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Multi-point Measurements of Ultra Low Frequency Waves in the Terrestrial Magnetosphere
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
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: urn:nbn:se:kth:diva-4404ISBN: 978-91-7178-615-9 (print)OAI: oai:DiVA.org:kth-4404DiVA: diva2:12167
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
List of papers
1. A statistical correlation of Pc5 pulsations and solar wind pressure oscillations
Open this publication in new window or tab >>A statistical correlation of Pc5 pulsations and solar wind pressure oscillations
2006 (English)In: Advances in Space Research, ISSN 0273-1177, Vol. 38, no 8, 1763-1771 p.Article in journal (Refereed) Published
Abstract [en]

The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow oscillations in the Pc5 frequency range with low-frequency oscillations in the solar wind pressure measured by the ACE spacecraft. Ten different days in 2000 and 2001 are analysed with respect to different frequencies and geomagnetic latitudes. Both data sets are bandpass filtered and a complex demodulation technique is used to calculate the correlation in each band. On a number of occasions the wave packet structure of the Pc5 pulsations is in good or excellent agreement with the wave packet structure of the solar wind pressure oscillations. This strongly suggests that the oscillations were directly driven by the solar wind. Particularly good correlation is found in the frequency band 0.8-1.2 mHz. Pulsations in this frequency range are hard to reconcile with the magnetospheric cavity mode model. We conclude that, at least on some occasions, Pc5 pulsations may be directly driven and the magnetosphere cavity/waveguide then assumes a more passive role.

Keyword
ULF waves; Pc5 pulsations; solar wind waves; magnetospheric waveguide
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-5812 (URN)10.1016/j.asr.2005.08.023 (DOI)000202988500029 ()2-s2.0-33845982221 (Scopus ID)
Note

QC 20100707. Uppdaterad från Accepted till Published 2010707. Konferens: 35th COSPAR Scientific Assembly Paris, FRANCE, JUL 18-25, 2004 Comm Space Res

Available from: 2005-08-25 Created: 2005-08-25 Last updated: 2012-09-17Bibliographically approved
2. Cluster satellite observations of mHz pulsations in the dayside magnetosphere
Open this publication in new window or tab >>Cluster satellite observations of mHz pulsations in the dayside magnetosphere
2006 (English)In: Advances in Space Research, ISSN 0273-1177, Vol. 38, no 8, 1730-1737 p.Article in journal (Refereed) Published
Abstract [en]

On 17 August 2002 the Cluster spacecraft moved through the dayside magnetosphere. Between 16:00 and 18:30 LIT clear monochromatic oscillations are seen in both electric field and magnetometer data. The frequency is 4.2 mHz in the spacecraft frame of reference. The oscillations have a clear spatial localisation. The magnetic field oscillations are radially polarised in the plane perpendicular to the background magnetic field, indicating that the wave is in the poloidal mode. From the difference in phase between the satellites we estimate the azimuthal wave number, in, to be about 130, consistent with the magnetic field polarisation. The frequency is stable for different L-values as well as over time. From the value of in, the Doppler shift due to satellite motion is estimated to 0.5 mHz. By looking at the phase of the electric and the magnetic field close to the equator we conclude that the oscillations are in a mode with an odd number of half wavelengths between the two ionospheres.

Keyword
ULF pulsations
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-5813 (URN)10.1016/j.asr.2005.04.103 (DOI)000202988500024 ()2-s2.0-33845991191 (Scopus ID)
Note

QC 20100707. Konferens: 35th COSPAR Scientific Assembly Paris, FRANCE, JUL 18-25, 2004 Comm Space Res.

Available from: 2005-08-25 Created: 2005-08-25 Last updated: 2012-09-17Bibliographically approved
3. Poloidal ULF oscillations in the dayside magnetosphere: a Cluster study
Open this publication in new window or tab >>Poloidal ULF oscillations in the dayside magnetosphere: a Cluster study
2005 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 7, 2679-2686 p.Article in journal (Refereed) Published
Abstract [en]

Three ULF wave events, all occurring in the dayside magnetopshere during magnetically quiet times, are studied using the Cluster satellites. The multi-point measurements obtained from Cluster are used to determine the azimuthal wave number for the events by means of the phase shift and the azimuthal separation between the satellites. Also, the polarisation of the electric and magnetic fields is examined in a field-aligned coordinate system, which, in turn, gives the mode of the oscillations. The large-inclination orbits of Cluster allow us to examine the phase relationship between the electric and magnetic fields along the field lines. The events studied have large azimuthal wave numbers (m similar to 100), two of them have eastward propagation and all are in the poloidal mode, consistent with the large wave numbers. We also use particle data from geosynchronous satellites to look for signatures of proton injections, but none of the events show any sign of enhanced proton flux. Thus, the drift-bounce resonance instability seems unlikely to have played any part in the excitation of these pulsations. As for the drift-mirror instability we conclude that it would require an unreasonably high plasma pressure for the instability criterion to be satisfied.

Place, publisher, year, edition, pages
European Geosciences Union (EGU), 2005
Keyword
ionosphere; wave propagation; magnetospheric physics; plasma waves and instabilities; instruments and techniques
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-5814 (URN)000233568900037 ()2-s2.0-27844539792 (Scopus ID)
Note

QC 20100707

Available from: 2005-08-25 Created: 2005-08-25 Last updated: 2016-07-21Bibliographically approved
4. On the excitation of ULF waves by solar wind pressure enhancements
Open this publication in new window or tab >>On the excitation of ULF waves by solar wind pressure enhancements
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.

Keyword
magnetospheric physics; MHD waves and instabilities; solar wind-magnetosphere interactions
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7222 (URN)000243431000031 ()2-s2.0-37849187942 (Scopus ID)
Note
QC 20100707Available from: 2007-05-29 Created: 2007-05-29 Last updated: 2010-07-07Bibliographically approved
5. Sunward propagating Pc5 waves observed on the post-midnight magnetospheric flank
Open this publication in new window or tab >>Sunward propagating Pc5 waves observed on the post-midnight magnetospheric flank
2008 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 6, 1567-1579 p.Article in journal (Refereed) Published
Abstract [en]

The overall focus on the driver of toroidal Pc5 waves has been on processes located at or acting on the dayside magnetopause and dayside flanks of the magnetosphere. These processes can generate waves that propagate tailward in the magnetosphere. However, an increasing number of studies, both theoretical and experimental, have looked at waves propagating sunward and that are caused by processes in the magnetotail. Here we present an ultra low frequency (ULF) wave observed in the post-midnight/morning sector of the magnetosphere at L=16 R-E. The wave has a toroidal mode polarization. We estimate the azimuthal wave number to m=3, consistent with a toroidal mode type pulsation. The positive sign indicates that the wave is propagating sunward and this is confirmed by looking at the Poynting flux of the wave. The frequency of the wave is not constant with time but shows a small increase in the beginning of the event up to over 2.0 mHz. Then the frequency decreases to 1.0 mHz. This decrease coincides with a drop in the total magnetic field strength and we speculate if this is related to an observed reversal of the sign of the interplanetary magnetic field (IMF) By-component. This event occurs during relatively quiet magnetospheric conditions with a solar wind speed of approximately 400 km/s. Thus this event is highly likely to be driven by a source in the magnetotail and the change in frequency is an excellent example that the frequency of an ULF wave may be modulated by changes of the plasma parameters on the resonant field line.

Keyword
magnetospheric physics; MHD waves and instabilities; solar wind-magnetosphere interactions
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7223 (URN)000256594000023 ()2-s2.0-45149086648 (Scopus ID)
Note
QC 20100707. Uppdaterad från Submitted till Published 20100707.Available from: 2007-05-29 Created: 2007-05-29 Last updated: 2010-07-07Bibliographically approved

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