The Maunder Minimum was a period with significantly reduced solar activity between 1645 and 1715, approximately. The transition between the low solar activity in the Maunder Minimum and the ...subsequent “normal” regime of solar activity was gradual. However, there are discrepancies in the solar activity level from sunspot number indices and solar activity proxies in that period. Among the contemporaneous observers, Johann L. Rost and Sebastian Alischer were two key sunspot observers to understand the solar activity in this transition just after the Maunder Minimum. We have revised all their sunspot records, counting the number of groups and individual sunspots to derive reliable data for the solar activity level for the period 1716–1726. We found significant misinterpretations of the sunspot group counting assigned to these astronomers in the existing group number databases. Our new group sunspot counting significantly reduces the number of groups for Rost and Alischer’s observations compared to entries in existing databases. Furthermore, our sunspot number estimates (obtained from the active day fraction methodology) of the maximum amplitude of Solar Cycles −3 and −4 are significantly lower than the amplitudes according to the official sunspot number, but they are compatible with sunspot number values obtained from solar activity proxies such as radioisotopes. Our result would imply that solar activity after the Maunder Minimum recovered more gradually and with a lower intensity than previously considered.
Abstract
We analyzed state-of-the-art observations of the solar atmosphere to investigate the dependence of the Ca
ii
K brightness of several solar features on spectral bandwidth and spatial ...resolution of the data. In particular, we study data obtained at the Swedish Solar Telescope with the Crisp Imaging Spectropolarimeter and Chromospheric Imaging Spectrometer instruments. The analyzed data, which are characterized by a spectral bandwidth of 0.12 Å and a spatial resolution of 0.″078, were acquired close to the disk center by targeting a quiet-Sun area and an active region. We convolved the original observations with Gaussian kernels to degrade their spectral bandwidth and spatial resolution to the instrumental characteristics of the most prominent series of Ca
ii
K observations available to date. We then studied the effect of data degradation on the observed regions and on parameters derived from Ca
ii
K line measurements that are largely employed as diagnostics of the solar and stellar chromospheres. We find that the effect of degrading the spectral resolution of Ca
ii
K observations and line profiles depends on both the employed bandwidth and observed solar region. Besides, we found that the spatial degradation impacts the data characterized by a broad bandwidth to a larger extent compared to those acquired with a narrow band. However, the appearance of the observed solar regions is only slightly affected by the spatial resolution of data with bandwidths up to 1 Å and in the range 3,10 Å. Finally, we derived relationships that can be used to intercalibrate results from observations taken with different instruments in diverse regions of the solar atmosphere.
Abstract
Angelo Secchi, an Italian Jesuit and prominent scientist of the 19th century, and one of the founders of modern astrophysics, observed the Sun regularly at the Collegio Romano in Rome, ...Italy, for more than 25 yr. Results from his observations are reported in articles published in the scientific journals of the time, as well as in drawings and personal notebooks that are stored in the historical archive of the Istituto Nazionale di Astrofisica Osservatorio Astronomico di Roma. The latter material, which reports solar observations performed from 1853–1878, includes original documents from Secchi and from a few of his close collaborators. The above unique material has recently been digitized for preservation purposes and for allowing the scientific exploitation of data not easily accessible so far. A total of more than 5400 digital images have been produced. Here we present the archival material and the new digital data derived from it. We also present results obtained from our primary analysis of the new digital data. In particular, we produced new measurements of the group number from 1853–1878, which will be available for future recalibration of the group number series.
We report progress on the ongoing recalibration of the Wolf sunspot number (SN) and group-sunspot number (GN) following the release of version 2.0 of SN in 2015. This report constitutes both an ...update of the efforts reported in the 2016 Topical Issue of Solar Physics and a summary of work by the International Space Science Institute (ISSI) International Team formed in 2017 to develop optimal SN and GN reconstruction methods while continuing to expand the historical sunspot-number database. Significant progress has been made on the database side while more work is needed to bring the various proposed SN and (primarily) GN reconstruction methods closer to maturity, after which the new reconstructions (or combinations thereof) can be compared with (a) “benchmark” expectations for any normalization scheme (e.g., a general increase in observer normalization factors going back in time), and (b) independent proxy data series such as F10.7 and the daily range of variations of Earth’s undisturbed magnetic field. New versions of the underlying databases for SN and GN will shortly become available for years through 2022 and we anticipate the release of the next versions of these two time series in 2024.
Context.
Coronal mass ejections (CMEs) are a manifestation of the Sun’s eruptive nature. They can have a great impact on Earth, but also on human activity in space and on the ground. Therefore, ...modelling their evolution as they propagate through interplanetary space is essential.
Aims.
EUropean Heliospheric FORecasting Information Asset (EUHFORIA) is a data-driven, physics-based model, tracing the evolution of CMEs through background solar wind conditions. It employs a spheromak flux rope, which provides it with the advantage of reconstructing the internal magnetic field configuration of CMEs. This is something that is not included in the simpler cone CME model used so far for space weather forecasting. This work aims at assessing the spheromak CME model included in EUHFORIA.
Methods.
We employed the spheromak CME model to reconstruct a well observed CME and compare model output to in situ observations. We focus on an eruption from 6 January 2013 that was encountered by two radially aligned spacecraft, Venus Express and STEREO-A. We first analysed the observed properties of the source of this CME eruption and we extracted the CME properties as it lifted off from the Sun. Using this information, we set up EUHFORIA runs to model the event.
Results.
The model predicts arrival times from half to a full day ahead of the in situ observed ones, but within errors established from similar studies. In the modelling domain, the CME appears to be propagating primarily southward, which is in accordance with white-light images of the CME eruption close to the Sun.
Conclusions.
In order to get the observed magnetic field topology, we aimed at selecting a spheromak rotation angle for which the axis of symmetry of the spheromak is perpendicular to the direction of the polarity inversion line (PIL). The modelled magnetic field profiles, their amplitude, arrival times, and sheath region length are all affected by the choice of radius of the modelled spheromak.
In 1876, Alfred Wolfer started observing the Sun and recording the properties of sunspots alongside Rudolf Wolf. Their observations became the basis for the construction of the sunspot-number series. ...After Wolf’s death in 1893, Wolfer became the primary observer for the sunspot-number series. Even though the observations of Wolf and Wolfer had an overlap of almost 17 years (1876–1893), this shift of primary observer from Wolf to Wolfer seems to have led to inconsistencies in the sunspot-number series, primarily due to inhomogeneities in Wolf’s observations. To address this issue we digitise Mittheilungen (Wolf’s journals) and analyse their tabulated datasets. These journals include the raw sunspot data from various observers that the Zürich Observatory used to compile the sunspot number series (SNV1). These datasets have been used as source data for the construction of the first version of the sunspot number (SNV1) series, but they were not digitally accessible for a recalibration of the sunspot-number series until recently. We have also acquired external datasets from recent archival investigations for contemporaneous sunspot observations. In this study, we use the Mittheilungen dataset to produce a new recalibration of the sunspot-number series covering 1816–1944, using four major observers (Tevel, Schwabe, Weber and Wolfer) as backbones. The availability of the raw data allows us to identify issues in the determination of the scaling factors or
k
-factors, between the records of different observers, but also the use of modern techniques for cross-calibrations. Our reconstruction for the years 1816–1944 is carried out with a novel method inspired by Chatzistergos et al. (
Astron. Astrophys
.
602
, A69,
2017
) allowing us to eliminate inconsistencies that resulted from the application of erroneous
k
-factors.
We study the distribution of the sunspot-group size (area) and its dependence on the level of solar activity. We show that the fraction of small groups is not constant but decreases with the level of ...solar activity so that high solar activity is mainly defined by large groups. We analyze the possible influence of solar activity on the ability of a realistic observer to see and report the daily number of sunspot groups. It is shown that the relation between the number of sunspot groups as seen by different observers with different observational acuity thresholds is strongly nonlinear and cannot be approximated by the traditionally used linear scaling (
k
-factors). The observational acuity threshold
A
th
is considered to quantify the quality of each observer, instead of the traditional relative
k
-factor. A nonlinear
c
-factor based on
A
th
is proposed, which can be used to correct each observer to the reference conditions. The method is tested on a pair of principal solar observers, Wolf and Wolfer, and it is shown that the traditional linear correction, with the constant
k
-factor of 1.66 to scale Wolf to Wolfer, leads to an overestimate of solar activity around solar maxima.
Archives of full-disc solar Ca II K observations covering even short periods of time can offer important data to fill observation gaps and to allow a better transition from historical to modern data. ...Two examples of such archives are those from the Kyoto and Sacramento Peak observatories. As most other historical Ca II K data, they suffer from artefacts that need to be accounted for to derive accurate plage areas. Here we present the results of our analysis of these archives of solar Ca II K images.
Abstract
The emission in the near-ultraviolet Ca
ii
H and K lines is modulated by stellar magnetic activity. Although this emission, quantified via the
S
-index, has been serving as a prime proxy of ...stellar magnetic activity for several decades, many aspects of the complex relation between stellar magnetism and Ca
ii
H and K emission are still unclear. The amount of measured Ca
ii
H and K emission is suspected to be affected not only by the stellar intrinsic properties but also by the inclination angle of the stellar rotation axis. Until now, such an inclination effect on the
S
-index has remained largely unexplored. To fill this gap, we develop a physics-based model to calculate
S
-index, focusing on the Sun. Using the distributions of solar magnetic features derived from observations together with Ca
ii
H and K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of the solar
S
-index over four activity cycles. Further, using the distribution of magnetic features over the visible solar disk obtained from surface flux transport simulations, we obtain
S
-index time series dating back to 1700 and investigate the effect of inclination on
S
-index variability on both the magnetic activity cycle and the rotational timescales. We find that when going from an equatorial to a pole-on view, the amplitude of
S
-index variations decreases weakly on the activity cycle timescale and strongly on the rotational timescale (by about 22% and 81%, respectively, for a cycle of intermediate strength). The absolute value of the
S
-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies.
We analysed state-of-the-art observations of the solar atmosphere to investigate the dependence of the \ca brightness of several solar features on spectral bandwidth and spatial resolution of the ...data. Specifically, we study data obtained at the Swedish Solar Telescope with the CRiSP and CHROMIS instruments. The analyzed data, which are characterized by spectral bandwidth of 0.12 \AA \ and spatial resolution of 0.078\arcsec, were acquired close to disc center by targeting a quiet Sun area and an active region. We convolved the original observations with Gaussian kernels to degrade their spectral bandwidth and spatial resolution to the instrumental characteristics of the most prominent series of \ca observations available to date. We then studied the effect of data degradation on the observed regions and on parameters derived from \ca line measurements that are largely employed as diagnostics of the solar and stellar chromospheres. We find that the effect of degrading the spectral resolution of \ca observations and line profiles depends on both the employed bandwidth and observed solar region. Besides, we found that the spatial degradation impacts the data characterized by a broad bandwidth to a larger extent compared to those acquired with a narrow band. However, the appearance of the observed solar regions is only slightly affected by the spatial resolution of data with bandwidths up to 1 \AA \ and in the range 3,10 \AA. Finally, we derived relationships that can be used to intercalibrate results from observations taken with different instruments in diverse regions of the solar atmosphere.
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