Lewis L. House

The High Altitude Observatory Coronagraph/Polarimeter on the Solar Maximum Mission

The High Altitude Observatory Coronagraph/Polarimeter, to be flown on the National Aeronautics and Space Administrations Solar Maximum Mission satellite, is designed to produce images of the solar corona in seven wavelength bands in the visible spectral range. The spectral bands have been chosen to specifically exclude or include ‘chromospheric’ spectral lines, so as to allow discrimination between ejecta at high (coronal) and low (chromospheric) temperatures, respectively. In addition, the instrument features spectral filters designed to permit an accurate color separation of the F and K coronal components, and a narrow band (5.5 Å) filter to observe the radiance and polarization of the Fe xiv 5303 Å line. The effective system resolution is better than 10 arc sec and the instrument images a selected quadrant (or smaller field) on an SEC vidicon detector. The total height range that may be recorded encompasses 1.6 to more than 6.0R⊙ (from Sun center). The instrument is pointed independently of the SMM spacecraft, and its functions are controlled through the use of a program resident within the onboard spacecraft computer. Major experimental goals include: (a) Observation of the role of the corona in the flare process and of the ejecta from the flare site and the overlying corona; (b) the study of the direction of magnetic fields in stable coronal forms, and, perhaps, ejecta; and (c) examination of the evolution of the solar corona near the period of solar maximum activity.

The determination of vector magnetic fields from stokes profiles

The application of Unnos (1956) solution of the transfer equation for polarized radiation to the determination of thevector magnetic field is investigated. An analysis procedure utilizing non-linear least squares techniques is developed that allows one to automate the reduction of measured spectral profiles of the Stokes parameters to determine the field angles, strength as well as other parameters. The method is applied to synthetic spectra generated using a model solar atmosphere and yields results of remarkably high accuracy. The influence of additional factors upon determination of the vector field are also considered. These factors include effects of asymmetric profiles, magneto-optical effects, magnetic field gradients, unresolved field elements, scattered light, and instrumental noise.

Studies of the corona with the Solar Maximum Mission coronagraph/polarimeter

The visible wavelength Coronagraph/Polarimeter on the Solar Maximum Mission (SMM) spacecraft is providing data on the flare processes manifested by coronal transients and on the degree of disruption of the evolutionary corona at the present epoch of the solar activity cycle. Among our first results are the discovery of frequent H..cap alpha.. emission from remnants of eruptive prominences in the outer corona and first observations of Fe XIV line emisson to 3.2 R/sub sun/. In the early stages of transients, cavities less dense than the ambient corona are occasionally found trailing the transient loops, with the loops being relatively thick and structureless. Some 22 transients have been identified in the initial survey of 52 days of observations; from this sample our preliminary conclusion is that transients during the SMM era (near solar maximum) occur over a wider range of latitude than, but with about the same range of speeds as, transients during the Skylab era (near solar minimum).

The association of radio noise storm enhancements with the appearance of additional material in the corona

Visible light observations of the corona have been combined with radioheliograph observations at metric wavelengths to examine the dynamic behavior of the corona during and after noise storm onsets and enhancements. For the period studied, the occurrence of such radio events is systematically associated with the addition of coronal material in the vicinity of the radio source. Some of the events correspond to mass ejection transients, but they more frequently represent merely a brightening, which grows in 1 hour or less with the coronal region and remains dense and stable for several hours.

The resonance fluorescence of polarized radiation—I. The general scattering function

The general function describing the scattering of linearly polarized radiation in the presence of magnetic fields and/or anisotropic radiation fields, as derived by Weisskopf,(1) is considered in detail in this paper. The function gives the probability that an incident photon of a given frequency and propagating in a given direction with a specified direction of linear polarization will scatter into any other specified frequency, direction, and polarization direction. Coherence effects arise in weak magnetic fields where there is significant overlap of levels or from hyperfine splitting. The coherence influences the frequency, direction, and polarization redistribution of the scattered radiation; these effects are explicitly taken into account. We have expanded the general scattering function and introduced the appropriate matrix elements so that the redistribution function may be easily applied to any J to J″, electric or magnetic dipole transition. Calculations are also given for the absorption and emission probabilities as well as for a frequency independent scattering function. Application of the general scattering function to a specific transition is given in a subsequent paper.

The resonance flourescence of polarized radiation—II. Scattering in the normal Zeeman triplet (J = 0–J″ = 1)

Abstract The general resonance flourescence scattering function discussed in the previous paper by House is reduced in the present paper to apply specifically to the normal Zeeman triplet. All coherence terms produced by the overlap of levels in weak magnetic fields are retained and calculations are given to illustrate the frequency dependent as well as the frequency independent scattering functions. Equations are derived, again for both the frequency dependent and independent cases, for the angle of maximum linear polarization and the degree of maximum polarization for a single scattering. From this, one may understand the importance of coherence in the interpretation of the measurement of weak magnetic fields, as for example, in solar prominences. The influence of the finite solid angle of the source of radiation (the solar disk) and of the contribution of scattering elements along the line of sight are also considered. Calculations demonstrate how measurements of linear polarization are expected to be influenced particularly by the finite solid angle. Application of the equations to the “strong field” case of the scattering of coronal forbidden lines is also discussed.

Vector magnetic fields in sunspots

AbstractObservations of a round, unipolar sunspot in the Zeeman triplet Fe i λ6302.5 with the High Altitude Observatory Stokes Polarimeter are used to derive the vector magnetic field in the spot. The behavior of the magnitude, inclination, and azimuth of the field vector B across the spot is discussed. A linear relation is found between the continuum intensity Ic and the field magnitude B. Time series obtained in the umbra show significant power in the magnitude of the field at a period of t ∼ 180 s but the other components of the field vector do not display this behavior.

A solar spicule model based upon calcium II K line radiative transfer studies

Monte Carlo radiative transfer techniques are used to develop a height-dependent spicule model based upon a more realistic configuration than has hitherto been considered. The spicule is represented by a uniform cylinder, of finite length, standing vertically upon a plane chromosphere. The observed, limb-darkened, anisotropic chromospheric flux incident upon the cylinder is incorporated into the transfer calculations.The resulting model is characterized by a random, line broadening velocity of 20 km/sec, with electron temperature increasing from 6 × 103 K at the base to about 1.5 × 104 K at 11500 km above the solar surface. The corresponding values of electron density are 8 × 1011 cm-3 and 4 × 1010 cm-3. Contrast curves of the spicule model against the chromospheric background are computed and indicate that spicules should appear both bright and dark on the disk, depending upon their position with respect to the limb, the spectral frequency of observation and the viewing height.

Coronal emission line polarization

A discussion of a program for the computation of coronal emission line polarization is presented. The starting point is a general formulation of the scattering function for magnetic dipole transitions between any two total angular momentum levels, J → J, J ± 1. Illustration of the behavior of the scattering function for different transitions is given. The integration of the scattering function over the solar disk and along the line of sight accounting for arbitrary distribution of magnetic fields as well as an inhomogeneous temperature and density structure of the corona is considered next.Sample results are presented for the numerical computation of the angle of maximum polarization and the degree of maximum polarization to be expected from idealized magnetic field configurations such as radial and dipole. A computation is included for a realistic field configuration predicted to exist at the time of the 1966 eclipse. The magnetic field input to the scattering calculation is based upon the potential field extension of photospheric magnetic fields. It is the purpose of the sample calculations to demonstrate how the measurement of emission polarization measurements can be interpreted in terms of the direction of coronal magnetic fields. Factors which lend ambiguity to such interpreations are clearly illustrated from the examples. These include the Hanle-effect depolarization and the depolarization at the Van Vleck angle.

The resonance fluorescence of polarized radiation—III. The stokes parameter and circulation polarization formulation of the scattering redistribution function☆

Abstract The scattering redistribution function for the resonance fluorescence of polarized radiation in the presence of magnetic fields is established in terms of the Stokes parameter representation, as well as for a circular polarization basis. This analysis extends the treatment by the author of the resonance fluorescence problem where previously only the linear polarization basis had been considered. The redistribution function is treated in generality so coherence effects are included that arise from the sublevels of the scattering atom being not far removed from degeneracy. In the presence of weak fields, these coherence effects alter the frequency, directional and polarization properties of the scattering event. Use is made of the density matrix formalism to arrive at a description of the scattering process in terms of Stokes parameters. For the Stokes parameter representation, the procedure is given for deriving the elements of the Mueller matrix for the resonance fluorescence process. Both frequency dependent and frequency averaged redistribution functions are given for the general case applicable to any dipole transition. In addition for examples, the general equations are reduced to those applicable to the transition J = 0 to J″ = 1.