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Author name code: schoolman
ADS astronomy entries on 2022-09-14
author:"Schoolman, Stephen A."
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Title: Low-resolution MG II H and K observations of solar type stars.
Authors: Barry, D. C.; Schoolman, S. A.
1982ApJ...261..220B Altcode:
Observations with the International Ultraviolet Explorer (IUE) have
made possible a comparison between chromospheric radiative loss rates in
the Mg II h and k lines and the Ca II H and K lines. Observations made
by Stencel et al. (1980) confirm the findings of Linsky et al. (1979)
that the chromospheric radiative loss rates in h and k and H and K are
about equal for supergiants, but the h and k loss rate is greater than
the H and K loss rate in giants. It is pointed out that this may result
from physical differences between the lower chromospheres of giants and
supergiants. The present investigation has the objective to explore
the possibility of determining relative chromospheric radiative loss
rates in F and G dwarf stars with low-resolution IUE spectra. It is
found that this is possible. Cluster stars were selected to provide
two temperature sequences at two slightly different metal abundances.
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Title: Chromospheric decay and the ages of solar type stars.
Authors: Barry, D. C.; Cromwell, R. H.; Hege, K.; Schoolman, S. A.
1981ApJ...247..210B Altcode:
A correlation is reported between the ages of solar type stars in
six clusters and their K and H line central depths. This correlation
suggests that chromospheric activity is being measured. The new method
includes simple spectral measurements requiring lower resolution and
consequently shorter observing times than previous methods. An effect on
the H and K lines caused by the time decay of chronomospheric activity
in solar type stars is illustrated in a graph. To the degree that the
fraction of chromospheric energy losses through the H and K features
remains constant in time, and the degree to which the response of KH0
to atmospheric structural changes is linear, these curves illustrate
the decay of the chromospheric activity itself. It is generally
believed that chromospheric activity is related to stellar rotational
velocity. With the above reservations, and the assumption that the
mean chromospheric activity level varies linearly with rotation,
the graph indicates also the time rate of decrease of the rotational
velocity for a solar type star.
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Title: Hα Flare Spectra
Authors: Schoolman, S. A.; Ganz, E. D.
1981SoPh...70..363S Altcode:
We have studied over 1000 Hα flare profiles from 24 solar flares
of Class 1 or brighter as recorded with Lockheed Solar Observatory's
multi-slit spectrograph. We find that the profiles can be classified
in terms of whether or not they show central self-reversals, whether
or not they have extended wings, and whether they are asymmetric. The
profile groupings were found not to be correlated with the size (class)
of the flares, nor do they seem to be well correlated with the kind
of chromospheric flare structure from which they are emitted. Although
different flare points within the same overall event may show profiles
of different types, any given element (bright patch) can usually be
described in terms of a single combination of the profile descriptors.
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Title: Observations of the 1980 April 30 limb flare by the ultraviolet
spectrometer and polarimeter on the Solar Maximum Mission
Authors: Woodgate, B. E.; Shine, R. A.; Brandt, J. C.; Chapman,
R. D.; Michalitsianos, A. G.; Kenny, P. J.; Bruner, E. C.; Rehse,
R. A.; Schoolman, S. A.; Cheng, C. C.
1981ApJ...244L.133W Altcode:
Observations of the M2 limb flare of 1980 April 30 by the ultraviolet
spectrometer and polarimeter in the C IV 1548 A line are described
and compared with observations from other SMM instruments and with
ground-based H-alpha data. Events observed during the 18 minutes leading
up to the flare impulsive phase include the filling of a small loop
with material moving at about 20 km/s, followed by a rapid brightening
in C IV, H-alpha, and hard X-rays, with a subsequent brightening of
a higher set of loops. The rapid brightening appears to be at the
junction of the small loop with the overlying magnetic structures,
which suggests the flare may be caused by their interaction.
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Title: Solar maximum mission experiment: Ultraviolet spectroscopy
and polarimetry on the solar maximum mission
Authors: Tandberg-Hanssen, E.; Cheng, C. C.; Woodgate, B. E.; Brandt,
J. C.; Chapman, R. D.; Kenney, P. J.; Michalitsianos, A. G.; Shine,
R. A.; Athay, R. G.; Beckers, J. M.; Bruner, E. C.; Rehse, R. A.;
Schoolman, S. A.; Gurman, J. B.; Hyder, C. L.; Henze, W.
1981AdSpR...1m.275T Altcode: 1981AdSpR...1..275T
We describe the Ultraviolet Spectrometer and Polarimeter (UVSP) on the
Solar Maximum Mission (SMM) spacecraft. The instrument, which operates
in the wavelength range 1150 - 3600 Å, has a spatial resolution of 2-3
arc sec and a spectral resolution of 0.02 Å FWHM in second order. A
Gregorian telescope, focal length 1.8 m, feeds a 1 m Ebert-Fastie
spectrometer. A polarimeter comprising rotating Mg F<SUB>2</SUB>
waveplates can be inserted behind the spectrometer entrance slit and
allows all four Stokes parameters to be determined. The observing
modes include rasters, spectral scans, velocity measurements, and
polarimetry. Finally, we present examples of initial observations made
since launch.
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Title: Solar Maximum Mission experiment: ultraviolet spectroscopy
and polarimetry on the Solar Maximum Mission.
Authors: Tandberg-Hanssen, E.; Woodgate, B. E.; Athay, R. G.; Beckers,
J. M.; Brandt, J. C.; Bruner, E. C.; Chapman, R. D.; Cheng, C. -C.;
Gurman, J. B.; Hyder, C. L.; Kenney, P. J.; Michalitsianos, A. G.;
Rehse, R. A.; Schoolman, S. A.; Shine, R. A.; Henze, W.
1981hea..conf..275T Altcode:
No abstract at ADS
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Title: The Development of Flares Observed in the Spectral Lines OV
1371 Å, FeXXI 1354 Å, and in Hard X-Rays
Authors: Woodgate, B. E.; Bruner, E. C.; Cheng, C. C.; Dennis, B. R.;
Gurman, J. B.; Frost, K. J.; Hyder, C. L.; Kiplinger, A.; Mason,
H. E.; Orwig, L. E.; Poland, A. I.; Schoolman, S. A.; Shine, R. A.;
Tandberg-Hanssen, E. A.
1980BAAS...12..911W Altcode:
No abstract at ADS
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Title: Status of the Spacelab 2 Solar Optical Universal Polarimeter
(SOUP)
Authors: Tarbell, T. D.; Finch, M. L.; Ramsey, H. E.; Schoolman,
S. A.; Title, A. M.
1980BAAS...12..915T Altcode:
No abstract at ADS
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Title: Transition Region Oscillations in Sunspots
Authors: Gurman, J. B.; Shine, R. A.; Woodgate, B. E.;
Tandberg-Hanssen, E. A.; Bruner, E. C.; Schoolman, S. A.; Athay, R. G.
1980BAAS...12..906G Altcode:
No abstract at ADS
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Title: SMM/UVSP Observations of Magnetic Fields in the Transition
Region above Sunspots
Authors: Henze, W.; Beckers, J. M.; Gurman, J. B.; Hyder, C. L.;
Schoolman, S. A.; Shine, R. A.; Tandberghanssen, E.; Woodgate, B. E.;
Hagyard, M. J.
1980BAAS...12R.896H Altcode:
No abstract at ADS
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Title: Sunspot Observations with the Ultraviolet Spectrometer and
Polarimeter Experiment on the Solar Maximum Mission
Authors: Gurman, J. B.; Woodgate, B. E.; Shine, R. A.; Brandt, J. C.;
Chapman, R. D.; Michalitsianos, A. G.; Kenny, P. J.; Bruner, E. C.;
Rehse, R.; Schoolman, S. A.; Cheng, C. C.; Tandberg-Hanssen, E. A.;
Athay, G. R.; Beckers, J. M.; Henze, W.; Brown, Teledyne; Hyder, C. L.
1980BAAS...12..535G Altcode:
No abstract at ADS
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Title: Density Diagnostic of Solar Active Region and Flare Plasmas
from Si IV/O IV Line Ratio as Observed from SMM
Authors: Bruner, E. C.; Rehse, R.; Schoolman, S. A.; Brandt, J. C.;
Chapman, R. D.; Kenny, P. J.; Michalitsianos, A. G.; Shine, R. A.;
Woodgate, B. E.; Cheng, C. C.; Tandberg-Hanssen, E. A.; Athay, G. R.;
Beckers, J. M.; Gurman, J.; Henze, W.; Brown, Teledyne; Hyder, C. L.
1980BAAS...12R.539B Altcode:
No abstract at ADS
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Title: Density diagnostic of solar active region and flare plasmas
from Si IV/O IV line ratio as observed from SMM (Solar Maximum
Mission).
Authors: Bruner, E. C.; Rehse, R.; Schoolman, S. A.; Brandt, J. C.;
Chapman, R. D.; Kenny, P. J.; Michalitsianos, A. G.; Shine, R. A.;
Woodgate, B. E.; Cheng, C. -C.; Tandberg-Hanssen, E. A.; Athay, G. R.;
Beckers, J. M.; Gurman, J. B.; Henze, W.; Hyder, C. L.
1980BAAS...12..534B Altcode:
No abstract at ADS
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Title: Solar Flare and Surge Image Sequences as Seen by the
Ultraviolet Spectrometer and Polarimeter on SMM
Authors: Henze, W.; Brown, Teledyne; Brandt, J. C.; Chapman,
R. D.; Kenny, P. J.; Michalitsianos, A. G.; Shine, R. A.; Woodgate,
B. E.; Bruner, E. C.; Rehse, R.; Schoolman, S. A.; Cheng, C. C.;
Tandberg-Hanssen, E. A.; Athay, G. R.; Beckers, J. M.; Gurman, J.;
Hyder, C. L.
1980BAAS...12..532H Altcode:
No abstract at ADS
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Title: The Ultraviolet Spectrometer and Polarimeter (UVSP) on the
Solar Maximum Mission and Initial Results in Polarimetry
Authors: Tandberg-Hanssen, E.; Athay, R. G.; Bruner, E. C.; Beckers,
J. M.; Brandt, J. C.; Chapman, R. D.; Cheng, C. C.; Gurman, J.;
Henze, W.; Brown, Teledyne; Hyder, C. L.; Michalitsianos, A. G.;
Shine, R. A.; Schoolman, S. A.; Woodgate, B. E.
1980BAAS...12..534T Altcode:
No abstract at ADS
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Title: The Dynamics of Solar Flares and Surges as Seen at the Solar
Limb in the Transition Zone
Authors: Woodgate, B. E.; Brandt, J. C.; Chapman, R. D.; Kenny,
P. J.; Michalitsianos, A. G.; Shine, R. A.; Bruner, E. C.; Rehse, R.;
Schoolman, S. A.; Cheng, C. C.; Tandbert-Hanssen, E. A.; Athay, G. R.;
Beckers, J. M.; Gurman, J.; Henze, W.; Brown, Teledyne; Hyder, C. L.
1980BAAS...12Q.535W Altcode:
No abstract at ADS
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Title: Active Region Morphology and Evolution Images from the
Ultraviolet Spectrometer and Polarimeter
Authors: Shine, R. A.; Brandt, J. C.; Chapman, R. D.; Kenny, P. J.;
Michalitsianos, A. G.; Woodgate, B. E.; Bruner, E. C.; Rehse, R.;
Schoolman, S. A.; Cheng, C. C.; Tandberg-Hanssen, E. A.; Athay, G. R.;
Beckers, J. M.; Gurman, J.; Henze, W.; Brown, Teledyne; Hyder, C. L.
1980BAAS...12R.531S Altcode:
No abstract at ADS
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Title: Radiative energy output of the 5 September 1973 flare
Authors: Canfield, R. C.; Cheng, C. -C.; Dere, K. P.; Dulk, G. A.;
McLean, D. J.; Schmahl, E. J.; Robinson, R. D., Jr.; Schoolman, S. A.
1980sfsl.work..451C Altcode: 1980sofl.symp..451C
Measurements of the radiative energy output of the solar flare of
Sept. 5, 1973, over a wavelength range of more than ten decades,
from below 1 A to above 1 m are presented. Observations of soft
X-rays (0.5-20 A), XUV and EUV lines (171-1863 A) and EUV continua
(1400-1960 A), H alpha radiation, visible lines and continua (3700-8700
A) and radio emission (centimeter to meter wavelengths) were obtained
concurrently by Skylab and ground-based instruments. Estimates of power
output at flare maximum are obtained for the observed wavelengths with
uncertainty of at least half an order of magnitude, due to corresponding
uncertainties in EUV and visible fluxes. Taking into account energy
radiated at unobserved wavelengths and the characteristic time of
the best-reduced data (the soft X-ray), calculations indicate a total
radiated flare energy of approximately 4 x 10 to the 29th erg.
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Title: Using Tunable Filters for Two-Dimensional Solar Spectroscopy
Authors: Ramsey, H. B.; Schoolman, S. A.; Smithson, R. C.; Tarbell,
T. D.; Title, A. M.
1979BAAS...11..640R Altcode:
No abstract at ADS
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Title: Ages of Solar-Type Field Stars From 4 A Resolution Spectra
Authors: Barry, D.; Cromwell, R.; Hege, K.; Schoolman, S.
1979BAAS...11..647B Altcode:
No abstract at ADS
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Title: Spectroscopic comparison of open clusters. II. The reddening,
blanketing, and metallicity of NGC 2264.
Authors: Barry, D. C.; Cromwell, R. H.; Schoolman, S. A.
1979ApJS...41..119B Altcode:
The spectral quantification procedure is used to determine the ratios
of the metallic line blanketing of selected members of the young open
cluster NGC 2264 to that of Hyades and Coma Cluster members. Intrinsic
colors are determined by two independent analyses, one involving the
Balmer lines and the other involving metallic features. The internal
scatter in the determinations of both Fe/H and intrinsic colors is found
to be remarkably small. Several interesting correlations are discovered
which provide information on the nature of pre-main-sequence stars and
demonstrate the value of quantitative spectroscopy even at moderate
resolution. Specifically, the results show that: (1) the metallicity
of NGC 2264 is the same as that of the Coma Cluster to within a
few hundredths dex; (2) there is not evidence of an increase in the
heavy-element abundance at the sun's galactocentric distance during the
last 5 billion years; and (3) B-V color excesses are correlated with the
distance (in magnitude) of an NGC 2264 member above the main sequence.
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Title: In search of the perfect magnetogram.
Authors: Ramsey, H. E.; Schoolman, S. A.; Smithson, R. C.; Tarbell,
T. D.; Title, A. M.
1979BAAS...11..611R Altcode:
No abstract at ADS
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Title: Weak and strong magnetic fields in the solar photosphere.
Authors: Tarbell, T. D.; Title, A. M.; Schoolman, S. A.
1979ApJ...229..387T Altcode:
Very high-resolution (0.5 arcsec) magnetograms of the quiet sun
and a plage are discussed which were obtained by using a tunable
birefringent filter in Fe I 6302A. A search for a turbulent bipolar
field with the use of co-added and spatially filtered frames is
unsuccessful. Statistical analysis sets an upper limit of 50 gauss on
the rms vertical component of such a field and probably rules out the
possibility of field strengths exceeding 100 gauss in the inner network
field observed at Kitt Peak. The area, total flux, and energy content
of the strong (kilogauss) network fields are measured and compared with
the upper limits for these properties of a hypothetical widespread
weak field. In the quiet photosphere, a weak background field may
contain interesting amounts of flux and energy, but the strong fields
are dominant at higher levels and in the plage. The total magnetic
energy in the quiet photospheric is roughly equal to the kinetic
energy of granular and oscillatory velocities at the same level. By
flux conservation, field strengths in the transition region are greater
than 25 gauss in the quiet network and 100 gauss in the plage.
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Title: The ratio of iron to hydrogen abundances in the young cluster
NGC 2264.
Authors: Barry, D. C.; Cromwell, R. H.; Schoolman, S. A.
1978PASP...90..491B Altcode:
No abstract at ADS
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Title: The solar metallicity and colors from spectral quantification.
Authors: Barry, D. C.; Cromwell, R. H.; Schoolman, S. A.
1978ApJ...222.1032B Altcode:
The method of spectral quantification is used to determine the UBV
colors of the sun. The summed strength of several Balmer lines and
the B-V color for solar-type stars in the Hyades are correlated with a
measure of the solar Balmer-line strength, yielding the color the sun
would have if it had the Hyades blanketing. The color the sun would
have for the blanketing of Coma stars is obtained from observations
of such stars, and a similar procedure is employed to obtain two
preliminary solar U-B colors appropriate for the Hyades and Coma
blanketing, respectively. Direct measures of the strength of various
metallic lines in the solar spectrum are then compared with those in
the Hyades and Coma spectra to determine the proper interpolation for
finding the colors appropriate for the actual solar blanketing. It
is shown that all significant differences between the reported UBV
colors and those available in previous literature are eliminated when
the latter are corrected for the observed solar line blanketing. The
results indicate that the solar spectrum, which defines spectral type
G2 V, apparently has UBV colors typical of stars classified about 0.1
spectral type later.
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Title: On the size, structure, and strength of the small-scale solar
magnetic field.
Authors: Ramsey, H. E.; Schoolman, S. A.; Title, A. M.
1977ApJ...215L..41R Altcode:
High-resolution magnetograms place an upper limit of 0.33 arcsec on
the smallest magnetic-field structures. These magnetograms show that
the active-region field is organized into roughly cellular patterns 2-3
arcsec in diameter and that the field structures occur in the centers of
'abnormal' granules. Comparison of these data and other magnetograms
with high signal-to-noise ratio indicates that there exists another
component of the field that is diffuse on the scale of an arc second
and has a maximum strength of less than 500 gauss.
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Title: Spectral quantification.
Authors: Barry, D. C.; Cromwell, R. H.; Schoolman, S. A.
1977ApJ...212..462B Altcode:
A fast, automated data reduction procedure has been developed for
quantitatively measuring stellar spectra to determine the fundamental
stellar parameters of temperature, luminosity, and metallicity. The
spectral quantification technique is an attempt to combine the
advantages of the various photoelectric photometry systems and spectral
classification while reducing or eliminating their disadvantages. The
speed and accuracy of the system depend on the choice of criteria
and the method of quantifying them, as well as on the detector. The
spectral quantification indices may be defined so as to be unaffected
by interstellar reddening and spectral emission features, a distinct
advantage over most photoelectric indices. The capabilities of the
technique are demonstrated through the quantification of image-tube
spectrograms of F and G stars. The present system is found to be
comparable in speed and accuracy to the uvbyfl photometry system. In
principle, a properly planned spectral quantification system can
accurately determine any fundamental stellar parameter obtainable from
the intensity versus wavelength record. Subfrct headings: photometry -
spectrophotometry - stars: spectral classification
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Title: The Lockheed Universal Filter.
Authors: Title, A. M.; Ramsey, H. E.; Schoolman, S. A.
1976BAAS....8R.535T Altcode:
No abstract at ADS
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Title: The Fine Scale Magnetic Structure of Plages.
Authors: Schoolman, S. A.; Title, A. M.; Ramsey, H. E.
1976BAAS....8..500S Altcode:
No abstract at ADS
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Title: The dark component of the photospheric network.
Authors: Schoolman, S. A.; Ramsey, H. E.
1976SoPh...50...25S Altcode:
We examine a high quality Zeeman-analyzed λ-scan through CaI
6103, using a 1/10 Å filter. We find that filter shifts of 0.01 Å
produce substantial changes in the contrast seen in the photospheric
network in the core of the line, implying that the Doppler velocity
in the network is constant to within 0.25 km s<SUP>−1</SUP> or
less. Relative line profiles constructed from the λ-scan indicate that
the bright network points have smaller equivalent widths than does the
background photosphere and are systematically Doppler shifted toward
the red. However, we also find numerous small dark points within the
magnetic network which exhibit increased equivalent widths. We infer
these to be due to magnetic flux tubes which are highly inclined
through the photosphere.
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Title: The Metallicity of the Sun Relative to Hyades and Coma
Cluster Stars.
Authors: Barry, D. C.; Cromwell, R. H.; Schoolman, S. A.
1976BAAS....8..524B Altcode:
No abstract at ADS
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Title: The Temporal and Spatial Extent of the Isoplanatic Patch.
Authors: Title, A.; Pope, T.; Schoolman, S.
1975BAAS....7..462T Altcode:
No abstract at ADS
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Title: Height of Helium Emission in the Chromosphere
Authors: Pope, T.; Schoolman, S. A.
1975SoPh...42...47P Altcode:
Filtergrams of the limb show the He I D<SUB>3</SUB> chromosphere as a
shell which is separated from the limb by a gap. The height of maximum
D<SUB>3</SUB> contribution occurs at about 1350 km above the limb
and is independent of the intensity of the D<SUB>3</SUB> emission. We
interpret this effect as the height to which coronal EUV radiation is
capable of penetrating the atmosphere.
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Title: Height of Helium Emission in the Chromosphere
Authors: Schoolman, Stephen A.; Pope, Thomas
1975BAAS....7..353S Altcode:
No abstract at ADS
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Title: Hα Observations of the Near-Limb Flare of 29 June 1973
Authors: Schoolman, Stephen A.; Ramsey, Harry E.
1974BAAS....6S.293S Altcode:
No abstract at ADS
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Title: Spectral characteristics of flares.
Authors: Schoolman, S.; Title, A.
1974sowi.conf..147S Altcode:
A technique is presented for obtaining H-alpha spectral profiles of
solar flares. A multislit spectrograph, which can take spectra every
15 seconds, is described along with a data reduction method utilizing a
very fast microdensometer and a computer. The technique is illustrated
with a spectral analysis of the solar flare of September 5, 1973.
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Title: Videomagnetograph Studies of Solar Magnetic Fields. II:
Field Changes in an Active Region
Authors: Schoolman, Stephen A.
1973SoPh...32..379S Altcode:
Using the Caltech videomagnetograph, we obtained a 61/2 h movie of the
magnetic fields in a young active region. The major contribution to
the short term magnetic evolution of the region was provided by many
discrete magnetic points which move in apparently random directions
with typical velocities of 0.4-1.0 km s<SUP>−1</SUP>. The majority
of these features appear to be footpoints of new EFR's, which emerge
at an observed rate of one to two per hour. The pattern of the motions
suggests that the magnetic evolution of a growing region cannot be
principally due to photospheric convective cells.
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Title: Contrast Elements in Birefringent Filters
Authors: Schoolman, Stephen A.
1973SoPh...30..255S Altcode: 1973SoPh...30..225S
Calculations were made to determine the effects which contrast
elements of various thicknesses have in Lyot filters. A filter which is
significantly narrower than the line at which it is looking produces
the best results. Therefore, if the filter is broad the addition of
as thick an element as possible is desired. However, if the filter is
already narrow, a contrast element whose thickness equals that of the
second Lyot element will produce the best performance.
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Title: Hα Profiles from the 2 August Events
Authors: Schoolman, Stephen A.
1973BAAS....5S.279S Altcode:
No abstract at ADS
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Title: The Dynamics of Magnetic Flux in a Young Active Region
Authors: Schoolman, Stephen A.
1972BAAS....4U.390S Altcode:
No abstract at ADS
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Title: Formation of the Solar Hα Profile
Authors: Schoolman, Stephen A.
1972SoPh...22..344S Altcode:
A series of non-LTE radiative transfer solutions for Hα was computed
using the integrodifferential equation technique of Athay and Skumanich
(1967). A model hydrogen atom consisting of three bound levels and a
continuum was assumed. It was found that increasing the temperature of
the chromosphere at the height of line formation decreases the central
intensity of the line. The density structure of the atmosphere primarily
affects the optical depth scale rather than the source function. The
temperature minimum region of the atmosphere was found to be transparent
to Hα radiation, so that the radiation in some part of the line
will arise from two distinct layers of the atmosphere, one above the
temperature minimum and one below it. The computed Hα profile was found
to be highly sensitive to the assumed 2-3 collisional cross-section.
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Title: Observation on the Detailed Correspondence of Magnetic and
H&alpha Features
Authors: Schoolman, Stephen A.
1971SoPh...21...57S Altcode:
No abstract at ADS
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Title: The Solar H-alpha Profile.
Authors: Schoolman, Stephen A.
1970BAAS....2R.343S Altcode:
No abstract at ADS
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Title: The Magnetic and Spectral Characteristics of Small Hα
Solar Plages.
Authors: Schoolman, Stephen Allan
1969PhDT........10S Altcode:
No abstract at ADS
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Title: The Magnetic Structure of Small Solar Plages.
Authors: Schoolman, Stephen A.
1968AJS....73R..76S Altcode:
Magnetograms of 18 small solar active regions were obtained at Climax
between June 1966 and May 1967. Most of the plages were bipolar magnetic
regions, although a few were monopolar remnants of older regions. The
contour map of the magnetic field generally exhibited a similarity
to the outline of the optical Ha plage, but the correspondence was
not one-to- one. This contrasts to the nearly exact correspondence
of a Ca II K plage to its field outline. Within a plage, some of the
structures undergo apparent changes of 30-50% in field intensity with
time scales of the order of 6 h. Other features remain quite constant
over this period. Over a period of weeks, the gradient of the field
between the main peaks of the two polarities tends to decrease. It
is speculated that these effects may be due to the appearance of
intense flux ropes within the plage and their subsequent diffusion
under the influence of the solar granulation. A magnetic threshold
for the formation of active region filaments is clearly indicated by
the data. This threshold is approximately 40 gauss.
