A two-wave dynamo model by Zharkova et al. (2015) disagrees with data on long-term solar variability
aa r X i v : . [ a s t r o - ph . S R ] D ec A two-wave dynamo model by Zharkovaet al. (2015) disagrees with data onlong-term solar variability
I.G. Usoskin , , G.A. Kovaltsov , ReSoLVE Centre of Excellence, University of Oulu, Finland Sodankyl¨a Geophysical Observatory (Oulu unit), University of Oulu, Fin-land Ioffe Physical-Technical Institute, St.Petersburg, Russia
Abstract
A two-wave dynamo model was recently proposed by Zharkova et al.(2015, Zh15 henceforth), which aims at long-term predictions of solar ac-tivity for millennia ahead and backwards. Here we confront the backwardpredictions for the last 800 years with known variability of solar activ-ity, using both direct sunspot observations since 1610 and reconstructionsbased on cosmogenic nuclide data. We show that the Zh15 model failsto reproduce the well-established features of the solar activity evolutionduring the last millennium. This means that the predictive part for thefuture is not reliable either.
A new view on the dynamo mechanism operated in the Sun’s convectionzone was proposed recently by Zharkova et al. ([1] Zh15, henceforth).According to Zh15, the model is able to predict variability of solar activity n millennial time scale. In particular, in Fig.3 of Zh15, a “prediction” ofsolar variability is shown for 1200 years ahead and 800 years backwards.While future predictions are unverifiable, the past solar activity is knownquite well for the last millennium and can be easily confronted with thepredictions by Zh15. As we show here, Zh15 work fails in reconstructionof the past solar activity and accordingly is not trustworthy in predictions. We confronted the results of the Zh15 “prediction” of the past solar activ-ity with available data obtained either directly from sunspot observationsfor the last 400 years or from cosmogenic radionuclide ( C in tree ringsand Be in ice cores) data, which form a direct proxy for cosmic raysvariability and thus for solar magnetic activity [2, 3]. Cosmogenic data,particularly radiocarbon C, cannot reproduce the 11-year cycle. Sincewe focus here on the centennial variability, we further discuss decadaldata which is the time resolution of many cosmogenic nuclide series (e.g.,[4]). Accordingly, the data from Zh15 and all other data with sub-decadaltime resolution were resampled to become 10-year averages. For the ZH15data we used the modulus of the “summary wave” of the Sun’s poloidalmagnetic field shown in their Figure 3.The comparison is shown in Figure 1. One can see that the predictedcurve by Zh15 (grey bars) disagrees with the real solar variability (colored Since the authors of Zh15 have strictly rejected to provide original data for an independenttest, we extracted their data from Figure 3 of Zh15 paper, which may lead to some minorinexactitudes but does not affect the centennial pattern. urves). For example, the linear Pearson’s correlation between the resultby Zh15 and that by [9] (St12) curve is 0 . ± .
26, which means no corre-lation. Other long-term curves also imply no or insignificant correlation.The Wolf minimum ca. 1300 is correctly reproduced by the model butit is the only success. Solar activity is predicted to be very high, compara-ble to the modern grand maximum, during the period of 1350 – 1650, butin fact the longest and deepest minimum during the last millennium, theSp¨orer minimum, took place during 1390 – 1550 (see Table 1). Thus, theSp¨orer minimum is not reproduce at all, on the contrary, a high activityis predicted by the Zh15 model. A suppression centered at the Maunderminimum is observed in the model prediction but it is much longer (about200 years) and disagrees with the periods of high activity ca. 1600 andin the second half of the 18th century [11, 14]. The Dalton minimum isnot reproduced correctly, as it falls on the period of the fast growth of thepredicted activity. The Gleissberg (or Modern) minimum of activity ca.1900 also disagrees with the predicted data, which reach the centennialmaximum at that time.Thus, we conclude that the model of Zh15 fails in reconstructing solaractivity in the past, especially during the 14th through 16th centurieswhen the prediction is in antiphase with the real solar activity. Thismakes long-term predictions based on this model unreliable. † Medieval Warm Period 1350 – 1530 950 – 1250 [12] †† see Table 5.1. Zh15 paper claims that the model prediction “corresponds very closely tothe sunspot data observed in the past 400... [and] predicts correctly manyfeatures from the past, such as: 1) an increase in solar activity during theMedieval Warm period; 2) a clear decrease in the activity during the LittleIce Age, the Maunder Minimum and the Dalton Minimum; 3) an increasein solar activity during a modern maximum in 20th century.” Here weshow that this statement, based on a visual inspection of Figure 3 of Zh15,is not correct. We noticed that most of the horizontal bars indicatingdifferent periods in Figure 3 of Zh15 paper are misleading. Since referencesto these bars are not provided, we copied the dates directly from Figure3 and collected them into Table 1 here along with the appropriate dates. ne can see that, for example the dates of the Medieval Warm Pe-riod (conventionally called the Medieval Climate Anomaly) was shiftedby Zh15 for several hundred years ahead. The appropriate dates of theAnomaly is beyond the frame of Figure 3 in Zh15. Other dates are alsomisplaced to ‘match’ the prediction curve. As shown here, the long-term reconstruction of solar activity, using thetwo-wave dynamo model, as presented by Zh15 fails in reproducing thewell-established pattern of solar variability during the last millennium.Thus this model cannot be trusted for the future predictions.
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200 1400 1600 1800 2000020406080100
LIAMCA DMMMSMWM A r b i t r a r y un i t s Years
Zharkova15 GSN ISN So04 Mu07 St12 Us14 In15
Figure 1: Deacadal (10-yr averaged) solar variability. The grey bars repre-sent the modulus of the “summary wave” from Figure 3 of Zh15 (see footnote1), while colored curves depict different measured and proxy data for solarvariability for the period 1200 – 2000 AD, as denoted in the legend: GSN –group sunspot number [5]; ISN – international sunspot number, v.2.0 [6]; So14– sunspot reconstruction from C [7]; Mu07 – modulation potential from C[8]; St12 – modulation potential from C and Be [9]; Us14 – sunspot recon-struction from C [10]; In15 – modulation potential from C and10