VLF Chorus Activity Associated with Substorm Particle Injections
2004 (English)Licentiate thesis, comprehensive summary (Other scientific)
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 stations 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.
Place, publisher, year, edition, pages
Stockholm: Alfvénlaboratoriet , 2004. , xii, 54 p.
space physics, magnetosphere, VLF waves, substorms
IdentifiersURN: urn:nbn:se:kth:diva-1760ISBN: 91-7283-769-1OAI: oai:DiVA.org:kth-1760DiVA: diva2:7751