An expanding high velocity hi cloud probably ejected from the galactic nucleus

An object located approximately atl=8°,b=–4° with a mean radial velocity of –212.3 km s^–1 has been observed in the 21 cm neutral hydrogen line. The mean weighted velocity dispersion is 11.2 km s^–1 and the total mass is estimated to be 190R ² (kpc) solar masses. We discuss possible interpretations of the origin and nature of this object. The most likely interpretation is that we observe an expanding object which has been ejected from the galactic nucleus.     

Magneto-radiative shock wave propagation in a conducting plasma

Similarity solutions describing the flow of a perfect gas behind a spherical and cylindrical shock wave in a magnetic field with radiation heat flux have been investigated. The total energy of the expanding wave has been assumed to remain constant. The solutions, however, are only applicable to a gaseous medium where the undisturbed pressure falls as the inverse square of the distance from the line of explosion.     

Civ plasma flow near active region filaments

Selected Civ V ₀ lines (lines separating adjacent regions of strong blue shift and strong red shift) located near the solar limb and oriented parallel to the radius vector from disk center are shown to be closely aligned with H dark filaments in active regions. The filaments, in turn, are known to lie in the vicinity of magnetic neutral lines. The radial orientation of the V ₀ lines minimizes uncertainties in image registration and their location near the limb ensures that the observed fluid motion has major components paralleling V ₀. It follows that the filaments are located at sites of velocity shear, and, by inference, of magnetic shear. For a case in which a given V ₀ line is observed near both east and west limbs, the gradient of Doppler velocity across the V ₀ line reverses sign from one limb to the other as is expected for horizontal steady flow. Thus, the velocity vectors remain fixed with respect to the filament.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Energy dissipation in wave propagation in general-relativistic plasma

Based on a recent communication by the present authors the question of energy dissipation in magnetohydrodynamical waves in an inflating background in general relativity is examined. It is found that the expanding background introduces a sort of dragging force on the propagating wave such that, unlike the Newtonian case, energy gets dissipated as it progresses. This loss in energy having no special-relativistic analogue is, however, not mechanical in nature as in case of an elastic wave. It is also found that the energy loss is model dependent and also depends on the number of dimensions.     

On an efficient shock wave generation mechanism in the quiet solar transition region

Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon.     

Impact cratering mechanics: Relationship between the shock wave and excavation flow

This paper describes the relationship between the shock wave produced by an impact and the excavation flow that opens the crater. The excavation flow velocity is shown to be a nearly constant fraction of the peak particle velocity in the wave. The existence of an excavation flow is due to thermodynamically irreversible processes in the shock. The excavation flow velocity is thus very sensitive to nonideal constitutive effects such as porosity, plastic yielding, and unreversed phase transformations. Cratering computations that do not model these effects correctly may produce misleading results.     

Relation between classical and relativistic shock wave theories by means of shock tube tests

The author proposes a laboratory simulation of cosmic shock waves by means of the mathematical correlations between the shock equations in the classical and the relativistic fluid dynamics, respectively. In the present note only the normal shock is treated.     

Small-scale Langmuir wave instability in preflare chromosphere of solar active region

Necessary conditions have been investigated for the appearance of instability of high-frequency electron Langmuir waves in plasma of solar chromosphere near the foot-point of loop structure. We have considered the earliest stage of a flare process in solar active region. At the chromospheric part of current circuit of a flare loop such instability can appear and develop as the result of combined action of large-scale electric field, Landau damping and collisional processes in preflare plasma. We have investigated the process of instability development for two possible scenarios: (a) when preflare loop plasma has a classical Coulomb conductivity and (b) when anomalous resistance appears due to saturation of Bernstein turbulence. The growth rates of instability have been obtained and analyzed in detail. It has been assumed in the process of calculation that preflare plasma can be described by the FAL model of the solar atmosphere, which takes into account the process of helium diffusion. It has been shown that Langmuir wave instability can appear in its marginal form in the area under investigation either in the presence of Coulomb conductivity or in the presence of saturated Bernstein turbulence. Existence of instability with the growth rate, which changes its sign, proves the principal possibility of generation of nondamping Langmuir waves with small amplitudes.     

The relationship between UV and X-ray active region structures

Yohkoh and the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) jointly observed two brightenings in active region NOAA 7981 on 6 August 1996. Combining the UV data from CDS with information from the high time resolution coronal images obtained with the Soft X-ray Telescope (SXT) on Yohkoh, provides us with important information on the relationship between the transition region and corona. Our observations show that cool plasma (T_e = 2.2 x 10^-5 K) can lie at the same altitude as the hot coronal plasma (T_e = 1–4 x 10⁶ K). The lower temperature structure is not formed from the cooling of the hotter coronal loop. We are also able to observe a low temperature cut-off of T_e = 1–4 x 10⁶ K for a loop which repeatedly brightened over the period of approximately one day.     

Interchange reconnection between an active region and a coronal hole

With data from the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory,we present a magnetic interaction between an isolated coronal hole(CH) and an emerging active region(AR).The AR emerged nearby the CH and interacted with it.Bright loops constantly formed between them,which led to a continuous retreat of the CH boundaries(CHBs).Meanwhile,two coronal dimmings respectively appeared at the negative polarity of the AR and the east boundary of the bright loops,and the AR was partly disturbed.Loop eruptions followed by a flare occurred in the AR.The interaction was also accompanied by many jets and an arc-shaped brightening that appeared to be observational signatures of magnetic reconnection at the CHBs.By comparing the observations with the derived coronal magnetic configuration,it is suggested that the interaction between the CH and the AR excellently agreed with the model of interchange reconnection.It appears that our observations provide obvious evidence for interchange reconnection.     

Flare stars in the taurus cloud region

In the Winters of 1975 and 1977, observations of flare stars in the Taurus Cloud region were made with the 40 cm double astrograph of the Purple Mountain Observatory. Two flare stars and one repeat outburst of FH Tau were identified. We give also a comparison between the flare stars of this region and the ϱ Oph region.     

Geoeffectiveness of magnetic cloud, shock/sheath, interaction region, high-speed stream and their combined occurrence

A subset of CMEs, called interplanetary magnetic clouds (MCs), are observed to have systematic rotation [northward to southward (NS) or southward to northward (SN)] in their field structures. These MCs identified in the heliospheric plasma and field data at 1 AU may have different features associated with them. These structures (NS/SN) may be isolated MC moving with the ambient solar wind. MCs (NS/SN) may also be associated with shock/sheath region, formed due to compression of the ambient plasma/field ahead of them. A fraction from each of these four types of MCs have additional features, being ‘pushed’ by fast solar wind streams from coronal holes, forming interaction region (IR) between MCs and high-speed solar wind streams (HSS). Using these different sets of MCs, we have done a detailed study of the geoeffectiveness of NS and SN turning MCs and their associated features (shock/sheath, IR and HSS). To study the process that produces the geomagnetic disturbances and influences its amplitude/duration, we have utilized the interplanetary plasma and field parameters, namely, plasma velocity, density, temperature, pressure, field strength and its north-south component, during the passage of these structures with different associated properties. Differences in the geoeffectiveness of MCs with different structural and dynamical properties have been identified. The possible role of high-speed stream in influencing the recovery time (and hence duration) of geomagnetic disturbance has also been investigated. A best-fit equation representing the relation between level of the geomagnetic activity (due to MCs) and interplanetary plasma/field parameter has been obtained.     

On the relation between chromospheric and photospheric fine structure in an active region

A comparative study was done on the bright fine structure in the upper photosphere and in the lower chromosphere of an active region. The results are shown in the following: (a) The bright points in the H wing are cospatial to the facular points, which confirms the result of Wilson (1981). (b) Some points bright in the H wing are associated with the facular granules which have larger sizes than the facular points, (c) The brightness enhancement in the Ha wing is positively correlated to the enhancement in the blue continuum. However, the correlation is not so strong. (d) The moustache points are also cospatial to the facular features. (e) The geometrical shape of a moustache point is like a funnel and diverging upward in the upper photospheric and the lower chromospheric levels.Some discussions are given on the nature of the heating mechanism of the fine structure based upon these results.On leave from Kyoto University and Hida Observatory, Japan.     

Long-period comet flux in the planetary region: Dynamical evolution from the Oort cloud

This study analyzes the evolution of 2 × 10⁵ orbits with initial parameters corresponding to the orbits of comets of the Oort cloud under the action of planetary, galactic, and stellar perturbations over 2 × 10⁹ years. The dynamical evolution of comets of the outer (orbital semimajor axes a > 10⁴ AU) and inner (5 × 10³ < a (AU) < 10⁴) parts of the comet cloud is analyzed separately. The estimates of the flux of “new” and long-period comets for all perihelion distances q in the planetary region are reported. The flux of comets with a > 10⁴ AU in the interval 15 AU < q < 31 AU is several times higher than the flux of comets in the region q < 15 AU. We point out the increased concentration of the perihelia of orbits of comets from the outer cloud, which have passed several times through the planetary system, in the Saturn-Uranus region. The maxima in the distribution of the perihelia of the orbits of comets of the inner Oort cloud are located in the Uranus-Neptune region. “New” comets moving in orbits with a < 2 × 10⁴ AU and arriving at the outside of the planetary system (q > 25 AU) subsequently have a greater number of returns to the region q < 35 AU. The perihelia of the orbits of these comets gradually drift toward the interior of the Solar System and accumulate beyond the orbit of Saturn. The distribution of the perihelia of long-period comets beyond the orbit of Saturn exhibits a peak. We discuss the problem of replenishing the outer Oort cloud by comets from the inner part and their subsequent dynamical evolution. The annual rate of passages of comets of the inner cloud, which replenish the outer cloud, in the region q < 1 AU in orbits with a > 10⁴ AU (～ 5.0 × 10^?14 yr^?1) is one order of magnitude lower than the rate of passage of comets from the outer Oort cloud (～ 9.1 × 10^?13 yr^?1).     

Efficiency of cosmic ray reflections from an ultrarelativistic shock wave

The process of cosmic ray acceleration up to energies in excess of 10²⁰ eV at relativistic shock waves with large Lorentz factors, Γ≫1, requires ∼Γ² particle energy gains at single reflections from the shock (cf. Gallant & Achterberg). In the present Letter , by applying numerical simulations we address an efficiency problem arising for such models. The actual efficiency of the acceleration process is expected to be substantially lower than the estimates of previous authors.     

Correlation between magnetic shear and magnetic tension in a solar active region

The difference between the magnetic tension and magnetic shear was calculated for four vector magnetograms of NOAA AR 4474. It was seen that this difference between the two independent angular measures of magnetic stress is less than 18° for more than 50% of the pixels. Magnetic tension is thus found to be fairly well correlated with magnetic shear for AR 4474.     

Origin of Jovian decameter wave emissions— Conversion from the electron cylotron plasma wave to the ordinary mode electromagnetic wave

Energy conversion rates from the extraordinary mode to the ordinary mode ofthe electromagnetic waves in the Jovian plasmasphere has been calculated for a model of the sharp boundary that is given in the vicinity of the position where ω = ω_p, for an angular frequency ω and the angular plasma frequency ω_p. The extraordinary mode electromagnetic wave that is obtained as a result of the transformation of a longitudinal propa- gating through an inhomogenous plasma is here considered. The results give conversion rates of 1–50 per cent, at the most, when a wave normal direction of an is nearly parallel to the boundary normal direction and when the Jovian magnetic field vector is close to the boundary normal direction within an angle range from 10° to 15°. The electric field intensity, in range from 7 to 70 mV/m, of the original electrostatic electron cyclotron plasma waves can give the power flux in a range from 10^-22 to 10^-20W/m² Hz for the Jovian decameter waves observed at the Earth's surface. Efficient energy conversion is possible only when the ray direction of the emitted wave is in nearly perpendicular direction with respect to the magnetic field; this is the origin of the sharp beam emission of the Jovian decameter wave burst.     

Comparison of the 9.1cm and soft X-ray emission from an active region

Thermal bremsstrahlung from the X-ray observed plasma accounts for most of the observed 9.1 cm emission from McMath 12336, an old, spotless active region, on June 2, 1973. This implies that only a small fraction of the emission measure within the active region is in the range around 10⁶ K and below.     

Implosive and re-explosive shock wave in magnetohydrodynamics

The present Letter deals with the problem of implosive and re-explosive shock wave in magnetohydrodynamics. A self-similar solution has been obtained neglecting thermal conduction and assuming a linear profile of the plasma velocity.