Solar Cycle 24 at hand!

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Number: 75
1st Author: Hugh Hudson
2nd Author:
Published: 6 January 2008
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A first conference on Cycle 24

It is quite an exciting prospect: solar activity is heating up and we can be pretty sure that another cycle of magnetic activity will happen. This has happened regularly at least since Carrington cycle numbers began to be used for identification - the next peak would be Carrington cycle 24. We are so confident magnetic activity will grow, and big flares will burst out, that we are organizing a conference to lay plans (Figure 1 is the official conference poster, prepared by Steele Hill and Bernhard Fleck of NASA and ESA). The meeting home page gives full details. Even if the cycle does not start on time, the meeting is in Napa Valley and we propose to enjoy it anyway.

The essence of this conference is to reflect upon the past cycles of solar activity (e.g., flares and CMEs) and to mull over what we have learned. But even more important - there are wonderful new observations from space and and there are always great ground-based observations; not all of these are easy to optimize for flare and CME observations because the interesting phenomena have such tiny spatial and temporal scales, and image dynamic ranges. Accordingly the conference should help to guide the observations of events about to happen.

The solar 10-cm radio flux: F10.7

But let us double-check the state of activity. Are we almost there? As early as January 2008, a RHESSI science nugget had noted the appearance of new-cycle magnetism. But as Carrington himself noted, "one swallow does not make a summer."

The 10.7-cm radio flux is a sensitive and well-determined index of solar activity. This is partly because radio wavelengths are much better for absolute photometry than visible ones - recall that the the cosmic microwave background was first directly observed by Penzias and Wilson simply as an unexplained photometric excess from the ostensibly dark sky.

In the solar domain, A. E. Covington developed the tools needed to make precise photometric observations of the unexpectedly variable Sun. He had picked 10.7 cm for his observations, simply because equipment for that wavelength was available. But later he showed that it was essentially the right wavelength anyway! Note that it is about the electron Larmor frequency at the magnetic field strength typical of a sunspot. The same Covington was also fortunate enough to make a definitive observation of the elusive and mysterious ball lightning.

What do F10.7 solar mimina look like?

Not considering flares, the solar 10 cm radio flux comes from two main mechanisms: gyroresonance emission and free-free emission. The latter is basically the same mechanism that forms the visible and soft X-ray continuum - it is bremsstrahlung. The former involves the magnetic field, and this helps F10.7 to reflect magnetic activity as well as it does.

Figure 2: The five recent solar minima as viewed in the F10.7 index. "SFU" means units of 10-22 W/m2. The data intervals are three years long, except for the current one, and are shifted arbitrarily to make them match roughly. The mean spacing is 10.8 years, about right. The heavy red line is our present minimum, with its absolute minimum in October 2007. The plots in Figure 2 show what monthly means of F10.7 look like during the years of solar minimum. There are noise-like variations, but these are largely due to magnetic activity. In principle the lower envelope of the curves - on the order of 70 SFU - should reflect the basic boring hydrodynamic Sun, with no magnetic influence. This may not be the case completely - it is a bit surprising to see the current minimum reading dipping down to 67.1. Solar mimimum may be more interesting than we had thought... the dashed line shows the approximate date of the Solar24 conference.

Biographical note: Hugh Hudson is a senior RHESSI researcher at the University of California, Berkeley.

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