G. W. Lockwood

Patterns of Variation among Sun-like Stars

We examine the patterns of variation among a sample of 35 stars that includes the Sun, particularly on the timescale of the 11 yr solar activity cycle. Our investigation uses contemporaneous photometric and chromospheric HK emission time series measurements from the Lowell and Mount Wilson Observatories, and comparable solar data. We find that the photometric and HK variability of the stars in our sample can be related to their average level of chromospheric activity by power laws. The photometric variability of the Sun may be somewhat subdued for its average activity level. We find that the younger, more active stars in our sample tend to become fainter as their HK emission increases, whereas the older, less active stars tend to become brighter as their HK emission increases, as the Sun does during its activity cycle.

Patterns of Photometric and Chromospheric Variation among Sun-like Stars: A 20 Year Perspective

We examine patterns of variation of 32 primarily main-sequence Sun-like stars [selected at project onset as stars on or near the main sequence and color index 0.42 ≤ (B - V) ≤ 1.4], extending our previous 7-12 yr time series to 13-20 yr by combining Stromgren b, y photometry from Lowell Observatory with similar data from Fairborn Observatory. Parallel chromospheric Ca II H and K emission data from the Mount Wilson Observatory span the entire interval. The extended data strengthen the relationship between chromospheric and brightness variability at visible wavelengths derived previously. We show that the full range of photometric variation has probably now been observed for a majority of the program stars. Twenty-seven stars are deemed variable according to an objective statistical criterion. On a year-to-year timescale, young active stars become fainter when their Ca II emission increases, while older less active stars such as the Sun become brighter when their Ca II emission increases. The Suns total irradiance variation, scaled to the b and y stellar filter photometry, still appears to be somewhat smaller than stars in our limited sample with similar mean chromospheric activity, but we now regard this discrepancy as probably due mainly to our limited stellar sample.

Europa's phase curve: Implications for surface structure

Ground-based photoelectric measurements taken by two of us (W.L. and D.T.) with narrow band filters at 0.47 and 0.55 μm at small phase angles clearly show the existence of a narrow opposition peak. We have combined these data with Voyager images to obtain a range of phase angle coverage from 0.3° to 119°. The integral phase data were analyzed using Hapkes photometric function. The photometric parameters are remarkably similar for both wavelengths and hemispheres. The following values at λ = 0.55 μm for the leading side are typical: single-scattering albedo w = 0.964, opposition effect amplitude B0 = 0.5, opposition effect width h = 0.0016, double-lobed Henyey-Greenstein factors b = −0.429, c = 0.113, and mean roughness angle θ = 10°. The photometric roughness is much smaller than on other bodies of the Solar System and implies that low surface relief seen at kilometer scales in the Voyager images extends down to centimeter scales. If the very narrow opposition surge is due to shadow hiding, the porosity must be large, on the order of 96%, which is much higher than that of a regolith generated by meteoritic gardening. Both the small roughness and high porosity imply that endogenic processes dominate surface formation even at centimeter scales. The single particle scattering function is more back scattering on the leading hemisphere, which contradicts the scattering properties expected of a planetary surface that is being preferentially bombarded by ions on its trailing hemisphere.

The Photometric Variability of Sun-like Stars: Observations and Results, 1984-1995

Using differential Stromgren b, y photometry, we monitored the brightness variations of 41 program stars and their 73 comparison stars from 1984 through 1995. The predominantly main-sequence program stars spanned ranges of temperature and mean chromospheric activity centered on solar values. About 40% of all the stars showed measurable variability, typically at levels below 0.01 mag (~1%), on both night-to-night and year-to-year timescales. The variability correlated with mean chromospheric activity and advancing spectral type. We present differential light curves and statistical descriptions of our observations.

The Chromospheric Activity and Variability of Cycling and Flat Activity Solar-Analog Stars

We present an analysis of more than 3700 observations of the Ca II H and K lines in 57 Sun-like stars and over 3000 analogous observations of the Sun. Ten of the 57 stars under consideration are observed in flat states, but these stars do not always exhibit overall Ca II H and K core brightness below that of solar minimum. Solar activity minimum lies near the lowest level observed for stars with cyclic or irregular variability, but many flat stars have HK activity levels comparable to or exceeding that of solar minimum. While flat activity stars may be in periods of extended activity minima analogous to the solar Maunder minimum, a significant reduction in magnetic activity during such periods is not implied (although it is also not rejected) by the data.

Stellar Activity and Brightness Variations: A Glimpse at the Sun's History

Radiometric measurements during the past decade from the Solar Maximum Mission and Nimbus 7 satellites have shown that the total solar irradiance varies in step with the suns 11-year magnetic activity cycle. Stellar observations from the Lowell and Mount Wilson observatories now confirm and elaborate this discovery. These measurements show that older stars similar to the sun tend to become brighter as their magnetic activity level increases, just as the sun does during its 11-year activity cycle. Younger stars, however, tend to become fainter as their magnetic activity level increases. This contrasting behavior suggests that the balance between the competing phenomena that influence solar brightness variability has shifted during the suns lifetime.

Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994

Images of Neptune taken at six wavelengths with the Hubble Space Telescope in October and November 1994 revealed several atmospheric features not present at the time of the Voyager spacecraft encounter in 1989. Furthermore, the largest feature seen in 1989, the Great Dark Spot, was gone. A dark spot of comparable size had appeared in the northern hemisphere, accompanied by discrete bright features at methane-band wavelengths. At visible wavelengths, Neptunes banded structure appeared similar to that seen in 1989.

The albedo of Titan

Photometric observations of Titan since 1972 show a cyclical variation of about 10 percent. A minimum value of brightness and albedo apparently occurred in 1984. Spectrophotometric observations, made annualy since 1980 at 8 A resolution, 3295-8880 A, were used to derive the value p-asterisk = 0.156 + or - 0.010 for the integrated geometric albedo in 1984. Variations of the equivalent widths of spectral features were not seen.

Secular brightness increases of Titan, Uranus, and Neptune, 1972–1976

Abstract The brightnesses of Titan, Uranus, and Neptune in b (4718 AA) and y (5508 AA) have increased linearly since 1972 at rates ranging from 0.005 to 0.025 mag yr −1 . The observations were made differentially on a number of nights each season with respect to a network of comparison stars whose relative magnitudes were determined by independent measurements. Solar phase coefficients were derived for each object, and all observations have been normalized to zero solar phase angle and mean heliocentric distances. No explanation for the changes has been found, but a possible influence of solar activity upon planetary albedo is suggested by the fact that all of the objects observed have brightened during the declining half of the solar cycle.

Long-term brightness variations of neptune and the solar cycle modulation of its albedo.

The visual brightness and albedo of Neptune vary periodically during the 11-year solar cycle with an amplitude of 4%, anticorrelated with the variation of solar ultraviolet output. A seasonal trend in color suggests that Neptune, like Uranus, may have a slightly reddened pole.