What implications could a low-activity sun have on earth? Fritz Vahrenholt’s and Frank Bosse’s latest solar report.
By Frank Bosse and Fritz Vahrenholt
Translated and edited by P Gosselin)
For solar observers there was quite a bit of activity last month, namely sunspots, and not too few of them. Activity even reached 84% of the mean value calculated from cycles 1-23. With an official SSN (sunspot number) of 90.3 the sun reached a second peak in December 2013, see the following chart:
Figure 1: Comparison of solar cycle 24 activity to the mean value of the previously observed cycles and to that of solar cycle 5.
As far as sunspot activity is concerned, the peak was comparable to what we observed 25 months ago (November 2011), but this time most activity occurring in the sun’s southern hemisphere: 82%. In the sun’s first cycle 24 outbreak, most of the activity happened in the northern hemisphere. To compare the activity of the current cycle to that of past solar cycles, we plot the sunspot anomalies from the mean for the number of sunspots up to 61 months into the cycle (blue curve in Figure 1). This yields Figure 2:
Figure 2: Accumulated number of sunspots since the beginning of the cycle, taking modified method of counting before 1945 though the “Waldmeier- Discontinuity” (see Cliver et al. 2013). Cycle 24 so far is the 5th least active cycle in over 250 years.
The low level of activity of the Dalton Minimum at the start of the 19th century is plain to see, as well as the rather relatively subdued activity that followed until about 1950. Then there was a significant increase until cycle 23 which heralded a drop that is now continuing today. Single months with increased activity hardly change the overall trend. As before we are currently seeing the lowest level of solar activity in terms of sunspots in the last 170 years.
So just what are sunspots?
From the sun’s moving plasma, magnetic fields are generated and they are able to strongly impact the energy flow in the sun’s upper convective layer.
Figure 3: The sun’s general structure. At the core the energy is generated by nuclear fusion. This energy is transported outwards by radiation, and then by convection through moving plasma. Source: Wikipedia. Kelvinsong: This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
These fields can cause convective energy flux to reduce locally, and so dark spots form on the sun’s surface. When that happens multiple times at nearby regions, different poled magnetic field lines interfere with each other and a short circuit is created. As a result the magnetic fields collapse and the pent up energy can suddenly burst. This is what we call a so-called flare – a solar explosion. When it is powerful enough, solar material gets ejected out into space with such violence that it literally escapes the sun’s gravitational field and flies out into space. This is the so-called coronal mass ejection (CME). It can have adverse consequences should it strike the earth and the material cloud reacts with the earth’s magnetic field and strong fluctuations occur. These in turn induce large currents within conductors at the earth’s surface (geomagnetic storms).
Our networked world is thus more prone to such events than 60 years ago. Power and communication networks and satellite outages can have consequences. However, very often such risks do not occur because several conditions have to be present before a serious outage takes place. A discussion on the impacts of CMEs in cycle 24 was held in December 2013 at the autumn American Geophysical Union conference. It was determined that that the latest geomagnetic storms have been really low in strength compared to what we observed before. They are even faster than they were in previous cycles. That can be traced back to a 40% reduced counter-pressure in the corona (see Figure 3). As a result the CMEs were able to move faster, yet have less impact. You can see the video of the presentation on the subject here.
On the other hand one can conclude: The reduced solar activity since 2006 reduced the density of the corona by a considerable amount. This not only has great impacts at the sun’s surface, but also out within the sun’s sphere of influence, which is also where the earth happens to orbit. So we can continue to wait with suspense what will be discovered next. Should it end up (as many scientists suspect) that solar activity continues to decline until the middle of the century, then there remains lots of potential. As Leif Svaalgard says at the end of the video: “None of us alive have ever seen such a weak cycle.”