| 1. |
- Pickering, Jonathan, et al.
(författare)
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Pluralizing Debates on the Anthropocene Requires Engaging with the Diversity of Existing Scholarship
- 2023
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Ingår i: Annals of the American Association of Geographers. - : ROUTLEDGE JOURNALS, TAYLOR & FRANCIS LTD. - 2469-4452 .- 2469-4460. ; 113:2, s. e-i-e-vi
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A recent article in this journal (Jackson 2021) validly emphasized that debates about the Anthropocene need to recognize a diverse range of perspectives, worldviews, and forms of knowledge. In doing so, however, the author mischaracterized scholarship on earth system governance as being antithetical to a critical and pluralistic stance on the Anthropocene. In this commentary we address key concerns about the article: selective and misleading quotations regarding the earth system governance literatures diversity; unwarranted insinuations that juxtapose the implications of this literature with those of slavery and holocausts; and neglect of the breadth and diversity of scholarship on earth system governance. We underscore the need for scholarly debates on the Anthropocene to be informed by a balanced and rigorous assessment of existing scholarship, and for a constructive dialogue between global and locally situated ways of understanding the earth.
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| 2. |
- Catapano, Filomena, et al.
(författare)
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In Situ Evidence of Ion Acceleration between Consecutive Reconnection Jet Fronts
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 908:1
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Tidskriftsartikel (refereegranskat)abstract
- Processes driven by unsteady reconnection can efficiently accelerate particles in many astrophysical plasmas. An example is the reconnection jet fronts in an outflow region. We present evidence of suprathermal ion acceleration between two consecutive reconnection jet fronts observed by the Magnetospheric Multiscale mission in the terrestrial magnetotail. An earthward propagating jet is approached by a second faster jet. Between the jets, the thermal ions are mostly perpendicular to magnetic field, are trapped, and are gradually accelerated in the parallel direction up to 150 keV. Observations suggest that ions are predominantly accelerated by a Fermi-like mechanism in the contracting magnetic bottle formed between the two jet fronts. The ion acceleration mechanism is presumably efficient in other environments where jet fronts produced by variable rates of reconnection are common and where the interaction of multiple jet fronts can also develop a turbulent environment, e.g., in stellar and solar eruptions.
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| 3. |
- Dokgo, Kyunghwan, et al.
(författare)
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High-Frequency Waves Driven by Agyrotropic Electrons Near the Electron Diffusion Region
- 2020
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 47:5
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Tidskriftsartikel (refereegranskat)abstract
- National Aeronautics and Space Administration's Magnetosphere Multiscale mission reveals that agyrotropic electrons and intense waves are prevalently present in the electron diffusion region. Prompted by two distinct Magnetosphere Multiscale observations, this letter investigates by theoretical means and the properties of agyrotropic electron beam-plasma instability and explains the origin of different structures in the wave spectra. The difference is owing to the fact that in one instance, a continuous beam mode is excited, while in the other, discrete Bernstein modes are excited, and the excitation of one mode versus the other depends on physical input parameters, which are consistent with observations. Analyses of dispersion relations show that the growing mode becomes discrete when the maximum growth rate is lower than the electron cyclotron frequency. Making use of particle-in-cell simulations, we found that the broadening angle Delta in the gyroangle space is also an important factor controlling the growth rate. Ramifications of the present finding are also discussed. Plain Language Summary Magnetospheric Multiscale mission has observed magnetic reconnection process, which converts magnetic energy to kinetic energy of charged particles. Extremely rapid time scale data reveal that electron scale high-frequency waves exist near the electron diffusion region of magnetic reconnection. Recently, two different types of waves observed; one is discrete electron-Bernstein waves, and the other is continuous beam modes. In this study, we formulated a unified theory for both types of waves. Comparing Magnetosphere Multiscale observations, the theory, and particle-in-cell simulations, this study shows that the same cause (agyrotropic electrons) can make two different wave structures depending on plasma parameters. The condition that the maximum growth rate of instabilities equals the electron cyclotron frequency can be considered as a threshold of the transition from discrete electron Bernstein waves to continuous beam modes.
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| 4. |
- Dokgo, Kyunghwan, et al.
(författare)
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The Effects of Upper-Hybrid Waves on Energy Dissipation in the Electron Diffusion Region
- 2020
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 47:19
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Tidskriftsartikel (refereegranskat)abstract
- Using a two-dimensional particle-in-cell simulation, we investigate the effects and roles of upper-hybrid waves (UHW) near the electron diffusion region (EDR). The energy dissipation via the wave-particle interaction in our simulation agrees withJ center dot E(')measured by magnetospheric multiscale (MMS) spacecraft. It means that UHW contributes to the local energy dissipation. As a result of wave-particle interactions, plasma parameters which determine the larger-scale energy dissipation in the EDR are changed. They-directional current decreases while the pressure tensorP(yz)increases/decreases when the agyrotropic beam density is low/high, where(x, y, z)-coordinates correspond the(L, M, N)-boundary coordinates. Because the reconnection electric field comes from- partial differential P-yz/ partial differential z, our result implies that UHW plays an additional role in affecting larger-scale energy dissipation in the EDR by changing plasma parameters. We provide a simple diagram that shows how the UHW activities change the profiles of plasma parameters near the EDR comparing cases with and without UHW.
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| 5. |
- Lee, Justin H., et al.
(författare)
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Application of Cold and Hot Plasma Composition Measurements to Investigate Impacts on Dusk-Side Electromagnetic Ion Cyclotron Waves
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:1
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Tidskriftsartikel (refereegranskat)abstract
- An extended interval of perturbed magnetospheric conditions in November 2016 supported increased convection and sunward transport of plasmaspheric material. During this period of time the Magnetospheric Multiscale satellites, with their apogees along Earth's dusk-side outer magnetosphere, encountered several cold plasma density structures at the same time as plasma bulk flows capable of accelerating hidden cold plasma occurred. Investigating the charged particle and fields data during two subintervals showed that the satellites made direct measurements of cold plasmaspheric ions embedded within multicomponent hot plasmas as well as electromagnetic emissions consistent with electromagnetic ion cyclotron (EMIC) waves. The complex in situ ion composition measurements were applied to linear wave modeling to interpret the impacts of cold and hot ion species on wave growth and band structure. Although the waves for both intervals were predicted to have peak growth rate below omega(He+), substantial differences were observed among all other dispersive properties. The modeling also showed EMIC waves generated in the presence of heavy ions had growth rates and unstable wave numbers always smaller than predicted for a pure proton-electron plasma. The results provide implications for future investigation of EMIC wave generation with and without direct measurements of the cold and hot plasma composition as well as of subsequent wave-particle interactions.
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| 6. |
- Lewis, Harry C., et al.
(författare)
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Magnetospheric Multiscale measurements of turbulent electric fields in earth's magnetosheath : How do plasma conditions influence the balance of terms in generalized Ohm's law?
- 2023
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 30:8
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence is ubiquitous within space plasmas, where it is associated with numerous nonlinear interactions. Magnetospheric Multiscale (MMS) provides the unique opportunity to decompose the electric field (E) dynamics into contributions from different linear and nonlinear processes via direct measurements of the terms in generalized Ohm's law. Using high-resolution multipoint measurements, we compute the magnetohydrodynamic ( E MHD ), Hall ( E Hall ), electron pressure ( E P e ), and electron inertia ( E inertia ) terms for 60 turbulent magnetosheath intervals, to uncover the varying contributions to the dynamics as a function of scale for different plasma conditions. We identify key spectral characteristics of the Ohm's law terms: the Hall scale, k Hall , where E Hall becomes dominant over E MHD ; the relative amplitude of E P e to E Hall , which is constant in the sub-ion range; and the relative scaling of the nonlinear and linear components of E MHD and of E Hall , which are independent of scale. We find expressions for the characteristics as a function of plasma conditions. The underlying relationship between turbulent fluctuation amplitudes and ambient plasma conditions is discussed. Depending on the interval, we observe that E MHD and E Hall can be dominated by either nonlinear or linear dynamics. We find that E P e is dominated by its linear contributions, with a tendency for electron temperature fluctuations to dominate at small scales. The findings are not consistent with existing linear kinetic Alfvén wave theory for isothermal fluctuations. Our work shows how contributions to turbulent dynamics change in different plasma conditions, which may provide insight into other turbulent plasma environments.
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| 7. |
- Nilsson, Hans, et al.
(författare)
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Birth of a Magnetosphere
- 2021
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Ingår i: Magnetospheres in the Solar System. - : John Wiley & Sons. ; , s. 427-440, s. 427-439
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Bokkapitel (refereegranskat)abstract
- A magnetosphere may form around an object in a stellar wind either due to the intrinsic magnetic field of the object or stellar wind interaction with the ionosphere of the object. Comets represent the most variable magnetospheres in our solar system, and through the Rosetta mission we have had the chance to study the birth and evolution of a comet magnetosphere as the comet nucleus approached the Sun. We review the birth of the comet magnetosphere as observed at comet 67P Churyumov–Gerasimenko, the formation of plasma boundaries and how the solar wind–atmosphere interaction changes character as the cometary gas cloud and magnetosphere grow in size. Mass loading of the solar wind leads to an asymmetric deflection of the solar wind for low outgassing rates. With increasing activity a solar wind ion cavity forms. Intermittent shock‐like features were also observed. For intermediate outgassing rate a diamagnetic cavity is formed inside the solar wind ion cavity, thus well separated from the solar wind. The cometary plasma was typically very structured and variable. The region of the coma dense enough to have significant collisions forms a special region with different ion chemistry and plasma dynamics as compared to the outer collision‐free region.
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| 8. |
- Wang, Shan, et al.
(författare)
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Whistler Waves Associated With Electron Beams in Magnetopause Reconnection Diffusion Regions
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:9
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Tidskriftsartikel (refereegranskat)abstract
- Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (a) Narrow-band waves with high ellipticities and (b) broad-band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of broadband waves are excited via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle-in-cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above similar to 3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left-hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities.
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| 9. |
- Zhang, L. Q., et al.
(författare)
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Anisotropic Vorticity Within Bursty Bulk Flow Turbulence
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:10
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing Magnetospheric Multiscale (MMS) observation in the tail plasma sheet, we study the vorticity field (omega = del x V) of the plasma bulk (convective) velocity within the bursty bulk flow (BBF) in detailed. Two typical events are presented. In the event on 25 June 2017, E-y is the main component. In the other event on 6 July 2017, E-z is the main component. For both cases, the BBF electric field is dominated by the convective electric field (Ec = - V x B). Our case studies show clearly the existence of the convective vorticity field within the BBF. The vorticity field has prominent anisotropy (quantified by the anisotropic angle (theta aa = arctan(omega(perpendicular to)/omega(parallel to))). More often, the BBF omega field has stronger perpendicular vorticity (omega(perpendicular to)) than the parallel vorticity (omega(parallel to)). The dominance of vorticity by omega(perpendicular to)-dominating BBF is confirmed in the statistical sense. In particular, event on 25 June shows the significant evolution of the ion flux energy with the strength of the omega field. The strong omega field corresponds to the ion flux enhancement at high energy (above 10 keV), while the weak omega field corresponds to the ion flux enhancement at medium energy (2-5 keV). Investigation of the subset of channel from fast plasma investigation partial moment measurement reveals that the ion behaviors in the strong and decayed BBFs are distinctly different. The channeled ions form the narrow band distribution in the strong BBF but the multiple-layer distribution in the weak BBF. Finally, spectrum analysis indicates that the BBF omega(perpendicular to) and omega(parallel to) have a similar scaling about -2.0 (below 0.2 Hz).
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| 10. |
- Zhang, L. Q., et al.
(författare)
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BBF Deceleration Down-Tail of X <-15 RE From MMS Observation
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 125:2
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Tidskriftsartikel (refereegranskat)abstract
- We report the direct observation on bursty bulk flow (BBF) deceleration down-tail of X < -15 R-E by MMS satellite. Two typical events are presented in the paper. In the first event on 05 June 2018, MMS1 is located at X similar to -16.1 R-E and records four individual bursty flows (BFs). Each burst flow has distinctly lower velocity than the preceding one. Accompanying with the decelerated BFs, the Bz/Bx continuously increases/decreases. Simultaneous Vx-decrease and Bz-increase are in coincidence with the scenario of the local BBF deceleration and formation of the magnetic pileup region. In the second event on 03 July 2017, MMS stays in the neutral sheet of X similar to -24.5 R-E, and encounters similar BBF deceleration process. For both events, the decelerated BF series exhibit prominent medium-energy ion component (2-10 KeV). Analyses show enhanced parallel current (J(//)) and Kinetic Alfvenic wave (KAW) emitting during the BF intervals. The strength of the emitted KAW has a clear tendency to decay with the BBF decreasing. Power spectra density analysis confirms the substantial Joule dissipation during the BBF deceleration, both J(//) and J(perpendicular to). Combined analyses support BBF dissipation via Joule heating as well as KAW emitting. Finally, we propose a possible mechanism on the BBF deceleration, i.e., "collision" with the tailward flow.
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| 11. |
- Zhang, L. Q., et al.
(författare)
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First observation of fluid-like eddy-dominant bursty bulk flow turbulence in the Earth's tail plasma sheet
- 2023
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence is a ubiquitous phenomenon in neutral and conductive fluids. According to classical theory, turbulence is a rotating flow containing vortices of different scales. Eddies play a fundamental role in the nonlinear cascade of kinetic energy at different scales in turbulent flow. In conductive fluids, the Alfvenic/kinetic Alfvenic wave (AW/KAW) is the new "cell" of magnetohydrodynamic (MHD) turbulence (frozen-in condition). Wave energy, which has equal kinetic and magnetic energy, is redistributed among multiple-scale Fourier modes and transferred from the large MHD scale to the small kinetic scale through the collision of counter-propagating Alfvenic wave packages propagating along the magnetic field line. Fluid-like eddy-dominant plasma flow turbulence has never been found in space since the launch of the first satellite in 1957. In this paper, we report the first observation of eddy-dominant turbulence within magnetic reconnection-generated fast flow in the Earth's tail plasma sheet by the Magnetospheric Multiscale Spacecraft (MMS). In eddy-dominant turbulent reconnection jet, ions dominate the flow field while electrons dominate current and magnetic fluctuations. Our findings shed new light on the nonlinear kinetic and magnetic energy cascade in MHD turbulence.
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| 12. |
- Zhang, L. Q., et al.
(författare)
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MMS Observation on the Cross-Tail Current Sheet Roll-up at the Dipolarization Front
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:4
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Tidskriftsartikel (refereegranskat)abstract
- We perform a case study on the evolution of the current sheet in different regions around the dipolarization front (DF), including magnetic-dip preceding the DF, front at the DF, and magnetic pileup region (MPR) behind the DF based on magnetospheric multiscale (MMS) observation on July 31, 2017. In this event, MMS1 stays inside the current sheet during the whole bursty bulk flow (BBF) interval. Our analysis reveals that the cross-tail current sheet at the DF is rolled up, signified by the depression (-V-z/-B-z) at the dip and elevation (+V-z/+B-z) at the front. The minimum variance analysis on the magnetic field method is applied to obtain the normal direction of the current sheet. The result confirms the roll-up, that is, downward at the depressed current sheet and upward at the elevated current sheet. The current sheet roll-up at the DF is asymmetric, with steeper elevation than depression. The elevation angle of the elevated current sheet is evaluated to be similar to 30 degrees. Strong duskward and predominantly perpendicular J spike (similar to 90 nA/m(2)) concentrate at the interface between the dip and the front. The strength of the current of the J-spike is about nine/three times the current at the dip/front. The front is characterized by positive E center dot J. In the dip/MPR, no such preference is seen. Ion/Electron pitch angle distributions exhibit significant and different evolutions in the roll-up current sheet from dip to front, including their energy-dependence and distributions. Finally, the roll-up current sheet could decelerate BBF and change the flow structure. The potential significance of the roll-up current sheet on BBF evolution is emphasized.
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| 13. |
- Zhang, L. Q., et al.
(författare)
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Turbulent current sheet frozen in bursty bulk flow : observation and model
- 2022
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing four-point joint observations by Magnetospheric Multiscale Spacecraft (MMS), we investigate the main features of the current sheet frozen in (CSFI) the bursty bulk flow. Typical event on the steady long-lasting BBF on July 23, 2017 shows the enhanced dawn-dusk current (Jy(0)) in the CSFI (beta similar to 10). The magnitude of the Jy(0) in the CSFI is about 5.5 nA/m(2). The CSFI is highly turbulent, with the ratio of Delta J/J(0) of similar to 2 (where Delta J is perturbed J). The turbulent CSFI is characterized by intermittent current coherent structures. The magnitude of the spiky-J at coherent structures is typically above 30 nA/m(2). Spectrum analysis exhibits that BBF turbulence follows distinct dissipation laws inside and outside the CSFI. Based on MMS observations, we propose a new model of the BBF in the framework of magnetohydrodynamics. In this model, the BBF is depicted as a closed plasma system with the localized current sheet frozen at the center of the flow (Taylor's hypothesis). In the light of principle of Helmholtz-decomposition, the BBF motion in the tail plasma sheet is explained. The model also predicts the thermal expansion of the BBF after leaving the reconnection source region.
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| 14. |
- Zhang, L. Q., et al.
(författare)
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Vorticity Within Bursty Bulk Flows : Convective Versus Kinetic
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:3
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Tidskriftsartikel (refereegranskat)abstract
- Based on four-point Magnetospheric Multiscale observation, we carefully analyze the vorticity field in the course of a reconnection jet on 27 June 2017. In this event, the convective electric field (E-c) is overwhelmed by the kinetic electric field (E-k). Accordingly, the omega-field in the course of the BBF is dominated by kinetic vorticity (omega(k)). The omega-field in the E-k-dominated bursty bulk flow (BBF) is characterized by perpendicular anisotropy. Comparison of velocity-curl vorticity (omega= backward difference xV $\mathbf{\omega }=\nabla \times \mathbf{V}$) with E/B induced vorticity confirms a greater contribution to the BBF vorticity by E (omega E=(B. backward difference )E/B2 ${\mathbf{\omega }}_{\mathbf{E}}=(\mathbf{B}\cdot \nabla )\boldsymbol{E}/{\boldsymbol{B}}<^>{2}$) than by B (omega B=(E. backward difference )B/B2 ${\mathbf{\omega }}_{\mathbf{B}}=(\mathbf{E}\cdot \nabla )\mathbf{B}/{\boldsymbol{B}}<^>{2}$). Power spectrum density reveals that in the E-k-dominated BBF, the E-c and E-k spectra have different power laws. E-c dominates the B-spectrum while E-k dominates the E-spectrum. The E-c(B) spectrum has a -5/3-like slope below 0.25 Hz but a -3-like above 0.25 Hz. The E-k(E) spectrum is -5/3-like at the low-frequency end (below 0.1 Hz) but -2-like at the high-frequency end (above 2 Hz). Within its medium frequency range, the spectrum is flat. Particularly, the E-k(E) spectrum exhibits the bump at 1-2 Hz in the frequency domain. The solitary/bipolar E-spikes, with typical temporal scale of 0.3-1 s, are likely responsible for the E-bump. Finally, we statistically analyze and compare the vorticity field in E-k-dominated and E-c -dominated BBFs. The result demonstrates that the E-k-dominated BBF tends to have stronger vorticity than the E-c -dominated BBF.
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