64 research outputs found
The potential of Ca II K observations for solar activity and variability studies
Several observatories around the globe started regular full-disc imaging of
the solar atmosphere in the Ca II K line in the early decades of the 20th
century. These observations are continued today at a few sites with either old
spectroheliographs or modern telescopes equipped with narrow-band filters. The
Ca II K time series are unique in representing long-term variations of the
Sun's chromospheric magnetic field. However, meaningful results from their
analysis require accurate processing of the available data and robust merging
of the information stored in different archives. This paper provides an
overview of the historical and modern full-disc Ca II K observations, with
focus on their quality and the main results obtained from their analysis over
the last decade.Comment: 6 pages, 2 figure
Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas
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 have analysed over
100,000 historical images from 8 digitised photographic archives of the
Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland,
and Wendelstein observatories, as well as 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.Comment: 30 pages, 22 figures, accepted in A&
Ca II K spectroheliograms for studies of long-term changes in solar irradiance
We address the importance of historical full disc Ca II K spectroheliograms
for solar activity and irradiance reconstruction studies. We review our work on
processing such data to enable them to be used in irradiance reconstructions.
We also present our preliminary estimates of the plage areas from five of the
longest available historical Ca II K archives.Comment: 5 pages, 3 figure
Recovering the unsigned photospheric magnetic field from Ca II K observations
We reassess the relationship between the photospheric magnetic field strength
and the Ca II K intensity for a variety of surface features as a function of
the position on the disc and the solar activity level. This relationship can be
used to recover the unsigned photospheric magnetic field from images recorded
in the core of Ca II K line. We have analysed 131 pairs of high-quality,
full-disc, near-co-temporal observations from SDO/HMI and Rome/PSPT spanning
half a solar cycle. To analytically describe the observationally-determined
relation, we considered three different functions: a power law with an offset,
a logarithmic function, and a power law function of the logarithm of the
magnetic flux density. We used the obtained relations to reconstruct maps of
the line-of-sight component of the unsigned magnetic field (unsigned
magnetograms) from Ca II K observations, which were then compared to the
original magnetograms. We find that both power-law functions represent the data
well, while the logarithmic function is good only for quiet periods. We see no
significant variation over the solar cycle or over the disc in the derived fit
parameters, independently of the function used. We find that errors in the
independent variable, usually not accounted for, introduce attenuation bias. To
address this, we binned the data with respect to the magnetic field strength
and Ca II K contrast separately and derived the relation for the bisector of
the two binned curves. The reconstructed unsigned magnetograms show good
agreement with the original ones. RMS differences are less than 90 G. The
results were unaffected by the stray-light correction of the SDO/HMI and
Rome/PSPT data. Our results imply that Ca~II~K observations, accurately
processed and calibrated, can be used to reconstruct unsigned magnetograms by
using the relations derived in our study.Comment: 18 pages, 22 figures, accepted in A&
Rome Precision Solar Photometric Telescope: precision solar full-disk photometry during solar cycles 23–25
The Rome Precision Solar Photometric Telescope (Rome/PSPT) is a ground-based telescope engaged in precision solar photometry. It has a 27-year database of full-disk images of the photosphere and chromosphere beginning in 1996 and continuing to 2022. The solar images have been obtained daily, weather permitting, with approximately 2 arcsec/pixel scale in Ca II K line at 393.3 nm, G-band at 430.6 nm, and continuum in the blue and red parts of the spectrum at 409.4 nm and 607.2 nm, respectively. Regular observations were also performed at the green continuum at 535.7 nm for a period of about 18 months. Since the first-light, Rome/PSPT operations have been directed at understanding the source of short-and long-term solar irradiance changes, spanning from 1 min to several months, and from 1 year to a few solar cycles, respectively. However, Rome/PSPT data have also served to study a variety of other topics, including the photometric properties of solar disk features and of the supergranulation manifested by the chromospheric network. Moreover, they have been unique in allowing to connect series of historical and modern full-disk solar observations, especially the Ca II K line data. Here, we provide an overview of the Rome/PSPT telescope and of the solar monitoring carried out with it from its first light to the present, across solar cycles 23–25. We also briefly describe the main results achieved with Rome/PSPT data, and give an overview of new results being derived with the whole time series of observations covering the period 1996–2022
Historical solar Ca II K observations at the Rome and Catania observatories
Here we present the little explored Ca II K archives from the Rome and the
Catania observatories and analyse the digitised images from these archives to
derive plage areas.Comment: 5 pages, 3 figures, to be published in "Nuovo Cimento C" as
proceeding of the Third Meeting of the Italian Solar and Heliospheric
Communit
Understanding the secular variability of solar irradiance: the potential of Ca II K observations
With the increasing concern about climate change, it is important to have accurate information on the individual contributions by the potential driving agents, solar variability being one of them. Long and reliable records of solar irradiance, which describe the solar radiative energy input into the climate system, are essential for assessing the role of the Sun. The short temporal extent (since the 1970s) of direct space-based irradiance measurements leaves reconstructions of the past variability with the help of models as the only avenue. Such models require information on the surface distribution and evolution of solar magnetic regions, dark sunspots and bright faculae, and network regions. However, such data become increasingly scarce and of diminishing quality further back in time. Prior to the period of direct irradiance measurements, reconstructions mainly rely on sunspot observations or other indirect data to describe facular and network regions. The resulting estimates of the long-term change between the Maunder minimum and the present diverge by about an order of magnitude. Having direct information on bright magnetic regions can help resolve these discrepancies. The by far most promising data for this purpose are the full-disc observations of the Sun in the Ca II K line. Despite the wealth of such data all the way back to 1892, their use up to now has been rather limited, owing to a number of intricacies of the data. Here we review the recent efforts to bring Ca II K datasets to their full potential. We briefly discuss the problems plaguing the data and processing methods that have been developed to account for them before switching to a summary of the products derived from them. Focus is given to reconstructions of total and spectral irradiance variations from Ca II K observations. We summarise the available such reconstructions and discuss various aspects requiring further attention in order to allow Ca II K observations to be used to their full potential and thus eventually more accurate irradiance reconstructions back to 1892
Full-disc Ca II K observations -- a window to past solar magnetism
First such observations were made in 1892 and since then various sites around
the world have carried out regular observations, with Kodaikanal, Meudon, Mt
Wilson, and Coimbra being some of the most prominent ones. By now, Ca II K
observations from over 40 different sites allow an almost complete daily
coverage of the last century. Ca II K images provide direct information on
plage and network regions on the Sun and, through their connection to solar
surface magnetic field, offer an excellent opportunity to study solar magnetism
over more than a century. This makes them also extremely important, among
others, for solar irradiance reconstructions and studies of the solar influence
on Earth's climate. However, these data also suffer from numerous issues, which
for a long time have hampered their analysis. Without properly addressing these
issues, Ca II K data cannot be used to their full potential. Here, we first
provide an overview of the currently known Ca II K data archives and sources of
the inhomogeneities in the data, before discussing existing processing
techniques, followed by a recap of the main results derived with such data so
far
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