[Seminar] Colloquia 3/29/2011

[Amy Barnes]

Dept. of Physics & Astronomy
Lewis Hall Rm 101
Tuesday, Mar 29th @ 4:00 p.m.
Refreshments 3:45 p.m.

Sumedhe Karunarathne
Department of Physics and Astronomy
University of Mississippi

Locating Initial Breakdown Pulses of Lightning Flashes

Lightning flashes often begin with a series of bipolar pulses, 1-5 us in width, called initial breakdown pulses or characteristic pulses. In this presentation we show electric field change data of initial breakdown pulses collected with a network of 5 flat-plate antennas with a bandwidth of 0 - 5 MHz. These pulses (see Figure 1) were obtained at the NASA/Kennedy Space Center (KSC) during the summer of 2010. To do the further studies, it is important to know where these pulses happen in clouds. There is a system called "Lightening Detection and Ranging" (LDAR2) with a center frequency of 63 MHz and a bandwidth of 6 MHz already up and running at KSC just to do this task. However, in most cases it misses these beginning pulses.

In this study we use our array of flat plate antenna to determine the location and time (x, y, z, t) of these characteristic pulses using the technique called time-of-arrival [Koshak and Solakiewicz, JGR, 1996]. In this presentation we are presenting locations of beginning pulses for several lightning flashes. In addition, we also collected magnetic field change data with a LINET system [e.g., Betz et al., GRL, 2004], which consisted of 7 crossed- loop sensors having a bandwidth of 5 - 200 kHz; the pulse locations detected by this system were also determined by time of arrival. The locations of the initial breakdown pulses from both systems will be compared to locations of VHF lightning sources made with the KSC LDAR2 system. Error analysis using a Monte-Carlo method will be presented to discuss the accuracy of the technique.



Phil Blom
Department of Physics and Astronomy
University of Mississippi

Using Relative Amplitude and Travel Times from Geometric Acoustics to Determine Nocturnal Effective Sound Speeds

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind structure lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal undergoes strong dispersion during propagation and is received at long ranges from the source as an extended wave train consisting of a series of distinct arrivals followed by a low frequency tail. The leading distinct arrivals have been shown to coincide with the direct and single reflection geometric ray paths. At a range of two kilometers from the source, these ray paths propagate through the lowest fifty to seventy meters of the atmosphere while larger range arrivals reach higher into the atmosphere. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the arrivals can be determined. Arrival details can then be used to fit a low order Taylor series approximation of the effective sound speed profile. Approximations of the atmospheric conditions in the duct will be presented along with both predicted and measured arrival waveforms and meteorological data from the time of propagation.


ALL GRADUATE STUDENTS ARE REQUIRED TO ATTEND




Amy Barnes
Administrative Secretary
Dept. of Physics & Astronomy
108 Lewis Hall
University, MS 38677

Office: 662-915-7046
Fax: 662-915-5045