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International audience; The unprecedented detail of measurements by the four Magnetospheric Multiscale (MMS) spacecraft enable deeper investigation of quasi‐perpendicular collisionless shocks. We compare shock normals, planarities, and Normal Incidence Frame (NIF) cross‐shock potentials determined from electric field measurements and proxies, for a subcritical (Fast Magnetosonic Mach number MF = 1.05 ± 0.05) interplanetary shock and a supercritical bow shock (MF = 2.13 ± 0.04). The low‐Mach shock's cross‐shock potential was 26±6V. The shock scale was 33km, too short to allow comparison with proxies from ion moments. Proxies from electron moments provided potential estimates of 40±5V. Shock normals from magnetic field minimum variance analysis were nearly identical, indicating a planar front. The high‐Mach shock's cross‐shock potential was estimated to be from 290 to 440V from the different spacecraft measurements, with shock scale 120km. Reflected ions contaminated the ion‐based proxies upstream, whereas electron‐based proxies yielded reasonable estimates of 250±50V. Shock normals from electric field maximum variance analysis differed, indicating a rippled front.

The Parker Solar Probe (PSP) spacecraft completed its second Venus gravity assist maneuver (VGA2) on 2019 December 26. For a 20 minute interval surrounding closest approach, the PSP/FIELDS Radio Fr...

We present a series of electron holes observed simultaneously on four Magnetospheric Multiscale spacecraft in the plasma sheet boundary layer. The multispacecraft probing shows that the electron ho ...

We consider intense electrostatic solitary waves (ESW) observed in a supercritical quasi-perpendicular Earth's bow shock crossing by the Magnetospheric Multiscale Mission. The ESW have spatial scales of a few tens of meters (a few Debye lengths) and propagate oblique to a local quasi-static magnetic field with velocities from a few tens to a few hundred kilometers per second in the spacecraft frame. Because the ESW spatial scales are comparable to the separation between voltage-sensitive probes, correction factors are used to compute the ESW electric fields. The ESW have electric fields with amplitudes exceeding 600 mV/m (oriented oblique to the local magnetic field) and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature. The negative electrostatic potentials indicate that the ESW are not electron phase space holes, while interpretation in terms of ions phase space holes is also questionable. Whatever is their nature, we show that due to the oblique electric field orientation the ESW are capable of efficient pitch-angle scattering and isotropization of thermal electrons. Due to the negative electrostatic potentials the ESW Fermi reflects a significant fraction of the thermal electrons streaming from upstream (downstream) back to upstream (downstream) region, thereby affecting the shock dynamics. The role of the ESW in electron heating is discussed. Plain Language Summary Processes governing electron thermalization across shock waves are not entirely understood. The high resolution particle and 3-D electric field measurements provided by the Magnetospheric Multiscale Mission make it possible to study the Earth's bow shock that is an excellent laboratory for addressing the electron thermalization across supercritical shock waves. Previous observations showed that electron heating across the bow shock is generally governed by macroscopic cross-shock electrostatic field. On the other hand, the role of the turbulence observed across the bow shock in the electron thermalization has remained unclear. In this letter we consider a particular bow shock crossing by the Magnetospheric Multiscale Mission and focus on the role of the high amplitude electrostatic solitary waves in the electron thermalization process. We accurately estimate the electrostatic solitary wave parameters and show that due to electric fields oriented oblique to a local DC magnetic field and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature, these Debye-scale structures are capable of efficient pitch angle scattering, Fermi reflection, and isotropization of thermal electrons.

We identify electromagnetic field variations from the Van Allen Probes which have the properties of Doppler shifted kinetic scale Alfvenic field line resonances. These variations are observed during injections of energetic plasmas into the inner magnetosphere. These waves have scale sizes perpendicular to the magnetic field which are determined to be of the order of an ion gyro-radius (ρi) and less. Cross-spectral analysis of the electric and magnetic fields reveals phase transitions at frequencies correlated with enhancements and depressions in the ratio of the electric and magnetic fields. Modeling shows that these observations are consistent with the excitation of field-line resonances over a broad range of wave numbers perpendicular to the magnetic field (k⊥) extending to k⊥ρi ≫ 1. The amplitude of these waves is such that E/Bo ≳ Ωi/k⊥ (E, Bo, and Ωi are the wave amplitude, background field strength, and ion gyro-frequency, respectively) leading to ion demagnetization and acceleration for multiple transitions through the wave potential.

On December 6, 1994, during a CIR (Corotating Interaction Region) event, the WIND 3D-Plasma and energetic particle experiment observed a burst (≤ 24 s) of 238-676 keV protons close to the electron foreshock boundary, followed by a 156-236 keV, 101-156 keV and 33-101 keV proton bursts about ∼40 s, 65 s and 85 s later, respectively. Similar dispersed bursts of helium with energies between -50 keV and ∼ 1 MeV followed the proton bursts. During this time, the IMF direction varied slowly with an almost monotonic decrease in θ Bn . The proton energy spectrum is initially peaked at -350 keV, and progresses to lower energies with time. The proton 3D angular distributions are peaked at ∼30° pitch-angle, propagating away from the shock, but they are non-gyrotropic. Finally, we show that the proton energy-spectra as well as the pitch angle distribution agree quantitatively with a model of a single adiabatic reflection of the incident energetic interplanetary ions by the quasi-perpendicular shock.

We present the first observation of the source region of an interplanetary type II radio burst, using instruments on the Wind spacecraft. Type II radio emission tracks the motion of a CME-driven interplanetary (IP) shock which encounters the spacecraft. Upstream of the IP shock backstreaming electrons are observed, first antiparallel to the interplanetary magnetic field (IMF), and then later parallel as well. Langmuir waves are observed concomitant with the shock-accelerated electrons. The electron energy spectrum and Langmuir wave amplitudes are very similar to those observed in the terrestrial electron foreshock. From the connection times to the shock, we infer the existence and characteristic size of large scale structure on the shock front. The type II radio emission seems to be generated in a small bay upstream of the shock, and this may account for some splitting structure observed in the frequency spectrum of many type II bursts.

We present observations of MF/HF ionospheric radio emission observed between 4-7 R e in the terrestrial magnetosphere by the Wind spacecraft. During a perigee pass, and AKR episode, two distinct radio signatures are observed: an intense, sporadic emission near f ≃ 1.8 MHz, and a more steady and weaker emission near f ≃ 4.4 MHz. These emissions show characteristics similar to ground observations of ionospheric 'auroral roar', although to propagate to the spacecraft they must have been generated above the ionospheric F-layer peak. We suggest that the emissions are generated on the topside ionosphere where the condition f uh ≃ nf ce is met. These observations represent a new component of the natural terrestrial radio spectrum as viewed from space.

We examine the phase relation between two orthogonal electric field components for several hundred waveform measurements of intense electron plasma waves in the terrestrial electron foreshock. In general, the phase shift at the carrier frequency is not zero or π as would be expected if the waves were purely electrostatic Langmuir waves, but is a function of the angle between the antennas and the interplanetary magnetic field (IMF). When the antennas are field aligned, the phase shift between the components is large; this value recedes smoothly to zero as the antenna is rotated away from the IMF direction. When solar wind density fluctuations are considered, this is consistent with the dispersion of the electromagnetic z-mode and we assert that the electron foreshock is populated by transverse z-mode waves, not purely longitudinal Langmuir waves. This has implications for conversion to freely propagating modes and large-amplitude saturation mechanisms.

We report observations of electrostatic ion acoustic waves and Langmuir waves during a recent lunar encounter by the Wind spacecraft. These waves are observed when Wind is magnetically connected to the nominal wake and at distances greater than 8 lunar radii from the wake. When interpreted in the context of a simple time-of-flight model, these observations imply the existence of a system of currents and disturbed particle distributions that extends far into solar wind.

Some theories of the generation of 2ωpe radiation, in the upstream solar wind, invoke the notion of nonlinear wave-wave interactions. We present observations, from the WIND spacecraft, of an event in the upstream solar wind with first and second harmonic emissions. The bicoherence spectrum of this event shows coherence amongst components that suggest three-wave interactions of the type described in (Cairns and Melrose, 1985). We discuss this result and its theoretical context.

[1] We investigate statistically the dependence of electron foreshock Langmuir wave activity on the properties of the upstream electron distribution function, using wave and electron measurements from the Wind spacecraft. We find that the presence of a strong strahl beam in the upstream solar wind leads to a significant increase in the occurrence of Langmuir wave activity in the sunward wing of the electron foreshock, generating an asymmetry in the foreshock between the sunward and anti-sunward wings. This asymmetry of electrostatic wave occurrence is likely to be reflected in the radio emission properties of the two foreshock wings. The mechanism behind the asymmetry was first postulated in the context of the Venus foreshock, and may be relevant to recent observations of Langmuir wave activity in solar wind magnetic holes. Furthermore, the likely absence of a strahl-related enhancement at fast forward interplanetary shocks points out a key difference between the physics of electron acceleration at interplanetary shocks and bow shocks.

[1] We report on the finding of a new periodicity in the Jovian decametric radio emission (DAM). Periodic bursts of non-Io component of DAM which recur with a period 1.5% longer than the Jupiter rotation (System III) have been found in the dynamic radio spectra acquired by STEREO/WAVES, Wind/WAVES and Cassini/RPWS during the years 2002–2008. Typically, the bursts appear very periodically over several Jovian days with a decreasing intensity and they display a negative frequency drift. All the bursts were detected within the same sector of Jovian Central Meridian Longitude (III), between 300° and 60° (via 360°) of CML (III), close to the region of the non-Io-C source. No correlation has been found with the position of Io. Considering the simultaneous stereoscopic observations onboard STEREO-A and STEREO-B, as well as Wind and Cassini we can conclude that the sources of the periodic bursts most probably sub-corotate with Jupiter.

International audience; Daily variations of terrestrial auroral kilometric radiation (AKR) are considered; an effect that is detected in STEREO/WAVES data. It has been found that the intensities of the AKR emitted from Northern and Southern sources are modulated with a period of ~24 hours. The occurrence frequency of the AKR has been shown to be strongly dependent on the orientation of the rotating oblique magnetic dipole of the Earth relative to the Sun. AKR is found to occur more often and emit in a broader frequency range when the axis of the terrestrial magnetic dipole in the given hemisphere is oriented toward the nightside. We suggest that the observed ~24 h variations of AKR are connected with diurnal changes of the ambient plasma density in the auroral region.

[1] The normal electric field structure of a supercritical (Mms = 5.2), quasiperpendicular (θBn = 70°) collisionless shock is examined using Cluster four-spacecraft observations of the terrestrial bow shock. Comparing the observed electric field with magnetic field and plasma observations, two different techniques find that the J × B/ne term in the generalized Ohm's law accounts for a majority of the large-scale normal electric field and potential drop encountered by the ions - the solar wind ion deceleration is in good empirical agreement with the observed potential drop, confirming earlier work. Large amplitude electric field fluctuations on shorter timescales, corresponding to fine scale structure, are not observed to contribute to the ion energization.

[1] During January, 2005, there were several large X-class solar flares and associated solar energetic particle (SEP) events. Coincidentally, the MINIS balloon campaign had multiple payloads aloft in the stratosphere above Antarctica measuring dc electric fields, conductivity and x-ray flux. One-to-one increases in the electrical conductivity and decreases to near zero of both the vertical and horizontal electric field components were observed in conjunction with an increase in particle flux at SEP onset. Combined with an atmospheric electric field mapping model, these data are consistent with a shorting out of the global electric circuit and point toward substantial ionospheric convection modifications. Additionally, two subsequent, rapid changes were detected in the vertical electric field component several hours after SEP onset. These changes result in similar fluctuations in the calculated vertical current density. We will describe how rigidity cut-off dynamics may be crucial in understanding these sudden jumps in the vertical electric field.

[1] We present Polar observations of high frequency (100 Hz ≤ f ≤ 4000 Hz) electrostatic (ES) waves at Earth's bow shock under extreme solar wind conditions. Although solitary waves are observed, the most prevalent structures in the magnetic ramp are coherent, large-amplitude (up to 80 mV/m) ES wave packets, which last 10-30 cycles, and propagate at varied obliquities relative to the magnetic field. The ES wave power is well correlated with maxima in the magnetic ramp substructure, suggesting that these maxima are important source regions. Detailed interferometric based analysis of waveforms show that they have wavelengths of a few hundred meters (e.g., ∼20λ D ∼ 0.5 ρe ) and phase speeds at the acoustic speed, suggesting that they are ion acoustic waves (IAW)s. The IAWs, having potentials ≤ 1V with no net change, are not likely to affect bulk plasma energization, though they may scatter the plasma and thus affect plasma thermalization.

[1] 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 Alfven 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.

[1] Nineteen electron diffusion regions at magnetic field reconnection sites have been found in one hour of Cluster satellite data near the subsolar magnetopause. Investigations on previously unachieved spatial and temporal scales show the following for the first time: direct conversion of magnetic energy to electron energy (The resulting accelerated electrons, their field-aligned currents, and their post-acceleration fate were measured); the spatial dimensions of the large electric fields in the electron diffusion regions have an average thickness of 0.3 times the electron skin depth, in agreement with theory; and these electron diffusion regions were topological boundaries between open and closed magnetic field geometries that contained different plasma and magnetic field line flows. The electron diffusion regions were located at the magnetospheric separatrix where the magnetic field was large and the electron beta was less than one.

[1] Electric fields with amplitudes to 140 mV/m and average durations of 7.5 milliseconds have been observed at the magnetospheric side of the dayside magnetopause. These fields are predominantly perpendicular to B, they are electrostatic, and they occur inside local minima in the plasma density. Because they have been observed at each of approximately seven opportunities, they must be a common feature of the magnetopause. They may be associated with the magnetospheric separatrix because computer simulations suggest such structures. Their complete characterization requires time domain measurements of plasma and three component fields with better than a few millisecond time resolution.

[1] The Cluster mission allows the determination not only of the bow shock crossing position but also, with a simple timing method and a reasonable confidence, the shock normal and the velocity along this normal. We apply this technique to a series of eleven consecutive bow shock crossings which occurred during a time interval of approximatively two and a half hours on 31 March 2001. We fit, on a distance versus time frame, the position of the bow shock subsolar point by imposing that the time derivatives at the crossings be equal to the shock speeds we determine. The curve we obtain this way represents global oscillations of the bow shock with a typical amplitude that compares quite well to the prediction of standard gasdynamic models which take into account the upstream solar wind plasma conditions.

[1] Intense lower hybrid waves are observed in the Hall current region of a reconnecting current sheet at the earth's magnetopause. Large measured cross-field drifts and density gradients are the likely sources of free energy through a lower hybrid drift instability (LHDI), which is stabilized near the neutral point where the plasma β is large. Harris neutral sheet and linear models are fitted to the data and inferred current density profiles are compared to the observed current density from particle measurements. We estimate the contribution of LHDI anomalous resistivity to the parallel electric field and show that it is more than 100 times smaller than the measured parallel field at the separator boundaries and essentially zero near the neutral point, leaving gradient electron pressure tensor and inertial terms or anomalous resistivity from higher frequency instabilities to support any parallel fields there and, hence, control the reconnection process.