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In the current work we have used the deep learning based long short-term memory model to predict the strength and peak time of solar cycle 25 by employing the monthly smoothed sunspot number data obtained from WDC-SILSO, Royal Observatory of Belgium, Brussels. We have used the stacked LSTM forecasting model to predict the upcoming cycle 25. From our analysis it has been shown that our proposed model is capable of capturing long term dependencies as well as trend within the data. For cycle 20 and 21 the error difference between predicted as well as observed peak value is 2.3 and 0.7 respectively while the peak prediction error is 1.47% and 0.30%. The RMSE of the model for cycle 20 and 21 is 3.97 and 4.34 respectively. For cycle 22, the AE and RE is 4.6 and 2.16% while the RMSE of the model for this case is 4.50. The predicted peak amplitude of solar cycle 23 and 24 from our proposed model has a relative error of 1.75% and 1.99% respectively from the observed value while the RMSE is 3.4 for cycle 23 and 4.2 for cycle 24. Our projected prediction of cycle 25 using the proposed LSTM model, says that it will be stronger than cycle 24 and weaker than cycle 23. The solar cycle 25 will peak with an amplitude of sunspot number at 171.9 ± 3.4 and will be 47 % stronger than cycle 24. The solar cycle 25 will reach its peak in August 2023 ± 2 months.

Currently, the problem of geospace storms and their components, geomagnetic storms, is one of the most important problems in solar-terrestrial physics and space geophysics. The exploration of the geospace and its use for the needs of civilization has led to the fact that our life is increasingly dependent on the manifestations of solar-terrestrial processes, the state of atmospheric-space weather, and ground-to-space systems of various purposes. The more technologically advanced our civilization becomes, the more vulnerable it is to the processes taking place on the Sun, manifestations of solar-terrestrial relationships, and variations in atmospheric-space weather. These circumstances determine the relevance and continuous scientific and practical significance of studies into the manifestations of solar-terrestrial processes and their consequences. Geospace (geomagnetic) storms can be accompanied by a number of effects: variations in the parameters of the atmosphere and geospace; deceleration of spacecraft; impact of increased cosmic radiation on crew and electronic equipment in spacecraft and aircraft; disturbances of conditions of radio wave propagation and channels of radio communication, radio navigation, radio ranging, radio position finding, radio astronomy, and remote sensing; the induction of currents in power transmission lines, cable lines, pipelines, automated railway systems; and the impact on weather- and climate-forming systems. In addition to the physical effects of individual geomagnetic storms, which are described in a large number of scientific papers, a statistical analysis of the parameters of the solar wind, interplanetary magnetic field, and geomagnetic storms over long periods is of interest. The purpose of this study is to statistically analyze the parameters of the solar wind disturbed by solar storms, the interplanetary magnetic field, and the geomagnetic activity indices over the period of solar cycle 24 (2009–2020). The main statistical characteristics of the disturbed solar wind parameters responsible for the geomagnetic storm origins (153 storms in total) over solar cycle 24 have been estimated. The main statistical characteristics of the components of the disturbed interplanetary magnetic field have been estimated, and the main statistical characteristics of the geomagnetic field indices have been obtained. With respect to magnetic activity, solar cycle 24 was quieter than solar cycle 23.

The paper provides a short historical overview of sunspot observations from their discovery until the present. The review goes beyond collecting all known historical information about the study of sunspots but highlights the research of five scientists of different epochs over five centuries since the 16th. Not as much attention is deliberately given to some well-known studies and discoveries. The focus is on the utmost long-term observations of sunspots, which provide information that expands the boundaries of classical Wolf numbers or the number of sunspots groups. Sunspots have been observed since ancient times and they were documented in ancient chronicles. Active observation of sunspots began after the invention of the telescope, probably by Hans Lippershey in the early 17th century. It is documented that Thomas Harriot was the first to observe sunspots with a telescope on December 8, 1610. It is probable that Galileo Galilei and Johann Fabricius observed sunspots almost simultaneously with him in December 1610 using a telescope, independently of each other and of Harriot. The first publication about sunspots was issued by Fabricius in June 1611. We dwell on the observations of Christoph Scheiner, Christian Horrebow, Heinrich Schwabe, and Hisako Koyama. Christoph Scheiner described his long-term observations and studies of sunspots from 1611 to 1630 in his book Rosa Ursina sive Sol, which became a model for the Sun observers for many years afterwards. Christian Horrebow was the first to speculate on the regularity of sunspots, and Heinrich Schwabe was the first in 1843 to discover the periodicity (with a period of approximately 10 years) of the number of groups of sunspots. In 1852 Rudolf Wolf, analyzing all available sources, clarified that solar activity has an 11-year periodicity. He introduced the concept of the relative sunspot number and organized regular observations and publication of their results. Hisako Koyama’s 40-year observations have helped reconcile current sunspot counts with earlier ones. Wolf’s system lasted until the beginning of the 21st century. In July 2015, a new version of the relative sunspot numbers was adopted (Version 2.0). In this paper, the ratio of “new” and “old” Wolf numbers is calculated and a table of characteristics of 11‑year cycles according to Version 2.0 is proposed. Two forecasts of the maximum of solar cycle 25 are also calculated. In the case when the precursor of the maximum is the value of the relative sunspot number in the cycle minimum (correlation coefficient r = 0.557 and P

The solar gravitational moments $J_{2n}$ are important astronomical quantities whose precise determination is relevant for solar physics, gravitational theory and high precision astrometry and celestial mechanics. Accordingly, we propose in the present work to calculate new values of $J_{2n}$ (for $n$=1,2,3,4 and 5) using recent two-dimensional rotation rates inferred from the high resolution SDO/HMI helioseismic data spanning the whole solar activity cycle 24. To this aim, a general integral equation relating $J_{2n}$ to the solar internal density and rotation is derived from the structure equations governing the equilibrium of slowly rotating stars. For comparison purpose, the calculations are also performed using rotation rates obtained from a recently improved analysis of SoHO/MDI heliseismic data for solar cycle 23. In agreement with earlier findings, the results confirmed the sensitivity of high order moments ($n>1$) to the radial and latitudinal distribution of rotation in the convective zone. The computed value of the quadrupole moment $J_{2}$ ($n=1$) is in accordance with recent measurements of the precession of Mercury's perihelion deduced from high precision ranging data of the MESSENGER spacecraft. The theoretical estimate of the related solar oblateness $\Delta_{\odot}$ is consistent with the most accurate space-based determinations, particularly the one from RHESSI/SAS., Comment: 6 pages, 3 figures, 1 table

The low magnetic field in the South Atlantic Anomaly (SAA) zone facilitates the entrance of high-energy particles from the magnetosphere. When particles that are drifting in closed orbits around the Earth reach the SAA zone, they can be quasi-trapped instead of being trapped in a stable way or precipitating. The effects over the total column ozone (TCO), NOx (= NO + NO2) and ozone profiles in the SAA zone during and after two intense geomagnetic storms that occurred during solar cycle 23 are analyzed in this work. The two periods occurred on November 20, 2003, and on November 08, 2004, during spring in the South American sector. Data of the Total Column Ozone Mapping Spectrometer (TOMS), Solar Backscatter Ultraviolet (SBUV) Merged Ozone Data and Upper Atmosphere Research Satellite (UARS-HALOE) are used. Three stations located in the SAA zone are chosen for the analysis with TOMS and SBUV data. During both storms, a statistically significant TOC decrease is observed. In the 2003 storm, the decrease reached values between −4.4 % and −6.0% one week after the day of the minimum value of the disturbance storm time (Dst) index, and in the 2004 storm the decrease is observed on November 17, with a value of the order of −4.0% over the three stations. In the case of ozone profiles from SBUV data, between 0.639 hPa and 101.3 hPa, a significant increase at middle stratospheric heights during both storms is detected. The following days also show significant decreases in the middle and lower stratosphere for the 2003 storm. On the other hand, from UARS-HALOE data, we observe that for both storms, the profiles of the relative difference of NOx show significant increases between 25 and 40 km, but ozone profiles do not present significant changes for the analyzed storms

A solar-flare-induced, high-latitude (peak at 70–75∘ geographic latitude – GGlat) ionospheric current system was studied. Right after the X9.3 flare on 6 September 2017, magnetic stations at 68–77∘ GGlat near local noon detected northward geomagnetic deviations (ΔB) for more than 3 h, with peak amplitudes of >200 nT without any accompanying substorm activities. From its location, this solar flare effect, or crochet, is different from previously studied ones, namely, the subsolar crochet (seen at lower latitudes), auroral crochet (pre-requires auroral electrojet in sunlight), or cusp crochet (seen only in the cusp). The new crochet is much more intense and longer in duration than the subsolar crochet. The long duration matches with the period of high solar X-ray flux (more than M3-class flare level). Unlike the cusp crochet, the interplanetary magnetic field (IMF) BY is not the driver, with the BY values of only 0–1 nT out of a 3 nT total field. The equivalent ionospheric current flows eastward in a limited latitude range but extended at least 8 h in local time (LT), forming a zonal current region equatorward of the polar cap on the geomagnetic closed region. EISCAT radar measurements, which were conducted over the same region as the most intense ΔB, show enhancements of electron density (and hence of ion-neutral density ratio) at these altitudes (∼100 km) at which strong background ion convection (>100 m s−1) pre-existed in the direction of tidal-driven diurnal solar quiet (Sq0) flow. Therefore, this new zonal current can be related to this Sq0-like convection and the electron density enhancement, for example, by descending the E-region height. However, we have not found why the new crochet is found in a limited latitudinal range, and therefore, the mechanism is still unclear compared to the subsolar crochet that is maintained by a transient redistribution of the electron density. The signature is sometimes seen in the auroral electrojet (AE = AU − AL) index. A quick survey for X-class flares during solar cycle 23 and 24 shows clear increases in AU for about half the > X2 flares during non-substorm time, despite the unfavourable latitudinal coverage of the AE stations for detecting this new crochet. Although some of these AU increases could be the auroral crochet signature, the high-latitude crochet can be a rather common feature for X flares. We found a new type of the solar flare effect on the dayside ionospheric current at high latitudes but equatorward of the cusp during quiet periods. The effect is also seen in the AU index for nearly half of the > X2-class solar flares. A case study suggests that the new crochet is related to the Sq0 (tidal-driven part) current.

The United States’ Wide Area Augmentation System (WAAS) broadcasts data to facilitate aircraft navigation. This paper examines the spatial dependence of ionospheric disturbances that have threatened the accuracy and reliability of position estimates derived from these data over the period 2000 – 2019. We address two distinct aspects of this spatial dependence: (1) the geographic distribution of these threats, in particular, in relation to geomagnetic latitude, and (2) the geometric dependence of threats relative to the locations of the receiver sites that comprise the WAAS network. We analyze threat distributions in terms of the various means that WAAS employs to mitigate these threats, including the Extreme Storm Detector, the Moderate Storm Detector, local irregularity detectors, and the ionospheric threat model. Distinct distributions are presented for threats occurring in Solar Cycle 23 and those of Solar Cycle 24. To study the geometric dependence of threats on receiver locations, we use as a metric the distance separating a threat from the centroid of the nearest Ncentroid receivers. Large values of this metric identify threats at or beyond the edge of coverage. We conclude by discussing the implications of our results for WAAS operations.

Time-dependent energy spectra of galactic cosmic rays (GCRs) carry fundamental information regarding their origin and propagation. When observed at the Earth, these spectra are significantly affected by the solar wind and the embedded solar magnetic field that permeates the heliosphere, changing significantly over an 11-year solar cycle. Energy spectra of GCRs measured during different epochs of solar activity provide crucial information for a thorough understanding of solar and heliospheric phenomena. The PAMELA experiment had collected data for almost ten years (15 June 2006 - 23 January 2016), including the minimum phase of solar cycle 23 and the maximum phase of solar cycle 24. Here, we present spectra for protons and helium-nuclei measured by the PAMELA instrument from 2006 to 2014. Time profiles of the proton-to-helium flux ratio at various rigidities were also investigated, allowing the study of all characteristic features resulting from their different mass-to-charge ratio and the difference in the shape of their respective local interstellar spectra. The force-field approximation of the solar modulation was used to relate these dependencies to the different shapes of the local interstellar proton and helium-nuclei spectra.

During Solar Cycle 23 16 ground-level enhancement events were registered by the global neutron monitor network. In this work we focus on the period with increased solar activity during late October – early November 2003 producing a sequence of three events, specifically on ground-level enhancement GLE 67 on 2 November 2003. On the basis of an analysis of neutron monitor and space-borne data we derived the spectra and pitch-angle distribution of high-energy solar particles with their dynamical evolution throughout the event. According to our analysis, the best fit of the spectral and angular properties of solar particles was obtained by a modified power-law rigidity spectrum and a double Gaussian, respectively. The derived angular distribution is consistent with the observations where an early count rate increase at Oulu neutron monitor with asymptotic viewing direction in the anti-Sun direction was registered. The quality of the fit and model constraints were assessed by a forward modeling. The event integrated particle fluence was derived using two different methods. The derived results are briefly discussed.

In this study, OI 630.0 nm nightglow image data obtained from an all-sky imaging station located at Sao Joao do Cariri (7.4°S, 36.5°W), have been used to study the occurrence of equatorial plasma bubbles (EPBs) over the Brazilian equatorial region. The observations, which took place from September 2000 to December 2010 (almost eleven years), covered a significant part of solar cycle 23 (descending phase of SC23) and the very beginning of solar cycle 24 (ascending phase of SC24). There were 1337 nights of observations with 666 nights with EPB events, which translates to an occurrence rate of ∼ 49.8%. Of these nights, 1290 were considered quiet (Dst ≥ -50 nT) with ∼ 50% plasma bubbles occurrence rate. The occurrence of EPBs was studied under three solar activity phases – high solar activity phase (HSA, September 2000 – March 2004), moderate solar activity phase (MSA or descending phase, March 2004 – October 2006) and low solar activity phase (LSA, October 2006 – December 2010). The low solar activity phase is part of the peculiar solar cycle 24, considered the weakest in over a century with most sunspotless days (2008–2009). The maximum occurrence of bubbles, equal to 54.2%, was found during the HSA phase, with percentages characterizing MSA and LSA being respectively 52.4% and 45.8%. The analysis also showed clear seasonal variation in the EPB occurrence with maximum rates in summer, spring, autumn, and the minimum rates in winter for all solar activity phases. Overall, there was observed solar cycle variation in each season with maximum occurrence in HSA followed by MSA and then LSA except in autumn where higher occurrence rate was observed in LSA phase than in MSA phase.

Responses of solar X-ray flares were observed in a layer of the Martian ionosphere at altitudes of ~110 km from 32 electron density profiles obtained by radio science experiment onboard Mars Global Surveyor (MGS) during solar cycle 23. Of the 32 profiles recorded during flare periods, 10 were associated with X-class flares, 12 with M class and 10 with C class flares. The flare E-peak densities vary with solar X-ray flux, Solar Zenith Angle (SZA), Solar Longitude (Ls) and latitudes. Ionospheric Electron Content (IEC) and E-peak electron production rates of these flare profiles are estimated in the E region ionosphere. We found a maximum increase of ~200%, ~140% and ~90% in the time series of IEC for X, M and C class flares respectively. The dependence of flare E-peak electron production rate with Ls is fitted by a sinusoidal function. We have also calculated biological doses ~0.1–1.0 × 10−1, 1–8 × 10−3 and 1–6 × 10−4 Gy for X, M, C class flares respectively to study the human risk for exploration to Mars. Among 10 X-class flares X1 is a strong solar flare that gives highest dose, which is potentially lethal for human risk in Mars' space.

We have searched for solar proton events consisting of both solar energetic particles (SEPs) accelerated near the Sun and energetic storm particles (ESPs) accelerated by interplanetary shocks driven by coronal mass ejections (CMEs) and observed near the time when the shock passes the observer. The purpose of this study is to investigate the possibilities and advantages of using energetic particle observations for mid-term (warning time several hours) forecasting of geomagnetic storms or as a support for longer-term forecasting methods based on solar observations. The study period extends from May 1996 to December 2017 covering the entire solar cycle 23 and the major part of solar cycle 24. Using two particle energies, 2 and 20 MeV, we found 95 SEP–ESP events of which 65 were associated with geomagnetic storms with Dst ⩽ −50 nT caused by CMEs. We performed correlation analysis between log10|Dst (nT)| and various parameters characterising the particle events or the associated CMEs. We found the best correlations for the single independent variables Δ t ESP - SEP ( r = - 0.47 ± 0.08 ), which is the difference between the ESP peak time and SEP onset time, the CME direction parameter DP ( r = 0.47 ± 0.10 ), and the logarithm of the maximum ESP energy log10[ E ESP max (MeV)] ( r = 0.44 ± 0.11 ). Using a linear combination of these three variables improves the correlation ( r = 0.68 ± 0.07 ). We suggest that an empirical equation based on these three parameters and requiring only coronagraph observations of CMEs and energetic particle measurements in interplanetary space can be used for mid-term forecasting of geomagnetic storm strengths. We found that 74% of the strongest storms (Dst ⩽ - 200 nT) during the study period were associated with energetic particle events. The average warning time and its standard deviation for all geomagnetic storms associated with SEP–ESP events was ( 15 ± 10 ) hours.

The irregularity of the solar activities based on sunspot numbers (SSNs) and areas (SSAs) over the solar disk has been considered. More activities phenomena occurred in one of the solar hemispheres than the other at the same time, which they are referred to as the North-South (N-S) asymmetry of solar activity. Daily averages of geomagnetic indices aa, Ap, Kp, and Dst over the period 1967–2016 have been examined according to the asymmetrical distribution of both solar hemispheric activities. The possible connection between the geomagnetic activity disturbances and the sign of non-uniformity of solar activity, i.e. the N-S asymmetry for each geomagnetic index has been confirmed. The dependence of the sign of geomagnetic disturbances on the solar activity cycle, has been examined. We have classified the considered data into two groups according to the activity of northern (N) or southern (S) solar hemisphere. The solar cycles 20 and 24 have northern hemisphere dominance. These activities shifted from northern to southern hemisphere in solar cycles 21, 22, and 23. The geomagnetic disturbances exhibited significant asymmetrical states at different periods. The most significant northern and southern peaks occurred at/near the minimum of the solar cycle or in periods of descending phase of the solar cycle. The geomagnetic activity has southern dominance during solar cycle 22 and it favoured the northern activity during solar cycle 23. The dependence of the geomagnetic activity disturbance of aa, Ap, Kp, and Dst indices over the solar activity cycle is confirmed and the sign of the asymmetry changes from one solar cycle to another.

Using data from the GPS and GLONASS navigation satellite systems, we analyze the responses of the mid-latitude ionosphere to the extreme solar flares that occurred at the maximum of solar cycle 23 (October 28, 2003) and at the minimum of solar cycle 24 (September 6, 2017) during the same season at close solar zenith angles. To obtain the response, we use the rate of change of the total electronic content, which is practically independent of characteristics of equipment and is determined only by parameters of a propagation medium (the ionosphere in our case). The ionospheric response is shown to be almost independent of the total duration of the flare. In both cases, the duration of the main response at a level of 0.5 is about 1.5–2 min, whereas the total duration of the response is about 10 min and fairly independent of solar flare importance.

During geomagnetic storms, the geomagnetic H component is depressed which is preceded by a sudden storm commencement (SSC) or not and this is categorized as sudden or gradual storms. Using the method of cross correlation analysis, we have studied the associations of geomagnetic H components at four low latitude stations at longitudinal separations of 145° - 215° with solar wind density and interplanetary magnetic field (IMF) B during the four most intense sudden and four most intense gradual geomagnetic storms in solar cycle 23. In addition to dawn-dusk responses, how the ionospheric and magnetospheric currents respond to these variations at low latitudes during geomagnetic storms will be determined. Results show that profiles of cross correlation coefficients against time lags were superposed and had peak associations at zero time lags during each event for both parameters. Also there was no dawn-dusk variation in the profiles which implies that the magnetosphere responds uniquely to sources of external origin during geomagnetic storms at low latitudes.. Key words: Geomagnetic storms, solar wind and interplanetary magnetic field (IMF) parameters, geomagnetic H components.

In order to statistically investigate the relationship between the low-latitude coronal holes (CHs), the solar wind speed, and the geomagnetic activity in solar cycles 23 (1996–2008) and 24 (2009–2016), we conducted a superposed epoch analysis of the variations in CH area, solar winds, the interplanetary magnetic field (IMF), and geomagnetic indices (AL, AU, and SYM-H) for the period from 1996 to 2016. We further divided the temporal variations of the IMF into four types and then investigated the variations in solar winds, the IMF, and the geomagnetic indices before and after the corotating interaction region (CIR) reached Earth’s magnetosphere in each case. As a result, we observed a north–south asymmetry in the CH area, which shows that the CH area was much larger in the southern hemisphere than in the northern hemisphere during solar cycles 23 and 24. In addition, the CH area for solar cycle 24 tended to appear in a wider latitude region compared with that for solar cycle 23. The maximum values of the CH area and the solar wind speed in solar cycle 24 tended to be smaller than those in solar cycle 23. The relationship between these maximum values showed a positive correlation for both solar cycles. The distribution was larger for solar cycle 23 than for solar cycle 24. The variations in solar wind speed and the geomagnetic indices (AE and SYM-H) associated with CIRs in solar cycle 24 tended to be smaller than those in solar cycle 23. We conclude that the geomagnetic activity for solar cycle 24 associated with CIRs was slightly lower compared with that for solar cycle 23. This decrease in geomagnetic activity was due to a decrease in the dawn-to-dusk solar wind electric field intensity, which is obtained as the product of the solar wind speed and the north–south component of the solar wind magnetic field.

At the present time, there is no generally accepted classification of the solar wind flows. There are various approaches to this problem depending on the goal of the study. In our paper, we propose the binary classification of the solar wind types by the main hydrodynamic parameters (velocity, temperature, and density) based on the statistical analysis of the solar wind. The analysis of the OMNIWeb one-minute data is performed for the period from 1996 to 2017, which encompasses solar cycle 23 and current solar cycle 24. Eight types of the solar wind are distinguished: fast-hot-dense, fast-hot-rarefied, fast-cold-dense, fast-cold-rarefied, slow-hot-dense, slow-hot-rarefied, slow-cold-dense, slow-cold-rarefied. These types occur with different frequency and are the consequences of different manifestations of solar activity. Of particular interest are the solar wind flows, the parameters of which deviate from the averages most significantly.

The declining phase of solar cycle 23 is an important case of study. Many high energetic solar flares occurred in 2003. The occurrence of high energetic solar X-ray flares (X-type) is related to solar cycle’s phases and the state of the active regions producing them. In some cases, the declining phases may be more active than the maximum phase. A statistical study is performed for solar flares (X-class) occurrence at declining and maximum phases of solar activity. The relationships between the X-class flares, SSN and active regions (ARs) will be investigated. The active region productivity of X-flares is in coherence with the number of days of (β – γ – δ) magnetic field. The active regions energies are consumed in the acceleration of protons and production of X-flares among other things (e.g. CME lifting up). The higher the energy of X-flares the lower the proton flux (pfu @ >10 MeV) and vice versa. Keywords: Solar cycle, Declining phase, X-class flare, Active region

We assess whether a formula obtained by Richardson et al. (2014,https://doi.org/10.1007/s11207-014-0524-8) relating the peak intensity of 14- to 24-MeV protons in a solar energetic particle (SEP) event at 1 AU to the solar event location and the speed of the associated coronal mass ejection (CME) may be used in a scheme to predict the intensity of an SEP event at any location at this heliocentric distance. Starting with all 334 CMEs in the CCMC/SWRC DONKI database in October 2011 to July 2012, we use the CME speed and direction to predict the proton intensity at Earth and the two Solar Terrestrial Relations Observatory spacecraft using this formula. Since most (85%)of these CMEs were not in fact associated with SEP events, many SEP events are predicted that are not actually observed. Such cases may be reduced by considering whether type II or type III radio emissions accompany the CMEs, or by selecting faster, wider CMEs. This method is also applied to predict the SEP intensities associated with 1,100 CMEs observed by the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph during 1997-2006 in solar cycle 23. Various skill scores are calculated, which assess different aspects of the skill of the SEP predictions. We conclude that the Richardson et al. (2014) formula has potential as a simple empirical SEP intensity prediction tool.

The Sun exhibits centennial-scale activity variations and sometimes encounters grand solar minimum when solar activity becomes extremely weak and sunspots disappear for several decades. Such an extreme weakening of solar activity could cause severe climate, causing massive reductions in crop yields in some regions. During the past decade, the Sun’s activity has tended to decline, raising concerns that the Sun might be heading for the next grand minimum. However, we still have an underdeveloped understanding of solar dynamo mechanisms and hence precise prediction of near-future solar activity is not attained. Here we show that the 11-year solar cycles were significantly lengthened before the onset of the Maunder Minimum (1645–1715 CE) based on unprecedentedly high-precision data of carbon-14 content in tree rings. It implies that flow speed in the convection zone is an essential parameter to determine long-term solar activity variations. We find that a 16 year-long cycle had occurred three solar cycles before the onset of prolonged sunspot disappearance, suggesting a longer-than-expected preparatory period for the grand minimum. As the Sun has shown a tendency of cycle lengthening since Solar Cycle 23 (1996–2008 CE), the behavior of Solar Cycle 25 can be critically important to the later solar activity.

We present a systematic time-series analysis of solar radio emission in nine different frequencies to compare with that of daily sunspot number (SSN) during Solar Cycle 23 (1996–2009). Owing to the contribution from quiet-sun emission, the total solar fluxes in microwaves do not decrease as significantly as the sunspot number does during 2006 to 2009. Lomb–Scargle (LS) and wavelet analysis techniques are employed to infer the various periodicities present in the time-series data. False alarm probability (FAP) levels are estimated by the use of background mean power spectrum in the global wavelet spectrum. The LS periodogram contains resolved period peaks, some of which are below FAP levels, for example a well-known rotational period. These peaks are assessed with global significance levels of the wavelet analysis. In all the data sets, the period for solar rotational modulation (26–31 days) is present. The periodogram for the SSN presents Rieger type periods (130–180 days), mid-term periods (300–400 days) and long-term periods (430–850 days). These periods in north and south are not similar, especially long term periods are missing in SSN data of the southern hemisphere. Corresponding to the SSN periodicities, Rieger and near Rieger type of oscillations (130–180 days), quasi-biennial periodicities in the range of 1.2 to 3 years were detected in the time-series data of radio frequencies. Several of these detected periods fall in the range of the periods that are suggested to be connected with magneto-Rossby wave spherical harmonics. Our analysis found reduced power levels in the LS periodograms of low frequencies because of the fact that these low frequency emissions originate higher up in the corona with diminishing contrast to small scale structures.

In the present paper, the relation between the CMEs and their linked solar flares is examined during the entire solar cycle 23. During this solar cycle and under certain temporal and spatial conditions, 521 CMEs linked with solar flares are selected. These selected CMEs are separated into two groups according to their time of detection, 254 Before Flare-ECME events (BF-CMEs) and 267 After Flare–ECME events (AF-CMEs). The results showed that the spatial separation angle between the CMEs and their linked flares has a peak between ± 25°. Also, it was found that there is a trend of decreasing the CME acceleration with the increasing of CME speed. Besides we obtained that the AF-CMEs are on average more massive and energetic than BF-CMEs. Finally, we found that the CME Mass-Width relation of AF-CMEs is more correlated to their associated flare events than BF-CMEs.

Studying the behavior of coronal mass ejections (CMEs) is important for both solar physics and space weather. The correlations of smoothed monthly mean daily integrated CME acceleration [ $a$ ], mass [ $M$ ], and the force [ $Ma$ ] to drive CMEs with sunspot activity [ $R_{\mathrm{I}}$ ] are analyzed for both slow and fast CMEs and for both Solar Cycles 23 and 24 separately. It is found that $a$ is inversely related to both $R_{\mathrm{I}}$ and $M$ . The correlation between $Ma$ and $R_{\mathrm{I}}$ for both slow and fast CMEs is negative at the rising phase of Solar Cycle 23 and positive otherwise. There is a sharp peak in $\gamma =Ma/R_{\mathrm{I}}$ near the solar minimum (December 2008) for both slow and fast CMEs. However, for fast CMEs, there is a sharp negative peak near the previous solar minimum (August 1996) and another positive peak near the current solar minimum (2019). The positive (negative) peak tends to be related to the solar minimum from a stronger (weaker) to a weaker (stronger) solar cycle. These results suggest that the CME acceleration depends more on the strength of solar activity than on the CME’s speed. Stronger magnetic activity may slow down the CMEs that are too massive or too fast and weaker activity may speed up the CMEs that are less massive or too slow. During a few years’ period of magnetic-field polarity reversal around the solar minimum, the force provided by large-scale magnetic-field structures may not be strong enough to constrain CME motions, leading to the “escape” of CMEs with large $|\gamma |$ .

Observational studies suggest that part of the North Atlantic Oscillation (NAO) variability may be attributed to the spectral ultra-violet (UV) irradiance variations associated to the 11-year solar cycle. The observed maximum surface pressure response in the North Atlantic occurs 2–4 years after solar maximum, and some model studies have identified that atmosphere–ocean feedbacks explain the multi-year lag. Alternatively, medium-to-high energy electron (MEE) precipitation, which peaks in the declining phase of the solar cycle, has been suggested as a potential cause of this lag. We use a coupled (ocean–atmosphere) climate prediction model and a state-of-the-art MEE forcing to explore the respective roles of irradiance and MEE precipitation on the NAO variability. Three decadal ensemble experiments were conducted over solar cycle 23 in an idealized setting. We found a weak ensemble-mean positive NAO response to the irradiance. The simulated signal-to-noise ratio remained very small, indicating the predominance of internal NAO variability. The lack of multi-annual lag in the NAO response was likely due to lagged solar signals imprinted in temperatures below the oceanic mixed-layer re-emerging equatorward of the oceanic frontal zones, which anchor ocean–atmosphere feedbacks. While there is a clear, yet weak, signature from UV irradiance in the atmosphere and upper ocean over the North Atlantic, enhanced MEE precipitation on the other hand does not lead to any systematic changes in the stratospheric circulation, despite its marked chemical signatures. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

International audience; The first human missions on Mars will likely involve several astrobiology‐driven science operations, at sites with high biosignature preservation potential. Here, we present a study of the radiation environment induced by Galactic Cosmic Rays and Solar Energetic Particles at Oxia Planum, landing site of the European Space Agency ExoMars 2022 mission, and at two different locations in Mawrth Vallis, using the Monte Carlo GEometry ANd Tracking 4‐based code dMEREM (detailed Martian Energetic Radiation Environment Model). The radiation environment for solar minimum in 2009 and a period close to solar maximum during the declining phase of solar cycle 23 appears similar at the different sites, with the deepest Mawrth Vallis location having a slightly enhanced γ‐ray contribution, due to a higher modulation of fast neutrons by the more water‐rich regolith. The comparison with the Dose Equivalent from an updated extrapolation of 7+ years data from the Radiation Assessment Detector (RAD) onboard the Curiosity rover highlights the importance of input modulation conditions, some drawbacks of the galactic cosmic ray model used here, and the need to include heavy ions, the three aspects affecting differently the estimations for solar maximum and minimum. The dependence of doses on surface pressure highlights a possible influence of the different dust loading at the different sites. Estimated exposure levels for a 1‐year stay and for a short stay in Arabia Terra, the latter including a October 28, 2003 event with a fluence an order of magnitude higher than the relevant September 2017 event detected by RAD, leave reasonable to large safety margins.

We studied temporal variation of the differential rotation and poleward meridional circulation during solar cycle 24 using the magnetic element feature tracking technique. We used line-of-sight magnetograms obtained using the helioseismic and magnetic imager aboard the Solar Dynamics Observatory from May 01, 2010 to March 26, 2020 (for almost the entire period of solar cycle 24, Carrington rotation from 2096 to 2229) and tracked the magnetic element features every 1 h. We also estimated the differential rotation and poleward meridional flow velocity profiles. The observed profiles are consistent with those of previous studies on different cycles. Typical properties resulting from torsional oscillations can also be observed from solar cycle 24. The amplitude of the variation was approximately ±10 m s$$^{-1}$$ - 1 . Interestingly, we found that the average meridional flow observed in solar cycle 24 is faster than that observed in solar cycle 23. In particular, during the declining phase of the cycle, the meridional flow of the middle latitude is accelerated from 10 to 17 m s$$^{-1}$$ - 1 , which is almost half of the meridional flow itself. The faster meridional flow in solar cycle 24 might be the result of the weakest cycle during the last 100 years.

The change of sound speed has been found at the base of the convection during the solar cycles, which can be used to constrain the solar internal magnetic field. We aim to check whether the magnetic field generated by the solar dynamo can lead to the cyclic variation of the sound speed detected through helioseismology. The basic configuration of magnetic field in the solar interior was obtained by using a Babcock-Leighton (BL) type flux transport dynamo. We reconstructed one-dimensional solar models by assimilating magnetic field generated by an established dynamo and examined their influences on the structural variables. The results show that magnetic field generated by the dynamo is able to cause noticeable change of the sound speed profile at the base of the convective zone during a solar cycle. Detailed features of this theoretical prediction are also similar to those of the helioseismic results in solar cycle 23 by adjusting the free parameters of the dynamo model., accepted for publication in RAA (Res. Astron. Astrophys.)

The relation between solar activity indices and earth atmosphere is studied in this current investigation using correlation as well as periodic analysis. The solar flare index and solar irradiance signal of India over different latitude values are considered in this study as solar activity indices and Earth atmospheric parameter respectively. To evaluate their relationship, last 33 years (1986 – 2018) of data is considered which covers solar cycle 22, 23 and almost the complete solar cycle 24. The solar irradiance of India over different considered geographical location has a positive correlation during solar cycle 23 and a negative correlation during solar cycle 24 with solar flare index. It has also been observed that the maximum correlation is exist between 5° - 10° latitude and 30° - 35° latitude data and the correlation has an increasing trend with higher latitude. The periodicity analysis of solar irradiance signal exhibits a similar type of inherent period with solar flare index as well as other activity indices. It can be suggested that the effect of the solar flare index is exists on the solar irradiance signal of India over different considered geographical location according to the current statistical investigation during solar cycle 22 – 24.

The behavior of sunspots is governed by the magnetic dynamo acting deep within the convection zone of the Sun. Therefore, knowledge of sunspot physical characteristics and how they evolve in time during a solar cycle can help to improve our understanding of the solar magnetic behavior. This work analyzes the physical characteristics of sunspots during the Solar Activity Cycle 23, detected using computer vision techniques. The main goal is to derive the relationships between sunspot properties. Images in visible light and magnetograms of the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) were used to detect sunspots and to extract their characteristics such as area, intensity (or temperature), and magnetic field magnitude. A total of 32,223 sunspots were analyzed, with longitudes between $-40^{\circ }$ and $40^{\circ }$ , throughout the entire Solar Cycle 23, from May 1996 through April 2008. These spots fell mainly into three categories. Most sunspots (85%) did not exhibit a well defined umbra, with average intensities larger than 0.657 relative to disk center, which corresponds to temperatures greater than 5200 K. A second group comprising less than 12% of the spots, was cooler (temperature less than 5200 K) and with more intense magnetic fields (2000 – 4000 G) and mostly presented a well defined umbra. Lastly, only 3% of the spots had large areas and strong magnetic fields with complex umbrae, but intermediate temperatures. The temporal behavior of sunspot physical characteristics throughout the 11 year cycle and the relationships between them were verified. Sunspot physical properties presented variations over time during Solar Cycle 23, such that larger, cooler sunspots with stronger magnetic fields occurred more frequently during periods of maximum activity. Linear correlations were found regarding the logarithm of the area and the intensity with extreme magnetic field, and of the temperature (or intensity) with the area; whereas a quadratic relation was found between magnetic field and temperature of spots.

We have compiled a catalog of solar flares as observed by the Extreme ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft and the GOES spacecraft over a span from 1997 to 2010. During mid-1998, the cadence of EIT images was revised from two images per day to 12 minutes. However, the low temporal resolution causes significant data gaps in capturing much of the flaring phenomenon. Therefore, we monitor possible errors in flare detection by flare parameters such as temporal overlap, observational wavelength, and considering full field of view (FOV) images. We consider the GOES flare catalog as the primary source. We describe the technique used to enhance the GOES detected flares using the Extreme Ultraviolet (EUV) image captured by the EIT instrument. In order to detect brightenings, we subtract the images with a maximum cadence of 25 minutes. We have downloaded and analyzed the EIT data via the Virtual Solar Observatory (VSO). This flare dataset from the SOHO/EIT period proves indispensable to the process of the solar flare predictions as the instrument has covered most of Solar Cycle 23., 11 pages, 6 figures

In April 2001, Mars Odyssey spacecraft with the High Energy Neutron Detector (HEND) onboard was launched to Mars. HEND/Odyssey was switched on measurement mode for most of transit to Mars to monitor variations of spacecraft background and solar activity. Although HEND/Odyssey was originally designed to measure Martian neutron albedo and to search for Martian subsurface water/water ice, its measurements during cruise phase to Mars are applicable to evaluate spacecraft ambient radiation background. The biological impact of the neutron component of this radiation background should be understood, as it must be taken into account in planning future human missions to Mars. We have modeled the spacecraft neutron spectral density and compared it with HEND measurements to estimate neutron dose equivalent rates during Odyssey cruise phase, which occurred during the maximum period of solar cycle 23. We find that the Odyssey ambient neutron environment during May – September 2001 yields 10.6 ± 2.0 μSv per day in the energy range from 0 to 15 MeV, and about 29 μSv per day when extrapolated to the 0–1000 MeV energy range during solar quiet time (intervals without Solar Particle Events, SPEs). We have also extrapolated HEND/Odyssey measurements to different periods of solar cycle and find that during solar minimum (maximum of GCR flux), the neutron dose equivalent rate during cruise to Mars could be as high as 52 μSv per day with the same shielding. These values are in good agreement with results reported for a similar measurement made with an instrument aboard the Mars Science Laboratory during its cruise to Mars in 2011–2012.

The hemispheric asymmetry of the solar-flare index during 1976 – 2018 from the Kandilli Observatory is studied in this investigation. The temporal duration covers Solar Cycles 21 – 23 and almost the whole of Solar Cycle 24. Different methodologies, such as cross-correlation analysis, rescaled-range analysis, empirical mode decomposition, and date-compensated discrete Fourier transform, have been used on the hemispheric solar-flare index as well as on absolute asymmetry data to study various inherent characteristics. We observed that: i) the temporal characteristics in the northern and southern hemispheres are different during the progression of a solar cycle; ii) the T-test indicates that Solar Cycles 21 and 23 do not have any dominant hemispheric effect, whereas Solar Cycle 22 and 24 have South-dominated hemispheric characteristics; iii) the southern hemisphere is leading by ten, three, and one months during Solar Cycles 21, 22, and 24, respectively, and for Solar Cycle 23 the hemispheres are in phase; iv) anti-persistence as well as short memory-dependent characteristics are present in both the hemispheric solar-flare index and the absolute asymmetry data; v) all of the time-series data have well-known periods of 11 years and 27 days as well as short-term periods around 7 days and 14 days. Apart from those, several mid-term periodicities such as the Rieger periodicity and quasi-biennial oscillations (QBOs) are also found in both hemispheric solar-flare index as well as absolute asymmetry index data; vi) the Waldmeier effect is also validated using solar-flare-index data. These results will enrich our knowledge about the distribution of hemispheric asymmetry in solar-flare-index data and may reveal some valuable points about asymmetry behaviors.

Context. How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. Aims. We aim to extend and improve on earlier studies that established that the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Methods. Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as “facular indices”, are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model, taking into account how active regions evolve and the possible non-linear relationship between plage emission, facular magnetic flux, and sunspot area. This model was incorporated into a model of total solar irradiance (TSI) to elucidate the implications for solar and stellar brightness variations. Results. The reconstruction of the facular indices from the sunspot indices with the model presented here replicates most of the observed variability, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. This explains why even though solar cycle 23 is significantly weaker than cycle 22, TSI rose to comparable levels over both cycles. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition. This happens as sunspot darkening starts to rise more rapidly with activity than facular and network brightening. This bolsters the interpretation of this behaviour of Sun-like stars as the transition from a faculae-dominated to a spot-dominated regime.

The Hubble Space Telescope (HST) has been operational for over 30 years and throughout that time it has been bombarded by high energy charged particles colloquially referred to as cosmic rays. In this paper, we present a comprehensive study of more than 1.2 billion cosmic rays observed with HST using a custom written python package, \texttt{HSTcosmicrays}, that is available to the astronomical community. We analyzed $75,908$ dark calibration files taken as part of routine calibration programs for five different CCD imagers with operational coverage of Solar Cycle 23 and 24. We observe the expected modulation of galactic cosmic rays by solar activity. We model the observed energy-loss distributions to derive an estimate of 534 $\pm$ 117 MeV for the kinetic energy of the typical cosmic ray impacting HST. For the three imagers with the largest non-uniformity in thickness, we independently confirm the overall structure produced by fringing analyses by analyzing cosmic ray strikes across the detector field of view. We analyze STIS/CCD observations taken as HST crosses over the South Atlantic Anomaly and find a peak cosmic-ray particle flux of $\sim1100$ $particle/s/cm^2$. We find strong evidence for two spatially confined regions over North America and Australia that exhibit increased cosmic-ray particle fluxes at the $5\sigma$ level., Comment: 48 pages, 30 figures, submitted to ApJ

The most regular of all daily geomagnetic field variations is the so-called solar quiet, or Sq, variation. It is attributed to the two current vortices flowing in the E-region of the dayside ionosphere. We present an investigation of the time-dependent parameters of Sq variation for the historical minimum of solar activity in 2008. We apply "Measure of Anomalousness" algorithm to detection of magnetically quiet days. The global maps of seasonal Sq amplitudes of the three orthogonal components are derived using 75 INTERMAGNET and 46 SuperMAG stations at low and middle latitudes. The global Sq amplitudes are compared to the previous Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model simulations and show good agreement. Significant variability was found in Sq(X) and Sq(Y) based on the solar activity and latitude, while almost no difference is observed in Sq(Z) for across all latitudes and seasons. We analyze equivalent Sq current system using observatory data from the Australian mainland and narrow European-African latitudinal segment. Sq current system also strongly depends on solar activity, as current vortices are strongest in the local summer-hemisphere and disintegrate during local winter. The dynamics of Sq variation along the solar cycles 23 and 24 is also discussed and compared to Swarm-based spherical harmonic Sq model.

During 1942–2017, 72 ground level events (GLEs) have been registered. Source location and solar cycle distribution of GLEs during 1942–2017 have been investigated. The GLEs that occurred during 1942–2017 are mainly distributed in the longitudinal area ranged from E30 to W150. The northern hemisphere has many more GLEs than the southern hemisphere during Solar Cycles 19 and 20, while the northern hemisphere has slightly more GLEs than the southern hemisphere of the Sun during Solar Cycles 21 and 22. However, the southern hemisphere has more GLEs than the northern hemisphere during Solar Cycle 23. The latitudinal area ranged from N20 to N40 of the northern hemisphere of the Sun has many more GLEs than the latitudinal area ranging from S20 to S40 of the southern hemisphere of the Sun. The latitudinal area ranging from N0 to N20 of the northern hemisphere of the Sun has slightly more GLEs than the latitudinal area ranged from S0 to S20 of the southern hemisphere of the Sun. 46 GLEs came from the northern hemisphere, while 26 GLEs came from the southern hemisphere of the Sun, suggesting that GLEs dominated in the northern hemisphere of the Sun during 1942–2017. Statistical results show that 44.3% and 55.7% of the GLEs appeared in the ascending and descending phases of the solar cycles, respectively, and about 83% of the GLEs appeared during the period from the two years before solar cycle peak and the three years after solar cycle peak time. The number of GLEs during a solar cycle has a poor correlation with the amplitude of the solar cycle.

The effect of precipitating high-energy particles on atmospheric physics and chemistry is extensively studied over the last decade. In majority of the existing models, the precipitating particles induced ionization plays an essential role. For such effects, it is necessary to possess enhanced increase in ion production, specifically during the winter period. In this study, we focus on highly penetrating particles - cosmic rays. The galactic cosmic rays are the main source of ionization in the Earth's stratosphere and troposphere. On the other hand, the atmospheric ionization may be significantly enhanced during strong solar energetic particle events, mainly over the polar caps. A specific interest is paid to the most energetic solar proton events leading to counting rate enhancement of ground-based detectors, namely the so-called ground level enhancements (GLEs). During solar cycle 23, several strong ground level enhancements were observed. A sequence of three GLEs was observed in October-November 2003, the Halloween events. Here, on the basis of 3-D Monte Carlo model, we computed the energetic particles induced atmospheric ionization, explicitly considering the contribution of cosmic rays with galactic and solar origin. The ion production rates were computed as a function of the altitude above sea level using reconstructed solar energetic particles spectra. The 24 hours and event averaged ionization effects relative to the average due to galactic cosmic rays were also computed., Comment: 21 pages, 11 figures

This study analyzes the temporal fluctuations of the z-component of the interplanetary magnetic field (IMF) as measured by the Advanced Composition Explorer (ACE) satellite located at 1.4 million kilometers from Earth. The data used in this study corresponds to IMF fluctuations during solar quiet days with no apparent solar flares. We have applied a path integral approach and showed that the fluctuations exhibit a non-Markovian stochastic process. By solving for the mean square deviation (MSD) of the fluctuating magnetic field together with the application of white noise calculus by Hida, we have obtained an exponentially damped memory function with a time constant of about 103 seconds. It was also observed that the Fourier transform of the IMF generally follows a power-law distribution. The computed power spectral density (PSD) has a “Kolmogorov-like” form with a scaling of f-2. The Kolmogorov spectrum is noticeably common in studies involving plasma turbulence. Furthermore, a probability density function (PDF) which satisfies the diffusion equation was derived and observed that it has a parameter-dependent diffusion coefficient. The amplitude coefficient of the equation has a direct proportionality relationship with the global planetary K-index. The time constant, (1/b) is approximately the same for all events considered in the study. Finally, using white noise analysis, a model signal describing the fluctuations of the IMF was obtained, which will be very useful in developing space weather forecasting models.

We analyse the topology of photospheric magnetic fields and sub-photospheric flows of several active regions (ARs) that are observed during the peak to descending phase of the solar cycle 23. Our analysis shows clear evidence of hemispheric preferences in all the topological parameters such as the magnetic, current and kinetic helicities, and the 'curl-divergence'. We found that 68\%(67\%) ARs in the northern (southern) hemisphere with negative (positive) magnetic helicities. Same hemispheric preference sign is found for the current helicities in 68\%(68\%) ARs. The hemispheric preferences are found to exist statistically for all the time except in few ARs observed during the peak and the end phases of the solar cycle. This means that magnetic fields are dominantly left(right)-helical in scales smaller than individual ARs of northern(southern) hemisphere. We found that magnetic and current helicities parameters show equator-ward propagation similar to the sunspot cycle. The kinetic helicity showed similar hemispheric trend to that of magnetic and current helicity parameters. There are 65\%(56\%) ARs with negative (positive) kinetic helicity as well as divergence-curl, at the depth of 2.4\,Mm, in the northern (southern) hemisphere. The hemispheric distribution of the kinetic helicity becomes more evident at larger depths, e.g., 69\%(67\%) at the depth of 12.6\,Mm. Similar hemispheric trend of kinetic helicity to that of the current helicity supports the mean field dynamo model. We also found that the hemispheric distribution of all the parameters increases with the field strength of ARs. The topology of photospheric magnetic fields and near surface sub-photospheric flow fields did not show good association but the correlation between them enhances with depths which could be indicating more aligned flows at deeper layers of ARs., Comment: 23 pages, 11 figures, Accepted for publication in Solar Physics