Magnetic Butterfly Diagram

Fig 1. The figure shows a magnetic butterfly diagram generated using NSO/Kitt-Peak magnetograms that depicts the net photospheric magnetic flux distribution on the sun for the last three solar cycles viz. cycles 21, 22 and 23. For better contrast in the image, we have limited the magnetic flux to ±30 Gauss. At the start of the solar cycle the bipolar magnetic regions appear with leading and trailing polarities that are oppositely oriented in both solar hemispheres in the sunspot belt zone (±35^osolar latitudes). In Figure 1 the positive polarities are shown in red while the negative polarities are shown in green. As the cycle progresses, the leading polarity flux move equatorwards where it eventually cancels the flux from opposite hemisphere while the trailing polarity flux move polewards.  This trailing polarity flux can be seen as magnetic surges (lateral motions of magnetic flux in the figure) moving polewards above latitudes of ± 45^o. These trailing fluxes, with polarities opposite to that of the previous polar cap field, on reaching the polar cap regions, cancel the per-existing polar cap fields thereby causing a solar polar field reversal.  Such field reversals occur around every solar maximum. Thus, at the start of every new cycle, the polarities of emerging bipolar regions will be opposite to that of the polarities in the previous cycle, as seen in Figure 1 where the polarities in cycle 22 are opposite to that of cycle 21 and 23.

 

Fig 2. A magnetic butterfly diagram similar to that of Fig 1 is shown.  The only difference is that it depicts the total magnetic flux distribution (absolute values) instead of the net flux (signed) distribution generated from same sets of NSO/Kitt-Peak synoptic magnetograms.

 

The butterfly diagram, depicting temporal photospheric magnetic flux distribution in solar latitudes, is generated using solar synoptic Carrington magnetic maps each of which covers a period of 27.2753 days. These synoptic magnetic maps are available online as standard FITS format files from the data bases of the National Solar Observatory at Kitt-peak, USA (NSO/Kitt-Peak) and the Synoptic Optical Long-term Investigation of the Sun (NOS/SOLIS) facility. The synoptic maps used here cover the period, in years, from 1975.14 to 2011.65.

Each of these Carrington synoptic maps is in the form 180×360 arrays in Sine of latitude and Carrington longitude format containing magnetic flux density values observed on the sun. For generation of the butterfly diagram, we have estimated magnetic flux values for each of the 180 latitude bins by taking a longitudinal average over the whole range of Carrington longitudes corresponding to every latitude bin. Thus, we now acquire images with array of 180×1 in Sine of latitude. Similarly, all 490 synoptic maps, spanning years 1975 to 2012, were processed to obtain 490 arrays each of 180×1 in Sine of latitude. Now these arrays can be laterally placed in time to produce a butterfly diagram covering three solar cycles. In short, a butterfly diagram presents longitudinally averaged latitudinal magnetic field maps in time. For generating the net flux butterfly diagram we have considered the signed fluxes while for total flux butterfly diagram we have considered the unsigned fluxes. We have limited the magnetic flux values to the range ±30 G for achieving a better contrast in the image.

Fig 1 and Fig 2 show respectively, the net flux butterfly diagram and total flux butterfly diagram and are described in the respective figure captions. The colour bar shown at the top in both images represent the variation in magnetic flux values. The net flux butterfly diagram is the real magnetic butterfly diagram that actually depicts the surface transport of flux on the solar surface, while the total flux butterfly diagram cannot be used to explain the same. Similar magnetic butterfly diagram can be found at NASA /Hathaway on web (http://solarscience.msfc.nasa.gov/images/magbfly.jpg).