N. A. Krivova

Max Planck Institute for Solar System Research

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Co-Authors

S. K. Solanki

Max Planck Institute for Solar System Research

284

A. I. Shapiro

Max Planck Institute for Solar System Research

K. L. Yeo

Max Planck Institute for Solar System Research

Theodosios Chatzistergos

Max Planck Institute for Solar System Research

I. Ermolli

Astronomical Observatory of Rome

Y. C. Unruh

Imperial College London

Ilya Usoskin

University of Oulu

W. Schmütz

Physikalisch-Meteorologisches Observatorium Davos

R. H. Cameron

Max Planck Institute for Solar System Research

Veronika Witzke

Max Planck Institute for Solar System Research

Cooperative Institutions

Purple Mountain Observatory

705

Centre National de la Recherche Scientifique

464

Max Planck Society

327

Max Planck Institute for Astronomy

187

Max Planck Institute for Solar System Research

185

National Institute for Astrophysics

172

Université Paris Cité

168

Observatoire de Paris

163

Sorbonne Université

149

Goddard Space Flight Center

140

Author Statistics

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Magnetic origin of the discrepancy between stellar limb-darkening models and observations

Nature Astronomy (2024)

Н. М. Костогрыз A. I. Shapiro Veronika Witzke R. H. Cameron L. Gizon

Abstract Stars appear darker at their limbs than at their disk centres because at the limb we are viewing the higher and cooler layers of stellar photospheres. Yet, limb darkening derived from state-of-the-art stellar atmosphere models systematically fails to reproduce recent transiting exoplanet light curves from the Kepler, TESS and JWST telescopes—stellar brightness obtained from measurements drops less steeply towards the limb than predicted by models. Previous models assumed stellar atmospheres devoid of magnetic fields. Here we use stellar atmosphere models computed with the three-dimensional radiative magnetohydrodynamic code MURaM to show that a small-scale concentration of magnetic fields on the stellar surface affects limb darkening at a level that allows us to explain the observations. Our findings provide a way forward to improve the determination of exoplanet radii and especially the transmission spectroscopy analysis for transiting planets, which relies on a very accurate description of stellar limb darkening from the visible to the infrared. Furthermore, our findings imply that limb darkening allows estimates of the small-scale magnetic field strength on stars with transiting planets.

Limb darkening

Starspot

Stellar physics

10.1038/s41550-024-02252-5

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Citations (3)

Solar Variability in the Mg ii h and k Lines

The Astrophysical Journal (2025)

K. Sowmya Martin Snow A. I. Shapiro N. A. Krivova Theodosios Chatzistergos

Abstract Solar irradiance and its variations in the ultraviolet (UV) control the photochemistry in Earth's atmosphere and influence Earth's climate. The variability in the 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, the 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 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 Spectral And Total Irradiance REconstruction (SATIRE) 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 the Mg ii index time series on climate-relevant timescales of decades and longer. Here we present the time series of the Mg ii index spanning nearly three centuries.

10.3847/1538-4357/ada6af

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Citations (0)

Long-term changes in solar activity and irradiance

Journal of Atmospheric and Solar-Terrestrial Physics (2023)

Theodosios Chatzistergos N. A. Krivova K. L. Yeo

Sunspot

Solar minimum

Solar maximum

Secular Variation

10.1016/j.jastp.2023.106150

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Citations (24)

CHAPTER XI: INDIVIDUAL MEMBERSHIP

Proceedings of the International Astronomical Union (2007)

M. G. Abadi E. Agüero A. V. Ahumada J. A. Ahumada M. V. Alonso

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Content (measure theory)

10.1017/s1743921308024393

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Citations (1)

New reconstruction of the sunspot group numbers since 1739 using direct calibration and “backbone” methods

Astronomy and Astrophysics (2017)

Theodosios Chatzistergos Ilya Usoskin G. A. Kovaltsov N. A. Krivova S. K. Solanki

Group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. It is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity. We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period. We use a daisy chain process with backbone (BB) observers and calibrate all overlapping observers to them. The calibration of each individual observer is performed with a probability distribution function (PDF) matrix, constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs is done in a similar manner. Propagation of errors is modelled with Monte Carlo simulations. The final series extends back to 1739 and includes data from 314 observers. It suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions. The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the Modern grand maximum of solar activity in the second half of the 20th century.

Sunspot

Sunspot number

10.1051/0004-6361/201630045

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Citations (94)

Analysis of Ca II K spectroheliogram time-series for solar irradiance studies

38th COSPAR Scientific Assembly (2010)

I. Ermolli N. A. Krivova S. K. Solanki

Source

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EMPIRE: A robust empirical reconstruction of solar irradiance variability

Journal of Geophysical Research Space Physics (2017)

K. L. Yeo N. A. Krivova S. K. Solanki

We present a new empirical model of total and spectral solar irradiance (TSI and SSI) variability entitled EMPirical Irradiance REconstruction (EMPIRE). As with existing empirical models, TSI and SSI variability is given by the linear combination of solar activity indices. In empirical models, UV SSI variability is usually determined by fitting the rotational variability in activity indices to that in measurements. Such models have to date relied on ordinary least squares regression, which ignores the uncertainty in the activity indices. In an advance from earlier efforts, the uncertainty in the activity indices is accounted for in EMPIRE by the application of an error-in-variables regression scheme, making the resultant UV SSI variability more robust. The result is consistent with observations and unprecedentedly, with that from other modelling approaches, resolving the long-standing controversy between existing empirical models and other types of models. We demonstrate that earlier empirical models, by neglecting the uncertainty in activity indices, underestimate UV SSI variability. The reconstruction of TSI and visible and IR SSI from EMPIRE is also shown to be consistent with observations. The EMPIRE reconstruction is of utility to climate studies as a more robust alternative to earlier empirical reconstructions.

Empirical modelling

Ordinary least squares

Empirical Research

10.1002/2016ja023733

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Citations (54)

Solar total irradiance in cycle 23

Astronomy and Astrophysics (2011)

N. A. Krivova S. K. Solanki W. Schmütz

Context. The most recent minimum of solar activity was deeper and longer than the previous two minima as indicated by different proxies of solar activity. This is also true for the total solar irradiance (TSI) according to the PMOD composite.

Maxima and minima

Solar maximum

Solar minimum

10.1051/0004-6361/201016234

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Citations (15)

Evolution of Large‐Scale Magnetic Fields From Near‐Earth Space During the Last 11 Solar Cycles

Journal of Geophysical Research Space Physics (2019)

Leonie Pick Monika Korte Yannik Thomas N. A. Krivova Chi‐Ju Wu

Abstract We use hourly mean magnetic field measurements from 34 midlatitude geomagnetic observatories between 1900 and 2015 to investigate the long‐term evolution and driving mechanism of the large‐scale external magnetic field at ground. The Hourly Magnetospheric Currents index (HMC) is derived as a refinement of the Annual Magnetospheric Currents index (HMC, Pick & Korte, 2017, https://doi.org/10.1093/gji/ggx367 ). HMC requires an extensive revision of the observatory hourly means. It depends on three third party geomagnetic field models used to eliminate the core, the crustal, and the ionospheric solar‐quiet field contributions. We mitigate the dependency of HMC on the core field model by subtracting only nondipolar components of the model from the data. The separation of the residual (dipolar) signal into internal and external (HMC) parts is the main methodological challenge. Observatory crustal biases are updated with respect to AMC, and the solar‐quiet field estimation is extended to the past based on a reconstruction of solar radio flux (F10.7). We find that HMC has more power at low frequencies (periods ≥ 1 year) than the Dcx index, especially at periods relevant to the solar cycle. Most of the slow variations in HMC can be explained by the open solar magnetic flux. There is a weakly decreasing linear trend in absolute HMC from 1900 to present, which depends sensitively on the data rejection criteria at early years. HMC is well suited for studying long‐term variations of the geomagnetic field.

Space Weather

QUIET

Ionospheric dynamo region

Solar observatory

10.1029/2018ja026185

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Citations (8)

How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations

Astronomy and Astrophysics (2020)

K. L. Yeo S. K. Solanki N. A. Krivova

How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. We extend earlier studies that found the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as 'facular indices', are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model that takes into account how active regions evolve. This model was incorporated into a total solar irradiance (TSI) model. The model presented here replicates most of the observed variability in the facular indices, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition as sunspot darkening starts to rise more rapidly with activity than facular brightening. This bolsters the interpretation of this behavior of Sun-like stars as the transition from a faculae-dominated to a spot-dominated regime.

Sunspot

Starspot

10.1051/0004-6361/202037739

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Citations (14)