Tour de France
posted: July 31, 2014

The Sun had its own mountain stage this month, with soaring sunspot numbers during the first week, then a fast decline until sea-level values (that is: zero sunspots) were reached on the 17th, followed by a gradual recovery to more moderate sunspot numbers. This can be seen in the evolution of the estimated daily sunspot number as compiled by SILSO, and shown in graph underneath.


The high number of sunspots early this month was already discussed in a previous STCE newsletter. Despite the numerous and complex sunspot groups, only 4 M-class flares were recorded during the first 10 days of July. Things got worse, as all the sunspot groups one-by-one rounded the west limb, and the solar surface got devoid of any freckles. The SDO-images underneath show the Sun on 7 and 17 July: Quite a difference! There was even an "all quiet alert" in place from 16-24 July (see this newsletter for more information).


The zero value of the sunspot number for the 17th is still preliminary. Indeed, observers early on the day correctly reported no spots, but observers taking a look at the Sun during the afternoon or evening hours could see the rise and fall of a sun speck, and -starting around 18:00UT- the development of a small bipolar group, as can be seen in the SDO-images underneath (resp. at 12:00 and 20:00UT).


Assuming the final sunspot number on 17 July will be zero, one could question how often this happens. After all, this is solar maximum, isn't it? Graph underneath puts things in perspective. It shows from 1818 onwards the (Meeus) smoothed monthly sunspot number (thick blue line), and for each month the highest and lowest value of the daily sunspot number (resp. green and red; data from WDC/SILSO). The values for the last 4 months are preliminary.


As can readily be seen, it has been quite a while since we had a cycle maximum with a spotless day. This was the case especially during SC 7 (+/- 1830), 12 (+/- 1883) and 14 (+/- 1905). Just like ongoing Solar Cycle 24, it concerned moderate to weak solar cycles. It's also not so unusual that we have very high and (near) zero daily sunspot numbers within one and the same month during a cycle maximum. In July 1905, the sunspot number was 148 on the 15th, but 0 only half a solar rotation later on the 28th.


A spotless interval of only one or a few days does not necessarily mean that the Sun has become completely inactive over a full solar rotation, as there can still be a lot of activity in a limited longitudinal band. Such configurations often occur during the ascending and descending phases of stronger cycles, and are favored by a strong asymmetry in activity between the northern and southern solar hemispheres, i.e. with only one hemisphere being active and almost no contribution from the other one as is currently the case with the weakly active northern hemisphere. So in this case, the longitudinal "gaps" of the active southern hemisphere were not filled by active longitudes of the northern hemisphere, whereas normally these gaps are filled at times of maximum activity. This can be seen in the above EUV-image from STEREO/SDO, showing nearly no active (bright) regions on the Earth-directed hemisphere, most of them being on the farside and in the southern hemisphere.