Upstream Magnetic Field Research Articles

Effects of the guide field on electron distribution functions in the diffusion region of asymmetric reconnection

Crescent electron distribution functions in the electron diffusion region in asymmetric reconnection with a guide field (strength less than 50% of the upstream magnetic field) are investigated by theory and fully kinetic simulations. Electron motion in the electron diffusion region is a combination of meandering across the current sheet and gyration around the guide field. Combining the two canonical momentum conservation laws in a one-dimensional model with the energy conservation law, we derive the shape of crescent electron distribution functions in both velocity planes: perpendicular to the magnetic field and coplanner with the magnetic field. There are two major effects of the guide field on crescent distribution functions: (1) widening the opening angle of the crescent in a reduced distribution function and (2) the linear cut-off of a distribution function nonparallel to the magnetic field. We also discuss a translational mapping (perpendicular to the linear cut-off) of distribution functions between the X-line and another position. In addition, considering the gyration due to the normal magnetic field, we predict the shape of distribution functions in general positions in the electron diffusion region. The predictions will be useful to identify features of magnetic reconnection in a guide field by satellite observations.

Corrugated Features in Coronal-mass-ejection-driven Shocks: A Discussion on the Predisposition to Particle Acceleration

The study of the acceleration of particles is an essential element of research in the heliospheric science. Here, we discuss the predisposition to the particle acceleration around coronal mass ejections (CMEs)-driven shocks with corrugated wave-like features. We adopt these attributes on shocks formed from disturbances due to the bimodal solar wind, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. In order to understand the role of a wavy shock in particle acceleration, we define three initial smooth shock morphologies each one associated with a fast CME. Using polar Gaussian profiles we model these shocks in the low corona. We establish the corrugated appearance on smooth shock by using combinations of wave-like functions that represent the disturbances from medium and CME piston. For both shock types, smooth and corrugated, we calculate the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions. We consider that corrugated shocks are predisposed to different process of particle acceleration due to irregular distributions of shock normal angles around of the shock. We suggest that disturbances due to CME irregular expansion may be a decisive factor in origin of particle acceleration. Finally, we regard that accepting these features on shocks may be the start point for investigating some questions in the sheath and shock, like downstream-jets, instabilities, shock thermalization, shock stability, and injection particle process.

Evolution of the Suprathermal Proton Population at Interplanetary Shocks

Abstract We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a “hump” profile peaking around ∼10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (5–10 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v −5 is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.

Magnetic Field in the Martian Magnetosheath and the Application as an IMF Clock Angle Proxy

AbstractWe present a study of the magnetic field in the Martian magnetosheath using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and provide a proxy for the upstream interplanetary magnetic field (IMF) clock angles based on the sheath field directions. The magnetic field data from MAVEN taken in the sheath region are organized in the Mars Solar Electric field coordinate system and sorted by the IMF polarity. The sheath magnetic field distributions clearly illustrate the morphology of draped IMF around the planetary obstacle and show strong asymmetries between the quasi‐perpendicular and quasi‐parallel bow shock quadrants as well as a slight asymmetry between the +E and –E quadrants. We have also studied the effects of different drivers on the strengths and clock angles of the sheath magnetic field and find that the sheath field is strongly affected by upstream IMF and solar wind and that Martian crustal fields also have small contributions to the sheath magnetic field strength. Based on the morphology of the draped field in the sheath, we developed a simple IMF clock angle proxy as the clock angle of the orbital‐averaged sheath magnetic field, which works well under steady IMF conditions. This proxy has an error <44° for 75% of all instances. The uncertainty of the proxy depends on upstream conditions and spacecraft orbit geometry. We also provide estimates of the proxy uncertainty based on sheath magnetic field variability and proxy clock angle.

Pulsar Radio Emission Mechanism: Radio Nanoshots as a Low-frequency Afterglow of Relativistic Magnetic Reconnection

Abstract In this Letter we propose that coherent radio emission of the Crab pulsar, other young energetic pulsars, and millisecond pulsars is produced in the magnetospheric current sheet beyond the light cylinder. We carry out global and local 2D kinetic plasma simulations of reconnection to illustrate the coherent emission mechanism. Reconnection in the current sheet beyond the light cylinder proceeds in the very efficient plasmoid-dominated regime, where the current layer gets fragmented into a dynamic chain of plasmoids that undergo successive coalescence. Mergers of sufficiently large plasmoids produce secondary perpendicular current sheets, which are also plasmoid unstable. Collisions of plasmoids with each other and with the upstream magnetic field eject fast magnetosonic waves, which propagate upstream across the background field and successfully escape from the plasma as electromagnetic waves that fall in the radio band. This model successfully explains many important features of the observed radio emission from the Crab and other pulsars with high magnetic field at the light cylinder: phase coincidence with the high-energy emission, nanosecond duration (nanoshots), and extreme instantaneous brightness of individual pulses.

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.

Observational Evidence of Magnetic Reconnection in the Terrestrial Bow Shock Transition Region

AbstractWe report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65°. An ion‐skin‐depth‐scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X‐line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field‐aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.

A Proxy for the Upstream IMF Clock Angle Using MAVEN Magnetic Field Data

AbstractWithout an upstream monitor at Mars to provide a contemporaneous measurement of solar wind conditions, it is useful to have techniques of inferring the upstream solar wind conditions using downstream data. We develop a method to estimate the upstream interplanetary magnetic field (IMF) clock angle, defined as the orientation of the IMF vector in the plane perpendicular to the solar wind velocity, at Mars using magnetometer data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The technique fits MAVEN magnetometer data from within the Martian magnetosheath to a model of the draped magnetic field direction in a coordinate system aligned with the motional electric field. The results provide a proxy for the clock angle that agrees with upstream measurements and reproduces the observed distribution of clock angles. Even if upstream observations are available for a given orbit, the proxy provides an estimate of the clock angle at the time when the spacecraft is in the magnetosheath, which may correct for inherent temporal variability, because the solar wind varies on timescales shorter than the 4.5‐hr MAVEN orbit. The clock angle proxy can be applied for any orientation of the MAVEN orbit, even when MAVEN does not traverse into the upstream solar wind.

Autogenous and efficient acceleration of energetic ions upstream of Earth's bow shock.

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a 'bow shock' forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth's are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as 'hot flow anomalies'4-9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11-13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1-10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth's bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi's 'injection problem', which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.

Strongly localized magnetic reconnection by the super-Alfvénic shear flow.

We demonstrate that the dragging of the magnetic field by the super-Alfvénic shear flows out of the reconnection plane can strongly localize the reconnection x-line in collisionless pair plasmas, reversing the current direction at the x-line. Reconnection with this new morphology, which is impossible in resistive-magnetohydrodynamics, is enabled by the particle inertia. Surprisingly, the quasi-steady reconnection rate remains of order 0.1 even though the aspect ratio of the local x-line geometry is larger than unity, which completely excludes the role of tearing physics. We explain this by examining the transport of the reconnected magnetic flux and the opening angle ma de by the upstream magnetic field, concluding that the reconnection rate is still limited by the constraint imposed at the inflow region. Based on these findings, we propose that this often observed fast rate value of order 0.1 itself, in general, is an upper bound value determined by the upstream constraint, independent of the localization mechanism and dissipation therein.

Discovery of Kolmogorov-like magnetic energy spectrum in Tycho’s supernova remnant by two-point correlations of synchrotron intensity

The spectral slope of the magnetic energy in supernova remnants (SNRs) can be obtained by analysis of spatial two-point correlation functions of synchrotron intensities. This method has been originally developed for the analysis of magnetic field structure in diffuse interstellar medium and applied when the geometry of the emission region is simple and known. In this paper, applying this correlation analysis to Tycho's SNR, for which the synchrotron emission region is known to be a spherical shell, we find that the magnetic energy spectrum shows the Kolmogorov-like scaling. Our results can be explained by turbulence developed downstream of the shock front via Richtmyer-Meshkov instability or an amplification of upstream magnetic field induced by cosmic rays. They could be discriminated by future observations with a sub arcsecond resolution such as Square Kilometer Array.

The Multi-Dimensional Structure of Radiative Shocks: Suppressed Thermal X-rays and Relativistic Ion Acceleration

Radiative shocks, behind which gas cools faster than the dynamical time, play a key role in many astrophysical transients, including classical novae and young supernovae interacting with circumstellar material. The dense layer behind high Mach number $\mathcal{M} \gg 1$ radiative shocks is susceptible to thin-shell instabilities, creating a "corrugated" shock interface. We present two and three-dimensional hydrodynamical simulations of optically-thin radiative shocks to study their thermal radiation and acceleration of non-thermal relativistic ions. We employ a moving-mesh code and a specialized numerical technique to eliminate artificial heat conduction across grid cells. The fraction of the shock's luminosity $L_{\rm tot}$ radiated at X-ray temperatures $kT_{\rm sh} \approx (3/16)\mu m_p v_{\rm sh}^{2}$ expected from a one-dimensional analysis is suppressed by a factor $L(>T_{\rm sh}/3)/L_{\rm tot} \approx 4.5/\mathcal{M}^{4/3}$ for $\mathcal{M} \approx 4-36$. This suppression results in part from weak shocks driven into under-pressured cold filaments by hot shocked gas, which sap thermal energy from the latter faster than it is radiated. Combining particle-in-cell simulation results for diffusive shock acceleration with the inclination angle distribution across the shock (relative to an upstream magnetic field in the shock plane$-$the expected geometry for transient outflows), we predict the efficiency and energy spectrum of ion acceleration. Though negligible acceleration is predicted for adiabatic shocks, the corrugated shock front enables local regions to satisfy the quasi-parallel magnetic field geometry required for efficient acceleration, resulting in an average acceleration efficiency of $\epsilon_{\rm nth} \sim 0.005-0.02$ for $\mathcal{M} \approx 12-36$, in agreement with modeling of the gamma-ray nova ASASSN-16ma.

Precursor Wave Emission Enhanced by Weibel Instability in Relativistic Shocks

Abstract We investigated the precursor wave emission efficiency in magnetized purely perpendicular relativistic shocks in pair plasmas. We extended our previous study to include the dependence of upstream magnetic field orientations. We performed two-dimensional particle-in-cell simulations and focused on two magnetic field orientations: the magnetic field in the simulation plane (i.e., in-plane configuration) and that perpendicular to the simulation plane (i.e., out-of-plane configuration). Our simulations in the in-plane configuration demonstrated that not only extraordinary but also ordinary mode waves are excited. We quantified the emission efficiency as a function of the magnetization parameter σ e and found that the large-amplitude precursor waves are emitted for a wide range of σ e . We found that especially at low σ e , the magnetic field generated by Weibel instability amplifies the ordinary mode wave power. The amplitude is large enough to perturb the upstream plasma, and transverse density filaments are generated as in the case of the out-of-plane configuration investigated in the previous study. We confirmed that our previous conclusion holds regardless of upstream magnetic field orientations with respect to the two-dimensional simulation plane. We discuss the precursor wave emission in three dimensions and the feasibility of wakefield acceleration in relativistic shocks based on our results.

The Morphology of the Solar Wind Magnetic Field Draping on the Dayside of Mars and Its Variability

AbstractThe magnetic field draping pattern in the magnetosheath of Mars is of interest for what it tells us about both the solar wind interaction with the Mars obstacle and the use of the field measured there as a proxy for the upstream interplanetary magnetic field (IMF) clock angle. We apply a time‐dependent, global magnetohydrodynamic model toward quantifying the spatial and temporal variations of the magnetic field draping direction on the Martian dayside above 500‐km altitude. The magnetic field and plasma are self‐consistently solved over one Mars rotation period, with the dynamics of the field morphology considered as the result of the rotation of the crustal field orientation. Our results show how the magnetic field direction on the plane perpendicular to the solar wind flow direction gradually departs from the IMF as the solar wind penetrates toward the obstacle and into the tail region. This clock angle departure occurs mainly inside the magnetic pileup region and tailward of the terminator plane, exhibiting significant dawn‐dusk and north‐south asymmetries. Inside the dayside sheath region, the field direction has the greatest departure from the IMF‐perpendicular component direction downstream of the quasi‐parallel bow shock, which for the nominal Parker spiral is over the dawn quadrant. Thus, the best region to obtain an IMF clock angle proxy is within the dayside magnetosheath at sufficiently high altitudes, particularly over subsolar and dusk sectors. Our results illustrate that the crustal field has only a mild influence on the magnetic field draping direction within the magnetosheath region.

Perpendicular relativistic shocks in magnetized pair plasma

Perpendicular relativistic ($\gamma_0=10$) shocks in magnetized pair plasmas are investigated using two dimensional Particle-in-Cell simulations. A systematic survey, from unmagnetized to strongly magnetized shocks, is presented accurately capturing the transition from Weibel-mediated to magnetic-reflection-shaped shocks. This transition is found to occur for upstream flow magnetizations $10^{-3}<\sigma<10^{-2}$ at which a strong perpendicular net current is observed in the precursor, driving the so-called current-filamentation instability. The global structure of the shock and shock formation time are discussed. The MHD shock jump conditions are found in good agreement with the numerical results, except for $10^{-4} < \sigma < 10^{-2}$ where a deviation up to 10\% is observed. The particle precursor length converges toward the Larmor radius of particles injected in the upstream magnetic field at intermediate magnetizations. For $\sigma>10^{-2}$, it leaves place to a purely electromagnetic precursor following from the strong emission of electromagnetic waves at the shock front. Particle acceleration is found to be efficient in weakly magnetized perpendicular shocks in agreement with previous works, and is fully suppressed for $\sigma > 10^{-2}$. Diffusive Shock Acceleration is observed only in weakly magnetized shocks, while a dominant contribution of Shock Drift Acceleration is evidenced at intermediate magnetizations. The spatial diffusion coefficients are extracted from the simulations allowing for a deeper insight into the self-consistent particle kinematics and scale with the square of the particle energy in weakly magnetized shocks. These results have implications for particle acceleration in the internal shocks of AGN jets and in the termination shocks of Pulsar Wind Nebulae.

Simulations of stellar winds and planetary bodies: Magnetized obstacles in a super-Alfvénic flow with southward IMF

This study addresses the issue of the electromagnetic interactions between a stellar wind and planetary magnetospheres with various dipole field strengths by means of hybrid simulations. Focus is placed on the configuration where the upstream plasma magnetic field is parallel to the planetary magnetic moment (also called “Southward-IMF” configuration), leading to anti-parallel magnetic fields in the dayside interaction region. Each type of plasma interaction is characterized by means of currents flowing in the interaction region. Reconnection triggered in the tail in such configuration is shown to affect significantly the structure of the magnetotail at early stages. On the dayside, only the magnetopause current is observable for moderate planetary dipole field amplitude, while both bow-shock and magnetotail currents are identifiable downtail from the terminator. Strong differences in term of temperature for ions are particularly noticeable in the magnetosheath and in the magnetotail, when the present results are compared with our previous study, which focused on “Northward-IMF” configuration.

Influence of asymmetries in the magnetic draping pattern at Titan on the emission of energetic neutral atoms

We model the emission of energetic neutral atoms (ENAs) that are generated by the interaction between energetic ions from Saturn's magnetosphere and neutrals from the upper atmosphere of the giant planet's largest moon Titan. The trajectories of the parent ions and the resulting ENA emission morphology are highly sensitive to the electromagnetic field configuration near the moon. We therefore compare the ENA emission pattern for spatially homogeneous fields to the emission obtained from a magnetohydrodynamic (MHD) and a hybrid (kinetic ions, fluid electrons) model of Titan's magnetospheric interaction, by computing the trajectories of several billion energetic test particles. While the MHD model takes into account the draping of the magnetic field lines around Titan, the hybrid approach also considers the significant asymmetries in the electromagnetic fields due to the large gyroradii of pick-up ions from Titan's ionosphere. In all three models, the upstream parameters correspond to the conditions during Cassini's TA flyby of Titan. The shape, magnitude, and location of the ENA emission maxima vary considerably between these three field configurations. The magnetic pile-up region at Titan's ramside deflects a large number of the energetic parent ions, thereby reducing the ENA flux. However, the draped magnetic field lines in Titan's lobes rotate the gyration planes of the incident energetic ions, thereby facilitating the observable ENA production. Overall, the ENA flux calculated for the MHD model is weaker than the emission obtained for the electromagnetic fields from the hybrid code. In addition, we systematically investigate the dependency of the ENA emission morphology on the energy of the parent ions and on the upstream magnetic field strength.

Radio signatures of shock-accelerated electron beams in the solar corona

Context.The Sun’s activity can appear in terms of radio bursts. In the frequency range 8−33 MHz the radio telescope URAN-2 observed special fine structures appearing as a chain of stripes of enhanced radio emission in the dynamic radio spectrum. The chain drifts slowly from 26 to 23 MHz within 4 min. The individual structures consist of a “head” at the high-frequency edge and a “tail” rapidly drifting from the “head” to lower frequencies over an extent of ≈10 MHz within 8 s. Since they resemble the well-known “herring bones” in type II radio bursts, they are interpreted as shock accelerated electron beams.Aims.The electron beams generating these fine structures are considered to be produced by shock drift acceleration (SDA). The beam electrons excite Langmuir waves which are converted into radio waves by nonlinear wave-plasma processes. That is called plasma emission. The aim of this paper is to link the radio spectral data of these fine structures to the theoretical results in order to gain a better understanding of the generation of energetic electrons by shocks in the solar corona.Methods.Adopting SDA for generating energetic electrons, the accelerated electrons establish a beam-like velocity distribution. Plasma emission requires the excitation of Langmuir waves, which is efficient if the velocity of the beam electrons exceeds a few times thermal electron speed. That is the case if the angle between the shock normal and the upstream magnetic field is nearly perpendicular. Hence, the Rankine-Hugoniot relationships, which describe the shock transition in the framework of magnetohydrodynamics, are evaluated for the special case of nearly perpendicular shocks under coronal circumstances.Results.The radio data deduced from the dynamic radio spectrum can be related in the best way to the theoretical results, if the electron beams, which generate these fine structures, are generated via SDA at an almost perpendicular shock, which is traveling nearly horizontally to the surface of the Sun.

Flows, Fields, and Forces in the Mars‐Solar Wind Interaction

AbstractWe utilize suprathermal ion and magnetic field measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, organized by the upstream magnetic field, to investigate the morphology and variability of flows, fields, and forces in the Mars‐solar wind interaction. We employ a combination of case studies and statistical investigations to characterize the interaction in both quasi‐parallel and quasi‐perpendicular regions and under high and low solar wind Mach number conditions. For the first time, we include a detailed investigation of suprathermal ion temperature and anisotropy. We find that the observed magnetic fields and suprathermal ion moments in the magnetosheath, bow shock, and upstream regions have observable asymmetries controlled by the interplanetary magnetic field, with particularly large asymmetries found in the ion parallel temperature and anisotropy. The greatest temperature anisotropies occur in quasi‐perpendicular regions of the magnetosheath and under low Mach number conditions. These results have implications for the growth and evolution of wave‐particle instabilities and their role in energy transport and dissipation. We utilize the measured parameters to estimate the average ion pressure gradient, J × B, and v × B macroscopic force terms. The pressure gradient force maintains nearly cylindrical symmetry, while the J × B force has larger asymmetries and varies in magnitude in comparison to the pressure gradient force. The v × B force felt by newly produced planetary ions exceeds the other forces in magnitude in the magnetosheath and upstream regions for all solar wind conditions.

Revisiting the structure of low-Mach number, low-beta, quasi-perpendicular shocks.

A study of the structure of 145 low-Mach number (M ≤ 3), low-beta (β ≤ 1), quasi-perpendicular interplanetary collisionless shock waves observed by the Wind spacecraft has provided strong evidence that these shocks have large-amplitude whistler precursors. The common occurrence and large amplitudes of the precursors raise doubts about the standard assumption that such shocks can be classified as laminar structures. This directly contradicts standard models. In 113 of the 145 shocks (~78%), we observe clear evidence of magnetosonic-whistler precursor fluctuations with frequencies ~0.1-7 Hz. We find no dependence on the upstream plasma beta, or any other shock parameter, for the presence or absence of precursors. The majority (~66%) of the precursors propagate at ≤45° with respect to the upstream average magnetic field and most (~87%) propagate ≥30° from the shock normal vector. Further, most (~79%) of the waves propagate at least 20° from the coplanarity plane. The peak-to-peak wave amplitudes (δB pk-pk) are large with a range of maximum values for the 113 precursors of ~0.4-13 nT with an average of ~2 nT. When we normalize the wave amplitudes to the upstream averaged magnetic field and the shock ramp amplitude, we find average values of ~40% and ~220%, respectively.