Predicting the behavior of a sunspot cycle is fairly reliable once the
cycle is well underway (about 3 years after the minimum in sunspot number occurs [see
Hathaway, Wilson, and Reichmann
Solar Physics; 151, 177 (1994)]).
Prior to that time the predictions are less reliable but nonetheless equally as
important. Planning for satellite orbits and space missions often require knowledge of
solar activity levels years in advance.
A number of techniques are used to predict the amplitude of a cycle during
the time near and before sunspot minimum. Relationships have been found between the size
of the next cycle maximum and the length of the previous cycle, the level of activity at
sunspot minimum, and the size of the previous cycle.
Among the most reliable techniques
are those that use the measurements of changes in the Earth's magnetic field at, and
before, sunspot minimum. These changes in the Earth's magnetic field are known to be
caused by solar storms but the precise connections between them and future solar activity
levels is still uncertain.
Of these "geomagnetic precursor" techniques three stand out. The
earliest is from Ohl and Ohl [Solar-Terrestrial Predictions Proceedings, Vol. II.
258 (1979)] They found that the value of the geomagnetic aa index at its minimum
was related to the sunspot number during the ensuing maximum. The primary disadvantage of
this technique is that the minimum in the geomagnetic aa index often occurs
slightly after sunspot minimum so the prediction isn't available until the sunspot cycle
An alternative method is due to a process suggested by Joan Feynman. She separates the
geomagnetic aa index into two components: one in phase with and proportional to
the sunspot number, the other component is then the remaining signal. This remaining
signal has, in the past, given good estimates of the sunspot numbers several years in advance. The
maximum in this signal occurs near sunspot minimum and is proportional to the sunspot number
during the following maximum. This method does allow for a prediction of the next sunspot
maximum at the time of sunspot minimum.
A third method is due to Richard Thompson [Solar Physics 148,
383 (1993)]. He found a relationship between the number of days during a sunspot cycle in
which the geomagnetic field was "disturbed" and the amplitude of the next
sunspot maximum. His method has the advantage of giving a prediction for the size of the
next sunspot maximum well before sunspot minimum.
We have suggested using the average of the predictions given by the Feynman-based method
and by Thompson's method. [See
Hathaway, Wilson, and Reichmann J. Geophys. Res. 104, 22,375 (1999)]
However, both of these methods were impacted by the "Halloween Events" of October/November
2003 which were not reflected in the sunspot numbers. Both methods give larger than average
amplitude to Cycle 24 while its delayed start and low minimum strongly suggest a much smaller cycle.
The smoothed aa index reached its minimum (a record low) of 8.4 in September of 2009.
Using Ohl's method now indicates a maximum sunspot number of 70 ± 18 for cycle 24.
We then use the shape of the sunspot cycle as described by Hathaway,
Wilson, and Reichmann [Solar Physics 151, 177 (1994)] and
determine a starting time and amplitude for the cycle
to produce a prediction of the monthly sunspot numbers through the next cycle.
We find a maximum of about 70 in the late Summer of 2013.
The predicted numbers are available in a text file, as a GIF
image, and as a pdf-file. As the cycle
progresses, the prediction process switches over to giving more weight to the fitting of
the monthly values to the cycle shape function. At this phase of cycle 24 we
now give 85% weight to the amplitude from curve-fitting technique of Hathaway, Wilson, and
Reichmann Solar Physics 151, 177 (1994).
That technique currently gives similar values to those of Ohl's method.
Another indicator of the level of solar activity is the flux of radio
emission from the Sun at a wavelength of 10.7 cm (2.8 GHz frequency). This flux has been
measured daily since 1947. It is an important indicator of solar activity because it tends
to follow the changes in the solar ultraviolet that influence the Earth's upper atmosphere
and ionosphere. Many models of the upper atmosphere use the 10.7 cm flux (F10.7) as input
to determine atmospheric densities and satellite drag. F10.7 has been shown to follow the
sunspot number quite closely and similar prediction techniques can be used. Our
predictions for F10.7 are available in a text file,
as a GIF image, and as a pdf-file. Current values for F10.7 can be found