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Sökning: L773:2156 2202 > Eriksson Anders

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1.
  • Cully, C.M., et al. (författare)
  • Electrostatic structure around spacecraft in tenuous plasmas
  • 2007
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A9, s. A09211-
  • Tidskriftsartikel (refereegranskat)abstract
    • Most satellite-based in situ plasma experiments are affected in some manner by the electrostatic structure surrounding the spacecraft. In order to better understand this structure, we have developed a fully three-dimensional self-consistent model that can accept realistic spacecraft geometry, including both thin (similar to 10(-4) m) wires and long (similar to 10(2) m) booms, with open boundary conditions. The model uses an integral formulation incorporating boundary element, multigrid and fast multipole methods to overcome problems associated with the large range in scale sizes and inherently three-dimensional structure. By applying the model to the Cluster spacecraft, we show that the electric potential structure is dominated by the charge on the wire booms, with the spacecraft body contributing at small distances. Consequently, the potential near the EFW ( Electric Fields and Waves experiment) probes at the end of the wire booms is typically significantly above the true plasma potential. For the Cluster spacecraft, we show that this effect causes a 19% underestimation of the spacecraft potential and 13% underestimation of the ambient electric field. We further assess the electric field due to the sunward-oriented photoelectron cloud, showing that the cloud contributes little to the observed spurious sunward field in the EFW data.
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2.
  • Edberg, Niklas, et al. (författare)
  • Statistical analysis of the location of the Martian magnetic pileup boundary and bow shock and the influence of crustal magnetic fields
  • 2008
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A8, s. A08206-
  • Tidskriftsartikel (refereegranskat)abstract
    • We use the data set from the magnetometer and electron reflectometer instruments on board the Mars Global Surveyor spacecraft to show that the crustal magnetic fields of Mars affect the location of the magnetic pileup boundary (MPB) and bow shock (BS) globally. We search for crossings of the MPB and BS in the data that were observed over the first 16 months of the mission. To identify the influence of the crustal magnetic fields, all crossings are extrapolated to the terminator plane in order to remove the solar zenith angle (SZA) dependence, and to make it possible to compare crossings independently of location. The MPB crossings that were observed over regions on Mars, which contain strong crustal magnetic fields, are on average located further out than crossings observed over regions with weak crustal fields. This is shown in three separate longitude intervals. We also find that the dayside BS crossings observed over the southern hemisphere of Mars are on average located further out than the BS crossings observed over the northern hemisphere, possibly because of the influence of the crustal fields. We also study the magnetic field strength and its variation at the inside of the MPB and their dependence on the SZA and altitude. We find that the magnitude of the magnetic field in the MPB is closely linked to the altitude of the MPB, with the magnitude increasing as the MPB is observed closer to the planet.
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3.
  • Garnier, P., et al. (författare)
  • The detection of energetic electrons with the Cassini Langmuir probe at Saturn
  • 2012
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A10202-
  • Tidskriftsartikel (refereegranskat)abstract
    • The Cassini Langmuir probe, part of the Radio and Plasma Wave Science (RPWS) instrument, has provided a wealth of information about the cold and dense plasma in the Saturnian system. The analysis of the ion side current (current for negative potentials) measured by the probe from 2005 to 2008 reveals also a strong sensitivity to energetic electrons (250-450 eV). These electrons impact the surface of the probe, and generate a detectable current of secondary electrons. A broad secondary electrons current region is inferred from the observations in the dipole L Shell range of similar to 6-10, with a peak full width at half maximum (FWHM) at L = 6.4-9.4 (near the Dione and Rhea magnetic dipole L Shell values). This magnetospheric flux tube region, which displays a large day/night asymmetry, is related to the similar structure in the energetic electron fluxes as the one measured by the onboard Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer (CAPS). It corresponds spatially to both the outer electron radiation belt observed by the Magnetosphere Imaging Instrument (MIMI) at high energies and to the low-energy peak which has been observed since the Voyager era. Finally, a case study suggests that the mapping of the current measured by the Langmuir probe for negative potentials can allow to identify the plasmapause-like boundary recently identified at Saturn, and thus potentially identify the separation between the closed and open magnetic field lines regions.
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4.
  • Haaland, S., et al. (författare)
  • Estimating the capture and loss of cold plasma from ionospheric outflow
  • 2012
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A07311-
  • Tidskriftsartikel (refereegranskat)abstract
    • An important source of magnetospheric plasma is cold plasma from the terrestrial ionosphere. Low energy ions travel along the magnetic field lines and enter the magnetospheric lobes where they are convected toward the tail plasma sheet. Recent observations indicate that the field aligned ion outflow velocity is sometimes much higher than the convection toward the central plasma sheet. A substantial amount of plasma therefore escapes downtail without ever reaching the central plasma sheet. In this work, we use Cluster measurements of cold plasma outflow and lobe convection velocities combined with models of the magnetic field in an attempt to determine the fate of the outflowing ions and to quantify the amount of plasma lost downtail. The results show that both the circulation of plasma and the direct tailward escape of ions varies significantly with magnetospheric conditions. For strong solar wind driving with a southward interplanetary magnetic field, also typically associated with high geomagnetic activity, most of the outflowing plasma is convected to the plasma sheet and recirculated. For periods with northward interplanetary magnetic field, the convection is nearly stagnant, whereas the outflow, although limited, still persists. The dominant part of the outflowing ions escape downtail and are directly lost into the solar wind under such conditions.
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5.
  • Morooka, Michiko W., et al. (författare)
  • Dusty plasma in the vicinity of Enceladus
  • 2011
  • Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 116
  • Tidskriftsartikel (refereegranskat)abstract
    • We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus in 2008 revealed the following cold plasma characteristics: (1) there is a large plasma density (both ions and electrons) within the Enceladus plume region, (2) no plasma wake effect behind Enceladus was detected, (3) electron densities are generally much lower than the ion densities in the E ring (n(e)/n(i) < 0.5) as well as in the plume (n(e)/n(i) < 0.01), and (4) the average bulk ion drift speed is significantly less than the corotation speed and is instead close to the Keplerian speed. These signatures result from half or more of the electrons being attached to dust grains and by the interaction between the surrounding cold plasma and the predominantly negatively charged submicrometer-sized dust grains. The dust and plasma properties estimated from the observations clearly show that the dust-plasma interaction is collective. This strong dust-plasma coupling appears not only in the Enceladus plume but also in the Enceladus torus, typically from about 20 R(E) (similar to 5000 km) north and about 60 R(E) (similar to 15,000 km) south of Enceladus. We also suggest that the dust-plasma interaction in the E ring is the cause of the planetary spin-modulated dynamics of Saturn's magnetosphere at large.
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6.
  • Nilsson, Hans, et al. (författare)
  • Hot and cold ion outflow : Observations and implications for numerical models
  • 2013
  • Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 118:1, s. 105-117
  • 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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7.
  • Nilsson, Hans, et al. (författare)
  • Hot and cold ion outflow : Spatial distribution of ion heating
  • 2012
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117
  • 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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8.
  • Pedersen, A., et al. (författare)
  • Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions
  • 2008
  • Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113:A7
  • Tidskriftsartikel (refereegranskat)abstract
    • Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic ( photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.
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9.
  • Pedersen, A., et al. (författare)
  • Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions
  • 2008
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A7, s. A07S33-
  • Tidskriftsartikel (refereegranskat)abstract
    • Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic ( photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.
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10.
  • Slapak, Rikard, et al. (författare)
  • Observations of oxygen ions in the dayside magnetosheath associated with southward IMF
  • 2012
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117
  • 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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