Of a total of 177 cool G, K, and M giants and supergiants, we measured the Mg II h+k line emission of extended chromospheres in high-resolution (LWR) IUE spectra by using the IUE final data archive at STScI, and derived the respective stellar surface fluxes. They represent the chromospheric radiative energy losses presumably related to basal heating by the dissipation of acoustic waves, plus a highly variable contribution due to magnetic activity. Thanks to the large sample size, we find a very well defined lower limit, the basal chromospheric Mg II h+k line flux of cool giant chromospheres, as a function of T_eff. A total of 16 giants were observed several times, over a period of up to 20 years. Their respective minimal Mg II h+k line fluxes confirm the basal flux limit very well because none of their emissions dip beneath the empirically deduced basal flux line representative for the overall sample. Based on a total of 15 to 22 objects with very low Mg II h+k emission, we find as limit: log F_Mg_h+k = 7.33 log T_eff - 21.75 (cgs units; based on B-V). Within its uncertainties, this is almost the same relation as has been found in the past for the geometrically much thinner chromospheres of main sequence stars. Concerning any residual dependence of the basal flux on the surface gravity it can be stated that over a gravity range of more than four orders of magnitude (main-sequence stars to supergiants), the basal flux does not appear to vary by more than a factor of 2. These findings are in good agreement with the predictions by previous hydrodynamic models of acoustic wave propagation and energy dissipation, as well as with earlier empirical determinations. Finally, we discuss the idea that the ample energy flux of the chromospheric acoustic waves in a cool giant may yield, as a by-product, the energy flux required by its cool wind (i.e., non-dust-driven, "Reimers-type" mass-loss).
The structure and energy balance of the solar chromosphere remain poorly known. We have used the imaging spectrometer IBIS at the Dunn Solar Telescope to obtain fast-cadence, multi-wavelength profile sampling of Halpha and Ca II 854.2 nm over a sizable two-dimensional field of view encompassing quiet-Sun network. We provide a first inventory of how the quiet chromosphere appears in these two lines by comparing basic profile measurements in the form of image displays, temporal-average displays, time slices, and pixel-by-pixel correlations. We find that the two lines can be markedly dissimilar i…
We perform magnetohydrodynamic (MHD) simulations to investigate the propagation of Alfven wave in magnetic chromosphere. We use the 1.5-dimensional expanding flux tube geometry setting and transverse perturbation at the bottom to generate the Alfven wave. Compared with previous studies, our expansion is that we include the radiative loss term introduced by Carlsson & Leenaarts (2012). We find that when an observation based transverse wave generator is applied, the spatial distribution of the time-averaged radiative loss profile in our simulation is consistent with that in the classic atmospher…
I first review trends in current solar spectrometry and then concentrate on comparing various spectroscopic diagnostics of the solar chromosphere. Some are actually not at all chromospheric but just photospheric or clapotispheric and do not convey information on chromospheric heating, even though this is often assumed. Balmer Halpha is the principal displayer of the closed-field chromosphere, but it is unclear how chromospheric fibrils gain their large Halpha opacity. The open-field chromosphere seems to harbor most if not all coronal heating and solar wind driving, but is hardly seen in optic…
The emphasis of observational and theoretical flare studies in the last decade or two has been on the flare corona, and attention has shifted substantially away from the flare's chromospheric aspects. However, although the pre-flare energy is stored in the corona, the radiative flare is primarily a chromospheric phenomenon, and its chromospheric emission presents a wealth of diagnostics for the thermal and non-thermal components of the flare. I will here review the chromospheric signatures of flare energy release and the problems thrown up by the application of these diagnostics in the context…
We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate plage from its surrounding regions compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that 1.25 mm wavelength free-free emission in plage regions observed with ALMA/Band6 may not form in the low chromo…
Aims: We demonstrate the universal character of the quiet-Sun chromosphere among inactive stars (solar-type and giants). By assessing the main physical processes, we shed new light on some common observational phenomena. Methods: We discuss measurements of the solar Mt. Wilson S-index, obtained by the Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the established chromospheric basal Ca II K line flux to the Mt. Wilson S-index data of inactive ("flat activity") stars, including giants. Results: During the unusually deep and extended activity minimum of 2009, the Sun…
Stellar Chromospheric Activity Database of Solar-like Stars Based on the LAMOST Low-Resolution Spectroscopic Survey III. Calibrating the Chromospheric Basal Flux and the Connection to Stellar Rotation
Nanxun Innovation Institute, Zhejiang University of Water Resources and Electric Power（浙江水利水电学院南浔创新研究院）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；University of Chinese Academy of Sciences（中国科学院大学）；School of Physics and Optoelectronics Engineering, Anhui University（安徽大学物理与光电工程学院）
Detailed numerical models of chromosphere and corona are required to understand the heating of the solar atmosphere. An accurate treatment of the solar chromosphere is complicated by the effects arising from Non Local Thermodynamic Equilibrium (NLTE) radiative transfer. A small number of strong, highly scattering lines dominate the cooling and heating in the chromosphere. Additionally, the recombination times of ionised hydrogen are longer than the dynamical timescales, requiring a non-equilibrium (NE) treatment of hydrogen ionisation. The MURaM code is extended to include the physical process…
Our knowledge of the heating mechanisms that are at work in the chromosphere of plage regions remains highly unconstrained from observational studies. The purpose of our study is to estimate the chromospheric heating terms from a plage dataset, characterize their spatio-temporal distribution and set constraints on the heating processes that are at work. We make use of NLTE inversions to infer a model of the photosphere and chromosphere of a plage dataset acquired with the Swedish 1-m Solar Telescope. We use this model atmosphere to calculate the chromospheric radiative losses from H i, Ca ii a…
The chromosphere is a complex region that acts as an intermediary between the magnetic flux emergence in the photosphere and the magnetic features seen in the corona. Large eruptions in the chromosphere of flares and filaments are often accompanied by ejections of coronal mass off the sun. Several studies have observed fast-moving progressive trains of compact bright points (called Sequential Chromospheric Brightenings or SCBs) streaming away from chromospheric flares that also produce a coronal mass ejection (CME). In this work, we review studies of SCBs and search for commonalties between th…
Numerical simulations of the solar chromosphere have become increasingly realistic over the past 5 years. However, many observed chromospheric structures and behavior are not reproduced. Current models do not show fibrils in Ca II 8542, and neither reproduce the Ca II 8542 bisector. The emergent H-alpha line core intensity computed from the models show granulation instead of chromospheric shocks or fibrils. I discuss these deficiencies and speculate about what physics should be included to alleviate these shortcomings.
Aims: To study the heating of solar chromospheric magnetic and nonmagnetic regions by acoustic and magnetoacoustic waves, the deposited acoustic-energy flux derived from observations of strong chromospheric lines is compared with the total integrated radiative losses. Methods: A set of 23 quiet-Sun and weak-plage regions were observed in the Mg II k and h lines with the Interface Region Imaging Spectrograph (IRIS). The deposited acoustic-energy flux was derived from Doppler velocities observed at two different geometrical heights corresponding to the middle and upper chromosphere. A set of sca…
We use radiation hydrodynamic simulations to examine two models of solar flare chromospheric heating: Alfvén wave dissipation and electron beam collisional losses. Both mechanisms are capable of strong chromospheric heating, and we show that the distinctive atmospheric evolution in the mid-to-upper chromosphere results in Mg II k-line emission that should be observably different between wave-heated and beam-heated simulations. We also present Ca II 8542A profiles which are formed slightly deeper in the chromosphere. The Mg II k-line profiles from our wave-heated simulation are quite different …
Recent observational and numerical studies show a variety of thermal structures in the solar chromosphere. Given that the thermal interplay across the transition region is a key to coronal heating, it is worth investigating how different thermal structures of the chromosphere yield different coronal properties. In this work, by MHD simulations of Alfvén-wave heating of coronal loops, we study how the coronal properties are affected by the chromospheric temperature. To this end, instead of solving the radiative transfer equation, we employ a simple radiative loss function so that the chromosphe…
Context. Magnetic reconnection during flares takes place in the corona but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 Å and He I 10830 Å. Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D3 line. Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyze the data using the inversion code HAZEL, and estimate the LOS velocity and t…
Oscillations in sunspot umbrae exhibit remarkable differences between the photosphere and chromosphere. We evaluate two competing scenarios proposed for explaining those observations: a chromospheric resonant cavity and waves traveling from the photosphere to upper atmospheric layers. We have employed numerical simulations to analyze the oscillations in both models. They have been compared with observations in the low (Na I D2) and high (He I 10830 Å) chromosphere. The nodes of the resonant cavity can be detected as phase jumps or power dips, although the identification of the latter is not su…
Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photo- sphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca II 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain depth-depen…
The million degree plasma of the solar corona must be supplied by the underlying layers of the atmosphere. The mechanism and location of energy release, and the precise source of coronal plasma, remain unresolved. In earlier work we pursued the idea that warm plasma is supplied to the corona via direct heating of the chromosphere by nanoflares, contrary to the prevailing belief that the corona is heated in-situ and the chromosphere is subsequently energized and ablated by thermal conduction. We found that single (low-frequency) chromospheric nanoflares could not explain the observed intensitie…
Vortex structures in the Sun's chromosphere are believed to channel energy between different layers of the solar atmosphere. We investigate the nature and dynamics of two small-scale quiet-Sun rotating structures in the chromosphere. We analyse two chromospheric structures that show clear rotational patterns in spectropolarimetric observations taken with the Interferometric Bidimensional Spectrometer (IBIS) at the Ca II 8542 Å~ line. We present the detection of spectropolarimetric signals that manifest the magnetic nature of rotating structures in the chromosphere. Our observations show two lo…
Using mainly the 1600 angstrom continuum channel, and also the 1216 angstrom Lyman-alpha channel (which includes some UV continuum and C IV emission), aboard the TRACE satellite, we observed the complete lifetime of a transient, bright chromospheric loop. Simultaneous observations with the SUMER instrument aboard the SOHO spacecraft revealed interesting material velocities through the Doppler effect existing above the chromospheric loop imaged with TRACE, possibly corresponding to extended non-visible loops, or the base of an X-ray jet.
Connectivity between the solar photosphere and chromosphere in a vortical structure. Observations of multi-phase, small-scale magnetic field amplification
Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare…
We examine the thermal free-free millimetre fluxes expected from non-dusty and non-pulsating K through mid-M giant stars based on our limited understanding of their inhomogeneous chromospheres. We present a semi-analytic model that provides estimates of the radio fluxes for the mm wavelengths (e.g., CARMA, ALMA, JVLA Q-band) based on knowledge of the effective temperatures, angular diameters and chromospheric Mg II h & k emission fluxes. At 250 GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and sub-mm range will hav…
The importance of the chromosphere in the mass and energy transport within the solar atmosphere is now widely recognised. This review discusses the physics of magnetohydrodynamic (MHD) waves and instabilities in large-scale chromospheric structures as well as in magnetic flux tubes. We highlight a number of key observational aspects that have helped our understanding of the role of the solar chromosphere in various dynamic processes and wave phenomena, and the heating scenario of the solar chromosphere is also discussed. The review focuses on the physics of waves and invokes the basics of plas…
We report on a detailed radiative transfer modeling of the observed scattering polarization in the H-alpha line, which allows us to infer quantitative information on the magnetization of the quiet solar chromosphere. Our analysis suggests the presence of a magnetic complexity zone with a mean field strength <B> > 30 G lying just below the sudden transition region to the coronal temperatures. The chromospheric plasma directly underneath is very weakly magnetized, with <B> ~ 1 G. The possible existence of this abrupt change in the degree of magnetization of the upper chromosphere of the quiet Su…
We propose a mechanism for efficient heating of the solar chromosphere, based on non-ideal plasma effects. Three ingredients are needed for the work of this mechanism: (1) presence of neutral atoms; (2) presence of a non-potential magnetic field; (3) decrease of the collisional coupling of the plasma. Due to decrease of collisional coupling, a net relative motion appears between the neutral and ionized components, usually referred to as "ambipolar diffusion". This results in a significant enhancement of current dissipation as compared to the classical MHD case. We propose that the current diss…
The semi-empirical models of the solar chromosphere are useful in the study of the solar radio emission at millimeter - infrared wavelengths. However, current models do not reproduce the observations of the quiet sun. In this work we present a theoretical study of the radiative transfer equation for four semi- empirical models at these wavelengths. We found that the Chromospheric Solar Milimeter-wave Cavity (CSMC), a region where the atmosphere becomes locally optically thin at millimeter wavelengths, is present in the semi-empirical models under study. We conclude that the CSMC is a general p…
I review some recent advances in methods to diagnose polarized radiation with which we may hope to explore the magnetism of the solar chromosphere and corona. These methods are based on the remarkable signatures that the radiatively induced quantum coherences produce in the emergent spectral line polarization and on the joint action of the Hanle and Zeeman effects. Some applications to spicules, prominences, active region filaments, emerging flux regions and the quiet chromosphere are discussed.
Context. Radiative losses are an indispensable part in the numerical simulation of flares. Detailed calculations could be computationally expensive, especially in the chromosphere. There have been some approximate recipes for chromospheric radiative losses in flares, yet their feasibility in flare simulations needs further evaluation. Aims. We aim to evaluate the performance of different recipes for chromospheric radiative losses in flare simulations. Methods. We compare the atmospheric structure and line profiles in beam-heated flares calculated with detailed radiative losses and the approxim…
We propose a method to derive the line-of-sight magnetic flux density from measurements in the chromospheric Ca II IR line at 854.2 nm. The method combines two well-understood techniques, the center-of-gravity and bisector methods, in a single hybrid technique. The technique is tested with magneto-static simulations of a flux tube. We apply the method to observations with the Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn Solar Telescope of the NSO/SP to investigate the morphology of the lower chromosphere, with focus on the chromospheric counterparts to the underlying…
Observations of the upper chromosphere shows an enormous amount of intricate fine structure. Much of this comes in the form of linear features which are most often assumed to be well aligned with the direction of the magnetic field in the low plasma beta regime thought to dominate the upper chromosphere. We use advanced radiative MHD simulations including the effects of ion-neutral interactions (using the generalized Ohm's law) in the partially ionized chromosphere to show that the magnetic field is often not well aligned with chromospheric features. This occurs where the ambipolar diffusion i…
In this paper, we study the heating of the magnetized solar chromosphere induced by the large fraction of neutral atoms present in this layer. The presence of neutrals, together with the decrease with height of the collisional coupling, leads to deviations from the classical MHD behavior of the chromospheric plasma. A relative net motion appears between the neutral and ionized components, usually referred to as ambipolar diffusion. The dissipation of currents in the chromosphere is enhanced orders of magnitude due to the action of ambipolar diffusion, as compared to the standard ohmic diffusio…
We present multilevel radiative transfer modeling of the scattering polarization observed in the solar O I infrared triplet around 777 nm. We demonstrate that the scattering polarization pattern observed on the solar disk forms in the chromosphere, far above the photospheric region where the bulk of the emergent intensity profiles originates. We study the sensitivity of the polarization pattern to the thermal structure of the solar atmosphere and to the presence of weak magnetic fields (0.01 - 100 G) through the Hanle effect, showing that the scattering polarization signals of the oxygen infra…
A particularly interesting line for exploring the physical conditions of the quiet solar chromosphere is H$α$, but its intensity profile is magnetically insensitive and the small circular polarization signatures produced by the longitudinal Zeeman effect come mainly from the underlying photosphere. Here we show that the Hanle effect in H$α$ provides quantitative information on the magnetism of the quiet chromosphere. To this end, we calculate the response function of the emergent scattering polarization to perturbations in the magnetic field.
The solar chromosphere can be observed well through strong absorption lines. We infer the physical parameters of chromospheric plasmas from these lines using a multilayer spectral inversion. This is a new technique of spectral inversion. We assume that the atmosphere consists of a finite number of layers. In each layer the absorption profile is constant and the source function varies with optical depth with a constant gradient. Specifically, we consider a three-layer model of radiative transfer where the lowest layer is identified with the photosphere and the two upper layers are identified wi…
Context. The origin of the heating of the solar atmosphere is still an unsolved problem. As the photosphere and chromosphere radiate more energy than the solar corona, it is challenging but important to reveal all the mechanisms that contribute to plasma heating there. Ion-neutral collisions could play an important role. Aims. We aim to investigate the impulsively generated two-fluid magnetoacoustic waves in the partially ionized solar chromosphere and to study the associated heating and plasma outflows, which higher up may result in nascent solar wind. Methods. To describe the plasma dynamics…
The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1-m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II 8542 line.…
The fine-structure of magnetic field of a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere. High spatial resolution spectropolarimetric observations were recorded with the 1.5-meter GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric He I triplet at 1083.0 nm and the photospheric Si I 1082.7 nm and Ca I 1083.3 nm spectral lines. The upper chromospheric magnetic field is obtained by inverting the He I triplet assuming a Milne-Eddington type model atmosphere. A height dep…
Aims. To investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere, observations in strong chromospheric lines are analyzed by comparing the deposited acoustic-energy flux with the total integrated radiative losses. Methods. Quiet-Sun and weak-plage regions were observed in the Ca II 854.2 nm and H-alpha lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope (GST) on 2019 October 3 and in the H-alpha and H-beta lines with the echelle spectrograph attached to the Vacuum Tower Telescope (VTT) on 2018 December 11 and 2019 June …
Context. The radiative losses in the solar chromosphere vary from 4~kW~m$^{-2}$ in the quiet Sun, to 20~kW~m$^{-2}$ in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aims. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We use observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm, H$α$, and Fe I 630.1 nm and 630.2 nm lines. We analyse the various line profiles and in add…
We investigate the upper chromosphere and the transition region of the sunspot umbra using the radio brightness temperature at 34 GHz (corresponding to 8.8-mm observations) as observed by the Nobeyama Radioheliograph (NoRH). Radio free-free emission in the longer millimeter range is generated around the transition region, and its brightness temperature yields the region's temperature and density distribution. We use the NoRH data at 34 GHz by applying the Steer-CLEAN image synthesis. These data and the analysis method enable us to investigate the chromospheric structures in the longer millimet…
We use spectropolarimetric observations of the Ca II 8542~Å line, taken from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic activity in a micro-flaring region near a sunspot via inversions. These inversions show localized mean temperature enhancements of $\sim$1000~K in the chromosphere and upper photosphere, along with co-spatial bi-directional Doppler shifting of 5 - 10 km s$^{-1}$. This heating also extends along a nearby chromospheric fibril, co-spatial to 10 - 15 km s$^{-1}$ down-flows. Strong magnetic flux cancellation is also apparent in one of the footpoints, c…
(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type …
The presence of the magnetic field is critical to transport energy through the solar atmosphere. The new generation of telescopes will provide new insight into how the magnetic field arrives into the chromosphere and its role in the energy balance of the solar atmosphere. We have used a 3D radiative MHD numerical model of the solar atmosphere with high spatial resolution (4~km) calculated with the Bifrost code. This code solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The model shows how the lower chromosphere in the…
We have performed 3D radiation MHD simulations extending from the convection zone to the corona covering a box 16 Mm$^3$ at 32 km spatial resolution. The simulations show very fine structure in the chromosphere with acoustic shocks interacting with the magnetic field. Magnetic flux concentrations have a temperature lower than the surroundings in the photosphere but higher in the low chromosphere. The heating is there mostly through ohmic dissipation preferentially at the edges of the flux concentrations. The magnetic field is often wound up around the flux concentrations. When acoustic waves t…
\textbf{Purpose:} This paper addresses long-standing solar physics problems, namely, the heating of the solar chromosphere and the origin of the solar wind. Our aim is to reveal the related mechanisms behind chromospheric heating and plasma outflows in a quiet-Sun. \textbf{Methods:} The approach is based on a two-fluid numerical model that accounts for thermal non-equilibrium (ionization/recombination), non-adiabatic, and non-ideal dynamics of protons+electrons and hydrogen atoms. The model is applied to numerically simulate the propagation and dissipation of granulation-generated waves in the…
We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that consid…
Chromosphere of the quiet sun: II. Atmospheric response to small-scale magnetic flux emergence
University of Oslo（奥斯陆大学）；KU Leuven（荷语鲁汶大学）；Sorbonne Université（索邦大学）；Observatoire de Paris – PSL（巴黎天文台-PSL）；École Polytechnique（巴黎综合理工学院）；Institut Polytechnique de Paris（巴黎理工学院）；CNRS（法国国家科学研究中心）
Chromospheric evaporation refers to dynamic mass motions in flare loops as a result of rapid energy deposition in the chromosphere. These have been observed as blueshifts in X-ray and extreme-ultraviolet (EUV) spectral lines corresponding to upward motions at a few tens to a few hundreds of km/s. Past spectroscopic observations have also revealed a dominant stationary component, in addition to the blueshifted component, in emission lines formed at high temperatures (~10 MK). This is contradictory to evaporation models predicting predominant blueshifts in hot lines. The recently launched Interf…
Most static 1D atmosphere models in the quiet Sun predict a rise of the gas temperature at chromospheric layers, but numerical simulations only yield an increase in the brightness temperature. We investigate the thermal structure in the solar chromosphere as derived from an LTE inversion of Ca II H spectra in QS and active regions. We investigate the temperature stratifications on differences between magnetic and field-free regions in the QS, and between QS and ARs. We determine the energy content of individual calcium bright grains (BGs). The rms temperature fluctuations are below 100 K in th…
We studied the acceleration mechanisms of chromospheric jets associated with emerging flux using a two dimensional magnetohydrodynamic (MHD) simulation. We found that slow mode shock waves generated by magnetic reconnection in the chromosphere and the photosphere play key roles in the acceleration mechanisms of chromospheric jets. An important parameter is the height of magnetic reconnection. When magnetic reconnection takes place near the photosphere, the reconnection out- flow collides with the region where the plasma beta is much larger than unity. Then the plasma moves along a magnetic fie…
The Unstable Chromosphere: Effects of the Thermal Farley-Buneman Instability Across a Broad Range of Solar Chromospheric Conditions
Boston University Center for Space Physics（波士顿大学空间物理中心）；SETI Institute（SETI研究所）；Lockheed Martin Solar & Astrophysics Laboratory（洛克希德·马丁太阳与天体物理实验室）；Rosseland Centre for Solar Physics, University of Oslo（奥斯陆大学罗斯兰太阳物理中心）；Institute of Theoretical Astrophysics, University of Oslo（奥斯陆大学理论天体物理研究所）
Oscillations in the solar atmosphere have long been observed in quiet conditions, and increasingly also in data taken during solar flares. The chromosphere is known for its 3-minute signals, which are particularly strong over sunspot umbrae. These signals are thought to be driven by photospheric disturbances and their periods determined by the chromosphere's acoustic cut-off frequency. A small number of observations have shown the chromospheric 3-minute signals to be affected by energetic events such as solar flares, however the link between flare activity and these oscillatory signals remains…
We examine spectra of the Ca II H line, obtained under good seeing conditions with the VTT Echelle Spectrograph in June of 2007, and higher resolution data of the Ca II 8542 Angstrom line from Fabry-Perot instruments. The VTT targets were areas near disk center which included quiet Sun and some dispersed plage. The infrared data included quiet Sun and plage associated with small pores. Bright chromospheric network emission patches expand little with wavelength from line wing to line center, i.e. with increasing line opacity and height. We argue that this simple observation has implications for…
Based on the near-IR spectra of 33 late-type stars in the wide metallicity range (mainly dwarfs and partly giants) obtained with IRCS+AO188 of the Subaru Telescope, we confirmed that He I 10830A line is seen in absorption in almost all moderately to extremely metal-poor stars of thick disk and halo population (from [Fe/H]~ -0.5 down to [Fe/H]~ -3.7), the strength of which is almost constant irrespective of the metallicity. This is an evidence that chromospheric activity at a basal level persists even for such old stars, despite that their rotations are considered to be slowed down and incapabl…
