Search

In this paper we conduct a data survey searching for well-defined streamer wave events observed by the Large Angle and Spectrometric Coronagraph (LASCO) on-board the Solar and Heliospheric Observatory (SOHO) throughout Solar Cycle 23. As a result, eight candidate events are found and presented here. We compare different events and find that in most of them the driving CMEs' ejecta are characterized by a high speed and a wide angular span, and the CME-streamer interactions occur generally along the flank of the streamer structure at an altitude no higher than the bottom of the field of view of LASCO C2. In addition, all front-side CMEs have accompanying flares. These common observational features shed light on the excitation conditions of streamer wave events. We also conduct a further analysis on one specific streamer wave event on 5 June 2003. The heliocentric distances of four wave troughs/crests at various exposure times are determined; they are then used to deduce the wave properties like period, wavelength, and phase speeds. It is found that both the period and wavelength increase gradually with the wave propagation along the streamer plasma sheet, and the phase speed of the preceding wave is generally faster than that of the trailing ones. The associated coronal seismological study yields the radial profiles of the Alfvén speed and magnetic field strength in the region surrounding the streamer plasma sheet. Both quantities show a general declining trend with time. This is interpreted as an observational manifestation of the recovery process of the CME-disturbed corona. It is also found that the Alfvénic critical point is at about 10 R, where the flow speed, which equals the Alfvén speed, is ∼ 200 km s. [ABSTRACT FROM AUTHOR]

This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields. In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered, where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network. As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging flux values is allowed (4×10 - 10 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux for emerging bipoles to be 4×10 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling. [ABSTRACT FROM AUTHOR]

The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al., 2009) obtains continuous EUV spectra of the Sun viewed as a star. Its primary objective is the characterization of solar spectral irradiance, but its sensitivity and stability make it extremely interesting for observations of variability on time scales down to the limit imposed by its basic 10 s sample interval. In this paper we characterize the Doppler sensitivity of the EVE data. We find that the 30.4 nm line of He ii has a random Doppler error below 0.001 nm (1 pm, better than 10 km s as a redshift), with ample stability to detect the orbital motion of its satellite, the Solar Dynamics Observatory (SDO). Solar flares also displace the spectrum, both because of Doppler shifts and because of EVE's optical layout, which (as with a slitless spectrograph) confuses position and wavelength. As a flare develops, the centroid of the line displays variations that reflect Doppler shifts and therefore flare dynamics. For the impulsive phase of the flare SOL2010-06-12, we find the line centroid to have a redshift of 16.8 ± 5.9 km s relative to that of the flare gradual phase (statistical errors only). We find also that high-temperature lines, such as Fe xxiv 19.2 nm, have well-determined Doppler components for major flares, with decreasing apparent blueshifts as expected from chromospheric evaporation flows. [ABSTRACT FROM AUTHOR]

Many models of eruptive flares or coronal mass ejections (CMEs) involve formation of a current sheet connecting the ejecting CME flux rope with a magnetic loop arcade. However, there is very limited observational information on the properties and evolution of these structures, hindering progress in understanding eruptive activity from the Sun. In white-light images, narrow coaxial rays trailing the outward-moving CME have been interpreted as current sheets. Here, we undertake the most comprehensive statistical study of CME-rays to date. We use SOHO/LASCO data, which have a higher cadence, larger field of view, and better sensitivity than any previous coronagraph. We compare our results to a previous study of Solar Maximum Mission (SMM) CMEs, in 1984 - 1989, having candidate magnetic disconnection features at the CME base, about half of which were followed by coaxial bright rays. We examine all LASCO CMEs during two periods of minimum and maximum activity in Solar Cycle 23, resulting in many more events, $\sim130$ CME-rays, than during SMM. Important results include: The occurrence rate of the rays is $\sim11~\%$ of all CMEs during solar minimum, but decreases to $\sim7~\%$ at solar maximum; this is most likely related to the more complex coronal background. The rays appear on average 3 - 4 hours after the CME core, and are typically visible for three-fourths of a day. The mean observed current sheet length over the ray lifetime is $\sim12~R_{\odot}$ , with the longest current sheet of $18.5~R_{\odot}$ . The mean CS growth rates are $188~\mbox{km}\,\mathrm{s}^{-1}$ at minimum and $324~\mbox{km}\,\mathrm{s}^{-1}$ at maximum. Outward-moving blobs within several rays, which are indicative of reconnection outflows, have average velocities of $\sim350~\mbox{km}\,\mathrm{s}^{-1}$ with small positive accelerations. A pre-existing streamer is blown out in most of the CME-ray events, but half of these are observed to reform within $\sim1$ day. The long lifetime and long lengths of the CME-rays challenge our current understanding of the evolution of the magnetic field in the aftermath of CMEs. [ABSTRACT FROM AUTHOR]

Galactic cosmic rays (GCRs) are energetic, charged particles coming from outside the solar system. These particles encounter an outward-moving turbulent solar wind with cyclic magnetic-field fluctuations when entering the heliosphere. This causes convection and diffusion in the heliosphere. The GCR counts detected by the ground-based neutron-monitor stations show intensity changes with a fluctuation of ∼ 11 years and are anti-correlated with the sunspot numbers with some time lags. GCRs experience various types of modulation from different solar activity features and are important components of space weather. The previous solar cycle, Cycle 23, has shown anomalous behavior with a prolonged deep minimum, which was characterized by a record-setting high Galactic cosmic-ray flux observed at Earth. Solar Cycle 24 started much later than expected and progressed sluggishly toward its maxima. In this paper, we study the heliospheric modulation and intermediate-term periodicities of GCRs during the ascending phase of Cycle 24. We utilize simultaneous solar, interplanetary plasma, magnetic field, and geomagnetic activity data including the tilt angle of the heliospheric current sheet, and we study their relation with GCRs. The wavelet power spectrum of GCRs exhibits the presence of a variety of prominent short- and mid-term periodicities including the well-known Rieger and quasi-biennial periodicities. Possible explanations of the observed results are discussed in the light of numerical models. [ABSTRACT FROM AUTHOR]

The aim of this paper is to test the model of oscillating magnetic traps (the OMT model), proposed by Jakimiec and Tomczak ( Solar Phys. 261, 233, 2010). This model describes the process of excitation of quasi-periodic pulsations (QPPs) observed during solar flares. In the OMT model energetic electrons are accelerated within a triangular, cusp-like structure situated between the reconnection point and the top of a flare loop as seen in soft X-rays. We analyzed QPPs in hard X-ray light curves for 23 flares as observed by Yohkoh. Three independent methods were used. We also used hard X-ray images to localize magnetic traps and soft X-ray images to diagnose thermal plasmas inside the traps. We found that the majority of the observed pulsation periods correlates with the diameters of oscillating magnetic traps, as was predicted by the OMT model. We also found that the electron number density of plasma inside the magnetic traps in the time of pulsation disappearance is strongly connected with the pulsation period. We conclude that the observations are consistent with the predictions of the OMT model for the analyzed set of flares. [ABSTRACT FROM AUTHOR]

We test the reliability of helioseismic far-side active-region predictions, made using Dopplergrams from both the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and the Global Oscillation Network Group (GONG), by comparison with far-side observation of solar activity from the Solar TErrestrial RElations Observatory (STEREO). Both GONG and HMI produce seismic Carrington maps that show strong magnetic-field regions, labeling predictions of far-side active regions that have a probability ≥ 70 %. By visual comparison of these prediction maps with STEREO extreme ultraviolet (EUV) Carrington maps, we determine whether or not solar activity, as evidenced as brightness in EUV, is observed at the predicted locations. We analyzed nine months of data from 2011 and 2012. For both GONG and HMI, we find that for approximately 90 % of the active-region predictions, activity/brightness is observed in EUV at the predicted location. We also investigated the success of GONG and HMI at predicting large active regions before they appear at the east limb as viewed from Earth. Of the 27 identified large east-limb active regions in the nine months of data analyzed, GONG predicted 15 (55 %) at least once within the week prior to Earth-side appearance and HMI predicted 13 (48 %). Based on the STEREO far-side EUV observations, we suggest that 9 of the 27 active regions were probably too weak to be predicted while on the far side. Overall, we conclude that HMI and GONG have similar reliability using the current data-processing procedures. [ABSTRACT FROM AUTHOR]

The aim of this work is to provide a physical explanation for the genesis of multiple coronal mass ejections (CMEs) in an asymmetric coronal field configuration. We analyze STEREO observations of a multiple eruption and compare the results from the data analysis with predictions provided by magnetohydrodynamic (MHD) simulations. To this end, the multiple CMEs (MCMEs) observed on 21 - 22 September 2009 were selected. Both eruptions originated from the same source region and showed approximately the same latitudinal deflection, by more than 15 degrees, toward the heliospheric current sheet (HCS) during their propagation in the COR1 field of view. Numerical MHD simulations of the MCMEs have been performed, starting from an asymmetric coronal field configuration that mimics the potential field source surface extrapolation for 21 September 2009. The results demonstrate that, by shearing the footpoints at the base of the southern arcade, we were able to reproduce the observed dynamics of the MCMEs. Both CMEs are deflected toward the HCS due to an imbalance in the magnetic pressure and tension forces; the global field strength turns out to be a crucial parameter in order to release two subsequent eruptions, and hence to reproduce the observed evolution. [ABSTRACT FROM AUTHOR]

The 6 June 2007 16:55 UT flare was well observed with high time-cadence sparse raster scans by the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft. The observation covers an active region area of 240 arcsec × 240 arcsec with the 1 arcsec slit in about 160 seconds. We describe here spectral properties of a 'pre-flare brightening' to this flare, which started about nine minutes prior to flare-ribbon onset. This flare brightening looks like two small loops apparently having a cusp-shape structure about 40 - 50 arcsec west of the main flaring loops, which show dynamic behavior in velocity during the early phases of the flare: The He ii line at 256.32 Å shows the existence of a bi-directional flow along the Sun-Earth line of sight of about −70 and +100 km s. On the other hand, the Fe xvi line at 262.98 Å formed at higher coronal temperatures shows only a slight increase in intensity at the location of these loops, and the Fe xxiii line at 263.76 Å barely appears. Electron density at the site derived from the intensity ratios of the Fe xiv line pair at 264.78 Å and 274.20 Å is lower than the average of 10 cm in other parts of the active-region outskirts. Combining a time series of STEREO-A/B SECCHI-EUVI 171 Å images, we conclude that the pre-flare-brightening region may be heated via magnetic reconnection taking place as a result of loop-loop interaction. [ABSTRACT FROM AUTHOR]

In this paper we examine the elemental composition of the 0.1 - 1 MeV nucleon interplanetary heavy ions from H to Fe in corotating interaction regions (CIRs) measured by the SIT ( Suprathermal Ion Telescope) instrument. We use observations taken on board the STEREO spacecraft from January 2007 through December 2010, which included the unusually long solar minimum following Solar Cycle 23. During this period instruments on STEREO observed more than 50 CIR events making it possible to investigate CIR ion abundances during solar minimum conditions with unprecedented high statistics. The observations reveal annual variations of relative ion abundances in the CIRs during the 2007 - 2008 period as indicated by the He/H, He/O and Fe/O elemental ratios. We discuss possible causes of the variability in terms of the helium focusing cone passage and heliolatitude dependence. The year 2009 was very quiet in CIR-event activity. In 2010 the elemental composition in CIRs were influenced by sporadic solar energetic particle (SEP) events. The 2010 He/H and He/O abundance ratios in CIRs show large event-to-event variations with values resembling the SEP-like composition. This finding suggests that the suprathermal SEPs could be the source population for CIR acceleration. [ABSTRACT FROM AUTHOR]

Data from the STEREO ( Solar Terrestrial Relations Observatory) mission are intensively used for 3D reconstruction of solar coronal structures. After the launch of the SDO ( Solar Dynamic Observatory) satellite, its additional observations give the possibility to have a third eye for more accurate 3D reconstruction in the very low corona (< 1.5 R). With our reconstruction code MBSR (Multi-view B-spline Stereoscopic Reconstruction), we use three view directions (STEREO A, B, and SDO) to perform the 3D reconstruction and evolution of a prominence which triggered a CME on 1 August 2010. In the paper we present the reconstruction of this prominence from the moment it starts to erupt until it leaves the field of view of the coronagraph. We also determine the evolution of the leading edge of the CME. Based on the temporal evolution, we analyze some of its properties, such as velocity, acceleration, opening and rotation angles and evolution of the cavity. [ABSTRACT FROM AUTHOR]

In this paper we present in situ observations of the heliospheric plasma sheet (HPS) from STEREO-A, Wind, and STEREO-B over four solar rotations in the declining phase of Solar Cycle 23, covering late March through late June 2007. During this time period the three spacecraft were located in the ecliptic plane, and were gradually separating in heliographic longitude from about 3 degrees to 14 degrees. Crossings of the HPS were identified using the following criteria: reversal of the interplanetary magnetic field sector, enhanced proton density, and local minima in both the proton specific entropy argument and in the alpha particle-to-proton number density ratio ( N/ N). Two interplanetary coronal mass ejections (ICMEs) were observed during the third solar rotation of our study period, which disrupted the HPS from its quasi-stationary state. We find differences in the in situ proton parameters at the HPS between the three spacecraft despite temporal separations of less than one day. We attribute these differences to both small separations in heliographic latitude and radial evolution of the solar wind leading to the development of compression regions associated with stream interaction regions (SIRs). We also observed a modest enhancement in the density of iron ions at the HPS. [ABSTRACT FROM AUTHOR]

Solar energetic particle (SEP) events are a key ingredient of solar-terrestrial physics both for fundamental research and space weather applications. Multi-satellite observations are an important and incompletely exploited tool for studying the acceleration and the coronal and interplanetary propagation of the particles. While STEREO uses for this diagnostic two identical sets of instrumentation, there are many earlier observations carried out with different spacecraft. It is the aim of the SEPServer project to make these data and analysis tools available to a broad user community. The consortium will carry out data-driven analysis and simulation-based data analysis capable of deconvolving the effects of interplanetary transport and solar injection from SEP observations, and will compare the results with the electromagnetic signatures. The tools and results will be provided on the web server of the project in order to facilitate further analysis by the research community. This paper describes the data products and analysis strategies with one specific event, the case study of 13 July 2005. The release time of protons and electrons are derived using data-driven and simulation-based analyses, and compared with hard X-ray and radio signatures. The interconnection of the experimental and the simulation-based results are discussed in detail. [ABSTRACT FROM AUTHOR]

In this paper, we extend our earlier work to provide additional evidence for an alternative scenario to explain the nature of events called 'explosive events'. The bidirected, fast Doppler motion of explosive events observed spectroscopically in the transition region emission is classically interpreted as a pair of bidirectional jets moving upward and downward from a reconnection site. We discuss the problems of this model. In our previous work, we focused basically on the discrepancy of fast Doppler motion without detectable motion in the image plane. We now suggest an alternative scenario for the explosive events, based on our observations of spectral line tilts and bifurcated structure in some events. Both features are indicative of rotational motion in narrow structures. We explain the bifurcation as the result of rotation of hollow cylindrical structures and demonstrate that this kind of sheath model can also be applied to explain the nature of the puzzling 'explosive events'. We find that the spectral tilt, the lack of apparent motion, the bifurcation, and a rapidly growing number of direct observations support an alternative scenario of linear, spicular-sized jets with a strong spinning motion. [ABSTRACT FROM AUTHOR]

This paper describes the methods used to obtain the thermal evolution and radiative output during solar flares as observed by the Extreme ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO). How EVE measurements, due to the temporal cadence, spectral resolution and spectral range, can be used to determine how the thermal plasma radiates at various temperatures throughout the impulsive and gradual phase of flares is presented and discussed in detail. EVE can very accurately determine the radiative output of flares due to pre- and in-flight calibrations. Events are presented that show that the total radiated output of flares depends more on the flare duration than the typical GOES X-ray peak magnitude classification. With SDO observing every flare throughout its entire duration and over a large temperature range, new insights into flare heating and cooling as well as the radiative energy release in EUV wavelengths support existing research into understanding the evolution of solar flares. [ABSTRACT FROM AUTHOR]

As part of the overall ground-based calibration of the Helioseismic and Magnetic Imager (HMI) instrument an extensive set of polarimetric calibrations were performed. This paper describes the polarimetric design of the instrument, the test setup, the polarimetric model, the tests performed, and some results. It is demonstrated that HMI achieves an accuracy of 1% or better on the crosstalks between Q, U, and V and that our model can reproduce the intensities in our calibration sequences to about 0.4%. The amount of depolarization is negligible when the instrument is operated as intended which, combined with the flexibility of the polarimeter design, means that the polarimetric efficiency is excellent. [ABSTRACT FROM AUTHOR]

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2010, incorporates a suite of instruments including the Extreme Ultraviolet Variability Experiment (EVE). EVE has multiple instruments including the Multiple Extreme ultraviolet Grating Spectrographs (MEGS) A, B, and P instruments, the Solar Aspect Monitor (SAM), and the Extreme ultraviolet SpectroPhotometer (ESP). The radiometric calibration of EVE, necessary to convert the instrument counts to physical units, was performed at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF III) located in Gaithersburg, Maryland. This paper presents the results and derived accuracy of this radiometric calibration for the MEGS A, B, P, and SAM instruments, while the calibration of the ESP instrument is addressed by Didkovsky et al. ( Solar Phys., , doi:). In addition, solar measurements that were taken on 14 April 2008, during the NASA 36.240 sounding-rocket flight, are shown for the prototype EVE instruments. [ABSTRACT FROM AUTHOR]

In the context of space weather forecasting, an automated detection of coronal mass ejections (CMEs) becomes more and more important for efficiently handling a large data flow which is expected from recently-launched and future solar missions. In this paper we validate the detection software package 'CACTus' by applying the program to synthetic data from our 3D time-dependent CME simulations instead of observational data. The main strength of this study is that we know in advance what should be detected. We describe the sensitivities and strengths of automated detection, more specific for the CACTus program, resulting in a better understanding of CME detection on one hand and the calibration of the CACTus software on the other hand, suggesting possible improvements of the package. In addition, the simulation is an ideal tool to investigate projection effects on CME velocity measurements. [ABSTRACT FROM AUTHOR]

The aim of this paper is to determine the flux emergence rate due to small-scale magnetic features in the quiet Sun using high-resolution Hinode SOT NFI data. Small-scale magnetic features are identified in the data using two different feature identification methods (clumping and downhill); then three methods are applied to detect flux emergence events. The distribution of the intranetwork peak emerged fluxes is determined. When combined with previous emergence results, from ephemeral regions to sunspots, the distribution of all fluxes are found to follow a power-law distribution which spans nearly seven orders of magnitude in flux (10 - 10 Mx) and 18 orders of magnitude in frequency. The power-law fit to all these data is of the form where Ψ=10 Mx and is used to predict a global flux emergence rate of ≈ 450 Mx cm day from all features with fluxes of 10 Mx or more. Since the slope of all emerged fluxes is less than −2, this implies that most of the new flux that is fed into the solar atmosphere is from small-scale emerging events. This suggests that the rate of flux emergence is independent of the solar cycle and is equivalent to a global rate of flux emergence of more than a few times 10 Mx day. The single power-law distribution over all emerged fluxes implies a scale-free dynamo, therefore indicating that a turbulent dynamo may act throughout the convection zone. Moreover, from the slope of the emerging flux distribution the (turbulent?) dynamo producing small-scale features produces considerably more flux than the active-region dynamo at the tachocline. [ABSTRACT FROM AUTHOR]

In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models. [ABSTRACT FROM AUTHOR]

Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra, time histories, and occurrence distribution patterns of GRB data in terms of “thermal feed-through”, the confusion of thermal soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 – X25, we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08. [ABSTRACT FROM AUTHOR]

The aim of this paper is to look at the magnetic helicity structure of an emerging active region and show that both emergence and flaring signatures are consistent with a same sign for magnetic helicity. We present a multiwavelength analysis of an M1.6 flare occurring in the NOAA active region 10365 on 27 May 2003, in which a large new bipole emerges in a decaying active region. The diverging flow pattern and the “tongue” shape of the magnetic field in the photosphere with elongated polarities are highly suggestive of the emergence of a twisted flux tube. The orientation of these tongues indicates the emergence of a flux tube with a right-hand twist ( i.e., positive magnetic helicity). The flare signatures in the chromosphere are ribbons observed in Hα by the MSDP spectrograph in the Meudon solar tower and in 1600 Å by TRACE. These ribbons have a J shape and are shifted along the inversion line. The pattern of these ribbons suggests that the flare was triggered by magnetic reconnection at coronal heights below a twisted flux tube of positive helicity, corresponding to that of the observed emergence. It is the first time that such a consistency between the signatures of the emerging flux through the photosphere and flare ribbons has been clearly identified in observations. Another type of ribbons observed during the flare at the periphery of the active region by the MSDP and SOHO/EIT is related to the existence of a null point, which is found high in the corona in a potential field extrapolation. We discuss the interpretation of these secondary brightenings in terms of the “breakout” model and in terms of plasma compression/heating within large-scale separatrices. [ABSTRACT FROM AUTHOR]

Mounted on the sides of two widely separated spacecraft, the two Heliospheric Imager (HI) instruments onboard NASA’s STEREO mission view, for the first time, the space between the Sun and Earth. These instruments are wide-angle visible-light imagers that incorporate sufficient baffling to eliminate scattered light to the extent that the passage of solar coronal mass ejections (CMEs) through the heliosphere can be detected. Each HI instrument comprises two cameras, HI-1 and HI-2, which have 20° and 70° fields of view and are off-pointed from the Sun direction by 14.0° and 53.7°, respectively, with their optical axes aligned in the ecliptic plane. This arrangement provides coverage over solar elongation angles from 4.0° to 88.7° at the viewpoints of the two spacecraft, thereby allowing the observation of Earth-directed CMEs along the Sun – Earth line to the vicinity of the Earth and beyond. Given the two separated platforms, this also presents the first opportunity to view the structure and evolution of CMEs in three dimensions. The STEREO spacecraft were launched from Cape Canaveral Air Force Base in late October 2006, and the HI instruments have been performing scientific observations since early 2007. The design, development, manufacture, and calibration of these unique instruments are reviewed in this paper. Mission operations, including the initial commissioning phase and the science operations phase, are described. Data processing and analysis procedures are briefly discussed, and ground-test results and in-orbit observations are used to demonstrate that the performance of the instruments meets the original scientific requirements. [ABSTRACT FROM AUTHOR]

We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90∘ in the case of the southward orientation and ≥ 90∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms ( D st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms. [ABSTRACT FROM AUTHOR]

This paper presents the techniques developed for the automated detection of filaments on Meudon Ha spectroheliograms, and, by extension, on any full-disk Ha Sun observations. Some cleaning processes are first applied to the images to correct them from defects characteristic of the instrument. Indeed, these defects may lead to spurious detections. From the cleaned images, filament areas are then segmented using a region growing method which efficiently returns the full extent of these dark areas. The filaments are finally described by means of their pruned skeleton. This representation allows one to compare the automatically segmented filaments with those manually recorded for Meudon Synoptic Maps. The very good agreement observed on a representative set of images confirms that this method can effectively be used in the frame of the EGSO (European Grid of Solar Observations) project in order to produce a reliable catalog dedicated to solar features. [ABSTRACT FROM AUTHOR]

We seek to quantify the relative contributions of Lorentz forces and aerodynamic drag on the propagation of solar coronal mass ejections (CMEs). We use Graduated Cylindrical Shell (GCS) model fits to a representative set of 38 CMEs observed with the Solar and Heliospheric Observatory (SOHO) and the Solar and Terrestrial Relations Observatory (STEREO) spacecraft. We find that the Lorentz forces generally peak between 1.65 and 2.45 R for all CMEs. For fast CMEs, Lorentz forces become negligible in comparison to aerodynamic drag as early as 3.5 - 4 R. For slow CMEs, however, they become negligible only by 12 - 50 R. For these slow events, our results suggest that some of the magnetic flux might be expended in CME expansion or heating. In other words, not all of it contributes to the propagation. Our results are expected to be important in building a physical model for understanding the Sun-Earth dynamics of CMEs. [ABSTRACT FROM AUTHOR]

Ground level enhancements (GLEs) of cosmic-ray intensity occur, on average, once a year. Because they are rare, studying the solar sources of GLEs is especially important to approach understanding their origin. The SOL2001-12-26 eruptive-flare event responsible for GLE63 seems to be challenging in some aspects. Deficient observations limited our understanding of it. Analysis of additional observations found for this event provided new results that shed light on the flare configuration and evolution. This article addresses the observations of this flare with the Siberian Solar Radio Telescope (SSRT). Taking advantage of its instrumental characteristics, we analyze the detailed SSRT observations of a major long-duration flare at 5.7 GHz without cleaning the images. The analysis confirms that the source of GLE63 was associated with an event in active region 9742 that comprised two flares. The first flare (04:30 - 05:03 UT) reached a GOES importance of about M1.6. Two microwave sources were observed, whose brightness temperatures at 5.7 GHz exceeded 10 MK. The main flare, up to an importance of M7.1, started at 05:04 UT and occurred in strong magnetic fields. The observed microwave sources reached a brightness temperature of about 250 MK. They were not static. After appearing on the weaker-field periphery of the active region, the microwave sources moved toward each other nearly along the magnetic neutral line, approaching the stronger-field core of the active region, and then moved away from the neutral line like expanding ribbons. These motions rule out an association of the non-thermal microwave sources with a single flaring loop. [ABSTRACT FROM AUTHOR]

We characterize the daytime sky quality in terms of brightness, cloud coverage, and main weather variables at the Carlos Ulrico Cesco station of the Felix Aguilar Astronomical Observatory (OAFA), located in El Leoncito National Park, San Juan, Argentina. We have collected more than 15 years of daily observations from the auxiliary sky brightness detectors of the Mirror Coronagraph for Argentina (MICA, in operations from 1997 to 2012), including daily observing reports. We additionally present data from two meteorological stations operated at the site from 2000 to 2020. We determine the main statistical properties and seasonal variability of daytime sky brightness, clear sky time fraction (CSTF), precipitable water vapor (WV), temperature, humidity, and wind speed, which are relevant for solar, particularly coronal observations. Our results confirm that El Leoncito is an excellent place to perform daytime astronomical observations. We measure a median sky brightness of 15.8 ppm, estimated at 526.0 ± 1.0 nm and 6 solar radii from the solar disk center; a median CSTF of 0.7; and a median WV below 6 mm . These values, and those of other relevant weather variables, are comparable to the levels found among the best astronomical observing sites in the world. Due to the extended period of time analyzed and high sampling frequency, the novel data and results presented in this report contribute to the analysis and interpretation of historical sky brightness data and are of great value for the future planning of daytime astronomical instrumentation at El Leoncito. [ABSTRACT FROM AUTHOR]

Zdeněk Švestka's research work influenced many fields of solar physics, especially in the area of flare research. In this article I take five of the areas that particularly interested him and assess them in a 'then and now' style. His insights in each case were quite sound, although of course in the modern era we have learned things that he could not readily have envisioned. His own views about his research life have been published recently in this journal, to which he contributed so much, and his memoir contains much additional scientific and personal information (Švestka in Solar Phys. 267, 235, ). [ABSTRACT FROM AUTHOR]

The solar filaments obtained from the daily Ca ii K3 observations at the Coimbra Astronomical Observatory, which is now called the Geophysical and Astronomical Observatory of the University of Coimbra (OGAUC), for the period 1929 – 1941 have been digitized and validated, and they are confirmed to be reliable for solar activity studies. We use these filament data to investigate the rotation and differential degree of the solar high chromosphere. The main results are as follows: (1) the rotation profile of filaments is somewhat steeper than the rotation profile for long-lived Hα filaments; (2) the high chromosphere filaments rotate faster than sunspots, the photosphere, and the medium-low chromosphere at middle-low latitudes; (3) solar filaments rotate at a higher rotation rate in the descending phase of the solar cycle than in the ascending phase, and their rotation velocities are relatively faster around the year of solar maximum. The possible explanation for the above results is given. [ABSTRACT FROM AUTHOR]

With the aim to examine whether the predicted solar gravitational redshift can be observationally confirmed under the influence of the convective Doppler shift due to granular motions, we attempted to measure the absolute spectral-line shifts on a large number of points over the solar disk based on an extensive set of 5188 - 5212 Å region spectra taken through an iodine cell with the Domeless Solar Telescope at Hida Observatory. The resulting heliocentric line shifts at the meridian line (where no rotational shift exists), which were derived by finding the best-fit parameterized model spectrum with the observed spectrum and corrected for the earth's motion, turned out to be weakly position-dependent as ≈ + 400 m s near the disk center and increasing toward the limb up to ≈ + 600 m s (both with a standard deviation of σ≈100 m s). Interestingly, this trend tends to disappear when the convective shift due to granular motions (≈ − 300 m s at the disk center and increasing toward the limb; simulated based on the two-component model along with the empirical center-to-limb variation) is subtracted, finally resulting in an averaged shift of 698 m s ( σ=113 m s). Considering the ambiguities involved in the absolute wavelength calibration or in the correction due to convective Doppler shifts (at least several tens m s, or more likely up to ≲ 100 m s), we may regard this value to be well consistent with the expected gravitational redshift of 633 m s. [ABSTRACT FROM AUTHOR]

A filament and its channel close to the solar disk were observed in the complete hydrogen Lyman spectrum, and in several EUV lines by the SUMER (Solar Ultraviolet Measurement of Emitted Radiation) and CDS (Coronal Diagnostic Spectrometer) spectrographs on the SoHO satellite, and in Hα by ground-based telescopes during a multi-instrument campaign in May 2005. It was a good opportunity to get an overview of the volume and the density of the cold plasma in the filament channel; these are essential parameters for coronal mass ejections. We found that the width of the filament depends on the wavelength in which the filament is observed (around 15 arcsec in Hα, 30 arcsec in Lα, and 60 arcsec in EUV). In Lα the filament is wider than in Hα because cool plasma, not visible in Hα, is optically thick at the Lα line center, and its presence blocks the coronal emission. We have derived physical plasma properties of this filament fitting the Lyman spectra and Hα profiles by using a 1D isobaric NLTE model. The vertical temperature profile of the filament slab is flat ( T≈7000 K) with an increase to ≈ 20 000 K at the top and the bottom of the slab. From an analysis of the Lα and Hα source functions we have concluded that these lines are formed over the whole filament slab. We have estimated the geometrical filling factor in the filament channel. Its low value indicates the presence of multi-threads. [ABSTRACT FROM AUTHOR]

Intensity ( I) and circular polarization ( V) spectra using the 854.2 nm line of Ca ii with 3.65 pm spectral and 1 arcsecond spatial sampling were obtained with the SOLIS vector spectromagnetograph on 8 November 2011. An active region filament showing Doppler shifts as large as 50 km s and an unrelated C1.8 flare were observed. Line-of-sight flux density estimates of the magnetic field (B) were mapped as a function of wavelength in both of these features using the weak-field Zeeman-splitting approximation that V∝d I/d λ. The filament had a large amount of structure in intensity and velocity but remarkably little B structure, which varied smoothly from 35 to 55 G (gauss). Two flare emission kernels showed average B values of 415 and − 215 G, about 84 % of the underlying photospheric fields. Counter to this modest strength decrease with increasing height, in nearly all parts of the brighter flare kernel there appears to be a substantial gradient of B over the heights corresponding to the core of the 854.2 nm line. This variation, if verified, may be related to compression of the chromosphere associated with flare-induced evaporation. Alternatively, it might be a spurious result if the profile of the line core is narrowed in the magnetic part of the flare kernel. The results presented here are generally similar to previous, rather sparse observations. Observations of the type reported here are made daily and could be used to greatly increase knowledge about the chromospheric magnetic field in active solar features. [ABSTRACT FROM AUTHOR]

We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find ten quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three – five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured blob velocity as a proxy for that of the mean flow, we suggest that the velocity of the background slow solar wind near the Sun can vary significantly within a few hours. This provides an observational manifestation of the large velocity variability of the slow solar wind near the Sun. [ABSTRACT FROM AUTHOR]

Photospheric ephemeral regions (EPRs) cover the Sun like a magnetic carpet. From this, we update the Babcock – Leighton solar dynamo. Rather than sunspot fields appearing in the photosphere de novo from eruptions originating in the deep interior, we consider that sunspots form directly in the photosphere by a rapid accumulation of like-sign field from EPRs. This would only occur during special circumstances: locations and times when the temperature structure is highly superadiabatic and contains a large subsurface horizontal magnetic field (only present in the Sun’s lower latitudes). When these conditions are met, superadiabatic percolation occurs, wherein an inflow and downflow of gas scours the surface of EPRs to form active regions. When these conditions are not met, magnetic elements undergo normal percolation, wherein magnetic elements move about the photosphere in Brownian-type motions. Cellular automata (CA) models are developed that allow these processes to be calculated and thereby both small-scale and large-scale models of magnetic motions can be obtained. The small-scale model is compared with active region development and Hinode observations. The large-scale CA model offers a solar dynamo, which suggests that fields from decaying bipolar magnetic regions (BMRs) drift on the photosphere driven by subsurface magnetic forces. These models are related to observations and are shown to support Waldmeier’s findings of an inverse relationship between solar cycle length and cycle size. Evidence for significant amounts of deep magnetic activity could disprove the model presented here, but recent helioseismic observations of “butterfly patterns” at depth are likely just a reflection of surface activity. Their existence seems to support the contention made here that the field and flow separate, allowing cool, relatively field-free downdrafts to descend with little field into the nether worlds of the solar interior. There they heat by compression to form a hot solar-type Santa Ana wind deep below active regions. [ABSTRACT FROM AUTHOR]

The Solar–Stellar Irradiance Comparison Experiment II (SOLSTICE II) is one of four experiments launched aboard the Solar Radiation and Climate Experiment (SORCE) on 25 January, 2003. Its principal science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5% and to determine solar variability with a relative accuracy of 0.5% per year during a 5-year long nominal mission. SOLSTICE II meets these objectives using a pair of identical scanning grating monochromators that can measure both solar and stellar irradiance. Instrument radiometric responsivity was calibrated to ∼3% absolute accuracy before launch using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute for Standards and Technology (NIST) in Gaithersburg, MD. During orbital operations, SOLSTICE II has been making daily measurements of both the Sun and an ensemble of bright, stable, main-sequence B and A stars. The stellar measurements allow the tracking of changes in instrument responsivity with a relative accuracy of 0.5% per year over the life of the mission. SOLSTICE II is an evolution of the SOLSTICE i instrument that is currently operating on the Upper Atmosphere Research Satellite (UARS). This paper reviews the basic SOLSTICE concept and describes the design, operating modes, and early performance of the SOLSTICE II instrument. [ABSTRACT FROM AUTHOR]

The Sun exhibits a well-observed modulation in the number of spots on its disk over a period of about 11 years. From the dawn of modern observational astronomy, sunspots have presented a challenge to understanding—their quasi-periodic variation in number, first noted 175 years ago, has stimulated community-wide interest to this day. A large number of techniques are able to explain the temporal landmarks, (geometric) shape, and amplitude of sunspot "cycles," however, forecasting these features accurately in advance remains elusive. Recent observationally-motivated studies have illustrated a relationship between the Sun's 22-year (Hale) magnetic cycle and the production of the sunspot cycle landmarks and patterns, but not the amplitude of the sunspot cycle. Using (discrete) Hilbert transforms on more than 270 years of (monthly) sunspot numbers we robustly identify the so-called "termination" events that mark the end of the previous 11-yr sunspot cycle, the enhancement/acceleration of the present cycle, and the end of 22-yr magnetic activity cycles. Using these we extract a relationship between the temporal spacing of terminators and the magnitude of sunspot cycles. Given this relationship and our prediction of a terminator event in 2020, we deduce that sunspot Solar Cycle 25 could have a magnitude that rivals the top few since records began. This outcome would be in stark contrast to the community consensus estimate of sunspot Solar Cycle 25 magnitude. [ABSTRACT FROM AUTHOR]

Since the Lyman- $\alpha$ rocket observations of Gabriel ( Solar Phys. 21, 392, 1971), it has been realized that the hydrogen (H) lines could be observed in the corona and that they offer an interesting diagnostic for the temperature, density, and radial velocity of the coronal plasma. Moreover, various space missions have been proposed to measure the coronal magnetic and velocity fields through polarimetry in H lines. A necessary condition for such measurements is to benefit from a sufficient signal-to-noise ratio. The aim of this article is to evaluate the emission in three representative lines of H for three different coronal structures. The computations have been performed with a full non-local thermodynamic-equilibrium (non-LTE) code and its simplified version without radiative transfer. Since all collisional and radiative quantities (including incident ionizing and exciting radiation) are taken into account, the ionization is treated exactly. Profiles are presented at two heights (1.05 and 1.9 solar radii, from Sun center) in the corona, and the integrated intensities are computed at heights up to five solar radii. We compare our results with previous computations and observations ( e.g. L $\alpha$ from Ultraviolet Coronal Spectrometer) and find a rough (model-dependent) agreement. Since the H $\alpha$ line is a possible candidate for ground-based polarimetry, we show that in order to detect its emission in various coronal structures, it is necessary to use a very narrow (less than 2 Å wide) bandpass filter. [ABSTRACT FROM AUTHOR]

Recent observations of magnetohydrodynamic oscillations and waves in solar active regions revealed their close link to quasi-periodic pulsations in flaring light curves. The nature of that link has not yet been understood in detail. In our analytical modelling we investigate propagation of slow magnetoacoustic waves in a solar active region, taking into account wave refraction and transmission of the slow magnetoacoustic mode into the fast one. The wave propagation is analysed in the geometrical acoustics approximation. Special attention is paid to the penetration of waves in the vicinity of a magnetic null point. The modelling has shown that the interaction of slow magnetoacoustic waves with the magnetic reconnection site is possible due to the mode transmission at the equipartition level where the sound speed is equal to the Alfvén speed. The efficiency of the transmission is also calculated. [ABSTRACT FROM AUTHOR]

Sunspot numbers are important tracers of historical solar activity. They are important in predicting the oncoming solar maximum, in the design of lifetimes of space assets, and in assessing the extent of solar-radiation impact on the space environment. Historically, sunspot numbers have been obtained visually from sunspot drawings. The availability of digital images from the US Air Force Improved Solar Optical Observing Network (ISOON) prototype telescope concurrent to observer-dependent sunspot numbers recorded at the National Solar Observatory (NSO) has provided a basis for comparing sunspot numbers determined from the two methods. We compare sunspot numbers from visual and digital methods observed nearly simultaneously. The advantages of digital imagery are illustrated. [ABSTRACT FROM AUTHOR]

Based on various diagnostics and corrections established in the framework of several Sunspot Number Workshops and described by Clette et al. ( Space Sci. Rev. 186, 35, 2014), we now assembled all separately derived corrections to produce a new standard version of the reference sunspot-number time series. We explain here the three main corrections and the criteria used to choose a final optimal version of each correction factor or function, given the available information and published analyses. We then discuss the differences between the new corrected series and the original sunspot number, including the disappearance of any significant rising secular trend in the solar-cycle amplitudes after this recalibration. We also introduce the new version management scheme now implemented at the World Data Center Sunspot Index and Long-term Solar Observations (WDC-SILSO), which reflects a major conceptual transition: beyond the rescaled numbers, this first revision of the sunspot number also transforms the former static data archive into a living observational series open to future improvements. [ABSTRACT FROM AUTHOR]

Specola Solare Ticinese is an observatory dedicated to sunspot number counting. It was constructed in 1957 in Locarno, Southern Switzerland, as an external observing station of the Zurich Federal Observatory. When the responsibility of the determination of the International Sunspot Number was assumed by the Royal Observatory of Belgium in 1981, Specola Solare Ticinese was given the role of pilot station, with the aim of preserving the continuity in the counting method. We report the observing procedure and counting rules applied in Locarno. [ABSTRACT FROM AUTHOR]

The Swiss Federal Observatory, which had been founded in 1863 by Rudolf Wolf, was dissolved in connection with the retirement of Max Waldmeier in 1979. The determination of the Zurich sunpot number, which had been a cornerstone activity of the observatory, was then discontinued by ETH Zurich. A smooth transition of the responsibility for the sunspot number from Zurich to Brussels was achieved in 1980, however, through which it was possible to avoid a discontinuity in this important time series. Here we describe the circumstances that led to the termination in Zurich, how Brussels was chosen for the succession, and how the transfer was accomplished. [ABSTRACT FROM AUTHOR]

We compile and analyze the sunspot observations made by John Flamsteed for the period 1672 - 1703, which corresponds to the second part of the Maunder Minimum. They appear in the correspondence of the famous astronomer. We include in an appendix the original texts of the sunspot records kept by Flamsteed. We compute an estimate of the level of solar activity using these records, and compare the results with the latest reconstructions of solar activity during the Maunder Minimum, obtaining values characteristic of a grand solar minimum. Finally, we discuss a phenomenon observed and described by Stephen Gray in 1705 that has been interpreted as a white-light flare. [ABSTRACT FROM AUTHOR]

On average, the international sunspot number ( $R_{\mathrm{I}}$ ) is 44 % higher than the group sunspot number ( $R_{\mathrm{G}}$ ) from 1885 to the beginning of the $R_{\mathrm{I}}$ series in 1700. This is the principal difference between $R_{\mathrm{I}}$ and $R_{\mathrm{G}}$ . Here we show that this difference is primarily due to an inhomogeneity in the Royal Greenwich Observatory (RGO) record of sunspot groups (1874 - 1976) used to derive observer normalization factors (called $k$ -factors) for $R_{\mathrm{G}}$ . Specifically, annual RGO group counts increase relative to those of Wolfer and other long-term observers from 1876 - 1915. A secondary contributing cause is that the $k$ -factors for observers who began observing before 1884 and overlapped with RGO for any years during 1874 - 1883 were not based on direct comparison with RGO but were calculated using one or more intermediary or additional observers. We introduce $R_{\mathrm{GC}}$ by rectifying the RGO group counts from 1874 - 1915 and basing $k$ -factors on direct comparison with RGO across the 1885 discontinuity, which brings the $R_{\mathrm{G}}$ and $R_{\mathrm{I}}$ series into reasonable agreement for the 1841 - 1885 interval (after correcting $R_{\mathrm{I}}$ for an inhomogeneity from 1849 - 1867 (to give $R_{\mathrm{IC}}$ )). Comparison with an independently derived backbone-based reconstruction of $R_{\mathrm{G}}$ ( $R_{\mathrm{BB}}$ ) indicates that $R_{\mathrm{GC}}$ over-corrects $R_{\mathrm{BB}}$ by 4 % on average from 1841 - 1925. Our analysis suggests that the maxima of Cycles 10 (in 1860), 12 (1883/1884), and 13 (1893) in the $R_{\mathrm{IC}}$ series are too low by ≈ 10 %. [ABSTRACT FROM AUTHOR]

In 1981, the production of the international sunspot number moved from the Zürich Observatory to the Royal Observatory of Belgium, with a new pilot station: the Specola Solare Ticinese Observatory in Locarno, Switzerland. This marked a profound transition in the history of the sunspot number. Those recent decades are particularly important as they provide the link between recent modern solar indices and the entire sunspot-number series extending back to the eighteenth century. However, large variations have recently been identified in the scale of the sunspot number during this recent time period. Here, we refine the determination of those recent inhomogeneities by reconstructing a new average sunspot-number series [ $S_{\mathrm{N}}$ ] from a subset of long-duration stations between 1981 and 2015. We also extend this reconstruction by gathering long time series from 35 stations over 1945 - 2015, thus straddling the critical 1981 transition. In both reconstructions, we also derive a parallel group number series [ $G_{\mathrm{N}}$ ] built by the same method from exactly the same data set. Our results confirm the variable trends associated with drifts of the Locarno pilot station, which start only in 1983. They lead to a fully uniform $S_{\mathrm{N}}$ -series over the entire 1945 - 2015 interval. By comparing the new $S_{\mathrm{N}}$ - and $G_{\mathrm{N}}$ -series, we find that a constant quadratic relation exists between those two indices over Cycles 19 to 23. Comparisons with a few other solar indices additionally validate this and reveal some possible undetected problems in those series. Using this new reference $S_{\mathrm{N}}$ , we find that observing stations are surprisingly grouped among distinct subsets that share similar personal $k$ -scaling coefficients. These various results also open the way to implementing a more advanced method for producing the sunspot number in the future. [ABSTRACT FROM AUTHOR]

Potential sources of inhomogeneity in the sunspot measurements published by the Royal Observatory, Greenwich, during the early interval 1874 - 1885 are examined critically. Particular attention is paid to inhomogeneities that might arise because the sunspot measurements were derived from solar photographs taken at various contributing solar observatories, which used different telescopes, experienced different seeing conditions, and employed different photographic processes. The procedures employed in the Solar Department at the Royal Greenwich Observatory (RGO), Herstmonceux, during the final phase of sunspot observations provide a modern benchmark for interpreting the early sunspot measurements. The different observing telescopes used at the contributing solar observatories during the interval 1874 - 1885 are discussed in detail, using information gleaned from the official RGO publications and other relevant historical documents. Likewise, the different photographic processes employed at the different solar observatories are reviewed carefully. The procedures used by RGO staff to measure the positions and areas of sunspot groups on photographs of the Sun having a nominal radius of either four or eight inches are described. It is argued that the learning curve for the use of the Kew photoheliograph at the Royal Observatory, Greenwich, actually commenced in 1858, not 1874. The RGO daily number of sunspot groups is plotted graphically and analysed statistically. Similarly, the changes of metadata at each solar observatory are shown on the graphical plots and analysed statistically. It is concluded that neither the interleaving of data from the different solar observatories nor the changes in metadata invalidates the RGO count of the number of sunspot groups, which behaves as a quasi-homogeneous time series. Furthermore, it is emphasised that the correct treatment of days without photographs is quite crucial to the correct calculation of Group Sunspot Numbers. [ABSTRACT FROM AUTHOR]

Long-term synoptic observations in the resonance line of Ca ii K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism. Synoptic Ca ii K observations began in late 1904 at the Kodaikanal Observatory in India. In the early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a new program of daily Sun-as-a-star observations in the Ca ii K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca ii K index time series that describes the average properties of the chromospheric emission over several solar cycles. We present such a Ca ii K composite and discuss its correlation with the new entirely revised sunspot number data series. For this preliminary investigation, the scaling factor between pairs of time series was determined assuming a simple linear model for the relationship between the monthly mean values during the duration of overlapping observations. [ABSTRACT FROM AUTHOR]

An M6.5-class flare was observed at N12E56 on the solar surface at 16:06 UT on July 8, 2014. In association with the flare, two neutron detectors located at high mountains, Mt. Sierra Negra in Mexico and Mt. Chacaltaya in Bolivia, recorded two neutron pulses, separated approximately by 30 min. Moreover, enhancements were also observed by the solar neutron detector onboard the International Space Station. We analyzed these data combined with solar images from Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. From these we noticed that the production mechanism of neutrons cannot be explained by a single model; at least one of the enhancements may be explained by an electric field generated by the collision of magnetic loops and the other by the shock acceleration mechanism at the front side of the CME. [ABSTRACT FROM AUTHOR]

The structure of the Sun's corona varies with the solar-cycle phase, from a near spherical symmetry at solar maximum to an axial dipole at solar minimum. It is widely accepted that the large-scale coronal structure is governed by magnetic fields that are most likely generated by dynamo action in the solar interior. In order to understand the variation in coronal structure, we couple a potential-field source-surface model with a cyclic dynamo model. In this coupled model, the magnetic field inside the convection zone is governed by the dynamo equation; these dynamo-generated fields are extended from the photosphere to the corona using a potential-field source-surface model. Assuming axisymmetry, we take linear combinations of associated Legendre polynomials that match the more complex coronal structures. Choosing images of the global corona from the Mauna Loa Solar Observatory at each Carrington rotation over half a cycle (1986 - 1991), we compute the coefficients of the associated Legendre polynomials up to degree eight and compare with observations. We show that at minimum the dipole term dominates, but it fades as the cycle progresses; higher-order multipolar terms begin to dominate. The amplitudes of these terms are not exactly the same for the two limbs, indicating that there is a longitude dependence. While both the 1986 and the 1996 minimum coronas were dipolar, the minimum in 2008 was unusual, since there was a substantial departure from a dipole. We investigate the physical cause of this departure by including a North-South asymmetry in the surface source of the magnetic fields in our flux-transport dynamo model, and find that this asymmetry could be one of the reasons for departure from the dipole in the 2008 minimum. [ABSTRACT FROM AUTHOR]