Preflare current sheets in the solar atmosphere

Neutral current sheets are expected to form in the solar atmosphere when photospheric motions or the emergence of new magnetic flux causes oppositely directed magnetic fields to be pressed together. Magnetic energy may thus be stored slowly in excess of the minimum energy associated with a purely potential field and released suddenly during a solar flare. For simplicity, we investigate the neutral sheet which forms between two parallel line dipoles when either the distance between them decreases or their dipole moments increase. It is found that, when the dipoles have approached by an amount equal to a tenth of their original separation distance, the stored energy is comparable with that released in a major flare. In addition, a similarity solution for one-dimensional magnetohydro-dynamic flow within such a neutral sheet is presented; it demonstrates that rapid conversion of magnetic energy into heat is possible provided conditions at the edge of the neutral sheet are changing sufficiently quickly.     

Probability distribution functions of microscale magnetic fluctuations during quiet conditions

A statistical study of microscale magnetic fluctuations in the interplanetary and magnetosheath region during quiet conditions is approached from the concept of probability distribution function. Magnetic field data from Explorer 34 were used to reconstruct the distribution functions and to calculate some of their moments. The distribution functions are found to be nearly tri-Maxwellian as the background field is relatively quiet. The direction of maximum fluctuations is found to be nearly perpendicular to that of the background magnetic field, but the fluctuations are rarely circularly polarized. Across the Earth's bow shock, the degree of fluctuation anisotropy increases, but no noticeable change in relative fluctuation intensity has been observed.     

Hill stability and distance curves for the general three-body problem

The notion of Hill stability is extended from the circular restricted 3-body problem to the general three-body problem; it is even extended to systems of positive energy and the Hill's curves with their corresponding forbidden zones are generalized.Hill stable systems of negative energy present a hierarchy: they have a close binary that can be neither approached nor disrupted by the third body. This phenomenon becomes particularly clear with the distance curves presentation.The three limiting cases, restricted, planetary and lunar are analysed as well as some real stellar cases.     

The dynamics of jovian white ovals from formation to merger

In early 1998 two of the three, long-lived anticyclonic, jovian white ovals merged. In 2000 the two remaining white ovals merged into one. Here we examine that behavior, as well as the dynamics of three earlier epochs: the Formation Epoch (1939-1941), during which a nearly axisymmetric band broke apart to form the vortices; the Kármán Vortex Street Epoch (1941-1994), during which the white ovals made up the southern half of two rows of vortices, and their locations oscillated in longitude such that the white ovals often closely approached each other but did not merge; and the Pre-merger Epoch (1994-1997), during which the three white ovals traveled together with intervening cyclones from the northern row of the Kármán vortex street in a closely spaced group with little longitudinal oscillation. We use a quasi-geostrophic model and large-scale numerical simulation to explain the dynamics. Our models and simulations are consistent with the observations, but none of the observed behavior is even qualitatively possible without assuming that there are long-lived, coherent cyclones longitudinally interspersed with the white ovals. Without them, the white ovals approach each other and merge on a fast, advective timescale (4 months). A necessary ingredient that allows the vortices to travel together in a small packet without spreading apart is that the strong, eastward-flowing jetstream south of the white ovals is coincident with a sharp gradient in background potential vorticity. The jet forms a Rossby wave and a trough of the wave traps the white ovals. In our simulations, the three white ovals were trapped before they merged. Without being trapped, the amount of energy needed to perturb two white ovals so that they merge exceeds the atmosphere’s turbulent energy (which corresponds to velocities of ∼1 m s⁻¹) by a factor of ∼100. The mergers of the white ovals BC and DE were not observed directly, so there is ambiguity in labeling the surviving vortices and identifying which vortices might have exchanged locations. The simulation and modeling make the identifications clear. They also predict the fate of the surviving white oval and of the other prominent jovian vortex chains.     

The propagation of gravity waves through a critical layer for conditions of moderate wind shear

Solutions of the linearised hydrodynamic equations for a viscous atmosphere using (i) a full-wave integration procedure and (ii) a simplified analytical approach are used to examine the attenuation of gravity waves passing through a critical layer, where the horizontal phase velocity is equal to that of the mean wind. Particular attenuation is paid to the variation of this attenuation with values of Richardson number, Ri, greater than unity. The two sets of results are in good agreement with the predictions of Booker and Bretherton (1967) for an inviscid fluid for values of Ri up to about 4. However, a marked discrepancy from these predicted values is found for larger values of Ri, the present results indicating substantially smaller attenuation. Further calculations suggest that the wave-amplitude attenuation factor predicted by the inviscid model is approached asymptotically in the limit of vanishingly small viscosity and thermal conductivity coefficients.The inclusion of viscosity and thermal conduction gives rise to three characteristic modes of propagation for each direction of energy flow, in place of the single mode occurring in the inviscid case. The importance of energy exchange between these modes in the propagation through the critical layer is demonstrated.     

Further analysis of the region of permissible motion in the problem of three bodies

In this paper we investigate the range of variation of the orbital inclination I between two bodies in the problem of three bodies. We give the conditions to determine whether the two bodies move in the same or opposite senses and give the maximum and minimum values of I when the three bodies are in collinear positions. We prove that the range of variation of I must attain maximum and minimum values when the bodies are collinear.     

Ulysses observations of anti-sunward flow on Jovian polar cap field lines

We examine the energetic (MeV) ion data obtained by the Anisotropy Telescopes instrument of the Ulysses COSPIN package during two northern high-latitude excursions prior to closest approach to Jupiter, when the spacecraft left the region of trapped fluxes on closed magnetic field lines at lower latitudes and entered a region of open field lines which we term the polar cap. During these intervals the ion fluxes dropped by 4–5 orders of magnitude to low but very steady values, and the ion spectrum was consistent with the observation of an essentially unprocessed interplanetary population. Ion anisotropies observed at these distances (within 16R_J, of Jupiter) indicate that in the low-latitude, high-flux regions the flows are principally azimuthail and in the sense of corotation, with speeds which are within a factor of 2 (in either direction) of rigid corotation. In the higher latitude trapped flux regions the flows rotate to become northward as the polar cap is approached, while in the polar cap itself the flows rotate further to become anti-corotational (and anti-sunward in the morning sector) and northward. These results provide primary evidence of the existence of solar wind-driven flows in the outer Jovian magnetosphere mapping to the high-latitude ionosphere. Investigation of concurrent magnetic data for the signatures of related field-aligned currents reveals only weak signatures with an amplitude of order 1 nT. The implication is that the height-integrated Pedersen conductivity of the ionosphere to which the spacecraft was connected was low, of order 0.01 mho or less. We also examine the ion observations during the two northern high-latitude excursions previous to those discussed above. These data indicate that the spacecraft approached but did not penetrate the open flux region during these intervals.     

The Winchcombe fireball—That lucky survivor

On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South-West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre-atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s⁻¹). A near-catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near-Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.     

The evolution of the lunar orbit revisited,II

We present here a model for the tidal evolution of an isolated two-body system. Equations are derived, including the dissipation in the planet as in the satellite, in a frequency dependent lag model. The set of differential equations obtained is still valid for large eccentricity, as well as for all inclinations. The reference plane chosen enables us to study the evolution for both the orbital plane and the equatorial plane.The results obtained show the Moon, after having approached the Earth with small variations for the inclination and the eccentricity, exhibits strong increase for the two parameters in the vicinity of the closest approach. In every case the eccentricity tends towards the value 1, whereas the variations of the in clinations are dependent on the magnitude of the dissipation in the satellite.Some qualitative results are also investigated for the final behaviour of satellites such as Triton and the Galilean satellites.     

Relative Equilibria for General Gravity Fields in the Sphere-Restricted Full 2-Body Problem

Equilibrium conditions for a mutually attracting general mass distribution and point mass are derived and their stability computed. The equilibrium conditions can be reduced to six equations in six unknowns, plus the existence of four integrals of motion consisting of the total angular momentum and energy of the system. The equilibrium conditions are further reduced to two independent equations, and their theoretical properties are studied. We derive three distinct conditions for a relative equilibrium which can be used to derive robust algorithms for solving these problems for non-symmetric gravity fields: a set of necessary conditions, a set of sufficient conditions, and a set of necessary and sufficient conditions. Each of these conditions is well suited for the computation of certain classes of equilibria. These equations are solved for non-symmetric gravity fields of interest, using a real asteroid shape model for the general gravity fields. Explicit conditions for the spectral and energetic stability of the resulting equilibria are also derived and computed for the shape of interest.     

Dynamics of shock waves in a contracting cloud

The contraction of an interstellar cloud is followed. The results indicates that there are shock waves appear during contraction. In order to study the effect of shock waves, two models have been studied. The post-shock temperature for the two models are, respectively, 3006 K and 2984 K. The density increases by more than three orders of magnitude. The gas is generally cooled by atoms, molecules, and grains. The dominant cooling process changes according to the chemical composition and the temperature of the gas. The thermal equilibrium depends on the existing physical conditions.     

Computation of the Liapunov Orbits in the Photogravitational RTBP with Oblateness

We study the periodic motion around the collinear equilibrium points of the restricted three-body problem when the primary is a source of radiation and the secondary is an oblate spheroid. In particular, the Liapunov families of two and three dimensional periodic orbits are computed. In order to gain the appropriate initial conditions a third-fourth order Lindstedt-Poincaré local analysis is used. The stability of these families is also computed.     

Symmetries, Reduction and Relative Equilibria for a Gyrostat in the Three-body Problem

The problem of three bodies when one of them is a gyrostat is considered. Using the symmetries of the system we carry out two reductions. Global considerations about the conditions for relative equilibria are made. Finally, we restrict to an approximated model of the dynamics and a complete study of the relative equilibria is made.     

On a perturbation theory using Lie transforms

Kamel has recently extended to non-Hamiltonian equations a perturbation theory using Lie transforms. We show here how Kamel's extension can be approached from an intrinsic viewpoint, which reformulation leads to a simpler algorithm. Then we complete Kamel's contribution by establishing the rules for inverting the transformation generated by the perturbation theory, and for composing two such transformations.     

关于太阳射电快速活动真伪信号识别问题

本文列举了云南天文台四波段太阳射电实测中得到的几种干扰实例及确认的太阳快速信号，在认识到太阳射电和干扰信号十分相似的基础上，探讨如何识别真伪信号问题。     

On the Chermnykh-Like Problems: II. The Equilibrium Points

Motivated by Papadakis (2005a, b), we study a Chermnykh-like problem, in which an additional gravitational potential from the belt is included. In addition to the usual five equilibrium points (three collinear and two triangular points), there are some new equilibrium points for this system. We studied the conditions for the existence of these new equilibrium points both analytically and numerically.     

Nonlinear oscillations in star-forming systems

A number of analytically formulated models of star-forming systems which include nonlinear interactions of stars and interstellar medium are considered. It is shown that two conditions are essential for the development of nonlinear oscillations: namely, the induced star formation and the presence of time-delay or strong nonlinearity in interacting star-cloud systems. The three component model with time-delay reproducing the temporal behaviour obtained in numerical simulations by Seidenet al. (1982) is proposed.     

Dynamical evolution of the nucleus of comet Hartley 2 and Asteroid Itokawa

Cometary nuclei are being actively studied using spacecrafts. In November 2010, the Deep Impact spacecraft (EPOXI project, NASA) approached the nucleus of comet 103P/Hartley 2 and returned images of this small celestial body having a dumbbell shape with a smooth neck. Since rotation of the nucleus leads to centrifugal forces, it is assumed that the dumbbell neck appeared as the result of their effect and the neck is lengthening slowly but continuously, which should eventually result in fragmentation of the nucleus. This paper considers dynamical evolution of the nucleus of comet Hartley 2. Calculations show that centrifugal forces exceed gravitational forces in the narrow part, and the nucleus can indeed undergo upcoming breakup and fragmentation into two parts. If there are no external perturbations, both parts of the celestial body will drift apart to a distance of less than 1 km from each other. The nucleus of comet Hartley 2 is an observed example of breakup of a celestial body. Asteroid Itokawa is considered, which has a similar feature but does not seem to undergo breakup.     

Visibility of the new Moon at two sites I: Maryland situated at northern geographical latitude. II: Sacramento peak situated at high altitude above sea level

The optimum conditions to see the crescent of the new Moon have been obtained at Sacramento Peak and Maryland. We have used the data of the sky twilight brightness given by Koomenet al. (1952) for the two sites. The results show that the crescent can not be seen at the two sites for sun's depression less than 4° and 8° elongation between Sun, Moon and Earth confirming the results obtained before by Asaadet al. (1976). The visibility conditions at Maryland and Sacramento Peak are better than that obtained before for the three sites Misallat, Helwan and Daraw at Egypt. The reason is mainly due to the decrease in the sky twilight brightness at sites having higher geographical northern latitudes and high elevation above sea level.