ORW (Dick White) Comments on HAO and Its Evolution
My involvement with HAO began unwittingly in 1951 at the University of Colorado. Each morning as I walked to my engineering classes from Willard Hall, I noticed the heliostat feeding a solar beam into laboratory on the south end of Temporary Building 8(TB-8). I was not curious enough, however, to explore these experiments at that time. During my final year studying Engineering Physics, Bill Rense and his students in the Physics Department were building and testing UV spectrometers and pointing systems for early solar measurements at White Sands. Once again I was intrigued by solar research being conducted in the basement of the Physics Building, but now I had a military service obligation to fulfill as a 2nd Lieutenant in a munitions development laboratory at Eglin Air Force Base, Florida.
My first real contact with HAO came in 1957 as the result of an advertisement for a new graduate study program at the University of Colorado. Placement of such a notice in the obscure magazine 'Ordnance',speaks to Walt Roberts' enthusiasm for the HAO program of both teaching and research at that time. Since my family ties were all in the Rocky Mountain region, I was immediately interested the new program. My USAF tour of duty was to end in October 1957, and Walt granted me an interview in August 1957. He talked to my wife, Lucy, and me about my CU engineering studies and our plans for the future. My application to study in AstroGeophysics was accepted, and I entered the AG program in November of 1957.
Since I entered too late in the semester to register for credit the first AG classes, Walt started my self study program with instructions to study Kuiper's book, The Sun, and to attend the HAO colloquia for the remainder of the semester. The first colloquium I attended was held in the class room at Sommers-Bausch Observatory overlooking the CU campus. There I met Dick Thomas, Grant Athay, John Jefferies, Don Billings, Hal Zirin, Jim Warwick, and Jean Claude Pecker. These exciting discussions of spectral line formation in the solar chromosphere made me aware of how much new physics I had to learn in the AstroGeophysics program.
Donald Menzel was the father of the HAO. As a member of the Harvard College Observatory, Prof. Menzel was among the elite in U.S. astronomy. His interests ranged through solar physics, stellar atmospheres, spectroscopy and astrophysics in general. He was very enthusiastic about the science and aggressive in pursuit of the means to support it. Menzel recognized the importance of Bernard Lyot's invention of the birefringent filter and the coronagraph in observing the Sun. R.R. McMath's first photographic movies of solar activity made at the University of Michigan demonstrated the technology needed to record changes on the Sun during the day. Menzel saw the opportunity for a new program in solar physics, but success in the program to study the corona required a special site with very clear skies free of dust.
Prof. Menzel was a native of Leadville, Colorado, and he knew first hand of the clarity of the sky above the high peaks in the Rockies. Thus, he set out to build the first high altitude solar observing station in the western hemisphere using movie cameras to record solar images made using a new coronagraph with a birefringent filter tuned to the H alpha line at 656.3 nm. He assigned his student, Walter Orr Roberts, to move to Climax and undertake this task in 1940.
This began HAO as the High Altitude Station of Harvard College Observatory. In light of subsequent events, Menzel's choice of Walt Roberts was prescient. Walt was always enthusiastic and determined about his studies. He enjoyed astronomy and understood the instrumentation required for good measurements. Because of the breadth of his interests, he could explain the solar project in terms understandable to people not familiar with the subject. Consequently, he was very persuasive in gaining support for his work.
Prof. Menzel was quite aware of the military interest in predicting disruptions of short wave communications due to solar activity. Bartel M regions and their geomagnetic effects prompted study of the Sun to determine their origin and occurrence. Such empirical connections between solar activity and properties of the ionosphere allowed Menzel and Roberts to secure support for their project throughout WWII.
In 1947 Walt moved his young family to Boulder and began operation of the HAO office there. Prof. Harlow Shapley, Director of Harvard College Observatory, suggested that the University of Colorado was the logical place to anchor this new enterprise. Thus, negotiations began between Harvard and CU to put the HAO offices on the CU campus and continue operation of the Climax site under the auspices of the University but without commitment of University funds. By 1951 HAO was operating from TB-8 where Jack Evans and Walt Roberts had their laboratory for building and testing new optics.
The US Air Force planned to establish an optical site in the Sacramento Mountains overlooking the White Sands Missile Range in southern New Mexico. Roberts and Evans saw this as an opportunity to develop larger aperture coronagraphs and high resolution spectrographs needed in their solar research program. The result was creation of the Sacramento Peak Observatory as a companion to the Climax site. Roberts and Evans secured sufficient monies to install 13 ft spars and larger aperture coronagraphs at both Sunspot and Climax. They were also able to outfit both installations with new spectrographs of sufficient resolution to show accurate line profiles needed in the development of sound techniques for interpreting solar spectra. By 1957, the new instrumentation was in use at both sites, and the vigorous exchange of theoretical ideas between Menzel's Harvard group and the two western observatories continued.
As a result of his exposure to the existence of solar effects in the upper atmosphere of the Earth during his early days at Climax, Walt became interested in more general problems the solar terrestrial interaction. He then began a lifelong study of effects of the Sun on our weather. He recognized the need for an academic program to train scientists needed to understand the properties of the Earth's atmosphere and the effect of the Sun on its behavior. Working with his staff and colleagues at CU, he succeeded in creating the Department of AstroGeophysics at the University of Colorado in 1957. The aim of this ambitious program was to train a generation of physicists capable of advancing our understanding the solar and terrestrial atmospheres as well as the solar-terrestrial interaction.
The breadth of this new program can be seen from the makeup of the AG staff at that time:
Walt Roberts, Grant Athay, Gordon Newkirk, Don Billings, Jim Warwick, Hal Zirin, Julius London, Bernard Haurwitz, Einar Tandberg-Hanssen, and Sadami Matsushita. Sydney Chapman spent six months each year in residence at HAO. We enjoyed frequent visits by Gene Parker, Don Menzel, Dick Thomas, Jean Claude and Charlotte Pecker, and John Jefferies. The Sacramento Peak staff grew to include Jack Zirker, Frank Orrall, Dick Dunn, Henry and Elske Smith, and Ed Dennison. As AG students, we had exposure to an extra-ordinary pool of talent in solar and terrestrial atmospheric physics as well as access to the observing opportunities at Climax and Sac Peak.
The first group of AG students in 1957 were Hollis Johnson, Kim Malville, Jack Eddy, Jerry Weinberg, Lewis House, Stewart Pottasch, and me. We also had two fellow students from Pakistan and India.
In the late 1950's, the atmospheric physics community recognized the need for substantial computing facilities in order to make accurate simulations of the global atmosphere of the Earth. Thus began discussion of the creation of a national center to house a state of the art computing system for use by the U.S. university community. Walt Roberts was involved in these early plans that led to the creation of the National Center for Atmospheric Research under the auspices of the University Corporation for Atmospheric Research funded under contract by the National Science Foundation. In 1962 NCAR became a reality as part of the Boulder community with Walt Roberts as its Director. NCAR was not only a computer complex but a true research institution with programs in atmospheric dynamics, atmospheric chemistry, solar physics, and instrument development. HAO came into the new Center as a research division.
Inclusion of HAO in NCAR meant withdrawal of the Observatory from the University despite the close tie between the two organizations. This also forced AG faculty members to choose between NCAR and CU as their employer. The resulting division of the AG faculty and the eventual move of HAO from the CU campus slowly weakened the AG program. The AG Department eventually ceased to exist, and teaching in solar terrestrial physics moved to the CU Department of Astrophysics, Physics, and Atmospheric Science.
5. Evolution of HAO Interest in solar terrestrial interaction: upper atmosphere, weather, climate, space weather
HAO was founded on a purely solar research program at the beginning of WWII. Because of the military interest in solar activity, Walt's knowledge of geomagnetic activity, ionospheric response to solar activity, and the solar terrestrial interaction in general grew and spread to his staff. The Observatory today remains strongly focussed in solar physics and upper atmospheric physics as part of the much broader research program of NCAR. The NCAR Directors recognize the obvious connection between solar inputs, atmospheric chemistsry, and global dynamics, and joint research programs are underway today.
Walt Roberts pursued his investigation into Sun-Weather connections in his collaboration with John Wilcox in the 1970's. After Jack Eddy's historical study of the Maunder Minimum, the solar connection to climate change seemed more obvious than direct effects on tropospheric weather in detail. Eddy's seminal study of the solar record from 1610 to current times and its similarity to the mean global temperature forced both new measurements of solar output as well as study of processes involved in the solar-terrestrial interaction.
Variability of the Total Solar Irradiance was the most obvious source of a direct heating effect in the troposphere. Abbot's failed attempt to measure such changes and their connection to terrestrial weather showed the need for new measurement techniques. This situation was discussed in the Big Bear Solar Observatory Workshop, "The Solar Constant and the Earth's Atmosphere", 19-21 May 1975, sponsored by the NSF. Following this workshop, Gordon Newkirk and Walt Roberts suggested that we needed a comprehensive review of our knowledge of climate variability and effects of known solar variations. With the help of Gordon Newkirk, Hal Zirin, and Julius London, I began planning a book to be entitled The Solar Output and its Variation. The book was completed in 1976 and published in 1977 by the Colorado Associated University Press. In it we summarized the state of observation of the solar spectral irradiance, the total solar irradiance, and variability of energetic particle inputs. Contributors to this volume also discussed possible mechanisms for variations in the solar photosphere and radiative input to the Earth. There you will also find discussions of insolation changes due to changes in the Earth's orbit and inclination to the ecliptic. A colleague from the Lamont-Doherty Geological Observatory discussed the effect of continental drift on ocean circulation and climate. I tried very hard to make the presentation broad and comprehensive.
By then, the need to make irradiance measurements in space, free of the effects of the Earth's atmosphere, was well known. Development of the Earth Radiaton Budget(ERB) experiment on Nimbus 7 and the Active Cavity Radiometric Irradiance Monitor I (ACRIM I) for measurement of the total solar irradiance were complete for the Solar Maximum Mission. John Hickey's thermopile began operation in November 1978 on the Nimbus 7 spacecraft. In 1979, John Hickey visited HAO to show his total irradiance data from the ERB. He pointed out the correspondence between decreases in the irradiance and central meridian passages of large sunspots. Gordon Newkirk and Jack Eddy immediately noted that the amount of the decrease (~0.1%) was about what would be expected from the fractional area of the solar disk occupied by the spots observed at that the same time. It appeared to us that the simplest mechanism of the passage of sunspots across the disk offered a defensible source of both the depth and duration of the downward excursions in Hickey's record. However, doubters claimed that these excursions could be instrumental noise. The issue was resolved after Dick Willson's ACRIM I radiometer came into operation in February 1980. Willson's ACRIM I measurement confirmed that the deep excursions corresponding to sunspot transits across the disk appeared in both ERB and ACRIM I records. In my opinion, John Hickey was the first to measure the effect of sunspots on the total solar irradiance, but the confirmation by Dick Willson was necessary to quiet the doubters.
Operation of Hickey's ERB experiment until January 1993 and Willson's ACRIM I until 1989 subsequently revealed that the total irradiance had its maximum at the same time as the maximum of the 11 yr activity cycle. This result forced consideration of bright faculae as the source of increased irradiance despite the presence of dark spots. Current empirical models have a bright (positive) facular component plus a dark (negative) sunspot blocking component to fit the net total irradiance..
We now know that the 11 yr Schwabe cycle causes a peak-to-peak variation in the total irradiance of about 0.2%. Radiometry problems remain in the absolute value of the total irradiance. Current measurements can differ by 0.5 W/m^2 even though they follow the short term variations to 100 ppm. The mean value of the total irradiance in current VIRGO measurements is ~1366W/m^2. The absolute scale problem may be in determination of the size of the radiometer aperture to the precision required. Rottman and Lawrence address this metrology problem in development of their SORCE instrument being built at the University of Colorado.
Solar cycle variability in the total irradiance at first glance appears too small to be a significant driver of change in the troposphere. However, UV variability below 400 nm is much larger than in the total irradiance. At wavelengths below 310 nm, the UV radiation is absorbed in the ozone layer where solar variability can be an important factor in determining chemistry of the upper atmosphere and its effect on dynamics.
An important question remains unanswered: What of secular change in the total irradiance? Although 23 years of measurements is far too short for an accurate determination of long-term solar change, observations at minima of solar activity in 1986 and 1996 offered the possibility to determine the rate of change of the irradiance from a quiet Sun. By combining his ACRIM II data with the Nimbus 7 ERB record in 1992, Willson found an increase of 0.6 W/m^2 in the quiet Sun output from 1986 to 1996. Mende and I independently confirmed Willson's finding. Lee's group at Langley and Chapman's group at San Fernando Observatory found that the ERB record had to be corrected downward by about 0.6 W/m^2 to match their empirical models. This correction reduces the secular variation to zero between the two solar minima. Thus, Frohlich and Lean base their composite total irradiance record on a constant solar minimum irradiance. Resolution of the question must wait until observation in the next minimum expected in 2007.
Current interpretation of total irradiance variations rests only on three surface features associated with magnetic structures in the atmosphere: sunspots, faculae, and plages. Kuhn continues to argue that both the network and the quiet atmosphere inside the network can contribute to the variability. At this point, the issue of a more global irradiance component remains unsettled.
Does the luminosity of the Sun change? Because the radiation from magnetic elements is highly directional, the irradiance measured at the Earth may vary differently from the luminosity of the entire star. The nearly constant energy generation rate in the Sun's nuclear core suggests that the luminosity determined from all outputs (radiation+plasma+high energy particles) must be constant over the duration of our current records. Since measurement of the radiative luminosity cannot be made over 4p steradians, we may never know how constant the luminosity really is by observations from the vicinity of the Earth.
What of solar effects in climate? Although convincing mechanisms for solar drivers in the Earth's climate do not exist, the empirical correspondences suggest that the increase in solar activity since the Maunder Minimum was a factor in increasing the mean global temperature of the Earth until about 1950. From that time onward, another factor appears to be needed to account for the observed increase in the global temperature. The rate of increase of the mean global temperature in the last fours decade has been too fast and too sustained relative to changes in the amplitude in the strength of the 11 yr solar cycle. The strongest solar cycle 19 ever observed went through its maximum in 1957(?). Subsequent cycles 20,21,22, and 23 are weaker, with the current cycle 23 being the weakest.
Solar-Climate effects were never a strong part of the HAO program, but the Observatory continues to supply solar observations and theory important in the atmospheric science community concerned with variability in the entire terrestrial atmosphere. The HAO research program contributes directly to the national space weather program through studies of coronal activity and the numerical simulation of the atmosphere from the thermosphere to the troposphere.. This is the natural evolution of Walt Roberts' vision for the understanding of the entire terrestrial atmosphere in its solar environment.
Proceedings of two earlier workshops help to appreciate the community involvement in irradiance measurement and solar-climate research. The 1983 Cal Tech Workshop, Solar Irradiance Variations on Active Region Time Scales, June 20-21, 1983,(NASA Conference Publication 2310) summarized work on how active regions contribute to variability in the total irradiance. In 1990, the NASA GSFC conference, Climate Impact of Solar Variability, (NASA Conference Publication 3086) was the first discussion of possible effects of irradiance variability on climate. Today, discussions of irradiance research occur annually at the AGU and AAS meetings as well as at other international meetings.
My account of the beginning and evolution of the High Altitude Observation belies difficulties over the years. Money was always a problem in the early days, particularly after the move to Boulder. Responsibility for raising the operating funds fell totally on Walt and his personal staff. He assiduously developed contacts with foundations and individuals interested in promoting scientific research. His personal enthusiasm for our program plus the skill in presenting it in understandable terms was more successful than not. However, there were times when salaries had to be temporarily reduced until new money could be found. As the staff grew and the demands for modernizing the observing machinery became more pressing, continued success for the Observatory required development of a more stable source of support. Creation of NCAR offered the opportunity for such a source from the national research program. In order to fit within the new Center, the Observatory staff view opened to the broader problems in solar-terrestrial physics and atmospheric physics. From that point forward, the Observatory became a more valuable contributor in the national research program. This was not a change in Walt's personal agenda: it enforced his original vision to understand the solar terrestrial interaction for the benefit of the U.S. public. Besides, he simply enjoyed winning the battle.