Solar Variability After Dark
Evidence and some dead ends from
stars and planets
Wes Lockwood, Lowell Observatory
Measuring (or at least trying to detect) solar variability using stellar photometry has been an obsession at Lowell Observatory since 1950. How this came about and
what we learned is the subject of this talk.
Abbots's claims of detected solar variability attracted little attention from night-time astronomers despite provocative claims. Prior to Lowell's entry into the fray,
only Joel Stebbins at Lick Observatory had attempted to detect variations in the sunlight reflected from the planet Uranus (in 1927) using a photocell. He saw
nothing of interest at a level of about 1% over several months.
Conflicting assumptions
1.The Sun is a variable star but if we work
hard enough, we can measure its
variations.
2. Cool dwarf stars like the Sun are so
inherently stable that we can use them as
trusted standards for astronomical
photometry.
The work I describe involves two conflicting assumptions:. First, we hypothesize that the Sun is a variable star and if we work hard enough we should be able to
detect its variations.
Then, second, we undertake differential photoelectric photometry using as comparison stars objects that, although conventionally regarded as stable by
astronomers and used as ‘standard stars,’ are themselves similar to the Sun.
1950s: In the beginning
In 1947 Harry Wexler, Chief of Scientific Services of the US Weather Bureau, approached the Observatory trustee, Roger Lowell Putnam, with a proposal to post
a couple of meteorologists to Lowell to study images of planets in an effort to understand better the Earth’s atmospheric circulation. Wexler evidently was a bit of
an impresario and matchmaker, intending in this work to arrange a “marriage between meteorology and astronomy.” In due course the Observatory’s first federal
contract arrived along with two young meteorologists, Ralph Shapiro and Seymour Hess. A 1951 Air Force report included the initial thinking about how solar
variability might be detected via photometry and what the constraints were. Henry Giclas, Gerard Kuiper and Harry Wexler laid out the issues. The main
advantage was the differential approach, comparing one object in the sky to another and thereby minimizing the effect of changing atmospheric transparency. The
early photometry was rough, with power fluctuations and primitive electronics hindering the demanding requirements of precision photometry. Salvation came
when Harold Johnson joined the Lowell staff in 1952. Johnson was the inventor of a widely used system of filter photometry a pioneer of early photometric
instrumentation and electronics.
Shown here:
(top) Harry Wexler, U.S. Weather Bureau Chief of Scientific Services
(bottom) Harold L. Johnson, photometrist at Lowell 1952-59
Press release about Lowell’s new telescope
1959: “The Sun as a Variable Star”
Johnson and Iriarte 1959
Lowell Observatory
Bulletin
Papers with the title “The Sun as a Variable Star” began to appear regularly in the Lowell Observatory Bulletins and the refereed literature. Here’s an early
example of jumping to conclusions, what Thomas Kuhn describes as the tendency of scientists to find what they are looking for. Shown here are light curves of
Uranus and Neptune and a 1959 press release. Lowell’s “first” Solar Variations program ended in 1966 with a final AF report and Lowell Observatory Bulletin.
After Johnson had departed for McDonald Observatory the work was very capably carried out by K. Serkowski and M. Jerzykiewicz , two scientists whose future
careers were quite distinguished. Their 1966 account of what they learned about Uranus and Neptune (and the Sun) was noncommittal and unnoticed by the wider
community (exception: Olin Wilson at Mount Wilson Observatory later cited this paper, as did the NOAA climatologist J. Murray Mitchell, Jr.)
1971: Planets and the solar cycle
Labs and Neckel look for a sun-planet
correlation in the Lowell planetary data
1%
In a review of solar constant calibration work, Labs and Neckel took notice of the Lowell Observatory photometry in a paper in Solar Physics. Here, they
examined the question of a connection to the solar cycle. (T. Sterne and N. Dieter had examined Abbot’s data in 1959 and found no spectral power at 11/22 years,
at the level of 0.2 percent.)
In this plot from the Labs and Neckel paper I have connected the sunspot number points with the red line.
1970: J. Murray Mitchell. Jr. writes…
•
In 1970, NOAA climatologist J. Murray Mitchell, Jr., began to fret about a lack of solar measurements anywhere. He was aware of the Lowell work and had earlier
invited Jerzykiewicz and Serlowski to report on it at a 1966 NCAR conference on climate change. Mitchell appealed to John Hall, Director of Lowell Observatory,
to resume the planetary measurements until a solar irradiance instrument could be launched (Nimbus 7, 1978). Jerzykiewicz returned to Lowell to start the new
program, adding Saturn’s moon Titan as a presumably reliable reflector of sunlight. Jerzykiewicz returned to his home institution in Wroclaw, Poland in 1973, but
Don Thompson (now retired) and I have continued the observations with uninterrupted grant funding ever since.
1972: We resume
observing planets
for solar variability
After four years we
think we’ve found
something
The first four years’ observations of Titan, Uranus, and Neptune showed a steady rise in all three objects. We couldn’t come up with an explanation that did not
involve the sun, so we hypothesized the possibility of a photochemical effect driven by varying solar UV. The lesson: “When you find a good correlation, stop
observing.” --Julius London (Colorado University).
Citation: Planetary brightness changes: Evidence for solar variability
Lockwood, G. W. 1975. Science, 190, 560–562
Planetary variability over 40 years
Unique long-term record, but no solar variability
Principal findings about planetary variability:
Titan – Cyclic seasonal variation characterized by a N-S albedo contrast difference that varies with insolation. Current thinking involved meridional circulation of
aerosol. The orbital period of Saturn (and Titan) is 29.5 years.
Uranus – Seasonal variation amplified by an equator-to-pole albedo gradient and a small secular or seasonal variation. The dashed line is the variation expected
just due to Uranus’ aspect and oblateness.
Neptune – Seasonal variation that attained a maximum at southern summer solstice, 2007. The dashed line is a lagged seasonal model originally proposed by L.
Sromovsky but now shown by recent observations to be incorrect.
Citations:
Seasonal photometric variability of Titan, 1972-2006
Lockwood, G. W., and Thompson, D. T. 2009. Icarus, 200, 616-626
Long-term atmospheric variability on Uranus and Neptune
Hammel, H. B., and Lockwood, G. W. 2007. Icarus, 186, 281-301
Photometric variability of Uranus and Neptune, 1950–2004
Lockwood, G. W., and Jerzykiewicz, M. 2006. Icarus, 180, 442-452
1966: A stellar byproduct
”If we assume the
sun acts in similar
fashion to each of
these stars, its
variability over a
15-year period
probably does not
exceed one half of
one percent. “
Jerzykiewicz and
Serkowski 1966
Lowell Observatory
Bulletin
When Harold Johnson put the Solar Variations program on a sound scientific and instrumental basis beginning in 1953, he thought it worthwhile to select a set of
sun-like “10-year standards”, his thinking being that stars of similar color would provide an alternate set of standards less susceptible to instrumental “color term”
effects. The variations of Sun-like stars were found to be quite small, less than 0.008 mag (0.8%) rms for most and less than 0.004 mag (0.4%) for several.
Jerzykiewicz and Serkowski concluded: “If we assume the Sun acts in similar fashion to each of these stars, its variability over a 15-year period probably does not
exceed one half of one percent.” More than a quarter century would pass before more precise observations of Sun-like stars would become available.
Measuring sunlike stars at Lowell
Brian Skiff:
1200 nights
6000 hours
1984-2000
night by night
year by year
In 1984 we obtained an Air Force contract (and later a series of NSF grants) to study field Sun-like stars drawn from the Mount Wilson HK survey. Brian Skiff
undertook the task of observing, 1200 nights, 15 years, 6000 observations. In retrospect, the detection of variability was straightforward but certainly at the
beginning we had no clue what we would find for solar age low activity stars. The illustration shows the six pairwise lightcurves of four stars of a quartet group
containing a star with a cyclic variation of about 0.06 mag.
Citations:
Patterns of photometric and chromospheric variation among sun-like stars: a 20-year perspective
Lockwood, G. W., Skiff, B. A., Henry, G. W., Henry, S., Radick, R. R., Baliunas, S. L., Donahue, R. A., and Soon, W. 2007. ApJS, 171, 260-303
Patterns of variation among sunlike stars
Radick, R. R., Lockwood, G. W., Skiff, B. A., and Baliunas, S. L. 1998. ApJS, 118, 239–258
The photometric variation of sunlike stars: Observations and results, 1984–1995
Lockwood, G. W., Skiff, B. A., and Radick, R. R. 1997. ApJ, 485, 789–811
New results from robotic photometry
The robotic telescope photometry program at Fairborn Observatory, beginning around 1993, overlapped the Lowell program that ended in 2000 and produces
differential photometry with about the same nightly internal precision as the Lowell data (0.0015 mag rms), but with 10x the density over a season, full nightly
coverage, and no time out for fatigue, illness or, ennui. The bottom line is a substantial gain in precision of the annual means (although the vexing problem of
variable comparison stars never goes away, and Greg Henry (Tennessee State University) has had to retool many groups and simply abandon others when no
suitable comparison star proved reliable,These two charts show, for 70 stars observed from 10-19 years at Fairborn, the rms dispersion of their annual mean b and
y magnitudes, corrected for comparison star variability. The x-axis is log R´HK, a measure of chromospheric activity that increases toward the right. The Sun has
log R´HK =-4.9. The y axis is rms variability of the annual means, expressed as stellar magnitudes on a log scale. The curved lines mark the 95% confidence
band.Note the position of our favorite solar twin, 18 Sco. On this diagram the Sun would lie very near 18 Sco (not allowing for a correction for solar inclination
and (b+y)/2 TSI). The two corrections, if applied, would raise the Sun up into the confidence band
Lower left: Lou Boyd at Fairborn Observatory. Boyd built the telescopes and operates the observatory, single-handed.
Upper right: Greg Henry, at Tennessee State University
b and y variation and the SIM surprise
b and y vary by the same
amount, year to year
SIM suggests that b and y
should vary differently
b
y
The relationship between the rms dispersion of annual mean magnitudes in b and y. On the y axis, 0.001 mag is approximately 0.1%. According to the result from
SORCE TIM seen at right, rms b variability on the chart at left should fall below the regression for stars behaving as the Sun does. This is a work in progress for
the stellar data,
Conclusions
1. The Sun is a variable star but measuring its
variation using astronomical photometry is
impossible.
2. Cool dwarf stars like the Sun are inherently
variable but if we work hard enough we can
measure their variations.
Despite the initial contradiction of trying to measure variability using objects that are themselves likely to be variable, we have found it to be feasible, with the
most recent refinement being our belated recognition (coming from the initial Lowell survey) that the late F stars are relatively more stable than the Sun-like G
stars we are trying to measure.
The 21-inch, still working today
The historic Lowell 21-inch reflector on Mars Hill remains in use for planetary photometry with continuing support from the NASA Planetary Astronomy
program. It’s the only remaining Lowell research telescope operated manually rather than from a heated control room. The photoelectric photometer with its
original 1972 EMI 6256S photomultiplier and b (472 nm) and y (551 nm) interference filters has been in steady use for 40 years. Over 55 years this telescope has
enabled more than 160 refereed papers.