Bow Shock Position 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.

Three‐dimensional multispecies MHD studies of the solar wind interaction with Mars in the presence of crustal fields

We present the results of model calculations using a multispecies MHD model of the interaction of the solar wind with Mars. The three ions considered are H+, O2+, and O+, representing the solar wind and the two major ionospheric ion species, respectively. The calculations indicate that the presence of a hot oxygen corona does not, within the resolution and accuracy of the model, lead to any significant effect on the dayside bow shock and ionopause positions. Next the trans‐terminator fluxes and escape fluxes down the tail were calculated neglecting the effects of the crustal magnetic field. The calculated flux values are consistent with the measured escape fluxes and the calculated limiting fluxes from the dayside ionosphere. Finally, a 60‐order harmonic expansion model of the measured magnetic field was incorporated into the model. The crustal magnetic field did not cause major distortions in the bow shock but certainly had an important effect within the magnetosheath and on the apparent altitude of the ionopause. The model results also indicated the presence of “minimagnetocylinders,” consistent with the MGS observations. We also recalculated the trans‐terminator and escape fluxes, for the nominal solar wind case, in the presence of the crustal magnetic field and found, as expected, that there is a decrease in the calculated escape flux; however, it is still reasonably close to the value estimated from the Phobos‐2 observations.

The Martian magnetosheath: how Venus-like?

The planets Mars and Venus, because of their weak global magnetic fields, have small-scale magnetosheaths amenable to detailed analysis and model comparisons. In this paper we examine some of the similarities and contrasts between the Venus and Mars cases based on Pioneer Venus Orbiter (PVO) magnetometer observations from the PVO prime mission, and Mars Global Surveyor magnetometer observations obtained during the MGS Science Phasing Orbits (March–September, 1998). The combination of a mass-loaded magnetohydrodynamic magnetosheath model and a data-based model of the Martian crustal fields is used to illustrate the differences produced by the presence of the Martian crustal fields. While Venus at solar maximum exhibits a nearly classical magnetosheath formed by the solar wind interaction with a practically impenetrable blunt body, Mars in late 1998 represents a complicated obstacle whose own magnetic fields compromise this simplicity within at least several hundred km of the nominal obstacle boundary inferred from the bow shock position. In particular, the results suggest the presence of a thick inner magnetosheath boundary layer when the strong southern hemisphere crustal fields are located on the sunward hemisphere.

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.

On the smooth surface of the northern bow shock of Mars, based on Mars Global Surveyor's measurements

A very smooth and time-invariable bow shock of Mars is revealed using Mars Global Surveyor's data. The bow-shock position has been identified using magnetic and electron flux data obtained by the Magnetometer and Electron Reflectometer (MAG/ER) experiment aboard Mars Global Surveyor, in the time period between days 87 and 255 of 1998. From the magnetic field and the electron flux measurements, 148 bow-shock crossings were detected, concentrated mostly on the northern hemisphere of the planet. With these results, a 3D configuration of the bow shock is constructed and presented. The results show that part of the observed bow shock is a surprisingly smooth surface. It is possible that the bow shock is smooth only in the northern hemisphere, since the southern surface is characterized by local magnetic anomalies. Its real shape can only be revealed in a 3D representation in the planetary centered solar ecliptic coordinate system and questions the theoretically expected variation of the bow shock.

The solar wind interaction with Mars: results of three-dimensional three-species MHD studies

The interaction of the solar wind with planets with no or only very weak intrinsic magnetic fields, such as Mars and Venus, involves their ionosphere/atmosphere system. Single-species MHD models, which incorporate some of the important mass-loading processes, have been useful in reproducing numerous observed features at these planets, such as the location of the bow shock, but they do have obvious limitations. Our present 3D MHD model uses three continuity equations in order to consider protons in the solar wind and the two dominant heavy ion species (O + 2 and O +) in the ionosphere, separately. We have used this model to study the interaction processes at Mars. The model results are in general agreement with the average observed bow shock shape and position and predict reasonable locations for the ionopause. The calculated oxygen escape flux down the tail was estimated to be about 3 × 10 25 s −1, which is consistent with Phobos-2 estimates. We also studied the possible effect of the surface crustal magnetic field on the bow shock and the ionosphere of Mars.

MHD effects in the venus magnetosheath including mass loading

We use the “magnetic string” approach to integrate the dissipationless MHD equations for a 3-D flow around planetary obstacles, and apply it to some aspects of the flow in the magnetosheath of Venus. Our method has both analytical and numerical components, and is particularly suited to study the structure of the magnetic barrier (depletion layer). Including pick-up processes, we study the contribution to the structure of the Venus magnetosheath made by the pick-up ions. As one main conclusion, we show that the used method leads to a stand-off bow shock position which in good agreement with observations.

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.

The solar wind interaction with Mars: Locations and shapes of the bow shock and the magnetic pile‐up boundary from the observations of the MAG/ER Experiment onboard Mars Global Surveyor

The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in‐situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile‐up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.

3D multiscale mass loaded MHD simulations of the solar wind interaction with Mars

Abstract We simulate the solar wind interaction with Mars with the help of a three-dimensional MHD model, which includes mass loading by photoionization, electron impact ionization and charge exchange. The planetary exosphere is composed of thermal and hot hydrogen, and thermal and hot oxygen. The model gives a good fit to the average observed bowshock shape and position. We obtain a significant dependence of the bow shock position on the mass production rates. Our results suggest that mass loading terms of hot oxygen are the most significant ones and that mass loading is not important at solar minimum.

3D multi‐fluid MHD studies of the solar wind interaction with Mars

The interaction of the solar wind with planets with no or only very weak intrinsic magnetic fields, such as Mars and Venus, involves their ionospheres. Single fluid MHD models, which incorporate some of the important mass‐loading processes, have been useful in reproducing numerous observed features at these planets, such as the location of the bow shock, but they do have obvious limitations. Our present 3D MHD model is a two‐fluid one, which considers protons and the dominant heavy ions in the ionosphere, separately. We have used this model to study the interaction processes at Mars. The model results are in general agreement with the average observed bow shock shape and position and predict reasonable locations for the ionopause. The calculated oxygen escape flux down the tail was estimated to be 2.7×1025s−1, which is consistent with Phobos‐2 estimates.

Aspects of MHD flow about Venus

We describe the “magnetic string” approach to integrating the dissipationless magnetohydrodynamic (MHD) equations for flow around planetary obstacles and apply it to some aspects of the flow in the magnetosheath of Venus. Our method has both analytical and numerical components and is particularly suited to study the structure of the magnetic barrier (depletion layer). We do not include ion pickup processes and thus discuss only the contribution to the structure of the Venus magnetosheath made by the flow of the shocked solar wind. We work with an interplanetary magnetic field which is directed orthogonal to the solar wind bulk velocity. Magnetic forces on the flow are strongly dependent on the Alfvén Mach number upstream of the bow shock, and one aim of this work is to study the dependence of field and flow quantities in the Venus magnetosheath on this parameter, thus allowing further future comparisons with data under a variety of interplanetary conditions. A second aim is to compare our MHD model results to a synopsis of observations made by the Pioneer Venus Orbiter. As one main conclusion, we show that this method leads, in principle, to a standoff bow shock position in good agreement with observations. We find, namely, that for a low but reasonable Alfvén Mach number, our MHD‐modeled magnetosheath is only ∼ 3.6% thinner in the Sun‐Venus direction than that given by observations. Our method is complementary to three‐dimensional, global MHD simulations of the solar wind‐Venus interaction and offers versatility to modeling other aspects of the complicated interaction of the solar wind with Venus.

A three‐dimensional MHD study of solar wind mass loading processes at Venus: Effects of photoionization, electron impact ionization, and charge exchange

Ionization of hot and cold planetary oxygen ions caused by solar radiation, electron impact, and charge exchange with the solar wind protons lead to a heavy plasma population near Venus which is accelerated and incorporated into the solar wind plasma. lonization of hot and cold hydrogen also enhances the solar wind density and reduces its momentum. Charge exchange of the solar wind plasma with hot and cold planetary hydrogen removes a part of this heavy ion population. We include these processes in a three‐dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Venus and analyze their impact on the magnetic and flow field geometry around Venus. We find that photoionization is the most important of these processes if the change in the plasma composition is taken into account. The ionization processes lead to a strong deceleration of the flow around the planet, which in turn results in changes of the magnetic field. Because of the increasing mean mass and density of the plasma near the ionopause velocity gradients develop in the near terminator region, which lead to a local increase of the magnetic field. The inclusion of mass loading in our model moves the bow shock position outward, close to the location observed by the Pioneer Venus Orbiter magnetometer at solar maximum.

Low-frequency electromagnetic waves and instabilities within the Martian bi-ion plasma

The Martian environment is characterized by the presence of heavy (oxygen) ions of planetary origin which strongly influence the solar wind dynamics, including the bow shock structure and position and may cause additional plasma boundaries in the magnetosheath. In this paper the dispersion characteristics of low-frequency electromagnetic waves (LFEW) in the proton gyrofrequency range are studied. The excitation of these waves results from the relative motion between the solar wind protons and planetary heavy ions, which are considered as unmagnetized and, therefore, may act like a beam in the solar wind. The model takes into account the small extension of the Martian magnetosphere compared with the pickup gyroradius of an exospheric ion. From the dispersion analysis it was found that the most unstable waves with relatively high growth rates propagate oblique to the ambient magnetic field. For small propagation angle to the magnetic field these are right-hand polarized whistler waves in the solar wind frame, and due to Doppler shift they appear near to the proton cyclotron frequency as left-hand polarized waves in the beam (spacecraft) frame. We suggest that the sporadic LFEW emission as seen in the upstream region of Mars by Phobos-2 may indicate the existence of localized “heavy ion bunches” whose origin is relatively unclear, but a possible relation to the Martian moons cannot be excluded. Especially, the so-called Phobos events marked by spectral peaks around the proton cyclotron frequency may be interpreted as signatures of the solar wind interaction with a tenuous gas torus. A comparable situation is known from the AMPTE Ba and Li releases where during the late stages of the experiments an enhanced proton cyclotron emission was observed as well. Another important aspect of LFEW excitation is its role in proton deceleration and heating upstream the bow shock where turbulent processes may provide a strong momentum coupling between the solar wind and the newly generated ions of planetary origin.

Martian planetopause as seen by the plasma wave system onboard Phobos 2

The existence of a plasma boundary between the bow shock and the atmosphere of Mars has been confirmed by the Phobos 2 observations. This boundary is called planetopause, magnetopause, ion‐composition boundary or protonopause, depending upon the authors. A careful examination of plasma wave system (PWS) data has revealed that planetopause signatures are generally clear and not questionable, enabling the calculation of the mean planetopause position up to 16 Martian radii in the distant tail. The planetopause model derived from PWS measurements is compared with models calculated from data sets obtained with other instruments on Phobos 2. With the exception of the transition plasma layer, called mass‐loading boundary by the Automatic Space Plasma Experiment with a rotating analyzer (ASPERA) investigators, the published planetopause, magnetopause/areomagnetopause and ion‐composition boundary locations are similar to the planetopause locations deduced from PWS data. This would suggest that, despite their different names, these boundaries correspond to the same plasma transition region. The PWS observations show that the planetopause position varies in the same way as does the bow shock position and is not significantly affected by the solar wind ram pressure. The absence of correlation between the planetopause position and solar wind ram pressure raises serious questions about the role that possible intrinsic magnetic field may play at the Martian planetopause. Finally, the planetopause is believed to be an ion‐composition discontinuity rather than the “obstacle” predicted by gasdynamic models.

Oxygen ionization rates at Mars and Venus: Relative contributions of impact ionization and charge exchange

Oxygen ion production rates above the ionopauses of Venus and Mars are calculated for photoionization, charge exchange, and solar wind electron impact ionization processes. The latter two require the use of the Spreiter and Stahara (1980) gas dynamic model to estimate magnetosheath velocities, densities, and temperatures. The results indicate that impact ionization is the dominant mechanism for the production of O+ ions at both Venus and Mars. This finding might explain both the high ion escape rates measured by Phobos 2 and the greater mass loading rate inferred for Venus from the bow shock positions.

The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three‐dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.

The Earth's bow shock and magnetopause position as a result of the solar wind-magnetosphere interaction

The paper deals with a study of the variations of the Earth's magnetopause and the bow shock position based on simultaneous measurements by the Prognoz 10 and IMP-8 spacecraft. The real boundary position determined by one spacecraft has been compared with the boundary position calculated from Formisano's model. The solar wind parameters, required for the calculation, have been taken from measurements of the other spacecraft. The differences between the calculated and observed boundary positions are discussed from the point of view of possible influences of different solar wind parameters on these deviations. From our discussion it follows that the magnetopause shape calculated by Olson [(1962), The shape of the tilted magnetopause. J. geophys. Res. 74, 5642.], Fairfield [(1971), Average and unusual location of the Earth's magnetopause and bow shock. J. geophys. Res. 76, 6700.] or Choe et al. [(1973), Precise calculation of the magnetosphere surface for a tilted dipole. Planet. Space Sci. 21, 485.] for a dipole magnetic field seems to be a better approximation than Formisano's three-dimensional fit. On the other hand, Formisano's fit to the bow shock shape can be used for determination of the bow shock position if the additional influence of the interplanetary magnetic field strength is taken into account.

The solar wind interaction with Mars: Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

Observations taken by Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 are used to model the shape, position, and variability of the Martian bow shock for the purpose of better understanding the interaction of this planet with the solar wind. Emphasis is placed upon comparisons with the results of similar analyses at Venus, the only planet known to have no significant intrinsic magnetic field. Excellent agreement is found between Mars bow shock models derived from the earlier Mariner‐Mars data set (24 crossings in 1964–1974) and the far more extensive observations recently returned by Phobos 2 (94 crossings in 1989). The best fit model to the aggregate data set locates the subsolar bow shock at a planetocentric distance of 1.56±0.04 RM. Mapped into the terminator plane, the average distance to the Martian bow shock is 2.66±0.05 RM. Compared with Venus, the bow wave at Mars is significantly more distant in the terminator plane, 2.7 RM versus 2.4 RV, and over twice as variable in location with a standard deviation of 0.49 RM versus 0.21 RV at Venus. The Mars 2, 3, and 5 and Phobos 2 data also contain a small number of very distant dayside shock crossings with inferred subsolar obstacle radii derived from gasdynamic modeling of 2000 to 4000 km. Such distant bow shock occurrences do not appear to take place at Venus and may be associated with the expansion of a small Martian magnetosphere under the influence of unusually low solar wind pressure. Finally, the altitude of the Venus bow shock has a strong solar cycle dependence believed to be due to the effect of solar EUV on the neutral atmosphere and mass loading. Comparison of the Phobos 2 shock observations near solar maximum (Rz = 141) with the Mariner‐Mars measurements taken much farther from solar maximum (Rz = 59) indicates that the Martian bow shock location is independent of solar cycle phase and, hence, solar EUV flux. These results are interpreted in terms of a hybrid solar wind interaction model in which this planet possesses a weak, but significant, intrinsic magnetic field.

Dynamics of the earth's bow shock position

The paper deals with a study of the variations in the Earth's bow shock position based on simultaneous measurements by the Prognoz 10 and IMP-8 satellites. Abrupt variations of solar wind parameters that have been registered by one apparatus are identified with shock crossings as seen by the other one. In the cases of solar wind - magnetosheath crossings the velocity has been calculated also from the data obtained by only one apparatus (the one observing the crossing). Assuming the velocity of discontinuity propagation the same as the velocity of the solar wind, we can calculate the mean shock velocity from the time interval between these events. The shock model proposed by Formisano has been used for the calculations of the bow shock position. Experimental data indicates that the bow shock position depends on the magnetic field more strongly than is usually supposed. On the other hand, the dependence on the solar wind dynamic pressure seems to be weaker than expected. Calculated velocities of the shock front motion lie in the interval of 2 to 60 km/s. For small variations of the solar wind dynamic pressure the calculated velocity is roughly proportional to the magnitude of the variation.