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  • 1
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    A THEMIS multicase study of dipolarization fronts in the magnetotail plasma sheet (2011)
    Wiley
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    Publication Date: 2011-05-24
    Description: We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (δt 〈 30 s), high-amplitude (δBz 〉 10 nT) increases in the northward magnetic field component, observed during six Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2–3 increase in high-energy (30–200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the Bz increase are, on average, 20 nA/m2, 5–7 times larger than the current density in the cross-tail current sheet. Because j · E 〉 0, the dipolarization fronts are kinetic-scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    THEMIS observations of ULF wave excitation in the nightside plasma sheet during sudden impulse events (2013)
    Wiley
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    Publication Date: 2013-01-25
    Description: [1]  Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI-induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East-west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest-amplitude ULF responses to SIs in the nightside plasma sheet are tailward-moving vortices, which have previously been reported, and the dynamic response of cross-tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfvén waves (toroidal modes) observed via the field-line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Electron dropout echoes induced by interplanetary shock: a statistical study (2017)
    Wiley
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    Publication Date: 2017-06-29
    Description: “Electron dropout echo” as indicated by repeated moderate dropout and recovery signatures of the flux of energetic electron in the outer radiation belt region has been investigated systematically. The electron moderate dropout and its echoes are usually found for higher energy (〉 300 keV) channels fluxes, whereas the flux enhancements are obvious for lower energy electrons simultaneously after the interplanetary shock arrives at the Earth's geosynchronous orbit. The electron dropout echo events are found to be usually associated with the interplanetary shocks arrival. 104 dropout echo events have been found from 215 interplanetary shock events from 1998 to 2007 based on LANL satellite data. In analogy to substorm injections, these 104 events could be naturally divided into two categories: dispersionless (49 events) or dispersive (55 events) according to the energy dispersion of the initial dropout. It is found that locations of dispersionless events are distributed mainly in the duskside magnetosphere. Further, the obtained locations derived from dispersive events with the time-of-flight technique of the initial dropout regions are mainly located at the duskside as well. Statistical studies have shown that the effect of shock normal, interplanetary magnetic field B z and solar wind dynamic pressure may be insignificant to these electron dropout events. We suggest that the ∼ 1 minute electric field impulse induced by the interplanetary shock produces a more pronounced inward migration of electrons at the dusk side, resulting in the observed dusk-side moderate dropout of electron flux and its consequent echoes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Magnetospheric ULF waves with increasing amplitude related to solar wind dynamic pressure changes: THEMIS observations (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: Ultra-low frequency (ULF) waves play an important role in transferring energy by buffeting the magnetosphere with solar wind pressure impulses. The amplitudes of magnetospheric ULF waves, which are induced by solar wind dynamic pressure enhancements or shocks, are thought to damp in one half a wave cycle or an entire wave cycle. We report in situ observations of solar wind dynamic pressure impulse-induced magnetospheric ULF waves with increasing amplitudes. We found six ULF wave events induced by solar wind dynamic pressure enhancements with slow but clear wave amplitude increase. During three or four wave cycles, the amplitudes of ion velocities and electric field of these waves increased continuously by 1.3 ~ 4.4 times. Two significant events were selected to further study the characteristics of these ULF waves. We found that the wave amplitude growth is mainly contributed by the toroidal mode wave. Three possible mechanisms of causing the wave amplitude increase are discussed. Firstly, solar wind dynamic pressure perturbations, which are observed in a duration of 20 ~ 30 minutes, might transfer energy to the magnetospheric ULF waves continually. Secondly, the wave amplitude increase in the radial electric field may becaused by superposition of two wave modes, a standing wave excited by the solar wind dynamic impulse and a propagating compressional wave directly induced by solar wind oscillations. When superposed, the two wave modes fit observations as does a calculation that superposes electric fields from two wave sources. Thirdly, the normal of the solar wind discontinuity is at an angle to the Sun-Earth line. Thus, the discontinuity will affect the dayside magnetopause continuously for a long time.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Average Thermodynamic and Spectral Properties of Plasma in and Around Dipolarizing Flux Bundles (2015)
    Wiley
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    Publication Date: 2015-05-09
    Description: Recent observations have suggested that spatially localized flows of high temperature, low density plasma carrying a dipolarized magnetic field (dipolarizing flux bundles, DFB) play a key role in hot plasma transport toward the inner magnetosphere? What controls plasma heating in DFBs and how do thermodynamic parameters (such as density, temperature, pressure, specific entropy) and spectral properties of the DFB population depend on ambient plasma sheet properties and geocentric distance R remains unknown. By statistical analysis of 271 DFB events detected by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission during the 2008–2009 tail seasons, we find that on average, plasma inside DFBs is a factor of 0.6 less dense and a factor of 1.5 to 2 hotter than ambient tail plasma. The radial profiles of average thermodynamic parameters inside and outside DFBs are similar; when fitted by the κ -function, their energy spectra have similar κ -exponents, but a factor of 2 larger peak energies inside DFBs. Our analysis suggests that average DFB plasma properties are closely linked to those of the ambient plasma sheet population. Estimations show that on average, adiabatic heating of the ambient plasma in the increased magnetic field is the major factor in DFB plasma heating.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    The Geometry of an Electron Scale Magnetic Cavity in the Plasma Sheet (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Electron scale magnetic cavities are electron vortex structures formed in turbulent plasma, while the evolution and electron dynamics of these structures have not been fully understood. Recently, high‐energy, angular, and temporal electron measurements from Magnetospheric Multiscale have enabled the application of an energetic particle sounding technique to these structures. This study analyzes an electron scale magnetic cavity observed by Magnetospheric Multiscale on 7 May 2015 in the plasma sheet. A comprehensive sounding technique is applied to obtain the geometry and propagation velocities of the boundaries. The result shows that the scale size of the structure is ∼90 km, and the leading and trailing boundaries are moving in the same direction but with different speeds (∼11.5 ± 2.2 and ∼18.1 ± 3.4 km/s, respectively). The speed difference suggests a shrinking of the structure that may play a significant role in magnetic energy dissipation and electron energization of electron scale magnetic cavities.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    ULF Waves Modulating and Acting as Mass Spectrometer for Dayside Ionospheric Outflow Ions (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Ionospheric outflow has been shown to be a dominant ion source of Earth's magnetosphere. However, most studies in the literature are about ionospheric outflow injected into the nightside magnetosphere. We still know little about ionospheric outflow injected into the dayside magnetosphere and its further energization after it enters the magnetosphere. Here, with data from Magnetospheric Multiscale mission, we report direct observations of the modulation of dayside ionospheric outflow ions by ultralow frequency (ULF) waves. The observations indicate that the modulation is mass dependent, which demonstrates the possibility of using ULF waves as a mass spectrometer to identify ion species. Moreover, the measurement suggests that polarization drift may play a role in O+ modulation, which may lead to a true acceleration and even nonadiabatic behavior of O+. This interaction scenario can work throughout the whole magnetosphere and impact upon the plasma environment and dynamics.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 8
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    Ultra-low-frequency wave-driven diffusion of radiation belt relativistic electrons (2015)
    Springer Nature
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    Publication Date: 2015-12-24
    Description: Article The Van Allen radiation belts are two zones of energetic particles encircling the Earth, but how electrons are accelerated to relativistic energies remains unclear. Here, the authors analyse a radiation belt event and provide evidence in favour of the ULF wave-driven radial diffusion mechanism. Nature Communications doi: 10.1038/ncomms10096 Authors: Zhenpeng Su, Hui Zhu, Fuliang Xiao, Q.-G. Zong, X.-Z. Zhou, Huinan Zheng, Yuming Wang, Shui Wang, Y.-X. Hao, Zhonglei Gao, Zhaoguo He, D. N. Baker, H. E. Spence, G. D. Reeves, J. B. Blake, J. R. Wygant
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 9
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    Compressional ULF wave modulation of energetic particles in the inner magnetosphere (2016)
    Wiley
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    Publication Date: 2016-05-31
    Description: We present Van Allen Probes observations of modulations in the flux of very energetic electrons up to a few MeV and protons between 1200 − 1400 UT on February 19th, 2014. During this event the spacecraft were in the dayside magnetosphere at L ⋆ ≈5.5. The modulations extended across a wide range of particle energies, from 79.80 keV to 2.85 MeV for electrons and from 82.85 keV to 636.18 keV for protons. The fluxes of π /2 pitch angle particles were observed to attain maximum values simultaneously with the ULF compressional magnetic field component reaching a minimum. We use peak-to-valley ratios to quantify the strength of the modulation effect, finding that the modulation is larger at higher energies than at lower energies. It is shown that the compressional wave modulation of the particle distribution is due to the mirror effect, which can trap relativistic electrons efficiently for energies up to 2.85 MeV , and trap protons up to ≈600 keV . Larger peak-to-valley ratios at higher energies also attributed to the mirror effect. Finally, we suggest that protons with energies higher than 636.18 keV can not be trapped by the compressional ULF wave efficiently due to the finite Larmor radius effect.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Interaction of ULF waves with different ion species: pitch angle and phase space density implications (2016)
    Wiley
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    Publication Date: 2016-09-18
    Description: ULF waves can accelerate/decelerate the charged particles including the ring current ions via drift-bounce resonance, which play an important role in the dynamics of ring current during storm times. This study compares the different behaviors of oxygen ions (10.5-35.1keV) and protons (0.3-12.3keV) which simultaneously interact with Pc5 ULF waves observed by Cluster on June 03, 2003. The ULF waves are identified as the fundamental mode oscillations. Both oxygen ions and protons show periodic energy dispersion and pitch angle dispersion signatures, which satisfy the drift-bounce resonance condition of N = 2. The different behaviors of oxygen ions and protons include: (1) the resonant energy of oxygen ions is higher than that of protons due to mass difference; (2) the phase space density (PSD) of oxygen ions show relative variations (3.6-6.3) that are much larger than that of protons ( 〈 0.4), which indicates a more efficient energy exchange between oxygen ions and ULF waves; (3) the PSD spectra show that oxygen ions are accelerated, while protons are decelerated, which depend on the radial gradient of their PSD; (4) the pitch angle distributions (PADs) of the oxygen ions and protons show negative slope and bidirectional field-aligned features, respectively, which is related to the preexisting state of ion PADs before the interaction with the ULF waves. In addition, the resonant ions with peak fluxes tracing back to the magnetic equator are always collocated with the accelerating (westward) electric field, which indicate that the ions are mainly accelerated near the magnetic equator and the electric field intensity of ULF waves peaks there.
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    Topics: Geosciences , Physics
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