We investigate the emergence of magnetic flux in the quiet Sun at very small spatial scales, focusing on the magnetic connection between the photosphere and chromosphere. The observational data consist of spectropolarimetric measurements and filtergrams taken with the Hinode satellite and the Dutch Open Telescope. We find that a significant fraction of the magnetic flux present in internetwork regions appears in the form of Omega-shaped loops. The emergence rate is 0.02 loops per hour and arcsec^{-2}, which brings 1.1 x 10^12 Mx s^{-1} arcsec^{-2} of new flux to the solar surface. Initially, the loops are observed as small patches of linear polarization above a granular cell. Shortly afterwards, two footpoints of opposite polarity become visible in circular polarization within or at the edges of the granule and start to move toward the adjacent intergranular space. The orientation of the footpoints does not seem to obey Hale's polarity rules. The loops are continuously buffeted by convective motions, but they always retain a high degree of coherence. Interestingly, 23% of the loops that emerge in the photosphere reach the chromosphere (16 cases out of 69). They are first detected in Fe I 630 nm magnetograms and 5 minutes later in Mg I b 517.3 nm magnetograms. After about 8 minutes, some of them are also observed in Ca II H line-core images, where the footpoints produce small brightness enhancements.
In the previous works the significance of the symmetric and non-symmetric ion-atom absorption processes in far UV and EUV regions within a model of the quiet Sun atmosphere, has been studied. The considered processes were the processes of the photo-dissociation of the molecular ions H$_{2}^{+}$ and H$X^{+}$ and absorption processes in H+H$^{+}$ and H$+X^{+}$ collisions, where $X$ denotes the metal atom. As the continuation of the previous investigation, these processes are considered also within the corresponding sunspot model. In this work the non-symmetric ion-atom absorption processes are c…
The NaD1 line in the solar spectrum is sometimes attributed to the solar chromosphere. We study its formation in quiet-Sun network and internetwork. We first present high-resolution profile-resolved images taken in this line with the imaging spectrometer IBIS at the Dunn Solar Telescope and compare these to simultaneous chromospheric images taken in Ca 8542 and Halpha. We then model NaD1 formation by performing 3D NLTE profile synthesis for a snapshot from a 3D radiation-magnetohydrodynamics simulation. We find that most NaD1 brightness is not chromospheric but samples the magnetic concentrati…
Three kinds of magnetic couplings in the quiet solar atmosphere are highlighted and discussed, all fundamentally connected to the Lorentz force. First the coupling of the convecting and overshooting fluid in the surface layers of the Sun with the magnetic field. Here, the plasma motion provides the dominant force, which shapes the magnetic field and drives the surface dynamo. Progress in the understanding of the horizontal magnetic field is summarized and discussed. Second, the coupling between acoustic waves and the magnetic field, in particular the phenomenon of wave conversion and wave refr…
Sunspots are the most spectacular manifestation of solar magnetism, yet, 99% of the solar surface remains 'quiet' at any time of the solar cycle. The quiet sun is not void of magnetic fields, though; they are organized at smaller spatial scales and evolve relatively fast, which makes them difficult to detect. Thus, although extensive quiet Sun magnetism would be a natural driver to a uniform, steady heating of the outer solar atmosphere, it is not clear what the physical processes involved would be due to lack of observational evidence. We report the topology and dynamics of the magnetic field…
We investigate the frequency of magnetic activities, namely flux emergence, splitting, merging, and cancellation, through an automatic detection in order to understand the generation of the power-law distribution of magnetic flux reported by Parnell et al. (2009). The quiet Sun magnetograms observed in the Na I line by Hinode/ SOT is used in this study. Investigated patches range from 10^{17}Mx to 10^{19}Mx. Emergence and cancellation are much less frequent than merging and splitting. The time scale for splitting is found to be ~33 minutes and is independent of the flux contained in the splitt…
We review (i) observations and numerical simulations of vortical flows in the solar atmosphere and (ii) measurements of the horizontal magnetic field in quiet Sun regions. First, we discuss various manifestations of vortical flows and emphasize the role of magnetic fields in mediating swirling motion created near the solar surface to the higher layers of the photosphere and to the chromosphere. We reexamine existing simulation runs of solar surface magnetoconvection with regard to vortical flows and compare to previously obtained results. Second, we review contradictory results and problems as…
The abundance of Stokes V profiles dominated by one lobe at the locations of emergence of Omega-shaped magnetic loops is evaluated. The emergence events were found in Hinode SOT/SP time-sequences of quiet-Sun regions. Such a study has the aim of confirming a prediction based on the basic geometrical and physical properties of emerging magnetic loops: Stokes V profiles dominated by one lobe are possibly the main polarimetric signature of these structures. In agreement with this prediction, 47 % of the Stokes V profiles analyzed has an unsigned amplitude asymmetry larger than 0.3, while in the q…
Small bipolar magnetic features are observed to appear in the interior of individual granules in the quiet Sun, signaling the emergence of tiny magnetic loops from the solar interior. We study the origin of those features as part of the magnetoconvection process in the top layers of the convection zone. Two quiet-Sun magnetoconvection models, calculated with the radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching from the top layers of the convection zone to the corona, are analyzed. Using 3D visualization as well as a posteriori spectral synthesis of Stokes parameters,…
We investigate the morphology and temporal variability of a quiet Sun network region in different solar layers. The emission in several EUV spectral lines through both raster and slot time series, recorded by EIS/Hinode is studied along with H$α$ observations and high resolution spectropolarimetric observations of the photospheric magnetic field. The photospheric magnetic field is extrapolated up to the corona showing a multitude of large and small scale structures. We show for the first time that the smallest magnetic structures both at the network and the internetwork contribute significantl…
We present observations of the formation process of a small-scale filament on the quiet Sun during 5-6 February 2016 and investigate its formation cause. Initially, a small dipole emerged and its associated arch filament system was found to reconnect with overlying coronal fields accompanied by numerous EUV bright points. When bright points faded out, many elongated dark threads formed bridging the positive magnetic element of dipole and external negative network fields. Interestingly, an anti-clockwise photospheric rotational motion (PRM) set in within the positive endpoint region of newborn …
Spectral lines allow us to probe the thermodynamics of the solar atmosphere, but the shape of a single spectral line may be similar for different thermodynamic solutions. Multiline analyses are therefore crucial, but computationally cumbersome. We investigate correlations between several chromospheric and transition region lines to restrain the thermodynamic solutions of the solar atmosphere during flares. We used machine-learning methods to capture the statistical dependencies between 6 spectral lines sourced from 21 large solar flares observed by NASA's Interface Region Imaging Spectrograph …
We study the evolution of a small-scale emerging flux region (EFR) in the quiet Sun, from its emergence to its decay. We track processes and phenomena across all atmospheric layers, explore their interrelations and compare our findings with recent numerical modelling studies. We used imaging, spectral and spectropolarimetric observations from space-borne and ground-based instruments. The EFR appears next to the chromospheric network and shows all characteristics predicted by numerical simulations. The total magnetic flux of the EFR exhibits distinct evolutionary phases, namely an initial subtl…
Small-scale internetwork (IN) features are thought to be the major source of fresh magnetic flux in the quiet Sun. During its first science flight in 2009, the balloon-borne observatory Sunrise captured images of the magnetic fields in the quiet Sun at a high spatial resolution. Using these data we measure the rate at which the IN features bring magnetic flux to the solar surface. In a previous paper it was found that the lowest magnetic flux in small-scale features detected using the Sunrise observations is 9 x 10^14 Mx. This is nearly an order of magnitude smaller than the smallest fluxes of…
In our studies of the dynamics and energetics of the solar atmosphere, we have detected, in high-quality observations from Hinode SOT/NFI, ubiquitous small-scale upflows which move horizontally with supersonic velocities in the quiet Sun. We present the properties of these fast moving upflows (FMUs) and discuss different interpretations.
Interpreting the Stokes profiles observed in quiet regions of the solar chromosphere is a challenging task. The Stokes Q and U profiles are dominated by the scattering polarisation and the Hanle effect, and these processes can only be correctly quantified if 3D radiative transfer effects are taken into account. Forward-modelling of the intensity and polarisation of spectral lines using a 3D model atmosphere is a suitable approach in order to statistically compare the theoretical and observed line profiles. Our aim is to present novel observations of the Ca 8542 line profiles in a quiet region …
Observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) reveal ubiquitous episodic outflows (jets) with an average speed around 120 km s-1 at temperatures often exceeding a million degree in plume-like structures, rooted in magnetized regions of the quiet solar atmosphere. These outflows are not restricted to the well-known plumes visible in polar coronal holes, but are also present in plume-like structures originating from equatorial coronal holes and quiet-Sun regions. Outflows are also visible in the "interplume" regions throughout the atmospher…
Observational and theoretical evidence suggest that beams of accelerated particles are produced in flaring events of all sizes in the solar atmosphere, from X-class flares to nanoflares. Current models of these types of particles in flaring loops assume an isolated 1D atmosphere. A more realistic environment for modelling accelerated particles can be provided by 3D radiative magnetohydrodynamics codes. Here, we present a simple model for particle acceleration and propagation in the context of a 3D simulation of the quiet solar atmosphere, spanning from the convection zone to the corona. We the…
This paper reviews some recent advances in the development and application of polarized radiation diagnostics to infer the mean magnetization of the quiet solar atmosphere, from the near equilibrium photosphere to the highly non-equilibrium upper chromosphere. In particular, I show that interpretations of the scattering polarization observed in some spectral lines suggest that while the magnetization of the photosphere and upper chromosphere is very significant, the lower chromosphere seems to be weakly magnetized.
The evolution of small magnetic features in quiet regions of the Sun provides a unique window to probing solar magneto-convection. Here we analyze small scale magnetic features in the quiet Sun, using the high resolution, seeing-free observations from the Sunrise balloon borne solar observatory. Our aim is to understand the contribution of different physical processes, such as splitting, merging, emergence and cancellation of magnetic fields to the rearrangement, addition and removal of magnetic flux in the photosphere. We employ a statistical approach for the analysis and the evolution studie…
\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…
Identification of the mechanisms responsible for heating the solar chromosphere and corona remains an outstanding problem, one of great relevance to late-type stars as well. There has been tremendous progress in the past decade, largely driven by new instruments, new observations, and sophisticated modeling efforts. Despite this progress, gaps remain. We briefly discuss the need for radio coverage of the 3D solar atmosphere and discuss the requirements.
The presence of magnetic field is crucial in the transport of energy through the solar atmosphere. Recent ground-based and space-borne observations of the quiet Sun have revealed that magnetic field accumulates at photospheric heights, via a local dynamo or from small-scale flux emergence events. However, most of this small-scale magnetic field may not expand into the chromosphere due to the entropy drop with height at the photosphere. Here we present a study that uses a high resolution 3D radiative MHD simulation of the solar atmosphere with non-grey and non-LTE radiative transfer and thermal…
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）
We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from {\sc Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and force-free extrapolation techniques. We find that most magnetic loops which reach into the chromosphere or higher have one foot point in relatively strong magnetic field regions in the photosphere. $91%$ of the magnetic energy in the mid chromosphere (at a height of 1 Mm) is in f…
The Fe I 6173 Å line is widely used in the measurements of vector magnetic fields by instruments including the Helioseismic and Magnetic Imager (HMI). We perform non-local thermodynamic equilibrium calculations of this line based on radiative hydrodynamic simulations in a flaring atmosphere. We employ both a quiet-Sun atmosphere and a penumbral atmosphere as the initial one in our simulations. We find that, in the quiet-Sun atmosphere, the line center is obviously enhanced during an intermediate flare. The enhanced emission is contributed from both radiative backwarming in the photosphere and …
Collision of the magnetic flux tubes in the Quiet Sun was proposed as one of the possible sources for the heating of the solar atmosphere (Furusawa and Sakai, 2000). The solar photosphere was observed using the New Solar Telescope ad Big Bear Solar Observatory. In TiO spectral line at 705.68 nm we approached resolution of 0.1". The horizontal plasma wave was observed spreading from the larger bright point. Shorty after this wave an increase in the oscillatory power appeared at the same location as the observed bright point. This behavior matches some of the results from the simulation of the c…
Short-lived (100s or less), sub-arcsec to a couple of arcsec size features of enhanced brightenings in the narrowband images at the $\mathrm{H_{2V}}$ and $\mathrm{K_{2V}}$ positions of the Ca II H and K lines in the quiet Sun are known as bright grains. With simultaneous observations of a quiet Sun internetwork region in the Fe I 6173 Å, Ca II 8542 {AA}, and Ca II K lines acquired by the CRisp Imaging Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on the Swedish 1-m Solar Telescope, we performed multi-line non-local thermodynamic equilibrium inversions using the STo…
Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high quality H$β$ observations indicate that Ellerman bombs are more common than previously thought and it was estimated that at any time about half a million Ellerman bombs are present in the quiet Sun. We performed an extensive statistical characterization of the quiet Sun Ellerman bombs (QSEBs) in these new H$β$ observations. We analyzed a 1 h dataset of quiet Sun observed with the Swedish 1-m Solar Telescope that consists of …
Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun (QSEBs) are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the solar lower atmosphere. We analyze high spatial resolution observations (0.1 arcsec) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the H$β$ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the FOV for a single moment in time; this is more than an or…
This work reviews our understanding of the magnetic fields observed in the quiet Sun. The subject has undergone a major change during the last decade (quiet revolution), and it will remain changing since the techniques of diagnostic employed so far are known to be severely biased. Keeping these caveats in mind, our work covers the main observational properties of the quiet Sun magnetic fields: magnetic field strengths, unsigned magnetic flux densities, magnetic field inclinations, as well as the temporal evolution on short time-scales (loop emergence), and long time-scales (solar cycle). We al…
Present-day semi-empirical models of solar irradiance (SI) variations employ spectra computed on one-dimensional atmosphere models (1D models) representative of various solar surface features to reconstruct SI changes measured on timescales greater than a day. Various recent studies have, however, pointed out that the spectra synthesized on 1D models do not reflect the radiative emission of the inhomogenous atmosphere revealed by high-resolution solar observations. We aimed to derive observational-based atmospheres from such observations and test their accuracy for SI estimates. We analysed sp…
The Large Area Telescope (LAT) on board Fermi has detected high-energy gamma rays from the quiet Sun produced by interactions of cosmic-ray nucleons with the solar surface and cosmic-ray electrons with solar photons in the heliosphere. Such observations provide a probe of the extreme conditions near the solar atmosphere and photosphere and permit the study of the modulation of cosmic rays over the inner heliosphere. For the first year of Fermi observations the solar modulation was at its minimum corresponding to a maximum cosmic-ray flux and, hence, maximum gamma-ray emission from the Sun. We …
Turbulent magnetic fields fill most of the volume of the solar atmosphere. However, their spatial and temporal variations are still unknown. Since 2007, during the current solar minimum, we are periodically monitoring several wavelength regions in the solar spectrum to search for variations of the turbulent magnetic field in the quiet Sun. These fields, which are below the resolution limit, can be detected via the Hanle effect which influences the scattering polarization signatures (Q/I) in the presence of magnetic fields. We present a description of our program and first results showing that …
The solar atmosphere is extremely dynamic, and many important phenomena develop on small scales that are unresolved in observations with the Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO). For correct calibration and interpretation of the observations, it is very important to investigate the effects of small-scale structures and dynamics on the HMI observables, such as Doppler shift, continuum intensity, spectral line depth, and width. We use 3D radiative hydrodynamics simulations of the upper turbulent convective layer and the atmosphere of the Sun, …
We present a series of numerical simulations of the quiet Sun plasma threaded by magnetic fields that extend from the upper convection zone into the low corona. We discuss an efficient, simplified approximation to the physics of optically thick radiative transport through the surface layers, and investigate the effects of convective turbulence on the magnetic structure of the Sun's atmosphere in an initially unipolar (open field) region. We find that the net Poynting flux below the surface is on average directed toward the interior, while in the photosphere and chromosphere the net flow of ele…
We studied the dynamics of the solar atmosphere in the region of a large quiet-Sun filament, which erupted on 21 October 2010. The filament eruption started at its northern end and disappeared from the H$α$ line-core filtergrams line within a few hours. The very fast motions of the northern leg were recorded in ultraviolet light by AIA. We aim to study a wide range of available datasets describing the dynamics of the solar atmosphere for five days around the filament eruption. This interval covers three days of the filament evolution, one day before the filament growth and one day after the er…
Solar observations at sub-mm, mm and cm wavelengths offer a straightforward diagnostic of physical conditions in the solar atmosphere because they yield measurement of brightness temperature which, for optically thick features, equals intrinsic temperature - much unlike solar diagnostics in other spectral ranges. The Atacama Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a new, hitherto underexplored, spectral window for studying the enigmatic solar chromosphere. In this review we discuss initial ALMA studies of the quiet chromosphere that used both single-dish and compa…
We study the radiative emission of various types of solar features, such as quiet Sun, enhanced network, plage, and bright plage regions, identified on filtergrams taken in the Ca II K line. We analysed fulldisk images obtained with the PSPT, by using three interference filters that sample the Ca II K line with different bandpasses. We studied the dependence of the radiative emission of disk features on the filter bandpass. We also performed a NLTE spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing both the PRD and CR…
This work presents a systematic investigation of the influence of weather conditions on the calibration errors by using Gaussian fitness, least chi-square linear fitness and wavelet transform to analyze the calibration coefficients from observations of the Chinese Solar Broadband Radio Spectrometers (at frequency bands of 1.0-2.0 GHz, 2.6-3.8 GHz, and 5.2-7.6 GHz) during 1997-2007. We found that calibration coefficients are influenced by the local air temperature. Considering the temperature correction, the calibration error will reduce by about $10\%-20\%$ at 2800 MHz. Based on the above inve…
Ubiquitous small-scale vortices have recently been found in the lower atmosphere of the quiet Sun in state-of-the-art solar observations and in numerical simulations. We investigate the characteristics and temporal evolution of a granular-scale vortex and its associated upflows through the photosphere and chromosphere of a quiet Sun internetwork region. We analyzed high spatial and temporal resolution ground- and spaced-based observations of a quiet Sun region. The observations consist of high-cadence time series of wideband and narrowband images of both H-alpha 6563 A and Ca II 8542 A lines o…
The magnetic fields of the quiet Sun cover at any time more than 90\% of its surface and their magnetic energy budget is crucial to explain the thermal structure of the solar atmosphere. One of the possible origins of these fields is due to the action of local dynamo in the upper convection zone of the Sun. Existing simulations of the local solar dynamo require an initial seed field, and sufficiently high spatial resolution, in order to achieve the amplification of the seed field to the observed values in the quiet Sun. Here we report an alternative model of seeding based on the action of the …
The interaction between emerging magnetic flux and the pre-existing ambient field has become a "hot" topic for both numerical simulations and high-resolution observations of the solar atmosphere. The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. We present an analysis of a small-scale flux emergence event in NOAA 10971, observed simultaneously with the Swedish 1-m Solar Telescope on La Palma and the \emph{Hinode} satellite during a joint campaign in September 2007. Extremely high-resolution G-band, H$α$…
Observations of the polar region of the Sun are critically important for understanding the solar dynamo and the acceleration of solar wind. We carried out precise magnetic observations on both the North polar region and the quiet Sun at the East limb with the Spectro-Polarimeter of the Solar Optical Telescope aboard Hinode to characterize the polar region with respect to the quiet Sun. The average area and the total magnetic flux of the kG magnetic concentrations in the polar region appear to be larger than those of the quiet Sun. The magnetic field vectors classified as vertical in the quiet …
We have examined the spatial and polarimetric properties of these events using a variety of data from the Hinode spacecraft. We have also inferred the atmospheric stratification of the physical parameters by means of the inversion of the observed Stokes profiles employing the Stokes Inversion based on Response functions (SIR) code. Finally, we analyzed their evolution using a time series from the same instrument. Blue-shifted events tend to appear over bright regions at the edge of granules, while red-shifted events are seen predominantly over dark regions on intergranular lanes. Large linear …
We observe a coronal jet around 22:08 UT on 23 rd March of 2017, in a quiet region towards the north-east of the solar disk. A minifilament eruption leads to this jet. We analyze dynamics of the minifilament from its formation until its eruption. We observe that the minifilament starts forming around 15:50 UT. Then at a later time, it disappears but again appears and finally erupts as a blowout jet around 22:08 UT. We study the evolution of photospheric magnetic field beneath the minifilament. Initially, we observe subsequent increase and decrease of positive magnetic flux. Prior to the initia…
We present a 3D simulation of the dynamic emergence of a twisted magnetic flux tube from the top layer of the solar convection zone into the solar atmosphere and corona. It is found that after a brief initial stage of flux emergence during which the two polarities of the bipolar region become separated and the tubes intersecting the photosphere become vertical, significant rotational motion sets in within each polarity. The rotational motions of the two polarities are found to twist up the inner field lines of the emerged fields such that they change their orientation into an inverse configura…
