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"Pollock, C."
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Direct observations of energy transfer from resonant electrons to whistler-mode waves in magnetosheath of Earth
Electromagnetic whistler-mode waves in space plasmas play critical roles in collisionless energy transfer between the electrons and the electromagnetic field. Although resonant interactions have been considered as the likely generation process of the waves, observational identification has been extremely difficult due to the short time scale of resonant electron dynamics. Here we show strong nongyrotropy, which rotate with the wave, of cyclotron resonant electrons as direct evidence for the locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves using ultra-high temporal resolution data obtained by NASA’s Magnetospheric Multiscale (MMS) mission in the magnetosheath. The nongyrotropic electrons carry a resonant current, which is the energy source of the wave as predicted by the nonlinear wave growth theory. This result proves the nonlinear wave growth theory, and furthermore demonstrates that the degree of nongyrotropy, which cannot be predicted even by that nonlinear theory, can be studied by observations.
Excitation of whistler-mode waves by cyclotron instability is considered as the likely generation process of the waves. Here, the authors show direct observational evidence for locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves in Earth’s magnetosheath.
Journal Article
The Jovian Auroral Distributions Experiment (JADE) on the Juno Mission to Jupiter
The Jovian Auroral Distributions Experiment (JADE) on Juno provides the critical
in situ
measurements of electrons and ions needed to understand the plasma energy particles and processes that fill the Jovian magnetosphere and ultimately produce its strong aurora. JADE is an instrument suite that includes three essentially identical electron sensors (JADE-Es), a single ion sensor (JADE-I), and a highly capable Electronics Box (EBox) that resides in the Juno Radiation Vault and provides all necessary control, low and high voltages, and computing support for the four sensors. The three JADE-Es are arrayed 120
∘
apart around the Juno spacecraft to measure complete electron distributions from ∼0.1 to 100 keV and provide detailed electron pitch-angle distributions at a 1 s cadence, independent of spacecraft spin phase. JADE-I measures ions from ∼5 eV to ∼50 keV over an instantaneous field of view of 270
∘
×90
∘
in 4 s and makes observations over all directions in space each 30 s rotation of the Juno spacecraft. JADE-I also provides ion composition measurements from 1 to 50 amu with
m
/Δ
m
∼2.5, which is sufficient to separate the heavy and light ions, as well as O+ vs S+, in the Jovian magnetosphere. All four sensors were extensively tested and calibrated in specialized facilities, ensuring excellent on-orbit observations at Jupiter. This paper documents the JADE design, construction, calibration, and planned science operations, data processing, and data products. Finally, the
Appendix
describes the Southwest Research Institute [SwRI] electron calibration facility, which was developed and used for all JADE-E calibrations. Collectively, JADE provides remarkably broad and detailed measurements of the Jovian auroral region and magnetospheric plasmas, which will surely revolutionize our understanding of these important and complex regions.
Journal Article
Simultaneous macroscale and microscale wave–ion interaction in near-earth space plasmas
Identifying how energy transfer proceeds from macroscales down to microscales in collisionless plasmas is at the forefront of astrophysics and space physics. It provides information on the evolution of involved plasma systems and the generation of high-energy particles in the universe. Here we report two cross-scale energy-transfer events observed by NASA’s Magnetospheric Multiscale spacecraft in Earth’s magnetosphere. In these events, hot ions simultaneously undergo interactions with macroscale (~
10
5
km) ultra-low-frequency waves and microscale (
~
10
3
km) electromagnetic-ion-cyclotron (EMIC) waves. The cross-scale interactions cause energy to directly transfer from macroscales to microscales, and finally dissipate at microscales via EMIC-wave-induced ion energization. The direct measurements of the energy transfer rate in the second event confirm the efficiency of this cross-scale transfer process, whose timescale is estimated to be roughly ten EMIC-wave periods about (1 min). Therefore, these observations experimentally demonstrate that simultaneous macroscale and microscale wave-ion interactions provide an efficient mechanism for cross-scale energy transfer and plasma energization in astrophysical and space plasmas.
Cross-scale energy transfers in collisionless plasmas help understanding involved mechanisms. Here, the authors show simultaneous macro- and micro-scale wave-ion interactions provide an efficient mechanism for cross-scale energy transfer and plasma energization in astrophysical and space plasmas.
Journal Article
Directly observing the magnetic rope contraction and expansion in space
One of the most fundamental hypotheses proposed to explain the electron acceleration in astrophysical and space plasmas is that the magnetic rope can contract and expand during a short period. However, such contraction and expansion of magnetic rope have never been directly evidenced hitherto. Targeting this longstanding problem, here we provide direct evidence for the magnetic rope contraction and expansion by utilizing the first-order Taylor expansion method and the magnetospheric multiscale measurements. The contraction and expansion of magnetic ropes happen during a few seconds in high-speed plasma flows, with the contraction related to an increase of pressure inside the rope and the expansion related to a decrease of pressure. Excitingly, during the magnetic rope contraction we observe electron acceleration, whereas during the magnetic rope expansion we observe electron deceleration. These findings have robustly validated the fundamental hypothesis in astrophysics, i.e., electrons can be accelerated by contracting magnetic ropes.
The magnetic flux rope is a crucial structure in astrophysical and space plasmas. Here, the authors show how this structure rapidly contracts/expands and consequently how it accelerates/decelerates electrons, by using an advanced analysis technique.
Journal Article
Observational evidence of accelerating electron holes and their effects on passing ions
As a universal structure in space plasma, electron holes represent an obvious signature of nonlinear process. Although the theory has a 60-year history, whether electron hole can finally accelerate ambient electrons (or ions) is quite controversial. Previous theory for one-dimensional holes predicts that net velocity change of passing electrons (or ions) occurs only if the holes have non-zero acceleration. However, the prediction has not yet been demonstrated in observations. Here, we report four electron holes whose acceleration/deceleration is obtained by fitting the spatial separations and detection time delays between different Magnetospheric Multiscale spacecraft. We find that electron hole acceleration/deceleration is related to the ion velocity distribution gradient at the hole’s velocity. We observe net velocity changes of ions passing through the accelerating/decelerating holes, in accordance with theoretical predictions. Therefore, we show that electron holes with non-zero acceleration can cause the velocity of passing ions to increase in the acceleration direction.
Electron holes with drift speeds comparable to local ion thermal velocity are called slow electron holes. Here, the authors show slow electron holes with non-zero acceleration can cause net velocity change of ions passing through.
Journal Article
Particle-sounding of the spatial structure of kinetic Alfvén waves
Kinetic Alfvén waves (KAWs) are ubiquitous throughout the plasma universe. Although they are broadly believed to provide a potential approach for energy exchange between electromagnetic fields and plasma particles, neither the detail nor the efficiency of the interactions has been well-determined yet. The primary difficulty has been the paucity of knowledge of KAWs’ spatial structure in observation. Here, we apply a particle-sounding technique to Magnetospheric Multiscale mission data to quantitatively determine the perpendicular wavelength of KAWs from ion gyrophase-distribution observations. Our results show that KAWs’ perpendicular wavelength is statistically 2.4
±
0.7
times proton thermal gyro-radius. This observation yields an upper bound of the energy the majority proton population can reach in coherent interactions with KAWs, that is, roughly 5.76 times proton perpendicular thermal energy. Therefore, the method and results shown here provide a basis for unraveling the effects of KAWs in dissipating energy and accelerating particles in a number of astrophysical systems, e.g., planetary magnetosphere, astrophysical shocks, stellar corona and wind, and the interstellar medium.
Kinetic Alfven Waves (KAWs) are ubiquitous in space plasmas. Here, the authors show that application of particle sounding technique to Magnetospheric Multiscale Mission data enables measuring perpendicular wavelength of KAWs.
Journal Article
Electron magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath
Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region
1
,
2
. On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed
3
–
5
. Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region
6
. In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales
7
–
11
. However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth’s turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.
Observations of electron-scale current sheets in Earth’s turbulent magnetosheath reveal electron reconnection without ion coupling, contrary to expectations from the standard model of magnetic reconnection.
Journal Article
Wave setup over a Pacific Island fringing reef
Measurements obtained across a shore‐attached, fringing reef on the southeast coast of the island of Guam are examined to determine the relationship between incident waves and wave‐driven setup during storm and nonstorm conditions. Wave setup on the reef flat correlates well (r > 0.95) and scales near the shore as approximately 35% of the incident root mean square wave height in 8 m water depth. Waves generated by tropical storm Man‐Yi result in a 1.3 m setup during the peak of the storm. Predictions based on traditional setup theory (steady state, inviscid cross‐shore momentum and depth‐limited wave breaking) and an idealized model of localized wave breaking at the fore reef are in agreement with the observations. The reef flat setup is used to estimate a similarity parameter at breaking that is in agreement with observations from a steeply sloping sandy beach. A weak (∼10%) increase in setup is observed across the reef flat during wave events. The inclusion of bottom stress in the cross‐shore momentum balance may account for a portion of this signal, but this assessment is inconclusive as the reef flat currents in some cases are in the wrong direction to account for the increase. An independent check of fringing reef setup dynamics is carried out for measurements at the neighboring island of Saipan with good agreement.
Journal Article
Efficient acceleration of energetic electrons upstream of Earth’s bow shock
It is widely believed that astrophysical shocks can accelerate particles to ultra-relativistic energy, via the well-established diffusive shock acceleration mechanism. However, this mechanism requires seed particles with kinetic energy sufficiently high, whose origin is still an enigma. Here we show observational confirmation of an efficient electron pre-acceleration mechanism at the Earth’s bow shock. This mechanism relies on a special V-shaped magnetic field configuration in the upstream solar wind, which channels the shock-reflected electrons back and thus enables them to be reflected by the shock many times. This special field configuration arises when a solar-wind discontinuity—an ubiquitous and inherent structure in space plasmas—approaches and intersects the shock. The acceleration scenario is further confirmed by test-particle and numerical methods. The results demonstrate its ability to accelerate low-energy (approximately 17 eV) solar-wind electrons to >200
k
B
T
e
. This study therefore provides important insights into the injection problem and generation of energetic particles in the universe.
How low-energy particles are pre-accelerated and injected into the diffusive shock acceleration process has long been a mystery. Here, the authors show the shock discontinuity interaction induces intense electron acceleration and may be a solution.
Journal Article