Many important properties of ultra-low frequency (ULF) magnetic pulsations (∼1 to 10 minute periods) observed in the magnetosphere and by ground-based magnetometers have been interpreted successfully in terms of resonant field line models. These models describe the pulsations as transverse hydromagnetic waves standing on dipole flux tubes with fixed ends in the ionosphere. Signal structure and frequency are consistently explained, but the models fail to provide convincing explanations of the discrete spectra often observed. In this letter we propose that the spectrum of excitations is dominated by frequencies that correspond to eigenfrequencies of the fast (compressional) mode in the outer magnetosphere. In the presence of plasma inhomogeneities, the eigenmodes of the fast mode damp, even in the absence of dissipation, through coupling to transverse waves. We discuss the properties of a damped eigenmode, pointing out the analogy to the properties of damped surface eigenmodes. The coupled waves exhibit field line resonance behavior on the magnetic shell where the transverse mode dispersion relation is satisfied, but the spectrum is dominated by fast mode eigenfrequencies.

Resonant ULF waves: A new interpretation

Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.

Theory of hydromagnetic waves in the magnetosphere

Magnetic pulsations within the magnetosphere: A review

Measurements at conjugate points on the ground near L = 4 of the power spectra of magnetic-field fluctuations in the frequency range 0.5 to 20 mHz are used as a means of estimating daily values for the relativistic-electron radial-diffusion coefficient DLL for two periods in December 1971 and January 1972. The values deduced for L−10 DLL show a strong variation with magnetic activity, as measured by the Fredricksburg magnetic index KFR. The radial-diffusion coefficient typically increases by a factor of ∼10 for a unit increase in KFR. When KFR ≲ 2, it is generally found that DLL ≲ 2 × 10−9 L10 day−1 for equatorially mirroring electrons having a first invariant M = 750 Mev/gauss; a value of DLL ∼4 × 10−7 L10 day−1 is deduced for one day on which the mean KFR was 4.5. The quantity L−10 DLL theoretically depends on energy and L as (L/M)(s−2)/2 for relativistic particles, where s is the logarithmic slope of the power-law spectrum of magnetic fluctuations observed on the ground. For the time period analyzed, s typically had values between 1 and 3.

ULF geomagnetic power near L = 4: 2. Temporal variation of the radial diffusion coefficient for relativistic electrons

Power spectra and digital dynamic power spectra of geomagnetic field fluctuations observed at a latitudinal network of stations near L = 4 were computed for the entire day of January 5, 1972, in order to study magnetospheric ULF pulsations on a nonstatistical basis. It was found that there are three dominant spectral peaks in the Pc 3–4 frequency range and another peak in the Pc 5 range. The peaks in the Pc 3–4 range are approximately higher harmonics of the spectral peak in the Pc 5 range. The contours of the peak intensities in the spectral bands have been determined as a function of geomagnetic latitude and universal time. These contour maps clearly show that the locations of the intensity maximums of the Pc 3 and Pc 4 peaks gradually change equatorward and poleward in parallel throughout the day and maintain the relationship that the intensity maximum of the Pc 4 peak is always located at a lower latitude than the intensity maximum of the Pc 3 peak. From these observational results it is suggested that the harmonic relationship results from the spectrum of the driving source and that the difference in the location of the intensity maximums of the Pc 3 and Pc 4 frequency bands arises from a sharp decrease in the phase velocity of the Alfven waves just inside the plasmapause. It is pointed out that these dynamic spectral analyses of Pc 3–4 pulsations near L = 4 open the possibility of studying the dynamics of the plasmapause by ground-based ULF techniques.

ULF pulsation evidence of the plasmapause: 1. Spectral studies of Pc 3 and Pc 4 pulsations near L = 4

An analysis of micropulsation spectra (Pc) obtained from the U.E. observatories

An empirical investigation has been conducted on the practical methods of obtaining the spectra of micropulsation activity, and the interpretational limits which can be applied to the detailed use of spectra computed by the Power Spectral Density (PSD) method. Results indicate that sufficiently high resolution may be obtained reliably from the PSD method for it to be regarded as a useful improvement over handscaling methods. The limitation is shown to remain in the selection of events rather than in computational or instrumental properties.

Vicki E Sherwood

Papers

An analysis of micropulsation spectra (Pc) obtained from the U.E. observatories

The Power Spectral Density technique applied to micropulsation analysis

The geomagnetic time dependence of Pi 2's and bays