Zonal jets in rotating convection with mixed mechanical boundary conditions     

Jonathan M. Aurnou  Moritz H. Heimpel 《Icarus》2004,169(2):492-498

Large-scale zonal flows, as observed on the giant planets, can be driven by thermal convection in a rapidly rotating spherical shell. Most previous models of convectively-driven zonal flow generation have utilized stress-free mechanical boundary conditions (FBC) for both the inner and the outer surfaces of the convecting layer. Here, using 3D numerical models, we compare the FBC case to the case with a stress free outer boundary and a non-slip inner boundary, which we call the mixed case (MBC). We find significant differences in surface zonal flow profiles produced by the two cases. In low to moderate Rayleigh number FBC cases, the main equatorial jet is flanked by a strong, high-latitude retrograde jets in the northern and southern hemispheres. For the highest Rayleigh number FBC case, the equatorial jet is flanked by strong reversed jets as well as two additional large-scale alternating jets at higher latitudes. The MBC cases feature stronger equatorial jets but, much weaker, small-scale alternating zonal flows are found at higher latitudes. Our high Rayleigh number FBC results best compare with the zonal flow pattern observed on Jupiter, where the equatorial jet is flanked by strong retrograde jets as well as small-scale alternating jets at high latitude. In contrast, the MBC results compare better with the observed flow pattern on Saturn, which is characterized by a dominant prograde equatorial jet and a lack of strong high latitude retrograde flow. This may suggest that the mechanical coupling at the base of the jovian convection zone differs from that on Saturn.  相似文献   

SDO Observations of Solar Jets     

S. P. Moschou  K. Tsinganos  A. Vourlidas  V. Archontis 《Solar physics》2013,284(2):427-438

We present an analysis of high cadence observations of solar jets observed in the Extreme Ultraviolet (EUV), at 304 Å, with the Atmospheric Imaging Assembly instrument aboard the Solar Dynamics Observatory (SDO). The jets in our sample lie very close to the solar limb to minimize projection effects. Two of the events show clear helical patterns during ejection. We also find that some of the jets are recurrent and that most of them cannot overcome solar gravity. We investigate the temporal evolution of the jets by measuring the height of their leading edge as a function of time. By fitting the resulting height–time diagrams, we derive the magnitude of their initial ejection speed and plasma acceleration by assuming ballistic motion. Moreover, we calculate the upward acceleration of the jets based on the dynamical velocity of the plasma, without assuming a ballistic motion. In both models, the acceleration profiles suggest the influence of forces other than gravity. In particular, we find indications of an upwards driving force which weakens the decelerating effect of the solar gravitational field along the motion of the jet. This force is larger in the dynamical model, which indicates that the ballistic approximation does not properly determine the rising motion of the plasma jets.  相似文献   

X-ray Jets in Interconnecting Loops     

Fárník  František  Švestka  Zdeněk 《Solar physics》2002,206(1):143-154

We present examples of X-ray jets, observed by the Soft X-ray Telescope on board Yohkoh, which followed trajectories of transequatorial interconnecting loops (TILs). All these TILs were preexisting, seen some time before, but were mostly invisible at the time of the onset of the jet which often made them bright along their total length. With few exceptions, these TIL-associated jets have properties very similar to other jets ejected inside active regions or along open field lines (footpoints in X-ray bright points, recurrence, strong collimation, average speed close to 350 km s⁻¹), but may reach larger lengths, in our examples up to 450 000 km. Exceptions are one jet that moved slower and one that had no brightened area at its supposed source region at the time of its origin (an X-ray bright point appeared there only 3 hours later). It appears that quite a high number of X-ray jets may be of this TIL-associated kind. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014963812437  相似文献   

Solar wind energy transfer regions inside the dayside magnetopause—I. Evidence for magnetosheath plasma penetration     

R. Lundin  E. Dubinin 《Planetary and Space Science》1984,32(6):745-755

PROGNOZ-7 high temporal resolution measurements of the ion composition and hot plasma distribution in the dayside high latitude boundary layer near noon have revealed that magnetosheath plasma may penetrate the dayside magnetopause and form high density, high β, magnetosheath-like regions inside the magnetopause. We will from these measurements demonstrate that the magnetosheath injection regions most probably play an important role in transferring solar wind energy into the magnetosphere. The transfer regions are characterized by a strong perpendicular flow towards dawn or dusk (depending on local time) but are also observed to expand rapidly along the boundary layer field lines. This increased flow component transverse to the local magnetic field corresponds to a predominantly radial electric field of up to several mV m^?1, which indicates that the injected magnetosheath plasma causes an enhanced polarization of the boundary layer. Polarization of the boundary layer can therefore be considered a result of a local MHD-process where magnetosheath plasma excess momentum is converted into electromagnetic energy (electric field), i.e. we have primarily an MHD-generator there. We state primarily because we also observe acceleration of “cold” ions inside the magnetopause as a result of this radial electric field. A few cases of polarity reversals suggest that the polarization is sometimes quite localized.The perhaps most significant finding is that the boundary layer is observed to be charged up to tens of kilovolts, a potential which may be highly variable depending on e.g. the presence of a momentum exchange by the energy transfer regions.  相似文献   

Turbulent convection in rapidly rotating spherical shells: A model for equatorial and high latitude jets on Jupiter and Saturn     

Moritz Heimpel  Jonathan Aurnou 《Icarus》2007,187(2):540-557

The origin of zonal jets on the jovian planets has long been a topic of scientific debate. In this paper we show that deep convection in a spherical shell can generate zonal flow comparable to that observed on Jupiter and Saturn, including a broad prograde equatorial jet and multiple alternating jets at higher latitudes. We present fully turbulent, 3D spherical numerical simulations of rapidly rotating convection with different spherical shell geometries. The resulting global flow fields tend to be segregated into three regions (north, equatorial, and south), bounded by the tangent cylinder that circumscribes the inner boundary equator. In all of our simulations a strong prograde equatorial jet forms outside the tangent cylinder, whereas multiple jets form in the northern and southern hemispheres, inside the tangent cylinder. The jet scaling of our numerical models and of Jupiter and Saturn is consistent with the theory of geostrophic turbulence, which we extend to include the effect of spherical shell geometry. Zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β-parameter and a jump in the Rhines length. This jump is manifest in the numerical simulations as a sharp equatorward increase in jet widths—a transition that is also observed on Jupiter and Saturn. The location of this transition gives an estimate of the depth of zonal flow, which seems to be consistent with current models of the jovian and saturnian interiors.  相似文献   

On the local magnetorotational instability of astrophysical jets     

M. Hanasz  H. Sol  C. Sauty 《Monthly notices of the Royal Astronomical Society》2000,316(3):494-506

We present the local linear stability analysis of rotating jets confined by a toroidal magnetic field. Under the thin flux tube approximation, we derive the equation of motion for slender magnetic flux tubes. In addition to the terms responsible for the conventional instability of the toroidal magnetic field, a term related to the magnetic buoyancy and a term corresponding to the differential rotation become relevant for the stability properties. We find that the rigid rotation stabilizes while the differential rotational destabilizes the jet in a way similar to the Balbus–Hawley instability. Within the frame of our local analysis, we find that if the azimuthal velocity is of the order of or higher than the Alfvén azimuthal speed, the rigidly rotating part of the jet interior can be completely stabilized, while the strong shearing instability operates in the transition layer between the rotating jet interior and the external medium. This can explain the limb-brightening effect observed in several jets. However, it is still possible to find jet equilibria that are stable all across the jet, even in the presence of differential rotation. We discuss observational consequences of these results.  相似文献   

HEOS-2 observations of the boundary layer from the magnetopause to the ionosphere     

V. Formisano 《Planetary and Space Science》1980,28(3):245-257

HEOS-2 low energy electron data (10 eV–3.7 keV) from the LPS Frascati plasma experiment have been used to identify three different magnetospheric electron populations. Magnetosheathlike electron energy spectra (35–50 eV) are characteristic of the plasma mantle, entry layer and cusps from the magnetopause down to 2–3 R_E Plasma sheet electrons (energy > 1 keV) are found at all local times, with strong intensities in the early morning quadrant and weaker intensities in the afternoon quadrant. The plasma sheet shows a well defined inner edge at all local times and latitudes, the inner edge coinciding probably with the plasmapause. The plasma sheet does not reach the magnetopause, but it is separated from it by a boundary layer electron population that is very distinct from the other two electron populations, most electrons having energies 100–300 eV.We map these three electron populations from the magnetopause down to the high latitude near earth regions, by making use of the HEOS-2 low latitude inbound passes and the high latitude outbound passes (in Solar Magnetic (SM) coordinates). The boundary layer extends along the magnetopause up to 5–7 R_E above the equator; at higher latitudes it follows the magnetic lines of force and it is found closer and closer to the earth, so that it has the same invariant latitudes of the system 1 currents observed by Iijima and Potemra (1976) in their region 1. The plasma sheet can be mapped into their region 2 and the cusp-entry layer-plasma mantle can be mapped into their cusp currents region. The boundary layer is observed for any Interplanetary Magnetic Field (IMF) direction. We speculate that magnetosheath particles penetrate into the magnetosphere everywhere along the magnetopause. The electron energization, however, is observed only in the boundary layer, on both dawn and dusk side and could be due to the polarization electric field at magnetopause generated by the magnetosheath plasma bulk motion in the region where such motion is roughly perpendicular to the magnetospheric magnetic field. The electron energization is absent in the regions (entry layer and plasma mantle) where the sheath plasma motion is roughly parallel or antiparallel to the magnetospheric magnetic field.  相似文献   

Active galactic nuclei jet mass loading and truncation by stellar winds     

A. Hubbard  E. G. Blackman 《Monthly notices of the Royal Astronomical Society》2006,371(4):1717-1721

Active galactic nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centres implies that there will be many jet/star interactions, which can mass load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass loading by stellar winds for a broader spectrum of wind mass-loss rates than has previously been considered. Given the observed stellar mass distributions in galactic centres, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses M _BH≲ 10⁴ M_⊙ are rapidly mass loaded and quenched by stellar winds. For  10⁴ M_⊙ < M _BH < 10⁸ M_⊙  , the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon-dominated composition on scales ≳2 kpc therein even if the jet is initially pair plasma dominated.  相似文献   

Radio circular polarization produced in helical magnetic fields in eight active galactic nuclei     

D. C. Gabuzda  V. M. Vitrishchak  M. Mahmud  S. P. O'Sullivan 《Monthly notices of the Royal Astronomical Society》2008,384(3):1003-1014

Homan & Lister have recently published circular polarization (CP) detections for 34 objects in the MOJAVE sample – a set of bright, compact active galactic nuclei (AGN) being monitored by the Very Long Baseline Array at 15 GHz. We report the detection of 15-GHz parsec-scale CP in two more AGN (3C 345 and 2231+114), and confirm the MOJAVE detection of CP in 1633+382. It is generally believed that the most likely mechanism for the generation of this CP is Faraday conversion of linear polarization (LP) to CP. A helical jet magnetic field ( B field) geometry can facilitate this process – linearly polarized emission from the far side of the jet is converted to CP as it passes through the magnetized plasma at the front side of the jet on its way towards the observer. In this case, the sign of the generated CP is essentially determined by the pitch angle and helicity of the helical B field. We have determined the pitch-angle regimes and helicities of the helical jet B fields in eight AGN for which parsec-scale CP has been detected, and used them to predict the expected CP signs for these AGN if the CP is generated via conversion in these helical fields. We have obtained the intriguing result that our predictions agree with the observed signs in all eight cases, provided that the longitudinal B field components in the jets correspond to south magnetic poles. This clearly non-random pattern demonstrates that the observed CP in AGN is directly associated with the presence of helical jet B fields. These results suggest that helical B fields are ubiquitous in AGN jets.  相似文献   

Characteristics of EUV Coronal Jets Observed with STEREO/SECCHI     

G. Nisticò  V. Bothmer  S. Patsourakos  G. Zimbardo 《Solar physics》2009,259(1-2):87-108

In this paper we present the first comprehensive statistical study of EUV coronal jets observed with the SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) imaging suites of the two STEREO spacecraft. A catalogue of 79 polar jets is presented, identified from simultaneous EUV and white-light coronagraph observations, taken during the time period March 2007 to April 2008, when solar activity was at a minimum. The twin spacecraft angular separation increased during this time interval from 2 to 48 degrees. The appearances of the coronal jets were always correlated with underlying small-scale chromospheric bright points. A basic characterization of the morphology and identification of the presence of helical structure were established with respect to recently proposed models for their origin and temporal evolution. Though each jet appeared morphologically similar in the coronagraph field of view, in the sense of a narrow collimated outward flow of matter, at the source region in the low corona the jet showed different characteristics, which may correspond to different magnetic structures. A classification of the events with respect to previous jet studies shows that amongst the 79 events there were 37 Eiffel tower-type jet events, commonly interpreted as a small-scale (～35 arc?sec) magnetic bipole reconnecting with the ambient unipolar open coronal magnetic fields at its loop tops, and 12 lambda-type jet events commonly interpreted as reconnection with the ambient field happening at the bipole footpoints. Five events were termed micro-CME-type jet events because they resembled the classical coronal mass ejections (CMEs) but on much smaller scales. The remaining 25 cases could not be uniquely classified. Thirty-one of the total number of events exhibited a helical magnetic field structure, indicative for a torsional motion of the jet around its axis of propagation. A few jets are also found in equatorial coronal holes. In this study we present sample events for each of the jet types using both, STEREO A and STEREO B, perspectives. The typical lifetimes in the SECCHI/EUVI (Extreme UltraViolet Imager) field of view between 1.0 to 1.7 R _⊙ and in SECCHI/COR1 field of view between 1.4 to 4 R _⊙ are obtained, and the derived speeds are roughly estimated. In summary, the observations support the assumption of continuous small-scale reconnection as an intrinsic feature of the solar corona, with its role for the heating of the corona, particle acceleration, structuring and acceleration of the solar wind remaining to be explored in more detail in further studies.  相似文献   

The radio jets in B2 0755+379     

M. Bondi  P. Parma  H. de Ruiter  R. Fanti  R. A. Laing  E. B. Fomalont 《Monthly notices of the Royal Astronomical Society》2000,314(1):11-22

We present and discuss observations of the radio galaxy 0755+379 made with the VLA at 1.4 and 5.0 GHz and with MERLIN at 1.7 GHz. These data allow us to image the radio jets over two orders of magnitude in linear size and to investigate the hypothesis that jets in low-luminosity radio galaxies start with velocities close to c and then slow down to subrelativistic speeds. We apply a model for an adiabatically expanding relativistic jet to the observed surface brightness and derive velocity profiles along the jet for various assumed starting conditions. We show that these profiles are consistent with the observed jet/counter-jet brightness ratios provided that the angle to the line of sight θ ≃27°. The inferred velocity at a distance of 0.5 kpc from the nucleus is ≃0.9 c . Finally, we show that the predicted velocity at 10 kpc from the nucleus is consistent with that obtained independently from energy-balance arguments.  相似文献   

Observations of molecular jets in Orion A     

Thomas Stanke 《Astrophysics and Space Science》2003,287(1-4):149-160

We report on the results of a wide field near infrared survey for protostellar jets identified via their emission in the 2.12μm line of shock heated molecular hydrogen, done over a 1.2 square degree area in Orion A. We derive an evolutionary sequence for protostellar jets, based on the observed lenghts and H₂ luminosities as well as the evolutionary stage and bolometric luminosity of their driving sources. Protostellar jets start from zero length, evolve quickly to parsec scale extents during the Class 0 phase, and shrink during the Class I and Class II phase. They are first very bright in H₂ emission, and fade later on. This is indicative of strongly time-variably mass accretion onto the driving protostar, with a peak early on, and a subsequent continous decay of accretion activity. Finally, we present evidence for a molecular CO jet from a Class 0 object, supporting the idea that a very efficient outflow phase at very early evolutionary stages should produce very dense, molecular jets.  相似文献   

What Drives Molecular Outflows from Young Stars?     

T.P. Ray 《Astrophysics and Space Science》2000,272(1-3):115-125

After briefly reviewing observations of molecular outflows from young stars, we discuss current ideas as to how they might be accelerated. Broadly speaking it is thought that such outflows represented either deflected accreted gas, or ambient material that has been pushed by a poorly collimated wind or accelerated by a highly collimated jet. Observations tend to favour the latter model, with jets being the clear favourite at least for the youngest flows. Jets from young stars may accelerate ambient gas either through the development of a boundary layer, where ambient and jet material are turbulently mixed, or at the working surface of the jet, i.e. the bow shock, via the prompt entrainment mechanism. Recently, we (Downes and Ray, 1999) have investigated, through simulations, the efficiency of prompt entrainment in jets from young stars as a means of accelerating ambient molecular gas without causing dissociation. Prompt entrainment was found to be very poor at transferring momentum from the jet to its surroundings in both the case of ``heavy' (not surprizingly) but also ``equi-density' (with respect to the ambient environment) jets. Moreover the transfer efficiency decreases with increasing density as the bow shock takes on a more aerodynamic shape. Models, however, in which jets are the ultimate prime movers, do have the advantage that they can reproduce several observational features of molecular outflows. In particular a power law relationship for mass versus velocity, similar to what is observed, is predicted by the simulations and the so-called ``Hubble Law' for molecular outflows is naturally explained. Pulsing of the jet, i.e. varying its velocity, is found to have little effect on the momentum transfer efficiency at least for the dynamically young jets we have studied. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Oscillations in solar jets observed with the SOT of Hinode: viscous effects during reconnection     

E. Tavabi  S. Koutchmy 《Astrophysics and Space Science》2014,352(1):7-15

Transverse oscillatory motions and recurrence behavior in the chromospheric jets observed by Hinode/SOT are studied. A comparison is considered with the behavior that was noticed in coronal X-ray jets observed by Hinode/XRT. A jet like bundle observed at the limb in Ca II H line appears to show a magnetic topology that is similar to X-ray jets (i.e., the Eiffel tower shape). The appearance of such magnetic topology is usually assumed to be caused by magnetic reconnection near a null point. Transverse motions of the jet axis are recorded but no clear evidence of twist is appearing from the highly processed movie. The aim is to investigate the dynamical behavior of an incompressible magnetic X-point occurring during the magnetic reconnection in the jet formation region. The viscous effect is specially considered in the closed line-tied magnetic X-shape nulls. We perform the MHD numerical simulation in 2-D by solving the visco-resistive MHD equations with the tracing of velocity and magnetic field. A qualitative agreement with Hinode observations is found for the oscillatory and non-oscillatory behaviors of the observed solar jets in both the chromosphere and the corona. Our results suggest that the viscous effect contributes to the excitation of the magnetic reconnection by generating oscillations that we observed at least inside this Ca II H line cool solar jet bundle.  相似文献   

Scaling laws for convection and jet speeds in the giant planets     

Adam P. Showman  Yohai Kaspi 《Icarus》2011,211(2):1258-1273

Three dimensional studies of convection in deep spherical shells have been used to test the hypothesis that the strong jet streams on Jupiter, Saturn, Uranus, and Neptune result from convection throughout the molecular envelopes. Due to computational limitations, these simulations must be performed at parameter settings far from jovian values and generally adopt heat fluxes 5-10 orders of magnitude larger than the planetary values. Several numerical investigations have identified trends for how the mean jet speed varies with heat flux and viscosity in these models, but no previous theories have been advanced to explain these trends. Here, we show using simple arguments that if convective release of potential energy pumps the jets and viscosity damps them, the mean jet speeds split into two regimes. When the convection is weakly nonlinear, the equilibrated jet speeds should scale approximately with F/ν, where F is the convective heat flux and ν is the viscosity. When the convection is strongly nonlinear, the jet speeds are faster and should scale approximately as (F/ν)^1/2. We demonstrate how this regime shift can naturally result from a shift in the behavior of the jet-pumping efficiency with heat flux and viscosity. Moreover, both Boussinesq and anelastic simulations hint at the existence of a third regime where, at sufficiently high heat fluxes or sufficiently small viscosities, the jet speed becomes independent of the viscosity. We show based on mixing-length estimates that if such a regime exists, mean jet speeds should scale as heat flux to the 1/4 power. Our scalings provide a good match to the mean jet speeds obtained in previous Boussinesq and anelastic, three-dimensional simulations of convection within giant planets over a broad range of parameters. When extrapolated to the real heat fluxes, these scalings suggest that the mass-weighted jet speeds in the molecular envelopes of the giant planets are much weaker—by an order of magnitude or more—than the speeds measured at cloud level.  相似文献   

赫比格-哈罗天体高分辨观测研究进展     

王红池 《天文学进展》2000,18(3):216-228

赫比格－哈罗天体（ＨＨ天体）包含了有关原恒星吸积和抛射过程的许多重要信息,ＨＨ天体高分辨观测研究取得了一系列新进展：分辨出激波峰面、马赫盘和辐射冷却区;分辨出喷流节点的结构,发现它们大多是内工作面,而不是由Ｋｅｌｖｉｎ－Ｈｅｌｍｈｏｌｔｚ不稳定性所产生的斜激波;发现喷流宽度随到激发源距离的减小仅缓慢减小,对喷流的准直和加速模型提供了限制条件;ＨＨ天体在小尺度上尚有复杂的激发结构。对这些进展进行了评  相似文献   

On the absence of jet features in the optical spectra of cataclysmic variables     

Christian Knigge  & Mario Livio 《Monthly notices of the Royal Astronomical Society》1998,297(4):1079-1086

Blue- and redshifted hydrogen and helium satellite recombination lines have recently been discovered in the optical spectra of at least two supersoft X-ray sources (SSSs), RX J0513−069 and RX J0019.8+2156, and, tentatively, also in one short-period cataclysmic variable star (CV), the recurrent nova T Pyx. These features are thought to provide evidence for the presence of highly collimated jets in these systems. No similar spectral signatures have been detected in the spectra of other short-period CVs, despite a wealth of existing optical data on these systems. Here, we ask if this apparent absence of 'jet lines' in the spectra of most CVs already implies the absence of jets of the kind that appear to be present in the SSSs and perhaps T Pyx, or whether the current lack of jet detection in CVs can still be ascribed to observational difficulties.
To answer this question, we derive a simple, approximate scaling relation between the expected equivalent widths (EWs) of the observed jet lines in both types of systems and the accretion rate through the disc, EW(line)∝˙M^4/3_acc. We use this relation to predict the strength of jet lines in the spectra of 'ordinary' CVs, i.e. systems characterized by somewhat lower accretion rates than T Pyx. Making the assumption that the features seen in T Pyx are indeed jet lines, and using this system as a reference point, we find that, if jets are present in many CVs, they may be expected to produce optical satellite recombination lines with EWs of a few hundredths to a few tenths of an angstrom in suitably selected systems. A similar prediction is obtained if the SSS RX J0513−069 is used as a reference point. Such EWs are small enough to account for the non-detection of jet features in CVs to date, but large enough to allow them to be detected in data of sufficiently high quality, if they exist.  相似文献   

Hydrodynamic simulations of rotating molecular jets     

Michael D. Smith  Alexander Rosen 《Monthly notices of the Royal Astronomical Society》2007,378(2):691-700

Molecular outflows and the jets which may drive them can be expected to display signatures associated with rotation if they are the channels through which angular momentum is extracted from material accreting on to protostars. Here, we determine some basic signatures of rapidly rotating flows through three-dimensional numerical simulations of hydrodynamic jets with molecular cooling and chemistry. We find that these rotating jets generate a broad advancing interface which is unstable and develops into a large swarm of small bow features. In comparison to precessing jets, there is no stagnation point along the axis. The greater the rotation rate, the greater the instability. On the other hand, velocity signatures are only significant close to the jet inlet since jet expansion rapidly reduces the rotation speed. We present predictions for atomic, H₂ and CO submillimetre images and spectroscopy including velocity channel maps and position–velocity diagrams. We also include simulated images corresponding to Spitzer IRAC band images and CO emission, relevant for APEX and eventual ALMA observations. We conclude that protostellar jets often show signs of slow precession but only a few sources display properties which could indicate jet rotation.  相似文献   

Jet–cloud collisions in the jet of the Seyfert galaxy NGC 3079     

E. Middelberg  I. Agudo  A. L. Roy  T. P. Krichbaum 《Monthly notices of the Royal Astronomical Society》2007,377(2):731-740

We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN.  相似文献   

Effect of expansion and magnetic field configuration on mass entrainment of jets     

《New Astronomy Reviews》2002,46(2-7):433-437

We investigate the growth of jet plus entrained mass in simulations of supermagnetosonic cylindrical and expanding jets. The entrained mass spatially grows in three stages: from an initially slow spatial rate to a faster rate and finally at a flatter rate. These stages roughly coincide with the similar rates of expansion in simulated radio intensity maps, and also appear related to the growth of the Kelvin–Helmholtz instability through linear, nonlinear, and saturated regimes. In the supermagnetosonic cylindrical jets, we found that a jet with an embedded primarily toroidal magnetic field is more stable than a jet with a primarily axial magnetic field. Also, pressure-matched expanding jets are more stable and entrain less mass than cylindrical jets with equivalent inlet conditions. We investigate the growth of jet plus entrained mass in simulations of supermagnetosonic cylindrical and expanding jets. The entrained mass spatially grows in three stages: from an initially slow spatial rate to a faster rate and finally at a flatter rate. These stages roughly coincide with the similar rates of expansion in simulated radio intensity maps, and also appear related to the growth of the Kelvin–Helmholtz instability through linear, nonlinear, and saturated regimes. In the supermagnetosonic cylindrical jets, we found that a jet with an embedded primarily toroidal magnetic field is more stable than a jet with a primarily axial magnetic field. Also, pressure-matched expanding jets are more stable and entrain less mass than cylindrical jets with equivalent inlet conditions.  相似文献