82. A Super-Synoptic Map of HMI Flux Density

Contributed by Leif Svalgaard. Posted on January 8, 2018

Leif Svalgaard
W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085

Roger Ulrich invented the concept of a ‘super-synoptic map’ [or chart] (http://obs.astro.ucla.edu/torsional.html ). To construct such a map, one reverses the time direction [from right to left] on ordinary synoptic maps to have the data run from left to right [i.e. increasing time], and then divides the map into pole-to-pole strips in longitude of a width sufficient to smooth out the smallest structures. Juxtaposing all the strips from rotation to rotation yields a time-history of the changing magnetic field on the surface. Figure 1 shows an example of a Mount Wilson Observatory [MWO] super-synoptic map for the years 2011-2012.

Figure 1| An MWO Super-Synoptic-Map (Chart) for years 2011-2012. Carrington Rotation numbers are given at the top of the plot. Flux densities in Gauss [=Mx/cm2] are color coded from -10 G (reddish colors) to +10 G (bluish colors). Each synoptic map was divided into 9 strips (of width 40°). The migration of flux from the sunspot zones towards to poles can be seen. Note that both polarities march towards the poles often in parallel. This is no news, of course (See Ref 1).

In this nugget I present a Super-Synoptic Map constructed for the Helioseismic and Magnetic Imager [HMI] for SDO, covering the time since launch (Spring 2010). As input synoptic maps I use the series hmi.synoptic_mr_720s [there is some confusion about the use of capital letters – they don’t matter]. The series starts with Carrington Rotation 2097 [starting on May 19, 2010] and ends with the latest one available at the time of writing CR2198 [ending on December 30, 2017]. The synoptic maps combine the 20 nearest-to-central-meridian flux-density measurements from magnetograms made every 720s (12 minutes); input values are assumed to be the line-of-sight component of a radial field. The radial-field synoptic maps are further averaged into 36 bins 10° in longitude (100 points each) and 10 values of sin(latitude), i.e. 144 bins from pole-to-pole. At least six data points must be present for a bin-value to be computed; that automatically excludes periods when a pole is not visible. To emphasize large-scale fearures, all flux density [‘field’] values above 5 G are plotted with the same color (blue for positive, out of the sun, and red for negative, into the sun), as shown in Figure 2.

Figure 2| An HMI Super-Synoptic-Map (Chart) for years 2010-2017. Carrington Rotation numbers are given at the top of the plot. Flux densities in Gauss [Mx/cm2] are color coded from -5 G (reddish colors) to +5 G (bluish colors). Each synoptic map was divided into 36 strips (of width 10°) and the 1440 values from pole-to-pole were averaged into 144 bins of sin(latitude). Click here for a larger view.

There are many interesting features that show up in this high-resolution representation, e.g. the recent migration of mid-latitude flux to the poles (see Figure 3).

Figure 3| The most recent rotations 2192-2198 for the last half of the year 2017. Negative flux (red ovals) can be seen as narrow ‘streaks’ moving towards the South Pole, helping to build a stable polar cap field there. Positive flux (dashed line blue ovals) is moving towards the North Pole, helping to establish a stable north polar cap field. The ordinate is the ordinal number of sin(latitude) projected on the 1440-pixel height of the synoptic maps. The color scale is as in Figure 2.

In Figure 2, the reversal of the polar fields is vividly displayed, especially the dominant contributions from just a very few flux concentrations. Another feature of note is the waning and waxing of the polar fields in place, rather than the often supposed varying ‘tilt’ of the dipole through the solar cycle2, or worse: ‘rotation’ of the dipole by 180° from pole to the other pole3.


[1] Topka, K., Moore, R. L., LaBonte, B. J., & Howard, R., 1980, BAAS, 12, 893
[2] Pipin, V. V., Moss, D., Sokoloff, D., & Hoeksema, J.T. 2014, A&A, 567, A90.
[3] Sanderson, T. R., T. Appourchaux, J. T. Hoeksema, & K. L. Harvey 2003, J. Geophys. Res., 108, 1035.

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