Andrew N. Wright

Excitation of magnetospheric waveguide modes by magnetosheath flows

Standard models of the Earths outer magnetospheric waveguide assume that a perfectly reflecting magnetopause can trap energy inside the waveguide. In contrast, we show that the near-noon magnetopause often acts as a leaky boundary, wave trapping only being possible for large magnetosheath flow speeds. Moreover, for sufficiently fast flow speeds, we show how waveguide modes may be energized by magnetosheath flows via the overreflection mechanism. Unbounded simulations of the growth of surface waves via the development of a Kelvin-Helmholtz instability (KHI) vortex sheet show growth rates which increase without limit proportional to wavenumber (ky), until the assumption of a thin boundary is no longer valid. For a bounded magnetosphere, however, overreflected body type waveguide modes can introduce wavenumber selection, that is, generate modes with maximum linear growth rates at finite ky. A necessary condition is that the wave is propagating in the magnetosphere, that is, the waves turning point lies inside the magnetosphere. By developing a new description of both KHI and waveguide mode growth in terms of overreflection and the propagation of negative energy waves, we show how the maximum growth rate can be understood in terms of the reflection coefficient of waves incident upon the magnetopause. Our model can also explain the observed local time dependence of Pc5 field line resonance wave power, and can explain the observed correlation between high solar wind speeds and Pc5 wave power. Finally, we show how a waveguide with a free magnetopause boundary supports quarter-wavelength modes. These modes have lower frequencies than the standard (magnetopause velocity node) half-wavelength modes, perhaps generating the millihertz waveguide mode eigenfrequencies which appear to drive field line resonances in HF radar data.

Dispersion and wave coupling in inhomogeneous MHD waveguides

The propagation of the fast mode is considered in nonuniform waveguides. We show how the natural dispersion inherent in a waveguide will select waveguide modes with a small wavenumber (ky) along the guide to remain near a localized source region of fast mode energy. It is these modes that are shown to have a coherent periodic time dependence over many cycles that are suitable for driving observable Alfven resonances (magnetic pulsations). We expect the frequencies of Alfven resonances to be very close to the eigenfrequencies of waveguide modes with ky = 0.

Coupling of magnetospheric cavity modes to field line resonances: A study of resonance widths

By using a box model for the magnetosphere and by using a matrix eigenvalue method to solve the cold linearized ideal MHD equations, we examine the temporal evolution of the irreversible coupling between fast magnetospheric cavity modes and field line resonances (FLRs). By considering the fast mode frequency to be of the form ωf = ωfr − iωfi, and using a Fourier transform approach, we have determined the full time-dependent evolution of resonance energy widths. We find that at short times the resonances are broad, and narrower widths continue to develop in time. Ultimately, an asymptotic resonance Alfven frequency full width at half maximum (FWHM) of ΔωA = 2ωfi develops on a timescale of τfi = ωfi−1. On timescales longer than τfi, we find that the resonance perturbations can continue to develop even finer scales by phase mixing. Thus, at any time, the finest scales within the resonance are governed by the phase mixing length Lph(t) = 2π (tdωA/dx)−1. The combination of these two effects naturally explains the localisation of pulsations in L shells observed in data, and the finer perturbation scales which may exist within them. During their evolution, FLRs may have their finest perturbation scales limited by either ionospheric dissipation or by kinetic effects (including the breakdown of single fluid MHD). For a continually driven resonance, we define an ionospheric limiting timescale τI in terms of the height-integrated Pedersen conductivity ΣP, and hence derive a limiting ionospheric perturbation scale LI = 2π (τIdωA/dx)−1, in agreement with previous steady state analyses. For sufficiently high ΣP, FLR might be able to evolve so that their radial scales reach a kinetic scale length Lk. For this to occur, we require the pulsations to live for longer than τk = 2π (LkdωA/dx)−1. For t < τk, τI, kinetic effects and ionospheric dissipation are not dominant, and the ideal MHD results presented here may be expected to model realistically the growth phase of ULF pulsations.

The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.

The nature of the obstructive muscular bundles in double-chambered right ventricle

OBJECTIVE Our goal was to establish the morphologic nature of the obstructive muscular lesions in double-chambered right ventricle. METHODS We based our morphologic observations on 10 normal hearts and on the surgical findings in 26 patients, aged 0.5 to 24 years, with a mean of 6.9 years (SD 5.8 years). In the normal hearts, we measured the distance from the pulmonary valve to the apex of the right ventricle and from the takeoff of the moderator band to the ventricular apex. From angiograms available in 20 patients, using the frontal view, we then measured the distance from the pulmonary valve to the apex of the right ventricle and from the midpoint of the obstructive lesion to the apex of the right ventricle. This permitted calculations of multiple ratios. RESULTS In the 10 normal hearts, the moderator band took origin at a mean ratio of 0.48 (SD 0.16) of the ventricular length. On the basis of the angiographic findings, we identified 2 basic forms of double-chambered right ventricle. In 9 patients, the obstructive muscular shelf was positioned low and diagonally across the apical component, with a mean ratio of 0.38 relative to the ventricular length (SD 0.02). In the other 11 patients, the obstructive shelf was high and horizontal, with a mean ratio of 0.27 (SD 0.02). The difference was statistically significant (P =.001). Surgical repair was performed successfully in all 26 patients through a right ventriculotomy. CONCLUSIONS Double-chambered right ventricle is the consequence of a high or low muscular division of the apical component of the right ventricle. The abnormal muscular bundle probably represents accentuated septoparietal trabeculations, rather than always being an abnormal moderator band.

Alfvén resonance excitation and fast wave propagation in magnetospheric waveguides

Magnetic pulsations are a robust feature of the Earths magnetosphere. It has been suggested recently that the magnetosphere is sometimes better modeled as a waveguide rather than a cavity. This paper presents numerical simulations of linear magnetohydrodynamic (MHD) waves in an inhomogeneous, low-β waveguide. Several features predicted by recent theoretical studies are confirmed in our simulations, notably that Alfven resonances are driven at frequencies corresponding to the natural frequency of the fast waveguide modes with Vg ≈ 0 (ky ≈ 0).

Coupled Alfvén and Kink Oscillations in Coronal Loops

Observations have revealed ubiquitous transverse velocity perturbation waves propagating in the solar corona. However, there is ongoing discussion regarding their interpretation as kink or Alfven waves. To investigate the nature of transverse waves propagating in the solar corona and their potential for use as a coronal diagnostic in MHD seismology, we perform three-dimensional numerical simulations of footpoint-driven transverse waves propagating in a low β plasma. We consider the cases of both a uniform medium and one with loop-like density structure and perform a parametric study for our structuring parameters. When density structuring is present, resonant absorption in inhomogeneous layers leads to the coupling of the kink mode to the Alfven mode. The decay of the propagating kink wave as energy is transferred to the local Alfven mode is in good agreement with a modified interpretation of the analysis of Ruderman & Roberts for standing kink modes. Numerical simulations support the most general interpretation of the observed loop oscillations as a coupling of the kink and Alfven modes. This coupling may account for the observed predominance of outward wave power in longer coronal loops since the observed damping length is comparable to our estimate based on an assumption of resonant absorption as the damping mechanism.

ULF pulsations in a magnetospheric waveguide: Comparison of real and simulated satellite data

The representation of the Earths magnetosphere as an MHD waveguide has prompted much recent speculation. In particular, the signatures associated with ULF pulsations sit well within the waveguide concept. We further explore the appositeness of the waveguide model by simulating data that a spacecraft would see when passing through a stimulated magnetospheric waveguide and by comparing the results with those obtained from a real spacecraft. We find that the waveguide results again compare favorably and can explain many features seen in the data. We also find that the magnetometer signature of the fast mode in a waveguide (unlike a cavity) does not have a regular oscillatory nature with constant period over a range of L shells.

Magnetotail waveguide: Fast and Alfvén waves in the plasma sheet boundary layer and lobe

Numerical simulations of MHD wave propagation and coupling in a realistic magnetotail are presented. Fast mode waves are observed to disperse and couple resonantly to Alfven waves over a broad layer rather than on an isolated field line. Indeed, the layer is likely to be so broad as to include the entire tail lobe as well as the plasma sheet boundary layer (PSBL). It appears that small k y modes (where k y is the cross-tail wave number) will provide the most efficient coupling as they will tend to propagate along the magnetotail field lines rather than across them and out of the tail boundaries. (Moreover, it is only small k y fast modes that will be able to penetrate the lobe.) Alfven waves in the PSBL are shown to phase mix rapidly resulting in strong field-aligned currents with an equatorward phase motion. These properties are in agreement with observations of optical auroral emissions. The lobe Alfven waves do not phase mix, and are not expected to excite optical emissions. They may, however, provide a significant ponderomotive force and could account for the transport of oxygen ions from the ionosphere into the distant tail lobes.

The dynamics of current carriers in standing Alfvén waves: Parallel electric fields in the auroral acceleration region

nonlinear dynamics of electrons responsible for carrying a few mAm � 2 field aligned current into the ionosphere. The solution shows how most of the electron acceleration in the magnetosphere occurs within 1 RE of the ionosphere, and that a parallel electric field of the order of 1 mVm � 1 is responsible for energising the electrons to 1 keV. The limitations of the electron fluid approximation are considered, and a qualitative solution including electron beams and a modified Ek is developed in accord with observations. We find that the electron acceleration can be nonlinear, (vekrk)vek > wvek, as a result of our nonuniform