An extensive spectroscopic study on ξBoo A (chromospherically active solar-type star) was conducted based on the spectra obtained in 2008 December though 2010 May, with an aim to detect any spectrum variability and to understand its physical origin. For each spectrum, the atmospheric parameters were spectroscopically determined based on Fe lines, and the equivalent widths (along with the line-broadening parameters) of selected 99 lines were measured. We could detect meaningful small fluctuations in the equivalent widths of medium-strength lines. This variation was found to correlate with the effective temperature (T_eff) consistently with the T-sensitivity of each line, which indicates that the difference in the mean temperature averaged over the disk of inhomogeneous condition is mainly responsible for this variability. It was also found that the macrobroadening widths of medium-strength lines and the equivalent widths dispersion of saturated lines tend to increase with the effective Lande factor, suggesting an influence of magnetic field. Our power spectrum analysis applied to the time-sequence data of V I/Fe II line-strength ratio and T_eff could not confirm the 6.4 d period reported by previous studies. We suspect that surface inhomogeneities of ξBoo A at the time of our observations were not so much simple (such as single star patch) as rather complex (e.g., intricate aggregate of spots and faculae).
The magnetic fields of low-mass stars are observed to be variable on decadal timescales, ranging in behaviour from cyclic to stochastic. The changing strength and geometry of the magnetic field should modify the efficiency of angular momentum loss by stellar winds, but this has not been well quantified. In Finley et al. (2018) we investigated the variability of the Sun, and calculated the time-varying angular momentum loss rate in the solar wind. In this work, we focus on four low-mass stars that have all had their surface magnetic fields mapped for multiple epochs. Using mass loss rates deter…
Chemical abundances in solar-type stars are a much debated topic. Planet-hosting stars are known to be metal-rich, but whether or not this peculiarity applies also to the chemical composition of the outer stellar atmospheres is still to be clarified. More in general, coronal and photospheric abundances in late-type stars appear to be different in many cases, but understanding how chemical stratification effects work in stellar atmospheres requires an observational base larger than currently available. We obtained XMM-Newton high-resolution X-ray spectra of Tau Bootis, a well known nearby star …
Binary stars in which oscillations can be studied in either or both components can provide powerful constraints on our understanding of stellar physics. The bright binary 12 Boötis (12 Boo) is a particularly promising system because the primary is roughly 60 per cent brighter than the secondary despite being only a few per cent more massive. Both stars have substantial surface convection zones and are therefore, presumably, solar-like oscillators. We report here the first detection of solar-like oscillations and ellipsoidal variations in the TESS light curve of 12 Boo. Though the solar-like os…
We have analysed around 170 000 individual photometric WASP measurements of 15 well established lambda Bootis stars to search for variability. The lambda Bootis stars are a small group of late-B to early-F, Pop. I, stars that show moderate to extreme (surface) underabundances (up to a factor 100) of most Fe-peak elements, but solar abundances of lighter elements (C, N, O and S). They are excellent laboratories for the study of fundamental astrophysical processes such as diffusion, meridional circulation, stellar winds, and accretion in the presence of pulsation. From the 15 targets, eight are …
Phase-resolved observations of the solar-type star Ksi Bootis A were obtained using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational modulation and the long-term evolution of various magnetic and activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and H alpha emission.
HR\,8799 is a $λ$ Bootis, $γ$ Doradus star hosting a planetary system and a debris disk with two rings. This makes this system a very interesting target for asteroseismic studies. This work is devoted to the determination of the internal metallicity of this star, linked with its $λ$ Bootis nature (i.e., solar surface abundances of light elements, and subsolar surface abundances of heavy elements), taking advantage of its $γ$ Doradus pulsations. This is the most accurate way to obtain this information, and this is the first time such a study is performed for a planetary-system-host star. We hav…
We report the results of intensive monitoring of the variability in the H$α$ line for two F-type stars, $τ$ Boo and $\upsilon$ And, during the last four years 2019-2022, in order to investigate their stellar magnetic activity. The 4-year H$α$ line intensity data taken with the 1.88-m reflector at Okayama Branch Office, Subaru Telescope, shows the existence of a possible $\sim$ 123-day magnetic activity cycle of $τ$ Boo. The result of the H$α$ variability as another tracer of the magnetic activity on the chromosphere is consistent with previous studies of the Ca II H&K line and suggests that th…
We present new wind models for τ Boötis (τ Boo), a hot-Jupiter-host-star whose observable magnetic cycles makes it a uniquely useful target for our goal of monitoring the temporal variability of stellar winds and their exoplanetary impacts. Using spectropolarimetric observations from May 2009 to January 2015, the most extensive information of this type yet available, to reconstruct the stellar magnetic field, we produce multiple 3D magnetohydrodynamic stellar wind models. Our results show that characteristic changes in the large-scale magnetic field as the star undergoes magnetic cycles produc…
We detect the second known lambda Bootis star (HD 54272) which exhibits gamma Doradus-type pulsations. The star was formerly misidentified as a RR Lyrae variable. The lambda Bootis stars are a small group (only 2 per cent) of late B to early F-type, Population I stars which show moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The photometric data from the Wide Angle Search for Planets (WASP) and All Sky Automated Survey (ASAS) projects were analysed. They have an overlapping time base of 1566 d…
New CCD photometric observations of the eclipsing system AR Boo were obtained from February 2006 to April 2008. The star's photometric properties are derived from detailed studies of the period variability and of all available light curves. We find that over about 56 years the orbital period of the system has varied due to a combination of an upward parabola and a sinusoid rather than in a monotonic fashion. Mass transfer from the less massive primary to the more massive secondary component is likely responsible for at least a significant part of the secular period change. The cyclical variati…
Hot Jupiters have been proposed as a likely population of low frequency radio sources due to electron cyclotron maser emission of similar nature to that detected from the auroral regions of magnetized solar system planets. Such emission will likely be confined to specific ranges of orbital/rotational phase due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio observations of the hot Jupiter Tau Bootis b, consisting of 40 hours carefully scheduled to maximize coverage of the planet's 79.5 hour orbital/rotational period in an effort to detect such rotationally modulated emi…
The lambda Bootis (LB) stars are located at the upper main sequence of the H-R diagram and exhibit a peculiar abundance pattern. The light elements (C, N, O, and S) present solar abundances whereas all other elements are moderately to strongly underabundant. It has not yet been determined whether that abundance pattern is intrinsic, or is restricted to the stellar surface. If we follow the hypothesis that the LB stars are intrinsically metal-weak, then there should be a connection with the intermediate Population II and F-weak objects. Such a possible affinity has not been previously investiga…
ZZ Boo is an F-type detached eclipsing binary system containing two almost-identical stars on a circular orbit with a period of 4.992 d. We analyse light curves from two sectors of observations with the Transiting Exoplanet Survey Satellite (TESS) and two published sets of radial velocities of the component stars to determine their physical properties to high precision. We find masses of 1.558 +/- 0.008 Msun and 1.599 +/- 0.012 Msun, and radii of 2.063 +/- 0.006 Rsun and 2.205 +/- 0.006 Rsun. The similarity in the primary and secondary eclipse depths has led to confusion in the past. The high …
In this paper, we present new spectropolarimetric observations of the planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL), respectively. We detected the magnetic field of the star at three epochs in 2008. It is a weak magnetic field of only a few Gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in June 2007. This is the second such reversal observe…
HR\,8799 is a $λ$ Bootis, $γ$ Doradus star hosting a planetary system and a debris disk with two rings. This makes this system a very interesting target for asteroseismic studies. In particular, this work is devoted to the determination of the internal metallicity of this star, linked with its $λ$ Bootis nature, and its age, taking the advantage of its $γ$ Doradus-type pulsations. To do so we have used the equilibrium code CESAM and the non-adiabatic pulsational code GraCo. We have applied the Frequency Ratio Method and the Time Dependent Convection theory to estimate the mode identification, …
The small group of lambda Bootis stars comprises late B to early F-type stars, with moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The main mechanisms responsible for this phenomenon are atmospheric diffusion, meridional mixing and accretion of material from their surroundings. Especially spectroscopic binary (SB) systems with lambda Bootis type components are very important to investigate the evolutionary status and accretion process in more details. For HD 210111, also delta Scuti type pulsa…
Aims. We observed the $τ$ Boo system with the HARPS-N spectrograph to test a new observational strategy aimed at jointly studying asteroseismology, the planetary orbit, and star-planet magnetic interaction. Methods. We collected high-cadence observations on 11 nearly consecutive nights and for each night averaged the raw FITS files using a dedicated software. In this way we obtained spectra with a high signal-to-noise ratio, used to study the variation of the CaII H&K lines and to have radial velocity values free from stellar oscillations, without losing the oscillations information. We develo…
The variation in the radiative output of the Sun, described in terms of solar irradiance, is important to climatology. A common assumption is that solar irradiance variability is driven by its surface magnetism. Verifying this assumption has, however, been hampered by the fact that models of solar irradiance variability based on solar surface magnetism have to be calibrated to observed variability. Making use of realistic three-dimensional magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art solar magnetograms from the Solar Dynamics Observatory, we present a model of t…
Solar five-minute oscillations have been detected in the power spectra of two six-day time intervals from soft X-ray measurements of the Sun observed as a star using the Extreme Ultraviolet Spectrophotometer (ESP) onboard the Solar Dynamics Observatory (SDO) Extreme Ultraviolet Variability Experiment (EVE). The frequencies of the largest amplitude peaks were found matching within 3.7 microHz the known low-degree (l = 0--3) modes of global acoustic oscillations, and can be explained by a leakage of the global modes into the corona. Due to strong variability of the solar atmosphere between the p…
The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections. We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. Our approach is based on the assumption that variability of the solar…
Knowing the variability of total solar irradiance (TSI) at the top of the atmosphere is crucial for specifying solar influence to climate variability. Satellite measured TSI data are available only for the last 32 years. But there is an opportunity to estimate approximate daily TSI values on the basis of the observed variability of solar activity. A common approach is based using data for Wolf sunspot numbers. Series of daily data from the DAVOS based TSI starting from 1978 have been compared with daily values of Wolf numbers and with the corresponding estimates for daily summary sunspot area.…
The unprecedented precision of broadband stellar photometry achieved with the planet-hunting missions CoRoT and \textit{Kepler} initiated a new era in examining the magnetically-driven brightness variations of hundreds of thousands of stars. Such brightness variations are well studied and understood for the Sun. The plethora of data allows to accurately compare solar and stellar brightness variations. An intriguing question is whether the observed trends in the stellar photometric variability (e.g. the dependence of the variability on the stellar rotation period) can be explained by utilising …
The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is a strong evidence that the SSI variability has an effect on the Earth's atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could led to a stronger response of the Earth's atmosphere as well as other solar system planets' atmospheres to the solar activity cycle. We calcu…
A two-wave dynamo model was recently proposed by Zharkova et al. (2015, Zh15 henceforth), which aims at long-term predictions of solar activity for millennia ahead and backwards. Here we confront the backward model predictions for the last 800 years with known variability of solar activity, using both direct sunspot observations since 1610 and reconstructions based on cosmogenic radionuclide data. We show that the Zh15 model fails to reproduce the well-established features of the solar activity evolution during the last millennium. This means that the predictive part for the future is not reli…
The level of solar magnetic activity, as exemplified by the number of sunspots and by energetic events in the corona, varies on a wide range of time scales. Most prominent is the 11-year solar cycle, which is significantly modulated on longer time scales. Drawing from dynamo theory together with empirical results of past solar activity and of similar phenomena on solar-like stars, we show that the variability of the solar cycle can be essentially understood in terms of a weakly nonlinear limit cycle affected by random noise. In contrast to ad-hoc `toy models' for the solar cycle, this leads to…
We examine Kepler light curve variability on habitable zone transit timescales for a large uniform sample of spectroscopically studied Kepler exoplanet host stars. The stars, taken from Everett et al. (2013) are solar-like in their properties and each harbors at least one exoplanet (or candidate) of radius $\le$2.5\re. The variability timescale examined is typical for habitable zone planets orbiting solar-like stars and we note that the discovery of the smallest exoplanets ($\le$1.2\re) with corresponding transit depths of less than $\sim$0.18 mmag, occur for the brightest, photometrically qui…
The Sun has long been considered a constant star, to the extent that its total irradiance was termed the solar constant. It required radiometers in space to detect the small variations in solar irradiance on timescales of the solar rotation and the solar cycle. A part of the difficulty is that there are no other constant natural daytime sources to which the Sun's brightness can be compared. The discovery of solar irradiance variability rekindled a long-running discussion on how strongly the Sun affects our climate. A non-negligible influence is suggested by correlation studies between solar va…
Utilizing the excellent stability of the Ozone Monitoring Instrument (OMI), we characterize both short-term (solar rotation) and long-term (solar cycle) changes of the solar spectral irradiance (SSI) between 265-500 nm during the on-going Cycle 24. We supplement the OMI data with concurrent observations from the GOME-2 and SORCE instruments and find fair-to-excellent, depending on wavelength, agreement among the observations and predictions of the NRLSSI2 and SATIRE-S models.
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 t…
Context. Solar spectral irradiance (SSI) variability is one of the key inputs to models of the Earth's climate. Understanding solar irradiance fluctuations also helps to place the Sun among other stars in terms of their brightness variability patterns and to set detectability limits for terrestrial exo-planets. Aims. One of the most successful and widely used models of solar irradiance variability is SATIRE-S. It uses spectra of the magnetic features and surrounding quiet Sun computed with the ATLAS9 spectral synthesis code under the assumption of Local Thermodynamic Equilibrium (LTE). SATIRE-…
We analyze the effect of solar features on the variability of the solar irradiance in three different spectral ranges. Our study is based on two solar-cycles' worth of full-disk photometric images from the San Fernando Observatory, obtained with red, blue and Ca II K-line filters. For each image we measure the photometric sum, Sigma, which is the relative contribution of solar features to the disk-integrated intensity of the image. The photometric sums in the red and blue continuum, Sigma_r and Sigma_b, exhibit similar temporal patterns: they are negatively correlated with solar activity, with…
Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behaviour of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR proper…
We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar and Heliospheric Observatory} and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find 10 quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three-five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured b…
We analyze and model total solar irradiance variability on time scales from minutes to months, excluding variations due to p-mode oscillations, using a combination of convective and magnetic components. These include granulation, the magnetic network, faculae and sunspots. Analysis of VIRGO data shows that on periods of a day or longer solar variability depends on magnetic activity, but is nearly independent at shorter periods. We assume that only granulation affects the solar irradiance variability on time scales from minutes to hours. Granulation is described as a large sample of bright cell…
Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability. To understand whether the broader pattern of stellar variations is consistent with the solar paradigm we assess relative contributions of faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness va…
The Sun's net radiative output varies on timescales of minutes to gigayears. Direct measurements of the total solar irradiance (TSI) show changes in the spatially- and spectrally-integrated radiant energy on timescales as short as minutes to as long as a solar cycle. Variations of ~0.01 % over a few minutes are caused by the ever-present superposition of convection and oscillations with very large solar flares on rare occasion causing slightly-larger measureable signals. On timescales of days to weeks, changing photospheric magnetic activity affects solar brightness at the ~0.1 % level. The 11…
The slow solar wind exhibits strong variability on timescales from minutes to days, likely related to magnetic reconnection processes in the extended solar corona. Higginson2017b presented a numerical magnetohydrodynamic simulation which showed interchange magnetic reconnection is ubiquitous and most likely responsible for releasing much of the slow solar wind, in particular along topological features known as the Separatrix-Web (S-Web). Here, we continue our analysis, focusing on two specific aspects of structured slow solar wind variability. The first type is present in the slow solar wind f…
Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of th…
Total solar irradiance and UV spectral solar irradiance have been monitored since 1978 through a succession of space missions. This is accompanied by the development of models aimed at replicating solar irradiance by relating the variability to solar magnetic activity. The NRLSSI and SATIRE-S models provide the most comprehensive reconstructions of total and spectral solar irradiance over the period of satellite observation currently available. There is persistent controversy between the various measurements and models in terms of the wavelength dependence of the variation over the solar cycle…
The Sun provides the energy required to sustain life on Earth and drive our planet's atmospheric circulation. However, establishing a solid physical connection between solar and tropospheric variability has posed a considerable challenge across the spectrum of Earth-system science. The canon of solar variability, the solar fiducial clock, lies almost exclusively with the 400 years of human telescopic observations that demonstrates the waxing and waning number of sunspots, over an 11(ish) year period. Recent research has demonstrated the critical importance of the underlying 22-year magnetic po…
Magnetic activity of the Sun and other stars causes their brightness to vary. We investigate how typical the Sun's variability is compared to other solar-like stars, i.e. those with near-solar effective temperatures and rotation periods. By combining four years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we measure photometric variabilities of 369 solar-like stars. Most of the solar-like stars with well-determined rotation periods show higher variability than the Sun and are therefore considerably more active. These stars appear n…
Dynamo models relying on the Babcock-Leighton mechanism are successful in reproducing most of the solar magnetic field dynamical characteristics. However, considering that such models operate only above a lower magnetic field threshold, they do not provide an appropriate magnetic field regeneration process characterizing a self-sustainable dynamo. In this work we consider the existence of an additional α-effect to the Babcock-Leighton scenario in a mean-field axisymmetric kinematic numerical model. Both poloidal field regeneration mechanisms are treated with two different strength-limiting fac…
HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799's limb-darkened angular diameter to be 0.342 +/- 0.008 mas; this is the smallest interferometrically measured stellar diameter to date, with an error of only 2%. By combining our measurement with the star's parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (719…
Observation of Interplanetary Scintillation (IPS) provides an important and effective way to study the solar wind and the space weather. A series of IPS observations were conducted by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The extraordinary sensitivity and the wide frequency coverage make FAST an ideal platform for IPS studies. In this paper we present some first scientific results from FAST observations of IPS with the L-band receiver. Based on the solar wind velocity fitting values of FAST observations on September 26-28, 2020, we found that the velocity decreases …
The rotational evolution of cool stars is governed by magnetised stellar winds which slow the stellar rotation during their main sequence lifetimes. Magnetic variability is commonly observed in Sun-like stars, and the changing strength and topology of the global field is expected to affect the torque exerted by the stellar wind. We present three different methods for computing the angular momentum loss in the solar wind. Two are based on MHD simulations from Finley & Matt (2018), with one using the open flux measured in the solar wind, and the other using remotely-observed surface magnetograms…
Several observatories around the globe started regular full-disc imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. These observations are continued today at a few sites with either old spectroheliographs or modern telescopes equipped with narrow-band filters. The Ca II K time series are unique in representing long-term variations of the Sun's chromospheric magnetic field. However, meaningful results from their analysis require accurate processing of the available data and robust merging of the information stored in different archives. This paper prov…
Synthesis performed under non Local Thermodynamic Equilibrium (non-LTE) conditions usually overestimate stellar spectra. An approach widely adopted in the literature to reduce the excess of UV radiation consists of artificially increasing the continuum opacity by using multiplicative fudge factors, which are empirically derived to impose the synthetized spectrum to match the observed one. Although the method was initially developed to improve non-LTE synthesis of spectral lines, it has been recently employed to model solar spectral irradiance variability. Such irradiance reconstruction techniq…
Superflares on solar-type stars has been a rapidly developing field ever since the launch of $\it Kepler$. Over the years, there have been several studies investigating the statistics of these explosive events. In this study, we present a statistical analysis of stellar flares on solar-type stars made using photometric data in 2-min cadence from $\it TESS$ of the whole southern hemisphere (sectors 1 - 13). We derive rotational periods for all stars in our sample from rotational modulations present in the lightcurve as a result of large starspot(s) on the surface. We identify 1980 stellar flare…
This topical collection summarizes recent advances in observing and modeling irradiance variations of the Sun and Sun-like stars, emphasizing the links between surface magnetic fields and the resulting solar and stellar variability. In particular, the articles composing this collection summarize recent progress in i) solar-irradiance measurements; ii) modeling of solar- and stellar-irradiance variability; and iii) understanding of the effects of such variability on Earth's climate and exoplanet environments. This topical-collection overview article gives background and more details on these as…
Solar activity has a cyclic nature with the ~11-year Schwabe cycle dominating its variability on the interannual timescale. However, solar cycles are significantly modulated in length, shape and magnitude, from near-spotless grand minima to very active grand maxima. The ~400-year-long direct sunspot-number series is inhomogeneous in quality and too short to study robust parameters of long-term solar variability. The cosmogenic-isotope proxy extends the timescale to twelve millennia and provides crucial observational constraints of the long-term solar dynamo modulation. Here, we present a brief…
It is generally accepted that densities of quiet sun and active region plasma are sufficiently low to justify the optically thin approximation, and it is commonly used in the analysis of line emissions from plasma in the solar corona. However, densities of solar flare loops are substantially higher, compromising the optically thin approximation. This study begins with a radiative transfer model that uses typical solar flare densities and geometries to show that hot coronal emission lines are not generally optically thin. Further, the model demonstrates that the observed line intensity should e…
Toward a live homogeneous database of solar active regions based on SOHO/MDI and SDO/HMI synoptic magnetograms.II.parameters for solar cycle variability
The energy in the solar acoustic spectrum is known to be correlated with flares, but it is not known if the same is true for stellar flares? In order to answer this question, we have analyzed 73 flares in 39 solar-like stars. These flares were identified in the 854 solar-like stars observed by the Kepler spacecraft that have stellar parameters measured with asteroseismology. Though we were not able to identify a statistically significant enhancement of the energy in the high-frequency part of the post-flare acoustic spectra compared to the pre-flare spectra of these stars, we did identify a la…
While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($\langle B_z\rangle=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $\langle B_z\rangle=100$ G. From these setups we compute the dependence of the outgoing …
