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On the excitation of ULF waves by solar wind pressure enhancements
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
Institute for Geophysics and Extraterrestrial Physics, Technical University of Braunschweig, Germany.
Institute for Geophysics and Extraterrestrial Physics, Technical University of Braunschweig, Germany.
2006 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 11, 3161-3172 p.Article in journal (Refereed) Published
Abstract [en]

We study the onset and development of an ultra low frequency (ULF) pulsation excited by a storm sudden commencement. On 30 August 2001, 14: 10 UT, the Cluster spacecraft are located in the dayside magnetosphere and observe the excitation of a ULF pulsation by a threefold enhancement in the solar wind dynamic pressure. Two different harmonics are observed by Cluster, one at 6.8 mHz and another at 27 mHz. We observe a compressional wave and the development of a toroidal and poloidal standing wave mode. The toroidal mode is observed over a narrow range of L-shells whereas the poloidal mode is observed to have a much larger radial extent. By looking at the phase difference between the electric and magnetic fields we see that for the first two wave periods both the poloidal and toroidal mode are travelling waves and then suddenly change into standing waves. We estimate the azimuthal wave number for the 6.8 mHz to be m = 10 +/- 3. For the 27 mHz wave, m seems to be several times larger and we discuss the implications of this. We conclude that the enhancement in solar wind pressure excites eigenmodes of the geomagnetic cavity/waveguide that propagate tailward and that these eigenmodes in turn couple to toroidal and poloidal mode waves. Thus our observations give firm support to the magnetospheric waveguide theory.

Place, publisher, year, edition, pages
2006. Vol. 24, no 11, 3161-3172 p.
Keyword [en]
magnetospheric physics; MHD waves and instabilities; solar wind-magnetosphere interactions
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-7222ISI: 000243431000031Scopus ID: 2-s2.0-37849187942OAI: oai:DiVA.org:kth-7222DiVA: diva2:12165
Note
QC 20100707Available from: 2007-05-29 Created: 2007-05-29 Last updated: 2010-07-07Bibliographically approved
In thesis
1. Multi-point Measurements of Ultra Low Frequency Waves in the Terrestrial Magnetosphere
Open this publication in new window or tab >>Multi-point Measurements of Ultra Low Frequency Waves in the Terrestrial Magnetosphere
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Waves in the mHz frequency range are prominent features of the terrestrial magnetosphere. In this frequency range the waves have wavelengths comparable to the lengths of the geomagnetic field lines. The waves are then standing waves along closed field lines with endpoints in the southern and northern ionosphere. Waves play an important role in the distribution of energy in the magnetosphere and mHz waves can accelerate electrons to MeV energies and have been proposed as driving mechanism for auroral arcs. They can also be used as diagnostic tools for determining the plasma density. There are two important classes of these low frequency waves. One has large azimuthal wavelength and is usually associated with driving mechanisms outside the magnetosphere, such as the Kelvin-Helmholtz instability at the magnetopause. The other has small azimuthal wavelength and is associated with plasma instabilities inside the magnetosphere. Both types of waves are studied in this thesis with a slight emphasis on the large azimuthal wavelength waves. For the type of wave with large azimuthal wavelength there is however, a considerable debate about the driving mechanism. One recently suggested driver is coherent magnetohydrodynamic waves in the solar wind. Part of this thesis studies this experimentally and we conclude that, at least on some occasions, this driving mechanism comes into play. The Cluster satellites are used to study the morphology of the waves. We demonstrate the ability of Cluster to determine the azimuthal wave number of the waves and also how the structure along the magnetic field lines can be determined. This gives information regarding the harmonic number of the standing waves, which in turn says something about the driver of the waves. We also look at possible excitation mechanisms for the small azimuthal wavelength waves.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. xi, 52 p.
Series
Trita-EE, ISSN 1653-5146 ; 2007:014
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-4404 (URN)978-91-7178-615-9 (ISBN)
Public defence
2007-06-08, Sal F3, KTH, Lindstedtsvägen 26, Stockhol, 10:00
Opponent
Supervisors
Note
QC 20100707Available from: 2007-05-29 Created: 2007-05-29 Last updated: 2010-07-07Bibliographically approved

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