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  • 1.
    Collier, Andrew
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    VLF and ULF Waves Associated with Magnetospheric Substorms2006Doctoral 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.

  • 2.
    Collier, Andrew
    KTH, Superseded Departments, Alfvén Laboratory.
    VLF Chorus Activity Associated with Substorm Particle Injections2004Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The Substorm Chorus Event (SCE) is a Very Low Frequency(VLF) radio phenomenon observed on the ground after the onsetof the substorm expansion phase. It consists of a band of VLFchorus emissions with rising upper and lower cutofffrequencies. These emissions are thought to result fromDoppler-shifted cyclotron resonance between whistler mode wavesand energetic electrons which drift into a station’s fieldof view from an injection site around midnight. The increasingfrequency of the emission envelope has been attributed to thecombined effcts of energy dispersion due to gradient andcurvature drifts, and the modification of the resonanceconditions resulting from the radial component of theE×Bdrift.

    A numerical simulation was developed which tracked themotion of individual electrons from the injection site, takinginto account both gradient-curvature andE×Bdrifts. Synthetic spectrograms were constructed for aground station located around dawn. The calculations indicatedthat although these two drifts are indeed sufficient toreproduce the SCE phenomenon, a substantial dawn-dusk electricfield would be required to produce the radial drift necessaryto generate events with slopes comparable to thoseobserved.

    The precise requirements for cyclotron resonance instabilitywere taken into account in a second set of calculations. Inorder to amplify whistler mode waves the distribution ofresonant particles must satisfy the conditions for instability,specifically its anisotropy should be larger than a criticalvalue which depends on both the wave frequency and the localelectron gyrofrequency. The wave growth rate was determined fora population of electrons subject only to gradient-curvaturedrifts. The dispersive azimuthal motion of the electronsresulted in an anisotropic distribution in the vicinity of theground station, and consequently wave growth. Furthermore, thetemporal evolution of the two factors influencing the growthrate, the anisotropy and relative number of particles inresonance, was such that amplification was initiated at lowerfrequencies and gradually progressed to higher frequencies,thereby reproducing the general frequency-time signature of theSCE. In addition, the results of these calculations placed somerestrictions on the particle source distribution which mightlead to a SCE, indicating that an injection with an enhancedhigh-energy tail produced more satisfactory results than aMaxwellian distribution.

  • 3.
    Collier, Andrew
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Hughes, Arthur Robert W
    KTH, Superseded Departments, Alfvén Laboratory.
    Modelling substorm chorus events in terms of dispersive azimuthal drift2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 12, p. 4311-4327Article in journal (Refereed)
    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.

  • 4.
    Collier, Andrew
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Hughes, Arthur Robert W
    University of KwaZulu-Natal, Durban, South Africa.
    Blomberg, Lars G.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sutcliffe, P. R.
    Evidence of standing waves during a Pi2 pulsation event observed on Cluster2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 10, p. 2719-2733Article in journal (Refereed)
    Abstract [en]

    Observations of Pi2 pulsations at middle and low latitudes have been explained in terms of cavity mode resonances, whereas transients associated with field-aligned currents appear to be responsible for the high latitude Pi2 signature. Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on 21 January 2003, when three of the satellites were within the plasmasphere (L=4.7,4.5 and 4.6) while the fourth was on the plasmapause or in the plasmatrough (L=6.6). Simultaneous pulsations at ground observatories and the injection of particles at geosynchronous orbit corroborate the occurrence of a substorm. Evidence of a cavity mode resonance is established by considering the phase relationship between the orthogonal electric and magnetic field components associated with radial and field-aligned standing waves. The relative phase between satellites located on either side of the geomagnetic equator indicates that the field-aligned oscillation is an odd harmonic. Finite azimuthal Poynting flux suggests that the cavity is effectively open ended and the azimuthal wave number is estimated as m similar to 13.5.

  • 5.
    Collier, Andrew
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Hughes, Arthur Robert W
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Lichtenberger, J
    Steinbach, P
    Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 2, p. 529-542Article in journal (Refereed)
    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.

  • 6.
    Collier, Andrew
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Hughes, A.R.W
    Modelling and Analysis of Substorm-Related Chorus Events2004In: Planetary Ionospheres and Atmospheres Including CIRA / [ed] Rodin A; Rees D; Gupta SP, 2004, Vol. 34, no 8, p. 1819-1823Conference paper (Refereed)
    Abstract [en]

    The substorm-related chorus event is a recognised VLF signature of the substorm expansion phase. Computer simulations support the contention that these events are generated by cyclotron resonance between whistler mode waves and energetic electrons which drift into a ground station's field of view from an injection site near midnight.

  • 7.
    Koen, Etienne J.
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Collier, Andrew B.
    SANSA Space Science, Hospital Street, Hermanus 7200, South Africa; School of Chemistry, University of KwaZulu-Natal, Durban, South Africa.
    Mid-latitude ionospheric signature of a weak solar flare in winter2013In: South African Journal of Science, ISSN 0038-2353, E-ISSN 1996-7489, Vol. 109, no 1-2, p. 83-87Article in journal (Refereed)
    Abstract [en]

    Measurements of the amplitude and phase of very low frequency transmitter signals were used to evaluate the effects on the ionosphere of a moderate intensity solar flare that occurred on 13 December 2007. These measurements were compared to modelled results from the Long Wave Propagation Capability code. The ionospheric effects were found to be delayed by ∼1 min with respect to the 0.1-0.8 nm solar X-ray flux.

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