The Creation of Outflowing Plasma in the Corona at Emerging Flux Regions: Comparing Observations and Simulations

In this paper we analyse the flux emergence that occurred in the following polarity area of an active region on 1 – 2 December 2006. Observations have revealed the existence of fast outflows at the edge of the emerging flux region. We have performed 3-D numerical simulations to study the mechanisms responsible for these flows. The results indicate that these outflows are reconnection jets or pressure-driven outflows, depending on the relative orientation of the magnetic fields in contact (i.e. the emerging flux and the active region’s field which is favourable for reconnection on the west side and nearly parallel with the pre-existing field on the east side of the emerging flux). In the observations, the flows are larger on the west side until late in the flux emergence, when the reverse is true. The simulations show that the flows are faster on the west side, but do not show the east flows increasing with time. There is an asymmetry in the expansion of the emerging flux region, which is also seen in the observations. The west side of the emerging flux region expands faster into the corona than the other side. In the simulations, efficient magnetic reconnection occurs on the west side, with new loops being created containing strong downflows that are clearly seen in the observations. On the other side, the simulations show strong compression as the dominant mechanism for the generation of flows. There is evidence of these flows in the observations, but the flows are stronger than the simulations predict at the later stages. There could be additional small-angle reconnection that adds to the flows from the compression, as well as reconnection occurring in larger loops that lie across the whole active region.     

On the absence of winds in advection-dominated accretion flows

We show that recently published assertions that advection-dominated accretion flows (ADAFs) require the presence of strong winds are unfounded because they assume that low radiative efficiency in flows accreting at low rates on to black holes implies vanishing radial energy and angular momentum fluxes through the flow (which is also formulated in terms of the 'Bernoulli function' being positive). This, however, is a property only of self-similar solutions which are an inadequate representation of global accretion flows. We recall general properties of accretion flows on to black holes and show that such, necessarily transonic, flows may have either d positive or negative Bernoulli function depending on the flow viscosity. Flows with low viscosities ( α ≲0.1 in the α -viscosity model) have a negative Bernoulli function. Without exception, all 2D and 1D numerical models of low-viscosity flows constructed to date experience no significant outflows. At high viscosities the presence of outflows depends on the assumed viscosity, on the equation of state and on the outer boundary condition. The positive sign of the Bernoulli function invoked in this context is irrelevant to the presence of outflows. As an illustration, we recall 2D numerical models with moderate viscosity that have positive values of the Bernoulli function and experience no outflows. ADAFs, therefore, do not differ from this point of view from thin Keplerian discs: they may have, but they do not have to have, strong winds.     

3个太阳活动区中的水平和垂直电流与耀斑的关系

对3个超级活动区（大的δ型黑子群）NOAA 5395、6659、6891中的电流分布作了系统计算；利用已发表的计算方法，首次用于实际活动区的水平电流分布；给出了电流与耀斑核的关系。将这种关系分为两类：密切相关和准相关，并同时给出了统计结果。结果显示：1）对于垂直电流和水平电流来说，密切相关率分别是29％和10％，准相关率分别是50％和30％；2）有些耀斑核与两种电流都相关，而大多数只与其中一种相关；3）与两种电流都不相关的耀斑核只占6％左右；4）两种电流起互补作用，因而对于预报耀斑具有一定的作用。通过分析还发现，磁场剪切强的地方相应于强的垂直电流，而磁中性线附近纵向磁场梯度大的地方相应于强的水平电流。     

High-Resolution Mapping of Flows in the Solar Interior: Fully Consistent OLA Inversion of Helioseismic Travel Times

To recover the flow information encoded in travel-time data of time?–?distance helioseismology, accurate forward modeling and a robust inversion of the travel times are required. We accomplish this using three-dimensional finite-frequency travel-time sensitivity kernels for flows along with a (2+1)-dimensional (2+1D) optimally localized averaging (OLA) inversion scheme. Travel times are measured by ridge filtering MDI full-disk Doppler data and the corresponding Born sensitivity kernels are computed for these particular travel times. We also utilize the full noise-covariance properties of the travel times, which allow us to accurately estimate the errors for all inversions. The whole procedure is thus fully consistent. Because of ridge filtering, the kernel functions separate in the horizontal and vertical directions, motivating our choice of a 2+1D inversion implementation. The inversion procedure also minimizes cross-talk effects among the three flow components, and the averaging kernels resulting from the inversion show very small amounts of cross-talk. We obtain three-dimensional maps of vector solar flows in the quiet Sun at horizontal spatial resolutions of 7?10 Mm using generally 24 hours of data. For all of the flow maps we provide averaging kernels and the noise estimates. We present examples to test the inferred flows, such as a comparison with Doppler data, in which we find a correlation of 0.9. We also present results for quiet-Sun supergranular flows at different depths in the upper convection zone. Our estimation of the vertical velocity shows good qualitative agreement with the horizontal vector flows. We also show vertical flows measured solely from f-mode travel times. In addition, we demonstrate how to directly invert for the horizontal divergence and flow vorticity. Finally we study inferred flow-map correlations at different depths and find a rapid decrease in this correlation with depth, consistent with other recent local helioseismic analyses.     

Cometary activity,active areas,and a mechanism for collimated outflows on 1P, 9P, 19P,and 81P

The properties of 50 jet and jet-filament outflows from 27 active areas observed on the four comet nuclei that have been visited by spacecraft (1P/Halley, 19P/Borrelly, 81P/Wild 2, and 9P/Tempel 1) are investigated and we propose a taxonomic categorization in which there are three types of active areas: Type I that is dominated by the sublimation of H₂O through the porous mantle; Type II that is controlled by the localized and persistent effusion of super-volatiles from the interior; and Type III that is characterized by episodic releases of super-volatiles.The zonally averaged distribution of active area locations associated with Type II outflows over the four comets is calculated and we find that they are distributed randomly in latitude. In longitude, the distribution shows a marginal tendency for active areas to occur more frequently in the region near the ends of the long axis or, alternatively, a tendency to avoid the region close to the ends of the intermediate axis.Combining observations of filamentary structure with exploratory hydrodynamic calculations we find that Type II outflows are likely to be relatively cold laminar flows (Re < 1000) of a mixture of CO₂, CO and H₂O that are highly collimated (6–10° full-cone angle) during the daytime as a result of being constrained by the ambient H₂O atmosphere. We propose that they become visible as a result of the turbulent momentum flux at the base of the filamentary structure that causes the friable surface to release dust at a higher rate than in surrounding areas.We present evidence that indicates that geophysical flows occur on cometary nuclei other than 9P/Tempel 1 and discuss a possible scenario for the long-term evolution of cometary surfaces near the Sun. We conclude with an exposition of a cometary activity paradigm brought up-to-date with discoveries made with recent space missions, associated Earth-based investigations, and the results of this work.     

Subsurface Meridional Circulation in the Active Belts

Temporal variations of the subsurface meridional flow with the solar cycle have been reported by several authors. The measurements are typically averaged over periods of time during which surface magnetic activity existed in the regions where the velocities are calculated. The present work examines the possible contamination of these measurements due to the extra velocity fields associated with active regions plus the uncertainties in the data obtained where strong magnetic fields are present. We perform a systematic analysis of more than five years of GONG data and compare meridional flows obtained by ring-diagram analysis before and after removing the areas of strong magnetic field. The overall trend of increased amplitude of the meridional flow towards solar minimum remains after removal of large areas associated with surface activity. We also find residual circulation toward the active belts that persists even after the removal of the surface magnetic activity, suggesting the existence of a global pattern or longitudinally-located organized flows.     

Interactions between molecular outflows and optical jets

Young stars produce both molecular outflows and, at a later evolutionary stage, well-collimated optical jets. The simplest explanation is that the molecular outflows are driven byobscured optical jets, rather than directly, by a disk wind for example, but the optical jets appear to have too small a momentum flux. Recent statistical studies however show that the molecular flows must be quasi-stationary, which means that the dynamical lifetime is a gross underestimate of the true age. As a consequence much less thrust is required. We present recent observations of RNO 43, which has well-defined optical and molecular outflows lying close to the plane of the sky. Excellent agreement with the observations is obtained with a simple kinematic model for the molecular material, which supposes that it lies in a parabolic shell around the optical jet with the highest velocities at the working surface. Together with our modelling of the NGC2024 outflow, this is very strong evidence that molecular outflows are produced by prompt entrainment of molecular material in a neutral or weakly-ionized jet.     

Outflows at the Edges of an Active Region in a Coronal Hole: A Signature of Active Region Expansion?

Outflows of plasma at the edges of active regions surrounded by quiet Sun are now a common observation with the Hinode satellite. While there is observational evidence to suggest that the outflows are originating in the magnetic field surrounding the active regions, there is no conclusive evidence that reveals how they are driven. Motivated by observations of outflows at the periphery of a mature active region embedded in a coronal hole, we have used a three-dimensional simulation to emulate the active region’s development in order to investigate the origin and driver of these outflows. We find that outflows are accelerated from a site in the coronal hole magnetic field immediately surrounding the active region and are channelled along the coronal hole field as they rise through the atmosphere. The plasma is accelerated simply as a result of the active region expanding horizontally as it develops. Many of the characteristics of the outflows generated in the simulation are consistent with those of observed outflows: velocities up to 45 km s⁻¹, properties akin to the coronal hole, proximity to the active region’s draining loops, expansion with height, and projection over monopolar photospheric magnetic concentrations. Although the horizontal expansion occurs as a consequence of the active region’s development in the simulation, expansion is also a general feature of established active regions. Hence, it is entirely possible and plausible that the expansion acceleration mechanism displayed in the simulation is occurring in active regions on the Sun and, in addition to reconnection, is driving the outflows observed at their edges.     

Interpreting Helioseismic Structure Inversion Results of Solar Active Regions

Helioseismic techniques such as ring-diagram analysis have often been used to determine the subsurface structural differences between solar active and quiet regions. Results obtained by inverting the frequency differences between the regions are usually interpreted as the sound-speed differences between them. These in turn are used as a measure of temperature and magnetic-field strength differences between the two regions. In this paper we first show that the “sound-speed” difference obtained from inversions is actually a combination of sound-speed difference and a magnetic component. Hence, the inversion result is not directly related to the thermal structure. Next, using solar models that include magnetic fields, we develop a formulation to use the inversion results to infer the differences in the magnetic and thermal structures between active and quiet regions. We then apply our technique to existing structure inversion results for different pairs of active and quiet regions. We find that the effect of magnetic fields is strongest in a shallow region above 0.985R _⊙ and that the strengths of magnetic-field effects at the surface and in the deeper (r<0.98R _⊙) layers are inversely related (i.e., the stronger the surface magnetic field the smaller the magnetic effects in the deeper layers, and vice versa). We also find that the magnetic effects in the deeper layers are the strongest in the quiet regions, consistent with the fact that these are basically regions with weakest magnetic fields at the surface. Because the quiet regions were selected to precede or follow their companion active regions, the results could have implications about the evolution of magnetic fields under active regions.     

A Multi-Wavelength Study of the Compact M1 Flare on October 22, 2002

In this paper we present a further study of the Ml class flare observed on October 22, 2002. We focus on the SOHO Coronal Diagnostic Spectrometer (CDS) spectral observations performed during a multi-wavelength campaign with TRACE and ground-based instruments (VTT, THEMIS). Strong blue-shifts are observed in the CDS coronal lines in flare kernels during the impulsive phase of this flare. From a careful wavelength calibration we deduce upflows of 140 km/s for the Fe XIX flare emission, with a pattern of progressively smaller flows at lower temperatures. Large line-widths were observed, especially for the Fe XIX line, which indicate the existence of turbulent velocities. The strong upflows correspond to full shifts of the line profiles. These flows are observed at the initial phase of the flare, and correspond to the “explosive evaporation”. The regions of the blueshifted kernels, a few arc seconds away from the flare onset location, could be explained by the chain reaction of successive magnetic reconnections of growing emerging field line with higher and higher overlying field. This interpretation is evidenced by the analysis of the magnetic topology of the active region using a linear force-free-field extrapolation of THEMIS magnetograms.     

Emerging and Decaying Magnetic Flux and Subsurface Flows

We study the temporal variation of subsurface flows of 788 active regions and 978 quiet regions. The vertical-velocity component used in this study is derived from the divergence of the measured horizontal flows using mass conservation. The horizontal flows cover a range of depths from the surface to about 16 Mm and are determined by analyzing about five years of GONG high-resolution Doppler data with ring-diagram analysis. We determine the change in unsigned magnetic flux during the disk passage of each active region using MDI magnetograms binned to the ring-diagram grid. We then sort the data by their flux change from decaying to emerging flux and divide the data into five subsets of equal size. The average vertical flows of the emerging-flux subset are systematically shifted toward upflows compared to the grand average values of the complete data set, whereas the average flows of the decaying-flux subset show comparably more pronounced downflows especially near 8 Mm. For flux emergence, upflows become stronger with time with increasing flux at depths greater than about 10 Mm. At layers shallower than about 4 Mm, the flows might start to change from downflows to upflows, when flux emerges, and then back to downflows after the active regions are established. The flows in the layers between these two depth ranges show no response to the emerging flux. In the case of decaying flux, the flows change from strong upflows to downflows at depths greater than about 10 Mm, whereas the flows do not change systematically at other depths. A cross-correlation analysis shows that the flows in the near-surface and the deeper layers might change about one day before flux emerges. The flows associated with the quiet regions fluctuate with time but do not show any systematic variation.     

Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small <Emphasis Type="Italic">δ</Emphasis> Spot

In recent high-resolution observations of complex active regions, long-lasting and well-defined regions of strong flows were identified in major flares and associated with bright kernels of visible, near-infrared, and X-ray radiation. These flows, which occurred in the proximity of the magnetic neutral line, significantly contributed to the generation of magnetic shear. Signatures of these shear flows are strongly curved penumbral filaments, which are almost tangential to sunspot umbrae rather than exhibiting the typical radial filamentary structure. Solar active region NOAA 10756 was a moderately complex β δ sunspot group, which provided an opportunity to extend previous studies of such shear flows to quieter settings. We conclude that shear flows are a common phenomenon in complex active regions and δ spots. However, they are not necessarily a prerequisite condition for flaring. Indeed, in the present observations, the photospheric shear flows along the magnetic neutral line are not related to any change of the local magnetic shear. We present high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO). Time series of speckle-reconstructed white-light images and two-dimensional spectroscopic data were combined to study the temporal evolution of the three-dimensional vector flow field in the β δ sunspot group. An hour-long data set of consistent high quality was obtained, which had a cadence of better than 30 seconds and subarcsecond spatial resolution.     

Helicity as a Component of Filament Formation

In this paper we seek the origin of the axial component of the magnetic field in filaments by adapting theory to observations. A previous paper (Mackay, Gaizauskas, and van Ballegooijen, 2000) showed that surface flows acting on potential magnetic fields for 27 days – the maximum time between the emergence of magnetic flux and the formation of large filaments between the resulting activity complexes – cannot explain the chirality or inverse polarity nature of the observed filaments. We show that the inclusion of initial helicity, for which there is observational evidence, in the flux transport model results in sufficiently strong dextral fields of inverse polarity to account for the existence and length of an observed filament within the allotted time. The simulations even produce a large length of dextral chirality when just small amounts of helicity are included in the initial configuration. The modeling suggests that the axial field component in filaments can result from a combination of surface (flux transport) and sub-surface (helicity) effects acting together. Here surface effects convert the large-scale helicity emerging in active regions into a smaller-scale magnetic-field component parallel to the polarity inversion line so as to form a magnetic configuration suitable for a filament.     

Hypersonic Swizzle Sticks: Protostellar Turbulence, Outflows and Fossil Outflow Cavities

The expected lifetimes for molecular clouds has become a topic of considerable debate as numerical simulations have shown that MHD turbulence, the nominal means of support for clouds against self-gravity, will decay on short timescales. Thus it appears that either molecular clouds are transient features or they are resupplied with turbulent energy through some means. Jets and molecular outflows are recognized as a ubiquitous phenomena associated with star formation. Stars however form not isolation but in clusters of different density and composion. The ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we present new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their abilityto drive turbulent flows in molecular clouds. Our studies focus on scales associated with young star forming clusters. In particular we first show that direct collisions between active outflows are not effective at stirring the ambient medium. We then show that fossil cavities from “extinct” outflows may provide the missing link in terms of transferring momentum and energy to the cloud.     

Dynamics of relativistic jets

We discuss the structure and relativistic kinematics that develop in three spatial dimensions when a moderately hot, supersonic jet propagates into a denser background medium and encounters resistance from an oblique magnetic field. Our simulations incorporate relativistic MHD in a four-dimensional spacetime and clearly show that (a) relatively weak, oblique fields (at 1/16 of the equipartition value) have only a negligible influence on the propagating jet and they are passively pushed away by the relativistically moving head; (b) oblique fields in equipartition with the ambient plasma provide more resistance and cause bending at the jet head, but the magnitude of this deflection and the associated backflow are small compared to those identified by previous studies. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently during the simulations. The effect is analogous to pushing Japanese “noren” or vertical Venetian blinds out of the way while the slats are allowed to bend and twist in 3-D space. Applied to relativistic extragalactic jets from blazars, the new results are encouraging since superluminal outflows exhibit bending near their sources and their environments are profoundly magnetized – but observations do not provide support for irregular kinematics such as large-scale vortical motions and pronounced reverse flows near the points of origin.     

Magnetohydrodynamic properties of extragalactic jets

The theory that magnetic fields are instrumental in the formation and propagation of jets in active galactic nuclei dates back four decades. Despite a recent growing consensus on this notion stemming from the results of numerical simulations of magnetohydrodynamic (MHD) flows near black holes, the precise dynamical role of magnetic fields in observed parsec and kiloparsec jets remains uncertain. Some of the unanswered fundamental questions about extragalactic jets include the location where the flow becomes relativistic and where acceleration and collimation terminate, as well as the specifics of how the flow interacts with the ISM. Such observed properties as superluminal motions and wiggled structures based on numerical simulations to constitute the foundation of an MHD paradigm for extragalactic jets. We particularly focus our attention to the M87 jet, which is one of the best candidates to investigate relativistic outflows in extragalactic system.     

Vorticity of Subsurface Flows of Emerging and Decaying Active Regions

We study the temporal variation of the vorticity of subsurface flows of 828 active regions and 977 quiet regions. The vorticity of these flows is derived from measured subsurface velocities. The horizontal flows are determined by analyzing high-resolution Global Oscillation Network Group Doppler data with ring-diagram analysis covering a range of depths from the surface to about 16 Mm. The vertical velocity component is derived from the divergence of the measured horizontal flows using mass conservation. We determine the change in unsigned magnetic flux density during the disk passage of each active region using Michelson Doppler Imager (MDI) magnetograms binned to the ring-diagram grid with centers spaced by 7.5° ranging ± 52.5° in latitude and central meridian distance with an effective diameter of 15° after apodization. We then sort the data by their flux change from decaying to emerging flux and divide the data into five subsets of equal size. We find that the vorticity of subsurface flows increases during flux emergence and decreases when active regions decay. For flux emergence, the absolute values of the zonal and meridional vorticity components show the most coherent variation with activity, while for flux decrease the strongest signature is in the absolute values of the meridional and vertical vorticity components. The temporal variation of the enstrophy (residual vorticity squared) is thus a good indicator for either flux increase or decrease. There are some indications that the increase in vorticity during flux emergence happens about a day later at depths below about 8 Mm compared to layers shallower than about 4 Mm. This timing difference might imply that the vorticity signal analyzed here is caused by the interaction between magnetic flux and turbulent flows near the solar surface. There are also hints that the vorticity decrease during flux decay begins about a day earlier at layers deeper than about 8 Mm compared to shallower ones. However, the timing difference between the change at different depths is comparable to the time step of the analysis.     

Self-similar solution of hot accretion flows with ordered magnetic field and outflow

Observations and numerical magnetohydrodynamic (MHD) simulations indicate the existence of outflows and ordered large-scale magnetic fields in the inner region of hot accretion flows. In this paper, we present the self-similar solutions for advection-dominated accretion flows (ADAFs) with outflows and ordered magnetic fields. Stimulated by numerical simulations, we assume that the magnetic field has a strong toroidal component and a vertical component in addition to a stochastic component. We obtain the self-similar solutions to the equations describing the magnetized ADAFs, taking into account the dynamical effects of the outflow. We compare the results with the canonical ADAFs and find that the dynamical properties of ADAFs such as radial velocity, angular velocity and temperature can be significantly changed in the presence of ordered magnetic fields and outflows. The stronger the magnetic field is, the lower the temperature of the accretion flow will be and the faster the flow rotates. The relevance to observations is briefly discussed.     

Hot accretion with outflow and thermal conduction

We present self-similar solutions for advection-dominated accretion flows with thermal conduction in the presence of outflows. Possible effects of outflows on the accretion flow are parametrized and a saturated form of thermal conduction, as is appropriate for the weakly-collisional regime of interest, is included in our model. While the cooling effect of outflows is noticeable, thermal conduction provides an extra heating source. In comparison to accretion flows without winds, we show that the disc rotates faster and becomes cooler because of the angular momentum and energy flux which are taking away by the winds. But thermal conduction opposes the effects of winds and not only decreases the rotational velocity, but increases the temperature. However, reduction of the surface density and the enhanced accretion velocity are amplified by both of the winds and the thermal conduction. We find that for stronger outflows, a higher level of saturated thermal conduction is needed to significantly modify the physical profiles of the accretion flow.     

What Drives Molecular Outflows from Young Stars?

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.