  Stoneyhurst  template.  Thereafter  heliographic latitudes were used for statistical analysis.

Observations  were  carried  out  during the rise  of sunspot  activity leading to the 2014 solar maximum, and the subsequent decline towards solar minimum. I am very grateful to the BAA  Solar Section for sunspot activity data, which were obtained from the Section’s monthly reports in  the  Journal.  Mean latitudes of  sunspot groups were derived from the NOAA  archive. (continued)

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