Synodic Rotation Period Research Articles

A new determination of the solar rotation rate

We use 'stackplot' displays to compare observations of the photospheric magnetic field during sunspot cycle 21 with simulations based on the flux-transport model. Adopting nominal rates of diffusion, differential rotation, and meridional flow, we obtain slanted patterns similar to those of the observed field, even when the sources of flux are assigned random longitudes in the model. At low latitudes, the slopes of the nearly vertical patterns of simulated field are sensitive to the rotation rate used in the calculation, and insensitive to the rates of diffusion and flow during much of the sunspot cycle. Good agreement between the observed and simulated patterns requires a synodic equatorial rotation period of 26.75 +/- 0.05 days.

Rotational properties of small asteroids - Photoelectric observations

In 1984 we started an observational program on small asteroids (diameter lower than about 50 km) with the aim to enlarge the available dataset of the rotational periods of this size range objects. Here, we report the results obtained from photometric observations of ten small asteroids from August 1984 to January 1989 at the European Southern Observatory (La Silla, Chile). Reliable synodic rotational periods were determined for 8 asteroids.

Galileo support observations of Asteroid 951 Gaspra

Observations of 951 Gaspra in support of the Galileo spacecraft encounter are reported. Photometric observations of the asteroid yield a synodic rotational period of 7.042 46 and a slope parameter G of 0.285 + or - 0.005. It is inferred from data obtained on May 18, 1990, that the subearth latitude was higher at that time than it was earlier in the opposition. This places a limit on the possible pole orientation of the asteroid. A slope parameter of 0.25 is proposed on the basis of a comparison of the present result for the slope parameter with that of Barucci et al. (1990). A low-quality 0.8-2.5-micron spectrum of 951 Gaspra suggests a high olivine/pyroxene ratio, which is indicative of a source region in the lower mantle of a differentiated asteroid, and similarities to 8 Flora and particularly 15 Eunomia.

Solar 'hot spots' are still hot

Longitude distributions of solar flares are not random but show evidence for active zones (or hot spots) where flares are concentrated. According to a previous study, two hot spots in the northern hemisphere, which rotate with a synodic period of about 26.72 days, produced the majority of major flares, during solar cycles 20 and 21. The more prominent of these two hot spots is found to be still active during the rising part of cycle 22, producing the majority of northern hemisphere major flares. The synodic rotation period of this hot spot is 26.727 + or - 0.007 days. There is also evidence for hot spots in the southern hemisphere. Two hot spots separated by 180 deg are found to rotate with a period of 29.407 days, with one of them having persisted in the same locations during cycles 19-22 and the other, during cycles 20-22.

66 Maja and 951 Gaspra: Possible flyby targets for Cassini and Galileo missions

The asteroids 66 Maja and 951 Gaspra, identified as possible flyby targets for Cassini and Galileo space missions, respectively, were observed during their apparitions in 1988. 66 Maja, a C-class asteroid with a diameter of 78 km, became the focus of a coordinated observing program carried out at Torino, Teramo, and Pic du Midi observatories using both photoelectric and CCD photometry. The computed synodic rotational period of Maja is 9.733 hr and the amplitude of the light variation was 0.45 mag. 951 Gaspra, an S-class asteroid with a diameter of 15 km, was observed about 2 months after opposition for two nights at the European Southern Observatory (La Silla, Chile) in August 1988. The best fit of the composite lightcurve was obtained with a rotational period of7.09 hr and a lightcurve amplitude of about 1.0 mag. An international program to coordinate Earth-based observations during the next oppositions of these asteroids is proposed.

Rotation of comet p/Tempel 2 from CCD and photoelectric photometry

Taking advantage of its favorable opposition, periodic comet Tempel 2 was extensively monitored during May and June 1988. Two lightcurves were obtained with a CCD camera on the 2.3-m telescope of the Steward Observatory on Kitt Peak, and five light-curves were obtained with the photoelectric photometer on the 1.5-m NASA telescope on Mt. Lemmon. The lightcurve amplitude (12-arcsec diaphragm) was nearly 0.5 mag and the synodic period of rotation was 8 hr 55.8 min. On each occasion two uneven maxima were observed, leading to the conclusion that a lightcurve is not a product of an active spot on the comet, but is caused by a true reflection from an elongated rotating nucleus seen through a transparent yet underdeveloped coma. A comparison is made of CCD and photoelectric photometry.

The nucleus of Comet P/Encke

We present charge-coupled device (CCD) images and spectra of P/Encke taken near aphelion in August and September 1988. The CCD photometry shows the brightness of the comet varying in a cyclic fashion about a mean apparent red magnitude m R − 19.8, with a photometric range Δm = 0.62 ± 0.04 mag. The periods T 0 = 15.08 ± 0.08 hr, T 0/2, 3 T 0/2 are consistent with the photometry; we argue that the nucleus synodic rotation period is equal to T 0. The cyclic variations, the conformation of the photometry to the inverse square law, and the absence of resolvable coma are all consistent with the detection of an aspherical rotating nucleus. CCD spectra of the nucleus reveal a reddened continuum with a reflectivity gradient S′ = 11 ± 2%/10 3 A ̊ . The compatibility of the observations with the published pole directions of P/Encke is discussed.

Physical studies of Apollo-Amor asteroids: UBVRI photometry of 1036 Ganymed and 1627 Ivar

Photoelectric lightcurves of the Amor asteroids 1036 Ganymed and 1627 Ivar were obtained during June–December 1985. For Ganymed a drastic change in the shape of the lightcurve was found together with a significant increase of the synodic rotation period. This indicates a large change in the viewing conditions during the apparition and a complex interrelationship between these changes and the probably very irregular shape of the asteroid. The mean synodic peroid is 10.3 hr and the sense of rotation is retrograde. For Ivar, a decrease of the synodic period (4.8 hr) is found, yielding a prograde sense of rotation. The phase curves for both asteroids show strong deviations from the H- G magnitude system phase function at large phase angles. Because of this fact and the lack of observations at phase angles <17° the determined absolute magnitude H(0°) and the slope parameter G are very uncertain. For Ganymed H = 9.50 and G = 0.33, and for Ivar H = 13.24 and G = 0.65. Pole solutions were tried for both asteroids, yielding unreliable results for Ganymed, while for Ivar two independent solutions are in good mutual agreement. We emphasize the opportunity to obtain additional observations to clarify the complex nature of both asteroids during the 1989 (Ganymed) and the 1990 (Ivar) apparitions. The UBVRI color indices found are typical for S asteroids.

2060 Chiron: CCD and electronographic photometry

R-band CCD photometry of 2060 Chiron was carried out on nine nights in November and December 1986 and on 23 March 1988. The synodic rotation period is 5.9180 ± 0.0001 hr and the peak-to-peak lightcurve amplitude is 0.088 ± 0.003 mag. From the 1986 data, the absolute magnitude is H R = 6.24 ± 0.02 mag and the slope parameter is G R = 0.70 ± 0.15, though formal errors may not be realistic. The lightcurve has two pairs of extrema, but its asymmetry, as evidenced by the presence of significant odd Fourier harmonics, suggests macroscopic surface irregularities and/or the presence of some large-scale albedo variegation. The 1986 observations show no evidence of periodic or nonperiodic brightness changes that can be ascribed to comet-like activity. In contrast, the 1988 observations indicate a brightening of Chiron by 0.6 ± 0.1 mag, which confirms the finding by D. J. Tholen, W. K. Hartmann, and D. P. Cruikshank 1988, IAU Circ. 4554) and is consistent with 1978 electronographic photometry, published here for the first time. The lightcurve amplitude appears to be unchanged, however, and image profiles are indistinguishable from those of stars in both 1986 and 1988. Chiron may recently have been varying nonasteroidally on time scales greater than a month or two; but it is also possible that the asteroid's intrinsic brightness has been bistable over the past decade, with an excursion of 0.56 ± 0.12 mag on a time scale of some years.

Distribution of flares on the sun during 1955-1985 - 'Hot spots' (active zones) lasting for 30 years

The coordinates of 'major solar flares' observed during the period from January 1955 through August 1985 are analyzed. About 100 'superactive' regions (large, complex, active regions containing large sunspots) produced 46 percent of the major flares during the period. Superactive regions appeared more frequently in certain areas of the sun called 'hot spots' or 'active zones'. The synodic rotation periods of the northern and southern hemisphere hot spots were 26.72 d and 26.61 d, respectively. One of the two hot spots persisted through three solar cycles, and the other was active during cycles 19 and 21 but was dormant during cycle 20. These findings suggest that the mechanism producing hot spots must be stable for two or three solar cycles or longer.

Asteroid 532 Herculina: Lightcurves, pole orientation and a model

Photoelectric lightcurves of 532 Herculina in 1984 show two maxima and two minima with a synodic rotation period of 0.39185 ± 0.00002 day (1σ). During some other oppositions the Herculina lightcurve has only one maximum and one minimum over that same rotation period. The absolute magnitude in V is 6.13 ± 0.02 mag, the phase coefficient in V is 0.037 ± 0.002, and the mean colors are B− V = +0.86 ± 0.04 and U− B = +0.43 ± 0.02. We applied photometric astrometry and the results indicate a sideral period of 0.3918711 ± 0.0000001 day with retrograde rotation for a north pole at 276° long and +1° lat. The uncertainty of the pole is ±1°. A model of Herculina is presented that generates lightcurves consistent with both the observed amplitudes and the timings of extrema over precisely 28,630 sideral rotations during 30 years. The model is a sphere with two dark regions that are each about 0.13 times the brightness of the surrounding surface. The regions are at 0° asterocentric longitude, +15° lat, with a radius of 30°, and 170° long, −38° lat, with a radius of 26°. With the photometric astrometry pole and the model with two dark regions, predicted lightcurves are shown for the next four oppositions.

Distribution of spin axes and senses of rotation for 20 large asteroids

Pole determinations for 20 large asteroids are presented. This is the first determination of the sense of rotation for 11 of the objects, and a sense of rotation opposite to previous results is obtained for two of the remaining nine asteroids. The spin axes are fairly isotropically distributed, with a statistically uncertain preference for prograde rotation. The mean of the component of the spin angular velocity vectors toward the north ecliptic pole is 〈 ω z 〉 = (0.8 ± 0.5) rev/day. This suggests that for large asteroids an original predominance of prograde rotators has not completely been randomized by collisions (the median diameter in the present sample is approximately 200 km). Two fundamentally different pole determination methods were combined in order to get as reliable results as possible. The first is an Amplitude-Magnitude method based on triaxial ellipsoidal models. The celestial sphere is scanned with trial poles and the one is chosen for which the best fit is obtained with semiempirical amplitude-aspect-phase and magnitude-aspect-phase relationships. Triaxial approximations to the true asteroidal shapes are also obtained with this method. The second method uses the variation of the observed synodic period of rotation to derive the axis and sense of rotation. A well-defined “standard feature” in the lightcurves is selected and is assumed to remain at a fixed rotational phase. An efficient algorithm for finding the correct number of rotational cycles between observations during different apparitions is used. This makes it possible to identify extrema observed during different apparitions with each other (it is not safe to assume that, e.g., the primary maximum at one opposition remains primary at other aspect angles). Discrimination between ambiguous rotation periods can also be made with this method. 4 Vesta is shown to have one maximum and one minimum per rotational cycle. The secular variations of the period of rotation for 7 Iris and 15 Eunomia are less than 3 × 10 −4 and 2 × 10 −4 sec/year, respectively.

Photoelectric photometry of asteroids 9 Metis, 18 Melpomene, 60 Echo, 116 Sirona, 230 Athamantis, 694 Ekard, and 1984 KD

Photoelectric observations of seven asteroids were made from Gila Observatory between October 14, 1983, and June 21, 1984. The following synodic rotational periods and amplitudes are reported: 9 Metis, P = 5.04 hr, ΔM = 0.05; 18 Melpomene, P = 11.570 hr, ΔM = 0.22; 60 Echo, P = 25.208 hr, ΔM = 0.22; 116 Sirona, P = 12.028 hr, ΔM = 0.42; 230 Athamantis, P = 23.99 hr, ΔM > 0.20; 694 Ekard, P = 5.925 hr, ΔM = 0.50; 1984 KD, P = 1.97 hr, ΔM = 0.26. The rotational periods reported for asteroid 60 Echo, 116 Sirona, 694 Ekard, and 1984 KD represent completely new results. The synodic rotational period reported for asteroid 1984 KD is tentative and is based largely upon the observations of a single night. The reported synodic periods of the remaining six asteroids are based upon a minimum of 3 nights of photometric observations.

Radar and photoelectric observations of asteroid 2100 Ra-Shalom

Results of 13-cm-wavelength radar observations and V-filter photoelectric observations of Ra- Shalom during its 1981 Aug–Sep apparition are reported. The radar data yid detections of echoes in the same sense of circular polarization as transmitted (i.e., the SC sense) as well as in the opposite (OC) sense. The estimate of the ratio of SC to OC echo power, μ c = 0.14 ± 0.02, indicates that most, but certainly not all, of the backscattering is due to single reflections from surface elements that are fairly smooth at decimeter scales. The value obtained for the OC radar cross section on Aug 26 (1.2 ± 0.3 km 2) is about three times larger than those obtained on Aug 23, 24, and 25. The echo bandwidth appears to be within about 1.5 Hz of 5.0 Hz on each date. The photoelectric data suggest a value, P syn = 19.79 hr, for the synodic rotation period, and yield a composite lightcurve with two pairs of extrema. Combining this value for P syn with a firm lower bound (4 Hz) on the maximum echo bandwidth yields a lower bound of 1.4 km on the maximum distance between Ra-Shalom's spin axis and any point on its surface.

The lightcurve and phase relation of the asteroid 133 Cyrene

The asteroid 133 Cyrene was observed photometrically on 17 nights during oppositions in 1979 and 1980. The synodic period of rotation was found to be 12. h708 ± 0. h001 with an amplitude of ∼0. m30 during both oppositions. At large phase angles, the phase relation is quite ordinary ( β v ≈ 0.025 mag/degree); however, the low phase angle observations reveal a dramatic opposition brightening, ∼0.2 mag/degree near zero phase angle. The absolute magnitude, V(1,0), extrapolated with the above linear phase coefficient, is 8.40. The following color indicates were also measured: B- V = 0.90, U-B = 0.51.

The influence of differential rotation on the equatorial component of the sun's magnetic dipole field

This paper examines the effect that solar differential rotation would have on a hypothetical large-scale equatorial dipole field. The evolving large-scale field pattern is expressed as a series of non-axisymmetric moments. As time increases, power is transferred to progressively higher order moments. In the 27d rotating coordinate system, each moment undergoes a small retrograde drift which remains nearly uniform until that mode begins to fade. The synodic rotation periods of the first few moments are comparable to the observed 28.5d period of the sun's large-scale field near sunspot maximum. Differential rotation may be the source of this 28.5d period, but the eruption of new flux is necessary to keep the pattern going.

Large-scale solar magnetic fields, coronal holes and high-speed solar wind streams

The connection between geomagnetic disturbances recurring with the 27 day synodic solar rotation period and streams of plasma emitted from particular regions on the Sun (so-called M-regions) has been one of the long-standing problems of solar terrestrial physics. The ‘ plasma streams ’ have been identified with long-lived streams of fast solar wind, imbedded in unipolar magnetic ‘ sectors', for more than a decade. The solar sources of these streams have been identified unequivocally only within the past few years as large-scale coronal regions of open, diverging magnetic fields and abnormally low particle densities, observed as ‘coronal holes’. The temporal evolution of holes and streams seems to reflect the evolution of the large-scale solar magnetic fields; the observed spatial pattern of holes suggests a grand three-dimensional structure of solar wind flow and interplanetary magnetic fields organized by a near-equatorial neutral sheet. The conclusion that much of the solar wind comes from coronal holes implies several important modifications of our ideas regarding the physical origins of the solar wind and any theoretical models of solar wind formation.

Photoelectric photometry and rotation periods of the asteroids 26 Proserpina, 194 Prokne, 287 Nephthys, and 554 Peraga

We present the photoelectric observations of the asteroids 26 Proserpina, 194 Prokne, 287 Nephthys, and 554 Peraga, performed in the years 1977–1978 at the Astronomical Observatory of Torino. The resulting synodic rotation periods are, for Proserpina, 13 h13; Prokne, 15 h67; and Peraga, 13 h63. The period of Nephthys is possibly ∼7 h or the double. A tentative value for the phase coefficient of Peraga was found: 0.043 mag/degree.

Rotation and lifetime of coronal features

A study on the differences of rotation properties, based on the lifetime of coronal features, has been performed for the period 1972–1974. The short-lived component of the green corona associated with solar activity is differentially rotating, while long-lived coronal features persisting more than one synodic rotation period, show little or no differential rotation. These two components coexist at a same latitude within a wide latitude range at least in one of the solar hemispheres.

1580 Betulia: An unusual asteroid with an extraordinary lightcurve

Results of UBV photometry and polarimetry of 1580 Betulia during its 1976 apparition are presented. The synodic period of rotation is found to be 6.130 hr. The linear phase coefficient and absolute magnitude of the primary maximum in V are 0.032 mag/deg and 14.88, respectively. No color variations with rotation or solar phase angle detected, the mean colors being B−V = 0.66 and U−B = 0.24. Betulia's lightcurve is unique among asteroids studied to date in that it displays three maxima and three minima within one rotational cycle, indicative of a region of greater roughness and/or a dark spot on one of its broad faces. Polarization results indicate a low albedo and a mean diameter of about 7 km, establishing Betulia as the first C type asteroid to be found among the Mars crossers. A model accounting for most features of Betulia's lightcurve is given by a prolate spheroid rotating about one of its shorter axes having an axis ratio of 1:1.21 with a major topographic feature on one of its broad faces.