arXiv subjects

T. Chatzistergos

Publications and source records attributed to T. Chatzistergos.

9 recordsLinked to original sources

arXiv2026

Reconstruction of annual solar irradiance over the last three millennia

Solar irradiance measurements are limited to the last few decades, requiring reconstructions to assess solar variability on longer timescales and its impact on Earth's climate. We present the first physics-based reconstruction of total solar irradiance (TSI) at annual resolution over the last three millennia. The reconstruction is obtained by extending the SATIRE-T model beyond the telescopic era using recently published, annually resolved sunspot number series derived from cosmogenic isotope records. This yields a continuous, physics-based TSI record extending from the satellite era back over the last three millennia, with annual resolution throughout the pre-telescopic period. Over the full three-millennia interval, the reconstructed TSI exhibits a maximum difference of $1.04_{-0.2}^{+0.14}\,\mathrm{W\,m^{-2}}$, defined as the difference between the maximum and minimum of the 50-yr running mean values.

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arXiv2026

Empirical flare energy limits for the largest historical sunspots

Extreme solar particle events reveal that the Sun can occasionally produce eruptions significantly more energetic than those observed in the modern era. These events are thought to originate from powerful coronal mass ejections, typically associated with large solar flares triggered by magnetic field reconnection in complex active regions. Stellar observations indicate that Sun-like stars can host superflares exceeding 10^34 erg roughly once per century, yet it remains uncertain whether the Sun can reach such flare energies. We empirically estimate the upper limit of solar flare energies using statistical relations between flare - ribbon areas and released energy derived from modern observations. By extrapolating these upper-envelope relations to the largest sunspot group recorded since 1859 - the Great Sunspot of 8 April 1947 - we find that exceptionally large and complex solar active regions could, in principle, produce flares with bolometric energies of a few 10^34 erg.

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arXiv2026

Solar Irradiance Reconstruction over the Telescopic Era Using a Revised Photospheric Magnetic Field Model

The Sun is the primary source of energy for Earth and one of the main external drivers of its climate. Solar irradiance -- the radiative power emitted by the Sun and received at 1-AU -- varies on all observable timescales. It is measured as total solar irradiance (TSI), the spectrally integrated flux, or as spectral solar irradiance (SSI), its wavelength-dependent distribution. However, direct space-based irradiance measurements span only about five decades and are too short to capture long-term trends, making reconstructions crucial for studying solar influence on climate. On climate-relevant timescales, irradiance variations are driven by changes in the solar surface magnetic field, which form the basis of reconstructions guided by physics. Here we present revised reconstructions of TSI and SSI over the past four centuries using the physics-based SATIRE-T (Spectral And Total Irradiance REconstruction, for the Telescopic era) model. SATIRE-T relates irradiance variability to the evolution of the solar surface magnetic field inferred from sunspot number records. In this work, we implement a recently revised description of magnetic field evolution that more realistically links the emergence of small-scale magnetic features to sunspot activity, constrained by modern observations. Using two independent sunspot number series as input, we obtain consistent reconstructions of magnetic flux and solar irradiance. The model reproduces the observed or independently reconstructed total and open magnetic flux, and agrees closely with satellite measurements of TSI and Lyman-$α$ irradiance, with correlation coefficients of 0.81-0.98 for 81-day-smoothed space-based TSI records, 0.69-0.85 for TSI at daily cadence, and 0.92 for daily Lyman-$α$ irradiance. On secular timescales, the reconstructed TSI increases by 0.67-0.75$\,\mathrm{W/m^2}$ between the 50-year means over 1650-1700 and 1967-2017.

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arXiv2025

Solar variability in the Mg II h and k lines

Solar irradiance and its variations in the ultraviolet (UV) control the photochemistry in Earth's atmosphere and influence Earth's climate. The variability of Mg II h and k core-to-wing ratio, also known as the Mg II index, is highly correlated with the solar UV irradiance variability. Because of this, Mg II index is routinely used as a proxy for solar UV irradiance variability, which can help to get insights into the influence of solar UV irradiance variability on Earth's climate. Measurements of the Mg II index, however, have only been carried out since 1978 and do not cover the climate relevant timescales longer than a few decades. Here we present a model to calculate the Mg II index and its variability based on the well-established SATIRE (Spectral And Total Irradiance REconstruction) model. We demonstrate that our model calculations yield an excellent agreement with the observed Mg II index variations, both on the solar activity cycle and on the solar rotation timescales. Using this model, we synthesize Mg II index timeseries on climate relevant timescales of decades and longer. Here we present the timeseries of the Mg II index spanning nearly three centuries.

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arXiv2023

Investigating the effect of solar ambient and data characteristics on Ca II K observations and line profile measurements

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 Å\ 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 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.

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arXiv2023

Re-calibration of the Sunspot Number: Status Report

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 re-construction 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 next versions of these two time series in 2024.

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arXiv2021

Modelling a multi-spacecraft coronal mass ejection encounter with EUHFORIA

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. 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. 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 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. 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. 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.

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arXiv2021

Modeling stellar Ca II H & K emission variations. I. Effect of inclination on the S-index

The emission in the near ultraviolet Ca II H & 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 & K emission are still unclear. The amount of measured Ca II H & 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 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 & K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of 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, both on 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 S-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies.

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arXiv2016

Dependence of the Sunspot-group Size on the Level of Solar Activity and its Influence on the Calibration of Solar Observers

The distribution of the sunspot group size (area) and its dependence on the level of solar activity is studied. It is shown that the fraction of small groups is not constant but decreases with the level of solar activity so that high solar activity is largely defined by big groups. We study 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 non-linear and cannot be approximated by the traditionally used linear scaling ($k-$factors). The observational acuity threshold [$A_{\rm th}$] is considered to quantify the quality of each observer, instead of the traditional relative $k-$factor. A nonlinear $c-$factor based on $A_{\rm th}$ 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.

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