The Experts below are selected from a list of 10098 Experts worldwide ranked by ideXlab platform
Alexei A. Pevtsov - One of the best experts on this subject based on the ideXlab platform.
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the effect of telescope aperture scattered light and human vision on early measurements of sunspot and group numbers
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: N V Karachik, Alexei A. Pevtsov, Yury A NagovitsynAbstract:Early telescopic observations of Sunspots were conducted with instruments of relatively small aperture. These instruments also suffered from a higher level of scattered light, and the human eye served as a "detector". The eye's ability to resolve small details depends on image contrast, and on average the intensity variations smaller than $\approx$ 3\% contrast relative to background are not detected even if they are resolved by the telescope. Here we study the effect of these three parameters (telescope aperture, scattered light, and detection threshold of human vision) on sunspot number, group number, and area of Sunspots. As an "ideal" dataset, we employ white-light (pseudo-continuum) observations from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory, and we model the appearance of Sunspots by degrading the HMI images to corresponding telescope apertures with an added scattered light. We discuss the effects of different parameters on sunspot counts and derive functional dependencies, which could be used to normalize historical observations of sunspot counts to common denominator.
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on the presence of two populations of Sunspots
The Astrophysical Journal, 2016Co-Authors: Yury A Nagovitsyn, Alexei A. PevtsovAbstract:Using historical (1894–1976) and more modern (1977–2014) observations, we investigate statistical properties of distributions of sunspot areas and their long-term variations. We confirm the presence of two populations of Sunspots with smaller and larger areas, and show that sunspot/group lifetime can be used to separate the two populations on small short-lived sunspot groups (SSG) and large long-lived groups (LLG). The area properties of LLG are nearly constant over the entire period of observations, while the SSGs show significant long-term variations. Based on the presence of long-term variations in one component and the absence of those in the other, we suggest that the production of two populations of Sunspots (SSG and LLG) may be affected by different processes.
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two populations of Sunspots and secular variations of their characteristics
Astronomy Letters, 2016Co-Authors: A Tlatov, Alexei A. Pevtsov, Yu. A. Nagovitsyn, A A Osipova, E V Miletskii, Yu E NagovitsynaAbstract:We investigate the magnetic fields and total areas of mid- and low-latitude Sunspots based on observations at the Greenwich and Kislovodsk (sunspot areas) and Mount Wilson, Crimean, Pulkovo, Ural, IMIS, Ussuriysk, IZMIRAN, and Shemakha (magnetic fields) observatories. We show that the coefficients in the linear form of the dependence of the logarithm of the total sunspot area S on its maximum magnetic field H change with time. Two distinct populations of Sunspots are identified using the twodimensional H–log S occurrence histogram: small and large, separated by the boundaries log S = 1.6 (S = 40 MSH) and H = 2050 G. Analysis of the sunspot magnetic flux also reveals the existence of two lognormally distributed populations with the mean boundary between them Φ = 1021 Mx. At the same time, the positions of the flux occurrence maxima for the populations change on a secular time scale: by factors of 4.5 and 1.15 for small and large Sunspots, respectively. We have confirmed that the Sunspots form two physically distinct populations and show that the properties of these populations change noticeably with time. This finding is consistent with the hypothesis about the existence of two magnetic field generation zones on the Sun within the framework of a spatially distributed dynamo.
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bimodal distribution of magnetic fields and areas of Sunspots
Solar Physics, 2014Co-Authors: A Tlatov, Alexei A. PevtsovAbstract:We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux density (B) and area (A). The results confirm an already known logarithmic relationship between the area of Sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux density (\(B_{\rm avg}\)) and sunspot area shows a bimodal distribution: for small Sunspots and pores (A≤20 millionth of solar hemisphere, MSH), \(B_{\rm avg} \approx 800~\mbox{G}\) (gauss), and for large Sunspots (A≥100 MSH), \(B_{\rm avg}\) is about 600 G. For intermediate Sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of Sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large Sunspots.
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bimodal distribution of magnetic fields and areas of Sunspots
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: A Tlatov, Alexei A. PevtsovAbstract:We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The results confirm a previously known logarithmic relationship between the area of Sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux (Bavg) and sunspot area shows a bimodal distribution: for small Sunspots and pores (A 100 MSH), Bavg is about 600 G. For intermediate Sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of Sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large Sunspots.
Dipankar Banerjee - One of the best experts on this subject based on the ideXlab platform.
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association of plages with Sunspots a multi wavelength study using kodaikanal ca ii k and greenwich sunspot area data
The Astrophysical Journal, 2017Co-Authors: Sudip Mandal, Subhamoy Chatterjee, Dipankar BanerjeeAbstract:Plages are the magnetically active chromospheric structures prominently visible in the Ca ii K line (3933.67 A). A plage may or may not be associated with a sunspot, which is a magnetic structure visible in the solar photosphere. In this study we explore this aspect of association of plages with Sunspots using the newly digitized Kodaikanal Ca ii K plage data and the Greenwich sunspot area data. Instead of using the plage index or fractional plage area and its comparison with the sunspot number, we use, to our knowledge for the first time, the individual plage areas and compare them with the sunspot area time series. Our analysis shows that these two structures, formed in two different layers, are highly correlated with each other on a timescale comparable to the solar cycle. The area and the latitudinal distributions of plages are also similar to those of Sunspots. Different area thresholdings on the "butterfly diagram" reveal that plages of area ≥4 arcmin2 are mostly associated with a sunspot in the photosphere. Apart from this, we found that the cyclic properties change when plages of different sizes are considered separately. These results may help us to better understand the generation and evolution of the magnetic structures in different layers of the solar atmosphere.
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association of plages with Sunspots a multi wavelength study using kodaikanal ca scriptsize textrm ii k and greenwich sunspot area data
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: Sudip Mandal, Subhamoy Chatterjee, Dipankar BanerjeeAbstract:Plages are the magnetically active chromospheric structures prominently visible in Ca $\scriptsize{\textrm{II}}$ K line (3933.67 A). A plage may or may not be associated with a sunspot which is a magnetic structure visible in the solar photosphere. In this study we explore this aspect of association of plages with Sunspots using the newly digitized Kodaikanal Ca $\scriptsize{\textrm{II}}$ K plage data and the Greenwich sunspot area data. Instead of using the plage index or fractional plage area and their comparison with the sunspot number, we use, to our knowledge for the first time, the individual plage areas and compared it with the sunspot area time series. Our analysis shows that these two structures formed at two different layers are highly correlated with each other on a time scale comparable to the solar cycle. The area and the latitudinal distributions of plages are also similar to that of the Sunspots. Different area thresholdings on the `Butterfly diagram' reveal that plages with area $\geq$4 arcmin$^2$ are mostly associated with a sunspot in the photosphere. Apart from this, we found that the cyclic properties change when different sized plages are considered separately. These results may help us to better understand the generation and the evolution of the magnetic structures in different layers of the solar atmosphere.
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sunspot sizes and the solar cycle analysis using kodaikanal white light digitized data
The Astrophysical Journal, 2016Co-Authors: Sudip Mandal, Dipankar BanerjeeAbstract:Sizes of the Sunspots vary widely during the progression of a solar cycle. Long-term variation studies of different sunspot sizes are key to better understand the underlying process of sunspot formation and their connection to the solar dynamo. The Kodaikanal white-light digitized archive provides daily sunspot observations for a period of 90 years (1921–2011). Using different size criteria on the detected individual Sunspots, we have generated yearly averaged sunspot area time series for the full Sun as well as for the individual hemispheres. In this Letter, we have used the sunspot area values instead of sunspot numbers used in earlier studies. Analysis of these different time series show that different properties of the sunspot cycles depend on the sunspot sizes. The "odd–even rule" double peaks during the cycle maxima and the long-term periodicities in the area data are found to be present for specific sunspot sizes and are absent or not so prominent in other size ranges. Apart from that, we also find a range of periodicities in the asymmetry index that have a dependency on the sunspot sizes. These statistical differences in the different size ranges may indicate that a complex dynamo action is responsible for the generation and dynamics of Sunspots with different sizes.
Laurent Gizon - One of the best experts on this subject based on the ideXlab platform.
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probing Sunspots with two skip time distance helioseismology
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Thomas L. Duvall, Paul Stuart Cally, Kaori Nagashima, Damien Przybylski, Laurent GizonAbstract:Previous helioseismology of Sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure. We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated. We study waves reflected almost vertically from the underside of a sunspot. Time-distance helioseismology was used to measure travel times for the waves. Ray theory and a detailed sunspot model were used to calculate travel times for comparison. It is shown that these large distance waves are insensitive to the magnetic field in the sunspot. The largest travel time differences for any solar phenomena are observed. With sufficient modeling effort, these should lead to better understanding of sunspot structure.
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probing Sunspots with two skip time distance helioseismology
Astronomy and Astrophysics, 2018Co-Authors: Thomas L. Duvall, Paul Stuart Cally, Kaori Nagashima, Damien Przybylski, Laurent GizonAbstract:Context. Previous helioseismology of Sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure. Aims. We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated. Methods. We study waves reflected almost vertically from the underside of a sunspot. Time–distance helioseismology was used to measure travel times for the waves. Ray theory and a detailed sunspot model were used to calculate travel times for comparison. Results. It is shown that these large distance waves are insensitive to the magnetic field in the sunspot. The largest travel time differences for any solar phenomena are observed. Conclusions. With sufficient modeling effort, these should lead to better understanding of sunspot structure.
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helioseismology of Sunspots how sensitive are travel times to the wilson depression and to the subsurface magnetic field
Astronomy and Astrophysics, 2013Co-Authors: H Schunker, Laurent Gizon, R. H. Cameron, A C BirchAbstract:To assess the ability of helioseismology to probe the subsurface structure and magnetic field of Sunspots, we need to determine how helioseismic travel times depend on perturbations to sunspot models. Here we numerically simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot models. Among the models we considered, a ±50 km change in the height of the Wilson depression and a change in the subsurface magnetic field geometry can both be detected above the observational noise level. We also find that the travel-time shifts due to changes in a sunspot model must be modeled by computing the effects of changing the reference sunspot model, and not by computing the effects of changing the subsurface structure in the quiet-Sun model. For p1 modes, the latter is wrong by a factor of four. In conclusion, numerical modeling of MHD wave propagation is an essential tool for interpreting the effects of Sunspots on seismic waveforms.
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helioseismology of Sunspots how sensitive are travel times to the wilson depression and to the subsurface magnetic field
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: H Schunker, Laurent Gizon, R. H. Cameron, A C BirchAbstract:In order to assess the ability of helioseismology to probe the subsurface structure and magnetic field of Sunspots, we need to determine how helioseismic travel times depend on perturbations to sunspot models. Here we numerically simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot models. Among the models we considered, a ~50 km change in the height of the Wilson depression and a change in the subsurface magnetic field geometry can both be detected above the observational noise level. We also find that the travel-time shifts due to changes in a sunspot model must be modeled by computing the effects of changing the reference sunspot model, and not by computing the effects of changing the subsurface structure in the quiet-Sun model. For p1 modes the latter is wrong by a factor of four. In conclusion, numerical modeling of MHD wave propagation is an essential tool for the interpretation of the effects of Sunspots on seismic waveforms.
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constructing semi empirical sunspot models for helioseismology
Solar Physics, 2011Co-Authors: R. H. Cameron, Laurent Gizon, H Schunker, A PietarilaAbstract:One goal of helioseismology is to determine the subsurface structure of Sunspots. In order to do so, it is important to understand first the near-surface effects of Sunspots on solar waves, which are dominant. Here we construct simplified, cylindrically-symmetric sunspot models that are designed to capture the magnetic and thermodynamics effects coming from about 500 km below the quiet-Sun τ5000=1 level to the lower chromosphere. We use a combination of existing semi-empirical models of sunspot thermodynamic structure (density, temperature, pressure): the umbral model of Maltby et al. (1986, Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989, Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used to derive the sound-speed profile. We smoothly merge the near-surface properties to the quiet-Sun values about 1 Mm below the surface. The umbral and penumbral radii are free parameters. The magnetic field is added to the thermodynamic structure, without requiring magnetostatic equilibrium. The vertical component of the magnetic field is assumed to have a Gaussian horizontal profile, with a maximum surface field strength fixed by surface observations. The full magnetic-field vector is solenoidal and determined by the on-axis vertical field, which, at the surface, is chosen such that the field inclination is 45° at the umbral – penumbral boundary. We construct a particular sunspot model based on SOHO/MDI observations of the sunspot in active region NOAA 9787. The helioseismic signature of the model sunspot is studied using numerical simulations of the propagation of f, p1, and p2 wave packets. These simulations are compared against cross-covariances of the observed wave field. We find that the sunspot model gives a helioseismic signature that is similar to the observations.
S K Solanki - One of the best experts on this subject based on the ideXlab platform.
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The relationship between bipolar magnetic regions and their Sunspots
'EDP Sciences', 2021Co-Authors: K. L. Yeo, Natalie A Krivova, S K Solanki, J JiangAbstract:Context. The relationship between bipolar magnetic regions (BMRs) and their Sunspots is an important property of the solar magnetic field, but it is not well constrained. One consequence is that it is a challenge for surface flux transport models (SFTMs) based on sunspot observations to determine the details of BMR emergence, which they require as input, from such data. Aims. We aimed to establish the relationship between the amount of magnetic flux in newly emerged BMRs and the area of the enclosed Sunspots, and examine the results of its application to an established SFTM. Methods. Earlier attempts to constrain BMR magnetic flux were hindered by the fact that there is no extensive and reliable record of the magnetic and physical properties of newly emerged BMRs currently available. We made use of the empirical model of the relationship between the disc-integrated facular and network magnetic flux and the total surface coverage by Sunspots reported in a recent study. The structure of the model is such that it enabled us to establish, from these disc-integrated quantities, an empirical relationship between the magnetic flux and sunspot area of individual newly emerged BMRs, circumventing the lack of any proper BMR database. Results. Applying the constraint on BMR magnetic flux derived here to an established SFTM retained its key features, in particular its ability to replicate various independent datasets and the correlation between the model output polar field at the end of each cycle and the observed strength of the following cycle. The SFTM output indicates that facular and network magnetic flux rises with increasing sunspot magnetic flux at a slowing rate such that it appears to gradually saturate. This is analogous to what earlier studies comparing disc-integrated quantities sensitive to the amount of faculae and network present to sunspot indices had reported. The activity dependence of the ratio of facular and network flux to sunspot flux is consistent with the findings of recent studies: although the Sun is faculae-dominated (such that its brightness is mostly positively correlated with activity), it is only marginally so as facular and network brightening and sunspot darkening appear to be closely balanced
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how faculae and network relate to Sunspots and the implications for solar and stellar brightness variations
arXiv: Solar and Stellar Astrophysics, 2020Co-Authors: S K Solanki, Natalie A KrivovaAbstract: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.
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reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
Astronomy and Astrophysics, 2016Co-Authors: M Dasiespuig, Y C Unruh, Natalie A Krivova, S K Solanki, J Jiang, K. L. YeoAbstract:Aims. We present a reconstruction of the spectral solar irradiance since 1700 using the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope era version 2) model. This model uses as input magnetograms simulated with a surface flux transport model fed with semi-synthetic records of emerging sunspot groups. Methods. The record of sunspot group areas and positions from the Royal Greenwich Observatory (RGO) is only available since 1874. We used statistical relationships between the properties of sunspot group emergence, such as the latitude, area, and tilt angle, and the sunspot cycle strength and phase to produce semi-synthetic sunspot group records starting in the year 1700. The semi-synthetic records are fed into a surface flux transport model to obtain daily simulated magnetograms that map the distribution of the magnetic flux in active regions (Sunspots and faculae) and their decay products on the solar surface. The magnetic flux emerging in ephemeral regions is accounted for separately based on the concept of extended cycles whose length and amplitude are linked to those of the sunspot cycles through the sunspot number. The magnetic flux in each surface component (Sunspots, faculae and network, and ephemeral regions) was used to compute the spectral and total solar irradiance (TSI) between the years 1700 and 2009. This reconstruction is aimed at timescales of months or longer although the model returns daily values. Results. We found that SATIRE-T2, besides reproducing other relevant observations such as the total magnetic flux, reconstructs the TSI on timescales of months or longer in good agreement with the PMOD composite of observations, as well as with the reconstruction starting in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI of 1.2 +0.2 -0.3 Wm -2 between 1700 and the present. The spectral irradiance reconstruction is in good agreement with the UARS/SUSIM measurements as well as the Lyman- α composite.
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reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: M Dasiespuig, Y C Unruh, Natalie A Krivova, S K Solanki, J Jiang, K. L. YeoAbstract:We present a reconstruction of the spectral solar irradiance since 1700 using the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope era version 2) model. This model uses as input magnetograms simulated with a surface flux transport model fed with semi-synthetic records of emerging sunspot groups. We used statistical relationships between the properties of sunspot group emergence, such as the latitude, area, and tilt angle, and the sunspot cycle strength and phase to produce semi-synthetic sunspot group records starting in the year 1700. The semisynthetic records are fed into a surface flux transport model to obtain daily simulated magnetograms that map the distribution of the magnetic flux in active regions (Sunspots and faculae) and their decay products on the solar surface. The magnetic flux emerging in ephemeral regions is accounted for separately based on the concept of extended cycles whose length and amplitude are linked to those of the sunspot cycles through the sunspot number. The magnetic flux in each surface component (Sunspots, faculae and network, and ephemeral regions) was used to compute the spectral and total solar irradiance between the years 1700 and 2009. This reconstruction is aimed at timescales of months or longer although the model returns daily values. We found that SATIRE-T2, besides reproducing other relevant observations such as the total magnetic flux, reconstructs the total solar irradiance (TSI) on timescales of months or longer in good agreement with the PMOD composite of observations, as well as with the reconstruction starting in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI of 1.2[+0.2, -0.3] Wm-2 between 1700 and the present. The spectral irradiance reconstruction is in good agreement with the UARS/SUSIM measurements as well as the Lyman-alpha composite.
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sunspot areas and tilt angles for solar cycles 7 10
Astronomy and Astrophysics, 2015Co-Authors: Senthamizh V Pavai, Natalie A Krivova, Rainer Arlt, M Dasiespuig, S K SolankiAbstract:Aims. Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims to estimate areas of Sunspots observed by Schwabe in 1825−1867 and to calculate the tilt angles of sunspot groups. Methods. The sunspot sizes in Schwabe’s drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of Sunspots was the same in the 19th century as it was in the 20th century. Results. Umbral areas for about 130 000 Sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, average to 5. ◦ 85 ± 0. 25, with the leading polarity located closer to the equator, in good agreement with tilt angles obtained from 20th century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided at the CDS.
A C Birch - One of the best experts on this subject based on the ideXlab platform.
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helioseismic holography of simulated Sunspots magnetic and thermal contributions to travel times
The Astrophysical Journal, 2016Co-Authors: T Felipe, D C Braun, A D Crouch, A C BirchAbstract:Wave propagation through Sunspots involves conversion between waves of acoustic and magnetic character. In addition, the thermal structure of Sunspots is very different than that of the quiet Sun. As a consequence, the interpretation of local helioseismic measurements of Sunspots has long been a challenge. With the aim of understanding these measurements, we carry out numerical simulations of wave propagation through Sunspots. Helioseismic holography measurements made from the resulting simulated wavefields show qualitative agreement with observations of real Sunspots. We use additional numerical experiments to determine, separately, the influence of the thermal structure of the sunspot and the direct effect of the sunspot magnetic field. We use the ray approximation to show that the travel-time shifts in the thermal (non-magnetic) sunspot model are primarily produced by changes in the wave path due to the Wilson depression rather than variations in the wave speed. This shows that inversions for the subsurface structure of Sunspots must account for local changes in the density. In some ranges of horizontal phase speed and frequency there is agreement (within the noise level in the simulations) between the travel times measured in the full magnetic sunspot model and the thermal model. If this conclusion proves to be robust for a wide range of models, it would suggest a path toward inversions for sunspot structure.
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evaluation of the capability of local helioseismology to discern between monolithic and spaghetti sunspot models
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: T Felipe, A D Crouch, A C BirchAbstract:The helioseismic properties of the wave scattering generated by monolithic and spaghetti Sunspots are analyzed by means of numerical simulations. In these computations, an incident f or p1 mode travels through the sunspot model, which produces absorption and phase shift of the waves. The scattering is studied by inspecting the wavefield, computing travel-time shifts, and performing Fourier-Hankel analysis. The comparison between the results obtained for both sunspot models reveals that the differences in the absorption coefficient can be detected above noise level. The spaghetti model produces an steep increase of the phase shift with the degree of the mode at short wavelengths, while mode-mixing is more efficient for the monolithic model. These results provide a clue for what to look for in solar observations to discern the constitution of Sunspots between the proposed monolithic and spaghetti models.
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helioseismology of Sunspots how sensitive are travel times to the wilson depression and to the subsurface magnetic field
Astronomy and Astrophysics, 2013Co-Authors: H Schunker, Laurent Gizon, R. H. Cameron, A C BirchAbstract:To assess the ability of helioseismology to probe the subsurface structure and magnetic field of Sunspots, we need to determine how helioseismic travel times depend on perturbations to sunspot models. Here we numerically simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot models. Among the models we considered, a ±50 km change in the height of the Wilson depression and a change in the subsurface magnetic field geometry can both be detected above the observational noise level. We also find that the travel-time shifts due to changes in a sunspot model must be modeled by computing the effects of changing the reference sunspot model, and not by computing the effects of changing the subsurface structure in the quiet-Sun model. For p1 modes, the latter is wrong by a factor of four. In conclusion, numerical modeling of MHD wave propagation is an essential tool for interpreting the effects of Sunspots on seismic waveforms.
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helioseismology of Sunspots how sensitive are travel times to the wilson depression and to the subsurface magnetic field
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: H Schunker, Laurent Gizon, R. H. Cameron, A C BirchAbstract:In order to assess the ability of helioseismology to probe the subsurface structure and magnetic field of Sunspots, we need to determine how helioseismic travel times depend on perturbations to sunspot models. Here we numerically simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot models. Among the models we considered, a ~50 km change in the height of the Wilson depression and a change in the subsurface magnetic field geometry can both be detected above the observational noise level. We also find that the travel-time shifts due to changes in a sunspot model must be modeled by computing the effects of changing the reference sunspot model, and not by computing the effects of changing the subsurface structure in the quiet-Sun model. For p1 modes the latter is wrong by a factor of four. In conclusion, numerical modeling of MHD wave propagation is an essential tool for the interpretation of the effects of Sunspots on seismic waveforms.
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Modeling the Subsurface Structure of Sunspots
Solar Physics, 2010Co-Authors: H. Moradi, Laurent Gizon, D C Braun, A C Birch, C. Baldner, R. H. Cameron, T. L. Duvall, D. Haber, S. M. Hanasoge, B. W. HindmanAbstract:While Sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of Sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model Sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. ( 2009a , 2009b ). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.
