Microbursts are impulsive (<1 s) injections of electrons into the atmosphere, thought to be caused by nonlinear scattering by chorus waves. Although attempts have been made to quantify their ...contribution to outer belt electron loss, the uncertainty in the overall size and duration of the microburst region is typically large, so that their contribution to outer belt loss is uncertain. We combine datasets that measure chorus waves (Van Allen Probes RBSP, Arase, ground‐based VLF stations) and microburst (>30 keV) precipitation (FIREBIRD II and AC6 CubeSats, POES) to determine the size of the microburst‐producing chorus source region beginning on 5 December 2017. We estimate that the long‐lasting (∼30 hr) microburst‐producing chorus region extends from 4 to 8 Δ ${\Delta}$MLT and 2–5 Δ ${\Delta}$L. We conclude that microbursts likely represent a major loss source of outer radiation belt electrons for this event.
Plain Language Summary
Microbursts are short‐duration (<1 s) bursts of electrons that precipitate from the magnetosphere into the atmosphere. Microbursts are thought to be a result of scattering by a plasma wave called chorus. Attempts have been made to understand the contribution microburst precipitation has on electron loss, which helps the outer radiation belt recover after enhancements during storms. The contribution depends on the overall size and duration of the microburst region. We combine datasets that measure chorus waves and microburst precipitation to determine the size and duration of a microburst region beginning on 5 December 2017. Our results show that microbursts are likely a significant source of electron loss.
Key Points
We use multipoint observations to estimate the size of a long‐lasting microburst‐producing chorus region beginning on 5 December 2017
We estimate that the microburst‐producing chorus region for this event extends from 4 to 8 Δ ${\Delta}$MLT and 2–5 Δ ${\Delta}$L
Microburst precipitation from this event likely constitutes a major source of electron loss from the outer radiation belt
Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of ...the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north‐south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres.
Plain Language Summary
Occasionally, the aurora suddenly brightens in local areas of the Northern and Southern Hemispheres, causing explosive and widespread phenomena. At this time, it is known that a part of the aurora travels toward east or west from the point of the explosion; however, the formation mechanism of this traveling aurora is unknown. Recently, this traveling aurora has been reproduced by computer simulation, and it has been shown that the direction of its traveling is determined by the state of the ionosphere rather than by the projected motion of the source region in the magnetosphere. In this paper, we observed traveling auroras with two identical high‐speed cameras installed in the Northern and Southern Hemispheres simultaneously and clarified for the first time that the state of the ionosphere determines the direction of the traveling aurora. Understanding the mechanism of such connection between the space and the Earth is important for predicting the space environment required for various human activities and technologies using the space.
Key Points
An auroral breakup was captured simultaneously by ground‐based all‐sky cameras at magnetically conjugate stations in both hemispheres
The auroral surge evolved asymmetrically regarding the longitudinal direction in both hemispheres
The surge evolved in the direction of the ionospheric Hall current, which is consistent with the expectation from a global simulation
A three‐dimensional fully kinetic particle‐in‐cell simulation of antiparallel magnetic reconnection is performed to investigate the three‐dimensional development of reconnection jet fronts treating ...three instabilities: the lower hybrid drift instability (LHDI), the ballooning/interchange instability (BICI), and the ion‐ion kink instability. Sufficiently large system size and high ion‐to‐electron mass ratio of the simulation allow us to see the coupling among the three instabilities in the fully kinetic regime for the first time. As the jet fronts develop, the LHDI and BICI become dominant over the ion‐ion kink instability. The rapid growth of the LHDI enhances the BICI growth and the resulting formation of finger‐like structures. The small‐scale front structures produced by these instabilities are similar to recent high‐resolution field observations of the dipolarization fronts in the near‐Earth magnetotail using Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Cluster spacecraft and pose important questions for a future full high‐resolution observation by the Magnetospheric Multiscale (MMS) mission.
Key Points
A large‐scale fully kinetic simulation shows the coupling among the LHDI, BICI, and the ion‐ion kink instability at reconnection jet fronts
The LHDI and BICI are dominant at sharp fronts, and the LHDI‐BICI coupling leads to the formation of finger‐like structures of the fronts
The results are reasonably consistent with observations of the dipolarization fronts by Cluster and THEMIS and give implications to MMS
Inner magnetospheric electrons are precipitated in the ionosphere via pitch‐angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic ...(ECH) waves. However, the PA scattering efficiency of low‐energy electrons (0.1–10 keV) has not been investigated via in situ observations because of difficulties in flux measurements within loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energy ranges using the Arase (ERG) satellite observation data and successively detected the moderate loss cone filling, that is, approaching strong diffusion. Approaching strong diffusion by LBC, UBC, and ECH waves occurred at ∼2–20 keV, ∼1–10 keV, and ∼0.1–2 keV, respectively. The occurrence rate of the approaching strong diffusion by high‐amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ∼70%, ∼40%, and ∼30% higher than that without simultaneous wave activity. The energy range in which the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory.
Key Points
The pitch‐angle scattering efficiencies by plasma waves are statistically investigated using in situ observations
Lower band chorus waves caused approaching strong diffusion with the highest occurrence rate in the energy range of ∼2–20 keV
Electrostatic electron cyclotron harmonic waves could contribute approaching strong diffusion in the ∼0.1–1 keV energy range
The Arase satellite observed clear dipolarization signatures at r~4.3–4.6 RE, GMLAT~16°–18°, and MLT~5.5–5.7 hr around 15:00 UT on 27 March 2017 when Dst~−70 nT. Strong magnetic field fluctuations ...were embedded and their characteristic frequency was close to the local gyrofrequency of O+ ions. After the dipolarization, O+ flux was enhanced at ≤15 keV, while H+ flux showed no clear variations. These observations provide evidence for the direct supply of O+ ions from the ionosphere. There were no clear signatures for the nonadiabatic local acceleration of O+ ions. We consider that a bump‐on‐tail structure in the energy spectrum around 30–50 keV due to a combination of charge exchange loss and drift motion of ions masks the nonadiabatic acceleration. Occurrence of the magnetic field dipolarization at dawn, which is far from the well‐known premidnight occurrence peak, may be due to an eastward skewing of partial ring current during the storm main phase.
Plain Language Summary
The Exploration of energization and Radiation in Geospace (ERG) “Arase” satellite was launched by Japan Aerospace Exploration Agency on 20 December 2016. Arase was placed in an elliptical orbit having initially a perigee of ~460‐km altitude and an apogee of 6.0 RE to survey the inner magnetosphere. This unique orbit makes it possible for Arase to successfully observe a clear magnetic field dipolarization and its associated ion flux variations in the dawnside deep inner magnetosphere, which have not been reported in previous studies. Observations provide evidence for the direct supply of O+ ions from the ionosphere to the deep inner magnetosphere. Occurrence of the magnetic field dipolarization at dawn, which is far from the well‐known premidnight occurrence peak, may be due to an eastward skewing of partial ring current during the storm main phase.
Key Points
Magnetic field dipolarization was observed by the Arase satellite at r~4.3–4.6 RE and dawn during the 27 March 2017 storm
The dipolarization was accompanied by strong magnetic fluctuations and O+ flux enhancements at ≤15 keV
Occurrence of the dipolarization at dawn may be due to an eastward skewing of partial ring current during the storm main phase
The temporal variation of the energetic electron flux distribution caused by whistler mode chorus waves through the cyclotron resonant interaction provides crucial information on how electrons are ...accelerated in the Earth's inner magnetosphere. This study employs a data‐driven test‐particle simulation which demonstrates that the rapid change of energetic electron distribution observed by the Arase satellite cannot be simply explained by a quasi‐linear diffusion mechanism, but is essentially caused by nonlinear scattering: the phase trapping and the phase dislocation. In response to upper‐band whistler chorus bursts, multiple nonlinear interactions finally achieve an efficient flux enhancement of electrons on a time scale of the chorus burst. A quasi‐linear diffusion model tends to underestimate the flux enhancement of energetic electrons as compared with a model based on the realistic dynamic frequency spectrum of whistler waves. It is concluded that the nonlinear phase trapping plays an important role in the rapid flux enhancement of energetic electrons observed by Arase.
Plain Language Summary
Energetic electrons could be a cause of satellite anomalies affected by electric discharge phenomena on its surface and interior materials. To minimize the anomalies through satellite operation, it is important to forecast the temporal variation of the energetic electron flux along the trajectories of a satellite. One of the causes of the variation of the electron flux is whistler mode waves, which are right‐handed, circularly polarized electromagnetic waves that can resonate with energetic electrons. To understand how the electrons are accelerated in realistic situations, we have performed a data‐driven numerical simulation to demonstrate electron scattering, by importing the observation data of the Arase satellite directly to the simulation. Results of the simulation reproduce the temporal variations of energetic electron flux distributions in burst of whistler mode waves. It is found that the nonlinear scattering contributes to the flux enhancement of energetic electrons. It is confirmed that a quasi‐linear diffusion model, which has been used in general so far, cannot explain such a rapid flux enhancement. We conclude that the nonlinear scattering caused by the whistler burst plays an important role in the rapid flux enhancement of energetic electrons observed by the Arase satellite.
Key Points
The data‐driven simulation of rapid flux enhancement has been performed using plasma/particle and wave data obtained by Arase
The simulation results reproduce the observed temporal variations of energetic electron flux distributions
The nonlinear phase trapping contributes to the flux enhancement of electrons above 20 keV
Storm‐time region‐2 field‐aligned currents (R2 FACs) are believed to be connected between the ring current region and the ionosphere, but this connection has not been clarified by simultaneous ...in situ observations. We confirmed the connection of upward R2 FACs during 16 July and 18 June 2017 storm events using coordinated magnetic observations by the Arase satellite in medium‐Earth orbit and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). The upward FACs were determined by drastic changes in the azimuthal magnetic field at Arase in the off‐equatorial (3‐ to 4‐RE radial distance and 1–2 RE above the magnetic equator) postmidnight inner magnetosphere. The magnetic latitude of the FAC observed by Arase projected onto the ionosphere was consistent with that of the ionospheric FAC observed by the AMPERE. Using the conservation of the ratio between the current density and the total magnetic field along the field line, we showed that the current between Arase and AMPERE was almost conserved, meaning that a large portion of the R2 FAC was generated in the low‐latitude inner magnetosphere. We also calculated the plasma pressures of H+ and O+ ions and pressure‐driven currents to examine their relationship for the first event. The O+ pressure contributed significantly to the inner part of the total azimuthal current. The peaks of combined pressure of H+ and O+, and pressure‐driven currents were located inside and outside the FAC, respectively. A simple model calculation indicated that this spatial relationship is controlled by the day‐night asymmetry of magnetic field.
Key Points
Connection of FACs in the inner magnetosphere and in the ionosphere was quantitatively confirmed by Arase and AMPERE observations
We present simultaneous in situ observations of storm‐time region‐2 FACs and the ring current derived from plasma pressures
The region‐2 FAC was located at the inner part of the ring current that was significantly contributed to by O+ ions
Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm‐associated magnetospheric plasma dynamics to the ...ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground‐based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east‐west structures with longitudinal scales of ∼0.5°–2.0°. Field‐aligned bi‐directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti‐phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6.
Plain Language Summary
A frequently occurring source of variations in the magnetosphere is the substorm, a process that causes energy dissipation into the atmosphere. Substorm is presented as the development of aurorae at high latitudes in the ionosphere. The study of substorm processes helps in understanding the near‐Earth space environment and the space weather. Along Earth's magnetic field lines, the aurora at a latitude of ∼65°N can be traced to ∼4–7 Earth radii away from the Earth at the equatorial plane in space. Using a ground‐based auroral camera, we can construct the correspondence between auroral motion and field and plasma variation at the satellite. This study reports such a unique event of substorm brightening arc observed at Tromsø, Norway, on 12 October 2017. Satellite observed bi‐directional electrons prove the connection between aurora break‐up at ∼100 km altitude and its source region in the magnetosphere at ∼30,000 km away from Earth. Based on the magnetic wave spectrograms, auroral bead‐like structures and other observational results, we suggest the possibility that a ballooning plasma instability occurred in the source region of the substorm brightening arc in the inner magnetosphere.
Key Points
Observation of plasma and field features in the source region of a sudden brightening auroral arc during a minor substorm onset at L ∼ 6
Energization of particles, field‐aligned electrons, and electromagnetic field fluctuations were observed during the arc crossing by Arase
Several observational facts indicate the possibility of ballooning instability occurring at this substorm onset
Recent simulation studies using the RAM‐SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the ...magnetosphere‐ionosphere coupling. In this study, we use the BATS‐R‐US + RAM‐SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave‐particle interactions. During the 27 May 2017 storm, Arase and RBSP‐A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk‐midnight sector. Simulation results show that H‐ and He‐band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP‐A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H‐band waves in the dusk sector are stronger than He‐band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS‐R‐US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm.
Plain Language Summary
During geomagnetic storms, plasma waves are generated in the Earth's magnetosphere. Among these waves, ElectroMagnetic Ion Cyclotron (EMIC) waves can scatter protons from the ring current, causing them to precipitate into the subauroral ionosphere. Such precipitation not only affects the midlatitude ionosphere but also impacts the dynamics of the magnetosphere. Understanding the origin of magnetospheric plasma waves and how they interact with the magnetospheric populations, along with their subsequent impact on the ionosphere, is crucial for predicting space weather accurately. In our study, we combined ground and satellite observations with simulations using the BATS‐R‐US + RAM‐SCB to investigate EMIC wave‐particle interactions in the inner magnetosphere and the resulting proton precipitation during the 27 May 2017 storm. We found that EMIC waves were excited in the dusk‐midnight sector during the storm's main phase, within the regions of strong temperature anisotropy. The simulations reproduced the proton precipitation observed in the dusk‐midnight sector by the Defense Meteorological Satellite Program /National Oceanic and Atmospheric Administration MetOP satellites fairly well. The model qualitatively captured the growth of the EMIC waves during the storm and showed that the EMIC waves, by scattering the ring current, were responsible for the proton precipitation into the dusk‐midnight sector during the storm.
Key Points
ElectroMagnetic Ion Cyclotron (EMIC) wave activity and proton precipitation were observed simultaneously in the dusk‐midnight sector during the 27 May 2017 storm
The BATS‐R‐US + RAM‐SCB model can capture the EMIC wave growth during the storm qualitatively
The EMIC wave scattering of ring current ions can account for the proton precipitation in the dusk‐midnight sector during the storm
An energy spectrum of electrons from 180 to 550 keV precipitating into the dayside polar ionosphere was observed under a geomagnetically quiet condition (AE ≤ 100 nT, Kp = 1‐). The observation was ...carried out at 73–184 km altitudes by the HEP instrument onboard the RockSat‐XN sounding rocket that has been launched from Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of −4.9 ± 0.4 and the electron flux does not vary much over the observation period (∼274.4 s). A nearby ground‐based VLF receiver observation at Lovozero, Russia shows the presence of whistler‐mode wave activities during the rocket observation. A few minutes before the RockSat‐XN observation, POES18/MEPED observed precipitating electrons, which also suggest whistler‐mode chorus wave activities at the location close to the rocket trajectory. A test‐particle simulation for wave‐particle interactions was carried out using the data of the Arase satellite as the initial condition which was located on the duskside. The result of the simulation shows that whistler‐mode waves can resonate with sub‐relativistic electrons at high latitudes. These results suggest that the precipitation observed by RockSat‐XN is likely to be caused by the wave‐particle interactions between whistler‐mode waves and sub‐relativistic electrons.
Plain Language Summary
Sub‐relativistic electrons precipitating into the Earth's dayside polar ionosphere are observed by a sounding rocket under geomagnetically quiet conditions. An energy spectrum of these electrons in an energy range from 180 to 550 keV is reported at the rocket altitude. A possible mechanism for generating this precipitation is the resonance scattering of electrons by whistler‐mode waves, which we conducted a test‐particle simulation based on the ground and satellite observations.
Key Points
A sounding rocket observed an energy spectrum of sub‐relativistic electron precipitation in the dayside polar ionosphere during quiet time
Ground and satellite observations suggest that the precipitation observed by RockSat‐XN was caused by the whistler‐mode waves
A test‐particle simulation for wave‐particle interactions based on the data of the Arase satellite supports the RockSat‐XN observation
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