Cloud Encounters Research Articles

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: Magnetohydrodynamic Simulation with Relativistic Electrons

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant (SNR) during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport. This was done using a simplified treatment of the diffusion-advection equation, thus allowing us to model injection and acceleration of cosmic-ray electrons at shocks and their subsequent transport. From this information we also model radio synchrotron emission, including spectral information. The simulations were carried out in spherical coordinates with azimuthal symmetry and compare three different situations, each incorporating an initially uniform interstellar magnetic field oriented in the polar direction on the grid. In particular, we modeled the SNR-cloud interactions for a spherical cloud on the polar axis, a toroidal cloud whose axis is aligned with the polar axis, and, for comparison, a uniform medium with no cloud. We find that the evolution of the overrun cloud qualitatively resembles that seen in simulations of simpler but analogous situations: that is, the cloud is crushed and begins to be disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. However, we demonstrate here that, in addition, the internal structure of the SNR is severely distorted as such clouds are engulfed. This has important dynamical and observational implications. The principal new conclusions we draw from these experiments are the following. (1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. (2) The internal flows of the SNR become highly turbulent once it encounters a large cloud. (3) An initially uniform magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily because of biased amplification in that region by Rayleigh-Taylor instabilities. A similar bias produces much greater enhancement to the magnetic energy in the SNR during an encounter with a cloud when the interstellar magnetic field is partially transverse to the expansion of the SNR. The enhanced magnetic fields have a significant radial component, independent of the field orientation external to the SNR. This leads to a strong equatorial bias in synchrotron brightness that could easily mask any enhancements to electron-acceleration efficiency near the magnetic equator of the SNR. Thus, to establish the latter effect, it will be essential to establish that the magnetic field in the brightest regions are actually tangential to the blast wave. (4) The filamentary radio structures correlate well with turbulence-enhanced magnetic structures, while the diffuse radio emission more closely follows the gas-density distribution within the SNR. (5) At these early times, the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray proton pressures. While that result is predictable, we find that this simple result can be significantly complicated in practice by SNR interactions with clouds. Those events can produce regions with significantly steeper spectra. Especially if there are multiple cloud encounters, this interaction can lead to nonuniform spatial spectral distributions or, through turbulent mixing, produce a spectrum that is difficult to relate to the actual strength of the blast wave. (6) Interaction with the cloud enhances the nonthermal electron population in the SNR in our simulations because of additional electron injection taking place in the shocks associated with the cloud. Together with point 3, this means that SNR-cloud encounters can significantly increase the radio emission from the SNR.

Dynamics of double‐theta aurora: Polar UVI study of January 10–11, 1997

During the much studied magnetic cloud encounter with Earth on January 10–11, 1997, pairs of polar cap arcs twice became widely separated from the auroral oval, that is, an extremely rare double θ aurora configuration arose. These events exhibit many dynamic features hitherto rarely if ever reported in the literature of θ aurora. The two independent bars proved capable of merging into a single bar, crossing the polar cap, then splitting again. Moreover, in both events, after completely crossing the polar cap, the transpolar arcs reversed direction, and crossed again. The dynamics of these double θ‐aurora events appear to constrain any models of their origin. Interestingly, in the January 10 event, a double θ‐aurora which had twice crossed the polar cap ended up in a configuration previously termed a double oval. Particle data from the DMSP F13 satellite demonstrates that an isolated plasma sheet fragment formed also in the southern hemisphere, at least for the January 11 event for which correlative data is available.

GOES imagery fills gaps in Montserrat volcanic cloud observations

GOES satellite imagery offers great potential to lessen the risk of volcanic ash clouds to aviation, and the situation at Montserrat in the Caribbean is providing the proof. Many transatlantic, commercial, and private aircraft use airspace around Montserrat, where the Soufriere Hills Volcano has been erupting since 1995.Worldwide over the last 15 years, more than 80 airplanes have reported encountering volcanic ash along flight paths. Encounters cannot be avoided because onboard radar cannot detect fine‐grained ash particles—those with a radius of 15 microns or less. In recent years volcanic cloud encounters are estimated to have caused hundreds of millions of dollars worth of damage and in a few cases have caused in‐flight engine failure [Casadevall et al., 1996].

Hubble Space Telescope Observations of Oxygen-Rich Supernova Remnants in the Magellanic Cloud. I. Narrow-Band Imaging of N132D in the LMC

view Abstract Citations (56) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Observations of Oxygen-Rich Supernova Remnants in the Magellanic Cloud. I. Narrow-Band Imaging of N132D in the LMC Morse, J. A. ; Blair, W. P. ; Dopita, M. A. ; Hughes, J. P. ; Kirshner, R. P. ; Long, K. S. ; Raymond, J. C. ; Sutherland, R. S. ; Winkler, P. F. Abstract We present Hubble Space Telescope (HST WFPC2 images of the young, oxygen- rich supernova remnant N132D in the Large Magellanic Cloud in the [O III]λ5007, [O II]λ3727 and [S II]λλ6724 emission lines, plus a visible continuum band. The ~0.1" resolution of HST allows us to analyze structure within N132D at scales comparable to ground-based images of nearby remnants such as Cas A. The oxygen-rich filaments (seen in [O III] and [O II] emission) are easily distinguished from shocked circumstellar clouds (seen in all three emission lines). Characteristic knots sizes and filament widths are ~0.2" - 0.5" (~1.5-4 x 10^17^ cm), at least partially resolved in our images. For the first time, we discern ionization structure within the shocked interstellar clouds and filaments, reminiscent of filaments in galactic SNRs that are thought to arise from recent shock/interstellar cloud encounters where the full cooling and recombination zone has not yet formed throughout the structure. Conversely, there appears to be fairly uniform ionization in the O-rich filaments. We also observe highly ionized, diffuse emission extending all the way around the outer edges of the remnant, as previously seen in ground-based studies. We compare our HST optical emission-line images of N132D to the ROSAT HRI soft X-ray image and find that the X-ray emission is closely traced optically by shocked circumstellar clouds; the O-rich filaments do not appear to be emitting X-rays. The limb-brightened X-ray shell aligns remarkably precisely along the inner edge of the diffuse outer rim. The diffuse emission probably represents ambient molecular gas that is being dissociated and photoionized by high-energy photons originating in the fast SNR shocks. We have used the MAPPINGS II code to model the X-ray producing main blast wave as an ~800 km s^-1^ shock moving into a circumstellar medium of density N_0_ ~ 3 cm^-3^. The preshock ISM density of N_0_ ~ 3 cm^-3^ inferred from our shock model agrees with the preshock density estimated from the [O III]λ5007 surface brightness in the diffuse outer rim. We also model the [O III]/[S II] emission-line ratios in the photoionized outer rim as a function of distance ahead of the main blast wave using the radiation field produced by fast shocks in the remnant as the ionizing source. According to our models, the ionizing radiation from the ~800 km s^-1^ main blast wave is not sufficient to generate the observed optical emission from the outer rim precursor. An extra source of EUV photons appears necessary to achieve the observed ionization of the preshock gas. We estimate that EUV photons produced in slower shocks moving into QSF-type clouds around the outer rim can probably contribute enough ionizing photons to achieve the observed ionization. Radiative shocks moving through the O-rich filaments may also be an important source of EUV photons. ======================================================================== 1996AJ....112.2350M Publication: The Astronomical Journal Pub Date: August 1996 DOI: 10.1086/118031 Bibcode: 1996AJ....112..509M Keywords: SUPERNOVA REMNANTS full text sources ADS | data products SIMBAD (8) NED (1) MAST (1) HEASARC (1) ESA (1) Related Materials (2) Part 2: 1996AJ....112.2350M Part 3: 2000ApJ...537..667B

Magnetic flux rope versus the Spheromak as models for interplanetary magnetic clouds

Magnetic clouds form a subset of interplanetary ejecta with well‐defined magnetic and thermodynamic properties. Observationally, it is well established that magnetic clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global magnetic field line topology of magnetic clouds: a magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the magnetic structure of magnetic clouds has been modeled by force‐free configurations. In a first step, we therefore analyze the ability of static force‐free models to account for the asymmetries observed in the magnetic field profiles of magnetic clouds. For a cylindrical flux tube the magnetic field remains symmetric about closest approach to the magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the magnetic field signatures along some spacecraft trajectories. The duration of typical magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus magnetic clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model magnetic clouds as self‐similarly evolving MHD configurations. In our theory, the interaction of the expanding magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the magnetic cloud is adequately described by both models, the “edges” of the cloud data are modeled better by the magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real magnetic configurations are never exactly force‐free and gas pressure plays an essential role. We consider a poly tropic gas. Our theoretical analysis shows that the self‐similar expansion of a magnetic flux tube requires the poly tropic index γ to be less than unity. For the spheromak, however, self‐similar, radially expanding solutions are known only for γ equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently (Osherovich et al., 1993a) that the polytropic relationship is applicable to magnetic clouds and that the corresponding polytropic index is ∼0.5. This observational result is consistent with the self‐similar model of the magnetic flux rope but is in conflict with the self‐similar spheromak model.

The evolution of CLO and NO along air parcel trajectories

Back trajectory analysis of Arctic and Antarctic aircraft data reveals that high ClO concentrations are associated with predicted polar stratospheric cloud (PSCs) encounters. The ClO concentrations within the Arctic and Antarctic polar vortices vary widely but appear to be inversely related to parcel solar exposure since the last PSC interaction. These results imply that production of NOx from HNO3 photolysis and reaction with OH is the mechanism, for the loss of chlorine radicals through the reformation of chlorine nitrate. Highly denitrified air parcels show no change in ClO with solar exposure. The recovery process is quantitatively duplicated using a model of chemistry along trajectories. Although PSC processing is the primary mechanism for producing elevated ClO amounts, back trajectories apparently unperturbed by PSC's also show slightly elevated ClO levels in 1992 compared to Arctic 1989 and Antarctic 1987 measurements presumably due to the presence of Pinatubo aerosol.

A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earth's magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP‐8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth‐Sun line. We note the presence of pulsations of ∼5‐hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self‐similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant‐α, force‐free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP‐8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5‐MeV ions while IMP‐8 was inside the cloud. These ions travel as a collimated, field‐aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well‐connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.

Concentrations and size distributions of Antarctic stratospheric aerosols

Particle Measuring Systems laser particle spectrometer (ASAS‐X and FSSP) probes were used to measure aerosol particle concentrations and size distributions during 11 out of 12 ER‐2 flights between Punta Arenas (53°S) and Antarctica (up to 72°S) from August 17 to September 22, 1987. The time resolution was 10 s, corresponding to a spatial resolution of 2 km. The data were divided into two size classes (0.05–0.25 and 0.53–5.5 μm radius) to separate the small particle from the coarse particle populations. Results show that the small particle concentrations are typical for a background aerosol during volcanic quiescence. This concentration is generally constant along a flight track; in only one instance did we measure a depletion of small particles during a polar stratospheric cloud (PSC) encounter, suggesting a nucleation of type I PSC particles on background aerosols. Temporary increases of the coarse particle concentrations indicated the presence of tenuous polar stratospheric clouds that were encountered most frequently at the southernmost portion of a flight track and when the aircraft descended to lower altitudes. During “particle events,” particle modes were found at 0.6 μm radius, corresponding to type I PSCs, and occasionally, a second mode at 2.0 μm radius, corresponding to type II PSCs.

Extra-solar Oort cloud encounters and planetary impact rates

Abstract If other stars possess Oort-like comet clouds, then some such clouds will pass sufficiently close to our Sun to induce an influx of Extra-Solar comets through the planetary region. We investigate this source, and find that while the expected number of planetary impacts due to Extra-Solar Oort comets will dominate impacts caused by free interstellar comets, only a few such comets have impacted the terrestial planets over time.

Cometary impacts, molecular clouds, and the motion of the Sun perpendicular to the galactic plane

Raup and Sepkoski1 have presented evidence from marine fossils for a 26-Myr periodicity in the occurrence of mass extinctions. Using the same data Rampino and Stothers2 obtained a different period, 30±1 Myr, which agrees with the 33±3-Myr half-period for the vertical oscillation of the Solar System about the plane of the galaxy3. To explain this agreement they suggest2 that encounters with molecular clouds perturb the Sun's family of comets, causing many to enter the inner Solar System where one or more collide with the Earth; the cloud encounter rate is modulated at twice the oscillation frequency, because the number density of clouds peaks at the galactic plane at the midpoint of the solar oscillation crossed by the Solar System twice per period. Notwithstanding an apparent objection to this that the most recent extinctions are not in phase with the solar oscillation4, their model, given its stochastic nature, can accommodate a few events with large phase discrepancies. The degree of modulation is crucial: it depends on the scale height of the population of molecular clouds relative to the amplitude of the solar motion and tends to zero if this ratio is large and encounters are entirely random. Here we present data from CO surveys of molecular clouds both within and beyond the solar circle, which permit explicit calculation of the strength of the modulation. The cloud layer near the Sun is too extended and, as a consequence, the modulation of cloud encounters is too weak for a statistically significant period to be extracted from the nine extinctions analysed by Rampino and Stothers.

GASP cloud encounter statistics - Implications for laminar flow control flight

The cloud observation archive from the NASA Global Atmospheric Sampling Program (GASP) is analyzed in order to derive the probability of cloud encounter at altitudes normally flown by commercial airliners, for application to a determination of the feasability of Laminar Flow Control (LFC) on long-range routes. The probability of cloud encounter is found to vary significantly with season. Several meteorological circulation features are apparent in the latitudinal distribution of cloud cover. The cloud encounter data are shown to be consistent with the classical midlatitude cyclone model with more clouds encountered in highs than in lows. Aircraft measurements of route-averaged time-in-clouds fit a gamma probability distribution model which is applied to estimate the probability of extended cloud encounter, and the associated loss of LFC effectiveness along seven high-density routes. The probability is demonstrated to be low.

Cloud Encounter and Particle Number Density Variabilities from GASP Data

Summary statistics and variability studies are presented for cloud encounter and particle number density data as part of the NASA Global Atmospheric Sampling Program (GASP) aboard commercial Boeing 747 airliners. On the average, cloud encounter is shown on about 15% of the 52,164 data samples available; this value varies with season, latitude, synoptic weather situation, and distance from the tropopause. The number density of particles (diameter greater than 3 microns) also varies with time and location, and depends on the horizontal extent of cloudiness.

Interstellar Cloud Material: Contribution to Planetary Atmospheres

A statistical analysis of the properties of dense interstellar clouds indicates that the solar system has encountered at least a dozen clouds of sufficient density to cause planets to accumulate nonnegligible amounts of some isotopes. The effect is most pronounced for neon. This mechanism could be responsible for much of the neon in Earth's atmosphere. For Mars, the predicted amount of neon added by cloud encounters greatly exceeds the present abundance.

The escape of stars from clusters III. Expansion versus contraction in the evolution of a star cluster

view Abstract Citations (23) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The escape of stars from clusters III. Expansion versus contraction in the evolution of a star cluster King, Ivan Abstract . A loose cluster expands as a result of tidal encounters with interstellar clouds, while the ejection of stars causes a dense cluster to contract. The evolution of a cluster of intermediate density can be studied by means of differ- ential equations that follow from the virial theorem and the conservation of energy. The solutions show that the balance between expansive and contractive tendencies is extremely unstable. In practice it is satisfactory to consider a critical radius to divide expanding clusters, to which the pure cloud-encounter theory applies, from contracting clusters, to which the pure ejection theory applies. The critical radius corresponds to the longest possible lifetime for a given number of stars. Present estimates put the critical radius, in the galactic plane, at I 21 pc, with a maximum lifetime of less than I0 years for 100 stars, and 3 or 4 >c I0 years for 1000 stars. A cluster that moves 500 pc above and below the galactic plane has its critical radius doubled and its maximum lifetime tripled. Encounters with field stars also add energy to a cluster, but their effect is always small compared with that of ejection, cloud encounters, or tidal forces. The evolu- tionary picture given here must give the observed distribution of cluster radii a strong bias in favor of values near the critical radius. Publication: The Astronomical Journal Pub Date: December 1958 DOI: 10.1086/107811 Bibcode: 1958AJ.....63..465K full text sources ADS | Related Materials (4) Part 1: 1958AJ.....63..109K Part 2: 1958AJ.....63..114K Part 4: 1959AJ.....64..351K Part 5: 1960AJ.....65..122K