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Modelling substorm chorus events in terms of dispersive azimuthal drift
KTH, Superseded Departments, Alfvén Laboratory.
KTH, Superseded Departments, Alfvén Laboratory.
2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 12, 4311-4327 p.Article in journal (Refereed) Published
Abstract [en]

The Substorm Chorus Event (SCE) is a radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into a ground station's field of view from an injection site around midnight. The increasing frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts, and the modification of resonance conditions and variation of the half-gyrofrequency cutoff resulting from the radial component of the E x B drift. A model is presented which accounts for the observed features of the SCE in terms of the growth rate of whistler mode waves due to anisotropy in the electron distribution. This model provides an explanation for the increasing frequency of the SCE lower cutoff, as well as reproducing the general frequency-time signature of the event. In addition, the results place some restrictions on the injected particle source distribution which might lead to a SCE.

Place, publisher, year, edition, pages
2004. Vol. 22, no 12, 4311-4327 p.
Keyword [en]
space plasma physics, wave-particle interaction, magnetospheric physics, plasma waves and instabilities, storms and substorms
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:kth:diva-24608DOI: 10.5194/angeo-22-4311-2004ISI: 000226831100025Scopus ID: 2-s2.0-13244259601OAI: oai:DiVA.org:kth-24608DiVA: diva2:352238
Note
QC 20100920Available from: 2010-09-20 Created: 2010-09-20 Last updated: 2017-12-12Bibliographically approved
In thesis
1. VLF and ULF Waves Associated with Magnetospheric Substorms
Open this publication in new window or tab >>VLF and ULF Waves Associated with Magnetospheric Substorms
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

A magnetospheric substorm is manifested in a variety of phenomena observed both in space and on the ground. Two electromagnetic signatures are the Substorm Chorus Event (SCE) and Pi2 pulsations.

The SCE is a Very Low Frequency (VLF) radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into an observer’s field of view from an injection site around midnight. The ascending frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts and the modification of the resonance conditions resulting from the radial component of the E × B drift. Two numerical models have been developed which simulate the production of a SCE. One accounts for both radial and azimuthal electron drifts but treats the wave-particle interaction in an approximate fashion, while the other retains only the azimuthal drift but rigorously calculates both the electron anisotropy and the wave growth rate. Results from the latter model indicate that the injected electron population should have an enhanced high-energy tail in order to produce a realistic SCE.

Pi2 are damped Ultra Low Frequency (ULF) pulsations with periods between 40 and 150 s. The impulsive metamorphosis of the nightside inner magnetosphere during the onset of the substorm expansion phase is accompanied by a broad spectrum of magnetohydrodynamic (MHD) waves. Over a limited range of local times around midnight these waves excite field line resonances (FLRs) on field lines connected with the auroral zone. Compressional waves propagate into the inner magnetosphere, where they generate cavity mode resonances. The uniform frequency of Pi2 pulsations at middle and low latitudes is a consequence of these cavity modes. A number of Pi2 events were identified at times when the Cluster constellation was located in the nightside inner magnetosphere. Electric and magnetic field data from Cluster were used to establish the existence of both cavity and field line resonances during these events. The associated Poynting flux indicated negligible radial or field-aligned energy flow but an appreciable azimuthal flux directed away from midnight.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 114 p.
Series
Trita-EE, ISSN 1653-5146 ; 2006:019
Keyword
Space physics, Subtorms, Magnetosphere, VLF waves, ULF waves
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-3939 (URN)91-7178-372-5 (ISBN)
Public defence
2006-05-23, H1, Teknikringen 33, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100920Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2010-09-20Bibliographically approved

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