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Foukal, P. V.

Publications and source records attributed to Foukal, P. V..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Solar irradiance variability from modern measurements

Direct measurements from satellites of the solar 'constant' (the total irradiance at mean sun-earth distance) during more than ten years show variations over time scales from minutes to years and decades. At high frequencies, solar oscillations contribute to the variance. The most important influences are related to solar activity: during the passage of active regions on the solar disk (sunspots and faculae) changes of a few 0.1 percent lasting for several days are observed. The effects of spots can be well reproduced by the projected sunspot index, whereas the influence of faculae have to be modeled from proxy data like the Ca-K plage index or the He I index. Long-term trends are detected which are connected to the 11-yr solar activity cycle.

Froehlich, C.↗
NASA NTRS1987

A photometric search for solar giant convection cells

The photometric contrast of solar giant convection cells is limited using 526.6 nm continuum images obtained on 15 days in May 1985. The rms of the giant cell intensity pattern must be less than or equal to the observed rms on spatial scales of 80 to 240 Mm, which is 0.023 percent or, equivalently, 0.33 K. However, the spatial scale and time-scale dependence of the variance demonstrate that giant cells are not the source of the observed variance. Consequently, a tighter constraint on the rms of the giant cell pattern may be placed, namely 0.016 percent or 0.23 K. This limit is consistent with temperature perturbations estimated from recent nonlinear simulations of global-scale solar convection. This limit on the rms of the giant cell pattern is used to estimate that the contribution of giant cells to the fluctuation of the solar irradiance on a one-month time-scale is less than 0.00003 S.

Chiang, W.-H.↗
NASA NTRS1980

A study of acoustic heating and forced convection in the solar corona

The S055 EUV spectra was used to perform emission measure and line intensity ratio analyses of loop plasma conditions in a study on the thermodynamics of magnetic loops in the solar corona. The evidence that loops contain plasma hotter than the background corona, and thus, require enhanced local dissipation of magnetic or mechanical energy is discussed. The S055 EUV raster pictures were used to study physical conditions in cool ultraviolet absorbing clouds in the solar corona, and optical data were used to derive constraints on the dimension, time scales and optical depths in dark opaque clouds not seen in H alpha and CaK as filaments or prominences. Theoretical modelling of propagation of magnetically guided acoustic shocks in the solar chromosphere finds it still unlikely that high frequency acoustic shocks could reach the solar corona. Dynamic modelling of spicules shows that such guided slow mode shocks can explain the acceleration of cool spicular material seen high in the corona.

Foukal, P. V.↗
NASA NTRS1977

The effect of sunspots and faculae on the solar constant

We study the available measurements of the solar constant made at ground sites and from recent space observations to determine whether sunspots or faculae produce a detectable modulation of either the solar flux or the earth's atmospheric transmission. The data from radiometers on Mariners 6 and 7 rule out any relative change of the solar constant in space due directly to faculae or spots exceeding 0.03%. This limit is two orders of magnitude smaller than previous values obtained from ground measurements. The measurements made at mountain stations of the Smithsonian Astrophysical Observatory between 1923 and 1952 show a marginally significant increase of solar constant at the level of 0.1%, related specifically to high projected facular area. Since this increase is not seen in the space measurements, we suggest that it may reflect a change in the earth's atmospheric transmission, possibly due to a change in ozone concentration induced by variation of facular ultraviolet radiation.

Foukal, P. V.↗
NASA NTRS1976

The pressure and energy balance of the cool corona over sunspots

The 22 largest sunspots observed with the Skylab SO55 spectrometer are studied for a relation between their EUV radiation and their umbral size or magnetic classification. The ultimate goal is to determine why the coronal plasma is so cool over a sunspot and how this cool plasma manages to support itself against gravity. Based on the time behavior of the EUV emission, a steady-state model is developed for the pressure and energy balance of the cool coronal-plasma loops over the spots. Analysis of the temperature structure in a typical loop indicates that the loop is exceedingly well insulated from the outside corona, that its energy balance is determined purely by internal heating and cooling processes, and that a heat input of about 0.0001 erg/cu cm per sec is required along the full length of the loop. It is proposed that: (1) coronal material flows steadily across the field lines at the tops of the loops and falls downward along both sides under gravity; (2) the corona is heated by mechanical-energy transport across the very thin transition region immediately over network-cell interiors; and (3) strong magnetic fields tend to inhibit mechanical-energy dissipation in the corona.

Foukal, P. V.↗
NASA NTRS1976

Plasma diagnostic techniques in the ultraviolet - The C III density-sensitive lines in the sun

Spectra and spectroheliograms of the C III transitions at 977 and 1176 A are obtained with the Harvard extreme-ultraviolet spectrometer on Skylab. Analysis of the intensities of these lines, and of their density-sensitive ratio, indicates a wide range of temperature gradients and electron densities in the transition region of various solar features. From values of the observed ratio, we suggest necessary revisions to the excitation rates, and propose a relationship between the ratio and density. The significantly higher ratio found in active regions indicates a density increase of about a factor 2 relative to the network. In the quiet sun, there is no significant difference in density between network and cell interiors, but the uncertainty is as large as a factor 3. The very central 10% of the areas of cell interiors shows a significantly higher density than the mean value for cell interiors.

Dupree, A. K.↗
NASA NTRS1976

Extreme-ultraviolet transients observed at the solar pole

Extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s. On the basis of a model developed from the EUV measurements, it is found that the energy required to produce each event is about 3 by 10 to the 26th power ergs, about two orders of magnitude more than that required to produce an ordinary spicule. This indicates that macrospicules may be an important factor in the energy balance of the chromosphere and corona.

Withbroe, G. L.↗
NASA NTRS1976

Initial results from the EUV spectroheliometer on ATM

The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to February 7, 1974. During this period, many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined, and the principal scientific results from a preliminary analysis of the data are described.

Reeves, E. M.↗
NASA NTRS1975

Time variations in extreme-ultraviolet emission lines and the problem of coronal heating

We have analyzed the time structure of the intensity of solar chromospheric and coronal extreme-ultraviolet lines, obtained by the Harvard College Observatory spectrometer aboard Skylab. We find changes in the intensity of up to 50 percent in times as short as 1 minute, but not periodic oscillations. Some evidence is found for the presence of shock waves in the chromosphere and the transition region. It is suggested that the solar chromosphere and corona are heated by nonperiodic waves.

Vernazza, J. E.↗
NASA NTRS1975

EUV observations of the active sun from the Harvard experiment on ATM

Some extreme UV observations of solar active regions made with a scanning spectroheliometer are described. Spectroheliograms constructed from digital data using a computer-driven cathode-ray tube display show clearly how the appearance of an active region changes as a function of temperature. Flare studies indicate that the impulsive rise in EUV emission occurs essentially simultaneously at all levels from the transition zone to the corona. Observations of sunspots reveal a very intense emission in transition zone lines. A matrix of Mg x rasters covering the entire sun reveals several hundred bright points having dimensions of 30 arc seconds or less. Other observations include coronal holes and prominences.

Noyes, R. W.↗
NASA NTRS1974

Extreme-ultraviolet observations of coronal holes - Initial results from Skylab

We compare the appearance and physical parameters of the solar chromosphere, transition zone, and corona in areas of coronal holes with that of quiet areas outside the hole. Measurements of the height of emission of various ions in a coronal hole appearing at the polar limb give a quantitative indication of the increased thickness of the transition zone underlying coronal holes.

Huber, M. C. E.↗
NASA NTRS1974

Solar prominences in the extreme ultraviolet as observed from the Apollo Telescope Mount

Observations of quiescent solar prominences with the Harvard College Observatory spectrometer aboard Skylab show that prominence material is optically thick in the Lyman alpha line and the Lyman continuum. The color temperature of the Lyman continuum has a mean of 6600 K and an upward gradient toward the top of the prominence. The departure coefficient of the ground state of hydrogen is found to be of the order of unity, as expected from theory. The optical depth of the C III sheath region is determined directly from the observation of the limb through the prominence and is used to infer the mean electron density and the temperature gradient of the sheath. The result implies that the sheath density is about 0.4, and the temperature gradient about 1.4 times the respective value in the C III transition zone of the quiet sun. The C III triplet-singlet ratio for the prominence is found to give a density compatible, within the uncertainty of the atomic parameters, with the density obtained from the optical depth.

Schmahl, E. J.↗
NASA NTRS1974

Initial results from the EUV spectroheliometer on ATM

The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to Feb. 7, 1974. During this period many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined and the principal scientific results from a preliminary analysis of the data are described.

Reeves, E. M.↗
NASA NTRS1974

Observations of the chromospheric network - Initial results from the Apollo Telescope Mount

A preliminary analysis of early data taken by the HCO spectrometer on Skylab shows that the solar chromospheric network can be clearly seen with varying contrast in the extreme-ultraviolet emission characteristic of temperatures between 10,000 K (the Lyman continuum) and 300,000 K (O VI). In the emission of Mg X, a coronal line formed at about 1,500,000 K, the network is generally unrecognizable. This is interpreted as being due to a spreading of the magnetic field lines of the network boundary in the height interval corresponding to the temperature difference between 300,000 and 1,500,000 K. We note that in certain anomalous cases, bright points of the network are seen to extend with high contrast and essentially unchanged in their cross-section through the full range of temperatures characteristic of the chromosphere, transition region, and low corona.

Reeves, E. M.↗