- Title
- Low latitude geomagnetic field line resonance
- Creator
- Waters, Colin L.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 1992
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In the past many attempts have been made to use geomagnetic pulsations for diagnostic studies of the magnetospheric plasma. These techniques have often involved the observation of Alfvén wave magnetic field line resonances which can be detected as geomagnetic pulsations in space and on the ground. Previous ground based measurements of these resonant wave structures have required large magnetometer arrays and detailed analysis of wave polarisation. The spectral difference in wave amplitude and phase between two closely spaced magnetometer sites is shown to be capable of identifying this resonant structure. A technique based on the cross spectral phase is described and shown to be particularly suitable for monitoring temporal variations in field line and plasma characteristics. Geomagnetic pulsation data at L=1.8 and L=2.8 show that resonance structure is present on most days and occurs during all daytime hours. The structure at L= 1.8 shows a single resonant frequency while at L=2.8 up to 4 harmonics are seen, depending on geomagnetic conditions. Data recorded for 4 months between July and October, 1989 at L=1.8 showed a day-to-day variation in the resonant frequency between 38-56 mHz with the most common frequency between 44-45 mHz occuring on 30 of the 138 samples. A distinguishing feature at L=1.8 is a local morning decrease in the resonant frequency commencing at sunrise and lasting approximately 3 hours. The diurnal variation in geomagnetic field line eigenfrequency at both L = 1.8 and L=2.8 on two consecutive days that exhibited different temporal variation has been modelled by solving the hydromagnetic wave equation for the Alfvén wave mode using a dipole magnetic field and the IRI-90 ionospheric and diffusive equilibrium (DE) plasma density models. These calculations failed to predict the observed harmonic spacing at L=2.8 with the eigenfrequencies found to be lower by more than 50% compared to the experimental values at both L=1.8 and 2.8. However, the importance of mass loading due to 0⁺ at ionospheric altitudes in determining the harmonic spacing is demonstrated. Helium was found to be unimportant in this regard. A comparison of eigenfrequencies calculated for L= 1.7 and 2.4 using the plasma density model of Bailey (1983) and those for the IRI-DE model show the model of Bailey (1983) used by Poulter et al. (1984a, b, 1988) is more realistic for these latitudes. A time domain phase analysis technique, similar in principle to the operation of a phase-locked loop, has been developed to study rapid phase changes in geomagnetic pulsation wave trains. The resulting phase-time representation shows the demodulated input signal and provides wave group structure information. A study of the pulsation group structure using a magnetometer array in eastern Australia spanning 300520 geomagnetic latitude (L=1.4-2.8) and 2.2 hours in longitude is presented. It is shown that the observation of phase skips in pulsation records do not necessarily imply an impulsive source. The phase-time characteristics of signals at three frequencies are compared across the array. Two waves (40 and 43 mHz) show similar characteristics across the latitudinal extent of the array while the other (34 mHz) shows evidence of geomagnetic field line resonance around L∼2. An examination of the pulsation phase-time representation with latitude across this array shows a low latitude region below L∼2 where the spectra at all stations exhibit a similar shape with decreasing power as the latitude decreases. The phase-time characteristics do not show evidence of a broadband wave energy source as the excitation mechanism of low latitude field line resonances. An analysis of phase skip propagation speeds also shows that the assumption of plane wave type propagation is an over simplification. Consequently geomagnetic pulsation activity was modelled by frequency modulating the driving frequency of a forced, lightly damped simple harmonic oscillator. Best agreement with the observed phase-time characteristics was obtained with the driving frequency modulated at 8 mHz and centred on a resonant frequency at 50 mHz.
- Subject
- magnetospheric plasma; magnetic field; Eastern Australia; wave amplitude
- Identifier
- http://hdl.handle.net/1959.13/1416267
- Identifier
- uon:37022
- Rights
- Copyright 1992 Colin L. Waters
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 166 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 2 MB | Adobe Acrobat PDF | View Details Download |