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- Sun, W. J., et al.
(författare)
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A Comparative Study of the Proton Properties of Magnetospheric Substorms at Earth and Mercury in the Near Magnetotail
- 2018
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 45:16, s. 7933-7941
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Tidskriftsartikel (refereegranskat)abstract
- The variations of plasma sheet proton properties during magnetospheric substorms at Earth and Mercury are comparatively studied. This study utilizes kappa distributions to interpret proton properties at both planets. Proton number densities are found to be around an order of magnitude higher, temperatures several times smaller, and kappa values broader at Mercury than at Earth. Protons become denser and cooler during the growth phase, and are depleted and heated after the dipolarizations in both magnetospheres. The changes of kappa at Earth are generally small (<20%), indicating that spectrum-preserving processes, like adiabatic betatron acceleration, play an important role there, while variations of kappa at Mercury are large (>60%), indicating the importance of spectrum-altering processes there, such as acceleration due to nonadiabatic cross-tail particle motions and wave-particle interactions. This comparative study reveals important intrinsic properties on the energization of protons in both magnetospheres. Plain Language Summary Earth and Mercury are the only two planets possessing global intrinsic magnetic fields among the four inner planets, which are Mercury, Venus, Earth, and Mars, within the solar system. The interactions between the intrinsic magnetic fields and the continual flow of high-speed solar wind from the Sun form similar magnetospheres at the two planets, although the scale of the magnetosphere is much smaller at Mercury than at Earth. Magnetospheric substorms, a result of solar wind-magnetosphere coupling, occur in both magnetospheres. Comparative study of a similar process between different planets is meaningful as it can help us in understanding the specific process further as well as help us in understanding the intrinsic properties of the magnetospheres. This research paper characterizes the proton properties of magnetospheric substorms of both planets, revealing that different mechanisms control the behavior of protons during the magnetospheric substorms of the two planets.
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- Yao, Shutao, et al.
(författare)
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Observations of kinetic-size magnetic holes in the magnetosheath
- 2017
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 122:2, s. 1990-2000
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic holes (MHs), with a scale much greater than ρi (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), previously called small-size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three-dimensional (3-D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10-20 ρe (electron gyroradii) and lasted 0.1-0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90 degrees pitch angle, the flux of electrons with energy 34-66 eV decreased, while for electrons of energy 109-1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self-consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10-20 ρe.
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- Yao, S. T., et al.
(författare)
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Propagating and Dynamic Properties of Magnetic Dips in the Dayside Magnetosheath : MMS Observations
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:6
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Tidskriftsartikel (refereegranskat)abstract
- The magnetosheath is inherently complex and rich, exhibiting various kinds of structures and perturbations. It is important to understand how these structures propagate and evolve and how they relate to the perturbations. Here we investigate a kind of magnetosheath structure known as a magnetic dip (MD). As far as we are aware, there have been no previous studies concerning the evolution (contracting or expanding) of these types of structures, and their propagation properties cannot be unambiguously determined. In this study, using Magnetospheric MultiScale (MMS) high-temporal resolution data and multispacecraft analysis methods, we obtain the propagation and dynamic features of a set of MDs. Four different types of MDs are identified: "frozen-in," "expanding," "contracting," and "stable-propagating." Significantly, a stable-propagation event is observed with a sunward propagation component. This indicates that the source of the structure in this case is closely associated with the magnetopause, which provides strong support to the contention in earlier research. We further reveal the mechanism leading to the MD contraction or expansion. The motion of the MDs boundary is found closely related with the dynamic pressure. The scale of the contracting and expanding events are typically similar to 5-20 rho(i) (ion gyroradius), significantly smaller than that of frozen-in events (similar to 40 rho(i)). The observations could relate large-scale (more than several tens of rho(i)) and kinetic-scale (less than rho(i)) MDs, by revealing an evolution that spans these different scales, and help us better understand the variation and dynamics of magnetosheath structures and plasmas.
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- Yao, S. T., et al.
(författare)
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Propagation of small size magnetic holes in the magnetospheric plasma sheet
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : Wiley-Blackwell. - 2169-9380 .- 2169-9402. ; 121:6, s. 5510-5519
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic holes (MHs), characteristic structures where the magnetic field magnitude decreases significantly, have been frequently observed in space plasmas. Particularly, small size magnetic holes (SSMHs) which the scale is less than or close to the proton gyroradius are recently detected in the magnetospheric plasma sheet. In this study of Cluster observations, by the timing method, the minimum directional difference (MDD) method, and the spatiotemporal difference (STD) method, we obtain the propagation velocity of SSMHs in the plasma flow frame. Furthermore, based on electron magnetohydrodynamics (EMHD) theory we calculate the velocity, width, and depth of the electron solitary wave and compare it to SSMH observations. The result shows a good accord between the theory and the observation.
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| 7. |
- Yao, S. T., et al.
(författare)
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Waves in Kinetic-Scale Magnetic Dips : MMS Observations in the Magnetosheath
- 2019
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 46:2, s. 523-533
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Tidskriftsartikel (refereegranskat)abstract
- Kinetic scale magnetic dips (KSMDs), with a significant depression in magnetic field strength, and scale length close to and less than one proton gyroradius, were reported in the turbulent plasmas both in recent observation and numerical simulation studies. These KSMDs likely play important roles in energy conversion and dissipation. In this study, we present observations of the KSMDs that are labeled whistler mode waves, electrostatic solitary waves, and electron cyclotron waves in the magnetosheath. The observations suggest that electron temperature anisotropy or beams within KSMD structures provide free energy to generate these waves. In addition, the occurrence rates of the waves are higher in the center of the magnetic dips than at their edges, implying that the KSMDs might be the origin of various kinds of waves. We suggest that the KSMDs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas. Plain Language Summary The Earth's magnetosheath is a turbulent plasma environment where energy conversion, particle acceleration, and mass and momentum transport take place. Many of these key processes involve kinetic-scale physics. However, in-depth studies from previous missions are limited by their lower spacecraft data resolution. The recent Magnetospheric Multiscale (MMS) mission provides us with a large amount of high-temporal cadence data for studying kinetic-scale physics in the magnetosheath. In this study, we report whistler mode waves, electrostatic solitary waves and electron cyclotron waves within kinetic-scale magnetic dips (KSMDs) that can be generated in the turbulent magnetosheath. These waves could be excited by electron temperature anisotropy or beams. As is well known, plasma waves are important processes in converting energy, accelerating and scattering electrons and ions, and modifying the distributions of charged particles. If plasma instabilities develop within the KSMDs, the resulting waves could absorb free energy from plasma particles and may propagate out of the KSMDs. Thus, our discoveries could significantly advance the understanding of energy conversion and dissipation for kinetic-scale turbulence. This study provides a new reference not only for observations in space physics but also for related basic plasma theories and numerical simulations.
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| 8. |
- Mailyan, B., et al.
(författare)
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Transpolar arc observation after solar wind entry into the high-latitude magnetosphere
- 2015
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:5, s. 3525-3534
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Tidskriftsartikel (refereegranskat)abstract
- Recently, Cluster observations have revealed the presence of new regions of solar wind plasma entry at the high-latitude magnetospheric lobes tailward of the cusp region, mostly during periods of northward interplanetary magnetic field. In this study, observations from the Global Ultraviolet Imager (GUVI) experiment on board the TIMED spacecraft and Wideband Imaging Camera imager on board the IMAGE satellite are used to investigate a possible link between solar wind entry and the formation of transpolar arcs in the polar cap. We focus on a case when transpolar arc formation was observed twice right after the two solar wind entry events were detected by the Cluster spacecraft. In addition, GUVI and IMAGE observations show a simultaneous occurrence of auroral activity at low and high latitudes after the second entry event, possibly indicating a two-part structure of the continuous band of the transpolar arc.
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| 9. |
- Xiao, Y.C., et al.
(författare)
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Statistical properties of the distribution and generation of kinetic-scale flux ropes in the terrestrial dayside magnetosheath
- 2023
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 50:23
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Tidskriftsartikel (refereegranskat)abstract
- The generation of kinetic-scale flux ropes (KSFRs) is closely related to magnetic reconnection. Both flux ropes and reconnection sites are detected in the magnetosheath and can impact the dynamics upstream of the magnetopause. In this study, using the Magnetospheric Multiscale satellite, 12,623 KSFRs with a scale <20 RCi are statistically studied in the Earth's dayside magnetosheath. It is found that they are mostly generated near the bow shock (BS), and propagate downstream in the magnetosheath. Their quantity significantly increases as the scale decreases, consistent with a flux rope coalescence model. Moreover, the solar wind parameters can control the occurrence rate of KSFRs. They are more easily generated at high Mach number, large proton density, and weak magnetic field strength of the solar wind, similar to the conditions that favor BS reconnection. Our study shows a close connection between KSFR generation and BS reconnection.
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| 10. |
- Yao, S.T., et al.
(författare)
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Ion-Vortex Magnetic Hole With Reversed Field Direction in Earth's Magnetosheath
- 2023
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 128:7
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Tidskriftsartikel (refereegranskat)abstract
- Plasma vortices are ubiquitous in space and play important roles in the transmission of energy and mass at various scales. For small-scale plasma vortices on the order of ion gyroradius, however, their properties and characteristics remain unclear. Here, we provide unique findings of an ion-scale vortex observed in the Earth's magnetosheath. The vortex is generated by the ion diamagnetic drift associated with an isolated magnetic hole (MH). The magnetic field in the axial direction is reversed in the vortex center, which is consistent with ring-shaped currents carried by the ions. The field strength becomes very weak (<1 nT) at the field reversal region, although the ion distributions vary rather continuously across the entire structure. A kinetic equilibrium model is then applied to reconstruct the above features. These findings can help us understand the plasma vortex and MH from magnetohydrodynamics to kinetic scales.
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