共查询到20条相似文献，搜索用时 687 毫秒
1.
We consider the stability of current sheets where a normal component of the field is present. It is well known that reconnection in such systems progresses orders of magnitude too slow to explain observations, even when full kinetic models are used. We consider here a new possible mechanism for fast reconnection in such systems. We consider the effect of the possible presence of velocity shear that can drive the Kelvin–Helmholtz instability (KHI). The effect of the KHI is shown to convert shear flow into compression flow that drives reconnection. Three scaling effects can be discerned in the simulations. First, the reconnection rate is directly controlled by the driving mechanism which is provided by the KHI. The result of this new mechanism is that fast reconnection can be achieved even in absence of anomalous resistivity. Second, the effect of varying the initial sheared flow along the main magnetic field direction enhances the reconnection process. Finally, the reconnection rate is insensitive to the value of resistivity. 相似文献
2.
Synthetic images of the dust tail are presented for a comet which has a rotating nucleus with one predominant dust source fixed to it. The images have been generated using a new computer model which, unlike similar models, allows for the study of dust tails caused by a rotating nucleus with an anisotropic distribution of sources.The dust tail is studied in the post-perihelion phase of a parabolic comet with a perihelion distance of 0.5 AU. One finds that in the case of a rotating nucleus with anisotropic emission characteristics streamers caused solely by the dynamics of the dust particles are forming in the dust tail even if there is no dependence between the solar irradiation angle of the source and the amount of dust emitted. If the dust emission depends on the solar irradiation angle of the dust source, then the brightest tail regions do not necessarily coincide with the synchrones for the times of maximum dust emission.As a consequence, a thorough analysis of streamer patterns in a cometary dust tail requires assumptions on the rotational state and the dust source distribution of the nucleus. Otherwise, it seems not possible to discern between streamers which are caused dynamically by nucleus rotation and others which reflect variations in the emission activity. 相似文献
3.
The paper presents the results of dynamic simulation for the dust tail formation of comet C/1995 O1 (Hale-Bopp). To simulate
the dust tail, the trajectories of 2 × 106 dust particles were traced. The sizes, ejection moments, outflow directions and velocities of the dust particles were defined
by the Monte Carlo algorithm. The obtained three-dimensional tail was projected on the sky plane to compare it with the observed
images. The brightness distribution in the comet tail was fitted to similar model parameters for three different dates. According
to our model experiments, the observed tails could be formed by particles with sizes from 0.3 to 8.0 μm, ejection velocities
from 0.155 to 0.670 km/s, and power index of the exponential size distribution from −3.6 to −3.7. It is shown that the inclusion
of the particles fragmentation processes leads to a noticeable improvement of the simulation results. 相似文献
4.
哈雷彗星在日彗距较大时出现长而直的主彗尾（尾流），这是很有趣的。尾流一般是指等离子体尾流；但是，当地球接近彗星轨道面时，尘埃尾流可能叠加到主彗尾上。在一般感光波段宽的彗星底片上很难区分这两种尾流。本文选取哈雷彗星在不同日彗距的5条主尾流，作了光度测量和比较分析。得出沿各尾轴及其垂直方向几个截面的亮度分布、亮度半极大全宽、尾轴的视风差角和真风差角及彗尾长度。在所分析的蓝敏底片上，过近日点前的2个尾流肯定是等离子体尾流，而5个尾流的相似性以及其他证据说明它们主要都是等离子体尾流，尘埃彗尾的污染是次要的。 相似文献
5.
Pavlo P. Korsun Irina V. Kulyk Oleksandra V. Ivanova Viktor L. Afanasiev Francois Kugel Claudine Rinner Yuriy M. Ivashchenko 《Icarus》2010,210(2):916-929
We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least. 相似文献
6.
Klaus Beisser 《Earth, Moon, and Planets》1990,48(2):127-137
The distinct structures ta comet Halley's dust tail around the beginning of March 1986 are analyzed by means of a computer simulation based on nucleus data obtained by the Giotto mission. It is shown that the assumption of a considerable free precession is required to understand the ground based dust tail observations from that time supposing a rotational period of some 50 hr. But a precession-free rotation with a period of about 7 days does not contradict an analysis of the dust tail structure. In both cases, an asymmetric distribution of the relevant emission sources is required. 相似文献
7.
Harker David E. Woodward Charles E. McMURTRY Craig W. Goetz J. A. Pipher J. L. Forrest W. J. 《Earth, Moon, and Planets》1997,78(1-3):259-264
We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion.
All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from
the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest
radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ)
in all the images closely follows the “steady state” ρ−1 model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant
implying the dust grain properties vary with coma distance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
W.T. Thompson 《Icarus》2009,200(2):351-357
The bright Kreutz Comet C/2007 L3 (SOHO) entered the fields of view of the twin Solar Terrestrial Relations Observatory (STEREO) COR1 telescopes on 7–8 June 2007. The 12° separation between the two spacecraft at the time afforded the opportunity to derive the position of the comet's tail in three-dimensional space using direct triangulation. The track of the comet's orbit is compared against more traditional orbital calculations using observations from the STEREO COR2 telescopes, and from the Large Angle and Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). The shape of the comet's tail shows that it is composed of dust particles released when the comet was between 18 and 22 solar radii, with no significant dust production after that. The comet did not survive perihelion passage, but a rare faint remnant of the comet tail persisted for several hours after the break-up, and was seen by both the SOHO and STEREO coronagraphs to drift slowly away from the Sun. This tail remnant was found to be composed of particles far back from the head of the comet. The motion of the tail remnant shows a loss of angular momentum during the passage through the solar corona. Atmospheric drag is estimated to account for a significant fraction of this change in angular momentum, but indications are that other mechanisms may be required to completely account for the total amount of change. 相似文献
9.
Masaki N. Nishino Hiroshi Hasegawa Yoshifumi Saito Iannis Dandouras Henri Rème Alessandro Retinò Elizabeth Lucek 《Planetary and Space Science》2011,59(7):502-509
Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth's magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn-dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn-dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth's magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles. 相似文献
10.
To explain the distinct transversal striae observed in the tail of comet C/2006 P1 (McNaught) near the perihelion, a dynamic
model for the formation of the dust tail of the comet has been developed. It is supposed that, on the surface of the nucleus,
there are three local active domains of the increased outflow of the material. Formation of the striated features is caused
by different rates of material outflow from the active areas depending on which side of the rotating nucleus, illuminated
or shadowed, these areas are located. It has been found that the period of the axial rotation of the comet is 21 h. 相似文献
11.
Yu. V. Sizonenko 《Kinematics and Physics of Celestial Bodies》2012,28(1):9-14
We studied variations in the structure of plasma and dust tails of the C/2006 M4 (SWAN) comet during a long observation period
(September–December 2006). We found sizes of grains ejected by the comet from the synchronic-syndynamic analysis of comet
images. We calculated solar wind speed for high heliographic latitudes from calculations of the aberration angle of the comet
plasma tail. Rapid changes in the calculated values of the solar wind speed are caused by its variable transversal component. 相似文献
12.
Twenty-two photographs of Comet Kohoutek (1973f) have been projected onto the comet's orbital plane under the assumption of a flat tail confined in that plane. The comet has a mixed-type tail; therefore the results concerning Type I and Type II components of the tail are presented separately. The axis of the Type I tail appears to sweep back and forth with respect to the prolonged radius vector in a rather periodical way. Interpretations advanced by some authors for the analogous case of Comet Burnham (1959k) are mentioned briefly. For the Type II tail, a comparison with a theoretical tail model by Sekanina has allowed us to establish when the onset of appreciable dust production occurred. Finally, mention is made of the fact that some peculiarities of the dust tail might be also explained by assuming a three-dimensional model. 相似文献
13.
It is shown that the sheared flow of electrons and ions in the presence of heavy stationary dust gives rise to unstable Alfvén waves. The coupling of newly studied low frequency electrostatic current-driven mode with the electromagnetic Alfvén and drift waves is investigated. The instability conditions and the growth rates of both inertial and kinetic Alfvén waves are estimated. The theoretical model is applied to the night side boundary regions of Jupiter’s magnetosphere which contain positive dust. The growth rates increase with increase in sheared flow speed. In the nonlinear regime, both inertial and kinetic Alfvén waves form dipolar vortices whose speed and amplitude depend upon the magnitude of the zero-order current. 相似文献
14.
The large-scale post-perihelion evolution of the dust tail in Comet C/1995 O1 (Hale–Bopp) was monitored with Schmidt telescope exposures at the European Southern Observatory (ESO) La Silla in Chile. In early October 1997 signatures of a peculiar dust-tail appeared as straight, but diffuse extensions in the northern and southern coma hemisphere. With the approach of the Earth to the orbital plane of the comet a needle-like anti-tail with a similar, but much longer counterpart in the region of the normal dust tail formed. In early January 1998, i.e., close to the plane crossing, the anti-tail pointed towards position angle 190° and was at least 0.4°long, its counter part extended over more than 5° into oppositedirection. During February and March 1998 the position angles of both features increased by more than 40$°– while their appearance became shorter, wider and more diffuse again. Thepeculiar dust tail was last observed in April 1998.Modeling of the dust tail dynamics shows that the anti-tail andpeculiar tail phenomenon observed is formed by a neck-line structureof heavy dust grains released around perihelion passage. Two scenariiare introduced to explain the extension of the anti-tail feature:(1) grains ejected about 20 days post-perihelion with a initial velocity of 170 m/s in radial direction towards the Sun and(2) grains released about 60–80 days before perihelion at a speed of70m/s into the orbital plane and against the motion directionof the comet. The out-of-plane velocity component of the dust wasless than 25 m/s. Both scenarios require dust of < 0.001to be involved, i.e., grains of at least mm-size. The rather symmetric shading of the neck-line structure before and after passage of theEarth through the orbital plane of the comet in early January 1998 supports the idea that dust of both scenarii may have contributed to the phenomenon. 相似文献
15.
We report the detection of Comet 67P/Churyumov-Gerasimenko's dust trail and nucleus in 24 μm Spitzer Space Telescope images taken February 2004. The dust trail is not found in optical Palomar images taken June 2003. Both the optical and infrared images show a distinct neck-line tail structure, offset from the projected orbit of the comet. We compare our observations to simulated images using a Monte Carlo approach and a dynamical model for comet dust. We estimate the trail to be at least one orbit old (6.6 years) and consist of particles of size ?100 μm. The neck-line is composed of similar sized particles, but younger in age. Together, our observations and simulations suggest grains 100 μm and larger in size dominate the total mass ejected from the comet. The radiometric effective radius of the nucleus is 1.87±0.08 km, derived from the Spitzer observation. The Rosetta spacecraft is expected to arrive at and orbit this comet in 2014. Assuming the trail is comprised solely of 1 mm radius grains, we compute a low probability (∼10−3) of a trail grain impacting with Rosetta during approach and orbit insertion. 相似文献
16.
Brandt J. C. Snow M. Yi Y. Larson S. M. Mikuz H. Petersen C. C. Liller W. 《Earth, Moon, and Planets》2002,90(1-4):15-33
The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt
and Snow (2000), Icarus
148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational
record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare.
In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months
in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region
and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly
near the head, but images taken with a CO+ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately
May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings
might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse
observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The
plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed. 相似文献
17.
Comet Kohoutek (1973f) has been observed photometrically and polarimetrically in the near-infrared region. The observed spectra revealed two components, scattered sunlight and thermal emission by dust particles. Color temperatures derived from intensities at 2.2 and 3.5 μm are close to the equilibrium temperature of a gray body with solar heating. Polarizations at 1.0 and 1.65 μm have been found to be ～15 to 20% and perpendicular to the tail direction. Properties of the dust particles in the comet are discussed in relation to these observations. 相似文献
18.
《Planetary and Space Science》1999,47(6-7):787-795
The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the β Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets. 相似文献
19.
D.K. Yeomans 《Icarus》1981,47(3):492-499
The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval. 相似文献
20.
A great deal of attention has been given to the production and spatial distribution of sodium in comets after the discovery
of the sodium tail, by Cremonese et al. (1997a), on Hale-Bopp. The sodium has been observed in several comets in the past,
but the Hale-Bopp represent the first time where it will be deeply analyzed considering the several data and scientists working
on that. The sodium tail stimulated different studies trying to explain the mechanism source and provided the new lifetime
for photoionization of the neutral sodium atom. We took into account other sodium observations performed in this century and
we focalized our attention to comet Hale-Bopp to understand the main sources responsible for the sodium features observed.
We analyzed the sodium tail observations performed by Cremonese et al. (1997b) and Wilson et al. (1998) finding that the Hale-Bopp
had four different tails. The wide field images and the high resolution spectroscopy performed along the sodium tail provided
very important clues to distinguish the two sodium tails observed and their possible sources. Considering most of the data
reported in several papers has been possible to draw a real sketch on what has occurred to the comet during March and April
1997. We are going to demonstrate that the sodium tail observed by Wilson et al. (1998) was not the same reportedby Cremonese
et al. (1997a) and in the images taken by the European Hale-Bopp Team there were two distinct sodium tails. The observations
allowed us to define “narrow sodium tail” the tail reported by Cremonese et al. (1997a), and “diffuse sodium tail” the tail
overimposed to the dust tail. We suggest that the narrow sodium tail was due to a molecular process instead of the diffuse
one due to the release of sodium atoms by the dust particles. Such a conclusion is supported by the spatial distribution of
sodium on the nucleus and in the coma as reported from other authors.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献