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  • 1. Hajra, Rajkumar
    et al.
    Henri, Pierre
    Vallières, Xavier
    More, Jeromé
    Gilet, Nicolas
    Wattieaux, Gaetan
    Goetz, Charlotte
    Richter, Ingo
    Tsurutani, Bruce T.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Nilsson, Hans
    Eriksson, Anders I.
    Nemeth, Zoltan
    Burchdegrees, James L.
    Rubin, Martin
    Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko2018Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 475, nr 3, s. 4140-4147Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov-Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of similar to 2.8 (+/- 1.9), skewness similar to 0.43 (+/- 0.36), mean duration of similar to 2.7 (+/- 0.9) min and relative density variation Delta N/N of similar to 0.5 (+/- 0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region.

  • 2.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Lindqvist, Per-Arne
    Royal Institute of Technology, Stockholm, Sweden.
    Ergun, Robert E.
    Laboratory of Atmospheric and Space Physics, Boulder, CO, USA.
    Giles, Barbara L.
    NASA Goddard Space Flight Center, Greenbelt, MD, USA.
    Bow shock generator current systems: MMS observations of possible current closure2018Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, s. 242-258Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use data from the first two dayside seasons of the Magnetospheric Multiscale (MMS) mission to study current systems associated with quasi‐perpendicular bow shocks of generator type. We have analyzed 154 MMS bow shock crossings near the equatorial plane. We compute the current density during the crossings and conclude that the component perpendicular to the shock normal (J⊥) is consistent with a pileup of the interplanetary magnetic field (IMF) inside the magnetosheath. For predominantly southward IMF, we observe a component Jn parallel (antiparallel) to the normal for GSM Y> 0 (<0), and oppositely directed for northward IMF. This indicates current closure across the equatorial magnetosheath, and it is observed for IMF clock angles near 0∘ and 180∘. To our knowledge, these are the first observational evidence for bow shock current closure across the magnetosheath. Since we observe no clear signatures of |J⊥| decreasing toward large |Y| we suggest that the main region of current closure is further tailward, outside MMS probing region. For IMF clock angles near 90∘, we find indications of the current system being tilted toward the north‐south direction, obtaining a significant Jz component, and we suggest that the current closes off the equatorial plane at higher latitudes where the spacecraft are not probing. The observations are complicated for several reasons. For example, variations in the solar wind and the magnetospheric currents and loads affect the closure, and Jn is distributed over large regions, making it difficult to resolve inside the magnetosheath proper.

  • 3.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Nilsson, Hans
    Swedish Institute of Space Physics.
    Simon Wedlund, Cyril
    University of Oslo, Department of Physics, Oslo, Norway.
    Kallio, Esa
    Aalto University, Department of Electronics and Nanoengineering, Espoo, Finland.
    Mann, Ingrid
    University of Tromsø, Department of Physics and Technology, Tromsø, Norway.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, Tomas
    KTH Royal Institute of Technology, School of Electrical Engineering, Stockholm, Sweden.
    Energy conversion in cometary atmospheres: Hybrid modeling of 67P/Churyumov-Gerasimenko2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets.

    Methods. We used a hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov-Gerasimenko at a heliocentric distance of 1.5 AU.

    Results. We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads.

    Conclusions. The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.

  • 4. Madsen, B.
    et al.
    Wedlund, C. Simon
    Eriksson, A.
    Goetz, C.
    Karlsson, T.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Spicher, A.
    Henri, P.
    Vallieres, X.
    Miloch, W. J.
    Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko2018Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, nr 9, s. 3854-3864Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The European Space Agency/Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual-probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well-studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low-frequency waves within two cavities. These low-frequency electrostatic waves are likely triggered by lower-hybrid waves observed in the surrounding magnetized plasma. Plain Language Summary As sunlight heats a comet nucleus, frozen volatile gases sublimate are ionized and interact with the solar wind and its embedded magnetic field, inducing a dynamical plasma environment around the comet. With the cornerstone European mission Rosetta and its 2years of near-continuous orbiting of comet 67P/Churyumov-Gerasimenko, the origin, structure, and evolution of this environment are only starting to be unveiled. Exciting are the numerous crossings of the diamagnetic cavity, the innermost plasma region from which the solar wind magnetic field is excluded. Whilst the magnetic field structure of the cavity crossings is well studied, the related electric field activity remains until now unexplored. Studying the electric field with the Langmuir probes onboard Rosetta, we find that whereas the large-scale electric field structure agrees well with the observed magnetic field behavior during cavity crossings, unexpected short-lived low-frequency electric field signals manifest themselves within the cavity. We interpret these as electrostatic waves triggered by a modulating of the cavity boundary caused by observed electrostatic waves at the same frequency in the surrounding magnetized plasma. This unravels a new aspect of the electromagnetic activity in the inner cometary environment, which is crucial for our understanding of the comet-solar wind-induced plasma environment.

  • 5. Nilsson, H.
    et al.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Karlsson, T.
    Brenning, N.
    Henri, P.
    Goetz, C.
    Eriksson, A. I.
    Behar, E.
    Wieser, G. Stenberg
    Vallieres, X.
    Size of a plasma cloud matters: The polarisation electric field of a small-scale comet ionosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.

    Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth's ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.

    Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.

    Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.

    Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.

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