Long-term Solar Variability Research Articles

Long-term oscillations and trends of the mesosphere derived from 60 Years of standard phase-heights measurements over Europe: An update

The time series of standard phase-height (SPH) and plasma scale-height (PSH) have been updated from a 60-year long-radio-wave measurement of the broadcasting station Allouis (France, 162 kHz). The signal was received at Kühlungsborn (54° N, 12° E, Mecklenburg, Northern Germany).The statistical analysis of the SPH series shows a significant overall trend with a decrease of 116 m/decade indicating a subsidence of the long-radio wave reflection height of about 700 m. With consideration of a stratopause altitude trend (-70 m/decade) follows an overall mesospheric shrinking of about 300 m over Western Europe.As expected the time series of SPH shows in its spectrum dominant modes which are typical for the solar cycle, ENSO and for QBO bands indicating solar and lower atmospheric influences. Solar cycle and ENSO (-QBO)-like band-pass show a growing increase of SPH up to 1987, followed by a decrease afterward. We found a strong reduction in the amplitude of the solar cycle band due to the weak solar cycle 24, but an increase in the ENSO band.For summer months during solar minimum years, and without stratopause altitude trend, a thickness temperature trend of the mesosphere is significant with a trend value of −0.47 ± 0.43 K/decade. The long-term solar variability and the stratopause altitude trend were excluded to determine a more realistic intrinsic mesospheric thickness temperature trend. The overall cooling of the intrinsic mesospheric temperature during 60 years of observation is in the order of 3 K.The long-term solar variability including the decreasing maximum of last solar cycle, and the stratopause altitude trend have to be excluded in order to determine an intrinsic mesospheric temperature trend, which may be caused by greenhouse gas increase in the middle atmosphere.

Katsue Misawa's sunspot observations in 1921–1934: a primary reference for the Wolfer-Brunner transition

Abstract Our knowledge of the long-term solar variability is largely based on the International Sunspot Number time series (SN), a composite index based on multiple visual sunspot observers from the 18th century onward and maintained by the World Data Center ‘Sunspot Index and Long-term Solar Observations’ (SILSO). However, over the period 1919–1944, our capacity to diagnose the homogeneity of this time series is currently limited because most of the archived source data of the Zürich Observatory are presently missing over that interval and were not published. Therefore, the recovery of any long-duration series from an individual sunspot observer active during this period is essential to bridge this Zürich data gap. In this context, Katsue Misawa has conducted regular sunspot observations from 1921 to 1934 (mean coverage of 25.4 days/month), which were not accessible for the Zürich Observatory and thus form a valuable addition to the database maintained by the WDC-SILSO. In this study, we digitized his observational records, documented his observing technique, and reconstructed his total and hemispheric sunspot numbers. We compared his data with the International Sunspot Number (current version V2) and evaluated their stability. Misawa's data series generally agrees well with SN V2. However, Misawa's data show a significant transitory drift in 1925–1928, when the Zürich pilot observer changed from Alfred Wolfer to William Otto Brunner.

Long-term solar variability: ISWAT S1 cluster review for COSPAR space weather roadmap

The Committee on Space Research (COSPAR) is updating its Roadmap on Space Weather. As input for this update, the COSPAR International Space Weather Action Teams (ISWAT) were asked to provide an overview of the current state-of-the-art and advancements since the last Roadmap (Schrijver et al., 2015), identifying gaps and opportunities for moving forward within the next 5 years — based on ongoing and planned missions, available modeling, and observational capabilities — and presenting an outlook beyond 5 years and recommendations on reaching long-term goals. While space weather is typically associated with short-term solar activity, knowledge of past solar variability observed and recorded through various parameters, including historical space weather events, informs us about the range of possible solar fluctuations. This long-term solar variability, belonging to the domain of space climate, is the prime focus of the ISWAT S1 Cluster. The goal of this paper is to describe the key objectives of the three S1 Action Teams, summarize the current state of knowledge of the topic that each team is focusing on, and identify the key science gaps that need to be addressed in each area.

Extending the sunspot area series from Kodaikanal Solar Observatory

Kodaikanal Solar Observatory (KoSO) possesses one of world’s longest and homogeneous records of sunspot observations that span more than a century (1904–2017). Interestingly, these observations (originally recorded in photographic plates/films) were taken with the same setup over this entire time period which makes this data unique and best suitable for long-term solar variability studies. A large part of this data, between 1921–2011, were digitized earlier and a catalog containing the detected sunspot parameters (e.g., area and location) was published in Mandal et al. (2017). In this article, we extend the earlier catalog by including new sets of data between 1904–1921 and 2011–2017. To this end, we digitize and calibrate these new datasets which include resolving the issue of random image orientation. We fix this by comparing the KoSO images with co-temporal data from Royal Greenwich Observatory. Following that, a semi-automated sunspot detection and automated umbra detection algorithm are implemented onto these calibrated images to detect sunspots and umbra. Additionally, during this catalog updation, we also filled data gaps in the existing KoSO sunspot catalog (1921–2011) by virtue of re-calibrating the “rouge” plates. This updated sunspot area series covering nearly 115 years (1904–2017) are being made available to the community and will be a unique source to study the long term variability of the Sun.

A candidate auroral report in the Bamboo Annals, indicating a possible extreme space weather event in the early 10th century BCE

Historical auroral reports extend our knowledge of solar eruptions and long-term solar variability in the millennial time scale beyond the chronological coverage of instrumental observations, in the decadal to centennial time scales. Such chronological extensions benefit the scientific community, increasing the number of case studies on extreme space weather events with lower frequency but higher potential impact on modern technological infrastructure. So far, the earliest known datable reports of candidate aurorae have reached back to the 7th century BCE. Beyond this time series, we have analysed a celestial report in the Chinese Bamboo Annals that has attracted little scientific interest, probably owing to the controversial interpretations for the physical identity and the chronology of the event. Our philological analysis establishes its probable auroral nature. The textual description can be compared with early modern accounts of visual auroral observations for the multi-colour aurorae. We have located the observational site around Hàojīng (N34°14′, E108°46′) and dated the event to 977 ± 1 or 957 ± 1 BCE. On this basis, we have computed the equatorward extension of the auroral visibility as ≤39.0° in magnetic latitude and reconstructed the equatorward boundary of the auroral oval as ≤45.5° in invariant latitude. Our investigations empirically associate this candidate aurora with an extreme geomagnetic storm. We have compared this space weather event with proxy-based reconstructions of long-term solar variability and characterised it as a unique space-weather reference before the Neo-Assyrian Grand Minimum (alternatively Homeric Grand Minimum) in ca. 810 – 720 BCE.

Nonlinear Mechanisms that Regulate the Solar Cycle Amplitude

Abstract The solar magnetic activity cycle has an amplitude that varies within a wide but limited range of values. This implies that there are nonlinear mechanisms that prevent runaway solutions. The purpose of this paper is to propose observable nonlinear mechanisms in the framework of the Babcock–Leighton-type dynamo. Sunspot emergences show systematic properties that strong cycles tend to have higher mean latitudes and lower tilt angle coefficients. We use the surface flux transport model to investigate the effect of these systematic properties on the expected final total dipolar moment, i.e., cancellation plus generation of dipole moment by a whole solar cycle. We demonstrate that the systematic change in latitude has similar nonlinear feedback on the solar cycle (latitudinal quenching) as tilt does (tilt quenching). Both forms of quenching lead to the expected final total dipolar moment being enhanced for weak cycles and saturated to a nearly constant value for normal and strong cycles. This explains observed long-term solar cycle variability, e.g., the Gnevyshev–Ohl rule, which, in turn, justifies the nonlinear mechanisms inherent in the Babcock–Leighton-type dynamo.

Sunspot area catalog revisited: Daily cross-calibrated areas since 1874

Context. Long and consistent sunspot area records are important for understanding long-term solar activity and variability. Multiple observatories around the globe have regularly recorded sunspot areas, but such individual records only cover restricted periods of time. Furthermore, there are systematic differences between these records and require cross-calibration before they can reliably be used for further studies. Aims. We produce a cross-calibrated and homogeneous record of total daily sunspot areas, both projected and corrected, covering the period between 1874 and 2019. In addition, we generated a catalog of calibrated individual group areas for the same period. Methods. We compared the data from nine archives: Royal Greenwich Observatory (RGO), Kislovodsk, Pulkovo, Debrecen, Kodaikanal, Solar Optical Observing Network (SOON), Rome, Catania, and Yunnan Observatories, covering the period between 1874 and 2019. Cross-comparisons of the individual records were done to produce homogeneous and inter-calibrated records of daily projected and corrected areas. As in earlier studies, the basis of the composite is formed by the data from RGO. After 1976, the only datasets used are those from Kislovodsk, Pulkovo, and Debrecen observatories. This choice was made based on the temporal coverage and the quality of the data. While there are still 776 days missing in the final composite, these remaining gaps could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the additional archives cover these periods. Results. In contrast to the SOON data used in previous area composites for the post-RGO period, the properties of the data from Kislovodsk and Pulkovo are very similar to those from the RGO series. They also directly overlap the RGO data in time, which makes their cross-calibration with RGO much more reliable. Indeed, comparing our area catalog with previous such composites, we find improvements both in data quality and coverage. We also computed the daily Photometric Sunspot Index, which is widely used, for example, in empirical reconstructions of solar irradiance.

Analysis of full-disc Ca II K spectroheliograms

Context. Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. The archives of full-disc Ca II K observations that have been performed more or less regularly at various sites since 1892 can serve as an important source of such information. Aims. We derive the plage area evolution over the last 12 solar cycles by employing data from all Ca II K archives that are publicly available in digital form, including several as-yet-unexplored Ca II K archives. Methods. We analysed more than 290 000 full-disc Ca II K observations from 43 datasets spanning the period between 1892–2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts affecting many of the images, including some very specific artefacts, such as bright arcs found in Kyoto and Yerkes data. Our employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here. Results. We produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. The ’backbone’ approach was used to bridge the series together. We have also shown that the selection of the backbone series has little effect on the final composite of the plage area. We quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during the activity maxima. Conclusions. We provide the most complete so far time series of plage areas based on corrected and calibrated historical and modern Ca II K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.

A New Mechanism for Maunder-like Solar Minima: Phase Synchronization Dynamics in a Simple Nonlinear Oscillator of Magnetohydrodynamic Rossby Waves

Abstract The long-term solar cycle variability and Grand solar minima remain open questions from a theoretical point of view. Recently, a growing basis of evidence points out to the role of the magnetic Rossby waves in the solar cycle. Here we present a simple deterministic model, based on a low-order spectral representation of the barotropic quasi-geostrophic-magnetohydrodynamic equations for the Solar tachocline. This model supports the idea of the long-term behavior of the solar activity as a result of nonlinear interaction of magnetic Rossby modes. Solutions show that Rossby waves undergo irregular switches between periods of high activity and periods of suppressed activity, resembling the Maunder minimum. Low-energy states in the model are associated with the synchronization of the dynamical phases of the waves. These irregular transitions in the amplitudes of the waves are reminiscent of the observed time series of the solar activity. This suggests that Maunder-like states arise from chaotic transitions between regimes with different degrees of organization in the system.

Visualization of the challenges and limitations of the long-term sunspot number record

The solar cycle periodically reshapes the magnetic structure and radiative output of the Sun and determines its impact on the heliosphere roughly every 11 years. Besides this main periodicity, it shows century-long variations (including periods of abnormally low solar activity called grand minima). The Maunder Minimum (1645-1715) has generated significant interest as the archetype of a grand minimum in magnetic activity for the Sun and other stars, suggesting a potential link between the Sun and changes in terrestrial climate. Recent reanalyses of sunspot observations have yielded a conflicted view on the evolution of solar activity during the past 400 years (a steady increase versus a constant level). This has ignited a concerted community-wide effort to understand the depth of the Maunder Minimum and the subsequent secular evolution of solar activity. The goal of this Perspective is to review recent work that uses historical data to estimate long-term solar variability, and to provide context to users of these estimates that may not be aware of their limitations. We propose a clear visual guide than can be used to easily assess observational coverage for different periods, as well as the level of disagreement between currently proposed sunspot group number series.

Global Solar Magnetic Field Evolution Over 4 Solar Cycles: Use of the McIntosh Archive

The McIntosh Archive consists of a set of hand-drawn solar Carrington maps created by Patrick McIntosh from 1964 to 2009. McIntosh used mainly H, He-I 10830A and photospheric magnetic measurements from both ground-based and NASA satellite observations. With these he traced polarity inversion lines (PILs), filaments, sunspots and plage and, later, coronal holes over a ~45-year period. This yielded a unique record of synoptic maps of features associated with the large-scale solar magnetic field over four complete solar cycles. We first discuss how these and similar maps have been used in the past to investigate long-term solar variability. Then we describe our work in preserving and digitizing this archive, developing a digital, searchable format, and creating a website and an archival repository at NOAA’s National Centers for Environmental Information (NCEI). Next we show examples of how the data base can be utilized for scientific applications. Finally, we present some preliminary results on the solar-cycle evolution of the solar magnetic field, including the polar field reversal process, the evolution of active longitudes, and the role of differential solar rotation.

Long-term variations in the heliosphere

AbstractReconstructions of long-term solar variability underpin our understanding of the solar dynamo, potential tropospheric climate implications and future space weather scenarios. Prior to direct spacecraft measurements of the heliospheric magnetic field (HMF) and solar wind, accurate annual reconstructions are possible using geomagnetic and sunspot records. On longer timescales, information about the HMF can be extracted from cosmogenic radionuclide records, particularly 14C in ancient trees and 10Be in ice sheets. These proxies, and what they reveal about the HMF and solar wind, are briefly reviewed here.

Anthropogenic Global Warming (AGW) or Natural Global Warming (NGM)

The concept of an anthropogenic global warming (AGW) driven by the increase in atmospheric CO2 is compared to the concept of a natural global warming (NGW) driven by solar variability. The application of the AGW concept only rests on models, whilst the NGW concept rests on multiple observational and evidence-based facts. Even more so, the long-term solar variability predicts a new Grand Solar Minimum with severe climatic conditions (type Little Ice Age) to occur in 2030-2050. This violates all talk about an increasing, even accelerating, global warming. Similarly, there is no true treat of a future sea level rise flooding lowlands and islands.

Evaluation of IRI predicted characteristics of ionospheric F1 layer by ionosonde observations in Nicosia, Cyprus

This paper presents an investigation of F1 layer characteristics derived from manually scaled digital ionosonde measurements at the low-middle latitude European station in Nicosia, Cyprus (geographical coordinates: 35°N, 33°E, geomagnetic lat. 29.38°N, I=51.7°) from low to high solar activity conditions (2009–2014) and their comparison with IRI-2012 predictions. It assesses the predictability of occurrence probability by employing all three options included in IRI-2012: IRI-95, Scotto-97 no L, and Scotto-97 with L. Results show that Scotto-97 no L option slightly overestimates the occurrence probability but predicts better than IRI-95 option, whereas IRI-95 option closely reflects the time range for F1 layer appearance. The Scotto-97 with L option slightly underestimates the occurrence probability of foF1+ledge type. A seasonal variation marked with higher occurrence probability in the equinoctial months than in winter is observed, both at low and high solar activity. IRI predictions match reasonably well observed values of foF1 with maximum 0.3MHz over/under estimation at low solar activity. At high solar activity, IRI slightly overestimates the values of foF1 throughout the year with a varying degree from 0.3 to 1.3MHz, except for the months of August and September, when the values are underestimated by 0.3MHz. A seasonal variation with higher foF1 values during equinoctial months than that of summer and winter is noticed in the observations at high solar activity in 2014. Long-term solar variation in the observed foF1 values were evident. Deviation of IRI-2012 predictions from observed foF1 values increases at high solar activity. Observed values of hmF1 demonstrate no distinct diurnal variation. However, these are lower in winter than those of summer and equinoctial months. No significant long-term solar variation in hmF1 was detected during the entire period under consideration. Throughout 2009–2014, with a few exceptions, IRI generally underestimates hmF1 during January-April but overestimates during September-December. Over/underestimation goes as high as 23km.

Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxima

Cosmogenic isotopes provide the only quantitative proxy for analyzing the long-term solar variability over a centennial timescale. While essential progress has been achieved in both measurements and modeling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. Here we improve the reconstruction of solar activity over the past nine millennia using a multi-proxy approach. We used records of the 14C and 10Be cosmogenic isotopes, current numerical models of the isotope production and transport in Earth's atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo-/paleointensity database. The obtained series were analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. A new reconstruction of the geomagnetic dipole field moment, GMAG.9k, is built for the last nine millennia. New reconstructions of solar activity covering the last nine millennia, quantified in sunspot numbers, are presented and analyzed. A conservative list of grand minima and maxima is provided. The primary components of the reconstructed solar activity, as determined using the SSA method, are different for the series based on 14C and 10Be. These primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. They have been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ~2400-yr quasi-periodicity, the so-called Hallstatt cycle, in both the 14C and 10Be based series. This Hallstatt cycle thus appears to be related to solar activity. Finally, we show that the grand minima and maxima occurred intermittently over the studied period, with clustering near highs and lows of the Hallstatt cycle, respectively.

An arch in the UK

Arches, streamers, polar lights, merry dancers… just a few of many names used to describe the aurora borealis in historical documents in the UK. We have compiled a new catalogue of 20591 independent reports of auroral sightings from the British Isles and Ireland for 1700–1975 using observatory yearbooks, the diaries of amateur observers, newspaper reports and the scientific literature. Our aim is to provide an independent data series that can aid understanding of longterm solar variability, alongside cosmogenic isotope data and historic records of geomagnetic activity and sunspots.

Cherry blossom phenological data since the seventeenth century for Edo (Tokyo), Japan, and their application to estimation of March temperatures.

The changes in March mean temperatures in Edo (Tokyo), Japan, since the seventeenth century, were reconstructed using phenological data for the cherry blossoms of Prunus jamasakura deduced from old diaries and chronicles. The observations of the time of full blossoming and of cherry blossom viewing parties were acquired and used to construct a full-blossoming phenological data series for P. jamasakura. Phenological data from 207 of the years from 1601 to 1905 were used for this study. The reconstructed temperatures suggested the existence of two cold periods (the second half of the seventeenth century and the beginning of the nineteenth century), during which times the estimated March mean temperatures were about 4°C and 5°C, respectively. These two cold periods at Edo coincided with those reconstructed at Kyoto in previous studies. These cold periods coincided with two less extreme periods, the Maunder and Dalton minima, in the long-term solar variation cycle.

Evidence of the solar Gleissberg cycle in the nitrate concentration in polar ice

Two sets of nitrate (NO3−) concentration data, obtained from Central Greenland and East Antarctic (Dronning Maud Land) ice cores, were analyzed statistically. Distinct century-scale (50–150yr) variability was revealed in both data sets during AD 1576–1990. It was found that century-type variation in Greenland and Antarctic nitrate correlates fairly significantly with the corresponding Gleissberg cycle: (a) in sunspot number over 1700–1970 AD; (b) in 10Be concentration in Central and South Greenland over 1576–1970 AD. Thus, presence of century-scale relationship between polar nitrate and solar activity was confirmed over the last 4 centuries. That proves that NO3− concentration in polar ice caps could serve as indicator of long-term solar variability.

Indirect comparison of Debrecen and Greenwich daily sums of sunspot areas

Sunspot area data play an important role in the studies of solar activity and its long-term variations. In order to reveal real long-term solar variations precise homogeneous sunspot area databases should be used. However, the measured areas may be burdened with systematic deviations, which may vary in time. Thus, there is a need to investigate the long-term variation of sunspot area datasets and to determine the time-dependent cross-calibration factors. In this study, we investigate the time-dependent differences between the available long-term sunspot databases. Using the results, we estimate the correction factor to calibrate the corrected daily sunspot areas of Debrecen Photoheliographic Data (DPD) to the same data of Greenwich Photoheliographic Results (GPR) by using the overlapping Kislovodsk and Pulkovo data. We give the correction factor as GPR=1.08(\pm 0.11)*DPD

Statistical Properties of Extreme Solar Activity Intervals

A study of long-term solar variability reflected in indirect indices of past solar activity leads to stimulating results. We compare the statistics of intervals of very low and very high solar activity derived from two cosmogenic radionuclide records and look for consistency in their timing and physical interpretation. According to the applied criteria, the numbers of minima and of maxima are 61 and 68, respectively, from the 10Be record, and 42 and 46 from the 14C record. The difference between the enhanced and depressed states of solar activity becomes apparent in the difference in their statistical distributions. We find no correlation between the level or type (minimum or maximum) of an extremum and the level or type of the predecessor. The hypothesis of solar activity as a periodic process on the millennial time scale is not supported by the existing proxies. A new homogeneous series of 10Be measurements in polar ice covering the Holocene would be of great value for eliminating the existing discrepancy in the available solar activity reconstructions.