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Cluster observations and theoretical identification of broadband waves in the auroral region
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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2005 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 12, 3739-3752 p.Article in journal (Refereed) Published
Abstract [en]

Broadband waves are common on auroral field lines. We use two different methods to study the polarization of the waves at 10 to 180 Hz observed by the Cluster spacecraft at altitudes of about 4 Earth radii in the nightside auroral region. Observations of electric and magnetic wave fields, together with electron and ion data, are used as input to the methods. We find that much of the wave emissions are consistent with linear waves in homogeneous plasma. Observed waves with a large electric field perpendicular to the geomagnetic field are more common (electrostatic ion cyclotron waves), while ion acoustic waves with a large parallel electric field appear in smaller regions without suprathermal (tens of eV) plasma. The regions void of suprathermal plasma are interpreted as parallel potential drops of a few hundred volts.

Place, publisher, year, edition, pages
2005. Vol. 23, no 12, 3739-3752 p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-93092DOI: 10.5194/angeo-23-3739-2005ISI: 000235008400015OAI: oai:DiVA.org:uu-93092DiVA: diva2:166464
Available from: 2005-05-11 Created: 2005-05-11 Last updated: 2012-08-21Bibliographically approved
In thesis
1. Cluster Observations and Theoretical Explanations of Broadband Waves in the Auroral Region
Open this publication in new window or tab >>Cluster Observations and Theoretical Explanations of Broadband Waves in the Auroral Region
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Broadband extremely low-frequency wave emissions below the ion plasma frequency have been observed by a number of spacecraft and rockets on auroral field lines. The importance of these broadband emissions for transverse ion heating and electron acceleration in the auroral regions is now reasonably well established. However, the exact mechanism(s) for mediating this energy transfer and the wave mode(s) involved are not well known. In this thesis we focus on the identification of broadband waves by different methods.

Two wave analysis methods, involving different approximations and assumptions, give consistent results concerning the wave mode identification. We find that much of the broadband emissions can be identified as a mixture of ion acoustic, electrostatic ion cyclotron and, ion Bernstein waves, which all can be described as different parts of the same dispersion surface in the linear theory of waves in homogeneous plasma.

A new result is that ion acoustic waves occur on auroral magnetic field lines. These are found in relatively small regions interpreted as acceleration regions without cold (tens of eV) electrons.

From interferometry we also determine the phase velocity and k vector for parallel and oblique ion acoustic waves. The retrieved characteristic phase velocity is of the order of the ion acoustic speed and larger than the thermal velocity of the protons. The typical wavelength is around the proton gyro radius and always larger than the Debye length which is consistent with ion acoustic waves.

We have observed quasi-static parallel electric fields associated with the ion acoustic waves in regions with large-scale currents. Waves, in particular ion acoustic waves, can create an anomalous resistivity due to wave-particle interaction when electrons are retarded or trapped by the electric wave-field. To maintain the large-scale current, a parallel electric field is set up, which then can accelerate a second electron population to high velocities.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. 42 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 55
Keyword
Space and plasma physics, ion acoustic waves, auroral region, anomalous resistivity, Cluster, Rymd- och plasmafysik
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-5809 (URN)91-554-6258-8 (ISBN)
Public defence
2005-06-01, Polhem, Ångströmslaboratoriet, Lägerhyddsv. 1, Uppsala, 10:15
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Available from: 2005-05-11 Created: 2005-05-11Bibliographically approved

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André, Mats

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