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Author name code: foukal
ADS astronomy entries on 2022-09-14
author:"Foukal, Peter V."
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Title: The Fiftieth Anniversary Celebration of the Solar Physics
Division of the American Astronomical Society
Authors: Pasachoff, Jay M.; Dupree, Andrea K.; Foukal, Peter; Weart,
Spencer; Zirker, Jack
2021JAHH...24.1057P Altcode:
The fiftieth anniversary celebration of the Solar Physics Division
of the American Astronomical Society was held virtually in August
2020 with joint sponsorship from the Historical Astronomy Division,
following the COVID-19-forced transformation of the original plan to
hold the meeting in Spokane, Washington. The presenters had given
papers at the first meeting in Huntsville, Alabama, in 1970, and
addressed related fields from the vantage point of fifty years.
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Title: Some developments in observational and theoretical solar
astronomy since 1970
Authors: Foukal, Peter
2021JAHH...24.1059F Altcode:
Studies of the Sun are widely considered to provide the 'Rosetta Stone'
for understanding the broader cosmos. Yet, the field has seldom been
entirely appreciated within astronomy. This is surprising given its
leadership in the organization of U.S. astronomy and in the development
of new observational techniques. We explore some developments since the
founding of the Solar Physics Division of the American Astronomical
Society in 1970 as observational and theoretical techniques evolved,
and note that the evolution towards ever 'Bigger Science' is unlikely
to be sustainable. We suggest that future advances may need to rely
more on thinking 'outside the box', and put forward some ideas that
might prove fruitful in this respect.
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Title: Solar Physics Then and Now
Authors: Foukal, P. V.
2020SPD....5110102F Altcode:
Solar research has consistently led in the organization of astronomy and
in introduction of new observational techniques. The founding of the
Mt Wilson Solar Observatory led the way in moving major observatories
from cities to mountaintops. The Sun provided the source for the first
radio, ultraviolet, and X-ray observations of cosmic sources. The
Orbiting Solar Observatories (OSO) produced the first satellite-borne
astronomical observations. The first cosmic observations by astronauts
were made of the Sun from the Skylab Space Station. So it is fitting
that the solar community was one of the first within the American
Astronomical Society to organize its own Division. This move, and
the founding of the journal Solar Physics shortly before, attracted
criticism from some who feared that it would isolate solar research. But
any such trend has been counter-balanced by increasing applications
of solar findings to solar-terrestrial studies represented within the
American Geophysical Union. The practice of solar research has also
evolved. Of the three most important solar observational advances
since WW II, the five-minute oscillation and its mode structure were
discovered using modest ground-based telescopes. But the Sun's total
irradiance variation and the discovery of the huge plasma eruptions
known as coronal transients, were first recognized using progressively
more elaborate space-borne instrumentation. The move towards Big Science
has continued inexorably in recent years but it is fast approaching
a Funding Wall set by budgetary limits. The future vitality of solar
research will be determined mainly by our ability to attract clever
and innovative minds to use the impressive instruments at our disposal.
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Title: The Umbra-Penumbra Area Ratio of Sunspots During the Maunder
Minimum
Authors: Carrasco, V. M. S.; García-Romero, J. M.; Vaquero, J. M.;
Rodríguez, P. G.; Foukal, P.; Gallego, M. C.; Lefèvre, L.
2018ApJ...865...88C Altcode: 2018arXiv180908670C
The Maunder Minimum (MM) was a prolonged period of low solar activity
that occurred between 1645 and 1715. The true level of solar activity
corresponding to this epoch is still a matter of debate. In order
to compare solar activity during the MM with that of other epochs,
we have evaluated the umbra-penumbra area ratio (U/P hereafter)
during the MM. Thus, we have analyzed 196 sunspot drawings, including
48 different sunspots observed during the period 1660-1709. The mode
value of the ratio obtained from the occurrence frequency distribution
lies between 0.15 and 0.25. Furthermore, the median and mean values
are equal to 0.24 ± 0.07 and 0.27 ± 0.08 with a sigma clipping,
respectively. These results are consistent with recent research
using more modern data. Higher U/P values mean faster sunspot decay
rates. From our results, the absence of sunspots during the MM could
not be explained by changes in the U/P since the values of the ratio
obtained in this work are similar to values found for other epochs.
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Title: An Explanation of the Vaughan - Preston Gap
Authors: Foukal, Peter
2018arXiv181006558F Altcode:
A plot of the calcium emission versus color of late type stars exhibits
a reduced population or gap at intermediate activity, somewhat
higher than that of the Sun. We suggest that this gap, first noted
by A. Vaughan and G. Preston in 1980 may result from a reduced area
of plages relative to spots, as observed at the highest levels of
solar activity. This reduced plage area weakens the calcium emission
and depletes the number of stars of intermediate calcium emission
index. We propose that, in the most active stars, the reduction
in relative plage area is offset by the increased filling factor of
photospheric magnetic fields. So the gap might simply be a consequence
of a gradual shift with age of the stellar dynamo towards production
of higher spatial frequencies.
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Title: Reddened Dimming of Boyajian’s Star Supports Internal
Storage of Its “Missing” Flux
Authors: Foukal, Peter
2017RNAAS...1...52F Altcode: 2017RNAAS...1a..52F; 2017arXiv171206637F
Two recent short term dimmings of KIC 8462852 (Boyajian's Star) exhibit
clear reddening in the B, r' and i' photometric passbands. We show that
the intensity ratios of the three pass bands agree well with cooling
of an approximately 6800 K black body by about 30K. This agreement,
together with other recent findings on the timing and longer term
dimmings of this star, support our previous argument that the star's
photometric behavior is caused by internal storage of impeded convective
flux, rather than by external sources of obscuration such the ISM or
circumstellar material.
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Title: An Explanation of the Missing Flux from Boyajian's Mysterious
Star
Authors: Foukal, Peter
2017ApJ...842L...3F Altcode: 2017arXiv170400070F
A previously unremarkable star in the constellation Cygnus has,
in the past year, become known as the most mysterious object in our
Galaxy. Boyajian’s star exhibits puzzling episodes of sporadic, deep
dimming discovered in photometry with the Kepler Mission. Proposed
explanations have focused on its obscuration by colliding exoplanets,
exocomets, and even intervention of alien intelligence. These hypotheses
have considered only phenomena external to the star because the
radiative flux missing in the dimmings was believed to exceed the
star’s storage capacity. We point out that modeling of variations
in solar luminosity indicates that convective stars can store the
required fluxes. It also suggests explanations for (a) a reported
time-profile asymmetry of the short, deep dimmings and (b) a slower,
decadal scale dimming reported from archival and Kepler photometry. Our
findings suggest a broader range of explanations of Boyajian’s star
that may produce new insights into stellar magneto-convection.
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Title: A Study of Solar Photospheric Temperature Gradient Variation
Using Limb Darkening Measurements
Authors: Criscuoli, Serena; Foukal, Peter
2017ApJ...835...99C Altcode: 2016arXiv161110201C
The variation in area of quiet magnetic network measured over the
sunspot cycle should modulate the spatially averaged photospheric
temperature gradient, since temperature declines with optical depth
more gradually in magnetic flux tube atmospheres. Yet, limb darkening
measurements show no dependence upon activity level, even at an rms
precision of 0.04%. We study the sensitivity of limb darkening to
changes in area filling factor using a 3D MHD model of the magnetized
photosphere. The limb darkening change expected from the measured
11-year area variation lies below the level of measured limb darkening
variations, for a reasonable range of magnetic flux in quiet network
and internetwork regions. So the remarkably constant limb darkening
observed over the solar activity cycle is not inconsistent with
the measured 11-year change in area of quiet magnetic network. Our
findings offer an independent constraint on photospheric temperature
gradient changes reported from measurements of the solar spectral
irradiance from the Spectral Irradiance Monitor, and recently, from
wavelength-differential spectrophotometry using the Solar Optical
Telescope aboard the HINODE spacecraft.
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Title: Investigation of photospheric temperature gradient variations
using limb darkening measurements and simulations
Authors: Criscuoli, Serena; Foukal, Peter V.
2016SPD....4730301C Altcode:
The temperature stratifications of magnetic elements and unmagnetized
plasma are different, so that changes of the facular and network
filling factor over the cycle modify the average temperature gradient
in the photosphere.Such variations have been suggested to explain
irradiance measurements obtained by the SIM spectrometers in he
visible and infrared spectral ranges. On the other hand, limb darkening
measurements show no dependence upon activity level. We investigate the
sensitivity of limb darkening to changes in network area filling factor
using a 3-D MHD model of the magnetized photosphere. We find that the
expected limb darkening change due to the measured 11- yr variation
in filling factor lies outside the formal 99% confidence limit of
the limb darkening measurements. This poses important constraints for
observational validation of 3D-MHD simulations.
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Title: Dimming of the Mid-20th Century Sun
Authors: Foukal, Peter
2015ApJ...815....9F Altcode: 2015arXiv150501040F
Area changes of photospheric faculae associated with magnetic active
regions are responsible for the bright contribution to variation in
total solar irradiance (TSI). Yet, the 102-year white light (WL) facular
record measured by the Royal Greenwich Observatory between 1874 and
1976 has been largely overlooked in past TSI reconstructions. We show
that it may offer a better measure of the brightening than presently
used chromospheric proxies or the sunspot number. These are, to varying
degrees, based on magnetic structures that are dark at the photosphere
even near the limb. The increased contribution of the dark component
to these proxies at high activity leads to an overestimate of solar
brightening around peaks of the large spot cycles 18 and 19. The WL
facular areas measure only the bright contribution. Our reconstruction
based on these facular areas indicates that TSI decreased by about
0.1% during these two cycles to a 20th century minimum, rather than
brightening to some of the highest TSI levels in four centuries,
as reported in previous reconstructions. This TSI decrease may have
contributed more to climate cooling between the 1940s and 1960s than
present modeling indicates. Our finding adds to previous evidence that
such suppression of solar brightening by an increased area of dark flux
tubes might explain why the Sun is anomalously quiet photometrically
compared to other late-type stars. Our findings do not change the
evidence against solar driving of climate warming since the 1970s.
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Title: An Explanation of the Differences Between the Sunspot Area
Scales of the Royal Greenwich and Mt. Wilson Observatories, and the
SOON Program.
Authors: Foukal, Peter V.
2014AAS...22442201F Altcode:
Several studies have shown that the sunspot areas recorded by the
Royal Greenwich Observatory (RGO) between 1874 - 1976 are about 40-50
percent larger than those measured by the NOAA/USAF Solar Observing
Optical Network(SOON) since 1966. We show here that, while the two
measurement sets provide consistent total areas for large spots, the
impossibility of recording small spots as anything except dots in the
SOON drawings leads to an underestimate of of small spot areas. These
are more accurately recorded by the RGO and other programs that use
photographic or CCD images. The large number of such small spots is
often overlooked. A similar explanation holds for the RGO umbral areas,
which amount to 40 percent more than those measured from Mt. Wilson
data between 1923 and 1982. The neglected small spots have a much lower
photometric contrast. Our explanation suggests, therefore, that the
adjustment to spot irradiance blocking at the 1976 transition from
RGO to SOON areas is smaller than the almost 50 percent correction
advanced by some recent, purely statistical, studies.
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Title: An Explanation of the Differences Between the Sunspot Area
Scales of the Royal Greenwich and Mt. Wilson Observatories, and the
SOON Program
Authors: Foukal, Peter
2014SoPh..289.1517F Altcode:
Several studies have shown that the sunspot areas recorded by the
Royal Greenwich Observatory (RGO) between 1874 - 1976 are about 40
- 50 % larger than those measured by the NOAA/USAF Solar Observing
Optical Network (SOON) since 1966. We show here that while the two
measurement sets provide consistent total areas for large spots, the
impossibility of recording small spots as anything except dots in the
SOON drawings leads to an underestimate of small spot areas. These
are more accurately recorded by the RGO and other programs that use
photographic or CCD images. The large number of such small spots is
often overlooked. A similar explanation holds for the RGO umbral areas,
which amount to 40 % more than those measured from Mt. Wilson data
between 1923 and 1982. The neglected small spots have a much lower
photometric contrast. Our explanation suggests, therefore, that the
adjustment to spot irradiance blocking at the 1976 transition from RGO
to SOON areas is smaller than the almost 50 % correction advocated by
some recent, purely statistical, studies.
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Title: An Explanation of the Difference in the Sunspot Area Scales
of the Royal Greenwich Observatory and the SOON Program
Authors: Foukal, Peter V.
2013SPD....44..154F Altcode:
Several studies have shown that the sunspot areas recorded by the Royal
Greenwich Observatory (RGO) between 1874 -1976 are about 40 - 50% larger
than those measured by the NOAA/USAF Solar Observing Optical Network
(SOON) since 1966. Many possible contributions to this surprisingly
large difference have been suggested, but no satisfying explanation
has emerged. We show here that, while the two measurement sets provide
consistent areas for large spots, the low resolution of the SOON
drawings leads to an underestimate of small- spot areas. These are more
accurately recorded by the RGO and other programs which use photographic
or ccd images. The large number of such small spots is often overlooked;
it appears sufficient to explain the reported scale difference. Our
explanation suggests that the sunspot blocking of solar irradiance
calculated from the RGO areas is over- estimated by approximately 20%
because the small spots have low photometric contrast. The smaller
SOON areas seem to under-estimate blocking by less than 10 %. Higher
accuracy of the blocking time series will require better accounting
of the spot area distribution, and a better measurement of the size
dependence of sunspot bolometric contrast. This work has been supported
at Heliophysics, Inc., under NASA grants NNX09AP96G and NNX10AC09G.
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Title: A New Look at Solar Irradiance Variation
Authors: Foukal, Peter
2012SoPh..279..365F Altcode:
We compare total solar irradiance (TSI) and ultraviolet (F<SUB>uv</SUB>)
irradiance variation reconstructed using Ca K facular areas since 1915,
with previous values based on less direct proxies. Our annual means for
1925 - 1945 reach values 30 - 50 % higher than those presently used
in IPCC climate studies. A high facula/sunspot area ratio in spot
cycles 16 and 17 seems to be responsible. New evidence from solar
photometry increases the likelihood of greater seventeenth century
solar dimming than expected from the disappearance of magnetic active
regions alone. But the large additional brightening in the early
twentieth century claimed from some recent models requires complete
disappearance of the magnetic network. The network is clearly visible
in Ca K spectroheliograms obtained since the 1890s, so these models
cannot be correct. Changes in photospheric effective temperature
invoked in other models would be powerfully damped by the thermal
inertia of the convection zone. Thus, there is presently no support
for twentieth century irradiance variation besides that arising from
active regions. The mid-twentieth century irradiance peak arising
from these active regions extends 20 years beyond the early 1940s
peak in global temperature. This failure of correlation, together
with the low amplitude of TSI variation and the relatively weak effect
of Fuv driving on tropospheric temperature, limits the role of solar
irradiance variation in twentieth century global warming.
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Title: Solar Bolometric Imager for Investigating the Sources of
Solar Irradiance Variability
Authors: Bernasconi, Pietro N.; Foukal, P. V.
2012AAS...22020114B Altcode:
The Solar Bolometric Imager is an innovative instrument for the
investigation of the sources of solar irradiance variability. It
makes precise, wavelength-integrated, photometric measurements of
the irradiance variations originating in the solar photosphere. It
provides images with spectrally flat response over the range 200-2600
nm, which includes about 95% of the total solar irradiance (TSI). It
is important to realize that the SBI measures broad band contrast
of thermal structures relative to their surroundings, so it does not
require absolute accuracy or even high long term reproducibility. Its
angular resolution (1 arcsecs/pixel) and field of view (320x240
arcsecs) are optimized to discriminate between TSI contributions from
different magnetic and non-magnetic solar regions. The detector is an
uncooled bolometric array with 320x240 ferro-electric pixels, coated
with gold-black to achieve uniform sensitivity at all wavelengths of
incident light. We are in the process of developing a space based
SBI that builds upon the heritage of a stratospheric balloon-borne
instrument successfully flown in 2003, and 2007. A space-based SBI
will directly attack one of the most challenging problems in solar
research: “What are the origins of long term solar total output
variation on centennial and millennial time scales?” In addition,
SBI measurements will continue to increase our understanding of solar
magneto-convection, and more generally the underlying physics of
solar magnetic variability. <P />Here we present the results of our
latest instrument development efforts aimed at bringing the current
SBI prototype to a Technology Readiness Level suitable for a SMEX or
a Mission of Opportunity.
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Title: A Different Pathway to the Stars
Authors: Foukal, Peter; Kováć, Štepán
2012S&T...123c..38F Altcode:
No abstract at ADS
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Title: Dimming of the 17th Century Sun
Authors: Foukal, Peter; Ortiz, Ada; Schnerr, Roald
2011ApJ...733L..38F Altcode: 2011arXiv1103.5442F
Reconstructions of total solar irradiance (TSI) rely mainly on linear
relations between TSI variation and indices of facular area. When these
are extrapolated to the prolonged 15th-17th century Spörer and Maunder
solar activity minima, the estimated solar dimming is insufficient to
explain the mid-millennial climate cooling of the Little Ice Age. We
draw attention here to evidence that the relation departs from linearity
at the lowest activity levels. Imaging photometry and radiometry
indicate an increased TSI contribution per unit area from small network
faculae by a factor of 2-4 compared with larger faculae in and around
active regions. Even partial removal of this more TSI-effective network
at prolonged minima could enable climatically significant solar dimming,
yet be consistent with the weakened but persistent 11 yr cycle observed
in Be 10 during the Maunder Minimum. The mechanism we suggest would
not alter previous findings that increased solar radiative forcing is
insufficient to account for 20th century global warming.
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Title: Dimming of the 17th Century Sun
Authors: Foukal, Peter V.; Ortiz, A.; Schnerr, R.
2011SPD....42.0702F Altcode: 2011BAAS..43S.0702F
Reconstructions of total solar irradiance (TSI) rely mainly on linear
relations between TSI variation and indices of facular area. When these
are extrapolated to the prolonged 15<SUP>th</SUP> - 17<SUP>th</SUP>
century Spörer and Maunder solar activity minima, the estimated solar
dimming is insufficient to explain the mid- millennial climate cooling
of the Little Ice Age. We draw attention here to evidence that the
relation departs from linearity at the lowest activity levels. Imaging
photometry and radiometry indicate an increased TSI contribution per
unit area from small network faculae by a factor of 2-4 compared to
larger faculae in and around active regions. Even partial removal of
this more TSI - effective network at prolonged minima could enable
climatically significant solar dimming, yet be consistent with the
weakened but persistent 11- yr cycle observed in Be 10 during the
Maunder Minimum. The mechanism we suggest would not alter previous
findings that increased solar radiative forcing is insufficient
to account for 20<SUP>th</SUP> century global warming. This work
was supported at Heliophysics, Inc. by NASA grants NNX09AP96G and
NNX10AC09G.
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Title: Dimming of the 17th Century Sun
Authors: Foukal, Peter V.; Ortiz, A.; Schnerr, R.
2011AAS...21822423F Altcode: 2011BAAS..43G22423F
Reconstructions of total solar irradiance (TSI) rely mainly on linear
relations between TSI variation and indices of facular area. When these
are extrapolated to the prolonged 15<SUP>th</SUP> - 17<SUP>th</SUP>
century Spörer and Maunder solar activity minima, the estimated solar
dimming is insufficient to explain the mid- millennial climate cooling
of the Little Ice Age. We draw attention here to evidence that the
relation departs from linearity at the lowest activity levels. Imaging
photometry and radiometry indicate an increased TSI contribution per
unit area from small network faculae by a factor of 2-4 compared to
larger faculae in and around active regions. Even partial removal of
this more TSI - effective network at prolonged minima could enable
climatically significant solar dimming, yet be consistent with the
weakened but persistent 11- yr cycle observed in Be 10 during the
Maunder Minimum. The mechanism we suggest would not alter previous
findings that increased solar radiative forcing is insufficient to
account for 20<SUP>th</SUP> century global warming.
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Title: Comment on “A homogeneous database of sunspot areas covering
more than 130 years” by L. A. Balmaceda et al.
Authors: Foukal, Peter
2010JGRA..115.9102F Altcode: 2010JGRA..11509102F
<A href="/journals/ja/ja1009/2010JA015293/">Abstract Available</A>
from <A href="http://www.agu.org">http://www.agu.org</A>
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Title: What Irradiance Studies Tell Us about Solar/Stellar Convection
and Magnetism
Authors: Foukal, Peter V.
2010AAS...21621901F Altcode:
Despite their enormous thermal inertia, many late - type stars exhibit
luminosity fluctuations caused by changing photospheric magnetic
structures. These fluctuations exist only because of the high heat
diffusivity of stellar convection. Were it lower, the dark spots
would be surrounded by intense bright rings, as Gene Parker pointed
out in 1974. These rings would cancel the spot - induced luminosity
dips. Conversely, dark rings around the bright faculae would cancel
their positive luminosity contribution. <P />Photometric measurements of
this heat diffusivity place independent constraints on solar magnetic
diffusivities - a key parameter in dynamo models. Irradiance studies
also suggest that the structure of emerging magnetic fields shifts
toward lower spatial frequencies with increasing activity. This finding
could provide new information on the field source function in dynamo
models. <P />Differential and near - IR imaging photometry reveal
the decreased temperature gradient of facular magnetic flux tubes and
the sunspot- like darkness of their deepest observable layers. Both of
these features support current mhd flux tube models. Bolometric imaging
measures the wide- band contribution to total irradiance variation,
of spot and facular magnetic flux tubes. The remarkably constant
solar limb - darkening measured over the past 33 years constrains
fluctuations in quiet photospheric temperature gradient and thus,
in global convective efficiency over the past three solar cycles. <P
/>Reconstruction of irradiance variation over past millennia relies
on radio- isotope studies. These provide many interesting insights,
but they assume that C14 and Be10 are formed only by solar modulation
of the galactic cosmic ray flux. This assumption would break down if
solar activity and particle fluxes much exceeded levels experienced in
cycle 19. Such a "hyperactive” Sun would vary more in its radiative
outputs, be dimmer in total irradiance, although brighter in the EUV
and X rays. <P />Work is supported by NASA grant NNX09AP96G
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Title: Recent Anomalous TSI Decrease Not Due To Low Polar Facula and
Network Areas: Time to Broaden Our View of Solar Luminosity Variation?
Authors: Foukal, Peter V.; Bernasconi, P.; Frohlich, C.
2009SPD....40.1113F Altcode:
Total solar irradiance (TSI) values measured during the present activity
minimum by the VIRGO, ACRIM, and TIM radiometers are significantly (
0.018% +/- 0.006 % rms) lower than reported during the last minimum
in 1996 (1). This decrease represents 1/4 the amplitude of 11 -
yr TSI variation. Differences in spots, faculae and active network
cannot account for this anomalous decrease. A sufficient difference
in the TSI contribution from quiet network also seems unlikely, since
the solar microwave flux index, F10.7, has dipped only 4 % below
its 1996 minimum. This is an order of magnitude less than required
to explain the TSI decrease by a decline in network area. <P />The
remaining explanation in terms of photospheric magnetic structures,
might lie in a decrease in the area of polar faculae, whose cycle
amplitude is presently at a minimum for this century. We evaluate
their TSI contribution using area and contrast measurements with
the Solar Bolometric Imager (SBI), together with polar facula counts
(2). We find that their TSI contribution between the present and 1996
activity minima, is below 0.002%. This is again, an order of magnitude
below the observed TSI decrease. <P />We conclude that the anomalous TSI
decrease is unlikely to be caused by photospheric magnetic changes. This
suggests that solar luminosity may be able to change significantly
over decadal time scales through an as- yet- unidentified, relatively
shallow mechanism that avoids the 10*5 year thermal relaxation time of
the solar convection zone. <P />This work was supported at Heliophysics,
Inc by NSF grant ATM 0718305, and at APL by NASA grant NNG 05WC07G <P
/>References: <P />1. Frohlich, C. 2008, AGU Fall Meeting, Abstract #
SH21C-05. <P />2. Sheeley, N. 2008, Ap.J. , 680, 1553.
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Title: A Century of Solar Ca II Measurements and Their Implication
for Solar UV Driving of Climate
Authors: Foukal, Peter; Bertello, Luca; Livingston, William C.;
Pevtsov, Alexei A.; Singh, Jagdev; Tlatov, Andrey G.; Ulrich, Roger K.
2009SoPh..255..229F Altcode:
Spectroheliograms and disk-integrated flux monitoring in the strong
resonance line of Ca II (K line) provide the longest record of
chromospheric magnetic plages. We compare recent reductions of the Ca II
K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson,
and US National Solar Observatories. Certain differences between the
individual plage indices appear to be caused mainly by differences
in the spectral passbands used. Our main finding is that the indices
show remarkably consistent behavior on the multidecadal time scales of
greatest interest to global warming studies. The reconstruction of solar
ultraviolet flux variation from these indices differs significantly
from the 20th-century global temperature record. This difference is
consistent with other findings that, although solar UV irradiance
variation may affect climate through influence on precipitation and
storm tracks, its significance in global temperature remains elusive.
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Title: Solar Network Bolometric Properties at Minimum of Activity
Observed by the Solar Bolometric Imager
Authors: Bernasconi, P. N.; Foukal, P. V.
2008AGUFMSH23A1625B Altcode:
On September 13 2007, the Solar Bolometric Imager (SBI) observed the Sun
in wide band spectrally integrated for 16 hours while suspended from
a balloon at ~120,000 feet altitude above New Mexico. SBI represents
a totally new approach in finding the sources of the solar irradiance
variation. Its detector is an array of 320x240 thermal IR elements
whose spectral sensitivity has been extended and flattened by a layer
of gold-black deposited on its IR sensitive surface. The combination
of bolometric array and telescope, a 30- cm Dall-Kirkham with uncoated
primary and secondary Pyrex mirrors, provide an image of the Sun with
constant spectral response between ~ 280 and 2600 nm, over a field of
view of 960 x 720 arcsec with a pixel size of 3 arcsec. The September
13, 2007 flight provided bolometric (integrated light) maps of the
photosphere when the Sun was near a minimum of activity. At the time of
the flight no active regions were present giving us the opportunity to
measure with high accuracy the bolometric contrast of the weak solar
magnetic network from Sun center to the limb. The network was easily
detectable by SBI near the limb. We measured an average bolometric
contrast of ~ 0.8 to 1.0 %, which is slightly above the 5-minute
oscillation brightness signal (the most prominent solar induced noise
source for us). We were also able to detect the bolometric brightness
signature of network near Sun center by averaging 720 bolometric images
taken close to Sun center over a period of 1 hour. The resulting RMS
noise was < 0.02% and most of the 5-minute oscillation brightness was
removed in the average. This enabled us to measure an average network
bolometric contrast at Sun center of 0.25% with a spread of about ±
0.05%. Ours is the first bolometric measurement (constant spectral
sensitivity from 280 to 2600 nm) of the center-to-limb contrast of
magnetic network. Our observations demonstrate that SBI can accurately
measure the bolometric contrast of even quiet network across the solar
disk. These measurements will enable a more precise estimate of the
TSI contribution from changes of the enhanced magnetic network, which
consist of larger elements than the quiet network. This will enable
us to determine whether other low level brightness sources besides
faculae and spots contribute to TSI and evaluate their possible long
term influence in TSI change and climate.
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Title: Barium strontium titanate (BST) pyroelectric detector for
bolometric solar imaging
Authors: Noble, M.; Bernasconi, P.; Francomacaro, A.; Eaton, H.;
Carkhuff, B.; Foukal, P.
2008SPIE.7055E..0AN Altcode: 2008SPIE.7055E...6N
The Solar Bolometric Imager (SBI) is an imaging solar telescope
assembly that employs a novel single-detector broadband bolometric
measurement technique. An uncooled thermal IR imaging detector is
coated with a thin gold-black film that absorbs over 98% of the solar
spectrum. The absorbed energy is then re-radiated in the thermal IR
and sampled by the detector array. This technique [4] provides an
evenly weighted integrated responsivity that spans the majority of the
solar spectrum (0.2-2.5μm). We present here performance results from
the follow-on gold-black deposition process investigation, radiation
testing results, spacecraft instrument design and some of the prototype
detector/imaging system's flight performance and calibration data from
our 2007 Ft. Sumner balloon flight that demonstrates the instrument
met or exceeded all of its specification.
---------------------------------------------------------
Title: Do Photospheric Brightness Structures Outside Magnetic Flux
Tubes Contribute to Solar Luminosity Variation?
Authors: Bernasconi, P. N.; Foukal, P. V.
2008AGUSMSP53B..07B Altcode:
Variations in total solar irradiance (TSI) correlate well with changes
in projected area of photospheric magnetic flux tubes associated with
dark sunspots and bright faculae in active regions and network. This
correlation does not, however, rule out possible TSI contributions from
photospheric brightness inhomogeneities located outside flux tubes,
and spatially correlated with them. Previous reconstructions of TSI
report agreement with radiometry that seems to rule out significant
"extra-flux tube" contributions. We show that these reconstructions are
more sensitive to the facular contrasts used than has been generally
recognized. Measurements with the Solar Bolometric Imager (SBI)
provide the first reliable support for the relatively high, wide-band,
disc-center contrasts required to produce 10% rms agreement. Longer-term
bolometric imaging will be required to determine whether the small
but systematic TSI residuals we see here are caused by remaining
errors in spot and facular areas and contrasts, or by extra-flux
tube brightness structures such as bright rings around sunspots, or
"convective stirring" around active regions.
---------------------------------------------------------
Title: Preliminary Results Of the 2007 Flight of the Solar Bolometric
Imager at Solar Minimum
Authors: Bernasconi, P. N.; Foukal, P. V.; Eaton, H. H.; Noble, M.
2008AGUSMSP41B..05B Altcode:
On September 13 2007, the Solar Bolometric Imager (SBI) successfully
observed the Sun for several hours while suspended from a balloon in
the stratosphere above New Mexico. The SBI represents a totally new
approach in finding the sources of the solar irradiance variation. The
SBI detector is an array of 320x240 thermal IR elements whose spectral
absorptance has been extended and flattened by a deposited layer of
gold-black. The telescope is a 30-cm Dall-Kirkham with uncoated primary
and secondary Pyrex mirrors. The combination of telescope and bolometric
array provide an image of the Sun with a constant spectral response
between ~ 280 and 2600 nm, over a field of view of 960 x 720 arcsec
with a pixel size of 3 arcsec. This is the second successful flight
of SBI, following a successful one on September 2003 which produced
the first measurements in broad band of the center-to-limb variation
of bolometric facular contrast (a flight attempt from Antarctica in
2006 was aborted). This latest flight provided bolometric (integrated
light) maps of the solar photosphere during a time of minimum of solar
activity. The SBI imagery will enable us to evaluate the photometric
contribution of weak magnetic structures (e.g. network) more accurately
than has been achievable with spectrally selective imaging over
restricted wavebands. It will also enable us to investigate the
presence, if any, of other thermal structures unrelated to magnetic
activity, such as e.g. giant cells and pole-to-equator temperature
gradients. During the 16 hour flight the SBI gathered several thousand
bolometric images that are now being processed to produce full-disk
maps of spatial variation in total solar output at solar minimum. The
SBI flight is also providing important engineering data to validate the
space worthiness of the novel gold-blackened thermal array detectors. In
this paper we will briefly describe the characteristics of the SBI,
its in-flight performance, and we will present the first results of
the analysis of the bolometric images.
---------------------------------------------------------
Title: Do Photospheric Brightness Structures Outside Magnetic Flux
Tubes Contribute to Solar Luminosity Variation?
Authors: Foukal, Peter; Bernasconi, Pietro N.
2008SoPh..248....1F Altcode: 2008SoPh..tmp...33F
Variations in total solar irradiance (TSI) correlate well with changes
in projected area of photospheric magnetic flux tubes associated with
dark sunspots and bright faculae in active regions and network. This
correlation does not, however, rule out possible TSI contributions from
photospheric brightness inhomogeneities located outside flux tubes
and spatially correlated with them. Previous reconstructions of TSI
report agreement with radiometry that seems to rule out significant
"extra-flux-tube" contributions. We show that these reconstructions are
more sensitive to the facular contrasts used than has been generally
recognized. Measurements with the Solar Bolometric Imager (SBI)
provide the first reliable support for the relatively high, wide-band,
disk-center contrasts required to produce 10% rms agreement. Longer term
bolometric imaging will be required to determine whether the small but
systematic TSI residuals we see here are caused by remaining errors in
spot and facular areas and contrasts or by extra-flux-tube brightness
structures such as bright rings around sunspots or "convective stirring"
around active regions.
---------------------------------------------------------
Title: Do Photospheric Brightness Structures Outside Magnetic Flux
Tubes Contribute to Solar Luminosity Variation?
Authors: Bernasconi, P. N.; Foukal, P.
2007AGUFMGC31B0345B Altcode:
Variations in total solar irradiance (TSI) correlate well with changes
in projected area of photospheric magnetic flux tubes associated with
spots, faculae and network. This correlation does not, however, rule out
possible TSI contributions from photospheric brightness inhomogeneities
located outside flux tubes, and spatially correlated with them. Previous
reconstructions report 10% amplitude agreement with radiometry that
seems to rule out significant extra-flux tube contributions. We show
that, while these reconstructions are insensitive to behavior of near-
limb facular contrast, their sensitivity to contrasts on the disc is
relatively high. Given this sensitivity, previously used observational
and theoretical approximations to wide-band facular contrast are too
uncertain to support claims of 10% reconstruction accuracy. Recent
measurements with the Solar Bolometric Imager (SBI) provide the first
observational support for the relatively high wide-band, disc-center
contrasts required to produce 10% rms agreement. Longer-term bolometric
imaging to measure areas and bolometric contrasts homogeneously
will be required to determine whether the systematic TSI residuals
we see are caused mainly by uncertainties in sunspot contrasts, or
by extra-flux tube brightness structures due to bright spot rings or
convective stirring.
---------------------------------------------------------
Title: Did the Sun's Prairie Ever Stop Burning?
Authors: Foukal, P.; Eddy, J.
2007SoPh..245..247F Altcode:
The presence of the red flash at total solar eclipses requires the
existence of an extended chromosphere and therefore of a photospheric
magnetic network that gives rise to spicules. We draw attention to
the earliest historical reports of a red flash at the 1706 and 1715
eclipses, which therefore imply a substantial, widespread photospheric
field during at least the last decade of the Maunder Minimum. Our
finding is consistent with reports of a persistent photospheric field
throughout the Maunder Minimum from analyses of <SUP>10</SUP>Be
radioisotope evidence. We note, however, that the last decade may
not be representative of conditions throughout the roughly 1645 -
1715 extent of that prolonged activity minimum.
---------------------------------------------------------
Title: Variations in solar luminosity and their effect on the
Earth's climate
Authors: Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L.
2006Natur.443..161F Altcode:
Variations in the Sun's total energy output (luminosity) are caused
by changing dark (sunspot) and bright structures on the solar
disk during the 11-year sunspot cycle. The variations measured from
spacecraft since 1978 are too small to have contributed appreciably to
accelerated global warming over the past 30 years. In this Review,
we show that detailed analysis of these small output variations
has greatly advanced our understanding of solar luminosity change,
and this new understanding indicates that brightening of the Sun is
unlikely to have had a significant influence on global warming since
the seventeenth century. Additional climate forcing by changes in the
Sun's output of ultraviolet light, and of magnetized plasmas, cannot
be ruled out. The suggested mechanisms are, however, too complex to
evaluate meaningfully at present.
---------------------------------------------------------
Title: Can Changing Sunspot and Facular Areas Reproduce the Amplitude
of Total Irradiance Variations?(Look,Mom; No Free Parameters!)
Authors: Foukal, P.; Bernasconi, P. N.; Walton, S. R.
2005AGUSMSH22B..02F Altcode:
Empirical models of total solar irradiance variation demonstrate a
high correlation between observed irradiance fluctuations and the
changing areas of spots and faculae. However, the contrast of these
structures (especially the faculae) in integrated light is still
uncertain. Consequently, the agreement in amplitude of the measured
and modeled irradiance time series remains poorly known. Recently, the
first measurements of facular contrast in broad - band integrated light
were obtained using the balloon -borne Solar Bolometric Imager (Foukal
et al., Ap.J. Letts 611,57,2004). These measurements, obtained over
approximately the same wavelength range accepted by radiometers such as
VIRGO or ACRIM, enable the first reconstruction of the total irradiance
expected from spots and faculae, with no free parameters. We compare
this reconstruction with the radiometric record to determine whether
other contributions besides the darkness of spots and brightness of
faculae are required to explain solar irradiance variation, at least
over rotational time scales.
---------------------------------------------------------
Title: Finding the sources of irradiance variation at sunspot
minimum .
Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M.; LaBonte, B. J.
2005MmSAI..76..907B Altcode:
In 2006-2007 the Solar Bolometric Imager (SBI) will operate in the polar
stratosphere where near-space conditions can be attained for 10 to 30
days. The instrument will provide bolometric (wavelength-integrated
light) and color temperature images of the Sun. At the upcoming sunspot
minimum, SBI observations will be able to detect subtle sources
of solar irradiance variation with the least confusion by signals
from the magnetic fields. This is the best observational approach
to characterizing potential causes of the long-term irradiance
variations. Possible predicted sources of secular variability
include torsional waves and meridional flow variations. SBI uses a
30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, and
neutral density filters to provide broadband (bolometric) sensitivity
that varies only by ±7% over the wavelengths from 0.31 mu m to 2.6
mu m. Inferred solar irradiance variations will be compared with space
based full-disk radiometric measurements.
---------------------------------------------------------
Title: Finding the Sources of Irradiance Variation at Sunspot Minimum
Authors: Rust, D. M.; Bernasconi, P. N.; Foukal, P. V.; Labonte, B. J.
2004AGUFMSH51E..02R Altcode:
In 2006-2007 the Solar Bolometric Imager (SBI) and the Multi-Spectral
Imager (MSI) will operate in the polar stratosphere where near-space
conditions can be attained for 10 to 20 days. The instruments will
provide bolometric (wavelength-integrated light) and color temperature
images of the Sun. At the upcoming sunspot minimum, SBI observations
will be able to detect subtle sources of solar irradiance variation
with the least confusion by signals from the magnetic fields. This is
the best observational approach to characterizing potential causes
of the long-term irradiance variations. Possible predicted sources
of secular variability include torsional waves and meridional flow
variations. SBI uses a 30-cm diameter F/12 Dall-Kirkham telescope with
uncoated mirrors, and neutral density filters to provide broadband
(bolometric) sensitivity that varies only by ±7 percent over the
wavelengths from 0.28 microns to 2.6 microns. The MSI is a CCD-based
imager that will provide diagnostics of solar magnetic and thermal
structures while SBI assesses their radiance. Sunspots, faculae
and magnetic network will be identified from the MSI images. Sonic
filtering of the MSI images will isolate the oscillatory signal. That
signal will be used to remove oscillations from SBI averages to reduce
the solar noise. Inferred solar irradiance variations will be compared
with SORCE/TIM and ACRIMSAT measurements. The images and data products
will be openly available via the Web.
---------------------------------------------------------
Title: Solar Irradiance Variation on Centennial to Millennial
Time Scales
Authors: Foukal, P.
2004AGUFM.U41B..04F Altcode:
Solar irradiance variation observed over the 11 yr sunspot cycle is
caused by the changing areas of dark and bright magnetic structures
(sunspots, faculae)on the solar disc, but its barely 0.1 % amplitude
is insufficient to drive existing climate models. Irradiance
reconstructions incorporating an additional slowly varying component
of sufficient amplitude to drive such models have been widely used
in recent climate studies. But these reconstructions were based on
results from photometry of Sun like stars which have now been largely
retracted. This changed evidence challenges our understanding how solar
luminosity variation could drive climate. Variation of UV flux may play
a role, but its correlation with global temperature seems low, at least
in the 20th century. The Sun's enormous thermal inertia restricts
sources of luminosity variation on centennial to millennial time
scales, to relatively superficial layers. This constraint diminishes
the likelihood that deeper lying structural changes associated with
e.g. the solar dynamo play a significant role. Still, some newly
discovered aspects of solar magnetic behavior suggest how luminosity
variation on these time scales might conceivably occur with the sign
and amplitude implied by the correlations between solar activity and
climate. More accurate solar and stellar observations and modeling
will be required to investigate such mechanisms at the frontier of
our understanding of the Sun.
---------------------------------------------------------
Title: Comment on “Variations of Total Solar Irradiance Produced
by Structural Changes in the Solar Interior”
Authors: Foukal, P.; Spruit, H.
2004EOSTr..85..524F Altcode:
In a recent Eos article, Sofia [2004] argues for the influence on
irradiance variation of global changes in the Sun's structure associated
with its magnetic dynamo. These changes would act in addition to the
relatively well understood modulation by dark sunspots and bright
faculae at the surface. His assessment of the present observational
evidence for such a global change agrees with our earlier conclusion
that it is not widely convincing at the present time [Foukal, 2003]. But
Sofia's article also claims (1) that the numerical results obtained by
him and his collaborators at Yale disagree with and correct earlier
work, and (2) that a hydrostatic approximation is not adequate for
variations on the 11-year solar cycle timescale. These surprising
claims are based on the results of recently published hydrostatic models
[e.g., Sofia and Li, 2004] using the same mixing length approximation
for convective heat transport used in earlier work [Spruit, 1982,
1991; Gilliland, 1988].
---------------------------------------------------------
Title: Broadband Measurements of Facular Photometric Contrast Using
the Solar Bolometric Imager
Authors: Foukal, Peter; Bernasconi, Pietro; Eaton, Harry; Rust, David
2004ApJ...611L..57F Altcode:
We present the first photometric measurements of solar faculae in
broadband light. Our measurements were made during the recent flight of
the Solar Bolometric Imager (SBI), a 30 cm balloon-borne telescope that
imaged the Sun with a spectrally constant response between about 0.31
and 2.6 μm. Our curve of facular contrast versus limb distance agrees
well with values obtained by the blackbody correction of monochromatic
measurements. This decreases uncertainty in the facular irradiance
contribution, which limits searches for other possible mechanisms of
solar luminosity variation, besides changes of photospheric magnetism.
---------------------------------------------------------
Title: Solar Astrophysics, 2nd, Revised Edition
Authors: Foukal, Peter V.
2004soas.book.....F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar bolometric imager
Authors: Bernasconi, P. N.; Eaton, H. A. C.; Foukal, P.; Rust, D. M.
2004AdSpR..33.1746B Altcode:
The balloon-borne Solar Bolometric Imager (SBI) will provide the first
bolometric (integrated light) maps of the solar photosphere. It will
evaluate the photometric contribution of magnetic structures more
accurately than has been possible with spectrally selective imaging
over restricted wavebands. More accurate removal of the magnetic
feature contribution will enable us to determine if solar irradiance
variation mechanisms exist other than the effects of photospheric
magnetism. The SBI detector is an array of 320 × 240 ferro-electric
thermal IR elements whose spectral absorptance has been extended and
flattened by a deposited layer of gold-black. The telescope itself is
a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex
mirrors. The combination of telescope and bolometric array provides
an image of the Sun with a flat spectral response between 0.28 and 2.6
μm, over a field of view of 917 × 687 arcsec, and a pixel size of 2.8
arcsec. After a successful set of ground-based tests, the instrument is
being readied for a one-day stratospheric balloon flight that will take
place in September 2003. The observing platform will be the gondola
previously used for the Flare Genesis Experiment (FGE), retrofitted
to house and control the SBI telescope and detector. The balloon
flight will enable SBI to image over essentially the full spectral
range accepted by non-imaging space-borne radiometers such as ACRIM,
making the data sets complementary. The SBI flight will also provide
important engineering data to validate the space worthiness of the
novel gold-blackened thermal array detectors, and verify the thermal
performance of the SBI's uncoated optics in a vacuum environment.
---------------------------------------------------------
Title: First Results Of The Solar Bolometric Imager
Authors: Bernasconi, P. N.; Foukal, P.; Eaton, H. H.; Rust, D. M.
2003AGUFMSH32A1101B Altcode:
On September 1 2003, the Solar Bolometric Imager (SBI) successfully
observed the Sun for several hours while suspended from a balloon in the
stratosphere above New Mexico. The SBI represents a totally new approach
in finding the sources of the solar irradiance variation. The mission
provided the first bolometric (integrated light) maps of the solar
photosphere, that will allow to evaluate the photometric contribution
of magnetic structures more accurately than has been achievable with
spectrally selective imaging over restricted wavebands. The more
accurate removal of the magnetic features contribution will enable
us to determine if solar irradiance variation mechanisms exist other
than the effects of photospheric magnetism. The SBI detector was an
array of 320 x 240 thermal IR elements whose spectral absorptance has
been extended and flattened by a deposited layer of gold-black. The
telescope was a 30-cm Dall-Kirkham with uncoated primary and secondary
pyrex mirrors. The combination of telescope and bolometric array
provided an image of the Sun with a flat spectral response between
0.28 and 2.6 microns, over a field of view of 917 x 687 arcsec with
a pixel size of 2.8 arcsec. The observing platform was the gondola
previously used for the Flare Genesis Experiment (FGE), retrofitted to
house and control the SBI telescope and detector. During the 9 hours
of flight the SBI gathered several thousand bolometric images that
are now being processed to produce the first maps of the total solar
irradiance. The SBI flight is also providing important engineering data
to validate the space worthiness of the novel gold-blackened thermal
array detectors, and to verify the thermal performance of the SBI's
uncoated optics in a vacuum environment. In this paper we will briefly
describe the characteristics of the SBI, its in flight performance,
and we will present the first results of the analysis of the bolometric
images. This work was funded by NASA under grant# NAG5-10998.
---------------------------------------------------------
Title: A Cryogenic Pyrheliometer for More Accurate Solar Irradiance
Measurements
Authors: Foukal, P.
2003SPD....34.2001F Altcode: 2003BAAS...35..844F
Space-borne pyrheliometry over the past two solar cycles has
demonstrated the variability of total solar irradiance in response
to photospheric magnetic structures such as sunspots or faculae, over
the 11-yr activity cycle.But the reproducibility of the measurements
remains marginal to detect or rule out possible trends in irradiance
below the 0.05-0.1% variation over the 11-yr cycle, but conceivably
dominant over multi-decadal time scales of greatest relevance to
climate. <P />In metrology laboratories,conventional radiometers similar
to those presently flown by NASA and ESA have been superseded in the
past ten years by cryogenic radiometers of ten times higher absolute
accuracy and long term reproducibility.But their helium cooling makes
them difficult to use in space. Recently, advances in superconducting
transition thermometry at NIST, and in high-temperature superconducting
materials,have presented the opportunity to reach cryogenic radiometer
performance at LN2 temperatures attainable with space qualified single
stage cryocoolers. <P />We report here on our results with a prototype
SCT-based radiometer, developed to investigate this opportunity
to improve the accuracy of space borne pyrheliometry.We show that
the sensitivity achieved is an order of magnitude better than with
conventional radiometers, although the noise threshold falls short of
values attainable with LHe cooling.The measured non-equivalence errors,
and results of monochromatic intercomparisons against trap detectors,
are both consistent with absolute accuracy at the 0.01% level, thus
comparable to LHe cooled radiometers. Improved thermal and mechanical
design will be required to reduce slow drifts, to test this accuracy
conclusively.
---------------------------------------------------------
Title: The Solar Bolometric Imager: Characteristics and Performance.
Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M.
2003SPD....34.2002B Altcode: 2003BAAS...35..844B
The Solar Bolometric Imager (SBI) is an innovative solar telescope
capable of recording the first bolometric (integrated light) maps of the
photosphere. It will enable evaluation of the photometric contribution
of magnetic structures more accurately than has been achievable with
spectrally selective imaging. The SBI has an angular resolution of 5",
sufficient to distinguish sunspots, faculae and enhanced network. These
photospheric magnetic structures are known to be linked closely to
irradiance variations. Accurate removal of irradiance variations linked
to the magnetic features will enable us to determine if other solar
irradiance variation mechanisms exist. <P />The SBI detector is an
array of 320 x 240 ferro-electric thermal IR elements whose spectral
absorptance has been extended and flattened by a deposited layer of
gold-black. The telescope is a 30-cm Dall-Kirkham design with uncoated
primary and secondary pyrex mirrors. The combination of telescope and
bolometric array provides an image of the sun with a flat spectral
response between 0.28 microns and 2.6 microns, over a field of view
of 917" x 687", and a pixel size of 2.8". After completion of ground
tests, the balloon-borne instrument will make a one-day stratospheric
flight in September 2003. <P />Observing from an altitude of over 30
km, the SBI will image the sun over nearly the full spectral range
accepted by non-imaging satellite-borne radiometers such as ACRIM,
making the data sets complementary. The SBI flight will also provide
important engineering data to validate the space worthiness of the
novel gold-blackened thermal array detectors, and to verify the
thermal performance of the SBI's optics in a vacuum environment. <P
/>Here we will describe the SBI in more detail and present the results
of various instrument performance tests, including solar observations
from the ground, in preparation for the balloon flight. <P />This work
is funded by NASA under grant NAG5-10998.
---------------------------------------------------------
Title: Reply: Evaluation of Climate Sensitivity to Solar Influences
Is an Important Goal
Authors: Foukal, Peter
2003EOSTr..84..532F Altcode:
The detection of an 11-year global temperature signal by Douglass
and Clader, and in other studies cited by David Douglass in his
letter, is an important achievement. However, these studies assume
that the driver is the measured 11-year variation in total solar
irradiance. They do not attempt to estimate the possible contributions
of the equally well-measured 11-year variations in solar ultraviolet
flux, and in solar modulation of galactic cosmic rays. Both of these
variable solar influences are under study as possible drivers of
11-year global temperature variation [e.g., Haigh, 1996; Svensmark and
Friis-Christensen, 1997]. These suggested mechanisms operate differently
from the direct coupling of total irradiance to climate. So it may
be premature to claim that the sensitivity to total irradiance has
been measured. Also, to the extent that the sign of possible climate
influences from solar UV [e.g., Shindell et al.; 1999] and plasma
output variations remains model-dependent, it seems uncertain in what
sense the reported sensitivities represent limits.
---------------------------------------------------------
Title: Can Slow Variations in Solar Luminosity Provide Missing Link
Between the Sun and Climate?
Authors: Foukal, Peter
2003EOSTr..84..205F Altcode:
Recent evidence from ocean and ice cores suggests that a significant
fraction of the variability in Northern Hemisphere climate since the
last Ice Age correlates with solar activity [Bond et al., 2001]. This
finding extends previous evidence connecting solar activity and climate
during the past millennium [Eddy, 1976, Lean et al., 1995]. The simplest
mechanism relies on increases of wavelength-integrated output of solar
heat and light (total irradiance, S) accompanying increases in solar
activity. But recent findings cast doubt on earlier evidence for
a sufficiently large variation of S. At the same time, advances in
instrumentation give promise of answering this question, to support
timely decisions on global warming. In this article, we assess the
status of the topic and suggest some new initiatives.
---------------------------------------------------------
Title: A comparison of variable solar total and ultraviolet irradiance
outputs in the 20th century
Authors: Foukal, Peter
2002GeoRL..29.2089F Altcode: 2002GeoRL..29w...4F
Differences in time- variation between total and ultraviolet solar
irradiance could help in separating their influence on climate. We
present the first models based on area measurements of magnetic plages
from CaK spectroheliograms obtained between 1915-1999. Correlation
of our time series of UV irradiance with global temperature, T,
accounts for only 20% of the global temperature variance during the
20th century. Correlation of our total irradiance time series with T
accounts statistically for 80% of the variance in global temperature
over that period, although the irradiance variation amplitude is
insufficient to influence global warming in present-day climate
models. This interesting difference has been obscured in past modelling
by additional components introduced to represent secular variations,
which are no longer supported by current observational evidence. Future
irradiance models emphasizing the more securely- based contributions
of photospheric magnetic structures seem to provide better prospects
for improved physical understanding of sun-climate links.
---------------------------------------------------------
Title: Investigation of the Sources of Irradiance Variation on the
Sun (ISIS)
Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.;
Hudson, H.; Spruit, H.
2002AAS...200.5608L Altcode: 2002BAAS...34..736L
There is a persistent correlation of the longterm climate change and
solar irradiance. ISIS is designed to understand the physical basis of
this correlation. ISIS combines an innovative bolometric imager and a
multiband CCD imager. The bolometric imager has uniform response from
200 nm to 3000 nm, spatial resolution < 5 arcseconds, and precision
of < 0.1% in a one minute integration. The multiband imager records
ultraviolet irradiance variation in the band from 200 to 350 nm,
measures photospheric temperature structure, and provides chromospheric
structure in Ca II K and H-alpha, with spatial resolution <1.0
arcsecond. Designed for flight on the Solar Dynamics Observatory,
ISIS will provide the comprehensive photometric measurements needed
to characterize the irradiance variation from identifiable structures
and challenge theoretical models of convection and the solar dynamo.
---------------------------------------------------------
Title: A Comparison of Variable Total and Ultraviolet Solar Irradiance
Inputs to 20 th Century Global Warming
Authors: Foukal, P. V.
2002AAS...200.2802F Altcode: 2002BAAS...34R.679F
Analysis of spaceborne radiometry has shown that the total
solar irradiance variation over the past two activity cycles was
approximately proportional to the weighted difference between areas
of dark spots and bright faculae and enhanced network. Empirical
models of ultraviolet irradiance variation indicate that its behavior
is dominated by changes in area of the bright component alone,
whose photometric contrast increases at shorter wavelength.This
difference in time behavior of total and UV irradiances could help to
discriminate between their relative importance in forcing of global
warming. Our recent digitization of archival Ca K images from Mt
Wilson and NSO provides the first direct measurement of variations
in area of the bright component, extending between 1915 and 1999
(previous models have relied on the sunspot number or other proxies
to estimate the bright - component contribution). We use these more
direct measurements to derive the time behavior of solar total and
UV irradiance variation, over this period .We find that they are
significantly different;the total irradiance variation accounts for over
80 percent of the variance in global temperature during this period,
while the ultraviolet irradiance variation accounts for only about 20
percent. The amplitude of total irradiance variation in our model is
smaller than required to influence global warming,in current climate
models.Also, the impact of sulfate aerosol variations on the extended
cooling between the 1940's and 1970's must be better understood before
the significance of correlations between 20 th century global warming,
and any solar activity index can be properly assessed. Despite these
caveats, the lower correlation we find between global temperature and
UV,compared to total, irradiance requires consideration in the search
for physical mechanisms linking solar activity and climate. This work
was supported in part under NASA grant NAG5-7607 to CRI, Inc., and
NAG5-10998 to the Applied Physics Laboratory, Johns Hopkins University.
---------------------------------------------------------
Title: The Solar Bolometric Imager
Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M.
2002AAS...200.5605B Altcode: 2002BAAS...34R.735B
The Solar Bolometric Imager (SBI) is an innovative solar telescope
capable of recording images in essentially total photospheric light,
with an angular resolution of 5", sufficient to distinguish sunspots,
faculae and enhanced network. These are the photospheric magnetic
structures so far linked most closely to irradiance variation. The
balloon-borne SBI will provide the first bolometric maps of the
photosphere, to evaluate the photometric contribution of magnetic
structures more accurately than has been achievable so far, using
spectrally selective imaging over restricted wavebands. More accurate
removal of the magnetic feature contribution will enable us to determine
whether other solar irradiance mechanisms exist besides the effects
of photospheric magnetism. The SBI detector is an array of 320 X 240
ferro-electric thermal IR elements whose spectral absorptance has
been extended and flattened by a deposited layer of gold-black. The
telescope itself is a 30-cm Dall-Kirkham design with uncoated primary
and secondary pyrex mirrors. The combination of telescope and bolometric
array provides an image of the solar irradiance with a flat spectral
response between 0.28 um and 2.6 um, over a field of view of 15.2' X
11.4', and a pixel size of 2.8". After a successful set of ground-based
tests, the instrument is being readied for a one-day stratospheric
balloon flight that will take place in September 2003. The observing
platform will be the gondola previously used for the Flare Genesis
Experiment project (FGE), retrofitted to house and control the SBI
telescope and detector. The balloon flight will enable SBI to image over
essentially the full spectral range accepted by non-imaging space borne
radiometers such as ACRIM, making the data sets complementary. The SBI
flight will also provide important engineering data to validate the
space worthiness of the novel gold-blackened thermal array detectors,
and verify the thermal performance of the SBI's uncoated optics in a
vacuum environment. This work was funded by NASA under grant NAG5-10998.
---------------------------------------------------------
Title: The solar bolometric imager
Authors: Rust, D.; Bernasconi, P.; Foukal, P.
2002cosp...34E1200R Altcode: 2002cosp.meetE1200R
The balloon-borne Solar Bolometric Imager (SBI) will provide the
first bolometric (integrated light) maps of the photosphere, to
evaluate the photometric contribution of magnetic structures more
accurately than has been achievable with spectrally selective imaging
over restricted wavebands. More accurate removal of the magnetic
feature contribution will enable us to determine if solar irradiance
variation mechanisms exist other than the effects of photospheric
magnetism. The SBI detector is an array of 320 x 240 ferro -electric
thermal IR elements whose spectral absorptance has been extended and
flattened by a deposited layer of gold- black. The telescope itself is
a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex
mirrors. The combination of telescope and bolometric array provides
an image of the sun with a flat spectral response between 0.28 microns
and 2.6 microns, over a field of view of 15.2 x 11.4 min, and a pixel
size of 2.8 arcsec. After a successful set of ground-based tests, the
instrument is being readied for a one-day stratospheric balloon flight
that will take place in September 2003. The observing platform will be
the gondola previously used for the Flare Genesis Experiment (FGE),
retrofitted to house and control the SBI telescope and detector. The
balloon flight will enable SBI to image over essentially the full
spectral range accepted by non-imaging space-borne radiometers such
as ACRIM, making the data sets complementary. The SBI flight will also
provide important engineering data to validate the space worthiness of
the novel gold-blackened thermal array detectors, and verify the thermal
performance of the SBI's uncoated optics in a vacuum environment. This
work was funded by NASA under grant NAG5-10998.
---------------------------------------------------------
Title: Radiometric and Photometric Tests of Solar Luminosity Variation
Mechanisms
Authors: Foukal, P.
2001AGUSM..SP31B04F Altcode:
A variety of diagnostics has been developed to discriminate among
competing physical explanations of solar luminosity variation, and
to use them for new insight into magneto-convection on the sun and
similar stars. The shape of spot -induced irradiance dips correlates
well with changes in spot projected area, but not with growth or decay
rate. This argues against storage of the spot's missing heat flux in
the spot magnetic field.Efficient heat flow blocking and storage in the
HCZ seems to provide the simplest mechanism.The weakness of sunspot
bright rings and thermal shadows is consistent with eddy thermal
diffusivities calculated from models of the HCZ. The contribution
of faculae and network is still too uncertain to decide whether they
account for all of the remaining variance in luminosity, after spot
dimming is removed.But the correlation of irradiance variations is
much higher with the difference of compensating spot and facular
contributions than with total magnetic flux. This argues against
"magnetic stirring" as an important factor in luminosity variation. The
darkness of small flux tubes in continuum near disc center, especially
near 1.63 microns,and their center - to - limb contrast variation,
seems to favor their interpretation as photospheric heat leaks.This is
supported by the lowered facular temperature gradient measured using
two -color photometric imaging. Photometric searches for large - scale
photospheric temperature inhomogeneities have yielded useful upper
limits.Possible global variations in effective temperature, studied
through monitoring of photospheric limb-darkening, and of temperature
- sensitive Fraunhofer lines, have not revealed any convincing
variations.But the percentage sensitivity to solar irradiance
change is limited to about 0.2 per year, and might be improved
with helioseismological techniques. The decrease in facular-to-spot
area ratio observed at high solar activity levels suggests a simple
explanation for the increased photometric variability of younger sun -
like stars in terms of photospheric magnetism. The apparent absence
of detectable solar luminosity variations outside modulation due to
photospheric magnetism poses an interesting new constraint on stellar
convection theory. Ongoing advances in cryogenic radiometry, thermal
imaging,and helioseismology are all likely to contribute to the search
for possible more subtle luminosity variations below the threshold of
present measurements.
---------------------------------------------------------
Title: Total-Light Imager with Flat Spectral Response for Solar
Photometric Measurements
Authors: Foukal, Peter; Libonate, Scott
2001ApOpt..40.1138F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A measurement of the quiet network contribution to solar
irradiance variation
Authors: Foukal, Peter; Milano, Leo
2001GeoRL..28..883F Altcode:
A large increase in quiet network area since the 17<SUP>th</SUP>
century Maunder Minimum has been suggested as a mechanism
for increasing solar irradiance sufficiently to drive global
warming. We show that this mechanism requires essentially complete
disappearance of network proceeding back in time to the beginning of
the 20<SUP>th</SUP> century. This disappearance is ruled out by the
many Ca K spectroheliograms taken since the discovery of the network
in the early 1890's. Furthermore, network area measurements we have
carried out on Ca K spectroheliograms digitized from the Mt. Wilson
and NSO/Sacramento Peak archives, for the nine solar activity minima
between 1914 and 1996, show no evidence of network area variations large
enough to produce a significant long-term component of total irradiance
variation. A network brightness variation of sufficient magnitude
is also unlikely, given the linear dependence of solar microwave
flux on area of bright structures.More generally, recent analyses
of cycle 21,22 pyrheliometry, and of broadband stellar photometry,
provide little support for any long-term irradiance component These
results do not rule out a secular irradiance increase. But they suggest
that high climate sensitivity to the relatively small changes in solar
total and UV irradiance that have been observed, provides a more likely
explanation of the global temperature-solar activity correlation.
---------------------------------------------------------
Title: Division II: Sun and Heliosphere
Authors: Benz, Arnold O.; Bogdan, T.; Foukal, P. V.; Melrose, D. B.;
Solanki, S.; Vandas, M.; Webb, D. F.
2001IAUTB..24..110B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Total Light Imager with Flat Spectral Response for Solar
Photometric Measurements
Authors: Libonate, S.; Foukal, P.
2000SPD....3102118L Altcode: 2000BAAS...32..832L
The Solar Bolometric Imager (SBI) is a novel solar imaging
system optimized for studying mechanisms of total irradiance
variation. Uncertain broad-band photometric contrasts of spots, and
especially faculae and network, currently present the main obstacle
to improved modeling of total irradiance fluctuations. After 20 years
of effort, accurate contrasts remain elusive because the photometric
response functions of conventional camera and telescope systems are
highly wavelength dependent, and difficult to remove from measurements
of structures having non-black-body radiance distributions. The SBI
can provide the required data in a single image because it has the same
spectrally `flat' (i.e. constant) photometric response as pyrheliometers
such as ACRIM over the wavelength range between approximately 0.26 um
and 2.6 um, containing over 96% of the total solar irradiance. The
prototype SBI system at CRI utilizes a 50,000-element uncooled
thermal imaging array whose spectral absorptance has been flattened
by gold-blacking, without significantly degrading its modulation
transfer. We use a 30 cm-aperture Dall-Kirkham design with uncoated
(i.e. bare glass) primary and secondary mirrors to provide uniform
spectral response, and to avoid solar heating and saturation of the
imager. The image quality ( 5' resolution over a 13 X 7 arc minute FOV)
is very satisfactory for our purpose of accurately discriminating the
total irradiance contributions of photospheric magnetic structures,
such as spots, faculae and network from other possible solar heat flow
inhomogeneities. We are currently redesigning the (commercial) camera
electronics to reduce non-linearities and improve calibration accuracy
in the telescope. We expect the improved accuracy provided by the SBI to
significantly improve the constraints on possible slow changes in solar
irradiance that may drive secular climate variations. Balloon flight of
the SBI is necessary to avoid the most serious atmospheric transmission
variations; useful measurements could be obtained from a short-duration
flight, and the full potential of the SBI would be realized with a
long-duration underflight of a spaceborne pyrheliometer. This work
has been performed at Cambridge Research and Instrumentation (CRI),
under NASA research grant NAG5-6979.
---------------------------------------------------------
Title: A Measurement of the Quiet Network Contribution to Solar
Irradiance Variation
Authors: Foukal, P.; Milano, L.
2000SPD....31.0805F Altcode: 2000BAAS...32..840F
We use recently digitized, archived Ca K images obtained between
1905-present, to measure variations in the area of quiet chromospheric
network at the 9 solar activity minima between 1911 and 1996. We find
only a marginally significant increase, an order of magnitude smaller
than required to provide an increase in total irradiance even comparable
in magnitude to the small 0.1% increase observed radiometrically during
cycles 21 and 22. Our results cast doubt on the hypothesis that an
irradiance decrease caused by disappearance of the magnetic network
during the Maunder Minimum could influence global warming since the
17th century. Together with recent findings from stellar photometry
and solar radiometry, this result calls into question previous evidence
for a significant long-term component of irradiance change beyond the
small 11-year modulation measured by space-borne pyrheliometers.
---------------------------------------------------------
Title: Commission 12: Solar Radiation and Structure (Radiation et
Structure Solaires)
Authors: Foukal, Peter; Solanki, Sami; Mariska, J.; Baliunas, S.;
Dravins, D.; Duvall, T.; Fang, C.; Gaizauskas, V.; Heinzel, P.;
Kononovich, E.; Koutchmy, S.; Melrose, D.; Stix, M.; Suematsu, Y.;
Deubner, F.
2000IAUTA..24...73F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Division II: The Sun and Heliosphere: (Le Soleil et
Heliosphere)
Authors: Foukal, Peter; Ai, Guoxiang; Benz, Arnold; Engvold, Oddbjorn;
Solanki, Sami; Vandas, Marek; Verheest, Frank
2000IAUTA..24...65F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Bolometric Imager -A New Direction in Solar
Irradiance Studies
Authors: Libonate, S.; Foukal, P. V.
1999AAS...194.7608L Altcode: 1999BAAS...31Q.957L
The Solar Bolometric Imager (SBI) is a novel solar imaging system with
spectrally constant photometric response over all wavelengths between
the UV and IR, which will provide a new tool for studying mechanisms
of total irradiance variation. The SBI utilizes an 80,000 pixel,
uncooled thermal IR imaging array whose spectral absorptance has been
modified by CRI to provide uniform response over the wavelength range
between at least 0.3 um and 2.5 um, containing 95% of the total solar
irradiance. We have demonstrated that ferro-electric uncooled arrays
can be modified to meet the SBI's spectral uniformity requirements
with the deposition of gold blacks, and we have also identified
two promising approaches for modifying the spectral absorptance of
uncooled microbolometer arrays. A modified 8-bit Raytheon ferro-electric
camera is being tested in the lab and on a telescope, while a 12-bit
camera that will accommodate either ferro-electric or microbolometer
arrays, is under development. The prototype SBI telescope utilizes
a Dall-Kirkham design with uncoated (i.e. bare glass) primary and
secondary mirrors in order to provide uniform spectral response and
reduce the irradiance at the focal plane. Our present research focuses
on image quality, photometric precision, stray light, and solar heating
in this ground-based, prototype SBI. Ultimately, the SBI will be used
to measure and remove temporal variations in solar irradiance due to
photospheric magnetic structures, so that the importance of residual
variations that may drive secular climate variations associated with
global warming, can be determined. Much of the science potential of
the SBI could be realized in a balloon experiment while the combination
of the SBI and a cavity radiometer would constitute an excellent SMEX
experiment to address a key challenge identified in the Sun-Earth
Connection Roadmap issued by NASA/OSS. This work is supported by NASA
research grant number NAG5-6979.
---------------------------------------------------------
Title: Bolometric imager for solar irradiance studies
Authors: Foukal, Peter V.
1998SPIE.3442...34F Altcode:
We are presently developing a solar imager with spectrally uniform
photometric response over all wavelengths between the UV and IR. Such a
Solar Bolometric Imager (SBI) will be capable of accurately measuring
heat flow inhomogeneities at the sun's photosphere and will provide
an innovative new tool for identifying mechanisms of long-term solar
luminosity variation. Our work builds on recent advances in uncooled,
relatively high-definition thermal arrays. We have shown that the
spectral absorptance of these arrays can be modified by deposition of
gold blacks, to provide spectrally uniform response over at least the
wavelength range between about 0.3(mu) and 2.5(mu) containing over 95
percent of the total solar irradiance. Our ongoing work is intended to
show that quantitative photometry of the solar disc can be performed
with such a modified array. We are constructing a breadboard SBI for
immediate use with an 8-bit ferro- electric camera, developing a 12-bit
camera to make full use of the ferro-electric array's capabilities,
and optimizing our process of gold-blacking the TI arrays. Much
of the science potential of the SBI could be realized in a balloon
experiment. The combination of the SBI and a cavity radiometer would
also constitute an excellent SMEX experiment to address a key challenge
identified in the Sun- Earth Connection Roadmap recently issued by
NASA/OSS.
---------------------------------------------------------
Title: Extension of the F10.7 Index to 1905 using Mt. Wilson Ca
K Spectroheliograms
Authors: Foukal, Peter
1998GeoRL..25.2909F Altcode:
The F10.7 index provides a daily record of solar microwave emissions,
which vary in rough proportion to the projected area of bright magnetic
structures called plages and network, and also sunspots, on the sun's
disk. The daily observations used to form the index only began in
1947. Recently, we digitized the archive of daily Ca K spectroheliograms
obtained at Mt. Wilson Observatory between 1905-1984, and measured the
area variations of plages and enhanced network, on these photographic
plates. We calibrated these variations against the F10.7 index between
1947-1984, so we are able to construct a full-disk proxy of F10.7
extending back to 1905. The behavior of this extended index indicates
that UV irradiance levels achieved near the peaks of sunspot cycles
15, 16, and 17 between 1915-1945, were 25-40% higher than would be
estimated from behavior of the Zurich sunspot number, R<SUB>z</SUB>.
---------------------------------------------------------
Title: What Determines the Relative Areas of Spots and Faculae on
Sun-like Stars?
Authors: Foukal, Peter
1998ApJ...500..958F Altcode:
We analyze newly digitized Ca K plage area data extending back to
1915, and also the white-light facular area data beginning in 1874,
to investigate further our earlier finding that the area ratio of
faculae to spots decreases at increasing activity levels. We find
that this ratio decreases in plage as well as facular data, so it
cannot be an artifact of the visibility function of limb faculae. The
decrease is also accentuated in daily data, compared to annual means;
we explain this as a consequence of the different dependences of
facular, plage, and spot lifetimes upon their emergent magnetic
flux. From this we show that subphotospheric field properties are
more likely to determine this ratio, rather than photospheric field
diffusion rates. Systematic, cycle-to-cycle variations in its value
suggest an origin in fluctuations of the field generation mechanism;
specifically, a mechanism that produces a positive correlation between
magnetic flux generation efficiency, and relative power in the spatial
spectrum at low frequencies. Our results also suggest that main-sequence
stars about 50% more magnetically active than the present Sun might
exhibit ratio values an order of magnitude lower than current solar
values. This evidence strengthens our earlier argument that a rapid
shift toward dark photospheric structures in both active regions and
network provides the most likely explanation of the recently reported
sharp increase of photometric variability in late-type stars somewhat
more active than the Sun.
---------------------------------------------------------
Title: Plasma Electric Field Measurements as a Diagnostic of Neutral
Sheets in Prominences
Authors: Foukal, P.
1998ASPC..150..119F Altcode: 1998npsp.conf..119F; 1998IAUCo.167..119F
No abstract at ADS
---------------------------------------------------------
Title: Chirality, Helicity, and Joy's Law
Authors: Foukal, P.
1998ASPC..150..446F Altcode: 1998IAUCo.167..446F; 1998npsp.conf..446F
No abstract at ADS
---------------------------------------------------------
Title: Solar Irradiance Variations and Climate
Authors: Foukal, Peter
1998fsam.conf..103F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Behavior of solar magnetic plages measured from Mt. Wilson
observations between 1915-1984
Authors: Foukal, P.
1996GeoRL..23.2169F Altcode:
We describe the digitization and reduction of the daily solar
chromospheric spectroheliograms obtained in the K-line of Ca II
between 1915-1984 at Mt. Wilson Observatory. Our results provide
the first reliable information on the behavior of solar magnetic
plage area prior to 1947, thus extending by almost 70% the length of
the primary data base needed to model past total, UV and EUV solar
irradiances. Comparison with other solar activity indices confirms the
remarkably linear relation between plage areas, and sunspot number
and area, found in the post-1947 data . Our analysis also shows a
markedly non-linear relation between the areas of chromospheric plage,
and of photospheric white-light faculae. We provide an explanation
of this surprising nonlinearity between measurements of two magnetic
structures that are known to be closely related in the sun's atmosphere.
---------------------------------------------------------
Title: Spectrapolarimetry of the 15-9 Transition of HI as a Diagnostic
of Plasma Electric Fields
Authors: Foukal, P.
1996AAS...188.3620F Altcode: 1996BAAS...28..876F
An IR observation of a solar prominence by Brault and Noyes (1982)
showed the surprising intensity of high HI transitions such as 7-6, 9-7,
10-7, 11-8, 12-8 and 15-9, in the 8-12 micron atmospheric window. The
15-9 transition at 11.539 microns is of particular interest as a
diagnostic of plasma electric fields (and also of electron density)
because of its very large calculated Stark splitting (Casini and Foukal,
1994). We present preliminary results of our spectrapolarimetric
measurements on the 15-9 line in prominences using the FTS at the
McMath telescope at Kitt Peak. Our observed line profiles agree with
the structure calculated for this line in a Holtsmark electric field
at the plasma density of the prominence, taking into consideration
Zeeman effect in the prominence magnetic field B=10G. We discuss how
further spectrapolarimetry of this line could significantly increase
the measurement sensitivity of the wave-related and d.c. macroscopic
electric fields in the sun and in laboratory plasmas. This work is
supported by the Solar-Terrestrial Program of the National Science
Foundation under grant ATM-9301832.
---------------------------------------------------------
Title: Calculated profiles of H I lines of interest for solar plasma
electric field measurements
Authors: Casini, Roberto; Foukal, Peter
1996SoPh..163...65C Altcode:
We present calculated Stark-polarized line profiles for a number of
H I lines observed in the visible and infrared emission spectrum of
solar prominences and other limb activity. For use in measurements
of possible electric fields in these structures, we also calculate
curves giving the difference in line width between the 1/2 (I ± Q)
profiles as a function of electric-field intensity. Our calculations
take into account magnetic fields in these structures, and incorporate
typical observed values of Doppler broadening. These calculations
explicitly consider the H I fine structure neglected in previous
work, and thus are more accurate in the range of low to intermediate
electric-field intensity likely to be encountered in solar plasmas
(E < 10<SUP>3</SUP> V cm<SUP>−1</SUP>). Our results enable us to
compare behavior when E and B are parallel, or perpendicular. We draw
particular attention to the high electric-field sensitivity of the
transitions between high levels such as 12-8 and 15-9 in H I, observed
in prominences at wavelengths around 11μ. Their sensitivity is roughly
an order of magnitude larger than that of the high Paschen-series
lines used in solar plasma electric field studies so far.
---------------------------------------------------------
Title: Daily Digital Mt Wilson CaK Images and Plage Areas 1915-1985
Authors: Foukal, P.; Harman, M.; Risacher, S.; Yang, R.
1995SPD....26..513F Altcode: 1995BAAS...27..959F
No abstract at ADS
---------------------------------------------------------
Title: Testing MHD Models of Prominences and Flares with Observations
of Solar Plasma Electric Fields
Authors: Foukal, Peter V.; Behr, Bradford B.
1995SoPh..156..293F Altcode:
We present measurements of electric fields in quiescent prominences
and in a small flare surge, obtained with the CRI electrograph at
the NSO/SP 40 cm coronagraph, in 1993 and 1994. Our results on the
9 brightest quiescent prominences enable us to place r.m.s. upper
limits ofE<SUB>t</SUB> < 2 − 5 V cm<SUP>−1</SUP> on the
component ofE transverse to the line of sight. We show that these
upper limits may be difficult to reconcile with non-ideal MHD models
of quiescent prominences formed in extended neutral sheets, whether
or not the tearing mode instability is present. They do, however,
seem consistent with ideal MHD models of prominence support. We point
out also that these upper limits are within a factor 4 of the minimum
value of anistropic electric field that exists due to motional Stark
effect in any thermal plasma permeated by a directed magnetic field.
---------------------------------------------------------
Title: Solar irradiance variability and luminosity changes
Authors: Foukal, P.
1995HiA....10..294F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reply to the comment on "Stellar luminosity variations and
global warming", by R. R. Radick.
Authors: Foukal, P.
1994Sci...266.1073F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar Variability and Global Warming
Authors: Radick, Richard R.; Foukal, Peter
1994Sci...266.1072R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar Luminosity Variations and Global Warming
Authors: Foukal, Peter
1994Sci...264..238F Altcode:
Recent studies indicate that variation in the sun's luminosity is
less than that observed in many other stars of similar magnetic
activity. Current findings also indicate that in more active stars,
the attenuation by faculae of sunspot luminosity modulation is less
effective than in the sun at present. The sun could thus become
photometrically more variable (and dimmer) if its magnetic activity
exceeded present levels. But the levels of solar activity required for
this to occur are not observed in carbon-14 and beryllium-10 records
over the past several millennia, which indicates that such an increase
in amplitude of surface magnetism-driven variations in solar luminosity
is unlikely in the present epoch.
---------------------------------------------------------
Title: Measurements of Electric Fields in Coronal Magnetic Structures
(Invited)
Authors: Foukal, P. V.; Behr, B. B.
1994scs..conf..177F Altcode: 1994IAUCo.144..177F
The authors review the use of Stark effect to measure d.c. and
wave-related electric fields predicted in models of coronal and flare
heating, prominence support, and related phenomena. They describe the
Mk II limb electrograph now in routine operation at NSO/Sac Peak.
---------------------------------------------------------
Title: Properties of Faculae from Observations Near the Opacity
Minimum
Authors: Foukal, P.; Moran, T.
1994IAUS..154...23F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Study of solar irradiance variations holds key to climate
questions
Authors: Foukal, P.
1994EOSTr..75..377F Altcode:
This article is part of a series in Eos that investigates issues
in space physics and aeronomy.In 1838, the French physicist Claude
Pouillet published the first measurement of the Sun's total light
and heat input to the Earth. He described his new instrument—the
pyrheliometer—and the corrections he made for attenuation of solar
light in the Parisian atmosphere. Similar measurements were carried
out by the English astronomer Sir John Herschel, working at about the
same time at the Cape of Good Hope.
---------------------------------------------------------
Title: The Curious Case of the Greenwich Faculae
Authors: Foukal, Peter
1993SoPh..148..219F Altcode:
We analyze the record of facular areas compiled by the Royal Greenwich
Observatory (RGO) from daily white-light observations between 1874
and 1976. Curiously, the relative amplitudes of the three largest
sunspot cycles 17, 18, and 19 in this record are reversed when they are
ranked by facular area. We show that this negative correlation arises
from a general decrease of the ratioA<SUB>F</SUB>/A<SUB>S</SUB>,
of facular to sunspot area, with increasingA<SUB>S</SUB>. Within
a given cycle,A<SUB>F</SUB>/A<SUB>S</SUB>decreases in active
regions of largeA<SUB>S</SUB>, butA<SUB>F</SUB>/A<SUB>S</SUB>is
also lower at allA<SUB>S</SUB>, in cycles of higher peak amplitude
inA<SUB>S</SUB>. This decrease ofA<SUB>F</SUB>/A<SUB>S</SUB>in
large spot groups is consistent with its decrease in younger, more
active solar-mass stars, and it may explain why stars only slightly
more magnetically active than the Sun tend to exhibit much greater
variability in broad-band photometry. We suggest that the physical
explanation is an increased spatial filling factor of magnetic flux,
favoring formation of sunspots over faculae. We also explain why the
decrease inA<SUB>F</SUB>/A<SUB>S</SUB>is not seen in the disc-integrated
Ca K plage areas, nor in theF10.7 microwave index, both of which exhibit
remarkable linearity when plotted against smoothed sunspot area. This
explanation suggests how complementary data on faculae and plages from
RGO and Mt. Wilson could be used to improve empirical models of total
irradiance variation, extending back to 1874.
---------------------------------------------------------
Title: Measurement of Plasma Electric Fields in Prominences
Authors: Foukal, P.; Behr, B.
1993BAAS...25Q1206F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The RISE Precision Solar Photometric Telescope Project
Authors: Kuhn, J. R.; Foukal, P. V.
1993BAAS...25.1184K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Photometric Study of Faculae and Sunspots Between 1.2-MICRONS
and 1.6-MICRONS
Authors: Moran, T.; Foukal, P.; Rabin, D.
1992SoPh..142...35M Altcode:
We investigate further the interpretation of dark magnetic faculae
observed in previous imaging of the solar photosphere at 1.63 μm. We
show that their contrast at 1.63 μm increases with magnetic flux
beyond a threshold value of Φ ∼ 2 × 10<SUP>18</SUP> Mx and blends
smoothly with the contrast vs flux relation measured at this wavelength
for larger structures of sunspot size. Not all facular structures that
are bright in Ca K are dark at 1.63 μm, apparently because their
magnetic flux is not large enough. After correction for blurring,
the contrast of the dark faculae observed near the disc center at
1.63 μm is approximately 4%. But our observations at 1.23 μm,
which probe slightly higher photospheric levels, do not show these
dark faculae. These results indicate that magnetic flux tubes of
diameter as small as 500 km significantly inhibit convective heat
flow to the photosphere, much as do sunspot flux tubes of much larger
diameter. They also suggest that, in even smaller flux tubes, the
inhibition becomes rapidly less significant. Finally, we show that the
sunspot-size dependence of umbral infrared contrast versus wavelength
that we observe can probably be explained in terms of instrumental
blurring. Observations with lower scattered light will be required
to determine whether a real decrease of contrast with diameter also
plays a role.
---------------------------------------------------------
Title: Darkness can illuminate
Authors: Foukal, Peter
1992Natur.358..285F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Case of the Missing Faculae
Authors: Foukal, P. V.
1992AAS...180.0703F Altcode: 1992BAAS...24..738F
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Solar Astrophysics
Authors: Foukal, P. V.
1992S&T....83..173F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Luminosity Variation
Authors: Foukal, Peter
1992ASPC...27..439F Altcode: 1992socy.work..439F
No abstract at ADS
---------------------------------------------------------
Title: Do Changes in the Photospheric Magnetic Network Cause the 11
Year Variation of Total Solar Irradiance?
Authors: Foukal, P.; Harvey, K.; Hill, F.
1991ApJ...383L..89F Altcode:
Changes in the area of the photospheric magnetic network over the
sunspot cycle have been put forward as the 'missing component'
required to explain the 11-yr variation of total solar irradiance
observed by space-borne radiometers. It is shown that this explanation
is consistent with recent measurements of the photometric contrast of
magnetic faculae and with the present measurement of the network area
change during cycle 21.
---------------------------------------------------------
Title: An Electrograph for Measurement of Macroscopic Electric Fields
in Prominences and Flares
Authors: Moran, T.; Foukal, P.
1991SoPh..135..179M Altcode:
We describe an `electrograph' instrument designed for measurement of
macroscopic electric fields in solar plasmas, using the polarization
dependence of line width in Stark-broadened hydrogen Paschen emission
lines. Observations of quiescent prominences and limb chromosphere with
our electrograph at the NSO/Sac Peak Evans Coronal Facility provide
upper limits of 5-10 V cm<SUP>−1</SUP> for transverse macroscopic
electric fields in these structures, averaged over an area of about 5
× 7 arc sec. Random thermal motions of hydrogen ions across magnetic
field lines generate a quasi-static electric field, which should be
distinguishable from pressure broadening in the intensely magnetized
chromosphere over a sunspot, given an electrograph sensitivity a factor
2-3 better than that achieved here. Future electrograph measurements
of limb flares, post-flare loops and eruptive prominences, even at 5
V cm<SUP>−1</SUP> sensitivity, could provide a useful new test of
reconnection and discharge effects in such dynamic structures.
---------------------------------------------------------
Title: Electric Fields in the Solar Atmosphere - a Review
Authors: Foukal, P.; Hinata, S.
1991SoPh..132..307F Altcode:
Macroscopic electric fields in the solar atmosphere have received
much less attention than magnetic fields, although they must play
a role of comparable importance in plasma heating, and in charged
particle acceleration and transport. We review various remote sensing
techniques that have been developed, whose sensitivity is now 5-10
V cm <SUP>−1</SUP> for measurement of the electric field component
transverse to the line-of-sight. Our review of the processes most likely
to produce observable fields in the solar atmosphere indicates that
quasi-static, macroscopic values of E<SUB>∥</SUB> (the electric field
component parallel to the magnetic vector) well above this detection
threshold are predicted by the discharge model of flares, by models of
return currents associated with flare particle beams, and by models
of neutral sheets associated with two-ribbon flares and post-flare
loops. In addition, both E<SUB>∥</SUB> and E<SUB>⊥</SUB> components
may be detectable in time dependent electric fields associated with MHD
and plasma waves, and with plasma turbulence. The emission measures
and time-scales associated with these electrified plasma volumes
are as highly uncertain as our present understanding of the volumes,
plasma conditions and processes involved in the liberation of flare
energy. Observations of electric field vector intensities, orientations,
time-behaviour and spatial distribution at the presently attained
electric field sensitivity levels could provide new, direct information
of great interest in the electrodynamics of solar magnetic structures.
---------------------------------------------------------
Title: On the Role of the Photospheric Magnetic Network in the
11-Year Variation of Total Solar Irradiance
Authors: Harvey, K. L.; Foukal, P. V.
1991BAAS...23.1068H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Objectives and Scope of RISE
Authors: Foukal, P.
1991BAAS...23.1040F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar astronomy
Authors: Rosner, Robert; Noyes, Robert; Antiochos, Spiro K.; Canfield,
Richard C.; Chupp, Edward L.; Deming, Drake; Doschek, George A.;
Dulk, George A.; Foukal, Peter V.; Gilliland, Ronald L.
1991aap..reptR....R Altcode:
An overview is given of modern solar physics. Topics covered include
the solar interior, the solar surface, the solar atmosphere, the Large
Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the
High Energy Solar Physics mission, the Space Exploration Initiative,
solar-terrestrial physics, and adaptive optics. Policy and related
programmatic recommendations are given for university research and
education, facilitating solar research, and integrated support for
solar research.
---------------------------------------------------------
Title: Measurement of polarization-dependent Stark broadening as a
diagnostic of electric fields in the solar atmosphere.
Authors: Moran, T.; Foukal, P.
1991sopo.work..393M Altcode:
Electric fields play a key role in models of energy dissipation and
charged particle acceleration in flares, as well as other dynamic
solar phenomena. An instrument designed to observe the transverse
(to the line of sight) component of such electric fields has been
constructed and installed at the 40 cm coronagraph at Sacramento
Peak. This "electrograph" measures the halfwidth of Stark-broadened
hydrogen emission lines as a function of polarization in the sky
plane. Thus, the instrument resembles a transverse magnetograph except
that it operates on Stark-broadened hydrogen emission lines in coronal
structures rather than on Zeeman-broadened absorption lines on the
disk. The authors describe the principles behind the instrument,
its design, and some first data.
---------------------------------------------------------
Title: Solar Irradiance Variability from Modern Measurements
Authors: Froehlich, C.; Foukal, P. V.; Hickey, J. R.; Hudson, H. S.;
Willson, R. C.
1991suti.conf...11F Altcode:
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.
---------------------------------------------------------
Title: The program on radiative inputs of the Sun to the Earth (RISE).
Authors: Foukal, Peter V.
1990NASCP3086..116F Altcode: 1990cisv.nasa..116F
General concerns about changes in the Earth's climate and in the ozone
layer have increased the importance of measuring and understanding
variations in the sun's radiative outputs. These outputs appear, for
instance, at the top of the list of global change forcing agents in the
recent FY-90 document on the U.S. Global Change Program. Significant
advances have been made over the past decade in radiometry of the total
solar irradiance. Photometry of light variations in stars similar to
the sun, but much younger, is providing new insights into the sun's
variations in luminosity and UV radiation at previous epochs of interest
to paleoclimate studies. Measurement of the sun's 11-year output
variability in the ultraviolet and extreme ultraviolet still possess
a challenge of great importance. All of these topics are addressed by
the program on Radiative Inputs of the Sun to Earth (RISE). RISE is a
5-year program of observations, data analysis, and theory, that has been
defined at two workshops held in Boulder, Colorado in November 1987,
and in Tucson, Arizona in October 1989. These meetings involved about 60
solar and atmospheric physicists, and stellar astronomers. A proceedings
of the 1987 workshop was issued in early 1988. A substantial component
of the program, consisting of ground-based observations, data analysis,
and theory, is directed at the National Science Foundation (NSF) and
the program is seeking a FY-92 funding start at the NSF. RISE also makes
recommendations on measurements required from NASA and NOAA satellites.
---------------------------------------------------------
Title: Solar cycle modulation of total irradiance: an empirical
model from 1874 to 1988.
Authors: Lean, J.; Foukal, P.
1990NASCP3086..197L Altcode: 1990cisv.nasa..197L
Evidence acquired during the past decade indicates that over time scales
of the solar cycle, enhanced emission from bright solar faculae cause
significant variations in the suns's total irradiance even though, on
shorter time scales, the most pronounced variations are those resulting
from the passage of dark sunspots across the solar disc. An empirical
model which accounts for the competing effects of dark sunspots and
bright faculae has been developed from the available radiometry in cycle
21, and extended back to the beginning of solar cycle 12. According to
this model, the largest 11-year modulation of total irradiance during
the C20th occurred in the most recent cycle 21.
---------------------------------------------------------
Title: Die veränderliche Sonne.
Authors: Foukal, P. V.
1990SpWis...4...66F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Luminosity Variations over Timescales of Days to the
past Few Solar Cycles
Authors: Foukal, P.
1990RSPTA.330..591F Altcode: 1990RSLPT.330..591F
Dips in the total irradiance of up to 0.2% and lasting typically 10
days are now well known to be caused by the transit of dark sunspots
across the photospheric disc. The large bright magnetic faculae usually
associated with spots cause irradiance increases of comparable magnitude
although the form of their signal is more subtle. Radiometry from five
satellites beginning in late 1978 indicates a minimum in irradiance
at the epoch of lowest magnetic activity between solar cycles 21 and
22. Analysis of these radiometric measurements indicates that this
irradiance decline between about 1981 and 1986 was caused mainly by
decay in the excess radiation of bright faculae in the magnetic network
outside of active regions. Empirical models of irradiance modulation
extending back to 1874 indicate that the Sun is typically about 0.05%
brighter at activity maximum than at minimum.
---------------------------------------------------------
Title: Astronomical Determinations of the Solar Variability:
Discussion
Authors: Stanford, J. S.; Ribes, Elizabeth; Foukal, P.
1990RSPTA.330..497S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Infrared Imaging of Faculae at the Deepest Photospheric Layers
Authors: Foukal, P.; Little, R.; Graves, J.; Rabin, D.; Lynch, D.
1990ApJ...353..712F Altcode:
The NOAO 58 x 62 InSb array and the National Solar Observatory McMath
telescope are used to image the deepest photospheric layers of three
active regions at the 1.63-micron opacity minimum. The faculae are
darker than the photosphere, with a measured contrast of at least 2
percent at positions on the disk with mu = 0.75-1.0. Near the limb,
they are brighter than the photosphere, as in the visible. At mu =
0.5-0.75, they are difficult to detect at 1.63 micron. The observation
that faculae and their immediate surroundings exhibit a clear deficit
of brightness temperature near disk center at 1.63 micron seems to
rule out the hillock model put forward to explain their center-to-limb
contrast variation.
---------------------------------------------------------
Title: An Electrograph for the Measurement of Electric Fields in
Coronal Structures
Authors: Moran, T.; Foukal, P.
1990BAAS...22..794M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Empirical Model of Total Solar Irradiance Variation Between
1874 and 1988
Authors: Foukal, P.; Lean, J.
1990Sci...247..556F Altcode:
An empirical model of variations in the total solar irradiance caused
by observed changes in photospheric magnetic activity between 1874 and
1988 is presented. The model provides a remarkably good representation
of the irradiance variations observed by satellite-borne radiometers
between 1980 and 1988. It suggests that the mean total irradiance has
been rising steadily since about 1945, with the largest peak so far at
about 1980 and another large peak expected during the current solar
cycle 22. But it is doubtful whether even this rise can contribute
significantly to global warming, unless the temperature increase of
about 0.02^circC that it produces in current energy balance models
seriously underestimates the sensitivity of climate to solar irradiance
changes.
---------------------------------------------------------
Title: The variable sun
Authors: Foukal, Peter V.
1990SciAm.262b..34F Altcode: 1990SciAm.262...34F
The variations in the sun's magnetic field, in its total light
output, and in its output of charged particles are discussed. Present
understanding of the causes of these variations is reviewed. The
effects of these variations on the earth are addressed.
---------------------------------------------------------
Title: The variable sun.
Authors: Foukal, P. V.
1990SciAm.262b..26F Altcode: 1990SciAm.262...26F
The sun's steady warmth and brightness are illusory; the sun's output
of radiation and particles varies. Systematic observations are beginning
to unveil the causes of these changes and their effects on the earth.
---------------------------------------------------------
Title: Solar astrophysics
Authors: Foukal, Peter
1990soas.book.....F Altcode: 1990QB521.F68......
The present work discusses advancements in such aspects of solar
astrophysics as radiative transfer in the solar atmosphere,
solar spectroscopic techniques, the dynamics of solar plasmas,
the solar photosphere, and the sun's internal structure and energy
generation. Also treated are solar rotation and advection, observations
of solar photospheric activity and magnetism, the solar chromosphere and
corona, solar prominences and flares, the dynamics of the solar magnetic
field, the solar wind and heliosphere, and the variability of the sun.
---------------------------------------------------------
Title: Solar luminosity variations over timescales of days to the
past few solar cycles.
Authors: Foukal, P.
1990ecvs.conf..591F Altcode:
Dips in the total irradiance of up to 0.2% and lasting typically 10
days are now well known to be caused by the transit of dark sunspots
across the photospheric disc. The large bright magnetic faculae usually
associated with spots cause irradiance increases of comparable magnitude
although the form of their signal is more subtle. Radiometry from five
satellites beginning in late 1978 indicates a minimum in irradiance
at the epoch of lowest magnetic actvity between solar cycles 21 and
22. Analysis of these radiometric measurements indicates that this
irradiance decline between about 1981 and 1986 was caused mainly by
decay in the excess radiation of bright faculae in the magnetic network
outside of active regions. Empirical models of irradiance modulation
extending back to 1874 indicate that the Sun is typically about 0.05%
brighter at activity maximum than at minimum.
---------------------------------------------------------
Title: Comment on `asymmetry and variations of solar limb darkening
along the diameter defined by diurnal motion in April 1981' by Neckel
and Labs (1987)
Authors: Foukal, P.
1989SoPh..120..249F Altcode:
In a recent paper (Neckel and Labs, 1987a) a strong claim is made
for detection of surprisingly large variations in photospheric limb
darkening over time-scales of minutes to hours. Some of this evidence
relies on re-interpretation of our measurements carried out at Kitt
Peak between 1980-1982 (Petro et al., 1984). The purpose of this
Comment is to draw attention to information we have published which
suggests that the variations noted by Neckel and Labs are more likely
to be of instrumental than solar origin.
---------------------------------------------------------
Title: Infrared Imaging of Faculae at the Deepest Photospheric Layers
Authors: Foukal, P.; Little, R.; Graves, B.; Rabin, D.; Lynch, D.
1989BAAS...21..828F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Radiative Output Variation Program
Authors: Foukal, P. V.
1989BAAS...21..832F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Infrared Imaging of Sunspots and Faculae at the Photospheric
Opacity Minimum
Authors: Foukal, P.; Little, R.; Mooney, J.
1989ApJ...336L..33F Altcode:
Continuum observations at 1.63-microns using a PtSi IR CCD camera
have yielded images of the deepest observable layers in an active
region. When these are compared with spectroheliograms in visible
continuum and in the Ca K wing, it becomes evident that faculae
are seen near disk center as low-contrast, dark structures; also,
the ratio of umbral intensity for small relative to large spots is
substantially lower at 1.63 microns than in the red continuum. These
findings suggest that, at the deepest observable layers, faculae as well
as spots contain plasma which is cooler than the surrounding photosphere
at equal optical depth; they can be explained straightforwardly if flux
tubes of all diameters exhibit convective heat flow along their axes.
---------------------------------------------------------
Title: Magnetic Modulation of Solar Luminosity by Photospheric
Activity
Authors: Foukal, P.; Lean, J.
1988ApJ...328..347F Altcode:
The authors study the behavior of slow changes in solar irradiance,
S, using measurements obtained with radiometers on the SMM and Nimbus
7 spacecraft. The analysis of the 1978 - 1984 ACRIM and ERB radiometry
reveals low-amplitude (0.04% - 0.07%) variations in S on time scales of
4 - 9 months that are well correlated between these two data bases. The
variations correlate very well with changes in facular radiations. It is
also shown that the slow downtrend in S seen since 1981 by the ACRIM and
ERB arises mainly from a decreasing irradiance contribution of bright
photospheric magnetic elements outside the large faculae included in
the daily CaK plage index. The finding that this network contribution
is unbalanced over several years shows that photospheric activity has
a net influence on solar luminosity, besides the more nearly balanced
contributions of the spots and the large faculae.
---------------------------------------------------------
Title: A Model of Solar Luminosity Modulation by Magnetic Activity
between 1954 and 1984
Authors: Lean, J.; Foukal, P.
1988Sci...240..906L Altcode:
A simple model based on the changes in excess radiation from bright
magnetic faculae and on changes in reduced radiation from dark spots is
remarkably successful in matching the slow variations of total solar
irradiance measured simultaneously by the ERB and ACRIM satellite
radiometers between 1981 and 1984. This model was extended back to
1954 to reconstruct the modulation of irradiance by magnetic activity
during the past three 11-year solar cycles. The model predicts that the
sun is consistently brighter at activity maximum than at minimum. The
0.07 percent brightening at the peak of the last cycle in 1980 was more
pronounced than the brightenings found for either of the two previous
cycles, even though cycle 19, which peaked around 1957, had the largest
sunspot number amplitude in the history of reliable sunspot records.
---------------------------------------------------------
Title: Infrared Imaging of Magnetic Faculae at the Photospheric
Opacity Minimum
Authors: Foukal, P.; Little, R.; Mooney, J.
1988BAAS...20..689F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Paschen-line Stark-broadening as an electric field diagnostic
in erupting prominences
Authors: Foukal, P.; Little, R.; Gilliam, L.
1988SoPh..114...65F Altcode:
We analyse the Stark broadening of the Balmer and Paschen lines emitted
in two bright eruptive prominences, to determine the total electric
field in these structures. We show that the Paschen lines provide a
significantly more sensitive and accurate electric field indicator than
the Balmer lines used previously in such studies. In the two eruptive
events analysed here, the total electric fields agree to within 5-10
V cm<SUP>-1</SUP> with the pressure-broadening fields expected from
local densities of the cool plasma, measured simultaneously and
co-spatially by a line-ratio diagnostic. We conclude that in such
structures the upper limit to any widespread macroscopic fields is
roughly 10 V cm<SUP>-1</SUP> or less. This is in agreement with the
motional electric field that might be associated with reconnection
at the observed rate of the prominences' outward motion of about 135
km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Solar Radiative Output Variation
Authors: Foukal, Peter
1988srov.proc.....F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Model of Solar Luminosity Modulation by Magnetic Activity
Between 1954-1984
Authors: Foukal, P.; Lean, J.
1988srov.proc..323F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Introduction
Authors: Foukal, P.
1988srov.procD...1F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theoretical interpretation of total solar irradiance variations
Authors: Foukal, P.
1988AdSpR...8g..43F Altcode: 1988AdSpR...8...43F
Analysis of the radiometric record of the total solar irradiance
obtained over the past ten years has revealed characteristic variations
caused by sunspots and faculae, the magnetic network, and also the
sun's internal oscillations and photospheric convection. Our physical
understanding of the variations caused by spots and faculae in active
regions is discussed in this review, and the results are applied to
interpretation of recently discovered solar irradiance changes over
time scales of several months to most of an 11 year activity cycle.
---------------------------------------------------------
Title: Measurement of macroscopic electric fields in solar plasma
structures
Authors: Foukal, Peter
1987crii.rept.....F Altcode:
The work performed in this effort has led to several important advances
in our investigation of a new technique for remote sensing of plasma
electric fields. The atomic physics underlying the technique is placed
on a firmer basis and the polarization structure of the high Balmer
and Paschen lines behaves as predicted by perturbation theory. Balmer-
and Paschen-line observations of erupting prominences are obtained
at Sacramento Peak Observatory and show that the marked increase in
sensitivity of the technique expected with the Paschen lines can
be realized. Understanding of the interpretation of the observed
hydrogen-line broadening is improved, and shown that the sensitivity
of the technique, when applied to structures emitting intense hydrogen
lines, can exceed 10 volts/cm. This sensitivity is more than adequate
to test predictions of transverse electric fields intensities and
orientations, in models of flares and other active phenomena. The goal
is to construct an instrument optimized for solar plasma electric field
measurements, to be installed at the SPO Big Dome. Such an electrograph
would make important advances toward understanding the mechanisms and
prediction of solar eruptions such as flares and active filaments.
---------------------------------------------------------
Title: Active region flows.
Authors: Foukal, Peter
1987NASCP2483...15F Altcode: 1987tphr.conf...15F
A wide range of observations has shown that active region phenomena
in the photospheric, chromospheric and coronal temperature regimes
are dynamical in nature. At the photosphere, recent observations of
full line profiles place an upper limit of about + or - 20/msec on
any downflows at supergranule cell edges. Observations of the full
Stokes 5 profiles in the network show no evidence for downflows in
magnetic flux tubes. In the area of chromospheric dynamics, several
models were put forward recently to reproduce the observed behavior of
spicules. However, it is pointed out that these adiabatic models do not
include the powerful radiative dissipation which tend to damp out the
large amplitude disturbances that produce the spicular acceleration
in the models. In the corona, loop flows along field lines clearly
transport mass and energy at rates important for the dynamics of
these structures. However, advances in understanding the heating
and mass balance of the loop structures seem to require new kinds
of observations. Some results are presented using a remote sensing
diagnostic of the intensity and orientation of macroscopic plasma
electric fields predicted by models of reconnective heating and also
wave heating.
---------------------------------------------------------
Title: Magnetic Modulation of Solar Luminosity by Photospheric
Activity
Authors: Foukal, P.; Lean, J.
1987BAAS...19..924F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flare Electric Fields Measured by Stark Effect in Paschen
and Balmer Lines
Authors: Foukal, P.; Little, R.; Gilliam, L.
1987BAAS...19..950F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Performance of a Schottky Barrier and HgCdTe Infrared
Arrays in Monochromatic Light
Authors: Graves, B.; Foukal, P.; Rieke, M.; Fowler, A.
1987BAAS...19..928G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Photometric Search for Solar Giant Convection Cells
Authors: Chiang, W. -H.; Petro, L. D.; Foukal, P. V.
1987SoPh..110..129C Altcode:
We limit the photometric contrast of solar giant convection cells
using λ525.6 nm continuum images obtained on 15 days in May 1985. The
r.m.s. of the giant cell intensity pattern must be less than or equal
to the observed r.m.s. on spatial scales 80 to 240 Mm which is 0.023%
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 r.m.s. of the giant cell pattern may be placed, namely 0.016%
or 0.23 K. This limit is consistent with temperature perturbations
estimated from recent nonlinear simulations of global-scale solar
convection. We use this limit on the r.m.s. of the giant cell pattern
to estimate that the contribution of giant cells to the fluctuation
of the solar irradiance on a one-month time-scale is less than 3 ×
10<SUP>−5</SUP> S.
---------------------------------------------------------
Title: Physical interpretation of variations in total solar
irradiance.
Authors: Foukal, P.
1987JGR....92..801F Altcode:
Radiometry from the Solar Maximum Mission and Nimbus 7 satellites
has demonstrated that the solar constant varies at a peak-to-peak
level of up to 0.2 percent on time scales of weeks. The rotation and
evolution of dark spots and bright faculae across the sun's disk
accounts for most of that variation. Reasonable explanations have
been put forward to explain how the spot-blocked heat flow might
be stored, and to explain the source of the intense radiation that
gives rise to the increased irradiance produced by the bright magnetic
faculae. Time-dependent models of the response of the solar convection
zone to small perturbations also indicate that slower variations
in total solar irradiance of comparable magnitude are likely. More
precise observations of the total solar irradiance and radius over
long time scales are required to demonstrate the existence of such
climatologically relevant changes, and to test models that would enable
the interpretation and prediction of these changes.
---------------------------------------------------------
Title: Paschen-line Stark-broadening as an electric field diagnostic
in erupting prominences
Authors: Foukal, P.; Little, R.; Gilliam, L.
1987SoPh..114...65F Altcode:
The authors analyse the Stark broadening of the Balmer and Paschen lines
emitted in two bright eruptive prominences, to determine the total
electric field in these structures. They show that the Paschen lines
provide a significantly more sensitive and accurate electric field
indicator than the Balmer lines used previously in such studies. In
the two eruptive events analysed here, the total electric fields
agree to within 5 - 10 V cm<SUP>-1</SUP> with the pressure-broadening
fields expected from local densities of the cool plasma, measured
simultaneously and co-spatially by a line ratio diagnostic. The authors
conclude that in such structures the upper limit to any widespread
macroscopic fields is roughly 10 V cm<SUP>-1</SUP> or less. This is in
agreement with the motional electric field that might be associated
with reconnection at the observed rate of the prominences' outward
motion of about 135 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Electric Fields and Plasma Structure in Coronal Magnetic Loops
Authors: Foukal, P.; Hoyt, C.; Gilliam, L.
1986ApJ...303..861F Altcode:
Solar flare loop spectra are analyzed by means of recently developed
density diagnostics to determine if the Stark effect observed in the
Balmer emission lines is due to pressure broadening or the presence
of macroscopic electric fields. The database covers five postflare
loops and three active prominences. The diagnostics include the
density sensitivity of the intensity ratios of strong Na I D, Mg
I b, and the Mg I lines relative to the Sr II lines. A total of 43
spectra were obtained in the 3500-6000 A interval, with most lines
falling in the 3600-4500 A region. A prism at the entrance to the
instrument permitted extracting additional data on the structure of
Stark-broadener Balmer lines. A dc electric field was detected, with
an energy level of 100 V/cm, oriented transverse to the loop magnetic
field vector. Electron densities of 1-10 trillion/cu cm were estimated
for the cool structures within loops and 100 billion/cu cm in active
prominences. The densities indicated that a pressure deficit may be
present in the loop structures in the corona, suggesting an inward
directed gas pressure force in cool loop structures.
---------------------------------------------------------
Title: Electric Fields in Coronal Loops
Authors: Foukal, P.; Hoyt, C.; Gilliam, L.
1986BAAS...18..708F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Influence of Faculae on Total Solar Irradiance and
Luminosity
Authors: Foukal, P.; Lean, J.
1986ApJ...302..826F Altcode:
The authors investigate the facular contribution to the total solar
irradiance, using the daily active cavity radiometer irradiance monitor
(ACRIM) radiometry for 1980 - 1982 and the Earth Radiation Budget
(ERB) radiometry for 1978 - 1982. A cross-correlation analysis of
the total irradiance, the UV flux measured by Nimbus 7 and calculated
contributions from observed sunspot and facular areas is presented. It
is found that the solar irradiance records, after subtraction of
the calculated sunspot-blocking contribution, exhibit a short-term
modulation that is better explained by faculae than by errors in the
sunspot-blocking function. The data for 1980 indicate that the facular
contribution was comparable to that of spots over time scales of active
region evolution in that year.
---------------------------------------------------------
Title: Differential photometry of magnetic faculae
Authors: Foukal, P.; Duvall, T., Jr.
1985ApJ...296..739F Altcode:
Previous photometric observations carried out at the KPNO vacuum
telescope showed that faculae become clearly visible near disk center
in the difference of two widely separated (green and red) continuum
passbands. The authors present new observations extending this technique
to the near-infrared, to demonstrate that the main factor determining
the facular visibility is the difference in H<SUP>-</SUP> opacity
between the continuum passbands. It is shown that the difference
signal is directly sensitive to changes in the temperature gradient
near τ<SUB>0.5</SUB> = 1 between quiet photosphere and faculae. The
measured facular temperature gradient is compared with that given
by empirical facular models based on Fraunhofer line observations,
and with theoretical predictions.
---------------------------------------------------------
Title: Photospheric Limb Darkening Signatures of Global Structure
Variations
Authors: Petro, L. D.; Foukal, P. V.; Kurucz, R. L.
1985SoPh...98...23P Altcode:
Observations of short-term irradiance variations and consideration
of mechanisms of the solar activity cycle suggest the possibility
of long-term variation of the solar flux. Since the limb darkening
is sensitive to effective temperature and convective efficiency,
observations of the solar limb darkening may provide a useful means
to detect and study long-term global variations. The limb-darkening
responses to impulsive variation (in depth) of the source function,
to effective temperature variation, and to convection variations are
presented. For the variations considered, the limb-darkening variation
is approximately linearly proportional to the associated parameters. The
minimum detectable amplitude of those parametric variations is derived
as a function of observational noise. Given our demonstrated errors
of observation, single-parameter sensitivies are 3 K for effective
temperature variation and 0.007 for local mixing-length variation for
year to year changes at 99% confidence.
---------------------------------------------------------
Title: The Influence of Faculae on Sunspot Heat Blocking
Authors: Chiang, W. -H.; Foukal, P.
1985SoPh...97....9C Altcode:
We study the influence of faculae on sunspot heat blockage using
a thermal model based on eddy heat diffusion through the convection
zone. The facula is represented as a localized area of excess emission
surrounding the sunspot, which is represented as a thermal plug. Our
computations using a range of reasonable combinations of spot and
facular depths show no significant influence of the facula on the long
storage times of heat blocked by sunspots. However, the local cooling
of surface layers produced by excess facular emission in this model
propagates globally within the convection zone in a similar way to
the heating produced by a spot. The net effect of spots and faculae on
L<SUB>⊙</SUB> over time scales longer than an active region lifetime
should thus be determined by the global sum of sunspot flux deficits
and facular excesses.
---------------------------------------------------------
Title: Investigation of Photospheric Limb-Darkening Variation Between
1980 and 1985
Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Pierce, A. K.;
Kurucz, R. L.
1985BAAS...17..644P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A study of solar photospheric limb-darkening variations
Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Kurucz, R. L.;
Pierce, A. K.
1984ApJ...283..426P Altcode:
The authors have obtained regular observations of photospheric limb
darkening, using the McMath Solar Telescope, to study possible slow
changes in the global temperature structure, in T<SUB>eff</SUB>, and in
the ultraviolet continuum flux from the quiet Sun. This paper reports
on the analysis of data obtained on 15 days between 1980 September and
1982 December in a continuum window at λ4451. There are no variations
of global limb darkening exceeding 0.1% at the 99% confidence level. The
implications of these measurements for slow changes in solar luminosity,
convection zone structure, and ultraviolet flux are discussed.
---------------------------------------------------------
Title: A photometric study of heat flow at the solar photosphere
Authors: Foukal, P.; Fowler, L.
1984ApJ...281..442F Altcode:
Results of a general survey of photospheric heat flow on scales between
granulation and global scale convection are presented. Photometric
rasters in the visible continuum show a prominent mottle pattern whose
scale and morphology resemble the CaK chromospheric network, but the
pattern's correlation with photospheric magnetic fields is too low
and its lifetime of 5-10 minutes is too short to support any physical
connection to the supergranulation. The network exhibits a slight excess
continuum brightness relative to the nonmagnetic photosphere at mu =
1. An upper limit of 2-3 K is placed on nonaxisymmetric brightness
inhomogeneities over scales of 50,000-200,000 km. This limit provides
a more direct constraint on models of global scale convection than do
existing limits on a pole-equator temperature difference. The absence
of a thermal shadow preceding an observed spot group seems to favor
the higher eddy thermal conductivity profile of Spruit's (1974) model
compared to that of Baker and Temesvary (1966).
---------------------------------------------------------
Title: On the Interpretation of Fraunhofer Line Doppler Shifts at
Supergranule Boundaries
Authors: Miller, P.; Foukal, P.; Keil, S.
1984SoPh...92...33M Altcode:
We have used the SPO tower telescope and echelle spectrograph to study
differences in the profiles of three FeI lines, between magnetic
network and cells. Ca K slit-jaw pictures were used to identify
the network and cell areas, and mean network and cell profiles were
computed from digitized spectra for the g = 0 lines λλ4065, 5434,
and the g = 1.5 line λ 5233. The profile bisectors show that the wings
of all three lines are red-shifted in the network by between 75-200
m s<SUP>−1</SUP> relative to the cell profiles. But the redshift
decreases in the line core and becomes less than the standard error of
20 m s<SUP>−1</SUP> near the line core minimum. This disappearance
of the redshift at the cores of all 3 lines formed over the height
range 250-500 km above τ<SUB>0.5</SUB> = 1, argues against a steady
downflow at supergranule boundaries. We show that such red-shifted wings
and a relatively unshifted core can result if granular convection is
suppressed near the network flux tubes, without implying any downflow
in the vicinity of these flux tubes. Our results also indicate that
searches for large-scale convective velocity patterns should measure
shifts of the line core, rather than the line wings which appear to
be very sensitive to inhomogeneities in granule structure.
---------------------------------------------------------
Title: Photometric studies of heat flow at the photosphere.
Authors: Foukal, P.
1984NASCP2310...97F Altcode: 1984siva.work...97F
The main subject of this review is the photospheric photometry carried
out at KPNO since 1980. Also described are some results obtained from
comparison of the photometry with a time-dependent model of heat
flow developed at Atmospheric and Environmental Research Inc. for
interpretation of sunspot effects on solar luminosity.
---------------------------------------------------------
Title: Erratum: A Photometric Study of Heat Flow at the Solar
Photosphere
Authors: Foukal, P.; Fowler, L.
1984ApJ...286..377F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiometric and photometric studies of Solar luminosity
variations
Authors: Foukal, P.
1984stp..conf..411F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interpretation of Electric Fields in Coronal Magnetic Loops
Authors: Foukal, P.; Landman, D.
1984uxsa.coll...25F Altcode: 1984uxsa.conf...25F; 1984IAUCo..86...25F
No abstract at ADS
---------------------------------------------------------
Title: Variation of Solar Limb Darkening
Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Kurucz, R. L.;
Pierce, A. K.
1983BAAS...15..951P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar constant
Authors: Foukal, Peter; Hoyt, Douglas
1983Natur.303..372F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspot Bright Rings and the Thermal Diffusivity of Solar
Convection
Authors: Fowler, L. A.; Foukal, P.; Duvall, T., Jr.
1983SoPh...84...33F Altcode:
We have used the 512 channel diode array and vacuum telescope at KPNO
to study the photospheric intensity distribution around sunspots,
for comparison with isotherms predicted by convective blocking models
of heat flow. Raster scan observations of 10 spots on 18 days were
carried out in 1980 and 1981. Continuum passbands of 0.25 Å width
were selected to avoid contamination by weak Fraunhofer lines, whose
strength is sensitive to the presence of magnetic faculae often found
near spots. Our observations show no evidence of extended bright rings
around the spots at the level of 1-2%, as reported in one recent study
using photographic photometry and much wider passbands. But 6 of the
10 spots we measured show marginally significant (2-3σ) bright rings
of peak amplitude 0.1-0.3%. We are not able to explain these rings as a
result of either residual facular signal, or instrumental effects. The
excess radiative flux in these rings is small compared to the missing
flux in the spot umbra and penumbra. We compare the brightness of the
observed rings with peak brightnesses calculated from models of heat
flow around spots of various depths and radii. Even if the spot is
assumed to be unrealistically shallow, a detectable bright ring requires
that the effective thermal conductivity (and/or its depth gradient)
in layers surrounding the spot be significantly lower than the values
indicated by mixing length models of the solar convection zone.
---------------------------------------------------------
Title: A thermal model of sunspot influence on solar luminosity
Authors: Foukal, P.; Fowler, L. A.; Livshits, M.
1983ApJ...267..863F Altcode:
It is shown that the observed properties of the solar irradiance dips
can be easily explained by the conventional thermal blocking model of
sunspots extended to include time dependence. The model does not rule
out energy transfer between spots and faculae, but it does not require
it. The analysis indicates that the heat blocked in proportion to a
spot's area and contrast is stored very efficiently in the slightly
increased thermal and potential energy of the solar convection zone. The
radiative flux blocked during high sunspot activity periods is only
radiated away over many subsequent 11-year cycles. It is pointed out
that this efficient storage implies a contribution to variation of the
solar luminosity and irradiance over the 1-year cycle, at an amplitude
that can be computed from the known variation of sunspot areas.
---------------------------------------------------------
Title: Interpretation of Fraunhofer-Line Red Shifts at Supergranule
Boundaries
Authors: Foukal, P.; Miller, P.; Keil, S.
1983BAAS...15..719F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Electric Fields in Coronal Magnetic Loops
Authors: Foukal, P.; Miller, P.; Gilliam, L.
1983SoPh...83...83F Altcode:
We analyze spectra taken with the 40 cm coronograph at Sacramento
Peak Observatory, for evidence of Stark effect on Balmer lines formed
in coronal magnetic structures. Several spectra taken near the apex
of a bright post-flare loop prominence show significant broadening
from H<SUB>10</SUB> to the limit of Balmer line visibility in these
spectra, at about H<SUB>20</SUB> The most likely interpretation of
the increasing width is Stark broadening, although unresolved blends
of Balmer emissions with metallic lines could also contribute to the
trend. Less significant broadening is seen in 3 other post-flare loops,
and the data from 5 other active coronal condensations observed in
this study show no broadening tendency at all, over this range of
Balmer number. The trend clearly observed in one post-flare loop
requires an ion density of n<SUB>i</SUB> ≃ 2 × 10<SUP>12</SUP>
cm<SUP>−3</SUP>, if it is to be explained entirely as Stark effect
caused by pressure broadening. But mean electron densities measured
directly from the Thomson scattering at λ3875 in the same SPO spectra,
yield n<SUB>e</SUB> ≲ 3−7 × 10<SUP>10</SUP> cm<SUP>−3</SUP>
for the same condensations observed within that loop. Comparison of
this evidence from electron scattering, with densities derived from
emission measures and line-intensity ratios, argues against a volume
filling factor small enough to reconcile the values of n<SUB>i</SUB>
and n<SUB>e</SUB> derived in this study. This discrepancy leads us to
suggest that the Stark effect observed in these loops, and possibly
also in flares, could be caused by macroscopic electric fields, rather
than by pressure broadening. The electric field required to explain
the Stark broadening in the brightest post-flare loop observed here is
approximately 170 V cm<SUP>−1</SUP>. We suggest an origin for such an
electric field and discuss its implications for coronal plasma dynamics.
---------------------------------------------------------
Title: A Summary of the Joint Discussion at Patras on Solar Luminosity
Variations
Authors: Eddy, J. A.; Foukal, P. V.
1983HiA.....6...79E Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Study of Variations in Photospheric Limb-Darkening and
Solar Luminosity
Authors: Rosen, W.; Petro, L.; Foukal, P.; Pierce, K.
1982BAAS...14..922R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Photometric Study of Low-Amplitude Temperature
Inhomogeneities at the Photosphere
Authors: Fowler, L. A.; Foukal, P.; Duvall, T., Jr.
1982BAAS...14..938F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Helium-Cooled Absolute Cavity Radiometer for Solar Irradiance
Measurement
Authors: Foukal, P.; Miller, P.
1982BAAS...14..922F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Electric Fields in Coronal Magnetic Loops
Authors: Foukal, P.; Miller, P.; Gilliam, L.
1982BAAS...14..923F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Sunspot Bright Rings and Comparisons with a
Convective Blocking Model
Authors: Fowler, L. A.; Foukal, P.
1982BAAS...14..624F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Variations in photospheric limb darkening as a diagnostic of
changes in solar luminosity
Authors: Rosen, W. A.; Foukal, P. V.; Kurucz, R. L.; Pierce, A. K.
1982ApJ...253L..89R Altcode:
The paper reports on photospheric limb-darkening measurements
obtained with the McMath Solar Telescope in July, September, and
October 1980 as part of a continuing program to investigate possible
long-term variations in the photospheric emergent flux. A total of
243 usable full-diameter scans were recorded over seven days in the
clean continuum window at 4451-25 A. The limb darkening was found to
decrease significantly between September 25 and 26. It is suggested
that this decrease was caused by a decrease of the temperature gradient
in the upper photosphere in the region above approximately tau(5000) =
0.5. The small increase in effective temperature that might accompany
this limb-darkening variation is estimated using a standard radiative
equilibrium photospheric model.
---------------------------------------------------------
Title: The thermal structure of solar coronal loops and implications
for physical models of coronae
Authors: Raymond, J. C.; Foukal, P.
1982ApJ...253..323R Altcode:
EUV spectra of three active region loops observed above the solar limb
with the SO55 spectrometer on Skylab are analyzed. It is noted that the
lengths, peak temperatures, and pressures of the loops are typical of
the X-ray coronal loops to which static models have been applied. It
is found that the physical parameters of the coronal loop plasma
derived from EUV spectra and raster pictures are not well represented
by the static models. Although the loops also contain a significant
quantity of cool plasma, no physical reason is found to differentiate
them from other active region loops of similar length, pressure,
and temperature. Several line ratios in the loop spectrum suggest
departures from ionization equilibrium caused by rapid cooling. The
source of this cooling material is discussed with reference to several
models of loop dynamics.
---------------------------------------------------------
Title: Variations in solar luminosity.
Authors: Foukal, P. V.
1982SAOSR.392B..17F Altcode: 1982csss....2...17F
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Loops in the Sun's Atmosphere
Authors: Foukal, P.
1981S&T....62..547F Altcode:
It is noted that the magnetic loops are again the subject of wide
attention because they are the basic features created by the sun's
magnetic field in the tenuous material of its outer atmosphere. Today
the structure of the larger loops can be resolved over a wide range of
plasma temperature and density. This offers the possibility of testing
the processes which have been proposed to carry wave or electric-current
energy from the photosphere to the much hotter chromosphere and corona
above. It is noted that evidence exists for large electric fields in
solar loops during and after flares. Their presence in general would
have a pronounced influence on problems ranging from the motions of
ionized material across magnetic fields to effects on the ionization
potentials of atomic species in the plasma.
---------------------------------------------------------
Title: Detection of a temperature deficit in magnetic faculae at
the solar photosphere
Authors: Foukal, P.; Duvall, T., Jr.; Gillipsie, B.
1981ApJ...249..394F Altcode:
A photometric technique is presented which allows the observation
of faculae across the entire solar disk. A clean continuum was
used and granular noise was reduced by subtracting simultaneous
spectroheliograms taken at widely separated points of the photospheric
radiation curve. Spectroheliographic observations taken from March
12-14, 1980, using two color-scans in the green at gamma 4980.96 and
gamma 5256.31 in the fourth order resulted in observation of clean
continua in the red at gamma 6641.85 and gamma 7008.8 in another
slot of the magnetograph. Approximately 20 raster scans were obtained
and indications were found that dark magnetic facular structures in
the Delta signal were cooler than the photosphere at equal optical
continuum depths
---------------------------------------------------------
Title: Erratum - the CI Opacity and Physical Structure of Cool Very
Dense Plasma in the Solar Corona
Authors: Foukal, P.
1981ApJ...247..382F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The C I opacity and physical structure of cool, very dense
plasma in the solar corona
Authors: Foukal, P.
1981ApJ...245..304F Altcode:
The physical structure of the cool material in the volume of the solar
corona is investigated. The observational evidence is summarized in a
table giving the brightness contrast in optical and EUV radiations. The
state of ionization and the opacity sources are discussed, and
a physical model is described that is consistent with the EUV and
optical data. It is noted that a comparison of the EUV raster pictures
with H-alpha and Ca K photoheliograms indicates that the material is
commonly injected from below into relatively low-lying magnetic loops
(fibrils) seen near sunspots and plages.
---------------------------------------------------------
Title: A Photometric Study of Heat Flow Inhomogeneities at the
Solar Photosphere
Authors: Foukal, P.; Fowler, L. A.; Duvall, T., Jr.
1981BAAS...13..879F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photospheric Limb Darkening as a Ground-based Diagnostic for
Variations in the Solar Effective Temperature
Authors: Rosen, W. A.; Foukal, P. V.; Kurucz, R.; Pierce, A. K.
1981BAAS...13..551R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Detection of a Temperature Deficit in Magnetic Faculae at
the Solar Photosphere
Authors: Foukal, P.; Duvall, T., Jr.; Gillespie, B.
1981BAAS...13..551F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Propagation of Magnetically Guided Acoustic Shocks in the
Solar Chromosphere
Authors: Foukal, P.; Smart, M.
1981SoPh...69...15F Altcode:
We study the propagation of a train of acoustic shocks guided by
diverging magnetic fields through a static model of the solar
chromospheric network and transition region. Our results show
that for initial flux densities of the order 10<SUP>6</SUP> ergs
cm<SUP>−2</SUP> s<SUP>−1</SUP> in the lower chromosphere, the
local efficiency of acoustic transmission into the corona can be much
higher than calculated for a plane parallel atmosphere. Thus acoustic
energy will tend to be deposited at higher chromospheric levels in
diverging magnetic fields, and magnetic guiding may well influence
the temperature profile of the network and plages. But the total flux
that can be transmitted into the corona along such diverging fields is
severely limited, since the magnetic elements occupy a small fractional
area of the photosphere, and the transmission efficiency is a rapidly
decreasing function of initial acoustic flux density. We conclude that
diverging magnetic fields and a varying ratio of specific heats are
not likely to allow high frequency shocks to dissipate high enough in
a static atmosphere, to contribute significantly to the coronal energy
balance. This result strengthens the view that acoustic waves do not
heat the solar corona. However, the conclusion may be sensitive to
the influence of observed mass motions, such as spicules.
---------------------------------------------------------
Title: Sunspots and changes in the global output of the sun
Authors: Foukal, P.
1981phss.conf..391F Altcode:
Solar radiometry data from the SMM and Nimbus-7 spacecraft are examined
for evidence that decreases in the solar radiance constant can be
accounted for by the presence of sunspot groups. Additionally, a storage
mechanism is described within the convective zone, where the energy is
held for later release. Short-time scale studies have revealed a 0.4%
peak-to-peak solar variation, while an rms variation in the radiance
has been set at about 0.05%. The results of numerical modeling of the
storage time expected (for time-dependent heat diffusion through the
upper convection zone of the sun) to validate the convection zone
as the storage medium show that adequate storage times do exist in
that region. The implications of the calculations for a possible 11-yr
modulation of the solar constant are discussed, as are further studies
to verify the model.
---------------------------------------------------------
Title: Solar Luminosity Variation on Directly Observable Time Scales :
Observational Evidence and Basic Mechanisms
Authors: Foukal, P. V.
1981sucl.conf..275F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comments on the thermodynamic structure and dynamics of the
cool solar corona over sunspots
Authors: Foukal, P.
1981phss.conf..191F Altcode:
The results of 9 mos of spectrophotometric observations of sunspots
with the Harvard S055 spectrophotometer on the ATM on board Skylab are
summarized. The corona over the sunspot was found to contain plasma 1-2
orders of magnitude cooler than surrounding active region media. The
extended sunspot atmosphere displays open or closed loops reaching
from 30,000-40,000 above the photosphere. The plumes and loops have
a temperature gradient transverse to the magnetic field, with the
coolest portion concentrated around the loop axis. No certain value
for the local density of the plasma was derived, particularly for a
two-order-of-magnitude increase with the coolest material, which is
necessary to define a constant gas pressure transverse to the magnetic
field. The plasma in the cooler atmosphere, lacking hydrostatic support,
was observed to stream downward at 10-100 km/sec.
---------------------------------------------------------
Title: On the secular behavior of seasonal changes in ozone column
density
Authors: Vernazza, J.; Foukal, P.
1980GeoRL...7..993V Altcode:
We have studied the daily Dobson measurements of ozone column made
between 1957 and 1975. The ozone column density measured at the seasonal
minima in the northern hemisphere, shows a clear increase over these
years. The amount of increase is approximately 4-5% between latitudes
of 15N and 60N, and 11% in the data obtained between 60N and 90N. The
seasonal maxima show no evidence for an increase during the same
time period, in the northern hemisphere. We conclude that the secular
increase of global ozone noted between approximately 1960 and 1970 by
a number of authors, is caused by a physical mechanism that leads to
an increase in the seasonal minima, while allowing the seasonal maxima
to remain roughly constant.
---------------------------------------------------------
Title: Comment on `average photospheric poloidal and toroidal magnetic
field components near solar minimum' by Duvall et al.
Authors: Foukal, P.; Duvall, T. L., Jr.
1980SoPh...67....9F Altcode: 1980STIA...8047634F
We discuss the dynamical interpretation of evidence for an azimuthal
tilt of the global magnetic field from the radial direction at the
photosphere. We point out that the Reynolds stresses of supergranular
convective motions might produce the required small tilt of intense flux
tubes, without implying an unacceptably large momentum flux across the
photospheric surface into the solar wind. Our calculations lead us to
conclude that there is little reason, at present, to infer (Duvall et
al., 1979) a separate low intensity constituent of the global magnetic
field, from the observational evidence for an azimuthal tilt. More
precise measurements of the vertical component of supergranular motions
would be useful in determining the actual torque exerted by the Reynolds
stresses on the magnetic field.
---------------------------------------------------------
Title: A study of acoustic heating and forced convection in the
solar corona
Authors: Foukal, P. V.
1980aeri.rept.....F Altcode:
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.
---------------------------------------------------------
Title: Does the Sun's Luminosity Vary?
Authors: Foukal, Peter
1980S&T....59..111F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar luminosity variation on short time scales - Observational
evidence and basic mechanisms
Authors: Foukal, P.
1980asfr.symp...29F Altcode:
Temporal variations in total solar luminosity on time scales between 100
and 10 to the 9th sec are discussed. Following a brief historical review
of solar constant measurements, observations of solar luminosity and its
variations made by ground-level radiometry, radiometry from balloons,
aircraft and rockets, continuous radiometry from space probes and
satellites, measurements of reflected light from solar system bodies,
and measurements of solar line depths and limb darkening are presented
which demonstrate solar constant variations of less than 1.2% since
1962, no variation over a period of 30 years in the range 0.34 and 2.4
microns, and an influence of magnetic activity. Specific processes which
may account for these variations are then examined, including heat flux
perturbations due to local variations in thermal impedance, variations
in convective heat transport efficiency, energy storage in magnetic
fields, and variations in wave heating at the photosphere. Comparison of
solar evidence with evidence of luminosity variations in other late-type
stars indicates that magnetic activity can influence luminosity on time
scales from days to tens of years. Future prospects for experimental
observations from spacecraft and from the ground are indicated.
---------------------------------------------------------
Title: A Doppler measurement with low scattered light of the higher
rotation rate of sunspot magnetic fields at the photosphere.
Authors: Foukal, P.
1979ApJ...234..716F Altcode:
We have measured the wavelength differences between two lines of Fe
I and Ti I formed in sunspot umbrae, and two reference lines of N I
and Na I, formed in the photosphere outside these spots. From repeated
observations of 10 spots, we find that in the eastern hemisphere, the
umbral lines are increasingly blueshifted relative to the photospheric
lines with increasing distance from the central meridian. In the
west, they show an increasing relative redshift. The Doppler shift
behavior caused by steady velocity fields around sunspots, and other
possible sources of systematic error, are considered in detail. The
observed change of sign of the relative Doppler shift at the central
meridian indicates that the umbral plasma and the magnetic flux
tubes are rotating faster than the local photosphere. The mean value
of the increased rotation speed over the latitude range 120 degrees,
derived from the 10 spots studied in 1978, is Av = 148 + 31 ms-1. Light
scattered in the coronagraph optical system and atmosphere during these
observations is very small, and, in any case, would tend to reduce the
value ofAVfound in this differential Doppler measurement. Thus these
results demonstrate that the higher rotation rate of sunspots (and
presumably of other rapidly rotating tracers) is not an instrumental
effect. This measurement also shows that the higher rotation rate is
the result of a systematic translation of magnetic flux tubes through
the local photosphere. Subject headings: Sun: magnetic fields - Sun:
rotation - Sun: sunspots
---------------------------------------------------------
Title: The effect of magnetic fields on solar luminosity.
Authors: Foukal, P.; Vernazza, J.
1979ApJ...234..707F Altcode:
The paper presents an investigation into the influence of magnetic
fields in sunspots and faculae on solar luminosity, using measurements
of the solar constant from ground level and from space. Attention
is given to an analysis that shows that it is difficult to devise an
atmospheric mechanism that would rapidly lower visible and infrared
transmission in response to sunspots, increase it in response to
faculae, and anticipate the magnetic development of these features
by about one day. It is shown that the phase shift of the luminosity
variation provides a promising new technique to determine the depth
at which the magnetic fields of sunspots and faculae redistribute the
flow of convective energy.
---------------------------------------------------------
Title: Angular velocity gradients in the solar convection zone.
Authors: Gilman, P. A.; Foukal, P. V.
1979ApJ...229.1179G Altcode:
Numerical calculations of Boussinesq nonaxisymmetric convection
in a rotating spherical shell are reported which were performed to
study how convection in the supergranule layer redistributes angular
momentum. It is found that supergranules are at best weakly influenced
by rotation and can be largely responsible for the radial gradient
of angular velocity observed in the thin supergranule layer below the
photosphere. The results indicate that convection in a thin spherical
shell weakly influenced by rotation can produce a substantial outward
decrease in rotational velocity that approaches the limit predicted for
radially moving particles that conserve their angular momentum. This
phenomenon is shown to provide a plausible explanation for the observed
difference in angular velocity between sunspots and the photospheric
plasma.
---------------------------------------------------------
Title: Solar luminosity variation: observational evidence and basic
mechanisms.
Authors: Foukal, P. V.
1979BAAS...11..423F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Luminosity Variation: Observational Evidence and Basic
Mechanisms
Authors: Foukal, P. V.
1979BAAS...11Q.421F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar luminosity variation on short time scales: Observational
evidence and basic mechanisms
Authors: Foukal, P. V.
1979LPICo.390...37F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effect of Sunspots and Faculae upon the Solar Constant
and Atmospheric Transmission
Authors: Foukal, P.; Vernazza, J.
1978BAAS...10R.620F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic loop, downflows, and convection in the solar corona.
Authors: Foukal, P.
1978ApJ...223.1046F Altcode:
Optical and extreme-ultraviolet observations of solar loop structures
show that flows of cool plasma from condensations near the loop apex
are a common property of loops associated with radiations whose
maximum temperature is greater than approximately 7000 K and less
than approximately 3,000,000 K. It is suggested that the mass balance
of these structures indicates reconnection by means of plasma motion
across field lines under rather general circumstances (not only after
flares). It is shown that the cool material has lower gas pressure than
the surrounding coronal medium. The density structure of the bright
extreme ultraviolet loops suggests that downflows of cool gas result
from isobaric condensation of plasma that is either out of thermal
equilibrium with the local energy deposition rate into the corona,
or is thermally unstable. The evidence is thought to indicate that
magnetic fields act to induce a pattern of forced convection.
---------------------------------------------------------
Title: Supergranulation and the dynamics of gas and magnetic field
below the solar photosphere.
Authors: Foukal, P.
1977ApJ...218..539F Altcode:
Results from recent calculations of convection are used to discuss
the interpretation of three observations which seem to bear upon the
dynamics of gas and magnetic fields below the photosphere but which
are not dependent on the difficult direct study of the wave number,
phase, and symmetry of large-scale low-amplitude motions on the
sun. These observations include the scale of supergranulation, the
rotation rate of sunspots, and the tilt of magnetic flux tubes at the
photosphere. It is suggested that the discrete scale of supergranular
flow, the high rotation rate of sunspots, and the azimuthal tilt of
magnetic fields from a radial orientation are most simply explained if
one supposes that a transition occurs in the structure of the magnetic
field at a depth of about 15,000 km below the photosphere. The gas
and magnetic field would be strongly coupled dynamically and comoving
below this depth; above the transition, the field would be confined to
a small fraction of the total volume, and the convecting gas would be
largely nonmagnetic. In this case, the supergranulation could be the
convection mode that reflect simply the full depth of the transition
layer. Implications of this layer are considered for the dynamics and
energy balance of magnetic features at the photosphere.
---------------------------------------------------------
Title: The effect of sunspots and faculae on the solar constant.
Authors: Foukal, P. V.; Mack, P. E.; Vernazza, J. E.
1977ApJ...215..952F Altcode:
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.
---------------------------------------------------------
Title: Magnetic fields and solar convection and rotation.
Authors: Foukal, P. V.
1977BAAS....9..375F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: INVITED PAPER - Magnetic Fields and Solar Convection and
Rotation.
Authors: Foukal, P.
1977BAAS....9..374F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of Gas and Magnetic Field in the Supergranular Layer
Authors: Foukal, Peter V.
1977lsms.proc..113F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The pressure and energy balance of the cool corona over
sunspots.
Authors: Foukal, P. V.
1976ApJ...210..575F Altcode:
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.
---------------------------------------------------------
Title: Plasma diagnostic techniques in the ultraviolet: the C III
density-sensitive lines in the sun.
Authors: Dupree, A. K.; Foukal, P. V.; Jordan, C.
1976ApJ...209..621D Altcode:
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.
---------------------------------------------------------
Title: The Relative Rotation of Gas and Magnetic Field at the
Photosphere
Authors: Foukal, P. V.
1976BAAS....8..344F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectroscopic evidence for a higher rotation rate of magnetized
plasma at the solar photosphere.
Authors: Foukal, P.
1976ApJ...203L.145F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Extreme-ultraviolet transients observed at the solar pole.
Authors: Withbroe, G. L.; Jaffe, D. T.; Foukal, P. V.; Huber, M. C. E.;
Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza,
J. E.
1976ApJ...203..528W Altcode:
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.
---------------------------------------------------------
Title: Initial results from the EUV spectroheliometer on ATM
Authors: Reeves, E. M.; Timothy, J. G.; Foukal, P. V.; Huber, M. C. E.;
Noyes, R. W.; Vernazza, J. E.; Withbroe, G. L.; Schmahl, E. J.
1976skls.conf...73R Altcode:
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.
---------------------------------------------------------
Title: Ultraviolet observations of C III transitions in the sun.
Authors: Dupree, A. K.; Foukal, P.; Jordan, C.
1976BAAS....8..292D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Temperature Structure and Pressure Balance of Magnetic
Loops in Active Regions
Authors: Foukal, P.
1975SoPh...43..327F Altcode:
EUV observations show many active region loops in lines formed at
temperatures between 10<SUP>4</SUP>K and 2×l0<SUP>6</SUP>K. The
brightest loops are associated with flux tubes leading to the umbrae
of sunspots. It is shown that the high visibility of certain loops in
transition region lines is due principallly to a sharp radial decrease
of temperature to chromospheric values toward the loop axis. The
plasma density of these cool loops is not significantly greater than
in the hot gas immediately surrounding it. Consequently, the internal
gas pressure of the cool material is clearly lower. The hot material
immediately surrounding the cool loops is generally denser than the
external corona by a factor 3-4. When the active region is examined in
coronal lines, this hot high pressure plasma shows up as loops that
are generally parallel to the cool loops but significantly displaced
laterally. In general the loop phenomenon in an active region is
the result of temperature variations by two orders of magnitude and
density variations of around a factor five between adjacent flux tubes
in the corona.
---------------------------------------------------------
Title: Time variations in extreme-ultraviolet emission lines and
the problem of coronal heating.
Authors: Vernazza, J. E.; Foukal, P. V.; Huber, M. C. E.; Noyes,
R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Withbroe, G. L.
1975ApJ...199L.123V Altcode:
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.
---------------------------------------------------------
Title: On the rotation of gas and magnetic fields at the solar
photosphere.
Authors: Foukal, P.; Jokipii, J. R.
1975ApJ...199L..71F Altcode:
We point out that observations of a 5 percent velocity difference
between photospheric gas and magnetic structures at a given latitude
may simply result from angular momentum conservation by fluid elements
in the convection zone. Estimates of the viscosity and magnetic drag
are considered, and we conclude that they probably are not large enough
to enforce strictly rigid rotation.
---------------------------------------------------------
Title: The Pressure Balance and Currents in Active Region Loop
Structures
Authors: Foukal, P. V.
1975BAAS....7..346F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Contribution of Active Regions to Solar Variation in the
Visible and Near Infrared
Authors: Foukal, P.
1975scea.conf..109F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: EUV Observations of the Active Sun from the Harvard Experiment
on ATM
Authors: Noyes, R. W.; Foukal, P. V.; Huber, M. C. E.; Reeves, E. M.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1975IAUS...68....3N Altcode:
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.
---------------------------------------------------------
Title: Extreme-Ultraviolet Observations of Coronal Holes: Initial
Results from SKYLAB
Authors: Huber, M. C. E.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1974ApJ...194L.115H Altcode:
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.
---------------------------------------------------------
Title: Extreme-Ultraviolet Observations of Sunspots with the Harvard
Spectrometer on the Apollo Telescope Mount
Authors: Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.;
Timothy, J. G.; Vernazza, J. E.; Wilhbroe, G. L.; Huber, M. C. E.
1974ApJ...193L.143F Altcode:
EUV spectroheliograms show that the areas directly above sunspot
umbrae are the brightest features in an active region by an order of
magnitude in the chromospherecorona transition region (1 K < T <
108 K.) Rarios of density-sensitive lines in the transition region
show a significant decrease in gas density over the umbra relative
to surrounding plage. We deduce that the temperature gradient in the
transition region over the spot is decreased by an order of magnitude
or more, relative to the plage. S#ject headings: chromosphere, solar -
corona, solar - spectroheliograms - sunspots
---------------------------------------------------------
Title: Initial results from the EUV spectroheliometer on ATM.
Authors: Reeves, E. M.; Timothy, J. G.; Foukal, P. V.; Huber, M. C. E.;
Noyes, R. W.; Schmahl, E. J.; Vernazza, J. E.; Withbroe, G. L.
1974aiaa.conf.....R Altcode:
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.
---------------------------------------------------------
Title: A three-Component Concept of the Chromosphere and Transition
Region
Authors: Foukal, P.
1974SoPh...37..317F Altcode:
It is proposed that present observations of the chromosphere and
transition region in EUV, optical and mm wavelengths are best described
by a three-component concept. The three components are taken to be: the
interiors of supergranular cells, the hot plagettes overlying faculae,
and the cooler, transient mottles which surround them in the network
boundaries. The enhanced emission of the hot plagettes in transition
ions is interpreted as a direct result of the increased pressure scale
height over faculae relative to the cell interiors.
---------------------------------------------------------
Title: Extreme Ultraviolet Solar Spectra from Skylab-Apollo Telescope
Mount.
Authors: Dupree, A. K.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.;
Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.;
Withbroe, G. L.
1974BAAS....6..349D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Measurement of Electron Temperature in the Orion Nebula
from the Intensity Ratio of Forbidden Transitions in S III
Authors: Foukal, Peter
1974PASP...86..211F Altcode:
The temperature dependence of the intensity ratio R = I( 9532 +
9069)!I( 6312) of three forbidden transitions within the 3p2 ground
configuration of S m was calculated, and the ratio has been observed
in the Orion nebula. The observed value of R = 56 i 25% leads to an
electron temperature of 97000 K i 10000 K, after a reasonable reddening
correction is made. This value of R is a factor of three higher than
that derived from the results of previous observers, which would give Te
= 1.5 X 1040 K even before correction for reddening. Comparison of the
theoretical and observed intensity ratios of the lines A9069 and A9532
leads to the conclusion that telluric H2O absorption is not likely to be
an explanation of the difference in observed values of R It is suggested
that observations at a spectral resolution too low to separate A6300
of 0i from A6312 of S in will underestimate the value of R by about
the factor noted above. Key words: nebula - electron temperature
---------------------------------------------------------
Title: Solar Prominences in the Extreme Ultraviolet as Observed from
the Apollo Telescope Mount
Authors: Schmahl, E. J.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.;
Reeves, E. M.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1974SoPh...39..337S Altcode:
Observations of quiescent solar prominences with the Harvard College
Observatory spectrometer abroad 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.
---------------------------------------------------------
Title: Observations of the Chromospheric Network: Initial Results
from the Apollo Telescope Mount
Authors: Reeves, E. M.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1974ApJ...188L..27R Altcode:
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 v K (the Lyman continuum) and 3 X 10 K (0
vi). In the emission of Mg x, a coronal line formed at about 1.5 X 108
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 3 X 10 and 1.5 X 108 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. Subject headings: granules and supergranules, solar-
spectra, ultraviolet
---------------------------------------------------------
Title: Solar EUV Photoelectric Observations from SKYLAB
Authors: Reeves, E. M.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1974IAUS...57..497R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Study of the Active Region McMath 12417 with the Harvard
ATM EUV Spectrometer.
Authors: Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1973BAAS....5..432F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: ATM Observations of Solar Flares in the Extreme Ultraviolet.
Authors: Noyes, R. W.; Foukal, P. V.; Huber, M. C. E.; Reeves, E. M.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1973BAAS....5..433N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Prominences-in the EUV as Observed from ATM.
Authors: Schmahl, E. J.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.;
Reeves, E. M.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1973BAAS....5..432S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of a Coronal Hole Boundary in the Extreme
Ultraviolet.
Authors: Huber, M. C. E.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.;
Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.
1973BAAS....5..446H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reply to `The relations between chromospheric features and
photospheric magnetic fields' by E. N. Frazier
Authors: Foukal, Peter; Zirin, Harold
1972SoPh...26..148F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Measurement of the Electron Temperature of Small 3-cm Radio
Bursts
Authors: Foukal, Peter
1972SoPh...24..411F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Coupling of the Active Chromosphere to the Solar
Interior
Authors: Foukal, Peter
1972ApJ...173..439F Altcode:
Evidence is summarized to show that the configuration of field lines
which governs the appearance of Ha fine structure in active regions
is set mainly by motions in the suhphotosphere where these lines are
anchored. It is shown that Ha fine structure is directly coupled
to a layer probably more than 5000 km below the photosphere, and
little distortion of the strong fields is expected in the intervening
layers. The shorter rotation period of active regions observed by
Howard and others (compared to the photospheric gas) is interpreted as
a result of this direct coupling of the strong field to a more rapidly
rotating solar interior. The effects of dragging such a field through
a photosphere of finite resistivity are briefly considered for features
of various observed cross-sections.
---------------------------------------------------------
Title: Observations of Diffuse Nebulae in the λ9532 Forbidden Line
of SIII.
Authors: Foukal, P.
1972BAAS....4..233F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Image-Tube Photography of Diffuse Nebulae in [s III] λ9532.
Authors: Foukal, Peter
1972ApJ...172..591F Altcode:
Exposures of M17 and the central core of MS show the potential
usefulness of the [S iii] 9532 radiation in the study of emission
nebulae. The line is of particular interest in studies of the obscured
central region of many of the bright northern objects, since it has
an optical penetration through dust about twice that of Ha, and is
produced by an ion known to be abundantly present in the central
regions of these diffuse nebulae.
---------------------------------------------------------
Title: Hα Fine Structure and the Chromospheric Field
Authors: Foukal, Peter
1971SoPh...20..298F Altcode:
The physical characteristics of the Hα structures previously defined as
fibrils and threads are studied. The interpretation of the fibrils as
ends of flux tubes is useful in tracing the behavior of the transverse
field component over the solar surface.
---------------------------------------------------------
Title: Morphological Relationships in the Chromospheric Hα Fine
Structure
Authors: Foukal, Peter
1971SoPh...19...59F Altcode:
A continuous relationship is proposed between the basic elements of the
dark fine structure of the quiet and active chromosphere. A progression
from chromospheric bushes to fibrils, then to chromospheric threads and
active region filaments, and finally to diffuse quiescent filaments,
is described. It is shown that the horizontal component of the field
on opposite sides of an active region quiescent filament can be in the
same direction and closely parallel to the filament axis. Consequently,
it is unnecessary to postulate twisted or otherwise complex field
configurations to reconcile the support mechanism of filaments with
the observed motion along their axis.
---------------------------------------------------------
Title: A morphological theory of the Halpha structure of the active
chromosphere.
Authors: Foukal, P.
1971BAAS....3R.261F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Development and Flaring of an Active Region Exhibiting
Unusual Magnetic Structure. II. Active Regions
Authors: Foukal, Peter
1970SoPh...13..330F Altcode:
The one-day development of a young bright region with loops (BRL)
led to the 1N flare of 00:35 UT, 25 April, 1968.
---------------------------------------------------------
Title: Erratum
Authors: Foukal, P.
1970Ap&SS...6..340F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Relation Between Chromospheric Structure, Magnetic Field
and Filaments
Authors: Foukal, Peter
1970BBSOP...9....1F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Electron Temperature Distributions and Internal Kinematics
of Seven Diffuse Nebulae
Authors: Foukal, Peter
1969Ap&SS...5..469F Altcode:
A Fabry-Pérot spectrophotometer is used to derive values of the
intensity ratio Hα/[N ii] at 98 points in the seven bright diffuse
nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen
in the singly ionized state is estimated in the different objects,
and is found to be sufficiently constant within any one nebula so
that the above intensity ratio may be used to derive accurate electron
temperature distributions. The position of the peak of the nebular line,
its excess non-thermal width, its shape and relative intensity are
used to derive kinematical models of these objects. It is found that
values of Hα/[N ii]≃1 are representative of the bright central cores
of these nebulae. Temperatures between 7000K and 12000K were derived
in the different objects. Although some of this apparent variation is
due to the different conditions of excitation in the various nebulae,
it is shown that a convincing progression of temperature in M8, M16,
M17 is supported by radio recombination line results. The temperature
variation within any one object was generally significantly less than
1500K. No evidence was found for velocities of mass motion at more
than twice the speed of sound. Relative radial velocities of generally
less than 15 km sec<SUP>-1</SUP> characterized the velocity fields of
M8, M20, M16, M42. The velocities in M17 were measured as about 20
km sec<SUP>-1</SUP>. Motions in NGC 7000 and IC 434 were much lower
(∼5 km sec<SUP>-1</SUP>) although here the number of points taken was
too small to construct meaningful kinematical models. It is concluded
that the internal motions of radiatively ionized Hii regions of Pop. I
will not significantly affect the results of existing surveys for
determining the rotation of the galaxy with radial velocities deduced
from nebular emission lines.
---------------------------------------------------------
Title: The Temperature and Internal Kinematics of M8
Authors: Foukal, Peter
1969Ap&SS...4..127F Altcode:
A photoelectric Fabry-Pérot spectrometer is used to record the line
profiles of Hα and [N ii] at 22 points in the nebula. The ratio
of intensity Hα/[N ii] is used to derive an electron temperature
distribution with values between 5700° and 9100° showing a peak
at the centre of density. These temperatures are compared with the
Hα Doppler temperatures to estimate excess velocities of mass
motion. Together with the shifts of the Hα line centres, these
lead to an evaluation of the velocity field in the nebula. It is
suggested that the nebula consists of a core expanding at about ±10
km/sec<SUP>-1</SUP> surrounded by a thick peripheral shell in which
large scale mass motions are small. Non-thermal broadening suggesting
turbulent velocities at about the speed of sound is observed in this
shell and is attributed to small scale dynamic effects in a non-smooth
density distribution. The effect of such expanding cores on heliocentric
velocities of galactic Hii regions is discussed.
---------------------------------------------------------
Title: A Possible observational correlation of lunar luminescence
with the K<SUB>p</SUB> index
Authors: Foukal, Peter
1968Icar....9..162F Altcode:
Simultaneous three-color photometry of the lunar craters Menelaus,
Copernicus, Kepler, and Gassendi was carried out on seven nights
between June 21st and July 1st 1967, to monitor possible short-period
color changes on a time scale of less than 10 min. Abnormally strong
brightness variations in the red and infrared bands, amounting to
deviations at high as 10% from the mean value were recorded on the
morning of July 28th at a phase angle of about 75%. Comparison with
K<SUB>p</SUB> values for these dates indicated a good, although not
perfect, correlation between the amount of time variation in the red
color index and the K<SUB>p</SUB> value. It is tentatively concluded
that if such observations can be continued it could be possible to
prove the irrelevance of magnetospheric effects to the question of
lunar luminescence.
---------------------------------------------------------
Title: The electron temperatures and internal motions of seven
diffuse nebulae
Authors: Foukal, Peter Vojtech
1968PhDT........61F Altcode:
No abstract at ADS
