| 1. |
- Fu, H. S., et al.
(författare)
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First observation of rising-tone magnetosonic waves
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:21, s. 7419-7426
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Tidskriftsartikel (refereegranskat)abstract
- Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90 degrees and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be "temporally continuous" in their time-frequency spectrogram. Here we show for the first time that MS waves actually have discrete wave elements with rising-tone features in their spectrogram. The frequency sweep rate of MS waves, similar to 1 Hz/s, is between that of chorus and electromagnetic ion cyclotron (EMIC) waves. For the two events we analyzed, MS waves occur outside the plasmapause and cannot penetrate into the plasmasphere; their power is smaller than that of chorus. We suggest that the rising-tone feature of MS waves is a consequence of nonlinear wave-particle interaction, as is the case with chorus and EMIC waves.
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| 2. |
- Fu, H. S., et al.
(författare)
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Pitch angle distribution of suprathermal electrons behind dipolarization fronts : A statistical overview
- 2012
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 117:12
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Tidskriftsartikel (refereegranskat)abstract
- We examine the pitch angle distribution (PAD) of suprathermal electrons (> 40 keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward-propagating DFs observed during 9 years (2001-2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the > 40 keV electrons inside the FPR. One group, characterized by the low flux (F < 500/cm(2) , s . sr . keV), consists of 153 events and corresponds to a broad distribution of IMF Bz components. The other group, characterized by the high flux (F >= 500/cm(2) . s . sr . keV), consists of 150 events and corresponds to southward IMF Bz components. Only the high-flux group is considered to investigate the PAD of the > 40 keV electrons as the low-flux situation may lead to large uncertainties in computing the anisotropy factor that is defined as A = F-perpendicular to/F-parallel to - 1 for F-perpendicular to > F-parallel to, and A = -F-parallel to/F-perpendicular to + 1 for F-perpendicular to < F-parallel to. We find that, among the 150 events, 46 events have isotropic distribution (vertical bar A vertical bar <= 0.5); 60 events have perpendicular distribution (A > 0.5), and 44 events have field-aligned distribution inside the FPR (A < -0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field-aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs.
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| 3. |
- Fu, H. S., et al.
(författare)
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Whistler-mode waves inside flux pileup region : Structured or unstructured?
- 2014
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 119:11, s. 9089-9100
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Tidskriftsartikel (refereegranskat)abstract
- During reconnection, a flux pileup region (FPR) is formed behind a dipolarization front in an outflow jet. Inside the FPR, the magnetic field magnitude and Bz component increase and the whistler-mode waves are observed frequently. As the FPR convects toward the Earth during substorms, it is obstructed by the dipolar geomagnetic field to form a near-Earth FPR. Unlike the structureless emissions inside the tail FPR, we find that the whistler-mode waves inside the near-Earth FPR can exhibit a discrete structure similar to chorus. Both upper band and lower band chorus are observed, with the upper band having a larger propagation angle (and smaller wave amplitude) than the lower band. Most chorus elements we observed are rising-tone type, but some are falling-tone type. We notice that the rising-tone chorus can evolve into falling-tone chorus within <3s. One of the factors that may explain why the waves are unstructured inside the tail FPR but become discrete inside the near-Earth FPR is the spatial inhomogeneity of magnetic field: we find that such inhomogeneity is small inside the near-Earth FPR but large inside the tail FPR.
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| 4. |
- Huang, S. Y., et al.
(författare)
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Electron acceleration in the reconnection diffusion region : Cluster observations
- 2012
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 39:L11103
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Tidskriftsartikel (refereegranskat)abstract
- We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.
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| 5. |
- Lu, H. Y., et al.
(författare)
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Electric structure of dipolarization fronts associated with interchange instability in the magnetotail
- 2013
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Ingår i: Journal of Geophysical Research: Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:10, s. 6019-6025
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Tidskriftsartikel (refereegranskat)abstract
- The electric structure of dipolarization fronts (DFs) is very important to both DF dynamics and particle acceleration. We performed two-dimensional Hall MHD simulation to study the electric structure of DF produced by interchange instability on the scale of ion inertial length in the flow braking region of near-Earth tail. The results indicate that the Hall effect makes the structures of plasma density and magnetic field deformed in the dawn-dusk direction. This deformation is caused by the induced Lorentz force along the tangent plane of DF, which is associated with the outward moving of demagnetized ions driven by the ion-scale Earthward electric field on DF. In addition, the x component of electric field contributed jointly by Hall and electron pressure gradient terms along with Bz can produce a dawnward ExB drift to the whole mushroom structure. Inside the DF, the electric field is mainly produced by Hall term, and the contributions from the convectional and electron pressure gradient electric fields are very small. This indicates that the ion frozen-in condition of magnetic field is violated inside the DF. Therefore, it is the electric field contributed by Hall term inside the DF that changes the overall MHD mushroom pattern. The comparison between the simulation results and the observations of THEMIS satellites demonstrates that the model of Hall MHD simulation can reproduce the plasma and electric field observed at DF.
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| 6. |
- Fu, Huishan, et al.
(författare)
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Occurrence rate of earthward-propagating dipolarization fronts
- 2012
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Ingår i: Geophysical Research Letters. - 2000 Florida Ave NW Washington, DC 20009, USA : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 39, s. L10101-
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence rate of earthward-propagating dipolarization fronts (DFs) is investigated in this paper based on the 9 years (2001-2009) of Cluster 1 data. For the first time, we select the DF events by fitting the characteristic increase in B-z using a hyperbolic tangent function. 303 earthward-propagating DFs are found; they have on average a duration of 4 s and a B-z increase of 8 nT. DFs have the maximum occurrence at Z(GSM) approximate to 0 and r approximate to 15 R-E with one event occurring every 3.9 hours, where r is the distance to the center of the Earth in the XYGSM plane. The maximum occurrence rate at Z(GSM) approximate to 0 can be explained by the steep and large increase of B-z near the central current sheet, which is consistent with previous simulations. Along the r direction, the occurrence rate increases gradually from r approximate to 20 to r approximate to 15 R-E but decreases rapidly from r approximate to 15 to r approximate to 10 R-E. This may be due to the increasing pileup of the magnetic flux from r approximate to 20 to r approximate to 15 R-E and the strong background magnetic field at r
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| 7. |
- Fu, Huishan S., et al.
(författare)
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Electric structure of dipolarization front at sub-proton scale
- 2012
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Ingår i: Geophysical Research Letters. - 2000 Florida Ave NW Washington, DC 20009, USA : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 39, s. L06105-
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Tidskriftsartikel (refereegranskat)abstract
- Using Cluster data, we investigate the electric structure of a dipolarization front (DF) - the ion inertial length (c/omega(pi)) scale boundary in the Earth's magnetotail formed at the front edge of an earthward propagating flow with reconnected magnetic flux. We estimate the current density and the electron pressure gradient throughout the DF by both single-spacecraft and multi-spacecraft methods. Comparison of the results from the two methods shows that the single-spacecraft analysis, which is capable of resolving the detailed structure of the boundary, can be applied for the DF we study. Based on this, we use the current density and the electron pressure gradient from the single-spacecraft method to investigate which terms in the generalized Ohm's law balance the electric field throughout the DF. We find that there is an electric field at ion inertia scale directed normal to the DF; it has a duskward component at the dusk flank of DF but a dawnward component at the dawn flank of DF. This electric field is balanced by the Hall (j x B/ne) and electron pressure gradient (del P-e/ne) terms at the DF, with the Hall term being dominant. Outside the narrow DF region, however, the electric field is balanced by the convection (V-i x B) term, meaning the frozen-in condition for ions is broken only at the DF itself. In the reference frame moving with the DF the tangential electric field is almost zero, indicating there is no flow of plasma across the DF and that the DF is a tangential discontinuity. The normal electric field at the DF constitutes a potential drop of similar to 1 keV, which may reflect and accelerate the surrounding ions.
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| 8. |
- Vaivads, Andris, et al.
(författare)
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EIDOSCOPE : particle acceleration at plasma boundaries
- 2012
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 33:2-3, s. 491-527
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Tidskriftsartikel (refereegranskat)abstract
- We describe the mission concept of how ESA can make a major contribution to the Japanese Canadian multi-spacecraft mission SCOPE by adding one cost-effective spacecraft EIDO (Electron and Ion Dynamics Observatory), which has a comprehensive and optimized plasma payload to address the physics of particle acceleration. The combined mission EIDOSCOPE will distinguish amongst and quantify the governing processes of particle acceleration at several important plasma boundaries and their associated boundary layers: collisionless shocks, plasma jet fronts, thin current sheets and turbulent boundary layers. Particle acceleration and associated cross-scale coupling is one of the key outstanding topics to be addressed in the Plasma Universe. The very important science questions that only the combined EIDOSCOPE mission will be able to tackle are: 1) Quantitatively, what are the processes and efficiencies with which both electrons and ions are selectively injected and subsequently accelerated by collisionless shocks? 2) How does small-scale electron and ion acceleration at jet fronts due to kinetic processes couple simultaneously to large scale acceleration due to fluid (MHD) mechanisms? 3) How does multi-scale coupling govern acceleration mechanisms at electron, ion and fluid scales in thin current sheets? 4) How do particle acceleration processes inside turbulent boundary layers depend on turbulence properties at ion/electron scales? EIDO particle instruments are capable of resolving full 3D particle distribution functions in both thermal and suprathermal regimes and at high enough temporal resolution to resolve the relevant scales even in very dynamic plasma processes. The EIDO spin axis is designed to be sun-pointing, allowing EIDO to carry out the most sensitive electric field measurements ever accomplished in the outer magnetosphere. Combined with a nearby SCOPE Far Daughter satellite, EIDO will form a second pair (in addition to SCOPE Mother-Near Daughter) of closely separated satellites that provides the unique capability to measure the 3D electric field with high accuracy and sensitivity. All EIDO instrumentation are state-of-the-art technology with heritage from many recent missions. The EIDOSCOPE orbit will be close to equatorial with apogee 25-30 RE and perigee 8-10 RE. In the course of one year the orbit will cross all the major plasma boundaries in the outer magnetosphere; bow shock, magnetopause and magnetotail current sheets, jet fronts and turbulent boundary layers. EIDO offers excellent cost/benefits for ESA, as for only a fraction of an M-class mission cost ESA can become an integral part of a major multi-agency L-class level mission that addresses outstanding science questions for the benefit of the European science community.
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