Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.     

The relations between chromospheric features and photospheric magnetic fields

High resolution photographic magnetograms are compared with H filtergrams (both on- and off - band) for a wide variety of solar features. It is verified that H filaments overlie neutral lines or bands and that H plages always occur at magnetic field clumps. However, the brightness of H plages bear no relation to magnetic field strength or polarity, and the direction of the magnetic field with respect to threads and filaments remains obscure. Counter-examples can be found for virtually every rule that has been formulated so far.Basic questions about the usefulness and final research goal of filtergrams and magnetograms are raised. It is shown that neither filtergram or magnetogram alone is capable of furnishing a unique solution. It is suggested that the proper direction for research is to use magnetograms, together with (as yet unspecified) additional sources of data, to understand H structures.     

On photospheric and chromospheric penumbral waves

The full magnetoatmospheric wave equation is numerically solved for the particular penumbral model of Nye and Thomas (1974). For chromospheric running penumbral waves it is found that the maximum vertical velocity occurs at the base of the Hα region (z ≈ 300 km) much lower than previous WKB estimates have suggested. The maximum ‘vertical’ kinetic energy occurs at z ≈ - 130 km. Very significant horizontal velocities are also found for these waves, and, in the absence of shear flow it appears that previous estimates of photospheric vertical velocities of order 10^?1 km s^?1 could be substantial underestimates. For the photospheric events of Musman et al. (1976), a high vertical velocity maximum is found in the corona, and the modes appear highly dispersive for periods ≥ 220 s. The effects of a sinusoidal shear flow profile on running penumbral waves are examined, and it is found that the eigenvalues (horizontal wavenumber k) are changed little, but the eigenmodes become significantly distorted; the position of the vertical velocity peak rises compared to the zero flow case, and the velocity below that peak drops significantly. This effect may well cancel the increased estimates based on zero flow.     

On the relation between chromospheric and photospheric fine structure in an active region

A comparative study was done on the bright fine structure in the upper photosphere and in the lower chromosphere of an active region. The results are shown in the following: (a) The bright points in the H wing are cospatial to the facular points, which confirms the result of Wilson (1981). (b) Some points bright in the H wing are associated with the facular granules which have larger sizes than the facular points, (c) The brightness enhancement in the Ha wing is positively correlated to the enhancement in the blue continuum. However, the correlation is not so strong. (d) The moustache points are also cospatial to the facular features. (e) The geometrical shape of a moustache point is like a funnel and diverging upward in the upper photospheric and the lower chromospheric levels.Some discussions are given on the nature of the heating mechanism of the fine structure based upon these results.On leave from Kyoto University and Hida Observatory, Japan.     

On some flare-sensitive high photospheric and low chromospheric lines

During a stay at the Kitt Peak National Observatory the writer has tried to find an influence of flare radiation on the high photospheric and low chromospheric lines of the area occupied by the flare. Observations have been made in the H region and in the region of the H and K lines. When flare emission is present in sunspots some of the faint (molecular) lines seem to be weakened. When a flare appears near the solar limb some of the Evershed-type (chromospheric) lines are strongly influenced.Kitt Peak National Observatory Contribution No. 481.Visiting Astronomer to the Solar Division, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Relationship between electric currents,photospheric motions,chromospheric activity,and magnetic field topology

Through coordinated observations made during the Max'91 campaign in June 1989 in Potsdam (magnetograms), Debrecen (white light and H), and Meudon (MSDP), we follow the evolution of the sunspot group in active region NOAA 5555 for 6 days. The topology of the coronal magnetic field is investigated by using a method based on the concept of separatrices - applied previously (Mandriniet al., 1991) to a magnetic region slightly distorted by field-aligned currents. The present active region differs by having significant magnetic shear. We find that the H flare kernels and the main photospheric electric current cells are located close to the intersection of the separatrices with the chromosphere, in a linear force-free field configuration adapted to the observed shear. Sunspot motions, strong currents, isolated polarities, or intersecting separatrices are not in themselves sufficient to produce a flare. A combination of them all is required. This supports the idea that flares are due to magnetic reconnection, when flux tubes with field-aligned currents move towards the separatrix locations.     

Local solar magnetic and velocity field development and related photospheric and chromospheric activity

We have compiled the results of our long-term studies of the local magnetic field and its activity development, derived from investigating sunspot group evolution, photoelectrically measured longitudinal magnetic and velocity fields, and measurements of sunspot proper motions. We estimate certain regularities according to which the magnetic and velocity fields, and photospheric, as well as chromospheric activities develop. We speculate about the physical background of such processes.Dedicated to Cornelis de Jager     

Preprocessing photospheric vector magnetograms for nonlinear force-free field extrapolation of the global corona

Nonlinear force-free magnetic field(NLFFF) extrapolation based on the observed photospheric magnetic field is the most important method to obtain the coronal magnetic field nowadays.However, raw photospheric magnetograms contain magnetic forces and small-scale noises, and fail to be consistent with the force-free assumption of NLFFF models. The procedure for removing the forces and noises in observed data is called preprocessing. In this paper, we extend the preprocessing code of Jiang Feng to spherical coordinates for a full sphere. We first smooth the observed data with Gaussian smoothing, and then split the smoothed magnetic field into a potential field and a non-potential field.The potential part is computed by a numerical potential field model, and the non-potential part is preprocessed using an optimization method to minimize the magnetic forces and magnetic torques. Applying the code to synoptic charts of the vector magnetic field from SDO/HMI, we find it can effectively reduce the noises and forces, and improve the quality of data for a better input which will be used for NLFFF extrapolations applied to the global corona.     

Green coronal intensity enhancements and their relation to the underlying photospheric/chromospheric activity

Analysis of the photospheric and chromospheric activity at the sites of enhanced 5303 Å coronal intensity revealed some important aspects of their association. We have examined the daily maps of 5303 Å coronal line intensity of Lomnický tít for the low sunspot activity years 1985 and 1986 in association with the cotemporal daily maps of sunspots, plages and Stanford magnetograms and identified strong field gradients at the sites of enhanced intensity regions. We found that the peak intensity does not depend on the strength of the underlying magnetic field though the coronal intensity-enhanced feature is almost sure to occur at the locations of sunspots with strong magnetic fields and at the locations of plages having larger areas.     

A comparison of vector magnetograms from the marshall space flight center and mees solar observatory

We compare completely independent vector magnetic field measurements from two very different polarimetric instruments. The Marshall Space Flight Center's imaging vector magnetograph is based on a birefringent filter, routinely measuring all four Stokes parameters integrated over the filter bandpass (1/8 Å) which is tunable across the Fei 5250 line in 10 mÅ steps. The Haleakala Stokes Polarimeter of the Mees Solar Observatory (MSO) is based on a spectrometer, routinely measuring all four Stokes parameters of the Fei 6302.5 line simultaneously and then spatially scanning to build up a vector magnetogram. We obtained active region magnetic field data with both the Marshall Space Flight Center (MSFC) and MSO systems on five days during June 1985. After interpolating the MSFC vector fields onto the more coarse spatial grid of MSO we make a point-by-point comparison of the two vector fields for data obtained on two of these days (June 8 and 9). From this comparison we conclude: (1) the spatially-averaged line-of-sight components agree quite well; (2) although the MSO spatial grid is coarser, the quality of the MSO image is better than that of the MSFC data because of better seeing conditions; (3) the agreement between the transverse magnitudes is affected by the poor image quality of the MSFC data; and (4) if the effects of Faraday rotation caused by including line-center linear polarization in the method of analysis are taken into account, the azimuths show good agreement within the scatter in the data caused by the averaging process.National Research Council Resident Research Associate.     

A comparison of the photospheric and interplanetary magnetic fields during the flight of Mariner IV

The polarity of the interplanetary magnetic field observed by the Mariner IV spacecraft is compared to the polarity of the photospheric magnetic field observed with the solar magnetograph at Mt. Wilson Observatory. Unlike the results obtained from observations during the flight of IMP-I, these polarities are not well correlated when the photospheric polarity is determined from data along a narrow latitudinal strip. It is suggested that the structure of the interplanetary field is often related to major features in the photospheric field that are observed over a broad range of solar latitudes.     

A comparison of the near-ultraviolet continuum and chromospheric emission line fluxes of the late-type giant stars

We investigate the near-ultraviolet high-resolution LWR spectra of the stars Cas, And, Tau, Gem, Cru, Boo, and Peg, obtained with the aid of the International Ultraviolet Explorer Satellite. We have given here a list of the strongest and most prevalent emission lines in the near-ultraviolet spectra of Boo, KlIIIp, and Peg, M2.5II-III which have the same luminosity class and different spectral type. The near-ultraviolet continuum flux measurements and integrated emission line fluxes of these stars for the 2500–3200 Å region are presented in order to compare the variations in the appearance of the near-ultraviolet flux distribution with the temperature structure of their chromospheres for K and M giant stars. We also discuss differences between observed and calculated fluxes found from the Planck function.     

Relationships between photospheric plasma angular velocity and solar activity

An analysis of relationships between latitudinal fine structures of the photospheric plasma differential rotation and solar activity shows that sunspot activity seems to be lower (as measured by the number and extension of sunspot groups) at latitudes where minima of angular velocity appear.     

Combined study between the chromospheric flare models and hard X-ray observation

By comparison between SMM HXRBS observation and ground observation of H and Caii K lines for the 2B flare on February 3, 1983, we found that there was a temporal correlation between H intensity and hard X-ray flux at the early stage of the impulsive phase while different peaks in the hard X-ray flux curve represented bursts at different locations. When we combined SMM HXRBS observation with chromospheric flare models, we further found that the temporal coincidence between H intensity and hard X-ray flux could be explained quantitatively by the fact that the H flare was indeed due to the heating by non-thermal electron beams responsible for the emission of hard X-rays. Together with the discussion on coronal density based on chromospheric flare models, it was also shown that the source of electrons seemed to be situated around the top of the flare loop and the column density at the top of the chromosphere in semi-empirical flare models could not be taken as the total material above the top of the chromosphere.     

A comparison of coronal X-ray structures of active regions with magnetic fields computed from photospheric observations

The appearances of several X-ray active regions observed on March 7, 1970 and June 15, 1973 are compared with the corresponding coronal magnetic field topology. Coronal fields have been computed from measurements of the longitudinal component of the underlying magnetic fields, under the current-free hypothesis. An overall correspondence between X-ray structures and calculated field lines is established, and the magnetic counterpart of different X-ray features is also examined. A correspondence between enhanced X-ray emission and the location of compact closed field lines is suggested by this study. Representative magnetic field values calculated under the assumption of current-free fields are given for heights up to 200″.     

A chromospheric response to pulse beam heating

Starting from the new flare models of Karlický (1990) and Karlický and Hénoux (1991), we present here the first time-dependent numerical simulations of hydrogen plasma excitation and ionization on time scales of less than one second. These time scales are consistent with the spiky behaviour of the kinetic temperature produced by non-thermal collisional processes. Such temperature spikes represent a chromospheric response to a series of short-duration electron beam pulses which are supposed to heat the flare atmosphere. Self-consistent numerical solution of a simplified, time-dependent, non-LTE problem for a three-level hydrogen atom model with continuum allows us to predict theoretically a qualitative behaviour of the H line intensity variations on very short time intervals. Our H temporal profiles, evaluated at the line center and for = 1 Å, can be qualitatively compared with some recent flare observations obtained with high temporal resolution.