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By comparing an electron‐only reconnection event with a traditional reconnection event observed in the magnetotail, we illustrate the differences between and similarities of the two events. The electron behaviors are very similar in both events, but intensities of the electron flows and temperature in the traditional reconnection are much stronger than those in the electron‐only reconnection. The Hall electric field in the traditional reconnection occurs on the ion‐scale and is deflected from the normal direction by the significant magnetic field reconnected, while this field varies on the electron‐scale and points to the middle plane in the electron‐only reconnection. The comparison indicates that the electrons are undergoing the same process in both events, and the electron‐only reconnection was prior to the traditional reconnection. The Hall electric field could control the form of reconnection: producing either electron‐only reconnection or traditional reconnection. Key Points: We illustrate the differences between and similarities of the electron‐only reconnection and the normal reconnectionThe electron reconnection was prior to the normal reconnectionThe Hall electric field and the reconnected magnetic field intensity could control the form of reconnection [ABSTRACT FROM AUTHOR]

Whistler mode chorus waves are coherent electromagnetic emissions in planetary magnetospheres, characterized by rising‐tone or falling‐tone chirping elements. Understanding the cause of different chirping directions and their properties is an important step in resolving the long‐standing problem of nonlinear chorus generation. We report here, for the first time, particle‐in‐cell simulations of bidirectional chirping of whistler waves in a uniform magnetic field and falling‐tone‐only chirping in an inhomogeneous field. Combined with previous simulations of rising‐tone‐only emissions, we demonstrate that the background magnetic field inhomogeneity is not required for chirping of chorus, but it plays a key role in determining the chirping direction. The findings of the present work also unveil the critical role of the dipole geometry of Earth's magnetic field in causing the statistical predominance of rising‐tone chorus and the oblique propagation of falling‐tone chorus. Plain Language Summary: Whistler mode chorus is a type of naturally occurring electromagnetic emission in planetary magnetospheres. This important wave is known to produce relativistic electrons in the hazardous radiation belts and to precipitate energetic electrons from space into the upper atmosphere to form diffuse aurora. Chorus consists of discrete spectral elements with fast frequency chirping in either upward (rising‐tone) or downward (falling‐tone) directions. A long‐standing problem is to understand the origin of different chirping directions and their distinctive observational properties. Here, we show, by first‐principle particle simulations, that the background magnetic field inhomogeneity plays a key role in determining the chirping direction and that the dipole geometry of Earth's magnetic field essentially controls the properties of chorus. Our results naturally account for the dominance of rising‐tone chorus and the oblique propagation of falling‐tone chorus and provide important insights into the fundamental mechanism of the nonlinear chirping process. The propagation properties of chorus are also expected to be similar in all planetary magnetospheres with a dipole‐type magnetic field. Key Points: We present the first PIC simulations of falling‐tone chorus waves and chirping with a uniform background magnetic fieldWe demonstrate that the background magnetic field inhomogeneity controls the chirping direction of parallel‐propagating chorusOur results explain the dominance of rising‐tone chorus and the oblique propagation of falling‐tone chorus in the magnetosphere [ABSTRACT FROM AUTHOR]

Near equatorial (fast) magnetosonic waves, characterized by high magnetic compressibility, are whistler‐mode emissions destabilized by proton shell/ring distributions. In the past, substorm proton injections are widely known to intensify magnetosonic waves in the inner magnetosphere. Here we report the unexpected observations by the Van Allen Probes of the magnetosonic wave quenching associated with the substorm proton injections under both high‐ and low‐density conditions. The enhanced proton thermal pressure distorted the background magnetic field configuration and the cold plasma density distribution. The reduced phase velocities locally allowed the weak growth or even damping of magnetosonic waves. Meanwhile, the spatially irregularly varying refractive indices might suppress the cumulative growth of magnetosonic waves. For intense injections, this wave quenching region could extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance. These results provide a new understanding of the generation and distribution of magnetosonic waves. Plain Language Summary: Magnetosonic waves are the near‐equatorially confined electromagnetic emissions between the proton gyrofrequency and the lower hybrid frequency in the magnetosphere. Theoretical and observational studies have demonstrated the potential of the magnetosonic waves to accelerate the radiation belt electrons. It is therefore important to understand the generation process and the spatiotemporal distribution of the magnetosonic waves. The substorm activities injecting hot protons into the inner magnetosphere are conventionally considered to intensify the magnetosonic waves. By analyzing the wave and particle data of the Van Allen Probes, we find a magnetosonic wave quenching region related to the distortion of the background magnetic field configuration and the cold plasma density distribution closely following the substorm injection front. We propose two possible causes for this phenomenon: (1) local weak growth or even damping of the magnetosonic waves with the reduced phase velocities and (2) suppression of the magnetosonic wave cumulative growth in the disturbed medium. This new finding may help refine the modeling of the magnetosonic waves and then the radiation belt electrons. Key Points: The enhanced thermal pressure of hot protons can distort the geomagnetic field configuration and the cold plasma density distributionThe magnetosonic wave quenching region can emerge closely following the substorm injection front under both high‐ and low‐density conditionsThe magnetosonic wave quenching region can extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance [ABSTRACT FROM AUTHOR]

The primary power gap of chorus waves is statistically investigated using 7‐year waveform data from THEMIS probes for the first time. Overall, ~2/3 of chorus events have a power gap between the lower and upper bands. Both the gap frequency and the frequency bandwidth of the gap have a broad distribution, which peaks at frequencies of ~0.49fce and ~0.07fce, respectively. The gap frequency tends to increase with increasing |MLAT| (magnetic latitude), while the frequency width gradually increases with L‐shell. In most of banded events, the peak frequency of upper band is roughly twice that of lower band. Two types of events are studied. For Type I events, one population is located around Pup/Plow = 10−1, and the other is around Pup/Plow = 10−3. However, Type II events are roughly concentrated around Pup/Plow = 10−3. The gap frequency positively correlates with the frequency of upper band. Our study provides detailed observational constraints on potential mechanisms of the power gap formation. Key Points: We provide the first statistical study to comprehensively describe this mysterious power gap of chorus waves in the Earth's magnetosphereOverall, there are about 2/3 of events identified to have a power gap between two bands, and 1/3 of events are observed without a power gapWe have studied the distribution of both the gap frequency and frequency width of the power gap and their dependencies on two bands [ABSTRACT FROM AUTHOR]

Abstract: The excitation of magnetosonic waves in the meridian plane of a rescaled dipole magnetic field is investigated, for the first time, using a general curvilinear particle‐in‐cell simulation. Our simulation demonstrates that the magnetosonic waves are excited near the equatorial plane by tenuous ring distribution protons. The waves propagate nearly perpendicularly to the background magnetic field along both radially inward and outward directions. Different speeds of inward and outward propagation result in the asymmetrical distribution about the source region. The waves are accompanied by energization of both cool protons and electrons near the wave source region. The cool protons are heated perpendicularly, while the cool electrons can be heated in the parallel direction and also experience enhanced perpendicular drift at the presence of intense wave power. The implications of simulation results to the observations of magnetosonic waves and related particle heating in the inner magnetosphere are also discussed. [ABSTRACT FROM AUTHOR]