Bow Shock Geometry Research Articles

Characterization of Foreshock Plasma Populations at Mercury

AbstractObservations of foreshock plasma populations at Mercury are presented utilizing measurements from the Fast Imaging Plasma Spectrometer aboard the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft. The magnetosphere and foreshock system at Mercury exists in a unique parameter space, due to the planet's relatively weak magnetic dipole and the proximity of its orbit to the Sun. Previous investigations have therefore questioned whether there is sufficient free energy at Mercury to generate foreshock populations, due to the small spatial scale of its bow shock. The observations presented in this work show that field‐aligned beam and diffuse populations similar to those seen in the terrestrial foreshock are able to form upstream of the Hermean bow shock. The observed populations are organized by the bow shock geometry, and are associated with magnetic wave activity previously detected in Mercury's foreshock with corollaries to the terrestrial foreshock.

Comparing Quasi-Parallel and Quasi-Perpendicular Configuration in the Terrestrial Magnetosheath: Multifractal Analysis

The terrestrial magnetosheath is characterized by large-amplitude magnetic field fluctuations. In some regions, and depending on the bow-shock geometry, these can be observed on several scales, and show the typical signatures of magnetohydrodynamic turbulence. Using Cluster data, magnetic field spectra and flatness are observed in two intervals separated by a sharp transition from quasi-parallel to quasi-perpendicular magnetic field with respect to the bow-shock normal. The multifractal generalized dimensions Dq and the corresponding multifractal spectrum f(α) were estimated using a coarse-graining method. A p-model fit was used to obtain a single parameter to describe quantitatively the strength of multifractality and intermittency. Results show a clear transition and sharp differences in the intermittency properties for the two regions, with the quasi-parallel turbulence being more intermittent.

Global Asymmetries of Hot Flow Anomalies

AbstractA three‐dimensional global hybrid simulation is conducted for the interaction of the Earth's bow shock with an interplanetary directional tangential discontinuity (TD) whose normal lies in the equatorial plane. Although the convection electric field points into the TD from its trailing side both north and south of the equator, the interaction only generates a hot flow anomaly (HFA) in the southern hemisphere. HFA generation results from bow shock‐reflected ions energized by the inward‐pointing electric field on one side of the TD and inward ion gyration on the other side. The latter only occurs south of the equator due to the global bow shock geometry. The global HFA asymmetry is driven by a reversal in the north‐south component of reflected ion velocities in the two hemispheres, such that ions gyrate into (away from) the TD in the southern (northern) hemisphere. Our results indicate HFAs must generally exhibit north‐south and dawn‐dusk asymmetries.

Statistical Study of Foreshock Transients in the Midtail Foreshock

AbstractIn the dayside foreshock, many foreshock transients have been observed and simulated. Because of their strong dynamic pressure perturbations, foreshock transients can disturb the local bow shock, magnetosheath, magnetopause, and thus the magnetosphere‐ionosphere system. They can also accelerate particles contributing to shock acceleration. Recent observations and simulations showed that foreshock transients also exist in the midtail foreshock, which can continuously disturb the nightside bow shock, magnetosheath, and magnetopause while propagating tailward for tens of minutes. To further understand the characteristics of midtail foreshock transients, we studied them statistically using Acceleration Reconnection Turbulence & Electrodynamics of Moon’s Interaction with the Sun observations. We selected 111 events that have dynamic pressure decrease along the local bow shock normal by more than 50%. We show that the dynamic pressure decrease is contributed by both density decrease and speed decrease. Around 90% of the events have electron temperature increase by more than 10% with a temperature change ratio proportional to the solar wind speed. Midtail foreshock transients more likely occur at the dawnside than the duskside. They are more significant closer to the bow shock and rather stable along the tailward direction. They have similar formation conditions compared to the dayside foreshock transients, except the ones related to the bow shock geometry. Our study indicates that the characteristics of foreshock transients based on dayside observations need to be generalized. Our study also implies that foreshock transients can exist for tens of minutes (even longer for larger planar shocks), continuously disturbing the local shock and accelerating/heating particles.

Dependence of Thermodynamic Processes on Upstream Interplanetary Magnetic Field Conditions for Magnetosheath Ions

AbstractUsing Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations over a 10‐year period from 2008 to 2017, we statistically investigate the thermodynamic properties for magnetosheath ions and their dependence on upstream interplanetary magnetic field (IMF) conditions. The thermodynamic properties for magnetosheath ions are estimated by using the polytropic index averaged over the subinterval that belongs to the same streamline ( ). The THEMIS observations show that the probability distribution of for magnetosheath ions has a major peak at ~1 (quasi‐isothermal conditions) with a longer left tail down to ~0 (quasi‐isobaric conditions). The spatial distributions of for two different types according to IMF spiral angle (i.e., Parker spiral and ortho‐Parker spiral IMF orientations) reveal that the ions in the downstream of a quasi‐perpendicular shock (quasi‐perpendicular magnetosheath) exhibit quasi‐isothermal processes, while those in the downstream of a quasi‐parallel shock (quasi‐parallel magnetosheath) show lower than unity (down to ~0.8) implying the anticorrelation between the ion temperature and the ion number density variations. Moreover, in the quasi‐parallel magnetosheath tends to decrease with increasing magnetic local time distance from the magnetic local noon. These results indicate that the thermodynamic properties for magnetosheath ions depend on the bow shock geometry (quasi‐perpendicular bow shocks versus quasi‐parallel bow shocks) and are presumably controlled by a variety of instabilities, waves, and turbulence in the magnetosheath.

VARIATION IN THE PRE-TRANSIT BALMER LINE SIGNAL AROUND THE HOT JUPITER HD 189733B

ABSTRACT As followup to our recent detection of a pre-transit signal around HD 189733 b, we obtained full pre-transit phase coverage of a single planetary transit. The pre-transit signal is again detected in the Balmer lines but with variable strength and timing, suggesting that the bow shock geometry reported in our previous work does not describe the signal from the latest transit. We also demonstrate the use of the Ca ii H and K residual core flux as a proxy for the stellar activity level throughout the transit. A moderate trend is found between the pre-transit absorption signal in the 2013 data and the Ca ii H flux. This suggests that some of the 2013 pre-transit hydrogen absorption can be attributed to varying stellar activity levels. A very weak correlation is found between the Ca ii H core flux and the Balmer line absorption in the 2015 transit, hinting at a smaller contribution from stellar activity compared to the 2013 transit. We simulate how varying stellar activity levels can produce changes in the Balmer line transmission spectra. These simulations show that the strength of the 2013 and 2015 pre-transit signals can be reproduced by stellar variability. If the pre-transit signature is attributed to circumplanetary material, its evolution in time can be described by accretion clumps spiraling toward the star, although this interpretation has serious limitations. Further high-cadence monitoring at Hα is necessary to distinguish between true absorption by transiting material and short-term variations in the stellar activity level.

Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath

AbstractCollisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion‐electron‐scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi‐parallel bow shock geometry. We observe multiple small‐scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

A new method for solving the MHD equations in the magnetosheath

Abstract. We present a new analytical method to derive steady-state magnetohydrodynamic (MHD) solutions of the magnetosheath in different levels of approximation. With this method, we calculate the magnetosheath's density, velocity, and magnetic field distribution as well as its geometry. Thereby, the solution depends on the geomagnetic dipole moment and solar wind conditions only. To simplify the representation, we restrict our model to northward IMF with the solar wind flow along the stagnation streamline. The sheath's geometry, with its boundaries, bow shock and magnetopause, is determined self-consistently. Our model is stationary and time relaxation has not to be considered as in global MHD simulations. Our method uses series expansion to transfer the MHD equations into a new set of ordinary differential equations. The number of equations is related to the level of approximation considered including different physical processes. These equations can be solved numerically; however, an analytical approach for the lowest-order approximation is also presented. This yields explicit expressions, not only for the flow and field variations but also for the magnetosheath thickness, depending on the solar wind parameters. Results are compared to THEMIS data and offer a detailed explanation of, e.g., the pile-up process and the corresponding plasma depletion layer, the bow shock and magnetopause geometry, the magnetosheath thickness, and the flow deceleration.

Cluster observations of hot flow anomalies with large flow deflections: 2. Bow shock geometry at HFA edges

Case and statistical studies of the bow shock geometry at hot flow anomaly (HFA) edges have been performed based on 87 HFAs with large flow deflections observed by the Cluster C1 spacecraft from 2003 to 2009. The results suggest that HFAs can be formed at both quasi‐parallel and quasi‐perpendicular shocks. In an accompanying paper, we show that the ions might be near‐specularly reflected at the bow shock and interact with the solar wind to form HFAs. The guiding center of specularly reflected ions will typically be swept downstream to the bow shock at quasi‐perpendicular shocks. However, this study shows that in at least 13 of these 87 (15%) HFAs, both the leading and trailing edges are at quasi‐perpendicular shocks. These HFAs show a high gyration velocity and a high fast magneto‐sonic Mach number, increasing the gyro‐radius and the possibility of pitch angle scattering, which might help the ions escape from the bow shock and move upstream. In addition, HFAs with both edges at quasi‐perpendicular shocks are closer to the bow shock than those with both edges at quasi‐parallel shocks. This might help the reflected ions at a quasi‐perpendicular shock interact with the incident solar wind immediately after the reflection and increase the possibility of HFA formation.

Investigation of MHD Impact on Argon Plasma Flows by Variation of Magnetic Flux Density

The interaction between a probe body and argon plasma flow is investigated to examine to what extent the probe head temperature and the bow shock distance can be influenced by applying a strong magnetic field. The experiments are performed using a strong permanent magnet installed inside a probe body with a spherical, coated probe head. Former investigations showed strong influence on the bow shock geometry but also on the inflow plasma jet. Several boundary conditions have been varied to evaluate their influence on the experiment. For an uncoated probe head the measured MHD impact was found to be of the same order of magnitude as for the coated case. Electrical isolation of the probe toward the vacuum chamber yielded only slight influence. The variation of the field strength was achieved by changing the amount of magnet segments installed. Pictures were analyzed to document the MHD interaction for each test case. It was found that the bow shock distance increased and the temperature of the probe head decreased while increasing the magnetic field density. This analysis precedes a thorough characterization of the plasma condition.

New approach for determining the normal of the bow shock based on Cluster four‐point magnetic field measurements

We introduce a new approach to determine the normal of Earth’s bow shock, based on four‐point magnetic field measurements. The method obtains the local bow shock normal, based on the assumption that the normal is in the opposite direction to the gradient of the magnetic pressure or magnetic strength (within the shock front). This gradient can be deduced from the four‐point magnetic field measurements of Cluster. Applying the method, we calculate the normal of the bow shock for 28 Cluster crossing events from March to April of 2002 and February to March of 2004 and compare the results for the boundary orientation to commonly known methods, i.e., minimum variance analysis and the coplanarity theorem. In addition, we compare results of the normal obtained by four spacecraft timing analysis, under the assumptions of local planarity and constant velocity of the boundary (triangulation). It is known that minimum variance analysis often fails because of the temporal variation of the bow shock and coplanarity analysis often fails because of the existence of an overshoot or poor determination of the downstream magnetic field due to plasma disturbance. We find that the gradient analysis agrees most closely with the timing analysis, and, in general, the shock orientations obtained fit the expected bow shock geometry. The gradient method is valid for quasi‐stable shocks and does not require the conditions of coplanarity or constant velocity of the bow shock to be satisfied. Its accuracy depends on this stability and the relative spatial scale of the shock, which should be large compared to the spacecraft configuration. A feature of the method, however, is that it provides an estimate of the normal point by point in time, allowing the nature of the spacecraft sampling to be monitored in terms of behavior as each shock event is traversed.

Multiple spacecraft observations of energetic ions during a high solar wind pressure event

On 28 June 1999 the Wind spacecraft (near the forward libration point) observed a large solar wind pressure spike from 0445 UT to 0600 UT. The Polar satellite at 7 hours magnetic local time detected an energetic particle event in the high‐altitude region associated with turbulent diamagnetic cavities from 0512 UT to 0627 UT. The particles and cavities are very similar to those that were previously found in the high‐altitude dayside cusp region. They are independent of both the solar wind velocity and the interplanetary magnetic field (IMF) vectors. The enhancements of the magnetic field fluctuations in the ultra‐low frequency range measured by the Wind were also observed by the Polar. Most of the time, during the event period, the IMF had a duskward component, suggesting that cusp diamagnetic cavities also existed in the postnoon sector in the Northern Hemisphere. Energetic ions of both ionospheric origin and solar wind origin were observed by the Polar spacecraft during this event period. The He++/H+ ratio in the diamagnetic cavities was a factor of four higher than in the quasi‐trapping region before the event onset, while the He+/He++ ratio in the cavities was more than one order of the magnitude lower. In this event, the measured cusp ions had energies up to 4 MeV. No clear relationship between the cusp energetic ion flux and the IMF cone angle was found. The Interball 1 spacecraft located just upstream of the bow shock in the prenoon sector measured an upstream ion event from about 0516 UT to 0600 UT. The onset of the energetic ions observed by Interball 1 in the upstream event was the same for different energies; the ion energy spectra were independent of the solar wind velocity and their intensities were independent of the bow shock geometry and the solar wind density. The energetic ion event onset was first detected in the cusp by Polar at 0512 UT, then near the bow shock in the prenoon by Interball 1 at 0516 UT, and then in the far upstream by Wind at 0523 UT. The measured energetic ion intensity decreased with increasing distance from the cusp diamagnetic cavities. These observational facts together with the IMF directions suggest that (1) this large solar wind pressure event produced an extremely large diamagnetic cavity (>10 RE) within the magnetosphere, (2) the bow shock was not the main source of both the cusp and upstream energetic ions, and (3) the upstream energetic ions most likely came from the cusp.

Stellar Bow Shocks in the Northern Arm of the Galactic Center: More Members and Kinematics of the Massive Star Population

We present new 2.2 μm diffraction-limited images from the W. M. Keck 10 m and Gemini 8 m telescopes of the cool Galactic center sources, IRS 1W, 5, 8, 10W, and 21, along with new proper motions for IRS 1W, 10W, and 21. These observations were carried out to test the bow shock hypothesis presented by Tanner et al. as an alternative to a very recent (104 yr) epoch of star formation within the tidal stream of gas and dust known as the northern arm. Resolved asymmetric structure is detected in all the sources, with bow shock morphologies associated with IRS 1W, 5, 8, and 10W. For IRS 1W and 10W, there is an agreement between the position angle of the asymmetry and that of the inferred relative velocity vector of the near-infrared source with respect to the northern arm gas, strengthening the bow shock hypothesis. We therefore conclude that the observed morphology is indeed a bow shock generated by sources plowing through the northern arm. Furthermore, the large extent of the resolved structures (310-1340 AU), along with their luminosities (~104-105 L☉), suggests that their central sources are Wolf-Rayet stars, comparable to the broad He emission-line stars, which have strong winds on the order of 1000 km s-1. The bow shock geometry, along with the proper motion measurements, provide three-dimensional orbital solutions for this enigmatic class of objects. The orientations of the orbital planes of IRS 1W and 10W are consistent with that of the putative clockwise plane which has been proposed as a solution for the He I emission-line stars. While these observations eliminate the need to invoke star formation within the northern arm, they increase by 10% the total known population of massive, young stars with strong winds, whose origin remains unexplained in the context of the nearby supermassive black hole.

Bow shock's geometry at the magnetospheric flanks

The database of IMP 8 bow shock crossings was used to investigate properties of the bow shock's cross section between 10 and 15 RE tailward of Earth in response to selected upstream and magnetospheric parameters. Best‐fitted ellipses were derived for each parameter subset and analyzed. We have found that for average solar wind conditions, the shock's cross section moves in the north‐south directions by 3.8 RE when the dipole tilt changes from sunward to antisunward orientations. Comparisons with results provided by global three‐dimensional MHD simulations of the magnetosphere have shown that the tilt angle effect is likely to be also important for the estimation of the dayside shock wave's position. We have found the orientation of the IMF with respect to the solar wind flow, expressed by the angle θBv, to influence the size and stability of the shock shape/position. The observed Mach number dependence agreed with previous studies and no other significant IMF‐induced asymmetries were found.

Comment on “MeV magnetosheath ions energized at the bow shock” by S.‐W. Chang et al.

[1] From a study of the 4 May 1998 storm event, Chang et al. [2001] (hereinafter referred to as CETAL01) suggested that ‘‘ions are accelerated at the quasi-parallel bow shock to energies as high as 1 MeVand subsequently transported into the magnetosheath during this event’’ and mentioned that ‘‘This is confirmed by a comparison of energetic ion fluxes simultaneously measured in the magnetosheath and at the quasi-parallel bow shock when both regions are likely connected by the magnetic field lines’’ (see their Abstract). After an inspection of the measured energetic ion data, however, one finds that CETAL01 have misplotted the observed ion energy spectrum in the ‘‘magnetosheath’’ (near the cusp) to lower energy which brings it in closer agreement to the flux measured near the quasi-parallel bow shock, making their analysis suspect. In fact, simultaneous measurements at this time indicate that (1) the energetic ion flux near the cusp was about one order of magnitude higher than that near the quasi-parallel bow shock, (2) the energetic ion time signatures were seen first near the cusp then near the bow shock, and (3) the energetic ion flux observed near the bow shock was independent of bow shock geometry. Each of these three facts is sufficient to demonstrate that the quasi-parallel bow shock was not the main source of the energetic ions near the cusp during this event. [2] CETAL01 stated that ‘‘A comparison of Interball and Polar ion spectra can potentially falsify our bow shock source hypothesis and is now the focus of our analysis.’’ In Figure 11 of CETAL01, accordingly, they compared the energetic ion flux measured by Interball near the quasiparallel bow shock with that measured by Polar near the cusp during the interval 1101–1142 UT on 4 May 1998, where their Polar/CEPPAD energetic ion data (open circles in their Figure 11) were taken only from the ion sensor that was looking 90 from the Polar spin axis. Our Figure 1 replots the Interball data (stars) and the Polar/CEPPAD data (open squares) for the same time interval. Comparing Figure 11 of CETAL01 to our Figure 1, we find that they have misplotted the CEPPAD ion energy spectrum near the cusp to the lower energies which reduces the difference between Interball and Polar ion fluxes. A closer inspection of their Figure 11 suggests that for each energy interval (channel) near the cusp they used the lower energy threshold to represent it without taking into account the effective energy passband [McKinnon and Fritz, 1976] of the steep energy spectrum, and the resulting energy spectrum near the cusp shown in their Figure 11 is thus lower than the actually observed spectrum as shown in our Figure 1. [3] This is not the only case where CETAL01 misplotted the observed ion energy spectrum around the cusp to lower than the actually observed spectrum, for in an earlier paper, Chang et al. [1998] (hereinafter referred to as CETAL98) misplotted the MICS (Magnetospheric Ion Composition Sensor) lower energy limit from 1 keV/e to 0.6 keV/e which brought the cusp fluxes into better alignment with ‘‘bow shock ion spectra,’’ and in addition, misplotted the HIT (Heavy Ion Telescope) helium data point in the cusp below the actual observed value of 0.22 He ions (cm-srs-keV/e) 1 at 0837:40-0845:00 UT on 27 August 1996 [CETAL98, Figure 3]. Based upon such misplotted data, CETAL01 stated in their introduction that CETAL98 ‘‘showed that cusp energetic ion spectra (<300 keV e ) matched very well with a large body of bow shock ion spectra.’’ [4] Yet even after misplotting the observed CEPPAD ion spectrum near the cusp to below the actual observed one, the Interball ion flux near the bow shock was still lower than the Polar ion flux near the cusp as shown in their Figure 11, so CETAL01 made another spectral adjustment by increasing the measured Interball ion flux near the bow shock by about 300% to match the repositioned Polar ion flux to obtain their Figure 13. They called it a ‘‘distance correction’’ and made it central to their argument stating that ‘‘an important piece of evidence for the bow shock source is demonstrated in Figure 13.’’ However, it has been reported [Lee, 1982] that the proton flux at 7 RE from the bow shock is almost the same as that at the bow shock at energies larger than 60 keV; that is, no correction is needed JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A8, 1311, doi:10.1029/2002JA009634, 2003

Multiwavelength Spectroscopy of the Bipolar Outflow from Cepheus E

Cepheus E is the site of an exceptional example of a protostellar outflow with a very young dynamical age and extremely high near-infrared luminosity. We combine molecular spectroscopic data from the submillimeter to the near-infrared in order to interpret the rotational excitation of CO and the rovibrational excitation of H2. We conclude that C-type shocks with a paraboloidal bow shock geometry can simultaneously explain all the molecular excitations. Extinction accounts for the deviation of the column densities from local thermodynamic equilibrium. A difference in the extinction between the red- and blueshifted outflow lobes may account for the measured flux difference. The outflow is deeply embedded in a clump of density 105 cm-3, yet a good fraction of atomic hydrogen, about 40%, is required to explain the excitation and statistical equilibrium. We propose that this atomic component arises, self-consistently, from the dissociated gas at the apex of the leading bow shocks and the relatively long molecule reformation time. At least 20 bow shocks are required in each lobe, although these may be subdivided into smaller bows and turbulent shocked regions. The total outflow mechanical power and cooling amounts to over 30 L☉, almost half the source's bolometric luminosity. Nevertheless, only about 6% of the clump mass has been set in outward motion by the outflow, allowing a collapse to continue.

Sudden Radiative Braking in Colliding Hot‐Star Winds

Hot, massive stars have strong stellar winds, and in hot-star binaries these winds can undergo violent collision. Because such winds are thought to be radiatively driven, radiative forces may also play an important role in moderating the wind collision. However, previous studies have been limited to considering how such forces may inhibit the initial acceleration of the companion stellar wind. In this paper we analyze the role of an even stronger radiative braking effect, whereby the primary wind is rather suddenly decelerated by the radiative momentum flux it encounters as it approaches a bright companion. We further show that the braking location and velocity law along the line of centers between the stars can be approximated analytically using a simple one-dimensional analysis. The results of this analysis agree well with a detailed two-dimensional hydrodynamical simulation of the wind collision in the WR + O binary V444 Cygni and demonstrate that radiative braking can significantly alter the bow-shock geometry and reduce the strength of the wind collision. We then apply the derived analytic theory to a set of 14 hot-star binary systems, and conclude that radiative braking is likely to be of widespread importance for wind-wind collisions in WR + O binaries with close to medium separation, D 100 R?. It may also be important in other types of hot-star binaries that exhibit a large imbalance between the component wind strengths.

Comparative study of numerical simulations of the solar wind interaction with Venus

The assumptions and peculiarities of the numerical codes, as well as the spatial resolution of the gas-dynamical and hybrid simulations which are available at present are discussed. As found the present day estimates of O + ion production rates which can effect the bow shock geometry as intensively as observed by Pioneer-Venus-Orbiter are controversial. The disagreement of the gas-dynamical simulations by Breus et al. (Planet. Space Sci. 40, 131–138, 1992) and Stahara and Spreiter ( Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions, Geophysical Monograph 66, p. 345. AGU, 1992) is a result of different numerical implementation of the flow tangency condition which does not permit the specification of the equation of state p = p( ϱ) at the impenetrable obstacle. The numerical code by Breus et al. uses the reflection procedure for the implementation of the flow tangency condition and is insensitive to the equation of state at the obstacle surface. The reflection procedure does not employ explicitly the boundary conditions at the obstacle which the Stahara and Spreiter code does. As a result Stahara and Spreiter's ( Venus and Mars: Atmoshheres, Ionospheres and Solar Wind Interactions, Geophysical Monograph 66, p. 345. AGU, 1992) simulation is in agreement with observations only in the case of an unreasonable value of the electron impact ionization: in several times of the photoionization within the magnetic barrier at Venus. Breus et al. (Planet. Space Sci. 40, 131–138, 1992) agree well with observations if the electron impact ionization is 60–70% of the photoionization there. Brecht and Ferrante ( J. geophys. Res. 96, 11209–11220, 1991) simulated an insignificant effect of the magnetic barrier on the shock geometry. However this simulation is unable to estimate the O + ionization rate to fit the hybrid model with observations. The cells employed by Moore et al. (J. geophys. Res. 96, 7779–7791, 1991) are larger than the thickness of the magnetic barrier region estimated from the magnetometer data by Zhang et al. (J. geophys. Res. 96, 11145–11153, 1991). Thus Moore et al. cannot simulate the magnetic barrier region properly and their prediction of an insignificant effect of the mass-loading under any expectable O + ion production rate should be revised.

Dependence of 50‐keV upstream ion events at IMP 7&amp;8 upon magnetic field bow shock geometry

We present the results of a statistical study of 4 years (1972–1976) of IMP 7 and 8 observations at ∼40 RE of 50‐ to 200‐keV upstream ion events measured with the JHU/APL Energetic Particle Experiment (EPE). We find a monotonic increase in the probability of observing upstream particle events with a decrease in the angle (θBn) between the interplanetary magnetic field (IMF) and the local shock normal at the point where the IMF intersects the bow shock, independent of the length of time of bow shock connection (if it exceeds ∼10 min). We find roughly equal probability of observing an event above a given flux from any portion of the bow shock with the same value of θBn, a growth time of the 50‐ to 200‐keV events of ∼10 min, a maximum attainable flux of ∼2.5×104 (cm²s sr)−1, and a positive correlation between the probability of exceeding a given flux and the 3‐hour Kp index. The results imply that the local structure of the bow shock in the immediate vicinity of the field line connection is the dominant influence in the generation process of energetic upstream particle events and that wave‐particle interactions produce a self‐throttling mechanism that limits the maximum flux of ions escaping the upstream foreshock.

Lower hybrid drift instability with temperature gradient in a perpendicular shock wave

The lower hybrid instability is studied in the perpendicular bow shock geometry including finite beta effects and an electron temperature gradient. The flute (k∥ = 0) mode, which for constant electron temperature is stable at the shock, is destabilized for sufficiently large temperature gradient (εTρi ≳ 1). Numerical solutions are presented for cases in which the ion distribution is either a drifting Maxwellian or consists of two Maxwellians to represent the effect of reflected ions at the shock. Implications of the results concerning ion and electron heating and electron acceleration at the bow shock are also discussed.