Cycle 24 - time to panic
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Nugget | |
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Number: | 99 |
1st Author: | Leif Svalgaard |
2nd Author: | and Hugh Hudson |
Published: | 13 April 2009 |
Next Nugget: | NUGGET 100 |
Previous Nugget: | Chree Analysis for Flares |
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Contents |
Introduction
We're in an extended period of minimal solar activity (see Nugget 91 for a previous look at this issue - without flares, RHESSI is missing its most important observational work, and there has not been even a C-class flare yet this year. Will sunspots and flares ever return? How unusual is this behavior? In this Nugget we conclude that it is too soon to panic, but that certainly we're seeing an interesting diminished level of activity which most of us has not seen before.
F10.7 and other indices
The 10-cm radio flux from the Sun, with its daily index F10.7, is one of the basic standard tools for gauging the level of solar activity. This index has been generated in an unbroken string since 1947. It is derived from careful radio flux measurements made in Canada and pioneered by A.E. Covington.
The figure on the left might give cause for alarm if one is interested in flare observation. It shows (red line) the variation of F10.7 monthly means for the two years prior to the present time. The black line shows the mean of the four previous cycles, registered by [summed epoch analysis] on simple Gaussian fits to their preceding maxima as references. The four time series from prior maxima from are averaged, using the Gaussian peak time as a reference, month by month; the range bars show the standard deviations of these means for each month. This procedure does not allow for the possibility of different cycle durations, something that seems fairly obvious from the sunspot record, but it is hard to be quantitative about this. Our approach here is to use the most direct approach to analysis of the most objective of the indices, and Figure 1 is the result. If one interprets the range bars as true error bars, and did not know (or believe) that cycles could have different lengths, this figure would provide compelling evidence that Cycle 24 is unique, and that it might be time for fans of solar flares to "panic." Our discussion below, however, shows that this would be premature.
A closer look (Figure 2 below) shows the daily values of three indices: F10.7, the total solar irradiance TSI, and the classical sunspot number. There is a clear apperance of an up-turn in the least "noisy" of these indices, F10.7, although there are other small variations that we do not understand well. But probably F10.7 is giving us an early warning about the sudden increase of Cycle 24 spots. These should appear within the next few weeks, though, so please check the daily updates of the indices on our Web page.
Counting the new and old spots
Historical fact
Conclusions
RHESSI Nugget Date | 13 April 2009 + |
RHESSI Nugget First Author | Leif Svalgaard + |
RHESSI Nugget Index | 99 + |
RHESSI Nugget Second Author | and Hugh Hudson + |