Proton Gyrofrequency Research Articles

Statistical characteristics and significance of low‐frequency instability associated with magnetic dipolarizations in the near‐Earth plasma sheet

Magnetic dipolarization has been considered as a key element of substorm phenomena. In this work we investigate the spectral features of the magnetic dipolarization fluctuations in frequency‐time space by using the continuous wavelet transform technique. We present details of the analysis for three specific examples and the statistical results for 82 magnetic dipolarizations that occurred at X > ∼−11.5 RE in the near‐Earth plasma sheet. We focus on a low‐frequency regime defined here as 0.005 to 0.03 Hz for convenience sake (typically well below local proton gyrofrequency). On the basis of the three specific events, we explicitly demonstrate that the magnetic dipolarization fluctuations can be dominated by intense waves at one or more (typically 2–3) discrete frequencies in the low‐frequency regime. Statistically, we find that this is the case for 59 (about 72%) out of the 82 events. In addition, we find that such a wave starts to grow in amplitude, thus implying occurrence of instability, typically minutes prior to the dipolarization onset time. The estimated exponential growth time is less than 2 min for ∼68% out of the 72% events. The statistically averaged frequency for the strongest wave is ∼0.01Hz, which we argue is in the regime of ballooning instability. All these features are most clearly seen in the compressional component of magnetic fluctuations. For two of the three example events, it is demonstrated that the magnetic fluctuations on the perpendicular plane are linearly polarized for a given frequency while a more comprehensive statistical study of polarization features is left for a future work. On the basis of the results obtained in this work we conclude that the association of low‐frequency instability with substorm‐associated dipolarizations can be quite significant from a statistical viewpoint.

Correlations between the proton temperature anisotropy and transverse high‐frequency waves in the solar wind

Correlations are studied between the power density of transverse waves having frequencies between 0.01 and 1 normalized to the proton gyrofrequency in the plasma frame and the ratio of the perpendicular and parallel temperature of the protons. The wave power spectrum is evaluated from high‐resolution 3D magnetic field vector components, and the ion temperatures are derived from the velocity distribution functions as measured in fast solar wind during the Helios‐2 primary mission at radial distances from the Sun between 0.3 AU and 0.9 AU. From our statistical analysis, we obtain a striking correlation between the increases in the proton temperature ratio and enhancements in the wave power spectrum. Near the Sun the transverse part of the wave power is often found to be by more than an order of magnitude higher than its longitudinal counterpart. Also the measured ion temperature anisotropy appears to be limited by the theoretical threshold value for the ion‐cyclotron instability. This suggests that high‐frequency Alfvén‐cyclotron waves regulate the proton temperature anisotropy.

ION CYCLOTRON WAVES IN THE SOLAR WIND OBSERVED BY STEREO NEAR 1 AU

Using high-resolution magnetic field data from the STEREO mission, we have observed strong narrow-band ion cyclotron waves (ICWs) in the solar wind near 1 AU. The waves propagate nearly parallel to the magnetic field and are below the local proton gyrofrequency in the solar wind frame. Because the twin STEREO spacecraft were far away from any planet, including the Earth, the waves do not have a planetary source. The ICWs often appear when the interplanetary magnetic field is more radial than the nominal Parker spiral. Since the wave frequency in the spacecraft frame is higher than the local proton gyrofrequency, the waves are not locally generated by ion pickup. Observations are consistent with wave generation closer to the Sun and outward transport by the super-Alfvenic solar wind. The waves are intrinsically left-hand polarized in the solar wind frame, but appear to be both left- and right-handed in the spacecraft frame, associated with outward and inward propagation, respectively. The relative amplitudes of the left-handed and right-handed waves support the closer-to-Sun generation scenario.

The Nonlinear Evolution of Ion Cyclotron Waves in the Earth's Magnetosheath

With one-dimensional (1-D) hybrid simulations we investigate the nonlinear evolution of the ion cyclotron waves excited by the H+ and He2+ temperature anisotropies, and analyze the evolution by using the wavelet analysis method. The results show that the proton cyclotron waves with the dominant frequency higher than the helium gyro-frequency (ΩHe = 0.5Ωp, with Ωp and ΩHe the proton and helium gyro-frequencies respectively) are firstly excited, and then the helium cyclotron waves with the dominant frequency lower than the helium gyro-frequency are excited. The relation of our simulation results to the BIF(bifurcated) (there are two peaks in the wave spectrum: one above and one below ΩHe) and CON(continuous) (continuous spectrum from 0.1 Ωp to 1.0 Ωp) wave spectra observed in the magnetosheath are discussed.

Lightning‐induced lower‐hybrid turbulence and trapped Extremely Low Frequency (ELF) electromagnetic waves observed in deep equatorial plasma density depletions during intense magnetic storms

During the early phase of the intense magnetic storm of 7–11 November 2004, the DEMETER satellite encountered large‐scale equatorial plasma density depletions with density decreases of two or three orders of magnitude. Wave measurements carried out inside these depletions show the occurrence of broadband and localized lower‐hybrid turbulence triggered by whistlers propagating from thunderstorm lightning occurring below the orbit path. High‐sample‐rate waveforms reveal that this lower‐hybrid turbulence can evolve into localized large‐amplitude quasi‐monochromatic wave packets similar to lower‐hybrid structures that were, up to now, only observed in the auroral regions, usually on high‐latitude magnetic field lines associated with discrete aurora. These equatorial structures have typical amplitudes of up to 20 mV/m and durations of ∼20–30 ms. Simultaneous thermal ion measurements show that these bursts are often correlated with small‐scale density depletions of 5–10%. Although the lower‐hybrid structures are less intense than those observed in the auroral zone and although their energy source is different, our observations lend support to the idea that the formation of lower‐hybrid structures is an universal mechanism operating in inhomogeneous magnetized space plasmas in the presence of VLF whistler mode turbulence. Besides the lower‐hybrid turbulence, another interesting feature is the occurrence of strong narrowband electromagnetic ELF emissions with amplitudes of a few millivolts per meter at frequencies below the proton gyrofrequency. They are continuously observed throughout the entire depletion. These emissions occur not only within the depletions but also, although less intense, outside of them over a large latitudinal range. They are tentatively identified as magnetospheric line radiations (MLRs) commonly observed during magnetically disturbed periods. Similar events were observed on 15 May 2005 and on 24 August 2005 during two other intense magnetic storms.

Resonant absorption of ULF waves at Mercury's magnetosphere

Ultra low frequency (ULF) waves, which are assumed to be standing waves on the field, are observed by the Mariner 10 spacecraft at Mercury. These waves are oscillating at 38% of the proton gyrofrequency. It is well known that heavy ions, such as Na+, are abundant in Mercury's magnetosphere. Because the presence of different ion species has an influence on plasma dispersion characteristics near the ion gyrofrequencies, magnetospheric eigenoscillations observed at Mercury with frequency in the gyrofrequency range require a multi‐fluid treatment for the plasma. Thus ULF waves at Mercury may have a distinct difference from typical ULF oscillations at Earth, which are often described in terms of magnetohydrodynamics. By adopting a multi‐fluid numerical wave model, we examine how magnetic eigenoscillations occur in Mercury's magnetosphere. Because protons and sodium ions are the main constituents at Mercury, we assume an electron‐proton‐sodium plasma in our model. Our results show: (1) the observed ULF waves are likely compressional waves rather than standing oscillations such as field line resonances (FLRs), (2) FLRs at Mercury are expected to occur when the ion‐ion hybrid and/or Alfvén resonance conditions are satisfied, (3) the magnetic field of FLRs at Mercury's magnetosphere oscillates linearly in the east‐west (azimuthal) meridian when the frequency is located between two ion gyrofrequencies, and (4) the resonance frequency enables us to estimate the local heavy ion concentration ratio.

On nonstationarity and rippling of the quasiperpendicular zone of the Earth bow shock: Cluster observations

Abstract. A new method for remote sensing of the quasiperpendicular part of the bow shock surface is presented. The method is based on analysis of high frequency electric field fluctuations corresponding to Langmuir, upshifted, and downshifted oscillations in the electron foreshock. Langmuir waves usually have maximum intensity at the upstream boundary of this region. All these waves are generated by energetic electrons accelerated by quasiperpendicular zone of the shock front. Nonstationary behavior of the shock, in particular due to rippling, should result in modulation of energetic electron fluxes, thereby giving rise to variations of Langmuir waves intensity. For upshifted and downshifted oscillations, the variations of both intensity and central frequency can be observed. For the present study, WHISPER measurements of electric field spectra obtained aboard Cluster spacecraft are used to choose 48 crossings of the electron foreshock boundary with dominating Langmuir waves and to perform for the first time a statistical analysis of nonstationary behavior of quasiperpendicular zone of the Earth's bow shock. Analysis of hidden periodicities in plasma wave energy reveals shock front nonstationarity in the frequency range 0.33 fBi<f<fBi, where fBi is the proton gyrofrequency upstream of the shock, and shows that the probability to observe such a nonstationarity increases with Mach number. The profiles observed aboard different spacecraft and the dominating frequencies of the periodicities are usually different. Hence nonstationarity and/or rippling seem to be rather irregular both in space and time rather than resembling a quasiregular wave propagating on the shock surface.

Interaction of positive and negative energy waves in a magnetized bi-ion plasma with differential ion streaming

AbstractIn this paper the concept of negative energy waves facilitates the analysis of instability in a magnetized bi-ion plasma with differential ion streaming. There are three frequency regimes in which instability may arise. For frequencies less than the alpha particle gyrofrequency, a negative energy alpha ion-cyclotron mode can interact with a positive energy proton-cyclotron mode. In the intermediate frequency regime lying between the alpha and proton gyrofrequencies, a negative energy alpha-acoustic mode interacts with a positive energy proton-cyclotron mode. In the high-frequency regime above the proton-cyclotron frequency a negative energy alpha-acoustic mode interacts with a positive energy proton-acoustic mode. The resonance (or coalescence) condition which lends itself to a simple geometrical interpretation as the intersection between the proton and alpha mode wave normal diagrams with differential streaming permits the evaluation of the instability growth rate in each frequency regime, which is calculated for both subsonic and supersonic differential streaming.

Linear and nonlinear electromagnetic and electrostatic instabilities in a plasma with two ion beams

Linear and nonlinear properties of a plasma system with a proton and an alpha‐particle beam, are studied. It is shown that the presence of a second beam can change the linear and nonlinear stability thresholds of the magnetosonic instabilities. Since the nonlinear dispersion relation of the nonlinear waves has 14 roots, the system is very rich in parametric decays. There are parametric decays which clearly cannot be present in a one beam system. These parametric decays are of the decay type, modulational, and essentially electromagnetic beat wave instabilities. In a previous paper (Gomberoff, 2007b), the stabilization properties of the magnetosonic instability triggered by the presence of a large amplitude Alfén waves for fix temperature were studied. Here the study is extended to large amplitude waves belonging to the dispersion branch having a resonance at the proton gyrofrequency, for several temperature values. It is shown that the threshold amplitude for stabilization can suffer drastic changes with respect to the one beam system. Finally, it is shown that when both beams trigger their own magnetosonic instabilities, the system can also be stabilized by the presence of the large amplitude wave.

Ground-based observations atL∼ 6 of multi-band structures in VLF hiss

We present ground‐based observations of banded structures (BS) in auroral hiss. This BS hiss was detected at Porojärvi station in Northern Finland, mainly in the local evening from 19 to 23 MLT, during magnetically quiet periods; half of all the events observed corresponded to AE < 50 nT. The structures, which resemble those observed by low‐altitude satellites, can be related to the cyclotron resonant interaction of VLF waves at frequencies near the lower‐hybrid resonance with suprathermal protons in the upper ionosphere and magnetosphere. The maximum probability distribution of band spacing for the events observed at Porojärvi is reached in the frequency range 200–300 Hz, which corresponds to proton gyrofrequencies at L ∼ 6 at altitudes of 4000–3000 km. We found that the BS hiss was detected in the region of evening‐side downward field‐aligned currents and rather intense precipitation of ions with energies 1–30 keV, detected by DMSP spacecraft. We propose that the observed BS hiss can be formed not only due to absorption at the proton gyroharmonics, as is usually assumed for similar structures observed onboard satellites, but also due to generation by energetic protons with an anisotropic velocity distribution.

Magnetic spectral signatures in the terrestrial plasma depletion layer: Hybrid simulations

The electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer (PDL) are characterized by three different spectral categories. They are LOW, where the ion cyclotron waves have a continuous spectrum with main power below 0.5Ωp (Ωp is the proton gyrofrequency); CON, where the main power in the continuous spectrum of the waves can extend from ∼0.1 up to 1.0Ωp; and BIF, where a diminution around 0.5Ωp occurs between two activity peaks in the spectrum. These magnetic fluctuations in the PDL are considered to be the combined effects of two types of ion cyclotron waves: proton cyclotron waves and helium cyclotron waves, which are excited by the H+ and He2+ temperature anisotropies, respectively. In this paper, with one‐dimensional (1‐D) hybrid simulations we investigate the nonlinear evolution of the ion cyclotron waves excited by the H+ and He2+ temperature anisotropies. The proton cyclotron waves with the dominant frequency (the amplitude at that frequency is largest in the spectrum) larger than 0.5Ωp are first excited, and then the helium cyclotron waves with the dominant frequency smaller than 0.5Ωp are excited. The frequencies of the proton cyclotron waves decrease in their corresponding nonlinear stage. For β∥p = 0.1 (where β∥p is the parallel proton plasma beta), the dominant frequency of the proton cyclotron waves remains around 0.62Ωp because of the He2+ absorption around the helium gyrofrequency. Therefore, after the helium cyclotron waves with the dominant frequency around 0.25Ωp are excited, there exist two activity peaks in the spectrum. At the quasi‐equilibrium stage, the amplitude of the proton cyclotron waves is very small because of the He2+ absorption, and a continuous spectrum with main power below 0.5Ωp is formed. When β∥p = 0.3, the effect of the He2+ absorption is very small and can be neglected. The frequencies of the proton cyclotron waves decrease in their nonlinear evolution. After the helium cyclotron waves are excited, their frequency band merges with that of the proton cyclotron waves and forms a continuous spectrum with main power extending above 0.5Ωp. At the quasi‐equilibrium stage, with the decrease of the frequencies of the ion cyclotron waves a continuous spectrum with main power below 0.5Ωp is formed. The relations between the three different spectral categories observed in the PDL and the spectra during the evolution of the ion cyclotron waves are also discussed. The results on the spectrum evolution of the ion cyclotron waves excited by the H+ and He2+ temperature anisotropies may help to explain the magnetic spectral signatures observed in the PDL.

Interferometric identification of ion acoustic broadband waves in the auroral region: CLUSTER observations

We determine the phase velocity and k vector for parallel and oblique broadband extremely low frequency, ELF, waves on nightside auroral magnetic field lines at altitudes around 4.6 RE. We use internal burst mode data from the EFW electric field and wave instrument onboard the Cluster spacecraft to retrieve phase differences between the four probes of the instrument. The retrieved characteristic phase velocity is of the order of the ion acoustic speed and larger than the thermal velocity of the protons. The typical wavelength obtained from interferometry is around the proton gyro radius and always larger than the Debye length. We find that in regions with essentially no suprathermal electrons above a few tens of eV the observed broadband waves above the proton gyro frequency are consistent with upgoing ion acoustic and oblique ion acoustic waves.

Low‐frequency plasma oscillations at Mars during the October 2003 solar storm

The powerful x‐class flare which occurred on the Sun on 28 October 2003 had important effects on plasma environments throughout the solar system. We present here observations of the effects at Mars from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer experiment. In particular we focus on the changes in the nature of the magnetic oscillations observed at an altitude of 400 km (MGS's current orbital altitude) during the passage of the solar storm. We find that strong, regular oscillations are observed in both the B∥ and B⊥ components of the magnetic field at all solar zenith angles. We emphasize in particular the powerful, coherent oscillations observed in the normally quiet nightside region. These oscillations carry power at the proton gyrofrequency and at and below the oxygen gyrofrequency. This implies that ions of planetary origin are interacting with the solar wind plasma and raises the possibility that significant atmospheric loss may occur during the passage of large solar storms at Mars.

Nonlinear inertial and kinetic Alfven waves

A two‐fluid (multifluid) model is used to study nonlinear kinetic (inertial) Alfven waves (solitons, oscillitons, periodic waves). The stationary wave dispersion equation yields the range of the wave speeds in which solitary waves are possible. Kinetic Alfven waves propagating almost perpendicular to the magnetic field exhibit a maximum of the phase speed (a positive dispersion followed by a negative one) which modifies the properties of solitons. At velocities greater than the maximum phase speed, soliton structure exists containing an oscillating core. Such structures, called oscillitons, also exist for other wave modes whose dispersion curves (ω − k) have an inflection point, e.g., the magnetoacoustic (whistler) mode, which gives rise to an extremum in the phase velocity. In a multifluid approach for a bi‐ion plasma, the main properties of oscillatory kinetic Alfven and magnetoacoustic waves are similar to those in a proton‐electron plasma. Assessment of kinetic effects (the Vlasov equations) shows that a maximum of the phase speed of kinetic Alfven wave remains but shifts to smaller k, and therefore the existence of oscilliton‐type solutions with embedded coherent oscillations at f ≤∼ 0.8 fp (fp is the proton gyrofrequency) is expected. In a multi‐ion Maxwellian plasma, a maximum moves further to smaller k (f ≤∼ 0.8fi, where fi is the heavy ion gyrofrequency) and even can cease when oxygen ions are injected to a proton‐electron plasma. The possibility of applying these results to observations carried out on the auroral field lines and near the magnetopause is discussed.

Kinetic Alfvén waves in plasmas with heavy ions

Since its wide identification in measurements by satellites in space plasmas as well as its potential importance in the energization of plasma particles, the kinetic Alfvén wave (KAW) has been discussed extensively in the two-component (electron-proton) plasma model. The component of heavy ions, however, can frequently be found in space and astrophysical plasmas and can affect considerably the properties of the KAWs in them. In this paper, based on a three-component (i.e., electrons, protons, and heavy ions) plasma model, characteristics of KAWs are investigated. The results show that the dispersion of KAWs splits up into two branches. In the long wavelength limit, one branch has an asymptotic solution of shear Alfvén waves, while the other approaches an electrostatic wave with a frequency between the proton and heavy ion gyrofrequencies. Their polarization and resonance are also discussed.

Multi-spacecraft determination of wave characteristics near the proton gyrofrequency in high-altitude cusp

Abstract. We present a detailed study of waves with frequencies near the proton gyrofrequency in the high-altitude cusp for northward IMF as observed by the Cluster spacecraft. Waves in this regime can be important for energization of ions and electrons and for energy transfer between different plasma populations. These waves are present in the entire cusp with the highest amplitudes being associated with localized regions of downward precipitating ions, most probably originating from the reconnection site at the magnetopause. The Poynting flux carried by these waves is downward/upward at frequencies below/above the proton gyrofrequency, which is consistent with the waves being generated near the local proton gyrofrequency in an extended region along the flux tube. We suggest that the waves can be generated by the precipitating ions that show shell-like distributions. There is no clear polarization of the perpendicular wave components with respect to the background magnetic field, while the waves are polarized in a parallel-perpendicular plane. The coherence length is of the order of one ion-gyroradius in the direction perpendicular to the ambient magnetic field and a few times larger or more in the parallel direction. The perpendicular phase velocity was found to be of the order of 100km/s, an order of magnitude lower than the local Alfvén speed. The perpendicular wavelength is of the order of a few proton gyroradius or less. Based on our multi-spacecraft observations we conclude that the waves cannot be ion-whistlers, while we suggest that the waves can belong to the kinetic Alfvén branch below the proton gyrofrequency fcp and be described as non-potential ion-cyclotron waves (electromagnetic ion-Bernstein waves) above. Linear wave growth calculations using kinetic code show considerable wave growth of non-potential ion cyclotron waves at wavelengths agreeing with observations. Inhomogeneities in the plasma on the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear effects or both of these should be taken into account.

Structure of the lunar wake: Two‐dimensional global hybrid simulations

We study the structure and properties of the lunar wake with these solar wind parameters: the angle θsw between directions of the solar wind velocity vsw and the ambient interplanetary magnetic field (IMF) B θsw = 45° and 90° and vsw = 6vA (where vA denotes solar wind Alfvén velocity). We examine the structure of the wake‐tail formed behind the obstacle. In agreement with in situ observations the lunar wake is formed by two counterstreaming beams which fill the wake with a relatively cold, inhomogeneous and highly anisotropic plasma. The results of this study suggest that for given solar wind conditions the downstream region of the lunar wake is dominated by electromagnetic turbulence with the frequencies near the local proton gyrofrequency. The properties and possible generating mechanisms of the low‐frequency electromagnetic turbulence are discussed.

Are Io's Alfvén wings filamented? Galileo observations

The interaction of Io with the Jovian magnetosphere generates auroral and radio emissions. The underlying electron acceleration process is not understood and few observations exist to constrain the theoretical models. The source of energy for the electron acceleration is in all likelihood supplied from the Alfvén wings that stretch out from both poles of Io into the two Jovian hemispheres. The form of the current system associated with the Alfvén wings has been disputed, some suggesting that the greatly slowed flow near Io implies that a steady current loop links Io to Jupiter's ionosphere, others arguing that the return waves appear only downstream of Io and others suggesting that both forms develop. Given the finite inclination of the Alfvén wings implied by the finite value of the Alfvén Mach number and the strong reflection that occurs at the boundary of the Io torus, we argue that no steady current loop can be invoked between Io and Jupiter's ionosphere. However, the energetics of the auroral and radio emissions imply that most of the energy in the Alfvén wings is transformed into electron acceleration at high-latitudes, that is, outside the Io torus. The dilemma then is to understand how a large fraction of the power penetrates the reflecting boundary. We present data from Galileo's multiple flybys of Io that suggest that the coupling with the Jovian ionosphere is mediated by filamentary Alfvén wings associated with electromagnetic waves propagating out of the torus. In particular, we report on the systematic observation, within the cross-section of Io's Alfvén wings and in their immediate vicinity, of intense electromagnetic waves at frequencies up to several times the proton gyrofrequency. We interpret these “high-frequency/small-scale” waves as the signature of a strong filamentation/fragmentation of the Alfvén wings before they reflect off of the sharp boundary gradient of the Io torus. As a consequence, we suggest that most of the primary energy is converted into “high-frequency/small-scale” electromagnetic waves that can propagate out from the torus toward Jupiter's ionosphere. Reaching high-latitudes, these waves are able to accelerate electrons to almost relativistic speeds.

Anomalous Nonresonant Pitch‐Angle Scattering of Low‐Energy Electrons in the Solar Wind

As the translational energy of particles decreases, so does the wavelength of the waves with which these particles can gyroresonate. Very low energy electrons in the solar wind resonantly interact with waves that contain only a tiny fraction of the turbulent wave energy. Their wavenumbers fall into the "dissipative" range of the solar wind turbulence, above the steepening observed at a fraction of the proton gyrofrequency. The problem is famous for MeV-GeV electrons of small pitch-angle cosine μ and is known as the problem of pitch-angle scattering through μ = 0. It can be solved by calculating the contribution of the nonresonant waves at small μ. At sub-MeV energies, the problem of the pitch-angle scattering of electrons is no longer limited to the small values of μ, but extends to the whole range of pitch angles. For these energies the most frequent assumption thus far has been that the electrons propagate scatter-free. The effect of the nonresonant waves is calculated here at larger μ. It is shown that the particle pitch-angle sines can subdiffuse or supradiffuse in logarithmic space as a function of the elapsed distance along the average magnetic field, and the dependence of this anomalous scattering is related to the spectral shape of the fast-mode wave turbulence. Using typical spectra of compressive turbulence in slow and fast solar wind in the inner heliosphere, as well as in the trailing edge of high-speed streams, it is found that the pitch-angle variation on a fraction of the distance traveled by the electrons is comparable to their initial pitch angle and exceeds, down to at least 0.1 AU, the pitch-angle reduction due to adiabatic focusing in the solar wind magnetic fields. Depending on both the level of nonresonant fast-mode wave turbulence and the injection distribution of the electrons, this can invalidate the scatter-free assumption for the propagation of sub-MeV electrons in the inner heliosphere.

Observations of low‐frequency magnetic oscillations in the Martian magnetosheath, magnetic pileup region, and tail

Observations are presented of low‐frequency magnetic oscillations in the Martian magnetosheath, magnetic pileup region, and tail as observed by the Magnetometer/Electron Reflectometer experiment on board Mars Global Surveyor. Within the dayside magnetosheath the oscillations are found to be predominantly compressional, elliptically polarized waves with wave vectors that have large angles relative to the mean field and dominant frequencies that are significantly below the local proton gyrofrequency. On the basis of these observations we identify these oscillations as mirror mode instabilities. In the nightside magnetosheath the oscillations are predominantly transverse and elliptical, and they propagate at smaller angles relative to the mean field at frequencies within a factor of 2–10 less than the local proton gyrofrequency. These characteristics lead us to associate these oscillations with ion/ion‐resonant instabilities that arise from counterstreaming plasma populations such as the solar wind and pickup ions of planetary origin. Within the magnetic pileup region and tail the oscillations have considerably smaller amplitudes and are linearly polarized, obliquely propagating, ultralow‐frequency oscillations which may be a mix of multiple wave modes.