A&A 441, 347-352 (2005)
DOI: 10.1051/0004-6361:20053201
I. G. Usoskin^{1} - S. V. Berdyugina^{2,3} - J. Poutanen^{2}
1 - Sodankylä Geophysical Observatory (Oulu unit),
90014 University of Oulu, Finland
2 -
Astronomy Division, PO Box 3000,
90014 University of Oulu, Finland
3 -
Institut für Astronomie, ETHZ, 8092 Zürich,
Switzerland
Received 6 April 2005 / Accepted 15 June 2005
Abstract
As recently found, the distribution of sunspots is non-axisymmetric and
spot group formation implies the existence of two persistent active longitudes
separated by 180.
Here we quantitatively study the non-axisymmetry of sunspot occurrence.
In a dynamic reference frame inferred from the differential
rotation law, the raw sunspot data show a clear clustering around the
persistent active longitudes.
The differential rotation describing the dynamic frame is quantified in
terms of the equatorial angular velocity and the differential rotation rate,
which appear to be significantly different from those for individual sunspots.
This implies that the active longitudes are not linked to the depth of
sunspot anchoring. In order to quantify the observed effect,
we introduce a measure of the non-axisymmetry of the sunspot
distribution. The non-axisymmetric component is found to be
highly significant, and the ratio
of its strength to that of the axisymmetric one is roughly 1:10.
This provides additional constraints for solar dynamo models.
Key words: Sun: activity - Sun: magnetic fields - Sun: sunspots
The question whether sunspots appear randomly in longitudes has been a long-standing issue since the early 20th century. Although the existence of preferred longitudes of sunspot formation (active longitudes) has been suggested long ago, the question of their persistency was still a subject of ongoing debates (e.g., Chidambara 1932; Lopez Arroyo 1961; Balthasar & Schüssler 1983; Vitinsky et al. 1986; Mordvinov & Kitchatinov 2004). A novel analysis of sunspot group data for the past 120 years revealed the existence of two persistent active longitudes separated by 180 (Berdyugina & Usoskin 2003, BU03 henceforth). In BU03 we have shown, using different filtering techniques, that the active longitudes are persistent on a century time scale. An important conclusion of our previous work is that the active longitudes are not fixed in any reference frame (e.g., in the Carrington system), but continuously migrate in longitude with a variable rate. Their migration is defined by changes of the mean latitude of the sunspot formation and the differential rotation. Neglecting this migration results in smearing of the active longitude pattern on time scales of more than one solar cycle. In contrast with the findings of BU03, most previous researchers assumed rotation of the active longitudes with a constant rate, which explains the diversity and contradictions of the previously published results.
The solar active longitudes and their behaviour are very similar to stellar activity phenomena, including the flip-flop cycle detected in binaries and solar-type stars (Berdyugina & Tuominen 1998; Berdyugina 2004; Berdyugina & Järvinen 2005). On the Sun the flip-flop phenomenon is observed as the alternation of the major spot activity between the opposite longitudes with a 3.7 year cycle (BU03). The similarity between the sunspot distribution and the activity patterns on cool active stars implies that non-axisymmetry is a fundamental feature of the solar and stellar dynamo mechanisms.
The persistent migrating active longitudes imply the existence of a non-axisymmetric component in the solar dynamo and provide new observational constraints for current solar dynamo models. Therefore, it is important to quantify this effect. As found by BU03, the migration of the active longitudes is defined by the differential rotation and mean latitude of sunspot formation. In the present paper we fit this model to raw sunspot data and determine the differential rotation of the active longitudes. Based on that, we introduce a dynamic reference frame and investigate the distribution of the sunspot area in this new frame. This allows us to account for the migration of the active longitudes. We reveal a double-peaked longitude distribution of the spot area for all sunspots without any filtering and also for the single, strongest spot group observed in each Carrington rotation. More impressively, such a distribution is also found for the single, strongest spot group observed in each Carrington rotation. Finally, we introduce a measure of the non-axisymmetry of the sunspot distribution and estimate a relative strength of the axisymmetric and non-axisymmetric components. The differential rotation law obtained and the measure of the non-axisymmetry can be used to constrain the corresponding dynamo models. Our new analysis confirms the previous conclusions by BU03 on a new basis and dispels the doubts expressed by Pelt et al. (2005) that the active longitude separation is an artefact of the data processing.
In this paper we analyse sunspot group data for the past 120 years. We use daily data on sunspot group locations and areas collected at the Royal Greenwich Observatory, the US Air Force and the National Oceanic and Atmospheric Administration for the years 1878-1996, covering 11 full solar cycles. Here we are primarily interested in the sunspot appearance rather than in their evolution. Accordingly, each spot was included into the analysis only once when it was first mentioned in the database (either on the day of its birth or when it appeared at the East limb), all later records of the spot were ignored. Therefore, in contrast to earlier studies, we analyze only the sunspot emergence. Because of the known asymmetry between the Northern and Southern hemispheres, we investigate them separately. About 40 000 sunspots occurred during about 1600 Carrington rotations have been considered in each hemisphere.
As suggested in BU03, the migration of the active longitudes
on the Sun is defined by the differential rotation and by changes of the
mean spot latitude.
A standard model of the differential rotation on the Sun relates
an angular velocity
with a helio-latitude
as follows:
Figure 1: The longitudinal distribution of the sunspot area during cycle No. 19. The vertical axis denotes the longitude and horizontal axis the time. The upper panel shows the observed Carrington longitudes and the expected migration path of the two active longitudes (shown by squares and triangles) given by Eq. (3) with B=3.40 and deg day^{-1}. The lower panel shows the same plot but after the longitude correction, i.e. subtraction of the migration path. For better visualization, each plot was filtered using the Lee filter (Lee 1986). | |
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The model predicts that for the time
the Carrington system
makes one full rotation, while the new reference frame does
rotations.
The difference in longitude accumulated between the two systems over many Carrington periods
results in migration of the active longitudes, including
a number of full rotations (see BU03).
In the following, we discard the full rotations and consider only the circular
longitude.
In the new reference frame, the longitude of a kth spot
in the ith Carrington rotation,
,
is found as follows:
The separation of the active longitudes in the dynamic reference frame is clearly seen in Fig. 2 which shows histograms of the area weighted sunspot occurrence in the corrected longitude for the whole data set in the Northern hemisphere. The sunspot distribution in the Carrington frame shows no preferred longitudes (Fig. 2a). The same distribution, but in the dynamic system, shows a clear preference to cluster at two corrected longitudes (Fig. 2b). Note that this histogram shows the longitude distribution of actual spot group areas in the dynamic frame (Eq. (4)), without any filtering, smoothing or other processing of the raw data. This is consistent with our earlier result (BU03) that the signature of the migrating active longitudes is totally smeared out in the Carrington system within 1-2 solar cycles, while a careful account for the migration of the active longitudes allows us to reveal their persistence. A more pronounced double-peaked distribution of the sunspot area is obtained when considering not all spots but only the largest spot within each Carrington rotation (Fig. 2c). Because of the flip-flop effect the two active longitudes are clearly revealed. We emphasize that further processing and averaging of the data significantly increase the revealed non-axisymmetry because of the suppression of the axisymmetric part. For instance, semiannual averaging produces a very significant double-peaked distribution (Figs. 2d and 5 in BU03). Very similar histograms are obtained for the Southern hemisphere (Fig. 3). The filtered distributions (panels c and d) are shown here only for the purpose of visualisation, and further we will deal only with the raw data distribution shown in Figs. 2b and 3b.
Figure 2: Longitudinal distributions of sunspot occurrence in the Northern hemisphere for the period 1878-1996. a) Actual sunspots (area weighted) in the Carrington frame. The distribution is nearly isotropic. b) The same as panel a) but in the dynamic reference frame, the non-axisymmetry measure is . c) Only the position of one dominant spot for each Carrington rotation is considered, . d) The same as in panel c) but for 6-months averages, . The solid line depicts the best fit double Gaussian. | |
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Figure 3: The same as Fig. 2 but for the Southern hemisphere. The value of the non-axisymmetry is 0.09, 0.15 and 0.39 for the panels b), c) and d), respectively. | |
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Let us now estimate the parameters of the differential rotation
(Eq. (2)) which fit the raw sunspot group data.
For this purpose we employ the least mean squares method.
First, we define the deviation between the model and the data as
(6) |
(7) |
In order to evaluate the confidence intervals for the best-fitting model parameters,
one needs to estimate the uncertainties of the observed data with respect
to the fitting model.
From the best-fitting parameters corresponding to the
we can
build the distribution of the raw data around the expected model
(Fig. 2b).
This distribution is nearly double Gaussian with the standard deviation
of about
.
We adopt this value as a measure of the random scattering of the data with respect to the
model.
Then we can compute an analogue of
for our model as follows,
(8) |
Figure 4: The distribution of the values in the space of the model parameters B and sidereal (Eq. (2)) for the Northern and Southern hemispheres. The black area depicts the 68% confidence region ( ). | |
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The best-fitting parameters are deg day^{-1} and deg day^{-1} for the Northern hemisphere, and deg day^{-1} and deg day^{-1} for the Southern hemisphere, where the quoted errors correspond to the 90% confidence interval for one parameter of interest with . The results for the North and South being close to each other are nevertheless somewhat different. The difference is formally significant but, taking the uncertainties of the parameter values obtained above as a rough estimate, we can only mention its indicative nature. The distributions in Figs. 2 and 3 in the dynamic reference frame were built using the above best-fitting values of the model parameters. The average value of B is close to that obtained by BU03. We note that the result from BU03 corresponds to a local minimum near and B=3.46 deg day^{-1} in Fig. 4.
When repeating the same procedure for individual cycles, we obtained the best-fitting parameters B varying from 1.5 to 4 deg day^{-1}, and from 13.7 to 14.7 deg day^{-1} (see Fig. 5). There is a general tendency that smaller B are paired with larger . Despite the large spread of the best-fitting parameters for individual cycles, the necessity for the differential rotation is apparent, because no good fit can be found for B=0. Formal averaging over individual cycles yields the values of deg day^{-1} and deg day^{-1}. The analysis for individual cycles could not possibly be used to check the persistence of the active longitudes, because each cycle is fitted independently. However, extending the studied interval not only systematically tightens the allowed area in the parameter space towards the small area determined for the entire data set, as illustrated in Fig. 5, but also proves the persistency of the phenomenon.
Figure 5: The area of parameters of the differential rotation in the Northern hemisphere defined using the entire studied interval of 1878-1996 (large cross) as well as from sub-intervals: individual cycles (dotted), 3-cycle intervals (dashed) and 5-cycle intervals (solid). | |
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In Fig. 6 we compare the present results for the differential rotation of the active longitudes with measurements obtained from sunspots (Balthasar et al. 1986) and surface plasma Doppler shifts (SOHO/MDI, Schou et al. 1998). Note that the SOHO/MDI data were originally approximated by Eq. (1). As seen from Fig. 6, at all latitudes the active longitudes rotate significantly slower than individual sunspots, which is in agreement with the previous finding by BU03. This implies that the active longitudes are not linked to the depth where developed sunspots are anchored. The difference in the rotation of the active longitudes and individual spots is about 0.2 deg day^{-1} which produces a lag of about 2 full rotations during a solar cycle as reported by BU03. A comparison with the SOHO/MDI measurements is more difficult because of the different shapes of the used approximations. It appears that the active longitudes rotate faster than the local plasma at low latitudes ( ) and vice-versa at higher latitudes.
The existence of the active longitudes implies a long-term asymmetry in
the sunspot longitudinal distribution which is related to a non-axisymmetric component
of the dynamo mechanism.
In order to quantify this we introduce a measure of the non-axisymmetry in the following way.
Let us choose the value of
so that the two active longitudes correspond to the
corrected longitudes of 0
and 180
(see Fig. 2b).
Depending on the value of the corrected longitude
,
the sunspot with the normalized area A_{ki} contributes to either of the two numbers
as follows.
N_{1} | = | ||
N_{2} | = | (9) |
Figure 6: The sidereal differential rotation of the active longitudes determined in this work compared with that obtained using surface Doppler shifts (SOHO/MDI, Schou et al. 1998) and sunspots (Balthasar et al. 1986). | |
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The non-axisymmetry of the area-weighted sunspot distribution for the entire studied series shown in Figs. 2b and 3b is 0.11 and 0.09 for the Northern and Southern hemispheres, respectively. Without normalizing the spot area (Eq. (5)), the distributions indicate the non-axisymmetry of 0.07. We conclude, therefore, that the strength of the nonaxisymmetric component is roughly 1:10 of that of the axisymmetric one as observed in the sunspot distribution for the last 120 years. For individual cycles takes values from 0.07 to 0.3. We note that at the best-fitting parameters minimizing discrepancy (see Fig. 4), does not necessarily reach the maximum value, being, however, rather close to it. The dependence of on the parameter B is shown in Fig. 7 (parameter is now chosen to provide the maximum possible ). One can see that the relation has a single peak at about B=3.42-3.43 deg day^{-1} (close to the best-fitting value B=3.39-3.40 deg day^{-1} minimizing ) and decreases when deviating from it. If the longitudes are not corrected for the migration (i.e., B=0), the non-axisymmetry is (see Fig. 7a) which is consistent with the null-hypothesis of the axisymmetric distribution (see below). This shows again that neglecting the differential rotation results in complete smearing of the pattern.
Figure 7: The measure of the non-axisymmetry as a function of the differential rotation parameter B for the Northern ( left panel) and Southern ( right panel) hemispheres. The values of B=3.42 and as denoted by the vertical lines, correspond to the maximum in the Northern and Southern hemispheres, respectively. | |
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In order to check the significance of the obtained non-axisymmetry measure we have performed the following test. For each Carrington rotation we have randomly permuted all the sunspots, i.e., a new random Carrington longitude has been ascribed to each actually observed sunspot while keeping its area. Then the value of has been calculated as described above. We have computed the non-axisymmetry for 5000 sets of such random-phase sunspot occurrence. The distribution of the value of shown in Fig. 8 depicts a nearly Poisson distribution. According to this simulation, the hypothesis of rotation of the active longitudes with a fixed rate (giving ; see Figs. 2a, 3a, and 7a,b) cannot be distinguished from the null hypothesis of the axisymmetric sunspot distribution. On the other hand, the probability to obtain (0.09) for an axisymmetric distribution is <10^{-6} (10^{-5}) for the Northern (Southern) hemispheres. This means that the non-axisymmetric component does really exist in the raw data of sunspot occurrence and at the very high significance level. For the case when only the dominant spot is considered (Figs. 2c and 3c) , implying that nearly 60% of the major spots appear in the vicinity of the active longitudes. Moreover, the non-axisymmetric mode dominates the semiannually averaged sunspot occurrence (Figs. 2d and 3d).
Active longitudes on the Sun were commonly expected to rotate with a constant rate. This a priori assumption led previous researchers to the conclusion that the active longitudes, if exist, are not stable in their appearance. The recent analysis of the sunspot distribution revealed however that there are two persistent active longitudes which migrate according to the differential rotation law (BU03). In this paper we have confirm this finding and determined the parameters of the differential rotation law affecting the active longitude migration. We emphasize that in the present paper we employ a different approach compared to that used in BU03, where the data were analysed to reveal the underlying regularities. Here we fit a theoretical model of the differential rotation to the raw data without any pre-processing or filtering of the latter. Previously (BU03) we investigated only the non-axisymmetric part of the sunspot distribution while efficiently suppressing the axisymmetric one. This was criticized by Pelt et al. (2005) who claimed that the active longitude pattern found in BU03 is an artefact of the used method. The present analysis, which is based on the raw observed sunspot areas, answers their criticism and confirms that the phenomenon of the persistent active longitudes is real.
Figure 8: The distribution of the non-axisymmetry calculated for 5000 sets of random-phase sunspot occurrence. | |
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The main conclusions obtained in the present paper are listed below.
Acknowledgements
We thank Dmitry Sokoloff and David Moss for stimulating and useful discussions. Solar data have been obtained from the GRO and USAF/NOAA web site http://science.nasa.gov/ssl/pad/solar/greenwch.htm. The Academy of Finland is acknowledged for the support, grant 43039.