Visible Solar Surface Research Articles

Dependence of Spicule Properties on the Magnetic Field—Results from Magnetohydrodynamics Simulations

Solar spicules are plasma jets observed in the interface region between the visible solar surface and the corona. At any given time, there is a forest of spicules originating in the chromosphere of the Sun. While various models attempt to elucidate their origin and characteristics, here, we consider the one driven by the magnetoconvection undulations. The radiative magnetohydrodynamic (rMHD) equations are solved using Pencil Code with a spatial resolution of 16 km using various magnetic field strengths. The obtained rMHD simulation data are investigated to unveil the various trends in spicular properties as a function of the applied magnetic fields. The important outcome of this study is the finding of a consistent reduction in both the number density and the maximum height reached by spicules as magnetic field strength increases. We also use parabolic fitting on time–distance curves of spicules that are taller than the 75th percentile in the distribution, in order to find a relation between the deceleration of the spicule tip and the magnetic field strength. Our results offer insights into the response of solar spicules to magnetic field strength.

Polymeric jets throw light on the origin and nature of the forest of solar spicules

Spicules are plasma jets, observed in the dynamic interface region between the visible solar surface and the hot corona. At any given time, it is estimated that about 3 million spicules are present on the Sun. We find an intriguing parallel between the simulated spicular forest in a solar-like atmosphere and the numerous jets of polymeric fluids when both are subjected to harmonic forcing. In a radiative magnetohydrodynamic numerical simulation with sub-surface convection, solar global surface oscillations are excited similarly to those harmonic vibrations. The jets thus produced match remarkably well with the forests of spicules detected in observations of the Sun. Taken together, the numerical simulations of the Sun and the laboratory fluid dynamics experiments provide insights into the mechanism underlying the ubiquity of jets: the nonlinear focusing of quasi-periodic waves in anisotropic media of magnetized plasma as well as polymeric fluids under gravity is sufficient to generate a forest of spicules on the Sun.

Coronal Heating Law Constrained by Microwave Gyroresonant Emission

Abstract The question why the solar corona is much hotter than the visible solar surface still puzzles solar researchers. Most theories of the coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure. This coupling is based on two facts: (i) the magnetic field is the main source of the energy in the corona and (ii) the heat transfer preferentially happens along the magnetic field, while is suppressed across it. However, most of the information about the coronal heating is derived from the analysis of extreme ultraviolet or soft X-ray emissions, which are not explicitly sensitive to the magnetic field. This paper employs another electromagnetic channel—the sunspot-associated microwave gyroresonant emission, which is explicitly sensitive to both the magnetic field and thermal plasma. We use nonlinear force-free field reconstructions of the magnetic skeleton dressed with a thermal structure as prescribed by a field-aligned hydrodynamics to constrain the coronal heating model. We demonstrate that the microwave gyroresonant emission is extraordinarily sensitive to details of the coronal heating. We infer heating model parameters consistent with observations.

The magnetic connectivity of coronal shocks from behind-the-limb flares to the visible solar surface during γ-ray events

Context. The observation of >100 MeV {\gamma}-rays in the minutes to hours following solar flares suggests that high-energy particles interacting in the solar atmosphere can be stored and/or accelerated for long time periods. The occasions when {\gamma}-rays are detected even when the solar eruptions occurred beyond the solar limb as viewed from Earth provide favorable viewing conditions for studying the role of coronal shocks driven by coronal mass ejections (CMEs) in the acceleration of these particles. Aims: In this paper, we investigate the spatial and temporal evolution of the coronal shocks inferred from stereoscopic observations of behind-the-limb flares to determine if they could be the source of the particles producing the {\gamma}-rays. Methods: We analyzed the CMEs and early formation of coronal shocks associated with {\gamma}-ray events measured by the Fermi-Large Area Telescope (LAT) from three eruptions behind the solar limb as viewed from Earth on 2013 Oct. 11, 2014 Jan. 06 and Sep. 01. We used a 3D triangulation technique, based on remote-sensing observations to model the expansion of the CME shocks from above the solar surface to the upper corona. Coupling the expansion model to various models of the coronal magnetic field allowed us to derive the time-dependent distribution of shock Mach numbers and the magnetic connection of particles produced by the shock to the solar surface visible from Earth. Results: The reconstructed shock fronts for the three events became magnetically connected to the visible solar surface after the start of the flare and just before the onset of the >100 MeV {\gamma}-ray emission. The shock surface at these connections also exhibited supercritical Mach numbers required for significant particle energization. [...] (Abridged)

BUOYANCY-DRIVEN MAGNETOHYDRODYNAMIC WAVES

ABSTRACT Turbulent motions close to the visible solar surface may generate low-frequency internal gravity waves (IGWs) that propagate through the lower solar atmosphere. Magnetic activity is ubiquitous throughout the solar atmosphere, so it is expected that the behavior of IGWs is to be affected. In this article we investigate the role of an equilibrium magnetic field on propagating and standing buoyancy oscillations in a gravitationally stratified medium. We assume that this background magnetic field is parallel to the direction of gravitational stratification. It is known that when the equilibrium magnetic field is weak and the background is isothermal, the frequencies of standing IGWs are sensitive to the presence of magnetism. Here, we generalize this result to the case of a slowly varying temperature. To do this, we make use of the Boussinesq approximation. A comparison between the hydrodynamic and magnetohydrodynamic cases allows us to deduce the effects due to a magnetic field. It is shown that the frequency of IGWs may depart significantly from the Brunt–Väisälä frequency, even for a weak magnetic field. The mathematical techniques applied here give a clearer picture of the wave mode identification, which has previously been misinterpreted. An observational test is urged to validate the theoretical findings.

HEIGHT VARIATION OF THE VECTOR MAGNETIC FIELD IN SOLAR SPICULES

Proving the magnetic configuration of solar spicules has hitherto been difficult due to the lack of spatial resolution and image stability during off-limb ground-based observations. We report spectropolarimetric observations of spicules taken in the He I 1083 nm spectral region with the Tenerife Infrared Polarimeter II at the German Vacuum Tower Telescope of the Observatorio del Teide (Tenerife; Canary Islands; Spain). The data provide the variation with geometrical height of the Stokes I, Q, U, and V profiles whose encoded information allows the determination of the magnetic field vector by means of the HAZEL inversion code. The inferred results show that the average magnetic field strength at the base of solar spicules is about 80 gauss and then it decreases rapidly with height to about 30 gauss at a height of 3000 km above the visible solar surface. Moreover, the magnetic field vector is close to vertical at the base of the chromosphere and has mid inclinations (about 50 degree) above 2 Mm height.

The Evolution of the Solar Magnetic Field

AbstractThe almost stately evolution of the global heliospheric magnetic field pattern during most of the solar cycle belies the intense dynamic interplay of photospheric and coronal flux concentrations on scales both large and small. The statistical characteristics of emerging bipoles and active regions lead to development of systematic magnetic patterns. Diffusion and flows impel features to interact constructively and destructively, and on longer time scales they may help drive the creation of new flux. Peculiar properties of the components in each solar cycle determine the specific details and provide additional clues about their sources. The interactions of complex developing features with the existing global magnetic environment drive impulsive events on all scales. Predominantly new-polarity surges originating in active regions at low latitudes can reach the poles in a year or two. Coronal holes and polar caps composed of short-lived, small-scale magnetic elements can persist for months and years. Advanced models coupled with comprehensive measurements of the visible solar surface, as well as the interior, corona, and heliosphere promise to revolutionize our understanding of the hierarchy we call the solar magnetic field.

Realistic magnetohydrodynamical simulation of solar local supergranulation

Three-dimensional numerical simulations of solar surface magnetoconvection using realistic model physics are conducted. The thermal structure of convective motions into the upper radiative layers of the photosphere, the main scales of convective cells and the penetration depths of convection are investigated. We take part of the solar photosphere with a size of 60×60 Mm2 in the horizontal direction and of depth 20 Mm from the level of the visible solar surface. We use a realistic initial model of the sun and apply the equation of state and opacities of stellar matter. The equations of fully compressible radiation magnetohydrodynamics (MHD) with dynamical viscosity and gravity are solved. We apply (i) the conservative total variation diminishing (TVD) difference scheme for MHD, (ii) the diffusion approximation for radiative transfer and (iii) dynamical viscosity from subgrid-scale modeling. In simulation, we take a uniform two-dimensional grid in the horizontal plane and a nonuniform grid in the vertical direction with the number of cells being 600×600×204. We use 512 processors with distributed memory multiprocessors on the supercomputer MVS-100k at the Joint Computational Centre of the Russian Academy of Sciences.

Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension

Aims. In this paper we analyse numerically the propagation and dispersion of acoustic waves in the solar-like sub-photosphere with localised non-uniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations. Methods. Numerical simulations of wave propagation through the solar sub-photosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height approximately corresponding to the visible surface of the Sun. We analyse the response of vertical velocity to changes in the interior due to magnetic field at the level corresponding to the visible solar surface, by the means of local time-distance helioseismology. Results. The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with localised magnetic field concentrations of various types are presented. Time-distance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases.

The rotation of the Sun as a star from the green-line emission of the entire corona

A new representation for the database created by J. Sykora on the 5303 A Fe XIV line emission observed from 1939 to 2001 is proposed. Observations of the corona at an altitude of 60″ above the limb reduced to a uniform photometric scale provide estimates of the emission of the entire visible solar surface. It is proposed to use the resulting series of daily measurements as a new index of the solar activity, GLSun (The Green-Line Sun). This index is purely observational and is free of the model-dependent limitations imposed on other indices of coronal activity. GLSun describeswell both the cyclic activity and the rotational modulation of the brightness of the corona of the Sun as a star. The GLSun series was subject to a wavelet analysis similar to that applied to long-term variability in the chromospheric emission of late-type active stars. We obtain that the brightness inhomogeneities in the solar corona rotate more slowly during epochs of high activity than their average rotational rate over the entire time observations. The time interval of slower rotation of the inhomogeneities is close to the epoch when the Sun’s field represents a horizontal magnetic dipole in each activity cycle, but is somewhat longer than the duration of the polarity reversal in both hemispheres. The difference between the periods for the slower and mean rotation exceeds three days, as is typical for some stars with higher but less regular activity than solar one. The importance of these findings for dynamo theory for the origin and evolution of the magnetic fields of the Sun and other late-type stars is briefly discussed.

Absorption Phenomena and a Probable Blast Wave in the 13 July 2004 Eruptive Event

We present a case study of the 13 July 2004 solar event, in which disturbances caused by eruption of a filament from an active region embraced a quarter of the visible solar surface. Remarkable are absorption phenomena observed in the SOHO/EIT 304 A channel; they were also visible in the EIT 195 A channel, in the H-alpha line, and even in total radio flux records. Coronal and Moreton waves were also observed. Multi-spectral data allowed reconstructing an overall picture of the event. An explosive filament eruption and related impulsive flare produced a CME and blast shock, both of which decelerated and propagated independently. Coronal and Moreton waves were kinematically close and both decelerated in accordance with an expected motion of the coronal blast shock. The CME did not resemble a classical three-component structure, probably, because some part of the ejected mass fell back onto the Sun. Quantitative evaluations from different observations provide close estimates of the falling mass, ~3 10^15 g, which is close to the estimated mass of the CME. The falling material was responsible for the observed large-scale absorption phenomena, in particular, shallow widespread moving dimmings observed at 195 A. By contrast, deep quasi-stationary dimmings observed in this band near the eruption center were due to plasma density decrease in coronal structures.

Origin of the climatic cycles from orbital to sub-annual scales

We have confirmed the solar origin of insolation cycles of 11,500, 4400, 3950, 2770, 2500, 2090, 1960, 1670, 1460, 1280, 1195, 1145, 1034, 935, 835, 750 and 610 yrs found in speleothem luminescence by their presence in other indices of the solar wind. The 11,500-yr solar cycle can produce climatic variations similar to those caused by the orbital variations. We discovered a sub-annual cycle of 27 days in one very high-resolution speleothem proxy paleoclimatic record, attributed to solar rotation, which causes the periodic appearance of active zones on the visible solar surface, which are the major emitters of solar wind. Solar wind modulates cosmic ray flux at the Earth, while cosmic rays influence the atmospheric transparency, thus producing a multiplication of solar variations in insolation. Small variations of the solar activity can produce a measurable influence on insolation.

A solar surface dynamo

Context. Observations indicate that the quiet solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear. Aims. We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux. Methods. We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary. Results. Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G. Conclusions. A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface.

Forward Modeling of Acoustic Wave Propagation in the Quiet Solar Subphotosphere

The results of numerical simulations of acoustic wave propagation and dispersion in the nonmagnetic solar subphotosphere are presented. Initial equilibrium density and pressure stratifications are taken from a standard solar model but modified to suppress convective instabilities in fully compressible two-dimensional ideal hydrodynamical modeling. Acoustic waves are generated by sources located below the height corresponding to the visible solar surface. The dynamic response of the solar interior to two acoustic source types, namely a harmonic source and one representing downward-propagating photospheric plumes, is studied. A large number of randomly distributed localized cooling sources with random amplitudes is also introduced. The methods used to analyze the simulation data are similar to ones used in observational studies in local helioseismology. Time-distance diagrams of the pressure and vertical velocity perturbations at the level corresponding to the solar surface show the appearance of wave packets propagating with different speeds, which are reflected at different depths beneath the subphotosphere. The (ω, kh) power spectra, derived from the vertical velocity data, show the existence of g-, f-, and p-modes; p-mode ridges are identifiable up to high radial orders of n ≈ 11; g-modes appear in the simulations, unlike in the real Sun, where they cannot propagate in the convectively unstable solar subphotosphere. Cross-correlation analysis of vertical velocity perturbations shows a good correspondence with the observed time-distance helioseismic data for quiet Sun. Thus, the ability of the implemented approach of forward modeling to investigate propagation of acoustic, internal, and surface gravity waves in a realistic solar interior model is shown.

Imaging sunspots using helioseismic methods

The origin of sunspots is one of the most fundamental and yet poorly understood areas in solar physics. Imaging local anomalous features in the solar interior offers a direct way to unravel the underlying physical processes of sunspots and the mechanisms behind their formation. The advent of local helioseismology in the last few years has, for the first time, made it possible to image local internal solar structures. High-resolution satellite and ground telescope data which reveal the details of the vibrations of the visible solar surface are essential in the development of local helioseismology. We are now in a position to transfer the seismic methods that have traditionally been used to study the Earth's interior to solar investigations. This interdisciplinary approach to developing seismic imaging techniques is opening up new ways of understanding the flow and other structural characteristics beneath sunspots. In this article, I review recent progress in the imaging of sunspots and the surrounding solar active regions. By highlighting the strengths of seismic methods and the challenges facing local helioseismology, I discuss some of the new research directions and possibilities that have arisen from this novel type of seismic imaging.

Large-Scale Activity in the Bastille Day 2000 Solar Event

We have analyzed dimmings, i.e., regions of temporarily reduced brightness, and manifestations of a coronal wave in the famous event of 14 July 2000 using images produced with the EUV telescope SOHO/EIT. Our analysis was inspired by a paper by Andrews (2001, Solar Phys. 204, 181 (Paper I)), in which this event was studied using running-difference EIT images at 195 A formed by subtraction of a previous image from each current one. Such images emphasize changes of the brightness, location, and configuration of observed structures occurring during the 12-min interval between two subsequent heliograms. However, they distort the picture of large-scale disturbances caused by a CME, particularly, dimmings. A real picture of dimmings can be obtained from fixed-base difference ‘de-rotated’ images. The latter are formed in two stages: first, the solar rotation is compensated using three-dimensional rotation of all images (‘de-rotation’) to the time of a pre-event heliogram, here 10:00 UT, and then the base heliogram is subtracted from all others. We show real dimmings to be essentially different from those described by Andrews (Paper I). The restructuring of large-scale magnetic fields in the corona in connection with the CME was accompanied by the appearance and growth of two large dimmings. One of them was located along the central meridian, southward of the eruption center, at the place of the pre-eruption arcade. Another dimming occupied the space between the flare region and a remote western active region. Several smaller dimmings were observed virtually over the whole solar disk, especially, within the northwest quadrant. We have also revealed a propagating disturbance with properties of a coronal wave in the northern polar sector, where no dimmings were observed. This fact is discussed in the context of probable association between dimmings and coronal waves. Having suppressed the ‘snowstorm’ produced in the EIT images by energetic particles, we have considered dimming manifestations in all four EIT pass bands of 171, 195, 284, and 304 A as well as the light curves of the main dimmings including several later images at 195 A. Our analysis shows that the major cause of the dimmings was density depletion that reached up to 30% in this event. The picture of dimmings implies that the CME in the Bastille Day event was an octopus-like bundle of some magnetic ropes, with the ‘arms’ being connected to several active regions disposed over almost the whole visible solar surface.

The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism

An attractive diagnostic tool for investigating the magnetism of the solar chromosphere is the observation and theoretical modeling of the Hanle and Zeeman effects in spicules, as shown in this letter for the first time. Here we report on spectropolarimetric observations of solar chromospheric spicules in the He I 10830 \AA multiplet and on their theoretical modeling accounting for radiative transfer effects. We find that the magnetic field in the observed (quiet Sun) spicular material at a height of about 2000 km above the visible solar surface has a strength of the order of 10 G and is inclined by approximately $35^{\circ}$ with respect to the local vertical direction. Our empirical finding based on full Stokes-vector spectropolarimetry should be taken into account in future magnetohydrodynamical simulations of spicules.

Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands

Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the G band, a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic structures, thus confirming the model of radiatively heated magnetic flux concentrations. These results provide a firm basis for observational studies of the evolution and dynamics of the small-scale solar magnetic field derived through proxy magnetometry with G-band images.

Modeling the coronal magnetic field, with a new method for obtaining boundary conditions on the farside of the Sun

A novel technique is presented that aids the reconstruction of solar coronal magnetic fields by augmenting the visible solar surface boundary conditions with an estimate of the simultaneous conditions on the far side of the Sun. In converting from line‐of‐sight measurements, two alternative assumptions are considered: (1) ignore the plasma beta differences between corona and photosphere and so find a radial field component that is consistent with an overlying potential (or force free) field corona; and (2) characterize the change in beta by a boundary layer, matching the coronal field to the photosphere where the field, seen in projection, is taken to be wholly radial. The radial magnetic field (or radial field component) over the unseen hemisphere of the Sun is deduced from information held within a time series of the Solar and Heliospheric Observatory Michelson Doppler imager photospheric magnetograms centered on the time of interest and combined with a full disc of visible data for that time. Comparison is made with the more usual synoptic map boundary conditions to test the time sensitivity of the problem. Several methods of extrapolation to the far side are assessed using data centered on an observation at the end of August 1996, when a large equatorial coronal hole was present on one side of the Sun. The corresponding magnetic field equilibrium in the corona is found, assuming a simple potential approximation, which employs an outer “source surface” in addition to the derived inner boundary condition. Together with the inherent assumption that the evolution of the global field is slow, the validity of the technique is confirmed by the self‐consistency of the results.

Numerical simulation of observations with GOLF on board SOHO

The main objective of the GOLF Experiment (Global Oscillations at Low Frequencies) on-board the SOHO (Solar and Heliospheric Observatory) space mis- sion is the quantitative knowledge of the internal struc- ture of the Sun by measuring the spectrum of its global oscillations in a wide frequency range (30 nHz to 6 mHz). There is special interest in detecting the low ' p- and g-modes (low frequency modes) which penetrate deeply down into the solar core. The instrument chosen is an im- proved disk-integrated sunlight resonant scattering spec- trophotometer. It obtains the line of sight velocity of the integrated visible solar surface by measuring the Doppler shift of the sodium doublet. Mainly, two innovations have been incorporated to standard earth-based similar appa- ratus (those from the networks IRIS and BISON). First, GOLF samples each line of the sodium doublet in prin- ciple at four points on its wings, using an extra small modulated magnetic eld. This new information enables an instantaneous calibration of the measured signal and also opens the possibility to correct from the background solar velocity noise. Second, the use of an extra xed quar- ter wave plate, placed at the entrance of the instrument, enables a selection of the circularly polarized solar light. Therefore, the disk averaged solar line-of-sight component of the magnetic eld can also be obtained. This is consid- ered as a secondary objective of the mission. In order to study the new information available due to these improve- ments in the apparatus, the necessity of fully understand- ing it and the need to write the appropriate software to analyze the data, a complete numerical simulation of the experiment has been built. Running the simulation has yielded two series of 12 months long each, one correspond- ing to a year of maximum solar activity and the other to a year of minimum solar activity. In this paper the numerical simulation of the GOLF experiment is presented, its sen- sitivity and instrumental response calculated and a power spectra of this two series have been obtained to show its