HR: 1330h
AN: SM12A-1190    [PDF]
TI: Slow, Fast and Mixed Compressible Modes near the Magnetopause
AU: * Scudder, J D
EM: jack-scudder@uiowa.edu
AF: University of Iowa, Room 203 Van Allen Hall, Iowa City, IA 52240 United States
AU: Maynard, N C
EM:
AF: Mission Research Corportation, Nashua, New Hampshire, Nashua, NH 03062 United States
AU: Burke, W J
EM:
AF: Hanscom Air Force Base, Bedford, Massachusetts, Acton, MA 01720 United States
AB: We motivate and illustrate a new technique to certify time variations, observed in spacecraft frame of reference, as compressible slow or fast magnetosonic waves. Like the Wal\'en test for Alfv\'en waves, our method for identifying compressible modes requires no Galilean transformation. Unlike the Wal\'en test, we use covariance techniques with magnetic field time series to select three special projections of {\bf B}(t). The projections of magnetic fluctuations are associated with three, usually non-orthogonal, wavevectors that, in principle, contribute to the locally sampled density fluctuations. Wavevector directions (${\hat {\bf k}}(CoV)$) are derived from eigenvectors of covariance matrices and mean field directions, ${\bf B}_o$. Linear theory for compressible modes indicates that these projections are proportional to the density fluctuations. Regression techniques are then applied to observed density and magnetic field profiles to specify coefficients of proportionality. Signs of proportionality constants, connecting the three projections of $\delta {\bf B}$ and $\delta \rho$, determine whether the compressional modes are of the fast (+) or slow (-) type. Within a polytropic-closure framework, the proportionality between magnetic and density fluctuations can be computed by relating ${\hat {\bf k}}$, the polytropic index, $\gamma$, and the plasma $\beta$. Our certification program validates the direct interpretation of proportionality constants comparing their best-fit and error values with the directions of wavevectors required by the dispersion relation, ${\hat {\bf k}}(Disp)$ inferred from experimental measurements of $\beta$ and $\gamma$. Final certification requires that for each mode retained in the correlation, the scalar product of wavevectors determined through covariance and dispersion-relation analyses are approximately unity $\hat{\bf k}(CoV)\cdot\hat{\bf k}{\rm (Disp)}\approx 1$. This quality check is the compressible-mode analogue to slope-one tests in the Wal\'en test expressed in Els\"asser [1950] variables. By products of completed certification include the assignment of various portions of time-domain data streams to the compression or rarefaction phases of fast/slow modes structures, the directions of wave-power propagation in the plasma frame and relative to the magnetic field direction as well as their phase speeds with respect to the background plasma. These certifications also imply temporal trains of electric fields of the ambipolar type, including spatially varying $E_{\parallel}(t)$, that may be the cause of some of the structured observations of $E_{\parallel}$ that have recently been detected near the diffusion region. Along with Wal\'en tests the new procedures enable surveys for the presence and roles of non-dispersive fast, intermediate, and slow MHD waves in geospace. Geophysical examples from the Polar satellite illustrate fast, slow and even admixtures of fast and slow magnetosonic waves retrieved through our analysis. On this experimental basis, we discuss the roles of compressible-mode structures in boundary layers associated with the magnetopause.
DE: 7831 Laboratory studies
DE: 7835 Magnetic reconnection
DE: 7867 Wave/particle interactions
DE: 7871 Waves and instabilities
SC: SPA - Magnetospheric Physics [SM]
MN: 2003 Fall Meeting