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1.
P. F. Browne 《Astrophysics and Space Science》1995,233(1-2):329-336
At an early stage in the lives of stars and galaxies when they are surrounded by discs, vorticity in the disc concentrates into a central vortex, thus converting a Keplerian velocity fieldu
ø r
–1/2 into an irrotational velocity fieldu
ør
–1, which implies inward transfer of angular momentum. Centrifugal forces due to spin-up of the inner region and gravity dominant in the outer region then squeeze gas at intermediate layers, increasing pressure gradient in the axial direction sufficiently to drive a wide-angle low-velocity bipolar outflow from the disc. A logarithmic singularity of vorticity at the axis implies strong centrifugal forces which expand plasma to radiusR where pressure gradient balances centrifugal force density of ions; the much weaker centrifugal force density of electrons cannot balance pressure gradient, so that electrons are driven inwards relative to ions until charge separation limits the relative displacement. Now the radial gradient ofu
øcauses ions to rotate at a different rate to electrons, generating an azimuthal current densityj
øwhich is the source of an axial magnetic fieldB
zin the core of the central vortex. Centrifuging carries lines of B to the core wall, where they are wound into helical force-free configuration with B j. An annular channel of radiusR and thickness R into which parallel helical lines ofj andB are compressed constitutes a magnetic vortex tube (MVT). An MVT separates an inner high-velocity highly collimated outflow from the outer low-velocity wide-angle outflow, and is responsible for jets. Magnetic pinches in the MVT may constrict the core flow at HH objects. 相似文献
2.
Valentí Bosch-Ramon 《Astrophysics and Space Science》2007,309(1-4):321-331
Microquasar (MQ) jets are sites of particle acceleration and synchrotron emission. Such synchrotron radiation has been detected
coming from jet regions of different spatial scales, which for the instruments at work nowadays appear as compact radio cores,
slightly resolvedradio jets, or (very) extended structures (e.g. Mirabel and Rodríguez, 1999; Fender, 2001; Corbel et al., 2002). Because of the presence of relativistic particles and dense photon, magnetic and matter fields, these outflows are also
the best candidates to generate the very high-energy (VHE) gamma-rays detected coming from two of these objects, LS 5039 and
LS I +61 303 (Aharonian, 2005; Aharonian et al., 2006a; and Albert, 2006, respectively), and may be contributing significantly to the X-rays emitted from the MQ core (e.g. Markoff et al., 2001; Bosch-Ramon et al., 2005a). In addition, beside electromagnetic radiation, jets at different scales are producing some amount of leptonic and hadronic
cosmic rays (CR), and evidences of neutrino production in these objects may be eventually found. In this work, we review on
the different physical processes that may be at work in or related to MQ jets. The jet regions capable to produce significant
amounts of emission at different wavelengths have been reduced to the jet base, the jet at scales of the order of the size
of the system orbital semi-major axis, the jet middle scales (the resolved radio jets), and the jet termination point. The
surroundings of the jet could be sites of multiwavelength emission as well, deserving also an insight. We focus on those scenarios,
either hadronic or leptonic, in which it seems more plausible to generate both photons from radio to VHE and high-energy neutrinos.
We briefly comment as well on the relevance of MQ as possible contributors to the galactic CR in the GeV–PeV range. 相似文献
3.
Ph. Nicolaï V. T. Tikhonchuk A. Kasperczuk T. Pisarczyk S. Borodziuk K. Rohlena J. Ullschmied 《Astrophysics and Space Science》2007,307(1-3):87-91
Under suitable conditions on laser intensity, focal spot radius and atomic number a radiative jet was launched from a planar
target. This jet was produced using a relatively low energy laser pulse, below 500 J and it presents similarities with astrophysical
protostellar jets. It lasts more than 10 ns, extends over several millimeters, has velocity more than 500 km/s, the Mach number
more than 10 and the density above 1018 cm−3. The mechanism of jet formation was inferred from the dimensional analysis and hydrodynamic two-dimensional simulations.
It is related to the radiative cooling while the magnetic fields play a minor role.
PACS numbers: 98.38.Fs, 52.50.Jm, 95.30.Qd 相似文献
4.
5.
We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward-tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward-tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein. 相似文献
6.
The dynamics of solitary Rossby waves (SRWs) embedded in a meridionally sheared, zonally varying background flow are examined using a non-divergent barotropic model centered on a midlatitude g -plane. The zonally varying background flow, which is produced by an external potential vorticity (PV) forcing, yields a modified Korteweg-de Vries (K-dV) equation that governs the spatial-temporal evolution of a disturbance field that contains both Rossby wave packets and SRWs. The modified K-dV equation differs from the classical equation in that the zonally varying background flow, which varies on the same scale as the disturbance field, directly affects the disturbance linear translation speed and linear growth characteristics. In the limit of a locally parallel background flow, equations governing the amplitude and propagation characteristics of SRWs are derived analytically. These equations show, for example, that a sufficiently large (small) translation speed and/or a sufficiently weak (strong) background zonal shear favor transmission (reflection) of the SRW through (from) the jet. Conservation equations are derived showing that time changes in the domain averaged amplitude ("mass") or squared amplitude ("momentum") are due to zonal variation in both the linear, long-wave phase speed and linear growth; dispersion and nonlinearity do not affect the "mass" or "momentum". Provided (1) the background PV forcing is sufficiently small, or (2) the background PV forcing is meridionally symmetric and the disturbance is a SRW, the dynamics of the disturbance field is Hamiltonian and mass and energy are thus conserved. Numerical solutions of the K-dV equation show that the zonally varying background flow yields three general classes of behavior: reflection, transmission, or trapping. Within each class there exists SRWs and Rossby wave packets. SRWs that become trapped within the zonally localized jet region may exhibit the following behaviors: (1) an oscillatory decay to a steady state at the jet center, (2) the creation of additional SRWs within the jet region, or (3) a steady-state wherein the solution has a smoothed step-like structure located downstream along the jet axis. 相似文献
7.
M. Birkinshaw 《Astrophysics and Space Science》1996,242(1-2):17-91
The observed properties of astrophysical jets are reviewed, and the techniques used to estimate the parameters of the underlying beams are described. This information is then used in a theoretical treatement of the Kelvin-Helmholtz instability of the flows, and the relevance of this instability to the persistence of the observed structures is emphasised. 相似文献
8.
Colin R. Masson 《Astrophysics and Space Science》1995,224(1-2):99-108
In the 3 decades since winds from young stars were discovered, there have been many observations of bipolar molecular flows and ionized jets, and it has been recognized that outflows are intimately linked to star formation. Despite many observational clues and theoretical ideas, we still do not have a fully coherent picture of the outflow process. 相似文献
9.
This study presents an assessment of the TropFlux and the National Centers for Environmental Prediction (NCEP) reanalysis air-sea fluxes in simulating the surface and subsurface oceanic parameters over the Bay of Bengal (BoB) region during 2002–2014 using the Regional Ocean Modelling System (ROMS). The assessment has been made by comparing the simulated fields with in-situ and satellite observations. The simulated surface and subsurface temperatures in the TropFlux forced experiment (TropFlux-E) show better agreement with the Research Moored Array for African-Asian-Australian Monsoon Analysis (RAMA) and Argo observations than the NCEP forced experiment (NCEP-E). The BoB domain averaged sea surface temperature (SST) simulated in the NCEP-E is consistently cooler than the satellite SST, with a root mean square error (RMSE) of 0.79 °C. Moreover, NCEP-E shows a limitation in simulating the observed seasonal cycle of the SST due to substantial underestimation of the pre-monsoon SST peak. These limitations are mostly due to the lower values of the NCEP net heat flux. The seasonal and interannual variations of SST in the TropFlux-E are better comparable to the observations with correlations and skills more than 0.80 and 0.90 respectively. However, SST is overestimated during summer monsoon periods mainly due to higher net heat flux. The superiority of TropFlux forcing over the NCEP reanalysis can also be seen when simulating the interannual variabilities of the magnitude and vertical extent of Wyrtki jets at two equatorial RAMA buoy locations. The jet is weaker in the NCEP-E relative to the TropFlux-E and observations. The simulated sea surface height anomalies (SSHA) from both the experiments are able to capture the regions of positive and negative SSHA with respect to satellite-derived altimeter data with better performance in the TropFlux-E. The speed of the westward propagating Rossby wave along 18°N in the TropFlux-E is found to be about 4.7 cm/s, which is close to the theoretical phase speed of Rossby waves. 相似文献
10.
A strong spring Wyrtki jet(WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastwardpropagating Madden–Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean. 相似文献