The Experts below are selected from a list of 21981 Experts worldwide ranked by ideXlab platform
S K Solanki - One of the best experts on this subject based on the ideXlab platform.
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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, Natalie A Krivova, S K Solanki, J Jiang, Y C Unruh, 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, Natalie A Krivova, S K Solanki, J Jiang, Y C Unruh, 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.
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Sunspot areas and tilt angles for solar cycles 7 10
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: Senthamizh V Pavai, Natalie A Krivova, Rainer Arlt, M Dasiespuig, S K SolankiAbstract:Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims at estimating areas of Sunspots observed by Schwabe in 1825-1867 and at calculating the tilt angles of Sunspot groups. 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. 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, place the leading polarity on average 5.85+-0.25 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 online.
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.
A G Kosovichev - One of the best experts on this subject based on the ideXlab platform.
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detection of fast moving waves propagating outward along Sunspots radial direction in the photosphere
The Astrophysical Journal, 2015Co-Authors: Junwei Zhao, Ruizhu Chen, Thomas Hartlep, A G KosovichevAbstract:Helioseismic and magnetohydrodynamic waves are abundant in and above Sunspots. Through cross-correlating oscillation signals in the photosphere observed by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we reconstruct how waves propagate away from virtual wave sources located inside a Sunspot. In addition to the usual helioseismic wave, a fast-moving wave is detected traveling along the Sunspot’s radial direction from the umbra to about 15 Mm beyond the Sunspot boundary. The wave has a frequency range of 2.5–4.0 mHz with a phase velocity of 45.3 km s{sup −1}, substantially faster than the typical speeds of Alfven and magnetoacoustic waves in the photosphere. The observed phenomenon is consistent with a scenario of that a magnetoacoustic wave is excited at approximately 5 Mm beneath the Sunspot. Its wavefront travels to and sweeps across the photosphere with a speed higher than the local magnetoacoustic speed. The fast-moving wave, if truly excited beneath the Sunspot’s surface, will help open a new window for studying the internal structure and dynamics of Sunspots.
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detection of fast moving waves propagating outward along Sunspots radial direction in the photosphere
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: Junwei Zhao, Ruizhu Chen, Thomas Hartlep, A G KosovichevAbstract:Helioseismic and magnetohydrodynamic waves are abundant in and above Sunspots. Through cross-correlating oscillation signals in the photosphere observed by the SDO/HMI, we reconstruct how waves propagate away from virtual wave sources located inside a Sunspot. In addition to the usual helioseismic wave, a fast-moving wave is detected traveling along the Sunspot's radial direction from the umbra to about 15 Mm beyond the Sunspot boundary. The wave has a frequency range of 2.5 - 4.0 mHz with a phase velocity of 45.3 km/s, substantially faster than the typical speeds of Alfven and magnetoacoustic waves in the photosphere. The observed phenomenon is consistent with a scenario of that a magnetoacoustic wave is excited at approximately 5 Mm beneath the Sunspot, and its wavefront travels to and sweeps across the photosphere with a speed higher than the local magnetoacoustic speed. The fast-moving wave, if truly excited beneath the Sunspot's surface, will help open a new window to study the internal structure and dynamics of Sunspots.
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effect of suppressed excitation on the amplitude distribution of 5 minute oscillations in Sunspots
The Astrophysical Journal, 2007Co-Authors: K V Parchevsky, A G KosovichevAbstract:Five minute oscillations on the Sun (acoustic and surface gravity waves) are excited by subsurface turbulent convection. However, in Sunspots the excitation is suppressed because a strong magnetic field inhibits convection. We use three-dimensional simulations to investigate how the suppression of excitation sources affects the distribution of the oscillation power in Sunspot regions. The amplitude of random acoustic sources was reduced in circular-shaped regions to simulate the suppression in Sunspots. The simulation results show that the amplitude of the oscillations can be approximately 2-4 times lower in the Sunspot regions in comparison to the quiet Sun, just because of the suppressed sources. Using SOHO MDI data we measured the amplitude ratio for the same frequency bands outside and inside Sunspots and found that this ratio is approximately 3-4. Hence, the absence of excitation sources inside Sunspots makes a significant contribution (about 50% or higher) to the observed amplitude ratio and must be taken into account in Sunspot seismology.
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effect of suppressed excitation on the amplitude distribution of 5 min oscillations in Sunspots
arXiv: Astrophysics, 2007Co-Authors: K V Parchevsky, A G KosovichevAbstract:Five-minute oscillations on the Sun (acoustic and surface gravity waves) are excited by subsurface turbulent convection. However, in Sunspots the excitation is suppressed because strong magnetic field inhibits convection. We use 3D simulations to investigate how the suppression of excitation sources affects the distribution of the oscillation power in Sunspot regions. The amplitude of random acoustic sources was reduced in circular-shaped regions to simulate the suppression in Sunspots. The simulation results show that the amplitude of the oscillations can be approximately 2-4 times lower in the Sunspot regions in comparison to the quiet Sun, just because of the suppressed sources. Using SOHO/MDI data we measured the amplitude ratio for the same frequency bands outside and inside Sunspots, and found that this ratio is approximately 3-4. Hence, the absence of excitation sources inside Sunspots makes a significant contribution (about 50% or higher) to the observed amplitude ratio and must be taken into account in Sunspot seismology.
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investigation of mass flows beneath a Sunspot by time distance helioseismology
The Astrophysical Journal, 2001Co-Authors: Junwei Zhao, A G Kosovichev, T. L. DuvallAbstract:A time-distance helioseismic technique is employed to analyze a set of high-resolution Dopplergram observations of a large Sunspot by SOHO/MDI on 1998 June 18. A regularized, damped least-squares inversion is applied to the measurements of travel times to infer mass flows around the Sunspot below the solar surface. Powerful converging and downward directed flows are detected at depths of 1.5-5 Mm, which may provide observational evidence for the downdrafts and vortex flows that were suggested by Parker for a cluster model of Sunspots. Strong outflows extending more than 30 Mm are found below the downward and converging flows. It is suggested that the Sunspot might be a relatively shallow phenomenon, with a depth of 5-6 Mm, as defined by its thermal and hydrodynamic properties. A strong mass flow across the Sunspot is found at depths of 9-12 Mm, which may provide more evidence in support of the cluster model, as opposed to the monolithic Sunspot model. We suggest that a new magnetic emergence that was found 5 hr after our analysis period is related to this mass flow.
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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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.
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on a possible explanation of the long term decrease in Sunspot field strength
The Astrophysical Journal, 2012Co-Authors: Yury A Nagovitsyn, Alexei A. Pevtsov, William LivingstonAbstract:Recent studies revealed a controversy in long-term variations in Sunspot field strengths. On one hand, the Sunspot field strengths computed by averaging both large and small Sunspots and pores show a gradual decrease over the declining phase of solar Cycle 23 and the rising phase of Cycle 24. On the other hand, the strongest Sunspot field strengths demonstrate only solar cycle variations with no long-term decline. Here, we investigate the field strength and area properties of Sunspots in an attempt to reconcile the presence of both tendencies in recent Sunspot field strength measurements. First, we analyze the data set from Penn & Livingston, and we show that in addition to the previously reported long-term decline, the data show the solar cycle variation when only Sunspots with the strongest magnetic fields are included. Next, we investigate the variations in the number of Sunspots of different sizes, and we find a negative correlation between the numbers of small and large Sunspots. Finally, we show that during the period of 1998-2011, the number of large Sunspots gradually decreased, while the number of small Sunspots steadily increased. We suggest that this change in the fraction of small and large Sunspots (perhaps, due to changes in the solar dynamo) can explain the gradual decline in average Sunspot field strength as observed by Penn & Livingston.
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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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.
