New applications of the H-reversal trajectory using solar sails

Advanced solar sailing has been an increasingly attractive propulsion system for highly non-Keplerian orbits.Three new applications of the orbital angular momentum reversal(H-reversal) trajectories using solar sails are presented:space observation,heliocentric orbit transfer and collision orbits with asteroids.A theoretical proof for the existence of double H-reversal trajectories(referred to as‘H2RTs’) is given,and the characteristics of the H2RTs are introduced before a discussion of the mission applicati...     

Asteroid body-fixed hovering using nonideal solar sails

The problem of body-fixed hovering over an asteroid using a compact form of nonideal solar sails with a controllable area is investigated.Nonlinear dynamic equations describing the hovering problem are constructed for a spherically symmetric asteroid.Numerical solutions of the feasible region for body-fixed hovering are obtained.Different sail models,including the cases of ideal,optical,parametric and solar photon thrust,on the feasible region is studied through numerical simulations.The influence of the asteroid spinning rate and the sail area-to-mass ratio on the feasible region is discussed.The required orientations for the sail and their corresponding variable lightness numbers are given for different hovering radii to identify the feasible region of the body-fixed hovering.An attractive scenario for a mission is introduced to take advantage of solar sail hovering.     

Transfer of solar angular momentum by inertial induction

Transfer of angular momentum from the Sun to the planetary system has been found to be inevitable in all evolutionary models for the origin of the solar system. In cold theories it has been proposed to be achieved through friction whereas electromagnetic forces are considered to be the agent for this transfer in hot theories. In the present paper it has been shown that the required order of magnitude of angular momentum can be transferred by another mechanism based on the principle of inertial induction. In the previous theories most of the transfer had been assumed to have taken place during the pre-Main-Sequence period whereas in this proposed theory most of the transfer takes place during the Main-Sequence period of the Sun. The paper does not intend to go into the details of planet formation and the evolutionary process but confines itself only to the problem of angular momentum transfer.     

The origin of the angular momentum distribution in the solar nebula

Supersonic non-central collision and coalescence of interstellar matter clouds is suggested as a physical process that could lead to the formation of a solar nebula with an appropriate distribution of the spinangular momentum.     

Protostars and the origin of the angular momentum of the solar system

Protostars in a group exert gravitational tidal torques on an aspherical nebula located in the group. The net torque transfers angular momentum from the orbital motions of the stars to rotation of the nebula. A relation can be derived between the parameters describing the protostars and the final angular momentum of the nebula. While the parameters concerned are uncertain, a conservative choice results in a value for the angular momentum equal to about 1/3 of that of the present solar system. This suggests that if the Sun formed in a group, tidal interactions with other protostars may account for a significant part of the angular momentum of the solar system.     

Rotation of pre-main-sequence stars in NGC 2264 and the solar angular momentum problem

Preliminary measurements of rotational velocities of pre-main sequence stars indicate that stars evolving into early F or late A spectral type have rotational velocities which are consistent with present Main-Sequence stars of similar spectral type. Stars evolving into G type, however, have rotational velocities which are as high as 100 km s^–1 and would reach the Main Sequence with velocities of 150 km s^–1. This requires the presence of a strong stellar wind to carry off considerable angular momentum in order to slow down the Sun to its present low rotational velocity.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Coronal expansion and the transfer of solar angular momentum to the interplanetary space

We discuss the question of loss of angular momentum through coronal expansion. From a large volume of data on Type-1 cometary tails we have confirmed the presence of a tangential component in the coronal expansion, which has not only a stochastic component but also a constant component of 9.8 km/s. Through coronal expansion the Sun has lost 80% of its angular momentum since it evolved on to the main sequence and the angular velocity of the Sun is decreasing exponentially. This result should have a large effect on the dynamical evolution of the Sun.     

The two rigid body interaction using angular momentum theory formulae

This work presents an elegant formalism to model the evolution of the full two rigid body problem. The equations of motion, given in a Cartesian coordinate system, are expressed in terms of spherical harmonics and Wigner D-matrices. The algorithm benefits from the numerous recurrence relations satisfied by these functions allowing a fast evaluation of the mutual potential. Moreover, forces and torques are straightforwardly obtained by application of ladder operators taken from the angular momentum theory and commonly used in quantum mechanics. A numerical implementation of this algorithm is made. Tests show that the present code is significantly faster than those currently available in literature.     

The influence of non-uniform solar wind expansion on the angular momentum loss from the Sun

The influence on the rate of angular momentum loss from the Sun of magnetic geometries which are not spherically symmetric is estimated. Departures from spherical symmetry are expected to influence significantly the loss rate by two effects - the presence of closed magnetic field regions with no loss and also the variability in the radial distance to the Alfvénic point, as stressed by Mestel (1968).The loss rate is calculated for an MHD solar wind model with a solar magnetic field whose normal component at the surface is that of a north-south dipole. In contrast to Mestel's work, where the field was assumed dipolar within a certain surface and radial outside, the coupling between the solar wind and magnetic field is here taken into account exactly. For equivalent boundary conditions at the surface, the resulting field configuration yields an angular momentum loss rate which is only 15% of that for the monopole field normally used in angular momentum loss estimates. If, instead of equating boundary conditions at the Sun, one equates the two losses at the equator to that observed at 1 AU by spacecraft, then the ratio of the total loss for the distended dipole to that for the monopole is about 40%.On Leave from the Department of Applied Mathematics, The University, St. Andrews, Scotland.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales

Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO₂ sublimation/condensation. The corresponding LOD variations (ΔLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe.     

Hard X-rays from solar flares — time characteristics in correlation with energy and angular distributions of electrons

Fine time variation of hard X-rays has been explained in terms of a spread in the angle of incidence of the source electrons in non-thermal thick-target model for bremsstrahlung generation. The electron energy and angular distributions have been calculated by combining small angle scatterings using analytical treatment with a large angle collision using Monte Carlo calculations as a function of column density. The incidence angles of electrons are taken as 0, 30, and 60°. Using the Bethe-Heitler cross section and the above calculated electron distributions, the bremsstrahlung flux for different photon energies as a function of column density has been studied. The computed X-ray pulse as a function of column density has been converted into time profile. It corresponds well with the observed fine time structure. The calculated spectra of X-rays at the peak and valley are also consistent with the observations. The variation of photon flux with time has also been computed for photon energies 20, 50, and 100 keV for 90 and 180° observation angles together with the changes in spectral shapes of photon energy spectrum at different times for 90 and 180° observation angles.     

Real time Kp predictions from solar wind data using neural networks

Multilayer feed-forward neural network models are developed to make three-hour predictions of the planetary magnetospheric Kp index. The input parameters for the networks are the B_z-component of the interplanetary magnetic field, the solar wind density n, and the solar wind velocity V, given as three-hour averages. The networks are trained with the error back-propagation algorithm on data sequences extracted from the 21^st solar cycle. The result is a hybrid model consisting of two expert networks providing Kp predictions with an RMS error of 0.96 and a correlation of 0.76 in reference to the measured Kp values. This result can be compared with the linear correlation between V(t) and Kp(t + 3 hours) which is 0.47. The hybrid model is tested on geomagnetic storm events extracted from the 22^nd solar cycle. The hybrid model is implemented and real time predictions of the planetary magnetospheric Kp index are available at http://www.astro.lu. se/-fredrikb.     

An optimal method for determining the time of the minimum of the 23rd solar cycle,July 2008

The time of the minimum of the solar cycle is usually determined by the minimum of the average monthly sunspot number smoothed over 13 months, i.e., with a large time delay, not earlier than eight months after the event. A new optimal method which allows one to establish the time of the minimum of the cycle as early as four months after the event is proposed. In the new method, the indicator of the time of a cycle minimum is the time of reaching the minimum level of average monthly values of the solar constant after which four succeeding values of this constant are larger than the pre-ceding minimum level. It is shown that the minimum of the past 23rd solar cycle took place in July, and the new 24th cycle started in August 2008.     

A study of the modulating effect of solar flares on the cosmic ray intensity using time series analysis

The cosmic ray 11-year variation for solar cycle 20 is attributed to the modulating effect of solar flare-induced shocks propagating through the interplanetary medium to the boundary of the heliosphere. The relative influence of these disturbances upon the cosmic ray intensity as a function of their travel time from the Sun is determined by a deconvolution of a linear system with the number of solar flares (importance 1) and the observed cosmic ray intensity as the input and output respectively of this system. The impulse response function so determined indicates that the solar flare - induced disturbances significantly modulate cosmic rays out to a distance of 70–90 AU where the modulating effect of the disturbances abruptly ends. This is interpreted as the boundary of the heliosphere.     

On experimental evidence of chaotic dynamics over short time scales in solar wind and cometary data using nonlinear prediction techniques

Strong indications of chaotic dynamics underlying in the interplanetary and cometary magnetic field fluctuations over short time scales are identified using HEOS-2 spacecraft (at 1 AU), Pioneer 10 (at 4.8 AU), Pioneer 11 (at 20 AU) and ICE (Giacobini-Zinner cometary environment) high-resolution measurements. Other non-chaotic candidate processes, such as linear deterministic models, fractal Brownian motion, and linear gaussian stochastic models are rejected at a high confidence level using nonlinear prediction methods. Experimental proofs of phase correlations are obtained. Assuming chaotic dynamics, estimations of the Kolmogorov-Sinai entropy are provided.