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Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations
KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
2006 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 2, 529-542 p.Article in journal (Refereed) Published
Abstract [en]

Lightning Imaging Sensor (LIS) data have been analysed to ascertain the statistical pattern of lightning occurrence over southern Africa. The diurnal and seasonal variations are mapped in detail. The highest flash rates (107.2 km(-2) y(-1)) occur close to the equator but maxima are also found over Madagascar (32.1 km(-2) y(-1)) and South Africa (26.4 km(-2) y(-1)). A feature of the statistics is a relatively steady contribution from over the ocean off the east coast of South Africa that appears to be associated with the Agulhas current. Lightning statistics are of intrinsic meteorological interest but they also relate to the occurrence of whistlers in the conjugate region. Whistler observations are made at Tihany, Hungary. Statistics reveal that the period of most frequent whistler occurrence does not correspond to the maximum in lightning activity in the conjugate region but is strongly influenced by ionospheric illumination and other factors. The whistler/flash ratio, R, shows remarkable variations during the year and has a peak that is narrowly confined to February and March.

Place, publisher, year, edition, pages
2006. Vol. 24, no 2, 529-542 p.
Keyword [en]
meteorology and atmospheric dynamics, lightning, radio science, radio wave propagation
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:kth:diva-24609DOI: 10.5194/angeo-24-529-2006ISI: 000237788400010Scopus ID: 2-s2.0-33751108980OAI: oai:DiVA.org:kth-24609DiVA: diva2:352239
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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