The level of geomagnetic activity near the time of solar activity minimum has been shown to be a reliable indicator for the amplitude of the following solar activity maximum. The geomagnetic activity index aa can be split into two components: one associated with solar flares, prominence eruptions, and coronal mass ejections which follows the solar activity cycle and a second component associated with recurrent high speed solar wind streams which is out of phase with the solar activity cycle. This second component often peaks before solar activity minimum and has been one of the most reliable indicators for the amplitude of the following maximum. The size of the recent maximum in this second component indicates that solar activity cycle 24 will be much higher than average - similar in size to cycles 21 and 22

Hathaway, D. H.

Wilson, R. M.

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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L18101, doi: 10.1029/2006GL027053, 2006 
e O t  4 
David H. Hathaway' and Robert M. Wilson' 
Received 31 May 2006; revised 1 August 2006; accepted 10 August 2006; published 16 September 2006. 
[i] The level of geomagnetic activity near the time of solar 
activity minimum has been shown to be a reliable indicator 
for the amplitude of the following solar activity maximum. 
The geomagnetic activity index aa can be split into two 
components: one associated with solar flares, prominence 
eruptions, and coronal mass ejections which follows the 
solar activity cycle and a second component associated with 
recurrent high speed solar wind streams which is out of 
phase with the solar activity cycle. This second component 
often peaks before solar activity minimum and has been one 
of the most reliable indicators for the amplitude of the 
following maximum. The size of the recent maximum in 
this second component indicates that solar activity cycle 24 
will be much higher than average - similar in size to cycles 
2 1 and 22 with a peak smoothed sunspot number, of 160 Ifr 25. 
Citation: Hathaway, D. H., and R. M. Wilson (2006), 
Geomagnetic activity indicates large amplitude for sunspot cycle 
24, Geophys. Res. Lett., 33, L18101, doi:l0.1029/2006GL027053. 
1. Introduction 
[2] Knowing the level of solar activity years in advance 
has important consequences. High levels of solar activity 
can heat and inflate the Earth's outer atmosphere. This 
increases the drag on satellites in low Earth orbit and can 
lead to their early reentry. Placing a satellite in a low orbit 
can lead to a costly early end of its mission. Placing a 
satellite in a higher orbit either increases the launch costs or 
decreases the payload weight. Accurate and reliable pre- 
dictions for solar activity levels are needed by the people, 
companies, and organizations that build, operate, and use 
satellites. 
[3] Geomagnetic activity near the time of sunspot cycle 
minimum has been shown to be a good indicator for the 
level of maximum activity during the following cycle. Oh1 
[1966] noted that the geomagnetic index aa reaches a 
minimum near (but usually after) the time of solar activity 
minimum and that this minimum in the aa index is well 
correlated with the amplitude of the following activity 
maximum. Feynman [ 19821 suggested that the geomagnetic 
activity indicated by the index aa could be split into two 
components - one in phase and proportional to the solar 
activity cycle and a second (residual) component associated 
with interplanetary disturbances that is out of phase with the 
activity cycle. This second, interplanetary component often 
peaks just before solar activity minimum and has been 
shown to be an even better indicator for the amplitude of 
the following cycle [Hathaway et al., 19991. Thompson 
'National Space Science and Technology Center, NASA, Huntsville, 
Alabama, USA. 
This paper is not subject to U S .  copyright. 
Published in 2006 by the American Geophysical Union. 
[ 19931 noted that the number of geomagnetically disturbed 
days during a cycle (as indicated by the number of days 
with Ap index 225 between solar activity minima) is 
proportional to the sum of the amplitudes of that cycle 
and the following cycle. While this method for predicting 
the amplitude of the next cycle requires waiting until cycle 
minimum occurs, it is also found to be a very reliable 
method [Hathaway et al., 19991. All three of these geo- 
magnetic precursor methods indicated a larger than 
achieved cycle for cycle 23 (Oh1 - 135 * 35; Feynman - 
154 * 25; Thompson - 153 f 35; achieved - 121) but with 
the predictions within or just outside the 2 a  errors. 
[4] The use of geomagnetic activity as a predictor for 
future solar activity seems counter-intuitive. The Sun is the 
source of the solar wind disturbances that drive geomag- 
netic activity and thus it would seem that solar activity 
should predict geomagnetic activity, not the other way 
around. Nonetheless, geomagnetic activity near the time 
of solar activity minimum has proved to be a reliable 
predictor for future solar activity. A likely explanation for 
this connection comes fi-om the sources of geomagnetic 
activity. Solar eruptions such as flares, filament eruptions 
and coronal mass ejections are active producers of geomag- 
netic activity. The frequency of these eruptions rises and 
falls with the solar activity cycle as indicated by the number 
of sunspots. These eruptions represent the solar cycle 
component of geomagnetic activity as described by 
Feynman [1982]. Additional drivers of geomagnetic activity 
include interplanetary shocks from high-speed solar wind 
streams associated with coronal holes that are out of phase 
with the sunspot cycle [cf. Luhmann et al., 20021. As the 
polar coronal holes expand during the approach to sunspot 
minimum their low-latitude extensions produce recurrent 
high-speed streams that give rise to geomagnetic activity. 
The magnetic field strengths and configurations that give 
rise to these coronal structures may provide a prelude to the 
strength of the ensuing sunspot cycle. 
[ 5 ]  Models for the Sun's magnetic dynamo may help to 
explain this connection. Recent models [cf. Dikpati and 
Charbonneau, 19991 incorporate the Sun's meridional cir- 
culation to transport strong, sunspot forming, magnetic field 
toward the equator at the base of the convection zone. This 
provides a simple explanation for the equatorward drift of 
the sunspot latitudes. It also suggests that evidence of the 
next cycle might be seen in the mid-latitudes prior to the 
appearance of sunspots. This "extended" solar cycle was 
suggested earlier by n%lson et al. 1119881 based on observa- 
tions of ephemeral active regions, coronal emission-line 
structures, and the torsional oscillation signal. These struc- 
tures appear at mid-latitudes prior to the first appearance of 
the sunspots of the new cycle and may very well contribute 
to geomagnetic activity at that time. 
[6] Predictions for the size for cycle 24 have already been 
published. Hathaway and Wilson [2004] predicted a large 
L18101 1 o f 3  
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L18101 HATHAWAY AND WILSON: LARGE AMPLITUDE FOR SUNSPOT CYCLE 24 18101 
35-' a ' " I I a ' ' I ' " 1 ,  " ' * ' I ' ' - I ' -  
I -aaR = 10.9 + 0.097 R 
30 1 .............. aa, = aa - aaR 19 - 
351 ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' j 
x 
S 
Q 
4 30 
25 
- 
.- c a 
C 9 20 
El 
5 10 
d 15 - 
(D 
3 
- aa, = 10.9 + 0.097 R a 
0 50 100 150 200 
Annual International Sunspot Number 
Figure 1. Annual values for the geomagnetic index aa as a 
function of the corresponding annual International Sunspot 
Number for the years 1868 to 2005. Both monthly activity 
indices are smoothed with a 24-month FWHM Gaussian 
and then sampled at yearly intervals. At a given sunspot 
number there is a baseline level of geomagnetic activity 
which is proportional to the sunspot number. Geomagnetic 
activity extends above this level - particularly late in each 
solar cycle. This baseline level, aaR, is determined by fitting 
a line through the lower boundary of activity. 
amplitude (145 f 30 for the maximum of the smoothed 
monthly sunspot number) for cycle 24 based on the equa- 
torward drift rate of the active latitudes during cycle 22. 
Svalgaard et al. [2005] predicted a small amplitude (75 * 8) 
based on the weak polar fields observed on the Sun during 
the decline of sunspot cycle 23. A significant new devel- 
opment in predicting the solar activity cycle is the use of a 
dynamo model with assimilated sunspot data as described 
by Dikpati et al. [2006]. Using historical records for sunspot 
areas and positions over the last 130 years as input for the 
source of the surface magnetic fields that seed the dynamo, 
they predict an amplitude of 150-180 for cycle 24. 
[7] In this letter we examine recent geomagnetic activity 
using the methods described by Feynman [1982] and find 
that this activity indicates a much larger than average cycle 
for solar cycle 24 - on par with the prediction of Hathaway 
and Wilson [2004] and Dikpati et al. [2006]. 
2. Data and Methodology 
[SI Geomagnetic activity is measured by noting the rapid 
changes in the geomagnetic field strength and direction. 
Numerous observatories have made these measurements 
and a number of indices have been constructed to charac- 
terize the level of activity. The most widely used long-term 
index is the aa index. This index is produced using two 
observatories at nearly antipodal positions on the Earth's 
surface. The index is computed fiom the weighted average 
of the amplitude of the field variations at the two sites over 
three-hour intervals. Monthly averages of this index began 
in January of 1868 with Greenwich, England and 
Melbourne, Australia as the two sites. Greenwich was 
replaced by Abinger, England in 1926 and by Hartland, 
England in 1957. Melbourne was replaced by Toolangui, 
1860 1880 1900 1920 1940 1960 1980 2000 
Date 
Figure 2. Solar cycle, aaR, and interplanetary, aaI, 
components of geomagnetic activity as functions of time. 
The solar cycle component is directly proportional to the 
sunspot number and illustrates the sequence of cycle 
amplitudes for the last 13 solar cycles (numbered). The 
interplanetary component has a similar sequence of peaks 
but they occur several years earlier - usually before the 
time of cycle minimum. 
250 
200 
150 
100 
50 
0 
0 5 10 15 20 
Max(aa,) near Minimum 
Figure 3. Maximum sunspot number for the following 
cycle as a function of the maximum in the interplanetary 
component of the aa index. The following cycle amplitude 
is well correlated with the earlier peak in aaz with little 
scatter about a linear relationship (solid line with 1-sigma 
limits shown with dotted lines). The prediction for sunspot 
cycle 24 is shown with the circled number 24 - a maximum 
sunspot number of 160 f 25. 
similar sequence of peaks but shifted back in time - 
usually to before sunspot cycle minimum. It is this 
behavior of the interplanetary component that provides 
predictive capability. 
[n] The relationship between the peaks in the interplan- 
etary component of the aa index and the amplitude of the 
following sunspot cycle is shown in Figure 3. There is a 
strong positive correlation (correlation coefficient 0.94) 
between the two quantities and the chance of getting this 
relationship from uncorrelated quantities is less than about 
0.1%. 
[i3] The relationship shown in Figure 3 can be used to 
predict the size of the next solar cycle. The smoothed 
interplanetary component of the aa index peaked at a level 
of 13.7 in October of 2003. This indicates a maximum 
sunspot number of about 160 for sunspot cycle 24. This is 
similar to the amplitudes of cycles 21 and 22 but less than 
that of cycle 19. The 90% prediction interval is 160 f 25; 
therefore, there is only a 5% chance that cycle 24 maximum 
amplitude will be smaller than 135. 
3. Conclusions 
[i4] The geomagnetic index aa can be split into two 
components - one proportional to, and in phase with, the 
sunspot number and another, interplanetary, component aaI 
which is out of phase with the sunspot cycle. This second 
component has peaks in activity that mimic those seen in 
the sunspot number but shifted in time several years earlier. 
These peaks in aaI usually occur before the time of sunspot 
cycle minimum and provide an accurate prediction for the 
amplitude of the following sunspot cycle. The recent 
(October 2003) peak in aaI indicates a sunspot number 
maximum for cycle 24 of about 160 i 14. This prediction is 
very much in line with the predictions of a large cycle 24 by 
Hathaway and Wilson [2004] and by Dikpati et al. [2006] 
but in contrast to the prediction of a small cycle 24 by 
Svalgaard et al. [2005]. 
[15] All four of these predictions are based on different 
methods. The prediction of a small cycle 24 by Svalgaard et 
al. [2005] is based on a correlation observed between 
directly measured polar fields and sunspot number for the 
last three cycles, following the method of Schatten et al. 
[1978]. The Dikpati et al. [2006] prediction is on the firmest 
physical ground - a dynamo model based on our current 
knowledge of the dynamics of the Sun's convection zone 
(note, however, comments by Tobais et al. [2006]). All 
three of the high predictions are consistent with each other 
and are based on data from the last 12-13 sunspot cycles. 
The Dikpati et al. [2006] prediction is based on a dynamo 
model with observational data on sunspot areas and posi- 
tions. The Hathaway and Wilson [2004] prediction is based 
on an observed, significant but loose, relationship between 
the equatorward drift rate of the active latitudes and future 
solar activity. The prediction presented in this letter is based 
on an observed, significant and tight, relationship between 
geomagnetic activity and future solar activity. The consis- 
tent results with these three methods strongly suggests that 
cycle 24 will indeed be a large cycle. 
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D. H. Hathaway and R. M. Wilson, National Space Science and 
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(david. hathaway@nasa.gov) 
18101 HATHAWAY AND WILSON: LARGE AMPLITUDE FOR SUNSPOT CYCLE 24 L18101 
3 o f 3  


Geomagnetic Activity Indicates Large Amplitude for Sunspot Cycle 24

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