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Recent in-situ observations from the Parker Solar Probe (PSP) mission in the inner heliosphere near perihelia show evidence of ion beams, temperature anisotropies, and kinetic wave activity, which are likely associated with kinetic heating and acceleration processes of the solar wind. In particular, the proton beams were detected by PSP/SPAN-I and related magnetic fluctuation spectra associated with ion-scale waves were observed by the FIELDS instrument. We present the ion velocity distribution functions (VDFs) from SPAN-I and the results of 2.5D and 3D hybrid-particle-in-cell (hybrid-PIC) models of proton and alpha particle super-Alfvenic beams that drive ion kinetic instabilities and waves in the inner-heliospheric solar wind. We model the evolution of the ion VDFs with beams, ion relative drifts speeds, and ion temperature anisotropies for solar wind conditions near PSP perihelia. We calculate the partition of energies between the particles (ions) along and perpendicular to the magnetic field, as well as the evolution of magnetic energy and compare to observationally deduced values. We conclude that the ion beam driven kinetic instabilities in the solar wind plasma near perihelia are important components in the cascade of energy from fluid to kinetic scale, an important component in the solar wind plasma heating process., Accepted for publication in The Astrophysical Journal

International audience; Heliophysics data analysis often involves combining diverse science measurements, many of them captured as time series. Although there are now only a few commonly used data file formats, the diversity in mechanisms for automated access to and aggregation of such data holdings can make analysis that requires intercomparison of data from multiple data providers difficult. The Heliophysics Application Programmer's Interface (HAPI) is a recently developed standard for accessing distributed time series data to increase interoperability. The HAPI specification is based on the common elements of existing data services, and it standardizes the two main parts of a data service: the request interface and the response data structures. The interface is based on the REpresentational State Transfer (REST) or RESTful architecture style, and the HAPI specification defines five required REST endpoints. Data are returned via a streaming format that hides file boundaries; the metadata is detailed enough for the content to be scientifically useful, e.g., plotted with appropriate axes layout, units, and labels. Multiple mature HAPI-related open-source projects offer server-side implementation tools and client-side libraries for reading HAPI data in multiple languages (IDL, Java, MATLAB, and Python). Multiple data providers in the US and Europe have added HAPI access alongside their existing interfaces. Based on this experience, data can be served via HAPI with little or no information loss compared to similar existing web interfaces. Finally, HAPI has been recommended as a COSPAR standard for time series data delivery.

While electromagnetic ion cyclotron (EMIC) waves have been long studied as a scattering mechanism for ultrarelativistic (megaelectron volt) electrons via cyclotron‐resonant interactions, these waves are also of the right frequency to resonate with the bounce motion of lower‐energy (approximately tens to hundreds of kiloelectron volts) electrons. Here we investigate the effectiveness of this bounce resonance interaction to better determine the effects of EMIC waves on subrelativistic electron populations in Earth's inner magnetosphere. Using wave and plasma parameters directly measured by the Van Allen Probes, we estimate bounce resonance diffusion coefficients for four different events, illustrative of wave and plasma parameters to be encountered in the inner magnetosphere. The range of electron energies and pitch angles affected is examined to better assess the realistic effects of EMIC‐driven bounce resonance on energetic electron populations based on actual, locally observed event‐based parameters. Significant local diffusion coefficients (~ > 10(exp −6) s(exp −1)) for 50‐ to 100‐keV electrons are achieved for both H+ band wave events as well as He+ band, with diffusion coefficients peaking for near‐90° pitch angles but remaining elevated for intermediate ones as well. Diffusion coefficients for higher‐energy 200‐keV electrons are typically multiple orders of magnitude lower (ranging from 10(exp −11) to 10(exp −6) s(exp −1)) and often peak at lower pitch angles (~20–30°). These results suggest that both H+ and He+ band EMIC waves can play a role in shaping lower‐energy electron dynamics via bounce‐resonant interactions, in addition to their role in relativistic electron loss via cyclotron resonance.

STR acknowledges support from the ISSI international team Cold plasma of ionospheric origin in the Earth's magnetosphere and of the Ministry of Economy and Competitiveness (MINECO) of Spain (grant FIS2017-90102-R). Research at IRAP was supported by CNRS, CNES and the University of Toulouse. JHL and DLT acknowledge support from NASA Grant 80NSSC18K1378. RED was supported by NASA grants 80NSSC19K070 and 80NSSC19K0254. MA was supported by SNSA Grant 56/18. SKV and RCA acknowledge support from NASA Grant 80NSSC19K0270. Work performed by MMS team members is supported by NASA contract NNG04EB99C., We report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen-in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the wave E field has small ellipticity and right-handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave., ISSI international team Cold plasma of ionospheric origin in the Earth's magnetosphere, Ministry of Economy and Competitiveness (MINECO) of Spain FIS2017-90102-R, Centre National de la Recherche Scientifique (CNRS), European Commission, Centre National D'etudes Spatiales, University of Toulouse, National Aeronautics & Space Administration (NASA) 80NSSC18K1378 80NSSC19K070 80NSSC19K0254 80NSSC19K0270 NNG04EB99C, SNSA 56/18

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time.

We report observations of the ion dynamics inside an Alfven branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are within 1% with the predictions of a dispersion solver, and indicate that the wave is an electromagnetic ion cyclotron wave with very oblique wave vector. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. The cold protons follow the magnetic field fluctuations and remain frozen-in, while the hot protons are at the limit of magnetization.The cold proton velocity fluctuations contribute to balance the Hall term in Ohm's law, allowing the wave polarization to be highly-elliptical and right-handed, a necessary condition for propagation at oblique wave normal angles. The dispersion solver indicates that increasing the cold proton density facilitates generation and propagation of these waves at oblique angles, as it occurs for the observed wave.

Interoperability between datasets in Heliophysics and Planetary archives is increasingly important to address complex science questions about space weather and planetary plasma environments. Yet fo...

Two-dimensional hybrid particle-in-cell (PIC) simulations are carried out on a constant L-shell (or drift shell) surface of the dipole magnetic field to investigate the generation process of near-e...

High-quality measurements of electromagnetic fields and electron velocity distributions by the Magnetospheric Multiscale (MMS) mission in Earth's magnetosheath present a unique opportunity to chara...

In this work we perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the '30 s' waves observed at the Earth's foreshock, the normalized wave amplitude (∼0.2) and occurrence rate (∼0.5%) are much smaller. This suggests significant lower backstreaming proton fluxes, due to the relatively low solar wind Alfvenic Mach number around Mercury. These differences could also be related to the relatively smaller foreshock size and/or more variable solar wind conditions. Furthermore, we estimate that the speed of resonant backstreaming protons in the solar wind reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the solar wind speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.

International audience; In the magnetosheath, intense whistler mode waves, called “Lion roars,” are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field‐aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of ~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure.Publication cover image

Apollo lunar samples reveal that the Moon generated its own global magnetosphere, lasting from ~4.25 to ~2.5 billion years (Ga) ago. At peak lunar magnetic intensity (4 Ga ago), the Moon was volcanically active, likely generating a very tenuous atmosphere, and, it is believed, was at a geocentric distance of ~18 Earth radii (RE). Solar storms strip a planet’s atmosphere over time, and only a strong magnetosphere would be able to provide maximum protection. We present simplified magnetic dipole field modeling confined within a paraboloidal-shaped magnetopause to show how the expected Earth-Moon coupled magnetospheres provide a substantial buffer from the expected intense solar wind, reducing Earth’s atmospheric loss to space.

The Heliophysics Application Programmer’s Interface (HAPI) represents a grass-roots effort to develop a common interface that data providers can use to offer time series data for computer-to-computer data access. Multiple data centers with Heliophysics and Planetary data are adopting this emerging standard. Several scientific analysis packages (Autoplot, SPEDAS) seamlessly pull from HAPI servers, and we have created and are enhancing Python libraries that can be used to read HAPI data into useful data structures within personalized analysis code. The HAPI specification and supporting software are available on GitHub. We will discuss the latest developments of the HAPI specification, current server and client implementations, all in the context of the push for increased interoperability within Heliophysics and Planetary data analysis. https://hapi-server.github.io/

Characterizing habitable exoplanets and/or their moons is of paramount importance. Here we show the results of our magnetic field topological modeling which demonstrate that terrestrial exoplanet-exomoon coupled magnetospheres work together to protect the early atmospheres of both the exoplanet and the exomoon. When exomoon magnetospheres are within the exoplanet's magnetospheric cavity, the exomoon magnetosphere acts like a protective magnetic bubble providing an additional magnetopause confronting the stellar winds when the moon is on the dayside. In addition, magnetic reconnection would create a critical pathway for the atmosphere exchange between the early exoplanet and exomoon. When the exomoon's magnetosphere is outside of the exoplanet's magnetosphere it then becomes the first line of defense against strong stellar winds, reducing the exoplanet's atmospheric loss to space. A brief discussion is given on how this type of exomoon would modify radio emissions from magnetized exoplanets., Comment: submitted to ApJ Letters

We analyze a wave event that occurred near noon between 07:03 and 07:08 UT on 23 February 2014 detected by the Van Allen Probes B spacecraft, where waves in the lower hybrid frequency range (LHFR) and electromagnetic ion cyclotron (EMIC) waves are observed to be highly correlated, with Pearson correlation coefficient of approximately 0.86. We assume that the correlation is the result of LHFR wave generation by the ions polarization drift in the electric field of the EMIC waves. To check this assumption the drift velocities of electrons and H+, He+, and O+ ions in the measured EMIC wave electric field were modeled. Then the LHFR wave linear instantaneous growth rates for plasma with these changing drift velocities and different plasma compositions were calculated. The time distribution of these growth rates, their frequency distribution, and the frequency dependence of the ratio of the LHFR wave power spectral density (PSD)parallel and perpendicular to the ambient magnetic eld to the total PSD were found. These characteristics of the growth rates were compared with the corresponding characteristics of the observed LHFR activity. Reasonable agreement between these features and the strong correlation between EMIC and LHFR energy densities support the assumption that the LHFR wave generation can be caused by the ions polarization drift in the electric field of an EMIC wave.

We present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 21 September 2012 to 1 August 2014. We show that statistically, the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (f(sub cP)) has a distinct funnel-shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extra-ordinary (whistler) mode at wave normal angles (theta(sub k)) near 90 deg. Performing polarization analysis on synthetic waveforms composed from a superposition of extra-ordinary mode plane waves with theta(sub k) randomly chosen between 87 and 90 deg, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to theta(sub k) of 60 deg, suggesting that another approach is necessary to estimate the true distribution of theta(sub k). We find that the histograms of the synthetically derived ellipticities and theta(sub k) are consistent with the observations of ellipticities and theta(sub k) derived using polarization analysis.We make estimates of the median equatorial theta(sub k) by comparing observed and model ray tracing frequency-dependent probability occurrence with latitude and give preliminary frequency dependent estimates of the equatorial theta(sub k) distribution around noon and 4 R(sub E), with the median of approximately 4 to 7 deg from 90 deg at f/f(sub cP) = 2 and dropping to approximately 0.5 deg from 90 deg at f/f(sub cP) = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE*, similar to findings of other studies.

Turbulence is a fundamental physical process through which energy injected into a system at large scales cascades to smaller scales. In collisionless plasmas, turbulence provides a critical mechanism for dissipating electromagnetic energy. Here we present observations of plasma fluctuations in low-β turbulence using data from NASA’s Magnetospheric Multiscale mission in Earth’s magnetosheath. We provide constraints on the partitioning of turbulent energy density in the fluid, ion-kinetic, and electron-kinetic ranges. Magnetic field fluctuations dominated the energy density spectrum throughout the fluid and ion-kinetic ranges, consistent with previous observations of turbulence in similar plasma regimes. However, at scales shorter than the electron inertial length, fluctuation power in electron kinetic energy significantly exceeded that of the magnetic field, resulting in an electron-motion-regulated cascade at small scales. This dominance should be highly relevant for the study of turbulence in highly magnetized laboratory and astrophysical plasmas.

International audience; Electron heating at Earth's quasiperpendicular bow shock has been surmised to be due to the combined effects of a quasistatic electric potential and scattering through wave-particle interaction. Here we report the observation of electron distribution functions indicating a new electron heating process occurring at the leading edge of the shock front. Incident solar wind electrons are accelerated parallel to the magnetic field toward downstream, reaching an electron-ion relative drift speed exceeding the electron thermal speed. The bulk acceleration is associated with an electric field pulse embedded in a whistler-mode wave. The high electron-ion relative drift is relaxed primarily through a nonlinear current-driven instability. The relaxed distributions contain a beam traveling toward the shock as a remnant of the accelerated electrons. Similar distribution functions prevail throughout the shock transition layer, suggesting that the observed acceleration and thermalization is essential to the cross-shock electron heating.

We present the first observations of Mercury's plasma mantle, a primary region for solar wind entry into the planetary magnetosphere, located in the high-latitude magnetotail. MESSENGER observations from two orbits on 10 November 2012 have been analyzed. The main plasma mantle features are (1) a steady decrease in proton density as MESSENGER moved deeper into the magnetotail; (2) frequent flux transfer events throughout the magnetosheath and into the magnetotail, suggesting that these events are the primary source for solar wind plasma injection; (3) a diamagnetic depression, due to the presence of plasma, as pressure balance is maintained; and (4) a clear proton velocity dispersion, resulting from lower-energy protons being transported deep into the magnetosphere as higher-energy protons escape downtail. From these velocity dispersions we infer cross-magnetosphere electric potentials of 23 kV and 29 kV, consistent with estimates determined from measurements of magnetopause reconnection rate and tail loading and unloading events.

During solar energetic particle (SEP) events, the inner heliosphere is bathed in MeV electrons. Through magnetic reconnection, these relativistic electrons can enter the magnetosphere of Mercury, nearly instantaneously filling the regions of open field lines with precipitating particles. With energies sufficient to penetrate solid aluminum shielding more than 1 mm thick, these electrons were observable by a number of sensors on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Because of its thin shielding, frequent sampling, and continuous temporal coverage, the Fast Imaging Plasma Spectrometer provided by far the most sensitive measurements of MeV electrons of all MESSENGER sensors. Sharp changes in energetic electron flux coincided with topological boundaries in the magnetosphere, including the magnetopause, polar cap, and central plasma sheet. Precipitating electrons with fluxes equal to ~40% of their corresponding upstream levels were measured over the entire polar cap, demonstrating that electron space weathering of Mercury's surface is not limited to the cusp region. We use these distinct precipitation signatures acquired over 33 orbits during 11 SEP events to map the full extent of Mercury's northern polar cap. We confirm a highly asymmetric polar cap, for which the dayside and nightside boundary latitudes range over ~50−70°N and ~30−60°N, respectively. These latitudinal ranges are consistent with average models of Mercury's magnetic field but exhibit a large variability indicative of active dayside and nightside magnetic reconnection processes. Finally, we observed enhanced electron fluxes within the central plasma sheet. Although these particles cannot form a stable ring current around the planet, their motion results in an apparent trapped electron population at low latitudes in the magnetotail.

We report an investigation of magnetic reconnection in Mercury’s magnetotail conducted with MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer measurements during seven “hot seasons” when the periapsis of the spacecraft orbit is on Mercury’s dayside. Flux ropes are formed in the cross-tail current sheet by reconnection. We have analyzed 49 flux ropes observed between 1.7 RM and 2.8 RM (where RM is Mercury’s radius, or 2440 km) down the tail from the center of the planet, for which minimum variance analysis indicates that the spacecraft passed near the central axis of the structure. An average Alfven speed of 465 km s−1 is measured in the plasma sheet surrounding these flux ropes. Under the assumption that the flux ropes moved at the local Alfven speed, the mean duration of 0.74±0.15 s determined for these structures implies a typical diameter of ~345 km, or ~0.14 RM, which is comparable to a proton gyroradius in the plasma sheet of ~380 km. We successfully fit the magnetic signatures of 16 flux ropes to a force-free model. The mean radius and core field determined in this manner were ~450 km, or ~0.18 RM, and ~40 nT, respectively. A superposed epoch analysis of the magnetic field during these events shows variations similar to those observed at Earth, including the presence of a post-plasmoid plasma sheet, filled with disconnected magnetic flux, but the timescales are 40 times shorter at Mercury. The results of this flux rope survey indicate that intense magnetic reconnection occurs frequently in the cross-tail current layer of this small but extremely dynamic magnetosphere.

This study presents a statistical overview of coherent wave activity in Mercury's magnetosheath. Left-handed electromagnetic ion cyclotron waves are commonly found behind the quasi-perpendicular section of the bow shock, where they are present in ~50% of the spacecraft crossings of the magnetosheath. Their occurrence distribution maximizes within the magnetosheath, approximately halfway between the bow shock and the magnetopause, and the waves are generally strongly Doppler shifted up to frequencies above the local ion cyclotron frequency. Downstream of the quasi-parallel shock, the magnetosheath often exhibits large-amplitude pulsations with wave periods around 10 s and peak-to-peak amplitudes of up to 100 nT that dominate the magnetic field structure. These waves are circularly left-hand polarized with wave vectors in the direction of the local shock normal. The data suggest that they have been generated upstream of the shock and transmitted into the downstream region. Their occurrence rates maximize at the near-parallel shock, where they are present approximately 10% of the time, and where they also show their largest wave powers. Some evidence is also found of waves with a right-handed polarization in the spacecraft frame. These consist of both whistler waves above the local ion cyclotron frequency and ion cyclotron waves propagating against the magnetosheath flow with Doppler shifts exceeding the intrinsic wave frequency, which results in a change in their apparent polarization. These waves are in minority compared to the left-handed observations, which indicates a preference for ion cyclotron waves propagating in the direction of the plasma flow.

Purely compressional electromagnetic waves (fast magnetosonic waves), generated at multiple harmonics of the local proton gyrofrequency, have been observed by various types of satellite instruments (fluxgate and search coil magnetometers and electric field sensors), but most recent studies have used data from search coil sensors, and many have been restricted to high harmonics. We report here on a survey of low-harmonic waves, based on electric and magnetic field data from the Electric Fields and Waves double probe and Electric and Magnetic Field Instrument Suite and Integrated Science fluxgate magnetometer instruments, respectively, on the Van Allen Probes spacecraft during its first full precession through all local times, from 1 October 2012 to 13 July 2014. These waves were observed both inside and outside the plasmapause (PP), at L shells from 2.4 to ~6 (the spacecraft apogee), and in regions with plasma number densities ranging from 10 to >1000 cm−3. Consistent with earlier studies, wave occurrence was sharply peaked near the magnetic equator. Waves appeared at all local times but were more common from noon to dusk, and often occurred within 3 h after substorm injections. Outside the PP occurrence maximized broadly across noon, and inside the PP occurrence maximized in the dusk sector, in an extended plasmasphere. We confirm recent ray-tracing studies showing wave refraction and/or reflection at PP-like boundaries. Comparison with waveform receiver data indicates that in some cases these low-harmonic magnetosonic wave events occurred independently of higher-harmonic waves; this indicates the importance of including this population in future studies of radiation belt dynamics.

We show that ~1 Hz magnetic compressional waves observed in Mercury's inner magnetosphere could be interpreted as ion-Bernstein waves in a moderate proton beta ~0.1 plasma. An observation of a proton distribution with a large planetary loss cone is presented, and we show that this type of distribution is highly unstable to the generation of ion-Bernstein waves with low magnetic compression. Ray tracing shows that as these waves propagate back and forth about the magnetic equator; they cycle between a state of low and high magnetic compression. The group velocity decreases during the high-compression state leading to a pileup of compressional wave energy, which could explain the observed dominance of the highly compressional waves. This bimodal nature is due to the complexity of the index of refraction surface in a warm plasma whose upper branch has high growth rate with low compression, and its lower branch has low growth/damping rate with strong compression. Two different cycles are found: one where the compression maximum occurs at the magnetic equator and one where the compression maximum straddles the magnetic equator. The later cycle could explain observations where the maximum in compression straddles the equator. Ray tracing shows that this mode is confined within ±12° magnetic latitude which can account for the bulk of the observations. We show that the Doppler shift can account for the difference between the observed and model wave frequency, if the wave vector direction is in opposition to the plasma flow direction. We note that the Wentzel-Kramers-Brillouin approximation breaks down during the pileup of compressional energy and that a study involving full wave solutions is required.

Observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in Mercury's magnetotail demonstrate for the first time that Na+ ions exert a dynamic influence on Mercury's magnetospheric system. Na+ ions are shown to contribute up to ~30% of the ion thermal pressure required to achieve pressure balance in the premidnight plasma sheet. High concentrations of planetary ions should lead to Na+ dominance of the plasma mass density in these regions. On orbits with northward-oriented interplanetary magnetic field and high (i.e., >1 cm−3) Na+ concentrations, MESSENGER has often recorded magnetic field fluctuations near the Na+ gyrofrequency associated with the Kelvin-Helmholtz (K-H) instability. These nightside K-H vortices are characteristically different from those observed on Mercury's dayside that have a nearly constant wave frequency of ~0.025 Hz. Collectively, these observations suggest that large spatial gradients in the hot planetary ion population at Mercury may result in a transition from a fluid description to a kinetic description of vortex formation across the dusk terminator, providing the first set of truly multiscale observations of the K-H instability at any of the diverse magnetospheric environments explored in the solar system.

We present Venus Express observations of the impact of a slow interplanetary coronal mass ejection (ICME), which struck Venus on 23 December 2006, creating unusual quasi steady state upstream conditions for the 2 h close to periapsis: an enhanced (∼ nT) interplanetary magnetic field (IMF), radially aligned with the Sun-Venus line; and a dense (∼ cm−3) solar wind. Contrary to our current understanding and expectations, the ionosphere became partially demagnetized. We also find evidence for shocked sheathlike solar wind protons and electrons in the wake of Venus, and powerful (≈ nT2/Hz) foreshock whistler mode waves radiating from the bow shock at an unexpectedly low frequency (0.6 Hz). Given the abnormally high density of escaping heavy ions at the magnetopause boundary (295 cm−3, one of the highest of the whole mission) and the enhanced density of escaping heavy ions in the wake, we find that even weak ICMEs with no driving shocks can increase atmospheric loss rates at Venus and suggests that the Bx component of the IMF may be a factor in atmospheric escape rates.

Alfvén waves are fundamental plasma wave modes that permeate the universe. At small kinetic scales, they provide a critical mechanism for the transfer of energy between electromagnetic fields and charged particles. These waves are important not only in planetary magnetospheres, heliospheres and astrophysical systems but also in laboratory plasma experiments and fusion reactors. Through measurement of charged particles and electromagnetic fields with NASA's Magnetospheric Multiscale (MMS) mission, we utilize Earth's magnetosphere as a plasma physics laboratory. Here we confirm the conservative energy exchange between the electromagnetic field fluctuations and the charged particles that comprise an undamped kinetic Alfvén wave. Electrons confined between adjacent wave peaks may have contributed to saturation of damping effects via nonlinear particle trapping. The investigation of these detailed wave dynamics has been unexplored territory in experimental plasma physics and is only recently enabled by high-resolution MMS observations., Alfvén waves are fundamental plasma modes that provide a mechanism for the transfer of energy between particles and fields. Here the authors confirm experimentally the conservative energy exchange between Alfvén wave fields and plasma particles via high-resolution MMS observations of Earth's magnetosphere.

The great geomagnetic storm of August 28 through September 3,1859 is, arguably, the greatest and most famous space weather event in the last two hundred years. For the first time observations showed that the sun and aurora were connected and that auroras generated strong ionospheric currents. A significant portion of the world's 200,000 km of telegraph lines were adversely affected, many of which were unusable for 8 hours or more which had a real economic impact. In addition to published scientific measurements, newspapers, ship logs, and other records of that era provide an untapped wealth of first hand observations giving time and location along with reports of the auroral forms and colors. At its height, the aurora was described as a blood or deep crimson red that was so bright that one "could read a newspaper by." At its peak, the Type A red aurora lasted for several hours and was observed to reach extremely low geomagnetic latitudes on August 28-29 (-25") and on September 2-3 (-18"). Auroral forms of all types and colors were observed below 50" latitude for -24 hours on August 28-29 and -42 hours on September 2-3. From a large database of ground-based observations the extent of the aurora in corrected geomagnetic coordinates is presented over the duration of the storm event.

Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by ~5 h: Probe A at 15 and Probe B at 9–11 h. Both spacecraft see similar frequency features, characterized by a periodic repetition at ~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition ~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features but not the quantitative details. The cause for this phenomena remains to be identified.

Direct measurement of low

CLJ and RMW acknowledge support from the Natural Sciences and Engineering Research Council of Canada, and CLJ acknowledges support from MESSENGER Participating Scientist grant NNX11AB84G. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW- 00002 to the Carnegie Institution of Washington and NAS5-97271 to The Johns Hopkins University Applied Physics Laboratory.

[1] We here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath-solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

[1] Mercury's bow shock is unique in our solar system as it is produced by low Mach number solar wind blowing over a small magnetized body. The availability of MESSENGER orbiter data enables us for the first time to conduct an in-depth study of upstream waves in Mercury's foreshock. This paper reports first results of an observational study of upstream ULF waves in Mercury's foreshock using high-time resolution magnetic field data from the MESSENGER spacecraft to understand the general morphology of these waves. We find that the most common wave phenomenon in Mercury's foreshock has frequencies ~ 2 Hz, with properties similar to the 1 Hz whistler waves in the Earth's foreshock. Their generation appears to be generic to the shock and not affected by the weak strength and small size of Mercury's bow shock. On the other hand, the most common wave phenomenon in the Earth's foreshock is the large-amplitude 30 second waves, identified as fast magnetosonic waves generated by backstreaming ions. Similar waves at Mercury have wave frequencies at ~ 0.3 Hz, but occur only sporadically. The general lack of strong “30 second” magnetosonic waves at Mercury can be attributed to the lack of strong backstreaming ions due to a weak bow shock and not enough time for wave growth due to the small foreshock size. Superposed on the “1 Hz” whistler waves, there are short bursts of spectral peaks at ~ 0.8 Hz that are new and have not been reported previously in Mariner 10 data.

We present results from a new three-dimensional empirical magnetopause model based on 15,089 magnetopause crossings from 23 spacecraft. To construct the model, we introduce a Support Vector Regression Machine (SVRM) technique with a systematic approach that balances model smoothness with fitting accuracy to produce a model that reveals the manner in which the size and shape of the magnetopause depend upon various control parameters without any assumptions concerning the analytical shape of the magnetopause. The new model fits the data used in the modeling very accurately, and can guarantee a similar accuracy when predicting unseen observations within the applicable range of control parameters. We introduce a new error analysis technique based upon the SVRM that enables us to obtain model errors appropriate to different locations and control parameters. We find significant east-west elongations in the magnetopause shape for many combinations of control parameters. Variations in the Earth's dipole tilt can cause significant magnetopause north/south asymmetries and deviation of the magnetopause nose from the Sun-Earth line nonlinearly by as much as 5Re. Subsolar magnetopause erosion effect under southward IMF is seen which is strongly affected by solar wind dynamic pressure. Further, we find significant shrinking of high-latitude magnetopause with decreased magnetopause flaring angle during northward IMF.

We report the simultaneous detection of a terrestrial myriametric radio burst (TMRB) by IMAGE and Geotail on 19 August 2001. The TMRB was confined in time (0830-1006 UT) and frequency (12-50kHz). Comparisons with all known nonthermal myriametric radiation components reveal that the TMRB might be a distinct radiation with a source that is unrelated to the previously known radiation. Considerations of beaming from spin-modulation analysis and observing satellite and source locations suggest that the TMRB may have a fan beamlike radiation pattern emitted by a discrete, dayside source located along the poleward edge of magnetospheric cusp field lines. TMRB responsiveness to IMF Bz and By orientations suggests that a possible source of the TMRB could be due to dayside magnetic reconnection instigated by northward interplanetary field condition.

[1] On 18 March 2011, MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) became the first spacecraft to orbit Mercury, providing a new opportunity to study the outer boundary of the planet’s magnetosphere—the magnetopause. Here we characterize Mercury’s magnetopause using measurements collected by MESSENGER’s Magnetometer and Fast Imaging Plasma Spectrometer. Analysis of measurements from two of MESSENGER’s “hot seasons,” when the orbital periapsis is on Mercury’s dayside and the magnetopause crossing takes place in the subsolar region, resulted in 43 events with well-determined boundary normals. The typical duration of a magnetopause traversal was ~5 s. The average normal magnetic field component was ~20 nT, and the dimensionless reconnection rate, i.e., the ratio of the normal magnetic field component to the total field magnitude just inside the magnetopause, was 0.15 � 0.02. This rate is a factor of ~3 larger than values found during the most extensive surveys at Earth. The ratio of the reconnection rate at Mercury to that of the Earth is comparable to the ratio of the solar wind Alfven speeds at their respective orbits. We also find that the magnetopause reconnection rate at Mercury is independent of magnetic field shear angle, but it varies inversely with plasma b, the ratio of total thermal pressure to magnetic pressure, in the magnetosheath. These results suggest that reconnectionatMercuryisnotonlymoreintensethanatEarthbutalsothatitoccursfornearly allorientationsofthe interplanetarymagnetic field duetothe low-bnatureof thesolar windin the inner heliosphere.

Mercury is exposed to the most dynamic heliospheric space environment of any planet in the solar system. The magnetosphere is particularly sensitive to variations in the interplanetary magnetic field (IMF), which control the intensity and geometry of the magnetospheric current systems that are the dominant source of uncertainty in determinations of the internal planetary magnetic field structure. The Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has made extensive magnetic field observations in the inner heliosphere over the heliocentric distances of Mercury's orbit, between 0.31 and 0.47 AU. In this paper, Magnetometer data from MESSENGER, obtained at rates of 2 and 20 vector samples per second, are used together with previous observations in the inner heliosphere by Helios and at Earth by the Advanced Composition Explorer, to study the characteristics of IMF variability at Mercury's orbit. Although the average IMF geometry and magnitude depend on heliocentric distance as predicted by Parker, the variability is large, comparable to the total field magnitude. Using models for the external current systems we evaluate the impact of the variability on the field near the planet and find that the large IMF fluctuations should produce variations of the magnetospheric field of up to 30% of the dipole field at 200 km altitude, corresponding to the planned periapsis of MESSENGER's orbit at Mercury. The IMF fluctuations in the frequency range 10 − 4 f 10 − 1 Hz are consistent with turbulence, whereas evidence for dissipation was observed for f > 1 Hz . The transition between the turbulent and dissipative regimes is indicated by a break in the power spectrum, and the frequency of this break point is proportional to the IMF magnitude.

To examine electron transport, energization, and precipitation in Mercury’s magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind