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The global distribution of chorus wave amplitudes and their wave normal angles is investigated using high‐resolution wave spectra and waveform data from THEMIS for lower‐band and upper‐band chorus separately. Statistical results show that large amplitude chorus (>300 pT) occurs predominantly from premidnight to postdawn and is preferentially observed at lower L shells (<8) near the magnetic equator. However, strong or moderate chorus extends further into the afternoon sector and to higher L shells. For lower‐band chorus, strong waves (>50 pT) tend to have wave normal angles of <20° and their wave normal angles become even smaller with increasing wave amplitudes. For modest waves, the wave normal angles are distributed over a broad range with a major peak at <20° and a secondary peak at 60°–80°. Wave normal angles of lower‐band chorus are generally smaller on the dayside than on the nightside possibly due to the more uniform and more compressed magnetic field configuration on the dayside. Lower‐band chorus becomes more oblique with increasing latitude on the dayside, whereas on the nightside the probability of observing oblique chorus decreases at higher latitudes. Compared to lower‐band chorus, the properties of upper‐band chorus are somewhat different. Upper‐band chorus is considerably weaker in magnetic wave amplitudes, shows tighter confinement to the magnetic equator (<10°), and occurs at smaller L shells (<8). Furthermore, wave normal angles of upper‐band chorus are generally larger than those of lower‐band chorus, but the occurrence rate still peaks at wave normal angles of <20°, particularly for strong upper‐band chorus. Key Points Large amplitude chorus occurs from premidnight to dawn near the equator Strong lower‐band chorus typically has small wave normal angles (less than 20 deg) Upper‐band chorus is more oblique and much weaker than lower‐band chorus

High-resolution measurements of electrons obtained by satellite during the geomagnetic storm of 9 October 2012 together with a data-driven global wave model are analysed to show that scattering by a magnetospheric electromagnetic emission, known as ‘chorus’, can explain the temporal evolution of the observed increase in relativistic electron flux. A local mechanism for magnetic storm A magnetic storm that occurred on 9 October 2012 has been analysed in detail using the array of instruments onboard NASA's two Van Allen probes, launched in August 2012 to study Earth's magnetosphere, including the Van Allen radiation belt. The nature of the force that accelerates electrons trapped in the radiation belts has been a topic of much debate centering on whether the electrons are accelerated locally or by radial diffusive transport between weak and strong magnetic fields. Initial results had favoured a local mechanism and now Richard Thorne et al . report high-resolution electron observations from Van Allen probe A, together with modelling studies that identify the likely source of accelerating energy as chorus scattering, an effect caused locally by structured wave formations. This powerful local acceleration is also likely to be a factor around Jupiter, Saturn and other bodies with significant magnetic fields. Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density 1 , which are compelling evidence for local electron acceleration in the heart of the outer radiation belt 2 , 3 , but are inconsistent with acceleration by inward radial diffusive transport 4 , 5 . However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration 6 , 7 , 8 , 9 , 10 , but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations 11 obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model 12 , that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth’s outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Recent analysis using quasilinear theory (QLT) has shown that magnetosonic (MS) waves are able to accelerate electrons to relativistic energies on fast time scales (∼1 day). However, the large obliquity of the wave and typical equatorial confinement of the MS wave power create conditions that bring into question the fundamental applicability of QLT to this problem. In this paper, a test particle code is used to model the interaction of energetic electrons with fast MS waves, to test the results of QLT analysis, and to investigate any potential nonlinear effects. It is found that in the expected Landau‐resonant region, test particle results show good agreement with QLT, but outside this region, the spatial confinement of the low‐frequency waves introduces a new source of scattering which we call “transit time diffusion.” Although this mechanism is weaker than resonant scattering, it is nevertheless able to interfere with the Landau resonance to create nulls in the energy‐pitch angle diffusion map, and the scattering persists even when resonant diffusion is completely removed.

The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth's magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA's Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.

Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler‐mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near‐equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity.

The characteristics of hiss‐like and discrete (rising and falling tones) whistler‐mode waves in the lower‐band wave frequency range (0.1–0.5 of equatorial electron gyrofrequency) are investigated using waveform data from near‐equatorially orbiting multiple THEMIS spacecraft outside the plasmasphere. Statistical results show that wave polarization properties of hiss‐like emissions are similar to rising tones, but are significantly different from falling tones. The magnetic wave amplitudes of hiss‐like bands and rising tones are generally larger than those of falling tones. Wave normal angles of broadband hiss‐like emissions and rising tones tend to be quasi field‐aligned, whereas falling tones are very oblique. Importantly, discrete emissions including rising and falling tones are predominantly observed in the region of lowfpe/fce(the ratio of plasma frequency to electron gyrofrequency), whereas hiss‐like bands alone preferentially occur in the region of highfpe/fce. These important features of hiss‐like and discrete whistler‐mode emissions should be considered when evaluating their interactions with energetic electrons. Key Points Hiss‐like band and rising tones have similar polarization properties Falling tone is oblique, but hiss‐like band and rising tone are field‐aligned Discrete (hiss‐like) emissions prefer to occur at low (high) fpe/fce

Since their discovery more than 50 years ago, Earth's Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

The Juno spacecraft acquired direct observations of the jovian magnetosphere and auroral emissions from a vantage point above the poles. Juno’s capture orbit spanned the jovian magnetosphere from bow shock to the planet, providing magnetic field, charged particle, and wave phenomena context for Juno’s passage over the poles and traverse of Jupiter’s hazardous inner radiation belts. Juno’s energetic particle and plasma detectors measured electrons precipitating in the polar regions, exciting intense aurorae, observed simultaneously by the ultraviolet and infrared imaging spectrographs. Juno transited beneath the most intense parts of the radiation belts, passed about 4000 kilometers above the cloud tops at closest approach, well inside the jovian rings, and recorded the electrical signatures of high-velocity impacts with small particles as it traversed the equator.

Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

A long-standing problem in the field of space physics has been the origin of plasmaspheric hiss, a naturally occurring electromagnetic wave in the high-density plasmasphere (roughly within 20,000 kilometers of Earth) that is known to remove the high-energy Van Allen Belt electrons that pose a threat to satellites and astronauts. A recent theory tied the origin of plasmaspheric hiss to a seemingly different wave in the outer magnetosphere, but this theory was difficult to test because of a challenging set of observational requirements. Here we report on the experimental verification of the theory, made with a five-satellite NASA mission. This confirmation will allow modeling of plasmaspheric hiss and its effects on the high-energy radiation environment.