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An atmospheric outburst on Neptune from 1986 through 1989
ICARUS
99,
363-367 (1992)
An Atmospheric
Outburst
on Neptune
from 1986 through
1989
H. B. HAMMEL, S. L. LAWSON, AND J. HARRINGTON Massachusetts
Institute
of Technology,
54416,
Department
of Earth, Atmospheric,
and Planetary
Sciences,
Cambridge,
Massachusetts
02139
G. W. LOCKWOOD AND D. T. THOMPSON Lowell
Observatory,
1400 West Mars Hill Road,
Flagstaff,
Arizona
86001
AND C. SWIFT’ California
Institute
of Technology,
Pasadena,
California
91109
Received March 6, 1992; revised June 1, 1992
dissipated slowly. In a review of Neptune’s long-term variation, Lockwood and Thompson (1991) reported sustained brightening since 1985, a departure from a 17-year anticorrelation with cyclic solar activity. They suggested that an outburst occurring now on Neptune might be responsible for this behavior. We report here observations that show that an outburst actually occurred about 1986, with Neptune returning to normal near-infrared standard deviations and brightnesses by 1989 and normal visual standard deviations by 1990.
In two independent long-term programs (visual photometry and methane-band imaging), we find that Neptune underwent a period of unusual atmospheric activity from 1986 through 1989. This outburst was characterized by (1) increased 8900-A methane-band brightness both for the dominant bright feature by 0.3 mag and for the hemisphere opposite that feature by 0.14 mag, and (2) increased standard deviation of the mean yearly magnitudes at visual and near-infrared wavelengths by over a factor of 2. By 1989 when Voyager 2 encountered Neptune, the near-infrared standard deviations and brightness had returned to normal, although the visual standard deviations were still marginally high. We interpret the observations as a localized source of upwelling, qualitatively similar to “great white spots” seen periodically on Saturn. o 1992 Academic Ress,
OBSERVATIONS 8900-A Photometry
Inc.
We obtained lightcurves of Neptune’s diurnal variation during each apparition from 1986 through 1991. The photometry was derived from CCD images of Neptune and a flux calibration standard star (SAO 083543 = BD + 26”2606; Oke and Gunn 1983) taken at the University of Hawaii 2.2-m telescope (Mauna Kea, Hawaii), using a methane-band filter centered at 8900 A with a full width at half maximum of 200 A. We also observed a second comparison star each year for extinction correction and a check of photometric accuracy. The second star differed from run to run, but was always a Landolt star chosen to be near Neptune (Landolt 1983). Hammel et al. (1989) described the analysis of the 1986-1987 data. We analyzed the 1988-1990 data using IRAF (Tody 1986) to obtain disk-integrated photometry of Neptune and photometry of the standard stars. Using the procedure described by Hammel et al. (1989), we determined and applied extinction corrections from the
INTRODUCTION Neptune’s atmospheric reflectivity varies on many timescales, ranging from tens of minutes in the highestresolution Voyager images (Smith et al. 1989, Limaye and Sromovsky 1991) to years (Lockwood and Thompson 1991). In March 1976, Joyce et al. (1977) observed an
unusual event termed an “outburst” by Cruikshank (1985). A substantial increase in Neptune’s reflectance betweeen 1 and 4 pm relative to 1975 values was followed by a gradual decrease in brightness to the earlier levels. Pilcher (1977) interpreted this event as the sudden formation of a relatively high-altitude haze layer, which then Presented at Neptune/Triton Conference in Tucson, Arizona, during January 6-10, 1992. ’ Current address: 420 Hao Street, Honolulu, HI, 96821. 363
0019-1035192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
364
HAMMEL ET AL.
combined photometry of the standards, and calculated magnitudes of Neptune relative to the SAO star. We then constructed rotational lightcurves for Neptune for each year. The images show that the transit of a bright discrete feature across Neptune’s disk causes the observed strong diurnal variability. (In 1989, simultaneous groundbased and Voyager 2 spacecraft images allowed identification of this feature as the “bright companion” south of the Great Dark Spot; see Fig. 8 of Smith et al. 1989.) For this analysis, we chose arbitrary epochs that put the feature’s transit at phase 0.5 in each lightcurve. We determined rotation periods by generating several lightcurves with different periods and choosing the one that provided the best visual fit. Experience has shown us that analytic techniques such as phase dispersion minimization are strongly dependent on the time sampling of the data, and often lead to wrong periods for undersampled data such as we have. Observations of feature transits in groundbased images provide an independent check of rotation periods: the time between feature transits must be an integral number of rotations of the planet. In addition, the technique eliminates confusion due to multiple features. Finally, in 1989, periods determined from the high-resolution Voyager imaging provided a third, unambiguous check of groundbased periods (Lockwood et al. 1991a), lending confidence to the periods found in other years. Periods ranged from near 17.0 hours (in 1986, 1987, and 1988) to 18.29 and 18.4 hr (in June and August 1989, respectively). The 1990 and 1991 data were too sparse to determine accurate rotation periods, but the data were consistent with periods of less than 17.5 hr. We determined yearly magnitudes for the antifeature hemisphere by calculating the weighted average of all measurements whose rotational phases were in the ranges of O-O.25 or 0.75-1.0. Similarly, the magnitude of the feature-centered hemisphere was defined as the weighted average of all measurements whose rotational phases were between 0.4 and 0.6; this value was not strongly dependent on the width of the “window” in rotational phase. The difference between the feature-centered and antifeature magnitudes is the flux contributed by the discrete bright feature. The yearly averages of the two hemispheres are shown in Fig. la. To compare more conveniently with the visual standard deviations, we also calculated the standard deviation of all 8900-A observations taken during each year (Fig. lb). b and y Photometry
As part of an independent program of long-term planetary monitoring, we obtained photometry of Neptune every year from 1972 through 1991 using intermediate-band filters at 5510 A (y) and 4720 A (6) on the Lowell 0.5-m telescope (Lockwood and Thompson 1986, 1991, Lock-
70 74 78 ~“‘I”‘I”‘I’.‘I”‘I”‘, L 0.9 1
82
(a) Magnitudes
86
90
94
at 8900 k,
1.0 1
f
Bright hemisphere
Dark hemisphere
0.200
L
0.160
-
0.120
-
0.080
-
0.040
-
0.010
(b) 8900A
.
.
.
(c) Stromgren y
0.008
\1 -
0.006
-
0.004
-
. .
.
.
. . l
l
l
.
.
. .
0.002
-
:
c
0.010 0.008
-
0.006
-
0.004
-
0.002
. .
.
(d) Stromgren b
. . . .
l
.
-
.
. .
. .
.
t...I...I...I...,...l...i 70
74
78
82
86
90
94
YEAR
FIG. 1. Neptune’s variability as a function of year of apparition. The observations were made over a period of several months (typically from Mav_ through centered on oouosition. (a) 8900-8, disk-integrated _ Aueust) _ ..
365
OUTBURST ON NEPTUNE
wood et al. 1991a, 1991b). We typically observed about 10 nights per apparition, but in 1989 and 1990 we made many more observations, expanding the normal l-hr nightly sequence to nearly 4 hr and including extra nights during the 4-month-long observation season that runs from May through August. Each year we observed Neptune and a different pair of comparison stars. We measured the stars repeatedly over several seasons, which allowed us to create a standard photometric network along Neptune’s orbital path (Lockwood and Thompson 1986). The observations yielded a time series of Neptune brightnesses adjusted to zero solar phase angle and a fixed opposition distance (cf. Lockwood 1977). The annual mean magnitudes of Neptune have an overall accuracy of about 0.003 mag, although there are differences from year to year depending mainly upon the photometric stability of the particular pair of comparison stars used. The long-term lightcurve (Lockwood and Thompson 1991) was highly anticorrelated with solar activity until about 1987, when increasing brightness levels marked a strong departure from the previous significant anticorrelation. This period of record visual brightness began at the time that the 8900-A photometry was unusually bright (1987) and has persisted to the present (see Fig. 1 of Lockwood and Thompson 1991), unlike the near-infrared brightness, which returned to near-normal values by 1989 (Fig. 1 of this paper). In this paper, we take the analysis of the long-term photometry one step further by looking not at the brightness variation of Neptune from year to year, but rather at the variability of Neptune within each year. The bottom two panels of Fig. 1 show the yearly intrinsic variability of Neptune at b and y derived from the observed differential magnitudes and expressed as the standard deviation of the annual mean. We corrected for variations of the standard stars by subtracting the observed variances of the comparison star magnitudes relative to one another from variances of the planet’s magni-
magnitudes relative to SAO 083543 for the bright feature-centered hemisphere (rotational phases 0.4 to 0.6) and the dark antifeature hemisphere (rotational phases 0 to 0.25 and 0.75 to 1). The lines indicate the weighted mean of the 1986, 1989, and 1991 values. (b, c, d) The standard deviations of the yearly mean magnitudes at 8900 A, y (5510 A), and b (4720 A), respectively. All 8900-A data were included in the average (i.e., rotational phases 0.0 to 1.O). The solid line in (b) is the average of 1986,1989, and 1991 values. The solid lines in (c) and (d) are the averages of the values for 1972-1985, 1990, and 1991. During 1990, poor weather prevented 8900-A measurements of the bright feature, thus the standard deviation for that year is unusually low. Neptune showed anomalous activity from 1986 through 1989 at y and b, and from 1987 to 1988 at 8900 A. The difference of activity as a function of wavelength probably indicates the depth of the activity’s level in the atmosphere.
tude relative to the comparison stars (cf. Table 1 of Lockwood et al. 1991b; new measurements for 1990 and 1991 are also included here). In some years, variability of one of the comparison stars masks the intrinsic variability of Neptune; we excluded those data from the figure. There is not sufficient temporal resolution within each year to separate the observations into feature-centered and antifeature hemispheres. During the apparitions that were coincident with the near-IR activity, visual photometry showed variations larger than normal by a factor of 2. This is probably due to rotational modulation by an unusually bright feature at visual wavelengths.
Combined
Results
Figure 1 summarizes our photometry over the past 20 years in visible light and over the past 6 years in the near infrared. Since 1989, the 8900-A disk-integrated magnitudes of Neptune’s antifeature hemisphere have been repeatable to within 0.05 mag; within the errors, the difference between magnitudes of the bright and dark hemispheres was roughly constant at -0.12 mag in the years 1986, 1989, and 1991. We will refer to this state of Neptune’s atmosphere as “normal.” Unfortunately, poor weather limited the 1990 observations, but we did get several measurements of the antifeature hemisphere consistent with the normal levels, given the errors. In 1987 and 1988, the discrete feature was unusually bright at 8900 A, up from normal by almost 0.4 mag and 0.2 mag, respectively. The antifeature hemisphere was also bright in 1987 by about 0.06 mag over the normal level. Interestingly, in 1988 the antifeature hemisphere was even brighter still, elevated above normal by nearly twice the 1987 amount (0.1 mag). The b and y photometry also showed unusual activity beginning somewhat earlier, in 1986. These data were not sampled frequently enough to generate diurnal lightcurves, but the yearly standard deviations give an indication of maximum diurnal variation. The standard deviation in each of the previous 14 apparitions (1972-1985) had been repeatable to within 0.004 mag in both colors. However, from 1986 through 1989 the amount of night-to-night variation was roughly double that.
DISCUSSION
We interpret our observations as an outburst of atmospheric activity. The b and y photometry indicate that the event began in 1986: the standard deviations at these wavelengths were both larger than normal by a factor of 2 for 11 nights of observation between May and July. The 1986 methane-band photometry obtained in late May and early June suggests a possible onset of increased activity
366
HAMMEL
in the antifeature hemisphere. The 8900-A data are sensitive to higher atmospheric levels than the b and y data. Thus the earlier detection of the activity at b and y may indicate that the outburst was an upwelling event that began relatively deep in the troposphere in mid-1986 and was detected at the higher levels in 1987. We can only put a lower limit on the upward velocity of material. In 1986, visual-wavelength observations were made until the first week of July (8900-A observations had ended by early June). In 1987, the earliest near-infrared observations were made in June. Assuming that the different wavelengths probe over a vertical depth of about 100 km (e.g., Smith et al. 1989), an 1l-month delay implies a minimum velocity around 4 mm/set. On the other hand, if the material reached the higher altitudes earlier (for example by August 1986), the velocities could be significantly larger. In 1987 the b standard deviation seems normal, although it is high in the years before and after. However, the observations at y are elevated, and the 8900-A observations show extremely large variations. As noted by Lockwood et al. (1991a), the y-filter bandpass includes the 5430-A methane band. A small increase in optical depth has a large effect at methane-band wavelengths. Thus the b filter may be insensitive to a change that strongly affects the other two wavelengths. In 1987, the disturbance appeared roughly longitudinally confined: even though the bright feature’s magnitude jumped by nearly 0.4 mag compared with the previous year, the antifeature hemisphere was not enormously brighter than the normal level. By 1988, however, the material responsible for the brightening had spread longitudinally: magnitudes at all rotational phases appeared significantly elevated relative to their normal levels. Because we obtained the photometry from images, we know that there was no single discrete feature contributing this excess flux in the antifeature hemisphere. By 1989, the year of the Voyager encounter, Neptune had returned to normal near-infrared brightness, but was still marginally more active than normal at visual wavelengths until 1990. Its visual brightness, as opposed to the standard deviation of the brightness discussed here, remained high into 1991 (Lockwood and Thompson 1991). Pilcher (1977) interpreted the 1976 infrared outburst reported by Joyce et al. (1977) as the formation of a global haze layer in an upper level of the atmosphere, which then partially dissipated. However, at that time Neptune was not known to be strongly variable on diurnal timescales, nor was it recognized then that Neptune’s atmospheric reflectivity, particularly in the near infrared, is dominated by one or more discrete features. Thus it is possible that the 1976 event, like the outburst reported here, began as a localized phenomenon and then spread longitudinally. This type of atmospheric disturbance may not be unusual on the outer planets. Saturn recently exhibited a
ET AL.
qualitatively similar event over a shorter timescale: a localized bright feature appeared in September 1990 (Beebe et al. 1991, 1992; Barnet et al. 1991). Over several months, Hubble Space Telescope images revealed that the material sheared away from the source location into a bright band of material. The two types of events are not completely analogous, however. For example, our observations spanned 19 years of Neptune’s 165year orbital period (roughly 12%); during this time, two outbursts occurred. In contrast, Saturn’s outbursts appear to occur roughly once per Saturnian year, suggesting that a seasonal change such as solar heating may be responsible (SanchezLavega and Battaner 1987, Sanchez-Lavega et al. 1991). It may be significant that both of Neptune’s recorded outbursts occurred near solar minimum. However, the visual brightness behaved differently after the two events, falling rapidly in 1977 but remaining high from 1988 to the present. Moreover, enhanced night-to-night variability was not seen in 1976 or 1977, and a search for rotational modulation in those years was unsuccessful at methaneband wavelengths in the visible (Lockwood, unpublished). Thus although the two events may have been similar in the near infrared, the residual effects seen at visible wavelengths were quite different. Rather than concentrating on the rarity of such events, future studies may want to focus on the repetitive nature of these phenomena, and look for insight into how insolation could be driving seasonal activity on the outer planets. The long timescales involved point out the pressing need for telescopic programs that monitor planetary atmospheres over years and even decades. ACKNOWLEDGMENTS H.B.H. acknowledges the following support over the years of this research: NASA Grant NGL 12-001-057, a National Research Council Resident Research Associateship, and NASA Grant NAGW-2385. S.L.L. was supported in part by MIT’s Undergraduate Research Opportunities Program. J.H. acknowledges support from NASA Grant NAGW-1494 and NSF Grant AST-8906011. G.W.L. and D.T.T. acknowledge the continuing support of the Division of Atmospheric Sciences of the National Science Foundation. CCD research at Hawaii’s Institute for Astronomy has been partially supported by NSF Grant AST-8615631. Finally, we acknowledge useful comments by referees C. Bamet and A. Sanchez-Lavega.
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23, 1147.
BEEBE, R. F., L. HUBER, C. B. BARNET,J. A. WESTPHAL,W. A. BAUM, AND R. LIGHT 1991. The nature of Saturn’s 1990 equatorial storm. Bull. Am. Astron.
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23, 1147.
BEEBE, R. F., C. BARNET, P. V. SADA, AND S. S. MURRELL 1992. The onset and growth of the 1990 equatorial disturbance on Saturn. Icarus 95, 163-172.
OUTBURST
ON NEPTUNE
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CRUIKSHANK,D. P. 1985. Variability of Neptune. Icarus 64, 107-l 11.
OKE, J. B., AND J. E. GUNN 1983. Secondary standard stars for absolute
HAMMEL, H. B., N. L. LARK, M. RIGLER, AND T. J. KREIDL 1989. Diskintegrated photometry of Neptune at methane-band and continuum wavelengths. Icarus 79, l-14.
PILCHER, C. B. 1977. Evidence for weather on Neptune. II. Astrophys. J. 214, 663-666.
JOYCE, R. R., C. B. PILCHER, D. P. CRUIKSHANK,AND D. MORRISON 1977. Evidence for weather on Neptune. I. Astrophys. J. 214,657-662.
SANCHEZ-LAVEGA,A., AND E. BATTANER 1987. The nature of Saturn’s atmospheric Great White Spots. Astron. Astrophys. 185, 315-326.
LANDOLT, A. 1983. UBVRI photometric standards around the celestial equator. Astron. J. 88, 439-460.
SANCHEZ-LAVEGA,A., F. COLAS, J. LECACHEUX, P. LAQUES, I. MIYAZAKI, AND D. PARKER 1991. The Great White Spot and disturbances in Saturn’s equatorial atmosphere during 1990. Nature 353,397-401.
LIMAYE, S. S., AND L. A. SROMOVSKY1991. Winds of Neptune: Voyager observations of cloud motions. .Z. Geophys. Res. 96, 18941-18960. LOCKWOOD, G. W. 1977. Secular brightness increases of Titan, Uranus, and Neptune, 1972-1976. Icarus 32,413-430. LOCKWOOD, G. W., AND D. T. THOMPSON 1986. Long-term brightness variations of Neptune and the solar cycle modulation of its albedo. Science 234, 1543-1545. LOCKWOOD, G. W., AND D. T. THOMPSON1991. Solar cycle relationship clouded by Neptune’s sustained brightness maximum. Nature 349, 593-594. LOCKWOOD, G. W., D. T. THOMPSON, H. B. HAMMEL, P. BIRCH, AND M. CANDY 199la. Neptune’s cloud structure in 1989: Photometric variations and correlation with ground-based images. Icarus 90, 299-307. LOCKWOOD, G. W., D. T. THOMPSON,B. L. LUTZ, AND E. S. HOWELL 199lb. The brightness, albedo, and temporal variability of Neptune.
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Astrophys. .I. 266, 713-717.
SMITH, B. A., L. A. SODERBLOM, D. BANFIELD, C. BARNET, A. T. BASILEVSKY,R. F. BEEBE, K. BOLLINGER,J. M. BOYCE, A. BRAHIC, G. A. BRIGGS, R. H. BROWN, C. CHYBA, S. A. COLLINS, T. COLVIN, A. F. COOK II, D. CRISP, S. K. CROFT, D. CRUIKSHANK,J. N. CUZZI, G. E. DANIELSON, M. E. DAVIES, E. DE JONG, L. DONES, D. GODFREY, J. GOGUEN, I. GRENIER, V. R. HAEMMERLE, H. HAMMEL, C. J. HANSEN, C. P. HELFENSTEIN,C. HOWELL, G. E. HUNT, A. P. INGERSOLL,T. V. JOHNSON,J. KARGEL, R. KIRK, D. I. KUEHN, S. LJMAYE, H. MASURSKY, A. MCEWEN, D. MORRISON, T. OWEN, W. OWEN, J. B. POLLACK, C. C. PORCO, K. RAGES, P. ROGERS, D. RUDY, C. SAGAN, J. SCHWARTZ, E. M. SHOEMAKER, M. SHOWALTER, B. SICARDY, D. SIMONELLI,J. SPENCER,L. A. SROMOVSKY,C. STOKER, R. G. STROM,~. E. SUOMI, S. P. SYNOTT, R. J. TERRILE, P. THOMAS, W. R. THOMPSON, A. VERBISCER,AND J. VEVERKA 1989. Voyager 2 at Neptune: Imaging science results. Science 246, 1422-1449. TODY, D. 1986. The IRAF data reduction and analysis system. InZnstrumentation in Astronomy VI: SPZE Proc. (D. L. Crawford, Ed.), Vol. 627, p. 733.
99,
363-367 (1992)
An Atmospheric
Outburst
on Neptune
from 1986 through
1989
H. B. HAMMEL, S. L. LAWSON, AND J. HARRINGTON Massachusetts
Institute
of Technology,
54416,
Department
of Earth, Atmospheric,
and Planetary
Sciences,
Cambridge,
Massachusetts
02139
G. W. LOCKWOOD AND D. T. THOMPSON Lowell
Observatory,
1400 West Mars Hill Road,
Flagstaff,
Arizona
86001
AND C. SWIFT’ California
Institute
of Technology,
Pasadena,
California
91109
Received March 6, 1992; revised June 1, 1992
dissipated slowly. In a review of Neptune’s long-term variation, Lockwood and Thompson (1991) reported sustained brightening since 1985, a departure from a 17-year anticorrelation with cyclic solar activity. They suggested that an outburst occurring now on Neptune might be responsible for this behavior. We report here observations that show that an outburst actually occurred about 1986, with Neptune returning to normal near-infrared standard deviations and brightnesses by 1989 and normal visual standard deviations by 1990.
In two independent long-term programs (visual photometry and methane-band imaging), we find that Neptune underwent a period of unusual atmospheric activity from 1986 through 1989. This outburst was characterized by (1) increased 8900-A methane-band brightness both for the dominant bright feature by 0.3 mag and for the hemisphere opposite that feature by 0.14 mag, and (2) increased standard deviation of the mean yearly magnitudes at visual and near-infrared wavelengths by over a factor of 2. By 1989 when Voyager 2 encountered Neptune, the near-infrared standard deviations and brightness had returned to normal, although the visual standard deviations were still marginally high. We interpret the observations as a localized source of upwelling, qualitatively similar to “great white spots” seen periodically on Saturn. o 1992 Academic Ress,
OBSERVATIONS 8900-A Photometry
Inc.
We obtained lightcurves of Neptune’s diurnal variation during each apparition from 1986 through 1991. The photometry was derived from CCD images of Neptune and a flux calibration standard star (SAO 083543 = BD + 26”2606; Oke and Gunn 1983) taken at the University of Hawaii 2.2-m telescope (Mauna Kea, Hawaii), using a methane-band filter centered at 8900 A with a full width at half maximum of 200 A. We also observed a second comparison star each year for extinction correction and a check of photometric accuracy. The second star differed from run to run, but was always a Landolt star chosen to be near Neptune (Landolt 1983). Hammel et al. (1989) described the analysis of the 1986-1987 data. We analyzed the 1988-1990 data using IRAF (Tody 1986) to obtain disk-integrated photometry of Neptune and photometry of the standard stars. Using the procedure described by Hammel et al. (1989), we determined and applied extinction corrections from the
INTRODUCTION Neptune’s atmospheric reflectivity varies on many timescales, ranging from tens of minutes in the highestresolution Voyager images (Smith et al. 1989, Limaye and Sromovsky 1991) to years (Lockwood and Thompson 1991). In March 1976, Joyce et al. (1977) observed an
unusual event termed an “outburst” by Cruikshank (1985). A substantial increase in Neptune’s reflectance betweeen 1 and 4 pm relative to 1975 values was followed by a gradual decrease in brightness to the earlier levels. Pilcher (1977) interpreted this event as the sudden formation of a relatively high-altitude haze layer, which then Presented at Neptune/Triton Conference in Tucson, Arizona, during January 6-10, 1992. ’ Current address: 420 Hao Street, Honolulu, HI, 96821. 363
0019-1035192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
364
HAMMEL ET AL.
combined photometry of the standards, and calculated magnitudes of Neptune relative to the SAO star. We then constructed rotational lightcurves for Neptune for each year. The images show that the transit of a bright discrete feature across Neptune’s disk causes the observed strong diurnal variability. (In 1989, simultaneous groundbased and Voyager 2 spacecraft images allowed identification of this feature as the “bright companion” south of the Great Dark Spot; see Fig. 8 of Smith et al. 1989.) For this analysis, we chose arbitrary epochs that put the feature’s transit at phase 0.5 in each lightcurve. We determined rotation periods by generating several lightcurves with different periods and choosing the one that provided the best visual fit. Experience has shown us that analytic techniques such as phase dispersion minimization are strongly dependent on the time sampling of the data, and often lead to wrong periods for undersampled data such as we have. Observations of feature transits in groundbased images provide an independent check of rotation periods: the time between feature transits must be an integral number of rotations of the planet. In addition, the technique eliminates confusion due to multiple features. Finally, in 1989, periods determined from the high-resolution Voyager imaging provided a third, unambiguous check of groundbased periods (Lockwood et al. 1991a), lending confidence to the periods found in other years. Periods ranged from near 17.0 hours (in 1986, 1987, and 1988) to 18.29 and 18.4 hr (in June and August 1989, respectively). The 1990 and 1991 data were too sparse to determine accurate rotation periods, but the data were consistent with periods of less than 17.5 hr. We determined yearly magnitudes for the antifeature hemisphere by calculating the weighted average of all measurements whose rotational phases were in the ranges of O-O.25 or 0.75-1.0. Similarly, the magnitude of the feature-centered hemisphere was defined as the weighted average of all measurements whose rotational phases were between 0.4 and 0.6; this value was not strongly dependent on the width of the “window” in rotational phase. The difference between the feature-centered and antifeature magnitudes is the flux contributed by the discrete bright feature. The yearly averages of the two hemispheres are shown in Fig. la. To compare more conveniently with the visual standard deviations, we also calculated the standard deviation of all 8900-A observations taken during each year (Fig. lb). b and y Photometry
As part of an independent program of long-term planetary monitoring, we obtained photometry of Neptune every year from 1972 through 1991 using intermediate-band filters at 5510 A (y) and 4720 A (6) on the Lowell 0.5-m telescope (Lockwood and Thompson 1986, 1991, Lock-
70 74 78 ~“‘I”‘I”‘I’.‘I”‘I”‘, L 0.9 1
82
(a) Magnitudes
86
90
94
at 8900 k,
1.0 1
f
Bright hemisphere
Dark hemisphere
0.200
L
0.160
-
0.120
-
0.080
-
0.040
-
0.010
(b) 8900A
.
.
.
(c) Stromgren y
0.008
\1 -
0.006
-
0.004
-
. .
.
.
. . l
l
l
.
.
. .
0.002
-
:
c
0.010 0.008
-
0.006
-
0.004
-
0.002
. .
.
(d) Stromgren b
. . . .
l
.
-
.
. .
. .
.
t...I...I...I...,...l...i 70
74
78
82
86
90
94
YEAR
FIG. 1. Neptune’s variability as a function of year of apparition. The observations were made over a period of several months (typically from Mav_ through centered on oouosition. (a) 8900-8, disk-integrated _ Aueust) _ ..
365
OUTBURST ON NEPTUNE
wood et al. 1991a, 1991b). We typically observed about 10 nights per apparition, but in 1989 and 1990 we made many more observations, expanding the normal l-hr nightly sequence to nearly 4 hr and including extra nights during the 4-month-long observation season that runs from May through August. Each year we observed Neptune and a different pair of comparison stars. We measured the stars repeatedly over several seasons, which allowed us to create a standard photometric network along Neptune’s orbital path (Lockwood and Thompson 1986). The observations yielded a time series of Neptune brightnesses adjusted to zero solar phase angle and a fixed opposition distance (cf. Lockwood 1977). The annual mean magnitudes of Neptune have an overall accuracy of about 0.003 mag, although there are differences from year to year depending mainly upon the photometric stability of the particular pair of comparison stars used. The long-term lightcurve (Lockwood and Thompson 1991) was highly anticorrelated with solar activity until about 1987, when increasing brightness levels marked a strong departure from the previous significant anticorrelation. This period of record visual brightness began at the time that the 8900-A photometry was unusually bright (1987) and has persisted to the present (see Fig. 1 of Lockwood and Thompson 1991), unlike the near-infrared brightness, which returned to near-normal values by 1989 (Fig. 1 of this paper). In this paper, we take the analysis of the long-term photometry one step further by looking not at the brightness variation of Neptune from year to year, but rather at the variability of Neptune within each year. The bottom two panels of Fig. 1 show the yearly intrinsic variability of Neptune at b and y derived from the observed differential magnitudes and expressed as the standard deviation of the annual mean. We corrected for variations of the standard stars by subtracting the observed variances of the comparison star magnitudes relative to one another from variances of the planet’s magni-
magnitudes relative to SAO 083543 for the bright feature-centered hemisphere (rotational phases 0.4 to 0.6) and the dark antifeature hemisphere (rotational phases 0 to 0.25 and 0.75 to 1). The lines indicate the weighted mean of the 1986, 1989, and 1991 values. (b, c, d) The standard deviations of the yearly mean magnitudes at 8900 A, y (5510 A), and b (4720 A), respectively. All 8900-A data were included in the average (i.e., rotational phases 0.0 to 1.O). The solid line in (b) is the average of 1986,1989, and 1991 values. The solid lines in (c) and (d) are the averages of the values for 1972-1985, 1990, and 1991. During 1990, poor weather prevented 8900-A measurements of the bright feature, thus the standard deviation for that year is unusually low. Neptune showed anomalous activity from 1986 through 1989 at y and b, and from 1987 to 1988 at 8900 A. The difference of activity as a function of wavelength probably indicates the depth of the activity’s level in the atmosphere.
tude relative to the comparison stars (cf. Table 1 of Lockwood et al. 1991b; new measurements for 1990 and 1991 are also included here). In some years, variability of one of the comparison stars masks the intrinsic variability of Neptune; we excluded those data from the figure. There is not sufficient temporal resolution within each year to separate the observations into feature-centered and antifeature hemispheres. During the apparitions that were coincident with the near-IR activity, visual photometry showed variations larger than normal by a factor of 2. This is probably due to rotational modulation by an unusually bright feature at visual wavelengths.
Combined
Results
Figure 1 summarizes our photometry over the past 20 years in visible light and over the past 6 years in the near infrared. Since 1989, the 8900-A disk-integrated magnitudes of Neptune’s antifeature hemisphere have been repeatable to within 0.05 mag; within the errors, the difference between magnitudes of the bright and dark hemispheres was roughly constant at -0.12 mag in the years 1986, 1989, and 1991. We will refer to this state of Neptune’s atmosphere as “normal.” Unfortunately, poor weather limited the 1990 observations, but we did get several measurements of the antifeature hemisphere consistent with the normal levels, given the errors. In 1987 and 1988, the discrete feature was unusually bright at 8900 A, up from normal by almost 0.4 mag and 0.2 mag, respectively. The antifeature hemisphere was also bright in 1987 by about 0.06 mag over the normal level. Interestingly, in 1988 the antifeature hemisphere was even brighter still, elevated above normal by nearly twice the 1987 amount (0.1 mag). The b and y photometry also showed unusual activity beginning somewhat earlier, in 1986. These data were not sampled frequently enough to generate diurnal lightcurves, but the yearly standard deviations give an indication of maximum diurnal variation. The standard deviation in each of the previous 14 apparitions (1972-1985) had been repeatable to within 0.004 mag in both colors. However, from 1986 through 1989 the amount of night-to-night variation was roughly double that.
DISCUSSION
We interpret our observations as an outburst of atmospheric activity. The b and y photometry indicate that the event began in 1986: the standard deviations at these wavelengths were both larger than normal by a factor of 2 for 11 nights of observation between May and July. The 1986 methane-band photometry obtained in late May and early June suggests a possible onset of increased activity
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in the antifeature hemisphere. The 8900-A data are sensitive to higher atmospheric levels than the b and y data. Thus the earlier detection of the activity at b and y may indicate that the outburst was an upwelling event that began relatively deep in the troposphere in mid-1986 and was detected at the higher levels in 1987. We can only put a lower limit on the upward velocity of material. In 1986, visual-wavelength observations were made until the first week of July (8900-A observations had ended by early June). In 1987, the earliest near-infrared observations were made in June. Assuming that the different wavelengths probe over a vertical depth of about 100 km (e.g., Smith et al. 1989), an 1l-month delay implies a minimum velocity around 4 mm/set. On the other hand, if the material reached the higher altitudes earlier (for example by August 1986), the velocities could be significantly larger. In 1987 the b standard deviation seems normal, although it is high in the years before and after. However, the observations at y are elevated, and the 8900-A observations show extremely large variations. As noted by Lockwood et al. (1991a), the y-filter bandpass includes the 5430-A methane band. A small increase in optical depth has a large effect at methane-band wavelengths. Thus the b filter may be insensitive to a change that strongly affects the other two wavelengths. In 1987, the disturbance appeared roughly longitudinally confined: even though the bright feature’s magnitude jumped by nearly 0.4 mag compared with the previous year, the antifeature hemisphere was not enormously brighter than the normal level. By 1988, however, the material responsible for the brightening had spread longitudinally: magnitudes at all rotational phases appeared significantly elevated relative to their normal levels. Because we obtained the photometry from images, we know that there was no single discrete feature contributing this excess flux in the antifeature hemisphere. By 1989, the year of the Voyager encounter, Neptune had returned to normal near-infrared brightness, but was still marginally more active than normal at visual wavelengths until 1990. Its visual brightness, as opposed to the standard deviation of the brightness discussed here, remained high into 1991 (Lockwood and Thompson 1991). Pilcher (1977) interpreted the 1976 infrared outburst reported by Joyce et al. (1977) as the formation of a global haze layer in an upper level of the atmosphere, which then partially dissipated. However, at that time Neptune was not known to be strongly variable on diurnal timescales, nor was it recognized then that Neptune’s atmospheric reflectivity, particularly in the near infrared, is dominated by one or more discrete features. Thus it is possible that the 1976 event, like the outburst reported here, began as a localized phenomenon and then spread longitudinally. This type of atmospheric disturbance may not be unusual on the outer planets. Saturn recently exhibited a
ET AL.
qualitatively similar event over a shorter timescale: a localized bright feature appeared in September 1990 (Beebe et al. 1991, 1992; Barnet et al. 1991). Over several months, Hubble Space Telescope images revealed that the material sheared away from the source location into a bright band of material. The two types of events are not completely analogous, however. For example, our observations spanned 19 years of Neptune’s 165year orbital period (roughly 12%); during this time, two outbursts occurred. In contrast, Saturn’s outbursts appear to occur roughly once per Saturnian year, suggesting that a seasonal change such as solar heating may be responsible (SanchezLavega and Battaner 1987, Sanchez-Lavega et al. 1991). It may be significant that both of Neptune’s recorded outbursts occurred near solar minimum. However, the visual brightness behaved differently after the two events, falling rapidly in 1977 but remaining high from 1988 to the present. Moreover, enhanced night-to-night variability was not seen in 1976 or 1977, and a search for rotational modulation in those years was unsuccessful at methaneband wavelengths in the visible (Lockwood, unpublished). Thus although the two events may have been similar in the near infrared, the residual effects seen at visible wavelengths were quite different. Rather than concentrating on the rarity of such events, future studies may want to focus on the repetitive nature of these phenomena, and look for insight into how insolation could be driving seasonal activity on the outer planets. The long timescales involved point out the pressing need for telescopic programs that monitor planetary atmospheres over years and even decades. ACKNOWLEDGMENTS H.B.H. acknowledges the following support over the years of this research: NASA Grant NGL 12-001-057, a National Research Council Resident Research Associateship, and NASA Grant NAGW-2385. S.L.L. was supported in part by MIT’s Undergraduate Research Opportunities Program. J.H. acknowledges support from NASA Grant NAGW-1494 and NSF Grant AST-8906011. G.W.L. and D.T.T. acknowledge the continuing support of the Division of Atmospheric Sciences of the National Science Foundation. CCD research at Hawaii’s Institute for Astronomy has been partially supported by NSF Grant AST-8615631. Finally, we acknowledge useful comments by referees C. Bamet and A. Sanchez-Lavega.
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