A two-dimensional solar model

We present a two-dimensional, nearly flux constant solar atmosphere which includes a physical model of granulation. If the inhomogeneous character of the solar photosphere is neglected in the construction of empirical models, the result is an underestimate of the temperature gradient in the layers below 0.5 and an overestimate of the convective flux penetration into the observable layers. The two-dimensional model adequately reproduces the mean limb darkening, the magnitude and center-to-limb variation of the intensity fluctuations, and the rms vertical and horizontal convective velocities.     

A two-dimensional model for a solar prominence

A two-dimensional magnetohydrostatic model of a vertical prominence sheet is set up by allowing slow variations of the magnetic field and plasma properties with height. The width of the prominence is found to decrease with height and in many cases the field lines become less curved, while the strength of the horizontal magnetic field increases with height, in agreement with observations. Since we are only considering a local analysis, the model applies to a general prominence sheet, whether of Kippenhahn-Schlüter or Kuperus-Raadu type. The challenge in the future is to understand the detailed fine-scale microstructure which takes place in the mould formed by the present global macro-models.On leave from: Departament de Fisica, Universitat de les Illes Balears, E-07071, Palma de Mallorca, Spain.     

Golf: A resonance spectrometer for the observation of solar oscillations

GOLF (Global Oscillations at Low Frequencies) is an instrument to study the line-of-sight velocity of the solar photosphere, to be flown on the SOHO satellite in 1995. It uses a sodium vapour cell in resonance scattering mode, in order to measure the absolute Doppler shift of the solar sodium absorption lines. We detail laboratory tests to determine the performances of the cell built for the experiment. The results are in good agreement with numerical simulations of the resonance processes. As a final result, we can conclude that the level of performances required for the flight instrument will be obtained.     

Digital multichannel spectrometer for solar microwave observations

Observations of solar microwave bursts have shown fine structures (e.g., the millisecond spikes), not resolvable in time and frequency by existing instruments. In order to investigate these features in greater detail we have developed a spectrometer with high temporal and spectral resolution. The frequency range from 3000 to 4000 MHz is covered by 32 channels with different bandwidths (0.1, 5, and 20 MHz). The instrument is fully controlled by a multiprocessor computer system and allows the recording of about 200 000 measurements per second. Thus it is possible to observe the intensity and the circular polarization of all the 32 channels with a time resolution of about 350 s. A very flexible frequency selection system allows the use of many different observation modes.     

The computer-controlled solar radio spectrometer ‘IKARUS’

The radiospectrometer IKARUS is a fully computer-controlled instrument covering the frequency band 0.11 to 1 GHz in steps of 1 MHz. It can automatically detect solar radio bursts and then write, on magnetic tape, 2000 measurements per second of intensity and circular polarization. The frequencies to be measured can be readily programmed in the band, compromising between frequency and time resolution. Reference noise sources are switched in automatically by the computer to calibrate the receiver at each frequency. The dynamic range is about 50 dB, recorded logarithmically with 8 bit resolution.The novelty of the instrument is its ability to measure broadband calibrated spectra (flux and degree of polarization) in the very interesting region of the lower corona.     

A study of solar preflare activity using two-dimensional radio and SMM-XRP observations

We present a study of type III activity at meter- decameter wavelengths in the preflare phase of the 1986 February 3 flare using data obtained with the Clark Lake Multifrequency Radioheliograph. We compare this activity with similar type III burst activity during the impulsive phase and find that there is a displacement of burst sources between the onset and end times of the activity. A comparison of this displacement at three frequencies suggests that the type III emitting electrons gain access progressively to diverging and different field lines relative to the initial field lines. The energetics of the type III emitting electrons are inferred from observations and compared with those of the associated hard X-ray emitting electrons. The soft X-ray data from SMM-XRP shows enhanced emission measure, density and temperature in the region associated with the preflare type III activity.On leave from Indian Institute of Astrophysics, Kodaikanal, Tamil Nadu, India.     

The performance of a large echelle grating in a solar spectrometer

A number of tests of a large diffraction grating are described. It is shown that useful information about the performance of the grating in a spectrometer can be obtained from an analysis of the diffracted wavefront by the method of Stroke. Comparison of the instrumental profile at three wavelengths strongly suggests that the influence of changes in the angle of diffraction on the profile cannot be neglected. A resolving power of 6.9 × 10⁵ is attained at 6328 Å. At the same wavelength the first order Rowland ghosts in the ninth order of diffraction have an intensity of only 8 × 10^-4 of the parent line. The influence of stray light on the central intensities of deep Fraunhofer lines is shown to be small but by no means negligible.     

The maintenance of solar differential rotation by two-dimensional turbulence

A numerical model has been made to test the theory that solar differential rotation is maintained by the Countergradient transport of energy peculiar to two-dimensional turbulence. After a brief discussion of this turbulent process, the numerical methods employed and their application to the sun are reviewed. The results of one problem are presented, indicating that this model can represent the observed large-scale nature of the sun's surface. The reader is referred to the author's dissertation for complete details of the methods and calculations.     

High-resolution models of solar granulation: the two-dimensional case

Using advanced numerical schemes and grid refinement, we present 2D high-resolution models of solar granulation with particular emphasis on downflowing plumes. In the high-resolution portion of our simulation, a box measuring 1.97 × 2.58 Mm² (vertical × horizontal), the grid size is 1.82 × 2.84 km². Calculations at the resolution usually applied in this type of simulations amount to only a few horizontal gridpoints for a downflowing plume. Due to the increased number of gridpoints in our high-resolution domain, the simulations show the development of vigorous secondary instabilities of both the plume's head and stem. The plume's head produces counterrotating vortex patches, a topology due to the 2D nature of the simulations. Below a depth of about 1 Mm, the plume's head and stem instabilities produce, in these 2D models, patches of low density, temperature, pressure and high vorticity which may last for all of our simulation time, ∼10 min, and probably considerably longer. Centrifugal forces acting in these patches counteract the strong inward pressure. Probably most importantly, the plume's instabilities give rise to acoustic pulses created predominantly down to ∼1.5 Mm. The pulses proceed laterally as well as upwards and are ubiquitous. Ultimately, most of them emerge into the photosphere. A considerable part of the photospheric 'turbulence' in these models is due to those pulses rather than to some sort of eddies. The upflows in granules are smooth where they reach the photosphere from below even in the present calculations; however, the pulses may enter in the photosphere also in granular upflows.     

Calculations of two-dimensional models of the lower solar chromosphere

Several two-dimensional models of the lower solar chromosphere are computed using relative RMS line centre intensity variations and mean limb darkening curves observed in Mg b and NaD. These calculations indicate that the small scale fluctuations observed at line centre in these lines may result from density and electron temperature fluctuations in the lower chromosphere, while the large scale fluctuations may result from either fluctuations in the deeper lying continuum or horizontal and vertical differential velocity fields.     

A two-dimensional model for a solar prominence: Effect of an external magnetic field

Using analytical approximations we study the effects of different external magnetic configurations on the half-width, mass, and internal magnetic structure of a quiescent solar prominence, modelled as a thin vertical sheet of cool plasma. Firstly, we build up a zeroth-order model and analyse the effects produced by a potential coronal field or a constant- force-free field. This model allows us to obtain the half-width and mass of the prominence for different values of the external field, pressure and shear angle. Secondly, the effects of these external magnetic configurations on a two-dimensional model proposed by Ballester and Priest (1987) are studied. The main effects are a change of the half-width with height, an increase of the mass, a decrease of the magnetic field strength with height and a change in the shape of the magnetic field lines.     

Magnetic field transfer by two-dimensional convection and solar ‘semi-dynamo’

A comparison of the equations for the magnetic field transfer and for the heat transfer by two-dimensional turbulent convection of a conducting compressible medium shows the magnetic field to be transported as a scalar admixture provided it is parallel to the convective rolls. At high magnetic Reynolds numbers the field strength in a convective zone varies proportionally to the density of the medium.A study of the distribution and amplification of the poloidal field in the two-dimensional convection zone of the Sun lying under the supergranulation, together with the processes of field pumping and amplification in other zones, reveals the importance of considering generation mechanisms of thesemi-dynamo type where the amplifying field is excited independently by weak e.m.f.'s of non-electric origin with no feedback which would otherwise produce MHD self-excitation of the field.An illustrative calculation of the solar poloidal field maintained by a weak Coriolis e.m.f. acting in a thin external layer of the convective envelope yields for the general near-polar field, if one somehow takes into account (1) field pumping by three-dimensional supergranulation, (2) field transfer and amplification by two-dimensional convection, and (3) ohmic diffusion of the field into a stable core, a value of the order of 10^–1 gauss.     

Measurements in the solar spectrum between 1400 and 1875 Å with a rocket-borne spectrometer

Intensity measurements in the solar continuum throughout the wavelength range 1400 Å–1875 Å are reported for a rocket flight from White Sands, New Mexico at 16:31 h UT on September 24, 1968. These intensities are approximately a factor of 3 lower than other published estimates and suggest a solar temperature minimum of 4400 K or lower which is in agreement with infrared observations.     

Golf: A resonance spectrometer for the observation of solar oscillations I) Numerical model of the sodium cell response

GOLF (Global Oscillations at Low Frequencies) is an instrument to study the line-of-sight velocity of the solar photosphere, to be flown on the SOHO satellite in 1995. It uses a sodium vapour cell in resonance scattering mode, in order to measure the absolute Doppler shift of the solar sodium absorption lines. We have developed a model of the resonance cell performance. We describe here the main characteristics of the model, and report the most important results concerning the performance of the cell and its dependance on temperature.     

LonGSp: A near infra-red spectrometer

We present a new cooled grating near infra-red spectrometer designed to acquire spectra at the TIRGO telescope in the 0.9÷2.5m region with a resolving power of 300÷4000, equipped with a Rockwell NICMOS 3 detector.     

Height dependence of the amplitudes of solar oscillations viewed with a potassium resonance scattering spectrometer

Over the course of the year, the variation in the solar line-of-sight velocity causes a change in the heights in the photosphere which are analysed by the red and blue passbands of a resonance scattering spectrometer. This in turn would be expected to produce a variation in the ratio of acoustic amplitudes measured in each passband. Using data from the Tenerife spectrometer of the Birmingham Solar Oscillations Network (BiSON), we demonstrate such a seasonal change. We then use the Eddington-Barbier approximation and an atmospheric model to convert the magnitude of this trend in velocity ratio with line-of-sight velocity into an approximate value of 580 km for the velocity-amplitude scale height. However, a simple argument suggests that Doppler imaging causes this to be an overestimate of the true value.     

A faint object spectrometer for the infrared

A cryogenic astronomical spectrometer for the NASA Infrared Telescope Facility is described. This spectrometer will employ an array of at least 20 detectors and provide a resolving power of 100 to 1500 at 1–5 m. The resolving power will be adjustable by changing gratings.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

A two-dimensional multi-band spectroheliograph

The present paper describes a two-dimensional multi-band spectrograph which is located at Yunnan Observatory (25° N 103° E), Kunming. The instrument consists of a coelostat system with an aperture of 40 cm, and spatial resolution better than 1 under excellent seeing, and a spectrograph equipped with a plane reflecting grating. It can simultaneously obtain spectral data in ten bands, including the Balmer lines, metallic lines [Fei 6173 ,D _{1, 2, 3}, Mg, and Caii (H + K)] and Heii 4686 . We are able to control the observational processes by means of a computer and obtain the following data synchronously or quasi-synchronously: multi-band spectra, H filtergrams, magnetic field, and white light. With the aid of H slit-jaw filtergrams, we can determine where and when the spectral data are obtained and, on the basis of analysing the spectral line profiles, we can understand the physical characteristics of an active region and derive the fields of physical parameters. For the line-of-sight velocities, the values measured are as high as hundreds of kilometers per second and as low as a few kilometers per second.     

Data acquisition, control, communication and computation system of solar X-ray spectrometer (SOXS) mission

The Solar X-ray Spectrometer (SOXS) mission onboard GSAT-2 Indian Spacecraft was launched on 08 May 2003 using GSLV-D2 rocket by Indian Space Research Organization (ISRO). SOXS aims to study solar flares, which are the most violent and energetic phenomena in the solar system, in the energy range of 4–56 keV with high spectral and temporal resolution. SOXS employs state-of-the-art semiconductor devices, viz., Si-Pin and CZT detectors to achieve sub-keV energy resolution requirements. In this paper, we present an overview of data acquisition, control, communication and computation of low energy payload of the SOXS mission.     

A space-borne solar stereoscope experiment in solar physics

A space experiment project is proposed, with the main purpose of obtaining 3-dimensional images of the solar atmosphere. We give a list of problems and objectives which can be resolved through the space-borne solar stereoscope.