Long-term Solar Activity Research Articles

Lessons learned from predictions of Solar Cycle 24

Solar Cycle 24 has almost faded and the activity of Solar Cycle 25 is appearing. We have learned much about predicting solar activity in Solar Cycle 24, especially with the data provided by SDO and STEREO. Many advances have come in the short-term predictions of solar flares and coronal mass ejections, which have benefited from applying machine learning techniques to the new data. The arrival times of coronal mass ejections is a mid-range prediction whose accuracy has been improving, mostly due to a steady flow of data from SoHO, STEREO, and SDO. Longer term (greater than a year) predictions of solar activity have benefited from helioseismic studies of the plasma flows in the Sun. While these studies have complicated the dynamo models by introducing more complex internal flow patterns, the models should become more robust with the added information. But predictions made long before a sunspot cycle begins still rely on precursors. The success of some categories of the predictions of Solar Cycle 24 will be examined. The predictions in successful categories should be emphasized in future work. The SODA polar field precursor method, which has accurately predicted the last three cycles, is shown for Solar Cycle 25. Shape functions for the sunspot number and F10.7 are presented. What type of data is needed to better understand the polar regions of the Sun, the source of the most accurate precursor of long-term solar activity, will be discussed.

Holocene vegetation patterns in southern Lithuania indicate astronomical forcing on the millennial and centennial time scales

The Earth’s biota originated and developed to its current complex state through interacting with multilevel physical forcing of our planet’s climate and near and outer space phenomena. In the present study, we focus on the time scale of hundreds to thousands of years in the most recent time interval – the Holocene. Using a pollen paleocommunity dataset from southern Lithuania (Čepkeliai bog) and applying spectral analysis techniques, we tested this record for the presence of statistically significant cyclicities, which can be observed in past solar activity. The time series of non-metric multidimensional scaling (NMDS) scores, which in our case are assumed to reflect temperature variations, and Tsallis entropy-related community compositional diversity estimates q* revealed the presence of cycles on several time scales. The most consistent periodicities are characterized by periods lasting between 201 and 240 years, which is very close to the DeVries solar cycles (208 years). A shorter-term periodicity of 176 years was detected in the NMDS scores that can be putatively linked to the subharmonics of the Gleissberg solar cycle. In addition, periodicities of ≈3,760 and ≈1,880 years were found in both parameters. These periodic patterns could be explained either as originating as a harmonic nonlinear response to precession forcing, or as resulting from the long-term solar activity quasicycles that were reported in previous studies of solar activity proxies.

Stable Sunspot Area Level of Debrecen Photoheliographic Data and Multivariate Correction Factor of SOON Data

We investigate the spatial and temporal variation of sunspot group areas reported by the Greenwich Photoheliographic Results (GPR), the Solar Optical Observing Network (SOON), the Kislovodsk Mountain Astronomical Station (KMAS), and the Debrecen Photoheliographic Data (DPD) databases. We identify improved correction factors for reconciling these individual records to a common scale. Our results show that the DPD sunspot group areas are stable over the studied interval (1974 – 2014). We find an improved fit between GPR and DPD sunspot group areas when using a correction factor such that $\mathrm{GPR} = 0.975(\pm 0.006) \times \mathrm{DPD}$ , independent of the position of the sunspot group on the solar disk. We also find that the scale of KMAS sunspot group areas fits that of DPD well, but has a small position-dependent trend near the limb. However, in order to set SOON sunspot group area records onto the scale of DPD, we find that there is a need for a multivariate correction factor. This multivariate correction factor has a value ranging between 1.1 and 1.9 and is dependent upon the time of the SOON observation, the distance of the group from disk center, and the observatory within the SOON network. Finally, we provide further context to the systematic bias in SOON sunspot group area observations toward lower values relative to those recorded in the GPR and DPD databases that has previously been reported in the literature. We have identified the two main contributors to the SOON area deficit; some penumbral parts are unobserved, and the spot areas are underestimated. Our analysis is vital for studies that require stable, long-term solar activity records such as solar irradiance models that estimate irradiance reduction from records of sunspot group numbers, areas, and locations.

Sunspot drawings by Japanese official astronomers in 1749–1750

AbstractSunspot observations with telescopes in the 18th century were carried out in Japan as well as elsewhere. One of these sunspot observations is recorded in an account called Sansaizusetsu narabini Kansei irai Jissoku Zusetsu (Charts of Three Worlds and Diagrams of Actual Observations since Kansei Era). We have analyzed manuscripts of this account to show a total of 15 sunspot drawings during 1749–1750. These observations are considered to be carried out by contemporary official astronomers in Japan, with telescopes covered by zongurasus (< zonglas in Dutch, corresponding to “sunglass” in English). We counted their group number of sunspots to locate them in long-term solar activity and show that their observations were situated near the solar maximum in 1750. We also computed their locations and areas, while we have to admit differences of the variant manuscripts with one another. These observational records show the spread of sunspot observations not only in Europe, but also in Japan, and hence may contribute to crosscheck, or possibly to improve the known sunspot indices.

Reprint of A deterministic model for forecasting long-term solar activity

A phenomenological model is presented for the quantitative description of individual solar cycles' features, such as onset, intensity, evolution, in terms of the number of M and X-class solar flares. The main elements of the model are the relative ecliptic motion of the planets Jupiter and Saturn, and its synergy with a quasi-periodic component of solar activity. Using as input the temporal distribution of flares during cycle 21, the general evolution of cycles 22–24 is reproduced in notable agreement with the observations, including the resurgence of activity in the last months of 2017, and further predictions are provided for cycle 25. This deterministic description could contribute to elucidating the responsible physical mechanisms and forecasting space weather.

A deterministic model for forecasting long-term solar activity

A phenomenological model is presented for the quantitative description of individual solar cycles' features, such as onset, intensity, evolution, in terms of the number of M and X-class solar flares. The main elements of the model are the relative ecliptic motion of the planets Jupiter and Saturn, and its synergy with a quasi-periodic component of solar activity. Using as input the temporal distribution of flares during cycle 21, the general evolution of cycles 22–24 is reproduced in notable agreement with the observations, including the resurgence of activity in the last months of 2017, and further predictions are provided for cycle 25. This deterministic description could contribute to elucidating the responsible physical mechanisms and forecasting space weather.

Long-term solar activity studies using microwave imaging observations and prediction for cycle 25

We use microwave imaging observations from the Nobeyama Radioheliograph at 17 GHz for long-term studies of solar activity. In particular, we use the polar and low-latitude brightness temperatures as proxies to the polar and active-region magnetic fields, respectively. We also use the locations of prominence eruptions as a proxy to the filament locations to study their time variation. We show that the polar microwave brightness temperature is highly correlated with the polar magnetic field strength and the fast solar wind speed. We also show that the polar microwave brightness in one solar cycle is correlated with the low latitude brightness with a lag of about half a solar cycle. We use this correlation to predict the strength of the solar cycle 25: the smoothed sunspot numbers in the southern and northern hemispheres can be predicted as 89 and 59, respectively. These values indicate that cycle 25 will not be too different from cycle 24 in its strength. We also combined the rush-to-the-pole data from Nobeyama prominences with historical data going back to 1860 to study the north-south asymmetry of sign reversal at solar poles. We find that the reversal asymmetry has a quasi-periodicity of 3–5 cycles.

Evolution of the Sunspot Number and Solar Wind B $B$ Time Series

The past two decades have witnessed significant changes in our knowledge of long-term solar and solar wind activity. The sunspot number time series (1700-present) developed by Rudolf Wolf during the second half of the 19th century was revised and extended by the group sunspot number series (1610–1995) of Hoyt and Schatten during the 1990s. The group sunspot number is significantly lower than the Wolf series before ∼1885. An effort from 2011–2015 to understand and remove differences between these two series via a series of workshops had the unintended consequence of prompting several alternative constructions of the sunspot number. Thus it has been necessary to expand and extend the sunspot number reconciliation process. On the solar wind side, after a decade of controversy, an ISSI International Team used geomagnetic and sunspot data to obtain a high-confidence time series of the solar wind magnetic field strength ( $B$ ) from 1750-present that can be compared with two independent long-term (> ∼600 year) series of annual $B$ -values based on cosmogenic nuclides. In this paper, we trace the twists and turns leading to our current understanding of long-term solar and solar wind activity.

Analysis of multiple time scales feature and chaotic property for time series of relative sunspot numbers

Using observational data of the relative sunspot numbers from January 1749 to December 2016, the multiple time scales and chaotic characteristics have been analyzed based on the ensemble empirical mode decomposition method and chaotic dynamics theory, then the differences between the relative sunspot numbers and the time series with the periodic components removed are especially compared. The results show that the relative sunspot numbers are mainly composed of periodic components, chaotic components and a trend component, and they have multi-time scale and chaotic characteristics. Therefore, the long-term solar activity is the result of the physical mechanism of periodic phenomenon and the factors of chaotic dynamical characteristics. In the mul-me scale analysis, the results are still the same as the relative sunspot numbers removed chaotic components, but chaotic characteristics become stronger if the periodic components are removed.

Nighttime lower ionosphere height estimation from the VLF modal interference distance

We have studied the dynamics of the nighttime lower ionosphere height through continuous monitoring of the VLF modal interference distance (so-called distance D). Since the distance D is related to the nighttime propagation modes within the Earth-Ionosphere waveguide, it provides information of the nighttime reflection height (hN). We have used a long-term VLF narrowband database of almost 8 years (2006–2014) from a long transequatorial VLF propagation path between the transmitter NPM (Hawaii, 21.4 kHz) and the receiver ATI (Atibaia, Brazil). Our results show that hN assumes lower values during northern hemisphere wintertime as compared with summertime. By using the Lomb-Scargle periodogram, periodicities around 180 (SAO), 365 (AO) and 800 (QBO) days have been found, being the periodicity around 180 days stronger than all other oscillations. Since these large-scale oscillations are commonly observed in several measurable parameters of the mesosphere-lower thermosphere (MLT) region, our results suggest that the nighttime lower ionosphere can be strongly influenced by the dynamics of the MLT region. The effect of the long-term solar activity on hN is also studied, resulting in high negative correlation (R = −0.91). This effect makes hN decrease around 1.2 km from low to high solar activity. This result suggests a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night.

Long-Term Pulses of Dynamic Coupling Between Solar Hemispheres

North-south (N-S) asymmetry of solar activity is a known statistical phenomenon but its significance is difficult to prove or theoretically explain. Here we consider each solar hemisphere as a separate dynamical system connected with the other hemisphere via an unknown coupling parameter. We use a non-linear dynamics approach to calculate the scale-dependent conditional dispersion (CD) of sunspots between hemispheres. Using daily Greenwich sunspot areas, we calculated the Neumann and Pearson chi-squared distances between CDs as indices showing the direction of coupling. We introduce an additional index of synchronization which shows the strength of coupling and allows us to discriminate between complete synchronization and independency of hemispheres. All indices are evaluated in a four-year moving window showing the evolution of coupling between hemispheres. We find that the driver-response interrelation changes between hemispheres have a few pulses during 130 years of Greenwich data with an at least 40 years-long period of unidirectional coupling. These sharp nearly simultaneous pulses of all causality indices are found at the decay of some 11-year cycles. The pulse rate of this new phenomenon of dynamic coupling is irregular: although the first two pulses repeat after 22-year Hale cycles, the last two pulses repeat after three and four 11-year cycles respectively. The last pulse occurs at the decay phase of Cycle 23 so the next pulse will likely appear during the decay of future Cycle 25 or later. This new phenomenon of dynamic coupling reveals additional constraints for understanding and modeling of the long-term solar activity cycles.

Multiple dynamo modes as a mechanism for long-term solar activity variations

Solar magnetic activity shows both smooth secular changes, such as the Grand Modern Maximum, and quite abrupt drops that are denoted as Grand Minima. Direct numerical simulations (DNS) of convection driven dynamos offer one way of examining the mechanisms behind these events. In this work, we analyze a solution of a solar-like DNS that has been evolved for roughly 80 magnetic cycles of 4.9 years, during which epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. The DNS employed is a semi-global (wedge) magnetoconvection model. For data analysis we use Ensemble Empirical Mode Decomposition (EEMD) and phase dispersion ($D^2$) methods. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like. This mode is accompanied by a higher frequency mode near the surface and a low-frequency mode in the bottom of the convection zone. The overall behavior of the dynamo solution is very complex exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum. We interpret the overall irregular behavior to be due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.

MODELING THE VARIATIONS OF DOSE RATE MEASURED BY RAD DURING THE FIRSTMSLMARTIAN YEAR: 2012–2014

The Radiation Assessment Detector (RAD), on board Mars Science Laboratory's (MSL) rover Curiosity, measures the {energy spectra} of both energetic charged and neutral particles along with the radiation dose rate at the surface of Mars. With these first-ever measurements on the Martian surface, RAD observed several effects influencing the galactic cosmic ray (GCR) induced surface radiation dose concurrently: [a] short-term diurnal variations of the Martian atmospheric pressure caused by daily thermal tides, [b] long-term seasonal pressure changes in the Martian atmosphere, and [c] the modulation of the primary GCR flux by the heliospheric magnetic field, which correlates with long-term solar activity and the rotation of the Sun. The RAD surface dose measurements, along with the surface pressure data and the solar modulation factor, are analysed and fitted to empirical models which quantitatively demonstrate} how the long-term influences ([b] and [c]) are related to the measured dose rates. {Correspondingly we can estimate dose rate and dose equivalents under different solar modulations and different atmospheric conditions, thus allowing empirical predictions of the Martian surface radiation environment.

Solar activity, cosmic rays, and earth temperature reconstructions for the past two millennia. Part 2. Analysis of the relation between the global temperature variations and natural processes

Regularities in the long-term global temperature variations for the past two millennia have been analyzed. It was shown that the periods of a prolonged temperature rise and drop were most pronounced before the industrial epoch. Analysis indicates that the extrema in the global temperature variations in the first and second millennia correspond to long-term solar activity increases and decreases. The performed analysis of temperature reconstructions, which were performed using different methods (including the geothermal method), show that the observed climate change during the past two millennia is in good agreement with the variations in the concentration of the 14C and 10Be cosmogenic isotopes modulated by time-varying solar activity.

What do the solar activity indices represent?

AbstractSunspot number, sunspot area, and radio flux at 10.7 cm are the indices which are most frequently used to describe the long‐term solar activity. The data of the daily solar full‐disk magnetograms measured at Mount Wilson Observatory from 19 January 1970 to 31 December 2012 are utilized together with the daily observations of the three indices to probe the relationship of the full‐disk magnetic activity respectively with the indices. Cross correlation analyses of the daily magnetic field measurements at Mount Wilson observatory are taken with the daily observations of the three indices, and the statistical significance of the difference of the obtained correlation coefficients is investigated. The following results are obtained: (1) The sunspot number should be preferred to represent/reflect the full‐disk magnetic activity of the Sun to which the weak magnetic fields (outside of sunspots) mainly contribute, the sunspot area should be recommended to represent the strong magnetic activity of the Sun (in sunspots), and the 10.7 cm radio flux should be preferred to represent the full‐disk magnetic activity of the Sun (both the weak and strong magnetic fields) to which the weak magnetic fields mainly contribute. (2) On the other hand, the most recommendable index that could be used to represent/reflect the weak magnetic activity is the 10.7 cm radio flux, the most recommendable index that could be used to represent the strong magnetic activity is the sunspot area, and the most recommendable index that could be used to represent the full‐disk magnetic activity of the Sun is the 10.7cm radio flux. Additionally, the cycle characteristics of the magnetic field strengths on the solar disk are given. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-dimensional chaos and fractal properties of long-term sunspot activity

Two primary solar-activity indicators–sunspot numbers (SNs) and sunspot areas (SAs) in the time interval from November 1874 to December 2012 – are used to determine the chaotic and fractal properties of solar activity. The results show that (1) the long-term solar activity is governed by a low-dimensional chaotic strange attractor, and its fractal motion shows a long-term persistence on large scales; (2) both the fractal dimension and maximal Lyapunov exponent of SAs are larger than those of SNs, implying that the dynamical system of SAs is more chaotic and complex than SNs; (3) the predictions of solar activity should only be done for short- to mid-term behaviors due to its intrinsic complexity; moreover, the predictability time of SAs is obviously smaller than that of SNs and previous results.

Long-term solar activity and its implications to the heliosphere, geomagnetic activity, and the Earth’s climate

The Sun’s long-term magnetic variability is the primary driver of space climate. This variability is manifested not only in the longobserved and dramatic change of magnetic fields on the solar surface, but also in the changing solar radiative output across all wavelengths. The Sun’s magnetic variability also modulates the particulate and magnetic fluxes in the heliosphere, which determine the interplanetary conditions and impose significant electromagnetic forces and effects upon planetary atmospheres. All these effects due to the changing solar magnetic fields are also relevant for planetary climates, including the climate of the Earth. The ultimate cause of solar variability, at time scales much shorter than stellar evolutionary time scales, i.e., at decadal to centennial and, maybe, even millennial or longer scales, has its origin in the solar dynamo mechanism. Therefore, in order to better understand the origin of space climate, one must analyze different proxies of solar magnetic variability and develop models of the solar dynamo mechanism that correctly produce the observed properties of the magnetic fields. This Preface summarizes the most important findings of the papers of this Special Issue, most of which were presented in the Space Climate-4 Symposium organized in 2011 in Goa, India.

Is there a planetary influence on solar activity?

Context. Understanding the Sun’s magnetic activity is important because of its impact on the Earth’s environment. Direct observations of the sunspots since 1610 reveal an irregular activity cycle with an average period of about 11 years, which is modulated on longer timescales. Proxies of solar activity such as 14 C and 10 Be show consistently longer cycles with well-defined periodicities and varying amplitudes. Current models of solar activity assume that the origin and modulation of solar activity lie within the Sun itself; however, correlations between direct solar activity indices and planetary configurations have been reported on many occasions. Since no successful physical mechanism was suggested to explain these correlations, the possible link between planetary motion and solar activity has been largely ignored. Aims. While energy considerations clearly show that the planets cannot be the direct cause of the solar activity, it remains an open question whether the planets can perturb the operation of the solar dynamo. Here we use a 9400 year solar activity reconstruction derived from cosmogenic radionuclides to test this hypothesis.Methods. We developed a simple physical model for describing the time-dependent torque exerted by the planets on a non-spherical tachocline and compared the corresponding power spectrum with that of the reconstructed solar activity record. Results. We find an excellent agreement between the long-term cycles in proxies of solar activity and the periodicities in the planetary torque and also that some periodicities remain phase-locked over 9400 years.Conclusions. Based on these observations we put forward the idea that the long-term solar magnetic activity is modulated by planetary effects. If correct, our hypothesis has important implications for solar physics and the solar-terrestrial connection.

A Study of Long-Term Solar Activity at 37 GHz

Abstract In this paper we investigate the solar activity at the radio frequency (37 GHz) using an extensive data series (solar radio maps) from the Metsähovi Radio Observatory. This paper aims to present this unique solar radio map collection to the public knowledge. The data set covers the years from 1978 to 2011 (solar cycles 21–24). We investigate the long-term solar activity on the ground of the distribution of solar radio brightenings and the differential rotation of the Sun.

Nitrate in Polar Ice: A New Tracer of Solar Variability

Knowledge of the long-term variability of solar activity is of both astrophysical and geoscientific interest. Reconstructions of solar activity over multiple millennia are traditionally based on cosmogenic isotopes 14C or 10Be measured in natural terrestrial archives, but the two isotopes exhibit significant differences on millennial time scales, so that our knowledge of solar activity at this time scale remains somewhat uncertain. Here we present a new potential proxy of solar activity on the centennial-millennial time scale, based on a chemical tracer, viz. nitrate content in an ice core drilled at Talos Dome (Antarctica). We argue that this location is optimal for preserving the solar signal in the nitrate content during the Holocene. By using the firn core from the same location we show that the 11-year and Gleissberg cycles are present with the variability of 10 – 25 % in nitrate content in the pre-industrial epoch. This is consistent with the results of independent efforts of modeling HNO3 and NO y in Antarctic near surface air. However, meteorological noise on the interannual scale makes it impossible to resolve individual solar cycles. Based on different processes of formation and transport compared to cosmogenic isotopes, it provides new, independent insight into long-term solar activity and helps resolve the uncertainties related to cosmogenic isotopes as diagnostics of solar activity.