| 1. |
- Chen, Li-Jen, et al.
(författare)
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Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A12, s. A12213-
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Tidskriftsartikel (refereegranskat)abstract
- We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two-dimensional particle-in-cell simulations with a mass ratio m(i)/m(e) = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy (Te-parallel to/Te-perpendicular to > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth (d(i)) above and below the ECS, the electron density decreases toward the ECS by a factor of 3-4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A di-scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron-scale structures in response to this electric field.
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| 2. |
- Chen, Li-Jen, et al.
(författare)
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Observation of energetic electrons within magnetic islands
- 2008
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Ingår i: Nature Physics. - : Nature Publishing Group. - 1745-2473 .- 1745-2481. ; 4:1, s. 19-23
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic reconnection is the underlying process that releases impulsively an enormous amount of magnetic energy(1) in solar flares(2,3), flares on strongly magnetized neutron stars(4) and substorms in the Earth's magnetosphere(5). Studies of energy release during solar flares, in particular, indicate that up to 50% of the released energy is carried by accelerated 20-100 keV suprathermal electrons(6-8). How so many electrons can gain so much energy during reconnection has been a long-standing question. A recent theoretical study suggests that volume-filling contracting magnetic islands formed during reconnection can produce a large number of energetic electrons(9). Here we report the first evidence of the link between energetic electrons and magnetic islands during reconnection in the Earth's magnetosphere. The results indicate that energetic electron fluxes peak at sites of compressed density within islands, which imposes a new constraint on theories of electron acceleration.
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| 3. |
- Nilsson, H, et al.
(författare)
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Centrifugal acceleration in the magnetotail lobes
- 2010
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 28:2, s. 569-576
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Tidskriftsartikel (refereegranskat)abstract
- Combined Cluster EFW and EDI measurements have shown that cold ion outflow in the magnetospheric lobes dominates the hydrogen ion outflow from the Earth's atmosphere. The ions have too low kinetic energy to be measurable with particle instruments, at least for the typical spacecraft potential of a sunlit spacecraft in the tenuous lobe plasmas outside a few R-E. The measurement technique yields both density and bulk velocity, which can be combined with magnetic field measurements to estimate the centrifugal acceleration experienced by these particles. We present a quantitative estimate of the centrifugal acceleration, and the velocity change with distance which we would expect due to centrifugal acceleration. It is found that the centrifugal acceleration is on average outward with an average value of about of 5 m s(-2). This is small, but acting during long transport times and over long distances the cumulative effect is significant, while still consistent with the relatively low velocities estimated using the combination of EFW and EDI data. The centrifugal acceleration should accelerate any oxygen ions in the lobes to energies observable by particle spectrometers. The data set also put constraints on the effectiveness of any other acceleration mechanisms acting in the lobes, where the total velocity increase between 5 and 19 R-E geocentric distance is less than 5 km s(-1).
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| 4. |
- Puhl-Quinn, P. A., et al.
(författare)
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Cluster and DMSP observations of SAID electric fields
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A5, s. A05219-
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Tidskriftsartikel (refereegranskat)abstract
- [1] We report on magnetically conjugate Cluster and the Defense Meteorological Satellite Program (DMSP) satellite observations of subauroral ion drifts (SAID) during moderate geomagnetic activity levels on 8 April 2004. To our knowledge, the field-aligned separation of DMSP and Cluster (approximate to 28,000 km) is the largest separation ever analyzed with respect to the SAID phenomenon. Nonetheless, we show coherent, subauroral magnetosphere-ionosphere (MI) coupling along an entire field line in the post-dusk sector. The four Cluster satellites crossed SAID electric field channels with meridional magnitude E-M of 25 mV/m in situ and latitudinal extent Delta Lambda approximate to 0.5 degrees in the southern and northern hemispheres near 07:00 and 07:30 UT, respectively. Cluster was near perigee (R approximate to 4 R-E) and within 5 degrees (15 degrees) of the magnetic equator for the southern ( northern) crossing. The SAID were located near the plasmapause-within the ring current-plasmasphere overlap region. Downward field-aligned current signatures were observed across both SAID crossings. The most magnetically and temporally conjugate SAID field from DMSP F16A at 07:12 UT was practically identical in latitudinal size to that mapped from Cluster. Since the DMSP ion drift meter saturated at 3000 m/s (or similar to 114 mV/m) and the electrostatically mapped value for E-M from Cluster exceeded 300 mV/m, a magnitude comparison of E-M was not possible. Although the conjugate measurements show similar large-scale SAID features, the differences in substructure highlight the physical and chemical diversity of the conjugate regions.
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| 5. |
- Matsui, H., et al.
(författare)
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Characteristics of storm time electric fields in the inner magnetosphere derived from Cluster data
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A11215-
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Tidskriftsartikel (refereegranskat)abstract
- Storm-time electric fields in the inner magnetosphere measured by Cluster are reported in this study. First, we show two events around the time when Dst index is at a minimum. The electric field possibly related to subauroral ion drifts and/or undershielding is measured inside the inner edge of the electron plasma sheet in the eveningside. For the second event observed in the nightside, the electric field is partly related to dipolarization and is considered as inductive. An electric field without coincident magnetic signatures is also observed. Spatial coherence of the electric field is not large when we check multispacecraft data. It is inferred that the electric field in the magnetotail penetrates inside the region 1 current, while it is not clear about the electric field within the region 2 current from our data. Then superposed epoch analyses using 71 storms are performed. Electric fields at R = 3.5-6R(E) and less than 25 degrees of magnetic latitudes are enhanced around the minimum Dst at all magnetic local times. Electric fields during the recovery phase decay on a time scale shorter than that of Dst index, which could be interpreted in terms of the relation between electric field and ring current during that storm phase. AC electric fields are generally larger than DC electric fields, indicating that the former component might play some role in accelerating ring current particles. These results will be useful to update our empirical electric field model.
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| 6. |
- Matsui, H., et al.
(författare)
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Derivation of inner magnetospheric electric field (UNH-IMEF) model using Cluster data set
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:9, s. 2887-2898
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Tidskriftsartikel (refereegranskat)abstract
- We derive an inner magnetospheric electric field (UNH-IMEF) model at L=2-10 using primarily Cluster electric field data for more than 5 years between February 2001 and October 2006. This electric field data set is divided into several ranges of the interplanetary electric field (IEF) values measured by ACE. As ring current simulations which require electric field as an input parameter are often performed at L=2-6.6, we have included statistical results from ground radars and low altitude satellites inside the perigee of Cluster in our data set (L similar to 4). Electric potential patterns are derived from the average electric fields by solving an inverse problem. The electric potential pattern for small IEF values is probably affected by the ionospheric dynamo. The magnitudes of the electric field increase around the evening local time as IEF increases, presumably due to the sub-auroral polarization stream (SAPS). Another region with enhanced electric fields during large IEF periods is located around 9 MLT at L>8, which is possibly related to solar wind-magnetosphere coupling. Our potential patterns are consistent with those derived from self-consistent simulations. As the potential patterns can be interpolated/extrapolated to any discrete IEF value within measured ranges, we thus derive an empirical electric potential model. The performance of the model is evaluated by comparing the electric field derived from the model with original one measured by Cluster and mapped to the equator. The model is open to the public through our website.
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| 7. |
- Matsui, H., et al.
(författare)
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Multi-spacecraft observations of small-scale fluctuations in density and fields in plasmaspheric plumes
- 2012
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 30:3, s. 623-637
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Tidskriftsartikel (refereegranskat)abstract
- In this event study, small-scale fluctuations in plasmaspheric plumes with time scales of similar to 10 s to minutes in the spacecraft frame are examined. In one event, plasmaspheric plumes are observed by Cluster, while IMAGE measured density enhancement at a similar location. Fluctuations in density exist in plumes as detected by Cluster and are accompanied by fluctuations in magnetic fields and electric fields. Magnetic fluctuations are transverse and along the direction of the plumes. The E/B ratio is smaller than the Alfv,n velocity. Another similar event is briefly presented. We then consider physical properties of the fluctuations. Alfv,n mode modulated by the feedback instability is one possibility, although non-local generation is likely. It is hard to show that the fluctuations represent a fast mode. Interchange motion is possible due to the consistency between measurements and expectations. The energy source could be a pressure or density gradient in plasmaspheric plumes. When more events are accumulated so that statistical analysis becomes feasible, this type of study will be useful to understand the time evolution of plumes.
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| 8. |
- Puhl-Quinn, P. A., et al.
(författare)
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An effort to derive an empirically based, inner-magnetospheric electric field model : Merging Cluster EDI and EFW data
- 2008
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 70:2-4, s. 564-573
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Tidskriftsartikel (refereegranskat)abstract
- A key ingredient for modelling many inner-magnetospheric processes is the realistic representation of the spatio-temporal dynamics of the inner-magnetospheric electric field, or, IMEF The Cluster Mission provides a unique opportunity to construct an IMEF model using electric field measurements from both the electron drift instrument (EDI) and the electric fields and waves instrument (EFW). A superset of IMEF data is formed by merging EDI and EFW data. Challenges presented by the merging process include the handling of compromised perpendicular electric field (E-L) calculations, electric field offsets, scaling problems, and spurious fields. The present goal is to produce the highest quality merged IMEF data set possible which is minimally affected by these issues. Preliminary investigation of the merging process on Cluster I for the years 2001-2003 has revealed that merging is a worthwhile exercise. The data sets are shown to be complementary, and the IMEF merged data set is superior to either data set alone in terms of improved spatial coverage, and coverage of a wider range of geomagnetic activity levels. Preliminary use of the merged IMEF data set to construct a parameterized, equatorial, electric field model for the inner magnetosphere, the UNH-IMEF model, is also presented. The electric field morphology produced by this preliminary version of the UNH-IMEF model shows the expected sensitivity to IMF orientation.
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