Prompt high-energy particle acceleration during two-current-loop collisions

We present a model of prompt high-energy particle acceleration during two-current-loop collisions. By investigating test proton and test electron motions in the electromagnetic field derived from the MHD equations, we found that high-energy particle acceleration occurs only in the case ofY-type, loop-loop collisions. The results depend strongly on the plasma and initial position of the test particle. When the plasma increases, the particle acceleration rate decreases. The particles near the edge of the collision region can be accelerated to higher energy than the ones inside it. It has been shown that both protons and electrons can be accelerated to 10 GeV within 0.001 s and 5 MeV within 10^–6 s, respectively. In the case ofY-type loop-loop collisions, one may expect that high-energy gamma-ray and neutrons will be generated from interaction between high-energy particles and the low atmospheric plasma.     

Dynamics of the physical state during two-current-loop collisions

A model of two-current-loop collisions is presented to explain the impulsive nature of solar flares. From MHD equations considering the gravity and resistivity effects we find self-consistent expressions and a set of equations governing the behavior of all physical quantities just after magnetic reconnection has taken place. Numerical simulations have revealed that the most important parameters of the problem are the plasma and the ratio of initial values of pressure gradient in the longitudinal and radial directions. Thus, the low plasma case during aY-type interaction (initial longitudinal pressure gradient is comparable with initial radial pressure gradient) shows a rapid pinch and simultaneous enhancement of all physical quantities, including the electric field components, which are important for high-energy particle acceleration. However, an increase of the plasma causes a weakening of the pinch effect and a decrease of extreme values of all physical quantities. On the other hand, for anX-type collision (initial longitudinal pressure gradient is much greater than initial radial pressure gradient), which is able to provide a jet, the increase of the plasma causes a high velocity jet. As for aI-type collision (initial longitudinal pressure gradient is much less than initial radial pressure gradient) it shows neither jet production nor very strong enhancement of physical quantities. We also consider direct and oblique collisions, taking into account both cases of partial and complete reconnection.     

Temperature variation of the plasma sheet during substorms

The temperature and density of the plasma in the Earth's distant plasma sheet at the downstream distances of about 20–25 R_e are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ (adiabatic process) in some instances, while by T ∝ N^?1 (isobaric process) in other cases. The total plasma and magnetic pressure decreases when $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ and increases when T ∝ N^?1. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward flow component near the central plasma sheet during substorms.     

Formation of fast magnetosonic shock waves during a two-current-loop collision in solar flares

We present a numerical simulation of the fast magnetosonic shock wave formation during a two-current-loop collision by using a magnetohydrodynamical model. It is shown that the rarefaction waves are generated in the initial stage when the two current loops start to collide. After the rarefaction waves propagate away from the excited region, the fast magnetosonic waves with density enhancement can be produced for the simulation when the current strength of the loop is weak. As the current becomes strong enough, the magnetosonic shock waves can be generated in the direction perpendicular to that of the two-loop collision.     

Changes in the gravitational energy of galaxies during collisions

The problem of the change in gravitational energy of a colliding galaxy due to tidal effects is considered. The change in the internal energy, the mass of escaping matter and the change in the mean radius of the test galaxy have been estimated for a relative velocity of 1000 km s^–1 for three distances of closest approach for the following four cases: (a) both galaxies centrally concentrated, (b) both galaxies homogeneous, (c) test galaxy centrally concentrated, field galaxy homogeneous, and (d) test galaxy homogeneous, field galaxy centrally concentrated. The masses and radii of the two galaxies are taken as 10¹¹ M and 10 kpc respectively. For simplicity, the galaxies are assumed to be spherically symmetric and the distribution of mass within a centrally concentrated galaxy is assumed to be that of a polytrope of indexn=4. The results also provide estimates for the minimum relative velocity a galaxy must have in order that it may not be captured by another to form a double system. It has been found that normally a relative velocity of less than about 500 km s^–1 will lead to the formation of a double galaxy by tidal capture. In the case of a head-on collision between two centrally concentrated galaxies even a relative velocity of about 1000 km s^–1 is small enough for tidal capture. The changes in the structure of the galaxies for relative velocities equal to velocity of escape are also indicated. These results show that there is no escape of matter from the test galaxy in cases (b) and (c). In the case (a) the escape of matter can be as high as 4% of the total mass. The head-on collision between galaxies are normally not accompanied by any escape of matter. All the gain in the internal energy of galaxies during such collisions results in increase in their dimensions. The fractional increase in the mean radius of the test galaxy in the head-on collision is 1.5 in the case (a), 3.2 in the case (b) and 0.01 in the case (c). In the case (d) the test galaxy will be disrupted by the tidal forces.     

Role of Coulomb collisions in the equatorial F-region plasma instabilities

The effect of collisions on electrostatic instabilities driven by gravity and density gradients perpendicular to the ambient magnetic field is studied. Electron collisions tend to stabilize the short wavelength (k_y?_i ? 1, where k_y is the perpendicular wavenumber of the instability and ?_i is the ion Larmor radius) kinetic interchange mode. In the presence of weak ion-ion collisions, this mode gets converted into an unmagnetized ion interchange mode which has maximum growth rate one order smaller than that of the collisionless mode. On the other hand, electron collisions can excite a long wavelength resistive interchange mode in a wide wavenumber regime ($\text{10}{}^{\mathrm{?}}\text{3 ? k}{}_{\mathrm{y}}\text{?}{}_{\mathrm{i}}\text{? 0.3}$) with growth rates comparable to that of the collisional Rayleigh-Taylor mode. The results may be relevant to some of the spread F irregularities.     

Diurnal variation of thermal plasma in the plasmasphere

All of the OGO-5 light ion density measurements (covering the period from March 1968 to May 1969) obtained from the Lockheed Light Ion Mass Spectrometer were used to determine the average global topology of the equatorial plasmasphere density distribution. The variation of the light ion equatorial density at L?3.2 with local time was deduced by determining the average density observed within one hour of a specific local time and within 0.1 of a given L coordinate. The average H⁺ density showed a semidiurnal variation with peaks near noon and midnight. The He⁺ observations also revealed multiple peaks throughout the day but with smaller amplitudes than those of H⁺. At L>3.2 plasma trough conditions increase the scatter of densities. The average variation of the H⁺ density with L within the plasmasphere is found to be steepest near midnight and can be least squares fitted equally well to either an exponential variation exp (?bL) where b is between 0.85 and 1.5 or to a power law L^?a where a varies from 3.2 to 5.     

Effects of ion collisions on the interaction of transverse waves with half-space plasma

For half-space (Z>0), homogeneous, collisonal and warm plasma, the expressions for fields and penetration depth δ/δ _e (in the unit of ion collisionless cold plasma penetration depth, i.e., when v _i =0, υ_0i =) are derived and discussed numerically. It is concluded that the propagation of transverse waves is only slightly affected by the ion collisions and the applied magnetic field when the plasma frequence is greater than the wave frequency (ω _pe >ω). For the case of ω _pe ≤ω, the damping of the wave is not affected by the changes in the ion collision frequency and the ion temperature. However, in this case, the propagation of the wave is drastically affected by the applied magnetic field and the wave damps quickly as the magnetic field strength or the gyrofrequency (Ω _e ) increases.     

Characteristics of enhanced and low-amplitude cosmic-ray diurnal variation

The occurrence of a large number of high- and low-amplitude cosmic-ray diurnal wave trains during the two solar cycles (20 and 21) over the years 1965–1990 has been examined as a function of solar activity. The high-amplitude days with the time of maximum in the 18:00 hr corotation direction do not indicate any significant correlation with solar activity. But, the low-amplitude days are inversely correlated with solar activity and the time of maximum shifts to earlier hours ( 15:00 hr direction). The slope of the power-specrum density roughly characterized by power spectral index n in the high-frequency range 3.5 x 10^–5 Hz to 8.3 x 10^–4 Hz (time scales of 20 min to 8 hr) is different for the two classes of events. A suggestion is made that the enhanced and low-amplitude cosmic-ray diurnal variations are produced by different types of interplanetary magnetic field distributions.     

Impact angle influence in high velocity dust collisions during planetesimal formation

We have examined the influence of impact angle in collisions between small dust aggregates and larger dust targets through laboratory experiments. Targets consisted of μm-sized quartz dust and had a porosity of about 67%; the projectiles, between 1 and 5 mm in diameter, were slightly more compact (64% porosity). The collision velocity was centered at 20 m/s and impact angles range from 0° to 45°. At a given impact angle, the target gained mass for projectiles smaller than a threshold size, which decreases with increasing angle from about 3 mm to 1 mm. The fact that growth is possible up to the largest angles studied supports the idea of planetesimal formation by sweep-up of small dust aggregates.     

The horizontal variation of temperature in the low solar photosphere

Observations of the rms intensity fluctuations in the continuum obtained by Pravdjuk et al. (Solnechnye Dannye, No. 2, p. 70, 1974) from white-light photographs made with the Soviet Stratospheric Solar Observatory are analyzed to obtain a horizontal temperature-fluctuation amplitude as a function of depth. The results indicate that temperature fluctuations increase with depth monotonically from a small value at ₅₀₀₀0.5 (cf. Figure 2). The initial rise of T appears quite steep, having a slope of approximately 20 K km^–1. The model of Wilson (Solar Phys. 9, 303, 1969) is incompatible with the data. Convective flux in the present model is approximately 6% of the total flux at ₅₀₀₀ = 1.     

Mass dispersal and angular momentum transfer during collisions between rubble-pile asteroids

We perform N-body simulations of impacts between initially non-rotating rubble-pile asteroids, and investigate mass dispersal and angular momentum transfer during such collisions. We find that the fraction of the dispersed mass (Mdisp) is approximately proportional to , where Qimp is the impact kinetic energy; the power index α is about unity when the impactor is much smaller than the target, and 0.5?α<1 for impacts with a larger impactor. Mdisp is found to be smaller for more dissipative impacts with small values of the restitution coefficient of the constituent particles. We also find that the efficiency of transfer of orbital angular momentum to the rotation of the largest remnant depends on the degree of disruption. In the case of disruptive oblique impacts where the mass of the largest remnant is about half of the target mass, most of the orbital angular momentum is carried away by the escaping fragments and the efficiency becomes very low (<0.05), while the largest remnant acquires a significant amount of spin angular momentum in moderately disruptive impacts. These results suggest that collisions likely played an important role in rotational evolution of small asteroids, in addition to the recoil force of thermal re-radiation.     

Effect of ion collisions on TM models in cylindrical wave guide filled with hot plasma

The expression for damping coefficients (K _i) is derived and discussed numerically, for a cylindrical wave guide, filled with hot collisional and uniaxially magnetised plasma. It is observed that TM modes suffer a very high damping for high values of plasma frequency (w _pe/w = 10) and low values of ion collision frequency (v _i/v _e = 10^?3), where as for low values of plasma frequency (w _pe/w = 0.1) the damping is low. The damping also increases as the ion temperature increases.     

Mass dispersal and angular momentum transfer during collisions between rubble-pile asteroids. II. Effects of initial rotation and spin-down through disruptive collisions

Expanding on our previous N-body simulation of impacts between initially non-rotating rubble-pile objects [Takeda, T., Ohtsuki, K., 2007. Icarus 189, 256-273], we examine effects of initial rotation of targets on mass dispersal and change of spin rates. Numerical results show that the collisional energy needed to disrupt a rubble-pile object is not sensitive to initial rotation of the target, in most of the parameter range studied in our simulations. We find that initial rotation of targets is slowed down through disruptive impacts for a wide range of parameters. The spin-down is caused by escape of high-velocity ejecta and asymmetric re-accumulation of fragments. When these effects are significant, rotation is slowed down even when the angular momentum added by an impactor is in the same direction as the initial rotation of the target. Spin-down is most efficient when the impact occurs in the equatorial plane of the target, because in this case most of the ejected fragments originate from the equatorial region of the target and a significant amount of angular momentum can be easily removed. In the case of impacts from directions inclined relative to the target's equatorial plane, spin-down still occurs with reduced degree, unless impacts occur onto the pole region from the vertical direction. Our results suggest that such spin-down through disruptive impacts may have played an important role in spin evolution of asteroids through collisions in the gravity-dominated regime.     

Matter temperature during cosmological recombination

The temperature of the atomic matter in the Universe is held to that of the cosmic background radiation until decoupling at   z ∼ 100  . After this, it cools faster than the radiation [  ∝ (1 + z )²  rather than  (1 + z )  ] and would have fallen to about 20 mK today if astrophysical feedback processes had not heated up the interglactic medium. We show how the derivative of the Compton coupling equation helps numerically to follow the decoupling process.     

Ion-ion collisions and element separation effects in the boundary region of a plasma surrounded by neutral gas

In the boundary region between a plasma and neutral gas, the temperature is low (some thousand degrees). Then the Coulomb cross-section is large compared with other collision cross-sections. The influence of ion-ion collisions in the boundary region is discussed. In particular a simple formula is derived for the separation between different kinds of constituents. This separation depends on different collision cross-sections for diffusion and seems to be especially large when resonant charge exchange collisions are involved. These diffusive processes of importance in impermeable plasmas are also compared with separation processes due to a difference in ionization potential, which is known to occur in permeable plasmas. Different isotopes have the same ionization potential and the same charge exchange cross-sections. The separation processes described above are, therefore, likely to give element, but not isotope, separation.     

Effects of ion collisions on TM modes in a cylindrical wave guide filled with hot moving plasma

For a cylindrical wave guide, filled with hot collisional and moving plasma, the expression for damping coefficientK _i (imaginary part of the wave vector) has been derived and discussed in case of TM modes. It is observed that due to the effects of ion collisions,K _i remains less than zero for all values of (=/c) and waves suffer strong reflections for 0.5. The damping is however not affected for moderate changes in collision frequencies, while for low values of electron collision frequency and plasma density, the damping coefficient varies in a parabolic fashion with sharp reflections at 0.3.