- ID:
- ivo://CDS.VizieR/J/A+A/627/A118
- Title:
- Optical activity indicators
- Short Name:
- J/A+A/627/A118
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Understanding stellar activity in solar-type stars is crucial for the physics of stellar atmospheres as well as for ongoing exoplanet programmes. We aim to test how well we understand stellar activity using our own star, the Sun, as a test case. We perform a detailed study of the main optical activity indicators (CaII H & K, Balmer lines, NaI D1, D2, and HeI D3 measured for the Sun using the data provided by the HARPS-N solar-telescope feed at the Telescopio Nazionale Galileo. We make use of periodogram analyses to study solar rotation, and we use the pool variance technique to study the temporal evolution of active regions. The correlations between the different activity indicators as well as the correlations between activity indexes and the derived parameters from the cross-correlation technique are analysed. We also study the temporal evolution of these correlations and their possible relationship with indicators of inhomogeneities in the solar photosphere like sunspot number or radio flux values. The value of the solar rotation period is found in all the activity indicators, with the only exception being H{delta}. The derived values vary from 26.29 days (H{gamma} line) to 31.23 days (HeI). From an analysis of sliding periodograms we find that in most of the activity indicators the spectral power is split into several "bands" of periods around 26 and 30 days, that might be explained by the migration of active regions between the equator and a latitude of ~30{deg}, spot evolution or a combination of both effects. In agreement with previous works a typical lifetime of active regions of ~ ten rotation periods is inferred from the pooled variance diagrams. We find that H{alpha}, H{beta}, H{gamma}, H{epsilon}, and HeI show a significant correlation with the S index. Significant correlations between the contrast, bisector span, and the heliocentric radial velocity with the activity indexes are also found. We show that the full width at half maximum, the bisector, and the disc-integrated magnetic field correlate with the radial velocity variations. The correlation of the S index and H{alpha} changes with time, increasing with larger sun spot numbers and solar irradiance. A similar tendency with the S index - radial velocity correlation is also present in the data. Our results are consistent with a scenario in which higher activity favours the correlation between the S index and the H{alpha} activity indicators and between the S index and radial velocity variations.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/507/417
- Title:
- Oxygen lines in solar granulation. I.
- Short Name:
- J/A+A/507/417
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We seek to provide additional tests of the line formation of theoretical 3D solar photosphere models. In particular, we set out to test the spatially-resolved line formation at several viewing angles, from the solar disk-centre to the limb and focusing on atomic oxygen lines. The purpose of these tests is to provide additional information on whether the 3D model is suitable to derive the solar oxygen abundance. We also aim to empirically constrain the NLTE recipes for neutral hydrogen collisions, using the spatially-resolved observations of the OI 777nm lines.
- ID:
- ivo://CDS.VizieR/J/A+A/508/1403
- Title:
- Oxygen lines in solar granulation. II.
- Short Name:
- J/A+A/508/1403
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- There is a lively debate about the solar oxygen abundance and the role of 3D models in its recent downward revision. These models have been tested using high-resolution solar atlases of flux and disk-centre intensity. Further testing can be done using centre-to-limb variations. Using high-resolution and high S/N observations of neutral oxygen lines across the solar surface, we seek to test that the 3D and 1D models reproduce their observed centre-to-limb variation. In particular we seek to assess whether the latest generation of 3D hydrodynamical solar model atmospheres and NLTE line formation calculations are appropriate to derive the solar oxygen abundance.
- ID:
- ivo://CDS.VizieR/J/A+A/635/A79
- Title:
- Pitch-angle distributions of solar wind
- Short Name:
- J/A+A/635/A79
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Suprathermal electron pitch-angle distributions (PADs) contain substantial information about the magnetic topology of the solar wind. Their characterisation and quantification allow us to automatically identify periods showing certain characteristics. This work presents a robust automatic method for the identification and statistical study of two different types of PADs: bidirectional suprathermal electrons (BDE, often associated with closed magnetic structures) and isotropic (likely corresponding to solar-detached magnetic field lines or highly scattered electrons). Spherical harmonics were fitted to the observed suprathermal PADs of the 119-193eV energy channel of STEREO/SWEA from March 2007 to July 2014, and they were characterised using signal processing analysis in order to identify periods of isotropic and bidirectional PADs. The characterisation has been validated by comparing the results obtained here with those of previous studies. Interplanetary coronal mass ejections (ICMEs) present longer BDE periods inside the magnetic obstacles. A significant amount of BDE remain after the end of the ICME. Isotropic PADs are found in the sheath of the ICMEs, and at the post-ICME region likely due to the erosion of the magnetic field lines. Both isotropy and BDE are solar-cycle dependent. The isotropy observed by STEREO shows a nearly annual periodicity, which requires further investigation. There is also a correspondence between the number of ICMEs observed and the percentage of time showing BDE. A method to characterise PADs has been presented and applied to the automatic identification of two relevant distributions that are commonly observed in the solar wind, such as BDE and isotropy. Four catalogues (STEREO-A and STEREO-B for isotropic and BDE periods of at least 10min) based on this identification are provided for future applications.
- ID:
- ivo://CDS.VizieR/J/A+A/625/A69
- Title:
- Plage area composite series
- Short Name:
- J/A+A/625/A69
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Reconstructions of past irradiance variations require suitable data on solar activity. The longest direct proxy is the sunspot number, and it has been most widely employed for this purpose. These data, however, only provide information on the surface magnetic field emerging in sunspots, while a suitable proxy of the evolution of the bright magnetic features, specifically faculae/plage and network, is missing. This information can potentially be extracted from the historical full-disc observations in the Ca II K line. We use several historical archives of full-disc Ca II K observations to derive plage areas over more than a century. Employment of different datasets allows the identification of systematic effects in the images, such as changes in instruments and procedures, as well as an assessment of the uncertainties in the results. We have analysed over 100000 historical images from eight digitised photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, and one archive of modern observations from the Rome/PSPT. The analysed data cover the period 1893-2018. We first performed careful photometric calibration and compensation for the centre-to-limb variation, and then segmented the images to identify plage regions. This has been consistently applied to both historical and modern observations. The plage series derived from different archives are generally in good agreement with each other. However, there are also clear deviations that most likely hint at intrinsic differences in the data and their digitisation. We showed that accurate image processing significantly reduces errors in the plage area estimates. Accurate photometric calibration also allows precise plage identification on images from different archives without the need to arbitrarily adjust the segmentation parameters. Finally, by comparing the plage area series from the various records, we found the conversion laws between them. This allowed us to produce a preliminary composite of the plage areas obtained from all the datasets studied here. This is a first step towards an accurate assessment of the long-term variation of plage regions.
- ID:
- ivo://CDS.VizieR/J/A+A/639/A88
- Title:
- Plage area composite series
- Short Name:
- J/A+A/639/A88
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. An important source of such information are the archives of full-disc CaII K observations performed more or less regularly at various sites since 1892. We derive the plage area evolution over the last 12 solar cycles employing data from all CaII K archives available publicly in digital form known to us, including several as yet unexplored CaII K archives. We analyse more than 290000 full-disc CaII K observations from 43 datasets spanning the period 1892-2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts plaguing many of the images, including some very specific artefacts such as bright arcs found in Kyoto and Yerkes data. The employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here. We have produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We have empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. "Backbone" series are used to bridge all the series together. We have also shown that the selection of the backbone series has little effect on the final plage area composite. We have quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during activity maxima. We provide the so far most complete time series of plage areas based on corrected and calibrated historical and modern CaII K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.
- ID:
- ivo://CDS.VizieR/J/A+A/491/561
- Title:
- Plasma filling factor of coronal bright points
- Short Name:
- J/A+A/491/561
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We determine the volumetric plasma filling factor of coronal bright points. Rastered spectra of the regions of the quiet Sun were recorded by the Extreme ultraviolet Imaging Spectrometer (EIS) during operations with the Hinode satellite. Calibrated intensities of Fe XII lines were obtained and images of the quiet corona were constructed. From the imaged spectra, the emission measures and geometrical widths of coronal bright points were obtained. Electron densities were determined from density-sensitive line ratios. A comparison of the emission measure and bright point widths with the electron densities yielded the plasma-filling factor.
- ID:
- ivo://CDS.VizieR/J/ApJ/793/L4
- Title:
- Polar network index for the solar cycle studies
- Short Name:
- J/ApJ/793/L4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Sun has a polar magnetic field which oscillates with the 11 yr sunspot cycle. This polar magnetic field is an important component of the dynamo process which operates in the solar convection zone and produces the sunspot cycle. We have direct systematic measurements of the Sun's polar magnetic field only from about the mid-1970s. There are, however, indirect proxies which give us information about this field at earlier times. The Ca-K spectroheliograms taken at the Kodaikanal Solar Observatory during 1904-2007 have now been digitized with 4kx4k CCD and have higher resolution (~0.86 arcsec) than the other available historical data sets. From these Ca-K spectroheliograms, we have developed a completely new proxy (polar network index, hereafter PNI) for the Sun's polar magnetic field. We calculate PNI from the digitized images using an automated algorithm and calibrate our measured PNI against the polar field as measured by the Wilcox Solar Observatory for the period 1976-1990. This calibration allows us to estimate the polar fields for the earlier period up to 1904. The dynamo calculations performed with this proxy as input data reproduce reasonably well the Sun's magnetic behavior for the past century.
- ID:
- ivo://CDS.VizieR/J/MNRAS/456/1542
- Title:
- Predicting coronal mass ejections transit times
- Short Name:
- J/MNRAS/456/1542
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Predicting transit times (TT) of coronal mass ejections (CMEs) from their initial parameters is a very important subject, not only from the scientific perspective, but also because CMEs represent a hazard for human technology. We used a neural network (NN) to analyse TT for 153 events with only two input parameters: initial velocity of the CME, v, and central meridian distance, CMD, of its associated flare. We found that transit time dependence on v is showing a typical drag-like pattern in the solar wind. The results show that the speed at which acceleration by drag changes to deceleration is v~500km/s. TT are also found to be shorter for CMEs associated with flares on the western hemisphere than those originating on the eastern side of the Sun. We attribute this difference to the eastward deflection of CMEs on their path to 1 au. The average error of the NN prediction in comparison to observations is ~12h which is comparable to other studies on the same subject.
- ID:
- ivo://CDS.VizieR/J/A+A/647/A60
- Title:
- Prominence average magnetic fields in cycle XXI
- Short Name:
- J/A+A/647/A60
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The aim of the present paper is to publish 24 synoptic maps of solar filaments, in which 296 prominence average unambiguous magnetic field vectors were determined by Pic-du-Midi observations between 1974 and 1982, which is the ascending phase of cycle XXI. The magnetic field was determined by interpretation of the Hanle effect observed in the HeI D3 line. Previou the prominence field polarity and prominence chirality were applied to solve the fundamental ambiguity. The meas averaged in each prominence for accuracy reasons. The result is twofold. First, alternating field directions can be observed from one neutral line to the next general field alignment is found along a solar North-South field distorted under the differential rotation effect. The about prominence and their magnetic field coordinates are provided as on-line material associated to this paper.
