Abs[rac[. Standard models of the Earth's 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growth rates which increase without limit proportional to wavenumber (ky), until the assumption of a thin boundary is no longer valid.

Observations have revealed ubiquitous transverse velocity perturbation waves propagating in the solar corona. However, there is ongoing discussion regarding their interpretation as kink or Alfvén 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 Alfvén mode. The decay of the propagating kink wave as energy is transferred to the local Alfvén 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 Alfvén 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.

By using a box model for the magnetosphere and by using a matrix eigenvalue method to solve the cold hnearized ideal MHD equations, we examine the temporal evolution of the irreversible coupling between fast magnetospheric cavity modes and field hne resonances (FLRs). By considering the fast mode frequency to be of the form wy = wy• -iwyi, 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 Alfv6n frequency full width at half maximum (FWHM) of Aw• = 2coyi develops on a timescale of ryi -c0•/1. On timescales longer than ryi, 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) -2•r (tdwA/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 wI in terms of the height-integrated Pealersen conductivity Ep, and hence derive a limiting ionospheric perturbation scale Li -2•r(ri&oA/dx) -1, in agreement with previous steady state analyses. For sufficiently high •p, 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 r/• -27r (LkdcoA/dx) -1. For t < r/•, ri, 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. ground-based magnetometers, in the form of ultralow frequency (ULF) waves standing on dipolar field lines. The Doppler signatures of these pulsations are also of-Copyright 1995 by the American Geophysical Union. Paper number 95JA00820. 0148-0227/95/95JA-00820505.00 ten observed by HF radar at the ionospheric footpoints of oscillating field lines. Dungey [1954, 1967] first suggested that pulsations were standing Alfvdn waves on dipolar field lines (toroidal modes). He also identified fast poloidal compressional waves, which should propagate across the background magnetic field, and subsequently completed the first decoupled studies of these modes. Southwood [1974] and Chen and Hasegawa [1974] independently presented the first attempts at a full theoretical analysis of the coupled pulsation problem. They proposed that the solar wind, incident upon the magnetospheric cavity and driving magnetosheath flows, could excite a travelling Kelvin-Helmholtz surface wave on the magnetopause. Having an evanescent structure within the magnetosphere, this wave mode...

A 58-amino acid polypeptide containing the functional core region, the x1 core, of the major transactivation domain of the human glucocorticoid receptor has been expressed in Escherichia coli and purified to homogeneity. The polypeptide retains 60-701% ofthe activity ofthe intact domain when assayed in vivo or in vitro. This report describes a structural characterization of the T1 core peptide fragment. Circular dichroism spectroscopy shows that the T1 core and a larger fragment encompassing the intact Tl domain are largely unstructured in water solution under a variety of pH conditions. The T1 core, however, acquires a significant a-helical structure when analyzed in the presence of trifluoroethanol, an agent that favors secondary structure formation in regions that have propensity for a-helical conformation.Two-and three-dimensional NMR spectroscopy of 1-N-labeled T1 core, in the presence of trifluoroethanol, has allowed sequential assignment of 'H and 15N resonances and identification of three protein segments with a-helical character.

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 Alfvén resonances (magnetic pulsations). We expect the frequencies of Alfvén resonances to be very close to the eigenfrequencies of waveguide modes with ky = 0.

We consider the resonant coupling of fast and Alfvén magnetohydrodynamic (MHD) waves in a 3D equilibrium. Numerical solutions to normal modes (∝ exp(−iωt)) are presented along with a theoretical framework to interpret them. The solutions we find are fundamentally different to those in 1D and 2D. In 3D there exists an infinite number of possible resonant solutions within a "Resonant Zone," and we show how boundary conditions and locally 2D regions can favour particular solutions. A unique feature of the resonance in 3D is switching between different permissible solutions when the boundary of the Resonant Zone is encountered. The theoretical foundation we develop relies upon recognising that in 3D the orientation of the resonant surface will not align in a simple fashion with an equilibrium coordinate. We present a method for generating the Alfvén wave natural frequencies for an arbitrarily oriented Alfvén wave, which requires a careful treatment of scale factors describing the background magnetic field geometry.

Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations that undergo strong damping as they propagate. These can be understood in terms of propagating kink waves that undergo mode coupling in inhomogeneous regions. Aims. The use of these propagating waves as a seismological tool for the investigation of the solar corona depends upon an accurate understanding of how the mode coupling behaviour is determined by local plasma parameters. Our previous work suggests the exponential spatial damping profile provides a poor description of the behaviour of strongly damped kink waves. We aim to investigate the spatial damping profile in detail and provide a guide to the approximations most suitable for performing seismological inversions. Methods. We propose a general spatial damping profile based on analytical results that accounts for the initial Gaussian stage of damped kink waves as well as the asymptotic exponential stage considered by previous authors. The applicability of this profile is demonstrated by a full parametric study of the relevant physical parameters. The implication of this profile for seismological inversions is investigated. Results. The Gaussian damping profile is found to be most suitable for application as a seismological tool for observations of oscillations in loops with a low density contrast. This profile also provides accurate estimates for data in which only a few wavelengths or periods are observed.

Aims. We investigate the damping process for propagating transverse velocity oscillations, observed to be ubiquitous in the solar corona, due to mode coupling. Methods. We perform 3D numerical simulations of footpoint-driven transverse waves propagating in a low β coronal plasma with a cylindrical density structure. Mode coupling in an inhomogeneous layer leads to the coupling of the kink mode to the Alfvén mode, observed as the decay of the transverse kink oscillations. Results. We consider the spatial damping profile and find a Gaussian damping profile of the form exp(−z 2 /L 2 g ) to be the most congruent with our numerical data, rather than the exponential damping profile of the form exp(−z/L d ) used in normal mode analysis. Our results highlight that the nature of the driver itself will have a substantial influence on observed propagating kink waves. Conclusions. Our study suggests that this modified damping profile should be taken into account when using coronal seismology to infer local plasma properties from observed damped oscillations.