J. Michael Picone

The interaction of a shock with a vortex: Shock distortion and the production of acoustic waves

Numerical simulations of a shock interacting with a compressible vortex are presented for shocks and vortices of various relative strengths. The simulations show the effects of the vortex on the shock structure and the structure of the acoustic field generated by the shock–vortex interaction. A relatively weak vortex perturbs the transmitted shock only slightly, whereas a strong vortex leaves the transmitted shock with a structure corresponding to either a regular or Mach reflection. The acoustic wave generated by the interaction consists of two components: a ‘‘quadrupolar’’ component produced by the initial shock–vortex interaction and the complex reflected shock system. When these waves merge, they form the asymmetric structure seen in experiments.

Evolution of the Orszag--Tang vortex system in a compressible medium. II. Supersonic flow

The numerical investigation of Orszag–Tang vortex system in compressible magnetofluids continues, this time using initial conditions with embedded supersonic regions. The simulations have initial average Mach numbers M=1.0 and 1.5 and β=10/3 with Lundquist numbers S=50, 100, or 200. Depending on the particular set of parameters, the numerical grid contains 2562 or 5122 collocation points. The behavior of the system differs significantly from that found previously for the incompressible and subsonic analogs. Shocks form at the downstream boundaries of the embedded supersonic regions outside the central magnetic X point and produce strong local current sheets that dissipate appreciable magnetic energy. Reconnection at the central X point, which dominates the incompressible and subsonic systems, peaks later and has a smaller impact as M increases from 0.6 to 1.5. Reconnection becomes significant only after shocks reach the central region, compressing the weak current sheet there. Similarly, the correlation b...

Inversion of Infrasound Signals for Passive Atmospheric Remote Sensing

During the past few years, significant progress has been made in our understanding of atmospheric propagation of infrasound signals from both natural and man-made impulsive events. In this chapter, we review this progress within the framework of the early history of infrasound remote sensing, including basic geophysical remote sensing theory and linear acoustic wave propagation. Also, we review the capabilities and limitations of current global atmospheric specification models used in propagation studies.

Nonlinear thermal instability in magnetized solar plasmas

The radiation-driven thermal instability might explain the formation and maintenance of cool dense regions embedded in a hotter more rarefied plasma. Structures of this type often are observed in astrophysical environments such as the solar corona or the interstellar medium. In the present work, the response of a magnetized solar transition-region plasma to a spatially random magnetic-field perturbation is simulated, where the magnetic field is perpendicular to the computational plane. It is found that the presence of the magnetic field, the value of the plasma beta, and the heating process significantly influence the number and size of the condensations as well as the evolutionary time scale. 24 refs.

Mid‐latitude temperatures at 87 km: Results from multi‐instrument Fourier analysis

Using a novel Fourier fitting method we com- bine two years of mid-latitude temperature measurements at 87 km from the High Resolution Doppler Imager, the Colorado State University lidar, and the Peach Mountain Interferometer. After accounting for calibration bias, sig- nificant local-time variations on the order of 10 K were ob- served. Stationary planetary waves with amplitudes up to 10 K were observed during winter, with weaker wave ampli- tudes occurring during other seasons. Because of calibration biases among these instruments, we could estimate the an- nual mean temperature to no better than 193.5 4- 8.5 K. (1991) and recent work by Leblanc et ed. (1999). We present a methodology for making statistical estimates of mid-latitude temperature fields using a combination of satel- lite and ground-based data. Results at a height of 87 km are presented here to illustrate the method and its potential future use with larger multi-instrument data sets. Figure la shows the combined longitudinal and tempo- ral sampling pattern of the temperature field between 41 o 4. 1 o N at 87 km over a two-year period from the High Resolution Doppler Interferometer (HRDI) (Ortland et at.,