Rotation and color properties of the nucleus of Comet 2P/Encke

We present results from CCD observations of Comet 2P/Encke acquired at Steward Observatory's 2.3 m Bok Telescope on Kitt Peak. The observations were carried out in October 2002 when the comet was near aphelion. Rotational lightcurves in B-, V-, and R-filters were acquired over two nights of observations, and analysed to study the physical and color properties of the nucleus. The average apparent R-filter magnitude across both nights corresponds to a mean effective radius of 3.95±0.06 km, and this value is similar to that found for the V- and B-filters. Taking the observed brightness range, we obtain a/b?1.44±0.06 for the semi-axial ratio of Encke's nucleus. Applying the axial ratio to the R-filter photometry gives nucleus semi-axes of [3.60±0.09]×[5.20±0.13] km, using the empirically-derived albedo and phase coefficient. No coma or tail was seen despite deep imaging of the comet, and flux limits from potential unresolved coma do not exceed a few percent of the total measured flux, for standard coma models. This is consistent with many other published data sets taken when the comet was near aphelion. Our data includes the first detailed time series multi-color measurements of a cometary nucleus, and significant color variations were seen on October 3, though not repeated on October 4. The average color indices across both nights are: (V−R)=0.39±0.06 and (B−V)=0.73±0.06 (). We analysed the R-filter time-series photometry using the method of Harris et al. [Harris, A.W., Young, J.W., Bowell, E., Martin, L.J., Millis, R.L., Poutanen, M., Scaltriti, F., Zappala, V., Schober, H.J., Debehogne, H., Zeigler, K.W., 1989. Icarus 77, 171-186] to constrain the rotation period of the comet's nucleus, and find that a period of ∼11.45 h will satisfy the data, however the errors bars are large. We have successfully linked our data with the September 2002 data from Fernández et al. [Fernández, Y.R., Lowry, S.C., Weissman, P.R., Mueller, B.E.A., Samarasinha, N.H., Belton, M.J.S., Meech, K.J., 2005. Icarus 175, 194-214]—taken just 2-3 weeks before the current data set—and we show that a rotation period of just over 11 h works extremely well for the combined data set. The resulting best-fit period is 11.083±0.003 h, consistent with the Fernández et al. value.     

Polarization changes with time during solar microwave impulsive bursts

The evolution with time of circular polarization (t) from solar bursts at 7 GHz presents, in the majority of cases, a polarization degree peak before the maximum flux time. The subsequent evolution of (t) is continuous and usually increasing. The changes could be caused by superimposed polarization effects, due to the fast emissive electrons (dominant in the first phase), and to the propagation effects caused by the coronal condensation where the event occurred (dominant in the second phase). In an approximate approach, (t) is connected to the movement of the source in the second phase, being qualitatively sound, but limited to the lack of knowledge on acceleration processes and on magnetic field topology in the active region where the flares take place.     

Emission features in the solar corona after the perihelion passage of Comet 1979 XI

On the coronograph spectrophotographic records taken on 31.372 UT, August 1979, some faint emission features were found which can be ascribed to Siii and Niii. These emissions were obviously a transient phenomena which were detected only 10 hr after the supposed fall of Comet 1979 XI in the Sun's photosphere. It cannot be excluded that the appearance of Si and Ni lines was triggered by the evaporation of dust particles with a high abundance of heavier elements in the solar corona. This assumption is also supported by intensity distribution of the Fex coronal line around the Sun's limb. The maximum coincide with the position angle of the projected path of the comet.     

Magnetic flux changes associated with the solar flares of August 1972

The active region associated with Mt. Wilson sunspot group 18 935 (McMath, 11 976) which had a central meridian passage on August 4 and 5, 1972 produced a number of flares during transit. These included two importance 3B flares on August 4 and 7 as well as several of importance 1 and 2. Calculations of the total magnetic flux in this region were made during the period July 31 through August 9 using data from six observatories. For the 3B flare on August 4, the total flux changed from about 7.2 × 10²² Mx just before onset to about 5.6 × 10²² Mx two hours after onset. For the 3B flare on August 7, the flux was about 6.4 × 10²² Mx three hours before onset and about 5.2 × 10²² Mx three hours after onset. An importance 2B flare on August 2 had no measurable effect on the flux nor did any of several 1N or 1B flares which also occurred in this region during the period. The flux changes measured for the 3B flares occurred in the umbral and penumbral fields and no significant changes were observed in facular fields.The Aerospace Corporation, P.O. Box 92957, Los Angeles, Calif. 90009, U.S.A.     

Helioseismic solar cycle changes and splitting coefficients

Using the GONG data for a period over four years, we have studied the variation of frequencies and splitting coefficients with solar cycle. Frequencies and even-order coefficients are found to change significantly with rising phase of the solar cycle. We also find temporal variations in the rotation rate near the solar surface.     

Permanent changes in filaments near solar flares

High resolution H images obtained before and after 57 importance 1N or larger flares have been examined for changes in the magnetic fields (B ) transverse to the line of sight. It was assumed that H chromospheric structures outline B . In 37% of the cases, there was a reconfiguration of segments of filaments or of chromospheric fibrils. Examination of data from 21 non-flare intervals shows such changes in 24% of cases. When changes of any kind, including total disappearance and length changes, are included, the proportions for flare and non-flare intervals increase to 58% and 52%, respectively. It is concluded that flares do not cause enduring magnetic field changes in the chromosphere.     

An interpretation of rapid changes in the magnetic field associated with solar flares

The energy source of a flare is the magnetic field in the corona. A topological model of the magnetic field is used here for interpreting the recently discovered drastic changes in magnetic field associated with solar flares. The following observational results are self‐consistently explained: (1) the transverse field strength decreases at outer part of active regions and increases significantly in their centers; (2) the center‐of‐mass positions of opposite magnetic polarities converge towards the magnetic neutral line just after flares onset; (3) the magnetic flux of active regions decreases steadily during the course of flares. For X‐class flares, almost 50% events show such changes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rocket sonde-derived stratospheric temperature changes associated with a major solar cosmic ray event

A significant cooling of the upper stratosphere at high latitudes following the major solar proton event of August 1972 is reported. The observed cooling is consistent with a decrease in upper stratospheric ozone after the same proton event that has been reported by Heath,et al. (1977). It is also in agreement with the cooling predicted by tentative model calculations of radiative equilibrium temperature changes following the deozonizing effect of nitric oxides produced at high latitudes by solar protons.     

The Close Encounter of Comet Wild 3 with Jupiter

Der periodische Komet Wild 3 hatte im Mai 1977 einen nahen Vorbeigang an Jupiter. Die Bewegung und Helligkeiten von Wild 3 vor und nach diesem Ereignis wurden untersucht.     

Solar wind interaction with the tail of Comet Kohoutek

Helical waves of large amplitude observed recently in the tail of Comet Kohoutek are interpreted as stable waves arising due to non-linear evolution of Kelvin-Helmholtz instability. The dispersion equation for waves of a finite amplitude shows that the phase velocity of these waves should approximately coincide with the velocity of the plasma outflow in the tail rather than with the Alfvén velocity. This fact is shown to be in agreement with observations. One may estimate the magnetic field in the Comet Kohoutek tail from both the amplitude of observed helical waves and the pressure balance at the tail boundary. The field turns out to be of the order of the interplanetary magnetic field or less, i.e. ?25 γ near ～0.5 AU.     

Solar wind interactions with Comet 19P/Borrelly

The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm³ at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO⁺/H₂O⁺ is lower than that observed with Giotto at 1P/Halley.     

Triaxiality of Halley's Comet

There are many aspects of observational evidence that cometary nuclei have irregular or nonspherical shape. The triaxial figure of the Halley's Comet nucleus is a well known fact. Therefore, the nucleus shape plays a significant role in consideration of the formation and evolution of comets and several attempts have been made to explain their nonsphericity. These studies were mainly based on the random-walk schemes for the aggregation processes. Although some results indeed lead to irregularities and deviation from sphericity, the spherical or irregular shape seem to be prevailing results. On the other hand the triaxial figure can be formed by the tidal and rotational forces. Thus, the assumption that the shape of the cometary nucleus due to some of these effects is in principle acceptable. In here assumed scenario already evolved cometary nucleus is situated as a satellite in the gravitation field of a planetary-like body. Since the rigidity of the nucleus is low, it may be easily transferred in the state of a synchronous satellite and in its shape could be imprinted the dynamical effects from this epoch. Here presented results indicate, that such a possibility should be seriously considered. The theory of this process is applied to the nucleus of comet Halley. It is shown, that the nucleus might be synchronously orbiting around a planetary-like hypothetical body with a period of 0.7 days. The minimal bulk tensile strength of the cometary material of about 10² N m^–2 is estimated.     

Estimation of the solar galactocentric distance and galactic rotation velocity from near-solar-circle objects

We have tested the method of determining the solar Galactocentric distance R ₀ and Galactic rotation velocity V ₀ modified by Sofue et al. using near-solar-circle objects. The motion of objects relative to the local standard of rest has been properly taken into account. We show that when such young objects as star-forming regions or Cepheids are analyzed, allowance for the perturbations produced by the Galactic spiral density wave improves the statistical significance of the estimates. The estimate of R ₀ = 7.25 ± 0.32 kpc has been obtained from 19 star-forming regions. The following estimates have been obtained from a sample of 14 Cepheids (with pulsation periods P > 5 ^d ): R ₀ = 7.66 ± 0.36 kpc and V ₀ = 267 ± 17 km s^?1. We consider the influence of the adopted Oort constant A and the character of stellar proper motions (Hipparcos or UCAC4). The following estimates have been obtained from a sample of 18 Cepheids with stellar proper motions from the UCAC4 catalog: R ₀ = 7.64 ± 0.32 kpc and V ₀ = 217 ± 11 km s^?1.     

Magnetic field changes associated with three successive M-class solar flares on 2002 July 26

With an extensive analysis,we study the temporal evolution of magnetic flux during three successive M-class flares in two adjacent active regions:NOAA 10039 and 10044.The primary data are full disk longitudinal magnetograms observed by SOHO/MDI.All three flares are observed to be accompanied by magnetic flux changes.The changes occurred immediately or within 1 ～ 10 minutes after the starting time of the flares,indicating that the changes are obvious consequences of the solar flares.Although changes in many ...     

用1.05m望远镜对海尔-波普彗星的照相定位观测

1996年7月和8月间,利用陕西天文台骊山天文观测站1．05m望远镜卡焦照相,对海尔-波普彗星进行了定位观测。该望远镜卡焦焦比为F／10,底片比例尺为20”／mm,无畸变象场约为1平方度天区．用蔡司ASCORECORDE2坐标量度促进行底片测量,归算时采用6常数模型,用PPMsouth星在做参考标准,最终给出在FK5基本系统上（J2000．0历元）各观测时刻海尔-波普彗星的α和δ观测值。     

Solar wind interaction with Comet Bennett (1969i)

This paper examines the relations between the solar-wind and Comet Bennett during the period March 23 to April 5, 1970. A large kink was observed in the ion tail of the comet on April 4, but no solar-wind stream was observed in the ecliptic plane which could have caused the kink. Thus, either there was no correlation between the solar wind at the Earth and that at Comet Bennett (which was 40° above the ecliptic) or the kink was caused by something other than a high-speed stream. The fine structure visible in photographs of the kink favors the second of these alternatives. It is shown that a shock probably passed through Comet Bennett on March 31, but no effect was seen in photographs of the comet. A stream preceded by another shock and a large abrupt change in momentum flux might have intercepted the comet between March 24 and March 28, but again no effect was seen in photographs of the Comet. In view of these results, one must seriously consider the possibility that a large, abrupt change in momentum flux of the solar-wind (such as that at a shock wave or ahead of a stream) is neither necessary nor sufficient to cause a large kink in a comet tail.On leave from Institute für Astrophysik Technische Universität Berlin West Germany.     

苏梅克－利维9号彗星撞击木星事件的光学观测

本文给出了１９９４年７月１７日至２２日，彗星碰撞期间，福州和乌鲁木齐两个观测点所取得的部分光学观测资料。     

Plasma clouds associated with Comet P/Borrelly dust impacts

The NASA DS1 spacecraft encountered Comet P/Borrelly on September 22, 2001 at a distance of ∼2171 km on the sunward side of the comet. The flyby speed was ∼16.5 km s⁻¹. Using high temporal resolution (50 μs) absolute electric field amplitude measurements from a ∼1 m dipole antenna, new features of plasma clouds created by cometary dust impacts have been detected. The pulses have 1/e exponential decays of ∼650 μs duration, exponentially shaped overshoots with rise times of ∼2 ms, and exponential-shaped overshoot decay times of ∼10 ms. Assuming a plasma temperature of 10⁴ K, these pulse features have been explained as plasma cloud space charge effects from the electron, proton and heavy ion portions of the clouds passing the antenna. Complex pulse shapes were also detected. These are believed to be due to either plasma cloud scattering off of the spacecraft, or to secondary impacts. Small electric pulses of duration 10-15 ms of cometary origin were detected but are presently unexplained. The electric component of the plasma wave spectra at closest approach had an f^−2.4 power law shape from 10 Hz to 1 kHz. The electron cyclotron frequency was approximately 1 kHz. One possible explanation of the wave spectrum is that whistler mode waves associated with phase steepened cometary plasma waves are dispersed, leading to the broad spectrum. Finally, based on the present results, a new type of low-cost, large-area dust detector is proposed.     

Evolution of Comet Nucleus Rotation

The secular evolution of comet nucleus rotation states subject to outgassing torques is studied. The dynamical model assumes that the nucleus inertia ellipsoid is axially symmetric. The outgassing torques acting on the surface are modeled using standard cometary activity formulae. The general rotational equations of motion are derived and separately averaged over the fast rotational dynamics terms and the comet orbit. Special cases where the averaging assumptions cannot be applied are evaluated separately. The modification of the comet orbit due to comet outgassing is neglected. Resulting from this analysis is a system of secular differential equations that describes the dynamics of the comet nucleus angular momentum and rotation state. We find that the qualitative secular evolution of the rotation state is controlled by a single parameter that combines parameters related to the comet orbit and parameters related to the nucleus surface geometry and activity. From this solution, we find qualitatively different evolutionary paths for comet nuclei whose entire surface is active, as compared to nuclei with only a single active region. For surface activity models between these extremes, we show that certain evolutionary paths are more likely than others. Additionally, our solution indicates that a comet nucleus' rotational angular momentum will tend to increase over time, potentially contributing to the observed phenomenon of comet nucleus splitting.     

Differential Corrections of Comet Orbits

On a calculě les éléments orbitaux corrigés de sept cométes en employant les observations données par les circulaires du Smithsonian Astrophysical Observatory.