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Numerical modelling of ENAs from stellar wind interactions
Institutet för rymdfysik (IRF).
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Energetic neutral atoms (ENAs) are produced whenever a stellar wind encounters a neutral atmosphere. If a stellar wind proton comes sufficiently close to a neutral a charge-exchange reaction may take place, transforming the proton into an ENA. Unaffected by magnetic and electric fields, ENAs provide an opportunity for global imaging of stellar wind interactions.

This thesis presents methods and results of numerical modelling of stellar wind interactions. In particular it treats in depth production of ENAs at comets, Mars and the extrasolar planet HD 209458b.

Sufficiently accurate numerical models of stellar wind interactions require extensive computations. Parallel computing has therefore been used throughout the work, both for fluid and particle simulations of space plasmas. This thesis describes the use of a general simulation tool, providing parallel computing for space plasma simulations.

The thesis presents estimations of the magnitude and morphology of the ENA production at comets and HD 209458b. It compares the results obtained with observations and analyzes them in the light of ENA production at similar objects. Also, simulated ENA images for Mars were produced and compared to observations.

Place, publisher, year, edition, pages
Kiruna: Institutet för rymdfysik (IRF) , 2008. , 52 p.
Series
IRF Scientific Report, ISSN 0284-1703 ; 297
Keyword [en]
Space physics, Space technology, Astrophysics
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:umu:diva-1804ISBN: 978-91-977255-5-2 (print)OAI: oai:DiVA.org:umu-1804DiVA: diva2:142032
Public defence
2008-09-26, Aulan, Institutet för rymdfysik, Etian 100, Kiruna, 10:00
Opponent
Supervisors
Available from: 2008-09-04 Created: 2008-09-03Bibliographically approved
List of papers
1. Energetic neutral atoms around HD 209458b: estimations of magnetospheric properties
Open this publication in new window or tab >>Energetic neutral atoms around HD 209458b: estimations of magnetospheric properties
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2010 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 709, no 2, 670-679 p.Article in journal (Refereed) Published
Abstract [en]

HD 209458b is an exoplanet found to transit the disk of its parent star. Observations have shown a broad absorption signature about the Ly alpha stellar line during transit, suggesting the presence of a thick cloud of atomic hydrogen around the "hot Jupiter" HD 209458b. This work expands on an earlier work studying the production of energetic neutral atoms (ENAs) as a result of the interaction between the stellar wind and the exosphere. We present an improved flow model of HD 209458b and use stellar wind values similar to those in our solar system. We find that the ENA production is high enough to explain the observations, and we show that-using expected values for the stellar wind and exosphere-the spatial and velocity distributions of ENAs would give absorption in good agreement with the observations. We also study how the production of ENAs depends on the exospheric parameters and establish an upper limit for the obstacle standoff distance at approximately 4-10 planetary radii. Finally, we compare the results obtained for the obstacle standoff distance with existing exomagnetospheric models and show how the magnetic moment of HD 209458b can be estimated from ENA observations.

Place, publisher, year, edition, pages
Bristol: Institute of Physics (IOP), 2010
Keyword
methods, data analysis, methods, numerical, stars, winds, outflows
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-3360 (URN)10.1088/0004-637X/709/2/670 (DOI)000273579800010 ()
Available from: 2008-09-04 Created: 2008-09-04 Last updated: 2017-01-23Bibliographically approved
2. The tailward flow of energetic neutral atoms observed at Mars
Open this publication in new window or tab >>The tailward flow of energetic neutral atoms observed at Mars
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2008 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, E12012Article in journal (Refereed) Published
Abstract [en]

The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.

Place, publisher, year, edition, pages
Washington: American Geophysical Union (AGU), 2008
Keyword
solar-wind interaction, subsolar ENA jet, 1st-ENA observations, Aspera-3 observations, upper atmosphere, charge-exchange, express mission, space plasmas, hydrogen, distributions
National Category
Geochemistry Geophysics
Identifiers
urn:nbn:se:umu:diva-3361 (URN)10.1029/2008JE003139 (DOI)000262043600001 ()
Available from: 2008-09-04 Created: 2008-09-04 Last updated: 2017-10-10Bibliographically approved
3. Energetic neutral atom imaging of comets
Open this publication in new window or tab >>Energetic neutral atom imaging of comets
2008 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 35, no L05103Article in journal (Refereed) Published
Abstract [en]

We present the first computer simulated images of energetic neutral atoms (ENAs) produced by charge exchange between solar wind protons and cometary neutrals. Proton bulk flow and temperature from MHD simulations are used along with a model of cometary neutral densities. The emission of hydrogen ENAs from the comet is then calculated using cross sections for the charge exchange collisions. The ENA production rate is integrated along lines of sight to produce an image of the fluxes. We find detection of hydrogen ENAs feasible and most favorable at solar zenith angles between 80 and 130 degrees.

Identifiers
urn:nbn:se:umu:diva-3362 (URN)10.1029/2007GL032955 (DOI)
Available from: 2008-09-04 Created: 2008-09-04Bibliographically approved
4. Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b
Open this publication in new window or tab >>Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b
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2008 (English)In: Nature, ISSN 0028-0836, Vol. 451, 970-972 p.Article in journal (Refereed) Published
Abstract [en]

Absorption in the stellar Lyman- (Ly) line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet1, 2. This has been interpreted as hydrogen atoms escaping from the planet's exosphere1, 3, possibly undergoing hydrodynamic blow-off4, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres5, 6, 7, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Ly absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations. line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet1, 2. This has been interpreted as hydrogen atoms escaping from the planet's exosphere1, 3, possibly undergoing hydrodynamic blow-off4, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres5, 6, 7, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Ly absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations.

Identifiers
urn:nbn:se:umu:diva-3363 (URN)10.1038/nature06600 (DOI)
Available from: 2008-09-04 Created: 2008-09-04 Last updated: 2009-06-22Bibliographically approved
5. MHD modeling of the interaction between the solar wind and solar system objects
Open this publication in new window or tab >>MHD modeling of the interaction between the solar wind and solar system objects
2006 In: Applied Parallel Computing: State of the Art in Scientific Computing, 2006, 554-562 p.Chapter in book (Other academic) Published
Identifiers
urn:nbn:se:umu:diva-3364 (URN)978-3-540-29067-4 (ISBN)
Available from: 2008-09-04 Created: 2008-09-04Bibliographically approved

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