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Mercury has a large loss cone difference in its two hemispheres due to the northward shifted magnetic dipole. The precipitation difference of energetic electrons in both hemispheres is poorly understood. We show that the northern precipitation is 2.5‐times higher than for a symmetric loss cone due to the effects of the enhanced whistler instability at the southern hemisphere with the larger loss cone. Simulations including nonlinear pitch angle scattering by the whistler‐mode waves show rapid (tens of milliseconds) electron flux modulation related to the wave subpacket structures by repeated interactions within a discrete wave element. The difference in the nonlinear whistler instability in the two hemispheres should enhance the electron precipitation, which, along with the direct impact effects of solar wind, contributes to Mercury's surface–magnetosphere coupling. Electrons hitting the planet's surface may be a possible factor in the formation of water through the formation of hydroxyl groups. Plain Language Summary: Mercury, the first planet from the Sun, has north–south asymmetric magnetic fields due to the northward shifted magnetic dipole from the planet's center. Computer simulations of plasma waves and electrons, taking into account Mercury's magnetic dipole offset, show that the northward precipitation of electrons is 2.5‐times higher than in the case of no magnetic dipole offset, which is the case like the Earth. This difference in electron precipitation fraction arises from a difference in the characteristics of wave growth due to the spatial characteristics of the planet's magnetic field, because plasma waves can efficiently push trapped electrons in Mercury's magnetosphere toward the planet's surface. This study contributes to the understanding of the electron precipitation fraction on Mercury and may help to estimate the production of water ice through the formation of hydroxyl groups not only by the direct impact of solar wind but also by the electron precipitation caused by plasma waves. Key Points: Whistler‐driven electrons at Mercury are simulated, as the electron precipitation on the planet surface may contribute to water productionFor an asymmetric (southward wider) loss cone, the northward precipitation fraction is 2.5‐times higher than for a symmetric loss coneRapid (tens of milliseconds) electron flux modulations can be observed as a signature of repeated interactions with wave subpackets [ABSTRACT FROM AUTHOR]

Nonlinear gyroresonant acceleration is an essential mechanism for the formation of relativistic electrons in Earth's radiation belts, yet we still have only limited knowledge of how electron motion interacts nonlinearly with plasma waves. Here we demonstrate the compelling suppression of whistler‐mode chorus emissions as a resonant wave with energetic electrons by tuning an external transmitter signal in space plasmas. Through forcible modification of electron motions via phase trapping by the external waves, the nonlinear gyroresonant current is suppressed along with the electron hole deformation. Moreover, this can decrease the population of relativistic electrons due to the lack of coherent resonant waves seen by the phase‐organized electrons. Understanding such nonlinear wave suppression effects is crucial for a potential active control of Earth's plasma environment. Plain Language Summary: Since the strong radiation in space can cause health risks for astronauts and satellite anomalies of electronics, understanding toward the active control of the particle populations is in increasing demand for sustainable space industry growth. The gyroresonant interactions between energetic electrons and whistler‐mode waves play important roles in the acceleration mechanism of the Earth's radiation belt electrons. We find that transmitter signals as an external wave source in space plasmas can utilize the changing of nonlinear electron motion by stimulating electrons in a phase space. We show that nonlinear whistler mode chorus wave excitation, which is the resonant wave source for energetic electrons, can effectively be suppressed by deforming the shape of electron holes to control resonant currents in the phase space. This mechanism is crucial for understanding the active control of Earth's plasma environments. Key Points: Chorus waves can be suppressed by inhibition of coherent interactions with resonant electrons due to adding external transmitter wavesThe external waves having a different frequency from the maximum linear growth can become incoherent from the point of view of electronsThe external waves can precipitate the seed electrons, then this reduces the anisotropy of particles and hence inhibits wave growth [ABSTRACT FROM AUTHOR]

Electron density enhancements in the ionospheric D‐region due to the precipitation of high‐energy electrons (>30 keV) have been measured as increases in cosmic radio noise absorption (CNA) using ground‐based riometers. CNA has been studied since the 1960s. However, there have been few studies of the spatiotemporal development of CNA at multi‐point ground stations distributed in longitude at subauroral latitudes, where plasma particles with a wide energy range are intermingled. In this study, we analyzed the longitudinal development of CNA steep increases using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland over 3 years from 2017 to 2020. The results revealed that the occurrence rate of steep increases in CNA was highest at midnight at 22‐08 magnetic local time (MLT), and lowest near dusk at 17–21 MLT. We also showed statistically that the CNA steep increases expanded eastward on the dawn side and westward on the dusk side. The CNA expansion velocity was slightly faster than the results of previous studies in the auroral zone. Correlation and superposed epoch analyses of CNA with solar wind and geomagnetic parameters revealed that CNA intensity was dependent on the Interplanetary Magnetic Field Bz, Interplanetary Electric Field Ey, SYM‐H index, and SME index. These results indicate that the CNA at subauroral latitudes is closely related to solar wind and geomagnetic activities, and its propagation characteristics correspond to the dynamics of high energy electrons in the inner magnetosphere. Plain Language Summary: The inner magnetosphere close to the Earth contains plasma particles with a wide range of energies. It is important to understand the dynamics of high‐energy electrons in the inner magnetosphere for safe space utilization and space weather forecast. This study focuses on the increase in electron density in the Earth's lower ionosphere caused by high‐energy electrons coming down into the atmosphere, based on measurement of cosmic radio noise absorption (CNA) in the ionosphere. Faint radio noise from external astronomical sources can show such absorption, and is slightly less intense, when the ionosphere is bombarded by such electrons. We studied the development in time and space of CNA at multiple ground stations at subauroral latitudes over 3 years. We found statistically that CNA was most likely to occur at midnight and expanded eastward on the dawn side and westward on the dusk side. We also found some correlation between CNA and solar wind and geomagnetic parameters. These results suggest that CNA is closely related to solar wind and geomagnetic activities and shows us the dynamics of high‐energy electrons in the inner magnetosphere. Key Points: This is the first statistical study of longitudinal development of cosmic noise absorption (CNA) using six riometers at subauroral latitudesThe occurrence rate of steep increases in CNA is high at 22‐08 magnetic local time (MLT) and low at 17–21 MLTCNA at subauroral latitudes is closely related to solar wind and geomagnetic activities [ABSTRACT FROM AUTHOR]