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NASA NTRS1977

International Sun-Earth Explorer - A three-spacecraft program

The International Sun-Earth Explorer (ISEE) is a three-spacecraft system developed by NASA and ESA to study the magnetospheric structure, paying attention to the quantitative mechanism of magnetospheric response to external perturbations, and the structure of its parts. In particular, the nature, structure, motion, and stability of magnetospheric boundaries will be studied, including bow shock, magnetopause, plasmapause, and neutral sheet. Spacecraft ISEE-A and ISEE-B will be launched into the same orbit (of apogee 23 earth radii) but separated by a small, controllable distance, and will make observations from within the magnetosphere. The ISEE-C spacecraft will be launched into a heliocentric orbit (234 earth radii upstream) and will make observations in the solar wind upstream of the earth.

Ogilvie, K. W.↗
NASA NTRS1977

Observations of the quiet sun at meter and decameter wavelengths

The new TeePee Tee array of the Clark Lake Radio Observatory has been used to observe the quiet sun at 121.5, 73.8 and 26.3 MHz. The equatorial brightness distributions at all three frequencies, and the polar brightness distributions at the two higher ones have been measured. From the observed total fluxes and half-power diameters we have derived the peak brightness temperatures of the solar disk as well as of some sources of the slowly varying component.

Kundu, M. R.↗
NASA NTRS1977

Evolution of open magnetic structures on the sun - The Skylab period

High-resolution harmonic analysis of the measured photospheric magnetic field of the sun is used to construct models of open magnetic structures over a period of 11 solar rotations. The models successfully reproduce the surface location and topology of all coronal holes during the Skylab period. In addition, there is persistent evidence in the models that open field lines are associated with active regions in a systematic way. These associations are listed for the period studied; they suggest that open field lines are a basic feature of solar magnetism. Specific examples of the evolution of coronal holes and of calculated open structures are presented. Quantitative study of the measured field strength within and neighboring a hole confirms the fact that coronal hole regions are indistinguishable by local magnetic properties. However, the calculated field strengths at the footpoints of open field lines within coronal holes show distinct evolutionary patterns and may indicate that, at least in young coronal holes, a significant amount of magnetic flux is closed. Problems of studying magnetic field divergence by using these models are discussed.

Levine, R. H.↗
NASA NTRS1977

The mean magnetic field of the sun - Observations at Stanford

A solar telescope has been built to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph connected to a 22.9-m vertical Littrow spectrograph. Sun-as-a-star integrated-light measurements of the mean solar magnetic field have been made daily since May 1975. The typical mean-field magnitude has been about 0.15 G, with a typical measurement error of less than 0.05 G. The mean-field polarity pattern is essentially identical to the interplanetary-magnetic-field sector structure (seen near earth with a four-day lag). The differences in the observed structures can be understood in terms of a 'warped current sheet' model.

Scherrer, P. H.↗
NASA NTRS1978

Energetic protons associated with interplanetary active regions 1-5 AU from the sun

Pioneer 11 has yielded data on approximately 100 energetic proton events at heliocentric distances between 1 and 2 AU. Measurements of absolute intensities, anisotropies, and crude energy spectra are studied in connection with interplanetary active regions (IAR's). It is found that in close vicinity to IAR's, the number of events observed per unit time interval is 10 times greater than in other areas of interplanetary space, and that the frequency of events has a maximum at plus or minus 5 hours of the time IAR edges are crossed. It is also noted that events in IAR vicinity have greater particle densities, softer energy spectra, and smaller time widths than other events. For many events associated with IAR's, particle anisotropies correspond to the net flow of particles along the interplanetary magnetic field toward the sun. This suggests that a mechanism in MHD shocks is responsible for local acceleration in the interplanetary medium.

Pesses, M. E.↗
NASA NTRS1978

On the accelerations of the moon and sun, the constant of gravitation, and the origin of mountains

On the assumption of a constant value for the gravitational constant G, the lunar and solar tidal couples and the rate of change of angular velocity of the earth are recalculated on the basis of the improved values for the apparent accelerations of the moon and sun arrived at by Muller and Stephenson (1977). These new values are then used to derive general dynamical equations with a changing moment of inertia and changing G connecting the apparent accelerations to the lunar and solar couples. The effect of a changing G is considered, and a value of (dG/dt)/G = -3 times 10 to the -11th power per yr can be reconciled only weakly with the improved accelerations, especially for the case of linear friction. For quadratic friction the situation is more favorable. The new accelerations, however, are best reconciled with a nonchanging G, and certainly cannot be reconciled with a value of (dG/dt)/G = -6 times 10 to the -11th power per yr.

Lyttleton, R. A.↗
NASA NTRS1977

The correction of Landsat data for the effects of haze, sun angle, and background reflectance

A technique has been developed for simulating the effects of haze, sun angle, and background reflectance in Landsat data and correcting for them. The atmospheric model assumes a two-layer atmosphere: a Rayleigh scattering molecular layer and a Mie scattering haze layer next to the earth's surface. Reflection and transmission matrices describe the reflection and transmission properties of the plane parallel scattering layers. The multispectral scanner response is computed for various values of the parameters under evaluation. This yields expressions for Landsat gray-scale levels used for determining the effect of changes in any parameter. The Atmospheric Correction computer program is used to determine the haze level from the data, to compute the reflectance, and to interpolate in order to find the correction coefficients necessary to make the desired correction.

Potter, J. F.↗
NASA NTRS1978

Sun tracking solar energy collector

A parabolic reflector is supported so that it can track the sun. The support for this reflector comprises an azimuth frame supported on two wheels and a central pivotal point which are positioned in a substantially triangular configuration. On top of the azimuth frame, there is provided an elevation frame. The reflector rides on wheels captured within curved rails. The wheels of the azimuth frame are driven by an azimuth actuator. The reflector structure is counterbalanced about its elevation axis by a pendulum cable system which is driven by a motor. At the focal point of the parabolic reflector, a heat engine or receiver is mounted independently on the reflector. Suitable means are provided for moving the reflector about its two axes.

Perkins, G. S.↗
NASA NTRS1978

Limb darkening 1945-3245 A for the quiet sun from Skylab data

Limb-darkening measurements of the quiet sun in the spectral range 1945-3245 A have been made using NRL's SO82B instrument on board Skylab. The spectral intensities have been averaged over 10 A wavelength bands, and coefficients for linear, quadratic, and logarithmic fits to the observed center-to-limb variations have been calculated and tabulated as a function of wavelength. The quadratic fits are found to represent the observations best, the observed and calculated values agreeing to better than plus or minus 15% on the average. Attention is drawn to the value of the results for ultraviolet photometry of eclipsing binaries.

Moe, O. K.↗
NASA NTRS1978

The strength of the sun's polar fields

The magnetic field strength within the polar caps of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. This paper reports measurements of the polar fields made at the Stanford Solar Observatory using the Fe I line at 525.02 nm. It is found that the average flux density poleward of 55 deg latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 x 10 to the 14th Wb. It is also shown that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields.

Svalgaard, L.↗
NASA NTRS1979

The Balmer 9 and Balmer 11 lines of He II in the sun

The Balmer 9 and 11 lines of He II at 959 A and 942 A in solar spectra are identified. These lines are produced mainly by recombination following photoionization of He II by coronal XUV radiation. From analysis of the line intensities, the theoretical model of Avrett et al. (1976), is confirmed where it was found that an appreciable amount of He (2+) is present at temperatures of 1 - 2 x 10 to the 4th K and that the anomalously strong He II 304 line is produced primarily by collisional excitation. The suggestion of Kohl (1977) that the photoionization-recombination process is more important in active regions than in the quiet sun is confirmed, and we find that the 304-A line is produced largely by recombination in solar flares.

Raymond, J. C.↗
NASA NTRS1979

Jovian electron propagation close to the sun /at about 0.5 AU/

On the basis of interplanetary electron flux measurements in the energy range 0.7-25 MeV made on the Mariner 10 spacecraft launched in 1973 for successive encounters with planet Mercury (R approximately equal 0.5 AU), it is shown that the dominant interplanetary flux of electrons close to the sun is Jovian in origin (except for times of solar flares) and that these observations are consistent with a three-dimensional diffusion-convection model which describes Jovian electron propagation in the outer solar system.

Eraker, J. H.↗
NASA NTRS1979

Coupling processes related to the Sun-weather problem

Physical mechanisms for coupling the energetics of solar activity to meteorological responses are reviewed. Although several hypotheses have been advanced, none can be said to be sufficiently complete to be applied to weather or climate prediction. Solar activity indicators potentially useful for forecasting are identified, including sunspots, solar flares, and magnetic sector boundary crossings. Additional experiments, studies, and analyses are required before Sun-weather concepts can be utilized for predicting meteorological responses.

Goldberg, R. A.↗
NASA NTRS1979

Planetary waves and Sun-weather effects

A brief outline about the theory of planetary waves is given and a review of space-time analysis, mainly at the 500 mbar pressure level, is presented. This analysis gives evidence that broad spectral bands of two types of waves exist within the troposphere: ultralong waves with zonal wave numbers M or approximately equal to 4 and periods tau or approximately equal to 5 days, propagating mainly to the west, and synoptic scale waves with M or approximately equal to 3 and tau or approximately equal to 10 days, propagating mainly to the east. These waves are generated by internal turbulent processes within the atmosphere and are quasi-persistent with lifetimes of several periods. It is shown that solar activity cannot generate planetary waves of significant amplitudes, and that the observed 'Sun-weather effects' can be interpreted within the framework of these internally generated planetary waves without any trigger mechanism from outside the atmosphere. It is suggested that a better knowledge of these persistent ultralong waves may help to improve long range weather forecasts.

Volland, H.↗
NASA NTRS1980

Utilization of multi-body trajectories in the Sun-Earth-Moon system

An overview of three uncommon trajectory concepts for space missions in the Sun-Earth-Moon System is presented. One concept uses a special class of libration-point orbits called 'halo orbits.' It is shown that members of this orbit family are advantageous for monitoring the solar wind input to the Earth's magnetosphere, and could also be used to establish a continuous communications link between the Earth and the far side of the Moon. The second concept employs pretzel-like trajectories to explore the Earth's geomagnetic tail. These trajectories are formed by using the Moon to carry out a prescribed sequence of gravity-assist maneuvers. Finally, there is the 'boomerang' trajectory technique for multiple-encounter missions to comets and asteroids. In this plan, Earth-swingby maneuvers are used to retarget the original spacecraft trajectory. The boomerang method could be used to produce a triple-encounter sequence which includes flybys of comets Halley and Tempel-2 as well as the asteroid Geographos.

Farquhar, R. W.↗
NASA NTRS1980

Sun Series program for the REEDA System

Modifications made to data bases and to four programs in a series of computer programs (Sun Series) which run on the REEDA HP minicomputer system to aid NASA's solar activity predictions used in orbital life time predictions are described. These programs utilize various mathematical smoothing technique and perform statistical and graphical analysis of various solar activity data bases residing on the REEDA System.

Shankle, R. W.↗
NASA NTRS1979

Optical spectrophotometry of comet Tempel 2 far from the sun

With the advent of mission planning for spacecraft encounters with comets in the 1980s, the lack of basic data on cometary nuclei has become quite apparent and a real problem. More effort is needed on all comets, with special attention given to probable space flight targets such as Tempel 2. The paper presents medium-spectral-resolution spectrophotometry of P/Tempel 2, made at heliocentric distances of 2.66 and 3.02 AU on two dates in 1978. It is shown that optical spectrophotometry of comet Tempel 2 near 3 AU heliocentric distance places strict limits on its activity, the modeled CN production rate having an upper limit of 6 times 10 to the 21st molecules per sec. The observations of P/Tempel 2 are compatible with a bare nucleus of 1.6 km radius (geometric albedo = 0.15) and somewhat redder than the sun.

Spinrad, H.↗
NASA NTRS1979

Making real-time sun reflectance measurements with a microprocessor-based spectroradiometer

A standard high-speed, field-portable spectroradiometric measurement system built around a programmable microprocessor has been adapted to the form of a Reflectometer/Comparator. In this configuration, the instrument makes passive measurements of the absolute reflectance of agricultural plant canopies over a spectral range of 0.4 to 2.5 micrometers. Real-time absolute measurements are made possible by an optical chopper which constantly compares the target with the sun, and makes extensive measurements of solar reflectance from a variety of these targets. The paper describes the instrumentation and measurement procedures, reviews the software programming and discusses the results.

Zweibaum, F. M.↗