Planetary Bow Shocks Research Articles

Planetary bow shocks: Gasdynamic analytic approach

A new analytical model of the bow shock surface is suggested for reasonably accurate and fast prediction of this boundary's position near obstacles of different shape. For axially symmetric obstacles the model was verified by comparison with experiments and results of gasdynamic code application for a wide range of upstream polytropic indexes, 1.15 < γ < 2, and Mach numbers, 1 < Ms < ∞. The model can also be used for prediction of the bow shock position around nonaxially symmetric obstacles.

On the existence of Alfvén waves in the terrestrial foreshock

Abstract. The terrestrial foreshock is characterised by the existence of large amplitude ultra low frequency waves. The majority of such waves are observed to be left-handed in the spacecraft frame, but are in fact intrinsically right-handed and have been identified as fast-magnetosonic waves. More rarely observed are waves that are right-handed in the spacecraft frame. Cluster four spacecraft observations of such waves are presented and analysed using multi-spacecraft techniques; in particular the k-filtering/wave telescope technique is used. The waves are found to be left-handed and propagating sunwards in the plasma rest frame, and are, therefore, identified as Alfvénic. The convection of the waves anti-sunward in the solar wind flow causes the observed polarisation to be reversed. Generation mechanisms are discussed.Key words. Interplanetary physics (MHD waves and turbulence; planetary bow shocks) – Space plasma physics (wave particle interactions)

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)

Acceleration of interstellar helium pickup ions at the Earth's bow shock: GEOTAIL observation

After ionization in the solar wind, He+ pickup ions of the interstellar origin are expected to be accelerated at heliospheric shocks (the termination shock, CIR (co‐rotating interaction region) shocks, and planetary bow shocks), and to become the seed population for anomalous cosmic rays. Although there are many theoretical attempts to treat the 'injection' process of the helium pickup ions at these shocks, there has been little direct observational evidence. In this report we present in situ identification of the initial acceleration process of pickup He+ at the bow shock.

Three-dimensional magnetohydrodynamic simulations on the interaction of interplanetary density pulses with the bow shock

Three-dimensional magnetohydrodynamic (MHD) simulations are carried out in order to study the interaction of upstream density pulses with the planetary bow shock. In this work, three distinguished types of the density pulse have been simulated: (1) Pure density pulse with no variations in all other variables, (2) density pulse in fast/slow mode-like type, and (3) another pure density pulse with a much stronger peak. For the first case, where the peak density is two times the background value, its interaction with the bow shock results in various MHD waves and a shock in the magnetosheath, including the fast mode wave which evolves into a fast shock, the entropy wave, and the slow mode wave. For the second case, the amplitude of the transmitted density pulse in the magnetosheath is reduced overall as compared to that in the first case. This is due to the generation of the stronger fast mode wave in the magnetosheath, and the effect is more pronounced for the fast mode-like pulse than for the slow mode-like pulse. When the third type pulse is incident with the peak density being five times the background value, the magnetosheath is more strongly compressed, which is followed by an expansion due to the sunward repulsion and then a second compression again. This still results in similar MHD waves and a shock in the magnetosheath, but they are generated in manners well-distinguished from the previous two cases. Based on the results from all three cases, it is suggested that the density amplitude and the properties of its related waves/shock in the magnetosheath can appear quite different depending on the observing spacecraft’s position, and thus necessitating a multispacecraft measurement for a reliable monitor.

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.

Cluster magnetic field observations of the bowshock: Orientation, motion and structure

Abstract. Four spacecraft Cluster magnetic field observations of the low <beta> quasi-perpendicular terrestrial bowshock are presented for the first time. Multiple quasi-perpendicular crossings on 25 December 2000 are analysed. By combining data from the four spacecraft, bowshock orientations and velocities can be calculated. It is shown that, even while in rapid motion, the bowshock normal direction remains remarkably constant, and that coplanarity estimates are accurate to, typically, around 20°. Magnetic field magnitude profiles are shown to be very well correlated between spacecraft although downstream waves with fluctuations perpendicular to the local field, while statistically similar at all four spacecraft, are poorly correlated on separation scales of several hundred km. Examples are shown of a number of bowshock phenomena, including non-standing fluctuations in the shock foot and the shock interacting with changing solar wind conditions.Key words. Interplanetary physics (planetary bow shocks) Space plasma physics (shock waves; waves and instabilities)

Observations of the spatial and temporal structure of field-aligned beam and gyrating ring distributions at the quasi-perpendicular bow shock with Cluster CIS

Abstract. During the early orbit phase, the Cluster spacecraft have repeatedly crossed the perpendicular Earth’s bow shock and provided the first multi-spacecraft measurements. We have analyzed data from the Cluster Ion Spectrometry experiment (CIS), which observes the 3D-ion distribution function of the major species in the energy range of 5 eV to 40 keV with a 4 s resolution. Beams of reflected ions were observed simultaneously at all spacecraft locations and could be tracked from upstream to the shock itself. They were found to originate from the same distribution of ions that constitutes the reflected gyrating ions, which form a ring distribution in the velocity space immediately upstream and downstream of the shock. This observation suggests a common origin of ring and beam populations at quasi-perpendicular shocks in the form of specular reflection and immediate pitch angle scattering. Generally, the spatial evolution across the shock is very similar on all spacecraft, but phased in time according to their relative location. However, a distinct temporal structure of the ion fluxes in the field-aligned beam is observed that varies simultaneously on all spacecraft. This is likely to reflect the variations in the reflection and scattering efficiencies.Key words. Interplanetary physics (planetary bow shocks; energetic particles; instruments and techniques)

Electron Acceleration in the Heliosphere

We review the evidence for electron acceleration in the heliosphere putting emphasis on the acceleration processes. There are essentially four classes of such processes: shock acceleration, reconnection, wave particle interaction, and direct acceleration by electric fields. We believe that only shock and electric field acceleration can in principle accelerate electrons to very high energies. The shocks known in the heliosphere are coronal shocks, traveling interplanetary shocks, CME shocks related to solar type II radio bursts, planetary bow shocks, and the termination shock of the heliosphere. Even in shocks the acceleration of electrons requires the action of wave particle resonances of which beam driven whistlers are the most probable. Other mechanisms of acceleration make use of current driven instabilities which lead to electron and ion hole formation. In reconnection acceleration is in the current sheet itself where the particles perform Speiser orbits. Otherwise, acceleration takes place in the slow shocks which are generated in the reconnection process and emanate from the diffusion region in the Petschek reconnection model and its variants. Electric field acceleration is found in the auroral zones of the planetary magnetospheres and may also exist on the sun and other stars including neutron stars. The electric potentials are caused by field aligned currents and are concentrated in narrow double layers which physically are phase space holes in the ion and electron distributions. Many of them add up to a large scale electric field in which the electrons may be impulsively accelerated to high energies and heated to large temperatures.

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.

Disintegration of trans‐Alfvénic shocks due to variable viscosity and resistivity

The evolution of magnetohydrodynamic shock waves depends on dissipation properties of the medium. The increase or decrease of dissipation coefficients results in a larger or smaller thickness of fast and slow shocks. This is not the case for trans‐Alfvénic shock waves (TASWs), i.e., such at which the flow velocity passes through the Alfvén velocity. In the present paper, it is shown that the variation of dissipation coefficients may cause disintegration of TASWs or diffusion‐like change of their upstream and downstream states. This mechanism is complementary to the accumulation of an Alfvén‐type perturbation inside the shock transition layer. It is also demonstrated that if this variation has a cyclic nature, the TASW undergoes oscillatory disintegration. In this process, it repeatedly transforms into another TASW and emits a nonlinear wave train consisting of shock and rarefaction waves. The implications for planetary bow shocks and shocks induced by CMEs are discussed. In the observational data the disintegration may show up as an unusual position of the bow shock under typical conditions of the solar wind and as a multiple, time‐varying structure of the CME leading edge.

Energy time dispersion of a new class of magnetospheric ion events observed near the Earth's bow shock

Abstract. We have analyzed high time resolution (\\geq6 s) data during the onset and the decay phase of several energetic (\\geq35 keV) ion events observed near the Earth's bow shock by the CCE/AMPTE and IMP-7/8 spacecraft, during times of intense substorm/geomagnetic activity. We found that forward energy dispersion at the onset of events (earlier increase of middle energy ions) and/or a delayed fall of the middle energy ion fluxes at the end of events are often evident in high time resolution data. The energy spectra at the onset and the decay of this kind of events show a characteristic hump at middle (50-120 keV) energies and the angular distributions display either anisotropic or broad forms. The time scale of energy dispersion in the ion events examined was found to range from several seconds to \\sim1 h depending on the ion energies compared and on the rate of variation of the Interplanetary Magnetic Field (IMF) direction. Several canditate processes are discussed to explain the observations and it is suggested that a rigidity dependent transport process of magnetospheric particles within the magnetosheath is most probably responsible for the detection of this new type of near bow shock magnetospheric ion events. The new class of ion events was observed within both the magnetosheath and the upstream region.Key words. Interplanetary physics (energetic particles; planetary bow shocks)

Spatial distribution of upstream magnetospheric ≥50 keV ions

Abstract. We present for the first time a statistical study of \\geq50 keV ion events of a magnetospheric origin upstream from Earth's bow shock. The statistical analysis of the 50-220 keV ion events observed by the IMP-8 spacecraft shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s-1 and values of the index Kp > 2. The statistical results are consistent with (1) preferential leakage of ~50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of ~50 keV ions within the magnetosheath, and (3) final escape of magnetospheric ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290-500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events, with the occurrence frequency ranging between ~16%-~34% in the upstream region.Key words. Interplanetary physics (energetic particles; planetary bow shocks)

Electron plasma waves beyond the Mars' bow shock: The PWS/Phobos-2 observations

Abstract Electron plasma waves are commonly observed at or near the ambient solar wind plasma frequencies beyond planetary bow shocks. They are correlated with times when the interplanetary magnetic field is connected to the shock front, i.e., when entering the electron foreshock region in which suprathermal electrons accelerated at the bow shock are streaming back from it. Such electron plasma oscillations have been observed upstream of the Martian bow shock by the plasma wave system, PWS, on the Phobos 2 spacecraft. Phobos 2 has been operational in Martian orbits for almost two months. From January 29 to March 27, 1989, the Martian shock was crossed at least a hundred times and the upstream region extensively probed. The aim of this paper is to present the characteristics of the Martian electron plasma waves and to compare them with the Earth's and Venus'ones.

Cyclic evolution of structurally unstable shock waves

Trans-Alfvénic shocks in a plasma with a finite Larmor radius are considered. It is shown that such shocks are subject to structural instability, i.e., the instability with respect to disintegration into several other structures. Furthermore, the disintegration is a repetitive process, and the shock evolution has a cyclic nature. Implications of the obtained results for interplanetary shocks and planetary bow shocks are discussed. It is suggested that the cyclic behavior is the reason why the trans-Alfvenic shocks are observed in the interplanetary space much more rarely than fast, stable, shocks.

Collisionless electrons in a thin high Mach number shock: dependence on angle and <font face="Symbol" ><b><i>b</i></b></font>

Abstract. It is widely believed that electron dynamics in the shock front is essentially collisionless and determined by the quasistationary magnetic and electric fields in the shock. In thick shocks the electron motion is adiabatic: the magnetic moment is conserved throughout the shock and v2^ ∝ B. In very thin shocks with large cross-shock potential (the last feature is typical for shocks with strong electron heating), electrons may become demagnetized (the magnetic moment is no longer conserved) and their motion may become nonadiabatic. We consider the case of substantial demagnetization in the shock profile with the small-scale internal structure. The dependence of electron dynamics and downstream distributions on the angle between the shock normal and upstream magnetic field and on the upstream electron temperature is analyzed. We show that demagnetization becomes significantly stronger with the increase of obliquity (decrease of the angle) which is related to the more substantial influence of the inhomogeneous parallel electric field. We also show that the demagnetization is stronger for lower upstream electron temperatures and becomes less noticeable for higher temperatures, in agreement with observations. We also show that demagnetization results, in general, in non-gyrotropic down-stream distributions.Key words. Interplanetary physics (interplanetary shocks; planetary bow shocks)

Injection and acceleration of H<sup>+</sup> and He<sup>2+</sup> at Earth's bow shock

Abstract. We have performed a number of one-dimensional hybrid simulations (particle ions, massless electron fluid) of quasi-parallel collisionless shocks in order to investigate the injection and subsequent acceleration of part of the solar wind ions at the Earth's bow shock. The shocks propagate into a medium containing magnetic fluctuations, which are initially superimposed on the background field, as well as generated or enhanced by the electromagnetic ion/ion beam instability between the solar wind and backstreaming ions. In order to study the mass (M) and charge (Q) dependence of the acceleration process He2+ is included self-consistently. The upstream differential intensity spectra of H+ and He2+ can be well represented by exponentials in energy. The e-folding energy Ec is a function of time: Ec increases with time. Furthermore the e-folding energy (normalized to the shock ramming energy Ep) increases with increasing Alfvén Mach number of the shock and with increasing fluctuation level of the initially superimposed turbulence. When backstreaming ions leave the shock after their first encounter they exhibit already a spectrum which extends to more than ten times the shock ramming energy and which is ordered in energy per charge. From the injection spectrum it is concluded that leakage of heated downstream particles does not contribute to ion injection. Acceleration models that permit thermal particles to scatter like the non-thermal population do not describe the correct physics.Key words. Interplanetary physics (planetary bow shocks) · Space plasma physics (charged particle motion and acceleration; numerical simulation studies)

Unsteady propagation of collisionless trans‐Alfvénic shocks at large timescales

In a collisional plasma, fast and slow shocks are time stationary. They evolve in such a way that the flow variations remain small under the action of a small perturbation. By contrast, in the case of a trans‐Alfvénic shock wave (TASW), at which the flow velocity passes over the Alfvén velocity, the small perturbation generates finite variation of the initial flow. As a result, the shock disintegrates or transforms into some other unsteady state. In the present paper, the problem of shock propagation is considered for a collisionless plasma with anisotropic pressure. Using a kinetic theory, it is shown that the transition of the flow velocity over the Alfvén velocity at a shock is crucial for its evolution. Namely, the small perturbation makes collisionless TASWs unsteady at timescales large enough compared to the ion gyroperiod. This conclusion is valid for both ordinary TASWs at which the Alfvén Mach number decreases and anomalous TASWs at which it increases. Possible manifestations of unsteady propagation of planetary bow shocks are discussed.

Collisionless electrons in a thin high Mach number shock: dependence on angle and β

<strong class="journal-contentHeaderColor">Abstract.</strong> It is widely believed that electron dynamics in the shock front is essentially collisionless and determined by the quasistationary magnetic and electric fields in the shock. In thick shocks the electron motion is adiabatic: the magnetic moment is conserved throughout the shock and <i>v</i>²<font face="Symbol">_^</font> &#x221D; <i>B</i>. In very thin shocks with large cross-shock potential (the last feature is typical for shocks with strong electron heating), electrons may become demagnetized (the magnetic moment is no longer conserved) and their motion may become nonadiabatic. We consider the case of substantial demagnetization in the shock profile with the small-scale internal structure. The dependence of electron dynamics and downstream distributions on the angle between the shock normal and upstream magnetic field and on the upstream electron temperature is analyzed. We show that demagnetization becomes significantly stronger with the increase of obliquity (decrease of the angle) which is related to the more substantial influence of the inhomogeneous parallel electric field. We also show that the demagnetization is stronger for lower upstream electron temperatures and becomes less noticeable for higher temperatures, in agreement with observations. We also show that demagnetization results, in general, in non-gyrotropic down-stream distributions.<br><br><b>Key words.</b> Interplanetary physics (interplanetary shocks; planetary bow shocks)

Wave observations at the foreshock boundary in the near-Mars space

Wave and plasma measurements carried out by the Phobos-2 spacecraft in the Martian environment indicate that a part of solar wind electrons and ions are reflected and accelerated at the Martian bow shock. These particles streaming back into the solar wind lead to the formation of the foreshock region upstream the planetary bow shock. It is shown that the general structure of the foreshock at Mars is similar to that of the foreshock region upstream the Earth’s bow shock. However, the electric emissions observed at frequencies around 100 Hz at the leading edge of electron foreshock at Mars have been never reported for observations in the similar region at Earth. The burst of these waves occurs before the spacecraft enters the electron foreshock identified with the onset of electron plasma waves and calculations of the magnetic connection to the planetary bow shock.