Solar Activity Proxies Research Articles

Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings

The Sun provides the principal energy input into the Earth system and solar variability represents a significant external climate forcing. Although observations of solar activity (sunspots) cover only the last about 400 years, radionuclides produced by cosmic rays and stored in tree rings or ice cores serve as proxies for solar activity extending back thousands of years. However, the presence of weather-induced noise or low temporal resolution of long, precisely dated records hampers cosmogenic nuclide-based studies of short-term solar variability such as the 11-yr Schwabe cycle. Here we present a continuous, annually resolved atmospheric 14C concentration (fractionation-corrected ratio of 14CO2 to CO2) record reconstructed from absolutely dated tree rings covering nearly all of the last millennium (ad 969–1933). The high-resolution and precision 14C record reveals the presence of the Schwabe cycle over the entire time range. The record confirms the ad 993 solar energetic particle event and reveals two new candidates (ad 1052 and ad 1279), indicating that strong solar events that might be harmful to modern electronic systems probably occur more frequently than previously thought. In addition to showing decadal-scale solar variability over the last millennium, the high-temporal-resolution record of atmospheric radiocarbon also provides a useful benchmark for making radiocarbon dating more accurate over this interval.

Characterisation of flare Soft X-ray distribution with solar magnetic activity

We analyse the 0.1 — 0.8 nm solar soft X-ray flux catalogue from Geostationary Operational Environmental Satellites (GOES), managed by NASA/NOAA, between September 1978 and September 2017, in order to investigate the possible role of solar activity and solar cycle epoch on the distribution of soft X-ray peak fluxes. We concentrate our attention on the last three solar cycles because solar activity proxies seem to indicate a decrease in the magnetic activity of our star. We know that flare soft X-ray peak fluxes are characterised by a power-law distribution with an index α ≃ 2 that shows a minor dependence on solar cycle. More in detail, we study the dependence of the power-law parameters during each single solar cycle (cycles 21-24) and during different regimes of solar activity defined using three different proxies: i) Sunspot Number (SSN), ii) Mg II core-to-wing ratio (Mg II Index), and iii) solar radio flux at 10.7 cm or 2800 MHz (F10.7). The power-law estimation analysis is performed in maximum likelihood estimation (MLE) fitting method with goodness-of-fit based on Kolmogorv- Smirnov test. Preliminary results indicate that the power-law index shows a slight decrease as solar activity decreases. This except for the F10.7 proxy. More in-depth statistical analysis is necessary to confirm our findings. A post-publication change was made to this article on 22 Jun 2020 to correct the webpage title to match the pdf.

FoF2 Long-Term Trend at a Station Located near the Crest of the Equatorial Ionization Anomaly

Critical frequency foF2 long-term trends at Dakar station (14.4°N, 342.74°E) located near the crest of the equatorial ionization anomaly EIA, are analysed taking into account geomagnetic activity, increasing greenhouse gases concentration and Earth’s magnetic field secular variation. After filtering solar activity effect using F10.7 as a solar activity proxy, we determined the relative residual trends slopes α values for three different levels of geomagnetic activity. For example, at 1200 LT, the value of α goes from &#450.27%/year for very magnetically quiet days to -0.19%/year for magnetically quiet days and to -0.13%/year for all days. It appears from the slopes α obtained, that they increase with the level of geomagnetic activity and their negative values are qualitatively consistent with the expected decreasing trend due to the increase in greenhouse gases concentration but are greater than 0.003%/year which would result from a 20% increase in CO2 emissions which actually took place during the analysis period. Regarding Earth’s magnetic field magnitude, B secular variation and the dip equator secular movement, Dakar station is located near the crest of the equatorial ionization anomaly, Earth’s magnetic field magnitude, B decreases there and the trough approaches the position of Dakar during the period of analysis. These two phenomena induce a decrease in foF2 which is in agreement with the decreasing trend observed at this station.

Probabilistic prediction of geomagnetic storms and theKpindex

Geomagnetic activity is often described using summary indices to summarize the likelihood of space weather impacts, as well as when parameterizing space weather models. The geomagnetic indexKpin particular, is widely used for these purposes. Current state-of-the-art forecast models provide deterministicKppredictions using a variety of methods – including empirically-derived functions, physics-based models, and neural networks – but do not provide uncertainty estimates associated with the forecast. This paper provides a sample methodology to generate a 3-hour-aheadKpprediction with uncertainty bounds and from this provide a probabilistic geomagnetic storm forecast. Specifically, we have used a two-layered architecture to separately predict storm (Kp ≥ 5−) and non-storm cases. As solar wind-driven models are limited in their ability to predict the onset of transient-driven activity we also introduce a model variant using solar X-ray flux to assess whether simple models including proxies for solar activity can improve the predictions of geomagnetic storm activity with lead times longer than the L1-to-Earth propagation time. By comparing the performance of these models we show that including operationally-available information about solar irradiance enhances the ability of predictive models to capture the onset of geomagnetic storms and that this can be achieved while also enabling probabilistic forecasts.

Overview of the NASA Solar Irradiance Science Team (SIST) Program Special Section

AbstractSolar irradiance represents the dominant energy source heating the Earth's atmosphere and climate. Both total solar irradiance and spectral solar irradiance vary over the 11‐year solar cycle. Characterizing these variations with sufficient accuracy for climate studies over multidecadal timescales requires a combination of multiple observational data sets, solar activity proxies, and irradiance models. NASA established the Solar Irradiance Science Team (SIST) program in 2015 to pursue this goal using a range of technical approaches. This paper summarizes those investigations, whose results are reported in separate papers in a special section of this journal.

Uncertainty Quantification in Sunspot Counts

Abstract Observing and counting sunspots constitutes one of the longest-running scientific experiments, with first observations dating back to Galileo (around 1610). Today the sunspot number (SN) time series acts as a benchmark of solar activity in a large range of physical models. An appropriate statistical modeling, adapted to the time series’ complex nature, is, however, still lacking. In this work, we provide the first comprehensive uncertainty quantification analysis of sunspot counts. We study three components: the number of sunspots (N s ), the number of sunspot groups (N g ), and the composite N c , defined as . Those are reported by a network of observatories around the world and are corrupted by errors of various types. We use a multiplicative framework to provide, for these three components, an estimation of their error distribution in various regimes (short-term, long-term, minima of solar activity). We also propose a robust estimator for the underlying solar signal and fit density distributions that take into account intrinsic characteristics such as overdispersion, excess of zeros, and multiple modes. The estimation of the solar signal underlying the composite N c may be seen as a robust version of the International Sunspot Number (ISN), widely used as a proxy of solar activity. Therefore, our results on N c may help characterize the uncertainty on ISN as well. Our results pave the way for a future monitoring of the observatories in quasi-real time, with the aim of alerting the observers when they start deviating from the network and preventing large drifts from occurring.

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.

Multidecadal‐ to Centennial‐Scale 10Be Variations in Holocene Sediments of Huguangyan Maar Lake, South China

AbstractWe report a new high‐resolution Holocene 10Be record from Huguangyan Maar Lake in subtropical‐tropical South China, aimed to detect the atmospheric 10Be production signal in low‐latitude regions. After minimizing climatic effects by regression analyses between 10Be concentration and climatic proxies from the same archive, we successfully distinguished variations in geomagnetic field intensity and solar activity using 2,000‐year low‐ and high‐pass filtering, respectively, of the residual 10Be record (a proxy of the atmospheric 10Be production rate). The resulting 10Be‐derived record of geomagnetic field intensity is generally comparable with geomagnetic models, and the solar‐modulated 10Be signal shows significant correlations with solar activity proxies. The preservation of 10Be production signal in the sediments of this low‐latitude maar lake highlights the largely unexplored potential as well as limitations of 10Be as a tool to reconstruct variations in solar activity and geomagnetic field intensity.

Cosmic Ray Modulation with the Maximum CME Speed Index During Solar Cycles 23 and 24

We analyzed modulation of cosmic-ray intensities (CRIs) during Solar Cycles 23 and 24 by using the international sunspot numbers (ISSN) and the maximum CME speed index (MCMESI) as proxies for solar activity. Temporal variations, cross-correlations, and hysteresis patterns of CRI, MCMESI, and ISSN data were investigated. As a result, we concluded that the MCMESI better describes solar modulation of the CRI as compared to the ISSN. This is mainly because the correlation between CRI and ISSN is caused by the general cyclic trend of solar activity, while the correlation between the CRI and the MCMESI is mainly due to short-term fluctuations related to Forbush decreases. In contrast to the ISSN, there is no time lag between the CRI and the MCMESI variations.

Notes on the correlation between sudden stratospheric warmings and solar activity

Abstract. A correlation between solar activity and normalized occurrence rate of sudden stratospheric warmings (SSWs) has been found. As a proxy for solar activity, the 10.7 cm solar radio flux has been used. In order to find the correlation, we derived a normalized occurrence rate of major sudden stratospheric warmings (MSSWs) based on both the ERA-40/ERA-Interim dataset and NCEP data. Based on this distribution, we calculated the correlation coefficient, which amounts to 0.63, with a significance of 90.68 %, for ERA-40/ERA-Interim, and 0.55 for the NCEP–NCAR-I reanalysis, with a significance of 83.80 %. Additionally, we calculate correlation coefficients for Lyman-alpha flux and sunspot numbers with the analogous method for the same period.

Do the Chinese Astronomical Records Dated AD 776 January 12/13 Describe an Auroral Display or a Lunar Halo? A Critical Re-examination

The enhancement of carbon-14 in tree rings around AD 774/775 has generated wide interest in solar activity at that time. The historical auroral records have been examined critically. Of particular interest was the “white vapour” observed in China on AD 776 January 12/13. Both Usoskin et al. (Astron. Astrophys.55, L3, 2013; U13) and Stephenson (Adv. Space Res.55, 1537, 2015; S15) interpreted this record as an auroral display. Subsequently, Neuhäuser and Neuhäuser (Astron. Nachr.336, 225, 2015; NN15) proposed five “criteria” for the likeliness of aurorae and on this basis rejected an auroral interpretation. Instead, they interpreted it as a lunar halo, and suggested there were no auroral records as a proxy of solar activity in the interval AD 774 – 785. We consider if their “lunar halo hypothesis” and their auroral criteria could be of use in future researches on historical auroral candidates. We first show a counter-example for the lunar halo hypothesis from a parallel record on 1882 November 17, which was seen as a whitish colour, in the southerly direction, and near the Moon. We then consider NN15’s criteria on colour, direction, and sky brightness and investigate other counter-examples from early-modern auroral observations. We also consider the extension of the white vapour in AD 776 according to the distribution of Chinese asterisms, and show that its large extension was inconsistent with the lunar halo hypothesis. Conversely, the streaks of white vapour penetrating the eight Chinese asterisms can be reproduced if we consider auroral-ray structures at altitudes between 97 km and 170 km, along geomagnetic field lines between the L-shells L=1.55 and 1.64. Our investigations show that we should consider candidate auroral records in historical documents not on the basis of the newly suggested a priori criteria by NN15 but based on all the available observational evidence.

A Solar Signature in Many Climate Indices

AbstractWe first apply singular spectrum analysis (SSA) to the international sunspot number (1849–2015) and the count of polar faculae (1906–2006). The SSA method finds 22‐, 11‐, and 5.5‐year components as the first eigenvectors of these solar activity proxies. We next apply SSA to the 10 Madden‐Julian oscillation (MJO; 1978–2016) indices. The first, most intense component SSA finds in all MJO indices has either a period of 5.5 or 11 years. The longer‐term modulation of amplitude is on the order of one third of the total variation. The 5.5‐year SSA component 1 of most MJO indices moreover follows the decreasing amplitude of solar cycles. We then apply SSA to climate indices Pacific Decadal Oscillation, El Nino Southern Oscillation precipitation index, Arctic Oscillation, Atlantic Multidecadal oscillation, Tropical Southern Atlantic oscillation, Western Hemisphere Warm Pool, and Brazil and Sahel rainfalls. We find that the first SSA eigenvectors are all combinations of rather pure 11, 5.5, and 3.6‐year pseudo‐cycles. The 5.5‐year component is frequently observed and is particularly important and sharp in the series in which it appears. All these periods have long been attributed to solar activity, and this by itself argues for the existence of a strong link between solar activity and climate. The mechanisms of coupling must be complex and probably nonlinear but they remain to be fully understood (UV radiation, solar wind, and galactic cosmic rays being the most promising candidates). We propose as a first step a Kuramoto model of nonlinear coupling that generates phase variations compatible with the observed ones.

Spatial distribution of temperature during geomagnetic disturbances

We propose an index of efficiency of the solar activity effect on the tropospheric temperature, which takes into account the spatial irregularity of the response to this effect. As a proxy of solar activity we take the PC index of geomagnetic activity, designed to monitor the geomagnetic field at high latitudes. Using NCEP/NCAR reanalysis data, we carry out a comparative analysis of variations in the proposed index and lower-troposphere temperature variations during geomagnetic disturbances. We identify the presence of a high degree of correlation between the temperature in the 925–700 hPa layer and the proposed index of solar activity effect. The spatio-temporal analysis of the index and temperature variations shows that the index of effi-ciency of the solar activity effect describes well both the value and the sign of the observed variations in the spa-tial distribution of the lower-troposphere temperature as compared to the frequently used index of geomagnetic activity.

Пространственное распределение температуры во время геомагнитных возмущений

We propose an index of efficiency of the solar activity effect on the tropospheric temperature, which takes into account the spatial irregularity of the response to this effect. As a proxy of solar activity we take the PC index of geomagnetic activity, designed to monitor the geomagnetic field at high latitudes. Using NCEP/NCAR reanalysis data, we carry out a comparative analysis of variations in the proposed index and lower-troposphere temperature variations during geomagnetic disturbances. We identify the presence of a high degree of correlation between the temperature in the 925–700 hPa layer and the proposed index of solar activity effect. The spatio-temporal analysis of the index and temperature variations shows that the index of effi-ciency of the solar activity effect describes well both the value and the sign of the observed variations in the spa-tial distribution of the lower-troposphere temperature as compared to the frequently used index of geomagnetic activity.

The Correlation of Synthetic UV Color versus Mg ii Index along the Solar Cycle

Abstract UV solar irradiance strongly affects the chemical and physical properties of the Earth’s atmosphere. UV radiation is also a fundamental input for modeling the habitable zones of stars and the atmospheres of their exoplanets. Unfortunately, measurements of solar irradiance are affected by instrumental degradation and are not available before 1978. For other stars, the situation is worsened by interstellar medium absorption. Therefore, estimates of solar and stellar UV radiation and variability often rely on modeling. Recently, Lovric et al. used Solar Radiation and Climate Experiment (SORCE)/Stellar Irradiance Comparison Experiment (SOLSTICE) data to investigate the variability of a color index that is a descriptor of the UV radiation that modulates the photochemistry of planets’ atmospheres. After correcting the SOLSTICE data for residual instrumental effects, the authors found the color index to be strongly correlated with the Mg ii index, a solar activity proxy. In this paper, we employ an irradiance reconstruction to synthetize the UV color and Mg ii index with the purpose of investigating the physical mechanisms that produce the strong correlation between the color index and the solar activity. Our reconstruction, which extends back to 1989, reproduces very well the observations, and shows that the two indices can be described by the same linear relation for almost three cycles, thus ruling out an overcompensation of SORCE/SOLTICE data in the analysis of Lovric et al. We suggest that the strong correlation between the indices results from the UV radiation analyzed originating in the chromosphere, where atmosphere models of quiet and magnetic features present similar temperature and density gradients.

Periodic behaviour of coronal mass ejections, eruptive events, and solar activity proxies during solar cycles 23 and 24

We report on the parallel analysis of the periodic behaviour of coronal mass ejections (CMEs) based on 21 years [1996–2016] of observations with the SOHO/LASCO–C2 coronagraph, solar flares, prominences, and several proxies of solar activity. We consider values of the rates globally and whenever possible, distinguish solar hemispheres and solar cycles 23 and 24. Periodicities are investigated using both frequency (periodogram) and time-frequency (wavelet) analysis. We find that these different processes, in addition to following the ≈11-year Solar Cycle, exhibit diverse statistically significant oscillations with properties common to all solar, coronal, and heliospheric processes: variable periodicity, intermittence, asymmetric development in the northern and southern solar hemispheres, and largest amplitudes during the maximum phase of solar cycles, being more pronounced during solar cycle 23 than the weaker cycle 24. However, our analysis reveals an extremely complex and diverse situation. For instance, there exists very limited commonality for periods of less than one year. The few exceptions are the periods of 3.1–3.2 months found in the global occurrence rates of CMEs and in the sunspot area (SSA) and those of 5.9–6.1 months found in the northern hemisphere. Mid-range periods of ≈1 and ≈2 years are more wide spread among the studied processes, but exhibit a very distinct behaviour with the first one being present only in the northern hemisphere and the second one only in the southern hemisphere. These periodic behaviours likely results from the complexity of the underlying physical processes, prominently the emergence of magnetic flux.

On Quasi-biennial Oscillations in Chromospheric Macrospicules and Their Potential Relation to the Global Solar Magnetic Field

Abstract This study aims to provide further evidence for the potential influence of the global solar magnetic field on localized chromospheric jets, the macrospicules (MS). To find a connection between the long-term variation of properties of MS and other solar activity proxies, including, e.g., the temporal variation of the frequency shift of solar global oscillations, sunspot area, etc., a database overarching seven years of observations was compiled. This database contains 362 MS, based on observations at the 30.4 nm of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Three of the five investigated physical properties of MS show a clear long-term temporal variation after smoothing the raw data. Wavelet analysis of the temporal variation of maximum length, maximum area, and average velocity is carried out. The results reveal a strong pattern of periodicities at around 2 years (also referred to as quasi-biennial oscillations—QBOs). A comparison with solar activity proxies that also possess the properties of QBOs provides some interesting features: the minima and maxima of QBOs of MS properties occur at around the same epoch as the minima and maxima of these activity proxies. For most of the time span investigated, the oscillations are out of phase. This out-of-phase behavior was also corroborated by a cross-correlation analysis. These results suggest that the physical processes that generate and drive the long-term evolution of the global solar activity proxies may be coupled to the short-term local physical processes driving the macrospicules, and, therefore modulate the properties of local dynamics.

Decadal radon cycles in a hot spring

AbsatrctA high-fidelity record covering nearly 40 years of water-dissolved radon from the hot spring site of BangLazhang (BLZ), Southwestern China is presented to study multi-year periodicities of radon. Ancillary observational data, i.e., water temperature, spring discharge rate, barometric pressure, combined with regional rainfall, galactic cosmic rays (GCR flux is modulated by solar wind and thus a proxy for solar activity) and regional seismicity from the same period are considered to identify potentially influencing factors controlling the changes in radon. Variations in radon concentration and ancillary observational data are studied using continuous Wavelet Power Spectrum (WPS), Wavelet Coherence (WTC), and Partial Wavelet Coherence (PWC). The results show that the long-period radon concentration is characterized by a quasi-decadal (8–11 years) cycle, matching well with the concurrent periodicity in water temperature, spring discharge rates and GCR. PWCs of radon, discharge rate and water temperature suggest that water temperature variations explain most of the coherent variability of radon and the discharge rate. We tentatively conclude that radon variations are mainly explained by variations in water temperature and spring discharge, which are modified and modulated by earthquakes and quasi-decadal variations of an unidentified process. The influence of solar activity on the decadal periodicity is discussed.

A Solar cycle correlation of coronal element abundances in Sun-as-a-star observations

The elemental composition in the coronae of low-activity solar-like stars appears to be related to fundamental stellar properties such as rotation, surface gravity, and spectral type. Here we use full-Sun observations from the Solar Dynamics Observatory, to show that when the Sun is observed as a star, the variation of coronal composition is highly correlated with a proxy for solar activity, the F10.7 cm radio flux, and therefore with the solar cycle phase. Similar cyclic variations should therefore be detectable spectroscopically in X-ray observations of solar analogs. The plasma composition in full-disk observations of the Sun is related to the evolution of coronal magnetic field activity. Our observations therefore introduce an uncertainty into the nature of any relationship between coronal composition and fixed stellar properties. The results highlight the importance of systematic full-cycle observations for understanding the elemental composition of solar-like stellar coronae.

Linear forecasting of the F10.7 proxy for solar activity

AbstractThe ability to accurately forecast variations in the solar extreme ultraviolet irradiance is important to many aspects of operational space weather. For example, variations in the Sun's radiative output at these wavelengths drive changes in thermospheric density, which perturbs the trajectories of objects in low Earth orbit. Thus, predicting the conjunction of an operational satellite with orbital debris requires accurate forecasts of solar activity. In this paper we present a simple linear forecasting model for the 10.7 cm radio flux (F10.7), a commonly used proxy for solar activity. Comparisons with simple reference models indicate that this linear model has positive skill for all forecast days that we have considered. We also examine the impact of the F10.7 forecast skill on empirical model predictions of thermospheric density and ionospheric total electron content.