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Numerical modeling of auroral processes
Umeå University, Faculty of Science and Technology, Department of Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

One of the most conspicuous problems in space physics for the last decades has been to theoretically describe how the large parallel electric fields on auroral field lines can be generated. There is strong observational evidence of such electric fields, and stationary theory supports the need for electric fields accelerating electrons to the ionosphere where they generate auroras. However, dynamic models have not been able to reproduce these electric fields. This thesis sheds some light on this incompatibility and shows that the missing ingredient in previous dynamic models is a correct description of the electron temperature. As the electrons accelerate towards the ionosphere, their velocity along the magnetic field line will increase. In the converging magnetic field lines, the mirror force will convert much of the parallel velocity into perpendicular velocity. The result of the acceleration and mirroring will be a velocity distribution with a significantly higher temperature in the auroral acceleration region than above. The enhanced temperature corresponds to strong electron pressure gradients that balance the parallel electric fields. Thus, in regions with electron acceleration along converging magnetic field lines, the electron temperature increase is a fundamental process and must be included in any model that aims to describe the build up of parallel electric fields. The development of such a model has been hampered by the difficulty to describe the temperature variation. This thesis shows that a local equation of state cannot be used, but the electron temperature variations must be descibed as a nonlocal response to the state of the auroral flux tube. The nonlocal response can be accomplished by the particle-fluid model presented in this thesis. This new dynamic model is a combination of a fluid model and a Particle-In-Cell (PIC) model and results in large parallel electric fields consistent with in-situ observations.

Place, publisher, year, edition, pages
Umeå: Fysik , 2007. , 65 p.
Keyword [en]
space plasma physics, auroral acceleration region, auroral current circuit, Alfvén waves, parallel electric fields, equation of state, fluid simulation, PIC simulation
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-1117ISBN: 978-91-7264-294-2 (print)OAI: oai:DiVA.org:umu-1117DiVA: diva2:140253
Public defence
2007-06-05, MA121, MIT-huset, Umeå University, 13:00
Opponent
Supervisors
Available from: 2007-05-08 Created: 2007-05-08 Last updated: 2017-03-27Bibliographically approved
List of papers
1. A linear auroral current-voltage relation in fluid theory
Open this publication in new window or tab >>A linear auroral current-voltage relation in fluid theory
2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 5, 1719-1728 p.Article in journal (Refereed) Published
Abstract [en]

Progress in our understanding of auroral currents and auroral electron acceleration has for decades been hampered by an apparent incompatibility between kinetic and fluid models of the physics involved. A well established kinetic model predicts that steady upward field-aligned currents should be linearly related to the potential drop along the field line, but collisionless fluid models that reproduce this linear current-voltage relation have not been found. Using temperatures calculated from the kinetic model in the presence of an upward auroral current, we construct here approximants for the parallel and perpendicular temperatures. Although our model is rather simplified, we find that the fluid equations predict a realistic large-scale parallel electric field and a linear current-voltage relation when these approximants are employed as nonlocal equations of state. This suggests that the concepts we introduce can be applied to the development of accurate equations of state for fluid simulations of auroral flux tubes.

Place, publisher, year, edition, pages
Paris: Gauthier-Villars, 2004
Identifiers
urn:nbn:se:umu:diva-12001 (URN)10.5194/angeo-22-1719-2004 (DOI)
Available from: 2007-04-17 Created: 2007-04-17 Last updated: 2017-12-14Bibliographically approved
2. Electron pressure effects on driven auroral Alfvén waves
Open this publication in new window or tab >>Electron pressure effects on driven auroral Alfvén waves
2005 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, no A01214, 11- p.Article in journal (Refereed) Published
Abstract [en]

Fluid models for the auroral electron acceleration processes have almost exclusively been derived by assuming cold or isothermal electrons. The consequences of these assumptions have never been thoroughly analyzed. In this study we compare results from an isothermal simulation with those obtained when the pressure is calculated from a double adiabatic approximation and from stationary kinetic theory. We find that the reflection of shear Alfvén waves, as well as the current-voltage relation, is very sensitive to the description of the electron pressure variations. Using pressures calculated from steady-state kinetic theory, we find that driven shear Alfvén waves can build up auroral currents and fields that are consistent with a linear current-voltage relation.

 

Place, publisher, year, edition, pages
Washington: American Geophysical Union., 2005
Keyword
auroral electron acceleration Alfvén waves, electron pressure, nonlocal kinetic theory
Identifiers
urn:nbn:se:umu:diva-12002 (URN)doi:10.1029/2004JA010610 (DOI)
Available from: 2007-04-17 Created: 2007-04-17 Last updated: 2017-12-14Bibliographically approved
3. Electrostatic potentials in the downward auroral current region
Open this publication in new window or tab >>Electrostatic potentials in the downward auroral current region
2005 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, no A08207, 1-9 p.Article in journal (Refereed) Published
Abstract [en]

Assuming a fixed ion density, adiabatic electron motion, and quasi-neutrality, we use the stationary Vlasov equation to derive the self-consistent potential in an auroral flux tube that carries downward current. Our model predicts downward electric fields ∼5 mV/m at an altitude near 2000 km, and around 4000 km the potential reaches ∼2.5 kV. A weak upward electric field at high altitudes reduces the potential, and the potential difference between the ionosphere and magnetosphere is much smaller.

Place, publisher, year, edition, pages
Washington: American Geophysical Union, 2005
Keyword
aurora, downward current, electrostatic, potential, electron density
Identifiers
urn:nbn:se:umu:diva-2306 (URN)doi:10.1029/2005JA011083 (DOI)
Available from: 2007-05-08 Created: 2007-05-08 Last updated: 2017-12-14Bibliographically approved
4. Particle-fluid simulation of the auroral current circuit
Open this publication in new window or tab >>Particle-fluid simulation of the auroral current circuit
2006 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A12201, 12- p.Article in journal (Refereed) Published
Abstract [en]

The incompatibility between stationary kinetic and dynamic fluid descriptions of auroral electron acceleration has been an outstanding problem in space physics for decades. In this study we introduce a new numerical simulation model that provides a unified picture by including electron temperature variations consistent with collisionless kinetic theory in the fluid description. We demonstrate that this new particle-fluid model can describe the partial reflection of Alfvén waves from the acceleration region, as well as the formation of a field-aligned potential drop proportional to the upward current. This study also suggests that for example ion dynamics and high-frequency waves must be added to the model before it properly can describe the return current region. Simulations based on the particle-fluid concept can be applied to various processes in space physics and astrophysics where strong currents flowing along an inhomogeneous magnetic field will cause temperature increases and field-aligned electric fields.

Place, publisher, year, edition, pages
Washington: American Geophysical Union., 2006
Keyword
Alfvén waves, auroral electron acceleration, electron heating
Identifiers
urn:nbn:se:umu:diva-12004 (URN)doi:10.1029/2006JA011826 (DOI)
Available from: 2007-03-18 Created: 2007-03-18 Last updated: 2017-12-14Bibliographically approved
5. Implementing a particle-fluid model of auroral electrons
Open this publication in new window or tab >>Implementing a particle-fluid model of auroral electrons
2007 (English)In: Applied Parallel Computing. State of the Art in Scientific Computing: 8th International Workshop, PARA 2006, Umeå, Sweden, June 18-21, 2006, Revised Selected Papers / [ed] Bo Kågström, Erik Elmroth, Jack Dongarra, Jerzy Wasniewski, Heidelberg: Springer Berlin/Heidelberg, 2007, 371-379 p.Conference paper, Published paper (Refereed)
Abstract [en]

The particle-fluid model of auroral electrons that is presented in [1] is a major step forward within the field of dynamic models of the auroral generation mechanisms. The model is, however, also an example where the implementation of a physical model requires a lot of knowledge from the field of computer science. Therefore, this paper contains a detailed description of the implementation behind the particle-fluid model. We present how the particles are implemented in doubly linked lists, how the fluid equations are solved in a time-efficient algorithm, and how these two parts are coupled into a single framework. We also describe how the code is parallelized with an efficiency of nearly 100%.

Place, publisher, year, edition, pages
Heidelberg: Springer Berlin/Heidelberg, 2007
Series
Lecture notes in computer science, ISSN 0302-9743 ; 4699
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-2308 (URN)10.1007/978-3-540-75755-9_45 (DOI)978-3-540-75754-2 (ISBN)
Conference
8th International Workshop, PARA 2006, Umeå, Sweden, June 18-21, 2006
Available from: 2007-05-08 Created: 2007-05-08 Last updated: 2012-08-10Bibliographically approved
6. Parallel electric fields: variations in space and time on auroral field lines
Open this publication in new window or tab >>Parallel electric fields: variations in space and time on auroral field lines
2008 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 74, no 1, 53-64 p.Article in journal (Refereed) Published
Abstract [en]

We present results from a particle–fluid simulation of auroral electrons and discuss the distribution of parallel electric fields along auroral field lines and the processes occurring during the build up of these electric fields. Neglecting field-aligned ion dynamics, the main potential drop has a width of about 5000, km and is centered at an altitude of roughly 5000, km. We find that the gradient in the potential becomes steeper and the build up of the potential drop becomes faster if the temperature of the magnetospheric electrons is lower.

Place, publisher, year, edition, pages
Cambridge University Press, 2008
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-2309 (URN)10.1017/S0022377807006538 (DOI)
Available from: 2007-05-08 Created: 2007-05-08 Last updated: 2017-12-14Bibliographically approved
7. Estimating properties of concentrated parallel electric fields from electron velocity distributions
Open this publication in new window or tab >>Estimating properties of concentrated parallel electric fields from electron velocity distributions
2007 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, no 16, L16107Article in journal (Refereed) Published
Abstract [en]

Information about the magnitude of the field-aligned potential drop along auroral field lines is usually derived from the velocity distribution of the particles. When the electrons are accelerated by a strong double layer their velocity distribution will have features different from those produced by a weak, spread-out, electric field. Quantifying these features, we obtain information about the strength and thickness of the double layer.

Place, publisher, year, edition, pages
Washington: American Geophysical Union (AGU), 2007
Keyword
auroral acceleration region, particle
National Category
Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:umu:diva-16550 (URN)10.1029/2007GL030162 (DOI)000249334400002 ()
Available from: 2007-10-05 Created: 2007-10-05 Last updated: 2017-12-14Bibliographically approved

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