Bow Shock Model Research Articles

A comparison of IMP 8 observed bow shock positions with model predictions

Over 12 years of IMP 8, data was searched for observed bow shock crossings. Out of the total 4562 crossings found, we used the 2293 unambiguous bow shocks for which upstream interplanetary magnetic field and solar wind parameters were available to study selected bow shock models under normal and unusual solar wind conditions. The chosen models were F79, NS91, FR94, FR94c, CL95, and P95 [Formisano, 1979; Němeček and Šafránková, 1991; Farris and Russell, 1994; Cairns and Lyon, 1995; Peredo et al., 1995]. This statistical study investigates these models' reliability not only for average solar wind plasma and interplanetary magnetic field (IMF) conditions but also for unusual conditions and as a result points out some deficiencies of these models. Statistically, the predictions of F79 and the phenomenological and MHD models FR94, FR94c, and CL95 are the most accurate, with F79 giving a slightly better result. The P95 model predicts standoff distances which are too large by ∼20%. For large values of the IMF and its components, all models except NS91 underestimate the bow shock distance. Furthermore, the models underestimate the bow shock distance when the upstream Mach numbers are low (≲5). The models also do not properly reflect changes in the relative orientation of the IMF and solar wind velocity vectors. An independent evaluation of the dawn and dusk sectors suggests an asymmetry in the bow shock shape and/or a different reaction of the flanks to solar wind deviations from a radial flow. Taking the upstream parameters from a distant solar wind monitor (the Wind spacecraft) resulted in the models predicting the shock farther away from the Earth, which is likely a result of the spacecraft separation perpendicular to the solar wind flow, or of calibrational differences of the plasma density measurements by the spacecraft.

Shocks in Protostellar Outflows

We present our analysis of four molecular outflows from Class 0 (Cep E, L 1448) sources and higher mass objects (Cep A, DR 21). The emission line spectra of these outflows were observed in the mid- and far-infrared using the spectrometers (SWS, LWS) and the camera (ISOCAM) aboard the ISO satellite. We interpret the spectra using J- and C-type bow shock models and infer properties of both the outflow and surrounding gas. We find C-type bows with a shape parameter of s = 1.4 as the best interpretation of the measured line fluxes, independent of the object. The emission is partly caused by fluorescence.

Cluster magnetic field observations at a quasi-parallel bow shock

Abstract. We present four-point Cluster magnetic field data from a quasi-parallel shock crossing which allows us to probe the three-dimensional structure of this type of shock for the first time. We find that steepened ULF waves typically have a scale larger than the spacecraft separation ( ~ 400–1000 km), while SLAMS-like magnetic field enhancements have different signatures in | B | at the four spacecraft, suggesting that they have a smaller scale size. In the latter case, however, the angular variations of B are similar, consistent with the space-craft making different trajectories through the same structure. The field enhancements have different orientations relative to a model bow shock normal, which might arise from different degrees of deceleration and deflection of the surrounding solar wind plasma. The observed rotation of the magnetic field rising from a direction approximately parallel to the model bow shock normal to a direction more perpendicular to the model normal across the field enhancement is consistent with previously published results. Successive magnetic field enhancements or ULF waves, and the leading and trailing edges of the same structure, are found to have different orientations.Key words. Interplanetary physics (planetary bow shocks)

An Hα nebula possibly associated with the central source in the G 266.2-1.2 supernova remnant

We report the discovery of a small Hα nebula positionally coincident with the candidate neutron star AX J0851.9-4617.4 located at the center of the supernova remnant G 266.2-1.2. The nebula has a roughly circular shape with a diameter of ~6'' and a flux of ~10-2 photons cm-2 s-1 in the Hα line. Considering the uncertainties in the distance and energy output from the putative neutron star, we find that such a flux can be explained either in a bow-shock model or assuming that the nebular emission is due to photo-ionization and heating of the ambient gas.

Four spacecraft measurements of the quasiperpendicular terrestrial bow shock: Orientation and motion

Measurements of the magnetic field at the four Cluster spacecraft, typically separated by ∼600 km, during bow shock crossings allow the orientation and motion of this structure to be estimated. Results from 48 clean and steady quasiperpendicular crossings during 2000 and 2001, covering local times from 0600 to 1700, reveal the bow shock normal to be remarkably stable, under a wide range of steady upstream conditions. Nearly 80% of normals lay within 10° of those of two bow shock models, suggesting that the timing method is accurate to around 10°, and possibly better, and therefore that four spacecraft timings are a useful estimator of the orientation and motion of quasiperpendicular bow shocks. These results show that models provide a good approximation to the bow shock surface and can therefore be used when four spacecraft data are not available. In contrast, only 19% of magnetic coplanarity vectors were within 10° of the model normal. The mean deviation of the coplanarity vector from the timing‐derived normal for shocks with θBN < 70° was 22 ± 4°. Typical shock velocities were ∼35 km s−1, although the fastest measured shock was traveling outbound at nearly 150 km s−1.

Feii] 1.257 μm and Hei1.083 μm Emission in the Central Region of the Orion Nebula: HiiRegion, HH Flows, Jets, and Proplyds

The [Fe II] 1.257 μm and He I 1.083 μm emission lines were observed in the central 6' × 8' region of the Orion Nebula, and their excitation in the photoionized H II region, HH flows, jets, and proplyds is investigated. Observations were carried out using the imaging Fabry-Perot spectrometer MUSE at the National Astronomical Observatory of Japan 1.5 m infrared telescope, which provides a 4' × 4' field of view and a spectral resolution λ/δλ of ~2000 at the observed wavelengths. The [Fe II] images exhibit (1) filamentary structures and diffuse emission, which presumably arise from ionization fronts of the photoionized H II region, and (2) a number of knots, some of which are newly identified. Centroidal velocities in most of the knots are negative relative to those in the ionization fronts by up to -60 km s-1, and observed line profiles in the bright knots exhibit blueshifted wings, agreeing with bow shock models. The He I 1.083 μm emission in the observed region is dominated by the photoionized H II region, and its distribution reflects the complicated nature of the excitation. The He I images also contain blueshifted emission from several HH flows and jets and redshifted emission associated with proplyds. Our results for the shocks suggest that the [Fe II] 1.257 μm and He I 1.083 μm emission reflects the ionization of the preshock gas: the [Fe II] 1.257 μm emission is prominent in shocks propagating in molecular/atomic gas, while the He I 1.083 μm is prominent in shocks in the photoionized H II region. Different line excitation in these shocks can be explained by the following physical properties and processes: (1) difference of the excitation energies from the ground state (1 and 20 eV for the [Fe II] and He I lines, respectively), (2) resonance scattering and collisional excitation from the metastable state, enhancing the He I 1.083 μm line in shocks in the photoionized H II region, (3) a large photoionization cross section of Fe+, causing the absence of the [Fe II] emission in the same shocks, and possibly (4) charge exchange reactions of Fe, which could enhance the [Fe II] emission in neutral gas.

Possible Source of the Difference between a Model Prediction and Observations of Bow Shock Crossings

The new data set of 1901 bow shock crossings by Geotail and Wind satellites in period 1999-2000 is selected for a wide range of distances and solar wind conditions observed by Wind and ACE upstream monitors. The data set is applied for verification of the semi-empirical bow shock model developed in the last solar minimum (1995-1997), using Geotail crossings of the bow shock and Wind upstream solar wind data. In general, the model accuracy is reasonably good in the dayside and flank regions of the bow shock. But, the model extrapolation in the tail region has much less accuracy due to restriction of the bow' shock crossing distances used in the model development. Some difference between the data of ACE and Wind upstream solar wind monitors leads also to the model discrepancies. Analysis of long time variations of the bow shock parameters during last three solar cycles shows noticeable difference in the heliospheric conditions controlling the bow shock size and shape in the solar minimum and maximum. In the solar maximum, the variations of the interplanetary conditions are much higher than in the solar minimum. This difference produces additional disagreement in prediction of the bow shock crossings in the current solar maximum by the model developed using a data set obtained in the solar minimum.

First Detections of Molecular Gas Associated with the Wolf‐Rayet Ring Nebula NGC 3199

This paper presents the first observations of molecular gas associated with the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. This includes first observations of the molecules HCN, HCO+, CN, and HNC seen in any Wolf-Rayet ring nebula. Our observations immediately suggest the presence of high-density molecular gas (>104 cm-3) in the nebula with significant amounts of associated molecular gas, which is in the form of clumpy ejecta and/or interstellar material. Molecular CO gas was mapped across the optically bright portion of the nebula and out into the diffuse ionized component using the 12CO J = 1 → 0 line. CO gas is not seen within the optically bright rim of NGC 3199 but adjacent to it. The optical emission rim therefore appears to mark regions of photodissociation. Velocity components in the CO data are consistent with those seen in high-resolution optical spectra of the Hα line but extend beyond the visible emission. A prior suggestion of the formation of the nebula via a bow shock appears unlikely since Hipparcos measurements show the proper motion of WR 18 is almost at right angles to the direction required for the bow shock model. Instead, line splitting toward the north of the nebula suggests that a possible blowout of the Wolf-Rayet wind through surrounding ejecta may be responsible for some of the velocity features observed. Preliminary estimates of molecular abundances in the nebula seen toward the central star are significantly higher than for the interstellar medium and are similar to those in planetary nebulae, although CN is distinctly underabundant in comparison to the very high values found in many planetary nebulae. The abundances found are consistent with the idea that at least a portion of the molecular material is associated with ejecta from the central star.

The location of low Mach number bow shocks at Earth

On April 26 and 27 and May 10–12, 1999, unusually low solar wind densities produced unusually low Alfven Mach numbers that moved the Earth's bow shock far out past its normal location. The shocks observed by the Wind spacecraft corresponded to shock subsolar distances of 45 and 42 RE, respectively, on these days which are the most distant locations at which the shock has ever been seen. Shock observations by three other spacecraft on these days along with 34 previously reported distant shocks are used to compare with the predictions of different models. A recent MHD bow shock model of Cairns and Lyon [1995] predicts the observed locations quite well as does a modified gasdynamic model of Farris and Russell [1994] if a new Mach‐number‐dependent shape parameter is used. A third model of Verigin et al. [1997] also predicts a shock shape and is also quite good. Bow shock predictions are limited by uncertainties in measurements of the very low densities and uncertainties in the position and shape of the magnetopause. Asymmetries in the shock shape caused by the interplanetary magnetic field direction and not accounted for by models are another likely source of uncertainty. These uncertainties make it impossible to clearly favor one theoretical model over another.

Kinematics of Herbig-Haro Objects and Jets in the Orion Nebula

We have surveyed the inner 5' of the Orion Nebula by means of Hα and [N II] Fabry-Perot imaging spectroscopy to present a kinematical study of the Herbig-Haro objects in the nebula. The objects studied in this work are HH 202, 203, 204, 529, 269, and other associated features. For HH 202 we find new features that, because of their high velocities (up to 100 km s-1), indicate the presence of an outflow that probably is an HH flow not cataloged previously. HH 202 could be only a part of this larger outflow. Large internal motions are found in the fainter regions of HH 203-204, as well as evidence of transverse density gradients that could account for the asymmetry in the brightness distribution of HH 204. We report for the first time a high blueshifted velocity (-118 km s-1) associated with HH 204 and show that the apex of HH 204 is indeed the zone of maximum velocity, in agreement with bow shock models. We also studied the radial velocity field of HH 269, finding features associated with the HH object. From our studies, we find kinematic evidence that suggests HH 203-204 and HH 202 are part of a big (~0.55 pc) bipolar HH outflow.

Wind observations of the terrestrial bow shock: 3-D shape and motion

Between late 1994 and early 2001 the Wind orbiter, generally targeted to stay in the solar wind, passed through the Earth’s magnetosphere ∼50 times. About 450 distinct bow shock crossings were collected during the inbound and outbound bracketing each Wind perigee. These crossings and corresponding vectorial upstream solar wind measurements by the Wind MFI and SWE instruments are used to study the 3-D shape of the bow shock and its motion. Mapping of bow shock crossings to the Sun-Earth line and to the terminator plane is realized using a recent analytical model of the planetary bow shock. The asymmetry of the terrestrial bow shock in the terminator plane is studied as a function of Friedrichs diagram anisotropy. Analysis of the subsolar bow shock position as a function of Alfvenic Mach number Ma during intervals of magnetic field aligned solar wind flow shows that the shock tends to approach the Earth when Ma is decreasing, while for non field-aligned flows bow shock moves from the planet.

Echelle spectroscopy of H2in the HH 111 jet

Near-Infrared echelle spectra of the Herbig-Haro knots F-P in the western lobe of the HH 111 outflow are presented. 10 adjacent, parallel slit positions were observed so that the kinematics could be mapped across the width of the flow. We find broad (FWZI km s-1), two-component H2 profiles in the knots nearest the source; the profiles converge to a single, intermediate-velocity peak ( km s-1) at knot P. The kinematic signature of the HH 111 jet in H2 is very similar to that seen at optical wavelengths (from both radial and tangential velocity meaurements). In conjunction with published proper-motion (knot/shock pattern speed) measurements, we interpret the data in terms of a simple geometrical bow shock model. The model infers a high pre-shock velocity, of the order of 200-250 km s-1, with H2 excitation in the extended bow wings in knots L, H and F, though exclusively near the bow head in knot P, with probably C-type shock excitation throughout.

Pulsar bow-shock nebulae

Pulsar bow-shock nebulae originate from the interaction of the ambient medium with the wind of a moving pulsar. The properties of these nebulae depend on both the physical conditions in the ambient medium and the characteristics of the pulsar wind, and may thus represent a powerful diagnostic tool. The main limits to this study are, on the observational side, the very limited sample of pulsar bow-shock nebulae known so far and, on the theoretical one, a still limited understanding of the physical conditions in these nebulae: in particular, classical bow-shock models are not appropriate to describe these objects. In this paper we outline the most likely physical regimes for nebulae associated with typical pulsars, and discuss why classical models cannot be applied to these objects. We also discuss how microphysical processes may aect both the dynamics of the flow and the properties of the emission (mostly in Balmer lines). We nally put forward a criterion to select among the known pulsars those with the highest probability of having a detectable bow-shock nebula.

Hydrodynamic Simulations of Jet‐ and Wind‐driven Protostellar Outflows

We present two-dimensional hydrodynamic simulations of both jet- and wind-driven models for protostellar outflows in order to make detailed comparisons to the kinematics of observed molecular outflows. The simulations are performed with the ZEUS-2D hydrodynamic code using a simplified equation of state, simplified cooling and no external heating, and no self-gravity. In simulations of steady jets, swept-up ambient gas forms a thin shell that can be identified as a molecular outflow. We find a simple ballistic bow shock model is able to reproduce the structure and transverse velocity of the shell. Position-velocity (PV) diagrams for the shell cut along the outflow axis show a convex spur structure with the highest velocity at the bow tip and low-velocity red and blue components at any viewing angle. The power-law index of the mass-velocity (MV) relationship ranges from 1.5 to 3.5, depending strongly on the inclination. If the jet is time-variable, the PV diagrams show multiple convex spur structures, and the power-law index becomes smaller than the steady jet simulation. In simulations of isothermal steady wide-angle winds, swept-up ambient gas forms a thin shell that at early stages has a similar shape to the shell in the jet-driven model; it becomes broader at later times. We find the structure and kinematics of the shell is well described by a momentum-conserving model similar to that of Shu et al. (1991). In contrast to the results from jet simulations, the PV diagrams for the shell cut along the outflow axis show a lobe structure tilted with source inclination, with components that are primarily either red or blue unless the inclination is nearly in the plane of sky. The power-law index of the MV relationship ranges from 1.3 to 1.8. If the wind is time-variable, the PV diagrams also show multiple structures, and the power-law index becomes smaller than the steady wind simulation. Comparing the different simulations with observations, we find that some outflows, e.g., HH 212, show features consistent with the jet-driven model, while others, e.g., VLA 0548, are consistent with the wind-driven model.

SiO Emission in the Multilobe Outflow Associated with IRAS 16293−2422

We have mapped the thermal emission line of SiO (v = 0; J = 2-1) associated with the quadrupolar molecular outflow driven by the very cold far-infrared source IRAS 16293-2422. The SiO emission is significantly enhanced in the northeastern red lobe and at the position ~50'' east of the IRAS source. Strong SiO emission observed at ~50'' east of the IRAS source presents evidence for a dynamical interaction between a part of the eastern blue lobe and the dense ambient gas condensation; however, such an interaction is unlikely to be responsible for producing the quadrupolar morphology. The SiO emission in the northeastern red lobe shows spatial and velocity structure similar to those of the CO outflow, suggesting that the SiO emission comes from the molecular outflow in the northeastern red lobe itself. The observed velocity structure is reproduced by a simple spatiokinematic model of bow shock with a shock velocity of 19-24 km s-1 inclined by 30°-45° from the plane of the sky. This implies that the northeastern red lobe is independent of the eastern blue lobe and that the quadrupolar structure is due to two separate bipolar outflows. The SiO emission observed in the western red lobe has a broad pedestal shape with low intensity. Unlike the SiO emission in the northeastern red lobe, the spatial extent of the SiO emission in the western red lobe is restricted to its central region. The spatial and velocity structures and the line profiles suggest that three different types of SiO emission are observed in this outflow: the SiO emission arising from the interface between the outflowing gas and the dense ambient gas clump, the SiO emission coming from the outflow lobe itself, and the broad SiO emission with low intensity observed at the central region of the outflow lobe.

Analysis of the 3-D shape of the terrestrial bow shock by interball/magion 4 observations

Location and shape of the terrestrial bow shock are analyzed using MAGION 4 (sub satellite of INTERBALL 1) crossings of this boundary and upstream solar wind parameters measured by the WIND spacecraft. Different crossing points were mapped to the Sun — Earth line and to the terminator plane using an analytical model of the planetary bow shock previously developed for the Martian bow shock investigation. Analysis of the subsolar bow shock position as a function of Alfvenic Mach number ( M a ) revealed fine effect that this boundary tends to approach the Earth when M a is decreasing for field-aligned flow of the solar wind, while for non field-aligned flow the bow shock moves away from the planet. Asymmetry of the terrestrial bow shock in the terminator plane is found for non field-aligned flow with anisotropic Friedrichs diagrams.

CO Outflows from Young Stars: Confronting the Jet and Wind Models

We have mapped the CO J = 1-0 emission from molecular outflows associated with five young stellar systems of class 0 to class II/III using the BIMA array and the FCRAO 14 m antenna. The systems, VLA 05487, HH 212, HH 240/241, HH 111, and RNO 91, are all relatively nearby and of low luminosity, and the majority have H2 emission or optical jet features. The CO outflow generally forms a shell structure around the outflow axis with the higher velocity emission further out from the source. Two distinctive kinematic features are evident in position-velocity (PV) diagrams: a parabolic structure originating at the driving source (e.g., VLA 05487 and HH 111) and a convex spur structure with the high-velocity tip near known H2 bow shocks (e.g., HH 212, HH 240/241 and HH 111). The parabolic PV structure can be produced by a wide-angle-wind model, while the velocity spur structure can be modeled with a jet-driven bow shock model. VLA 05487, which is not associated with any H2 bow shocks, shows only the parabolic structure and kinematics consistent with the wide-angle wind-driven model. HH 212, which is associated with a series of H2 bow shock structures, shows a striking morphological coincidence between the H2 and CO emission and velocity spurs in the PV diagram. It is our best example of the jet-driven bow shock model, and its kinematics can be qualitatively explained in that context. HH 240/241 is similar to HH 212 and shows a close relationship between the H2 and CO emission. The kinematics of its western lobe can also be explained with the jet-driven model. The kinematics of RNO 91 are similar to VLA 05487 and are broadly consistent with a wide-angle wind-driven model. HH 111 has both parabolic and spur PV structures, a combination that is not easily explained in the simplest version of either model.

A milliarcsecond-resolution image of the OH masers in G34.3+0.2

We present measurements of the distribution of the OH masers at 1665 and 1667 MHz towards the cometary ultracompact H ii region in the complex G34.3+0.2. The results are based on observations made in both senses of circular polarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5×20 mas2. 38 maser features are identified in the region. 33 of these lie on an arc at the edge of the cometary H ii region. Five are located in a cluster offset toward the north-east by 3 arcsec, and are probably associated with an independent ultracompact H ii region. There is a velocity gradient of 30 km s−1 pc−1 across the arc. We identify five Zeeman pairs and determine that the magnetic field varies between 1 and 7 mG, but is always directed away from the Earth. The OH masers may arise in clumps in a shell of gas in a bow shock caused by the motion of the exciting star through the molecular cloud. The stand-off distance and the thickness of the shocked shell are roughly consistent with those predicted by such a bow-shock model. Also, the position of the exciting star(s), as estimated from the focus of the parabolic bow shock, closely matches that of the peak emission from the cometary H ii region. However, the north-south velocity gradient in the ionized material remains difficult to explain in the context of the bow-shock model.

Electron temperature and de Hoffmann‐Teller potential change across the Earth's bow shock: New results from ISEE 1

We present a survey of the trends between the electron temperature increase ΔTe and the de Hoffmann‐Teller frame (HTF) electrostatic potential jump ΔΦHT and their correlation with other parameters that characterize the shock transition using a new ISEE 1 database of 129 Earth bow shock crossings. A fundamental understanding of the HTF potential is central to distinguishing the reversible and irreversible changes to electron temperature across collisionless shocks. The HTF potential is estimated using three different techniques: (1) integrating the steady state, electron fluid momentum equation across the shock layer using high time resolution plasma and field data from ISEE 1, (2) using the steady state, electron fluid energy equation, and (3) using an electron polytrope approximation. We find that ΔΦHT and ΔTe are strongly and positively correlated with |Δ(mpUn2/2)|, which is in good qualitative agreement with earlier experimental surveys [Thomsen et al., 1987b; Schwartz et al., 1988] that used bow shock model normals and used the flow in the spacecraft frame. There is a strong linear organization of the ΔTe with ΔΦHT, which suggests an average effective electron polytropic index of 〈γe〉 ≈ 2. In addition, ΔTe and ΔΦHT are organized by βe, although our results may be biased by our limited sampling of shock conditions. Comparisons indicate that the differentials in the HTF potential δΦHT are proportional to the differentials in the magnetic field intensity δB across the shock, with a proportionality constant κ that is a fixed constant for a given shock crossing.

Interaction between an interplanetary magnetic cloud and the Earth's magnetosphere: Motions of the bow shock

An interplanetary magnetic cloud (IMC) is an important solar‐terrestrial connection event. It is an ideal object for the study of solar‐terrestrial relations and space weather because the Earth's space environment can be affected considerably during an IMC passage. An IMC was observed to pass the Earth during October 18–20, 1995. Wind recorded its interplanetary characteristics at ∼ 175 RE upstream of the Earth's bow shock, and ∼ 45 min later, Geotail, being near the nominal location of the dawn bow shock, detected IMC‐related multiple bow shock crossings. Using simultaneous measurements from Wind and Geotail, we analyzed, with a semiempirical bow shock model with two parameters, the bow shock motion caused by the interaction of the IMC with the magnetosphere during the passage. We also compared the bow shock motion predicted by the model, and hence the predicted Geotail bow shock crossings, with Geotail observations of the actual crossings. The results showed that the observed multiple bow shock crossings, which were obviously due to temporal variations of the upstream solar wind, can be well explained by the model‐predicted bow shock motion.