| 1. |
- Nilsson, Hans, et al.
(författare)
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Hot and cold ion outflow : Observations and implications for numerical models
- 2013
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 118:1, s. 105-117
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Tidskriftsartikel (refereegranskat)abstract
- Cluster observations of oxygen ion outflow and low-frequency waves at high altitude above the polar cap and cold ion outflow in the lobes are used to determine ion heating rates and low-altitude boundary conditions suitable for use in numerical models of ion outflow. Using our results, it is possible to simultaneously reproduce observations of high-energy O+ ions in the high-altitude cusp and mantle and cold H+ ions in the magnetotail lobes. To put the Cluster data in a broader context, we first compare the average observed oxygen temperatures and parallel velocities in the high-altitude polar cap with the idealized cases of auroral (cusp) and polar wind (polar cap) ion outflow obtained from a model based on other data sets. A cyclotron resonance model using average observed electric field spectral densities as input fairly well reproduces the observed velocities and perpendicular temperatures of both hot O+ and cold H+, if we allow the fraction of the observed waves, which is efficient in heating the ions to increase with altitude and decrease toward the nightside. Suitable values for this fraction are discussed based on the results of the cyclotron resonance model. Low-altitude boundary conditions, ion heating rates, and centrifugal acceleration are presented in a format suitable as input for models aiming to reproduce the observations
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| 2. |
- Nilsson, Hans, et al.
(författare)
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Hot and cold ion outflow : Spatial distribution of ion heating
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117
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Tidskriftsartikel (refereegranskat)abstract
- Ions apparently emanating from the same source, the ionospheric polar cap, can either end up as energized to keV energies in the high-altitude cusp/mantle, or appear as cold ions in the magnetotail lobes. We use Cluster observations of ions and wave electric fields to study the spatial variation of ion heating in the cusp/mantle and polar cap. The average flow direction in a simplified cylindrical coordinate system is used to show approximate average ion flight trajectories, and discuss the temperatures, fluxes and wave activity along some typical trajectories. It is found that it is suitable to distinguish between cusp, central and nightside polar cap ion outflow trajectories, though O+ heating is mainly a function of altitude. Furthermore we use typical cold ion parallel velocities and the observed average perpendicular drift to obtain average cold ion flight trajectories. The data show that the cusp is the main source of oxygen ion outflow, whereas a polar cap source would be consistent with our average outflow paths for cold ions observed in the lobes. A majority of the cusp O+ flux is sufficiently accelerated to escape into interplanetary space. A scenario with significant oxygen ion heating in regions with strong magnetosheath origin ion fluxes, cold proton plasma dominating at altitudes below about 8 RE in the polar cap, and most of the cusp oxygen outflow overcoming gravity and flowing out in the cusp and mantle is consistent with our observations.
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| 3. |
- Sadeghi, Soheil, et al.
(författare)
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Cluster Observations of Quasi-Static Potential Structures Overlapping with Alfvénic Regions
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Results are presented from an equatorward crossing of the Cluster spacecraft through the high-altitude auroral acceleration region, over a system of East-West aligned auroral arcs in the Southern hemisphere auroral oval. The event occurred during quiet geomagnetic conditions during the expansion phase of a weak substorm. Acceleration potential profiles of the quasi-static structures are determined from the particle and field data. The observations reveal a dynamically developing system of small-scale and large-scale quasi-static structures overlapping with Alfvénic regions. Such overlaps are found within the PSBL and inside the Region 2 of downward currents. No density cavities are seen in the overlap regions. Growths of small-scale potential structures were observed in the PSBL and in the middle of Region 2, during the ~1.5 minutes between the Cluster 4 and Cluster 3 passages. During this period, the Alfvénic regions retreated both at the poleward and equatorward oval boundaries, and the large-scale quasi-static potentials intensified.
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| 4. |
- Sadeghi, Soheil, et al.
(författare)
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Spatiotemporal features of the auroral acceleration region as observed by Cluster
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:12, s. A00K19-
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Tidskriftsartikel (refereegranskat)abstract
- A pair of negative electric potential structures associated with inverted-V aurora is investigated using electric and magnetic field, ion and electron data from the Cluster spacecraft, crossing the auroral acceleration region (AAR) at different altitudes above the Northern hemisphere midnight auroral oval. The spatial and temporal development of the acceleration structures is studied, given the magnetic conjunction opportunity and the one minute difference between the Cluster spacecraft crossings. The configuration allowed for estimation of characteristic times of development for the two structures and of the parallel electric field and potential drop for the more stable one. The first potential structure had a width of similar to 80 km (projected to the ionosphere) and was relatively short-lived, developing in less than 40 s and decaying in one minute. The parallel potential drop increased between altitudes of 1.13 R(E) and 1.3 R(E), whereas the acceleration potential above 1.3 R(E) remained almost unchanged during that time. This intensification occurred mainly after the time when the associated upward current had reached its maximum value. The second structure had a width of similar to 50 km and was subject to an increase by a factor of 3 of the parallel potential drop below 1.3 R(E), during about 40 s, after which it remained rather stable for one minute or more. Similarly here, the acceleration potential above 1.3 R(E) remained roughly unchanged. For the more stable second structure, an average parallel electric field between 1.13 and 1.3 R(E) could be estimated (similar to 0.56 mV/m). The conductance along the flux tube was also stable for one minute or more.
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| 5. |
- Slapak, Rikard, et al.
(författare)
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Observations of oxygen ions in the dayside magnetosheath associated with southward IMF
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117
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Tidskriftsartikel (refereegranskat)abstract
- We present a case study of high energy oxygen ions (O+) observed in the dayside terrestrial magnetosheath, in the southern hemisphere. It is shown that the presence of O+ is strongly correlated to the IMF direction: O+ is observed only for Bz<0. Three satellites observe O$^+ immediately at both sides of the magnetopause and about 2 RE outside the magnetopause. These conditions indicate escape along open magnetic field lines. We show that if outflowing O+ is heated and accelerated sufficiently in the cusp, it takes 15-20 minutes for it to reach the magnetopause, allowing the ions to escape along newly opened field lines on the dayside. Earlier studies show evidence of strong heating and high velocities in the cusp and mantle at high altitudes, strengthening our interpretation. The observed magnetosheath O+ fluxes are of the same order as measured in the ionospheric upflow, which indicates that this loss mechanism is significant when it takes place.
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| 6. |
- Westerberg, Lars-Göran, et al.
(författare)
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3D MHD reconnection model coupled with Cluster multi-spacecraft data : 3D reconnection model
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113
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Tidskriftsartikel (refereegranskat)abstract
- We present a model where a three-dimensional viscous/resistive analytical model of the large scale plasma flow in the outflow region about a reconnection site at the dayside magnetopause, is coupled with multi-spacecraft measurements from the Cluster satellite armada. A total of ten magnetopause crossings have been analyzed during the period 10:29-11:05 UT January 26, 2001, where nine are shown to be suitable to be coupled with the analytical model. Five of these nine reconnection events results in a successful coupling. By fitting the analytical expressions of the plasma velocity and magnetic field behaviour during the transition of the magnetopause, with data from the Cluster spacecraft we obtain estimates of the location of the X-line together with estimates on the anomalous transport coeffcients of kinematic viscosity (v) and magnetic diffusivity (ηd). We also obtain the development of the magnetopause transition layer away from the X-line. It is shown that the distance from the spacecraft to the X-line varies between 0.01 and 1 Earth radii, while v + ηd have a value between 4 . 104 km2/ s and 4 . 106 km2/ s. Furthermore it is shown that the transition layer thickness follows the analytical pattern where it grows proportional to the square root of the distance from the reconnection site. The estimated thickness obtained from each spacecraft is shown to follow the analytical structure well.
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