| 1. |
- Nakamura, R., et al.
(författare)
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Flow bouncing and electron injection observed by Cluster
- 2013
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Ingår i: Journal of Geophysical Research-Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:5, s. 2055-2072
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Tidskriftsartikel (refereegranskat)abstract
- Characteristics of particles and fields in the flow-bouncing region are studied based on multipoint observations from Cluster located at 13-15R(E) downtail during a substorm event around 12:50 UT on 7 September 2007. The Cluster spacecraft were separated by a distance of up to 10,000 km and allowed to determine the mesoscale evolution of the current sheet as well as the development of the dipolarization front. We show that the flow bouncing took place associated with a tailward-directed j x B force in a disturbed current sheet in addition to an enhanced tailward pressure gradient force. Multiple Earthward propagating dipolarization fronts accompanied by enhanced flux of energetic electrons were observed before the flow bouncing. The sequence of events started with a localized dipolarization front and ended with a large scale (>10R(E)) dipolarization front accompanied by a major increase in energetic electrons at all spacecraft and immediately followed by flow bouncing. Multiple dipolarization fronts result in the formation of compressed magnetic field with a plasma bulge bounded by thin ion-scale current layers, a favorable condition for flow bouncing. These observations suggest that to understand the flow bouncing and related acceleration of plasma in the near-Earth tail, both the large-scale MHD properties and the transient and small-scale effect of the plasma interaction with the Earth-dipole field need to be taken into account.
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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. |
- Li, K., et al.
(författare)
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On the ionospheric source region of cold ion outflow
- 2012
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L18102-
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies have shown that low energy ions constitute a significant part of the total ion population in the Earth's magnetosphere. In this study, we have used a comprehensive data set with measurements of cold (total energy less than 70 eV) ion velocity and density to determine their source. This data set is derived from Cluster satellite measurements combined with solar wind and interplanetary magnetic field measurements and geomagnetic indices. By using the guiding center equation of motion, we were able to calculate the trajectories and thus determine the source region of the cold ions. Our results show that the polar cap region is the primary source for cold ions. We also found that the expansion and contraction of the polar cap as a consequence of changes in solar wind parameters were correlated with the source region size and intensity of the cold ion outflow. Elevated outflow fluxes near the nightside auroral zone and the dayside cusps during disturbed conditions suggest that energy and particle precipitation from the magnetosphere or directly from the solar wind can enhance the outflow of cold ions from the ionosphere. Citation: Li, K., et al. (2012), On the ionospheric source region of cold ion outflow, Geophys. Res. Lett., 39, L18102, doi:10.1029/2012GL053297.
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| 4. |
- Li, Kun, et al.
(författare)
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Transport of cold ions from the polar ionosphere to the plasma sheet
- 2013
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Ingår i: Journal of Geophysical Research: Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:9, s. 5467-5477
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Tidskriftsartikel (refereegranskat)abstract
- Ionospheric outflow is believed to be a significant contribution to the magnetospheric plasma population. Ions are extracted from the ionosphere and transported downtail by the large-scale convection motion driven by dayside reconnection. In this paper, we use a comprehensive data set of cold ion (total energy less than 70 eV) measurements combined with simultaneous observations from the solar wind to investigate the fate of these ions. By tracing the trajectories of the ions, we are able to find out where in the magnetotail ions end up. By sorting the observation according to geomagnetic activity and solar wind parameters, we then generate maps of the fate regions in the magnetotail and investigate the effects of these drivers. Our results suggest that, on overall, for about 85% of the cases, the outflowing ions are transported to the plasma sheet. The region where the ions are deposited into the plasma sheet is larger during geomagnetic quiet time than during disturbed conditions. A persistent dawn-dusk asymmetry in the plasma sheet deposition is also observed.
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| 5. |
- Wang, Rongsheng, et al.
(författare)
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Observation of multiple sub-cavities adjacent to single separatrix
- 2013
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:11, s. 2511-2517
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Tidskriftsartikel (refereegranskat)abstract
- We investigate a direct south-north crossing of a reconnection ion diffusion region in the magnetotail. During this crossing, multiple electron density dips with a further density decrease within the cavity, called sub-cavities, adjacent to the northern separatrix are observed. The correlation between electron density sub-cavities and strong electric field fluctuations is obvious. Within one of the sub-cavities, a series of very strong oscillating perpendicular electric field and patchy parallel electric field are observed. The parallel electric field is nearly unipolar and directs away from X line. In the same region, inflow electrons with energy up to 100keV are injected into the X line. Based on the observations, we conclude that the high-energy inflowing electrons are accelerated by the patchy parallel electric field. Namely, electrons have been effectively accelerated while they are flowing into the X line along the separatrix. The observations indicate that the electron acceleration region is widely larger than the predicted electron diffusion region in the classical Hall magnetic reconnection model.
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