Density Compression Ratio Research Articles

Magnetic fields in barred galaxies

We present λ3.5 cm and A6.2 cm radio continuum maps in total and polarized intensity of the barred galaxies NGC 1097 (at 2-15 resolution) and NGC 1365 (at 9-25 resolution). A previously unknown radio galaxy southwest of NGC 1097 is reported. Apart from a smooth faint envelope and a bright central region, both galaxies exhibit radio ridges roughly overlapping with the massive dust lanes in the bar region. The contrast in total intensity across the radio ridges is compatible with compression and shear of an isotropic random magnetic field, where the gas density compression ratio is approximately equal to 4 and the cosmic ray density is constant across the ridges. The contrast in polarized intensity is significantly smaller than that expected from compression and shearing of the regular magnetic field: this could be the result of decoupling of the regular field from the dense molecular clouds. The regular field in the ridge is probably strong enough to reduce significantly shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic field amplification by shearing. This contributes to the misalignment of the observed field orientation with respect to the velocity vectors of the dense gas. Our observations, for the first time, indicate that magnetic forces can control the flow of the diffuse inlerstellar gas at kilopursec scales. The total radio intensity reaches its maximum in the circumnuclear starburst regions, where the equipartition field strength is about 60μG, amongst the strongest fields detected in spiral galaxies so far. The regular field in the inner region has a spiral shape with large pitch angle, indicating the action of a dynamo. Magnetic stress leads to mass inflow towards the centre, sufficient to feed the active nucleus in NGC 1097. - We detected diffuse X-ray emission, possibly forming a halo of hot gas around NGC 1097.

Diamagnetic solar wind ions changing the MHD conditions at the heliospheric termination shock

With the two Voyager spacecraft presently flying at solar distances larger than 80 AU, it not only becomes very interesting to study the changing particle environments but also to study the behavior of the distant magnetic fields which are swept out with the solar wind. It is especially challenging to look for the predicted shock features of the magnetic fields which are expected at the solar wind termination shock. While classical MHD theories clearly predict a strong abrupt increase in the magnetic field magnitude at perpendicular MHD shocks, Voyager‐1 either has recently not crossed the termination shock, as claimed by Krimigis et al. [2003], or it did not see the predicted field increase, as stated by Burlaga et al. [2003]. To perhaps clarify on these open points, we here investigate the role of diamagnetism of the solar wind ions approaching the termination shock when becoming shocked there. As we shall show, the magnetic field induction is influenced by the magnetic moments of the gyrating solar wind ions. The magnetic moment per unit volume thereby is determined by the ion pressure and influences the effective magnetic fields. On the basis of that we here derive new MHD shock relations which show that deviating from classical MHD shock expectations the magnetic field compression ratio for a perpendicular shock is different from the density compression ratio and actually is determined by the preshock Alfvénic and sonic solar wind Mach numbers. If these data are carefully taken into account, it may become explainable why Voyager‐1 in fact did cross the termination shock without seeing the expected magnetic field increase. Energetic neutral atoms (ENAs) originating from decharged pick‐up ions beyond the termination shock, by their spectra could serve as messengers of these new features of the shock [see Fahr and Scherer, 2004a].

Scanning, compression and land cover classification of astronaut-acquired orbital photographs

Studies that utilize astronaut-acquired orbital photographs for visual or digital classification require high-quality data to ensure accuracy. The majority of images available must be scanned from film and electronically transferred to scientific users. This study examined the effect of scanning spatial resolution (1200, 2400 pixels per inch (21.2 and 10.6 µm/pixel)), scanning density range option (Auto, Full) and compression ratio (non-lossy: Tagged-Image File Format (TIFF); and lossy: Joint Photographic Experts Group (JPEG) 10:1, 46:1, 83:1) on digital classification results of an orbital photograph from the National Aeronautics and Space Administration (NASA)–Johnson Space Center archive. Qualitative results suggested that 1200 ppi was acceptable for visual interpretative uses for major land cover types. Moreover, Auto scanning density range was superior to Full density range. Quantitative assessment of the processing steps indicated that, while 2400 ppi scanning spatial resolution resulted in more classified polygons as well as a substantially greater proportion of polygons ≤0.2 ha, overall agreement between 1200 ppi and 2400 ppi was quite high. JPEG compression up to approximately 46:1 also did not appear to have a major impact on quantitative classification characteristics. We conclude that both 1200 and 2400 ppi scanning resolutions are acceptable options for this level of land cover classification, as well as a compression ratio at or below approximately 46:1. Auto range density should always be used during scanning because it acquires more of the information from the film. The particular combination of scanning spatial resolution and compression level will require a case-by-case decision and will depend upon memory capabilities, analytical objectives and the spatial properties of the objects in the image.

Injection and acceleration of thermal protons at quasi‐parallel shocks: A hybrid simulation parameter survey

Collisionless shocks that propagate along the mean magnetic field are known to accelerate some fraction of the incident charged particles directly from the thermal pool to energies that are considerably higher than the energy at which the plasma rams into the shock. Using hybrid simulations, we address two issues: (1) the dependence of the injection/acceleration of thermal protons to energies much higher than the plasma ram energy on various shock parameters such as Mach number, plasma beta, etc., and (2) the effect of the high‐energy particles, accelerated directly from the thermal population by the shock, on the macroscopic properties of the shock, most notably, on the density compression. We find that for supercritical Mach numbers the acceleration of the thermal plasma is efficient enough that the back pressure due to the energetic particles can significantly increase the density compression across the shock, above the value expected from the simple Rankine‐Hugoniot prediction. Additionally, at low Alfvén Mach number, where the acceleration of the thermal plasma is inefficient, the density compression is smaller than the simple Rankine‐Hugoniot prediction owing to the nonresonant fire hose instability. The acceleration efficiency increases with Mach number except at very high Alfvén Mach numbers, where it begins to decrease for Mach numbers greater than ∼ 10. This is due to the presence of a fixed, free‐escape boundary that limits the size of the foreshock region measured in units of the mean‐free paths of the accelerated particles. Additionally, we find that regardless of the upstream plasma parameters, the acceleration efficiency increases with both the density compression ratio across the shock and the distance to the free‐escape boundary measured in units of the mean‐free path of the energetic particles. Both of these are consistent with analytic theory and numerical models that use a phenomenological scattering law.

Nonlinear instabilities in shock-bounded slabs

(substantial changes to section 3.2, otherwise minor) We present an analysis of the hydrodynamic stability of a cold slab bounded by two accretion shocks. Previous numerical work has shown that when the Mach number of the shock is large the slab is unstable. Here we show that to linear order both the bending and breathing modes of such a slab are stable. However, nonlinear effects will tend to soften the restoring forces for bending modes, and when the slab displacement is comparable to its thickness this gives rise to a nonlinear instability. The growth rate of the instability, above this threshold but for small bending angles, is $\sim c_sk (k\eta)^{1/2}$, where $\eta$ is the slab displacement. When the bending angle is large the slab will contain a local vorticity comparable to $c_s/L$, where $L$ is the slab thickness. We discuss the implications of this work for gravitational instabilities of slabs. Finally, we examine the cases of a decelerating slab bounded by a single shock and a stationary slab bounded on one side by thermal pressure. The latter case is stable, but appears to be a special case. The former case is subject to a nonlinear overstability driven by deceleration effects. We conclude that shock bounded slabs with a high density compression ratio generically produce substructure with a strong local shear, a bulk velocity dispersion like the sound speed in the cold layer and a characteristic scale comparable to the slab thickness.

Location of the radio emitting regions of interplanetary shocks

In this study we analyze 20 interplanetary type II radio bursts to determine the location of the type II source region relative to the interplanetary shock. We also report the first determination of a density‐distance relationship (density model) appropriate for interplanetary type II source regions. To determine source location we compare densities in type II source regions, derived from observed type II emission frequencies, to ambient solar wind densities to determine whether source regions are in the ambient, upstream solar wind or in the compressed plasma behind the shock. We compute densities in the ambient solar wind upstream of each shock by using a simple model to extrapolate solar wind plasma densities measured at 1 AU back to the shock front. Densities in the type II source regions are found to be enhanced relative to densities in the ambient solar wind by a factor which is close to the average shock density compression ratio. The simplest interpretation of this result is that the source is located in the compressed plasma within or behind the shock. Although it is possible that the emission is produced in enhanced density regions of the upstream solar wind, we feel that the weight of evidence favors the compressed postshock plasma as the source site.

Density ratios in compressions driven by radiation pressure

It has been recently suggested (Hora & Miley 1984) that in the cannonball scheme of laser compression the pellet may be considered to be compressed by the ‘brute force’ of the radiation pressure. For such a radiation-driven compression, this paper applies an energy balance method to give an equation fixing the radius compression ratio κ which is a key parameter for such intense compressions. A shock model is used to yield specific results. For a square-pulse driving power compressing a spherical pellet with a specific heat ratio of 5/3, a density compression ratio Γ of 27 is computed. Double (stepped) pulsing with linearly rising power enhances Γ to 1750. The value of Γ is not dependent on the absolute magnitude of the piston power, as long as this is large enough. Further enhancement of compression by multiple (stepped) pulsing becomes obvious. The enhanced compression increases the energy gain factor G for a 100 μm DT pellet driven by radiation power of 1016 W from 6 for a square pulse power with 0·5 MJ absorbed energy to 90 for a double (stepped) linearly rising pulse with absorbed energy of 0·4 MJ assuming perfect coupling efficiency.

Structure of the November 12, 1978, quasi‐parallel interplanetary shock

An objective of this paper is to determine the jump in plasma parameters across the November 12, 1978, interplanetary shock, sufficiently accurately to test in a subsequent paper a major prediction of shock acceleration theory: the dependence of the energetic ion spectral index upon the density compression ratio. We use ISEE 1 and 3 measurements of the magnetic field and electron and proton densities, temperatures, and bulk velocities, as well as ISEE 3 alpha particle measurements, and confirm the ISEE 1 electron densities using plasma wave measurements. We solve for the shock normal using four independent methods and show that the upstream and downstream flow parameters are consistent to better than 10% with γ = 5/3 Rankine‐Hugoniot jump conditions. We conclude that the November 12, 1978, shock was a high‐speed (612 km s−1), supercritical, quasi‐parallel (θBn = 41°) shock of moderate strength (fast Mach number of 2.8) propagating into an upstream plasma whose total β was 1.14 and whose electron‐to‐proton temperature ratio was 2.8. This shock had three dissipative scale lengths one of a few Larmor radii associated with its magnetic field jump, one of about 10 RE associated with electron equilibration, and one of about 30 RE associated with an energetic proton foreshock.

Experimental study of a continuous flow pinch

A continuous flow pinch formed in a coaxial plasma device with pulsed gas admission has been investigated. Maximum temperatures of 50 000 K and densities of 3 × 1017 cm−3 have been measured spectroscopically during the 40 μs stable lifetime of the pinch. The properties of the low-β plasma which was formed in the interelectrode space of the device have been determined and used to calculate the degree of compression which occurred beyond the end of the centre electrode. The number density compression ratio was found to be 25 and the temperature was increased by a factor of 3·5. An approximate energy balance for the flow process has been established using measured values of the speed of flow of the plasma before and after compression.