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Träfflista för sökning "WFRF:(Reme H.) "

Sökning: WFRF:(Reme H.)

  • Resultat 1-10 av 52
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1.
  • Deng, X. H., et al. (författare)
  • Dynamics and waves near multiple magnetic null points in reconnection diffusion region
  • 2009
  • Ingår i: Journal of Geophysical Research. - : Blackwell Publishing. - 0148-0227 .- 2156-2202. ; 114:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.
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2.
  • Fuselier, S. A., et al. (författare)
  • Ion chemistry in the coma of comet 67P near perihelion
  • 2016
  • Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 462, s. S67-S77
  • Tidskriftsartikel (refereegranskat)abstract
    • The coma and the comet-solar wind interaction of comet 67P/Churyumov-Gerasimenko changed dramatically from the initial Rosetta spacecraft encounter in 2014 August through perihelion in 2015 August. Just before equinox (at 1.6 au from the Sun), the solar wind signal disappeared and two regions of different cometary ion characteristics were observed. These 'outer' and 'inner' regions have cometary ion characteristics similar to outside and inside the ion pileup region observed during the Giotto approach to comet 1P/Halley. Rosetta/Double-Focusing Mass Spectrometer ion mass spectrometer observations are used here to investigate the H3O+/H2O+ ratio in the outer and inner regions at 67P/Churyumov-Gerasimenko. The H3O+/H2O+ ratio and the H3O+ signal are observed to increase in the transition from the outer to the inner region and the H3O+ signal appears to be weakly correlated with cometary ion energy. These ion composition changes are similar to the ones observed during the 1P/Halley flyby. Modelling is used to determine the importance of neutral composition and transport of neutrals and ions away from the nucleus. This modelling demonstrates that changes in the H3O+/H2O+ ratio appear to be driven largely by transport properties and only weakly by neutral composition in the coma.
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3.
  • Fuselier, S. A., et al. (författare)
  • ROSINA/DFMS and IES observations of 67P : Ion-neutral chemistry in the coma of a weakly outgassing comet
  • 2015
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 583
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun. Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma. Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), COP, and CO, (masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions include H3O+ and HCO+ (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H3O /H2O+ and HCO /CO+ ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network. Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO /CO+ ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H3O /H2O+ ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densities can exceed that of water.
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4.
  • Hietala, H., et al. (författare)
  • Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock
  • 2009
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:24, s. 245001-
  • Tidskriftsartikel (refereegranskat)abstract
    • The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.
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5.
  • Hietala, H., et al. (författare)
  • Supermagnetosonic subsolar magnetosheath jets and their effects : from the solar wind to the ionospheric convection
  • 2012
  • Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 30:1, s. 33-48
  • Tidskriftsartikel (refereegranskat)abstract
    • It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than similar to 6 R-E transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.
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8.
  • Waara, Martin, 1980-, et al. (författare)
  • Oxygen ion energization observed at high altitudes
  • 2010
  • Ingår i: Annales Geophysicae. - : Copernicus Publications on behalf of the European Geosciences Union. - 0992-7689 .- 1432-0576. ; 28, s. 907-916
  • Tidskriftsartikel (refereegranskat)
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9.
  • Waara, M., et al. (författare)
  • Oxygen ion energization observed at high altitudes
  • 2010
  • Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 28:4, s. 907-916
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a case study of significant heating (up to 8 keV) perpendicular to the geomagnetic field of outflowing oxygen ions at high altitude (12 R-E) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2-0.4 R-E in altitude). This is a clear example of local ion energization at much higher altitude than usually reported. In contrast to many events at lower altitudes, it is not likely that the locally observed wave fields can cause the observed ion energization. Also, it is not likely that the ions have drifted from some nearby energization region to the point of observation. This suggests that additional fundamentally different ion energization mechanisms are present at high altitudes. One possibility is that the magnetic moment of the ions is not conserved, resulting in slower outflow velocities and longer time for ion energization.
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10.
  • Yamauchi, M., et al. (författare)
  • Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT
  • 2009
  • Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 27:7, s. 2947-2969
  • Tidskriftsartikel (refereegranskat)abstract
    • In the evening equatorial magnetosphere at about 4 R-E geocentric distance and 19 MLT, the four Cluster spacecraft observed a solitary structure with a width of about 1000 similar to 2000 km in the propagation direction. The solitary structure propagates sunward with about 5 similar to 10 km/s carrying sunward electric field (in the propagation direction) of up to about 10 mV/m (total potential drop of about 5 similar to 10 kV), depletion of magnetic field of about 25%, and a duskward E x B convection up to 50 km/s of He+ rich cold plasma without O+. At the same time, auroral images from the IMAGE satellite together with ground based geomagnetic field data showed a westward (sunward at this location) propagating auroral bulge at the magnetically conjugate ionosphere with the solitary structure. The solitary structure is maintained by flux enhancement of selectively 3000 km/s ions (about 50 keV for H+, 200 keV for He+, and 750 keV for O+). These ions are the main carrier of the diamagnetic current causing the magnetic depletion, whereas the polarization is maintained by different behavior of energetic ions and electrons. Corresponding to aurora, field-aligned accelerated ionospheric plasma of several keV appeared at Cluster from both hemispheres simultaneously. Together with good correspondence in location and propagation velocity between the auroral bulge and the solitary structure, this indicates that the sunward moving auroral bulge is caused by the sunward propagation of the solitary structure which is maintained by energetic ions. The solitary structure might also be the cause of Pi2-like magnetic variation that started simultaneously at Cluster location.
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  • Resultat 1-10 av 52
  • [1]23456Nästa

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