Department of Physics Colloquia of Spring 2013
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Jan 15 X-ray observations of galaxy clusters: the gas mass fraction and dynamical state Max Bonamente, Department of Physics, UAHuntsville |
Jan 22 Gamma-ray Bursts:connecting the macro with the micro Michael Burgess, Department of Physics, UAHuntsville [Abstract] |
Jan 29 Ground-based Ionospheric Observations from Equatorial Africa Linda Krause, NASA MSFC [Abstract] |
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Feb 5 On spectral hardening in solar flares --- diffusive shock acceleration of electrons at a finite width termination shock Gang Li, Department of Physics & CSPAR, UAHuntsville [Abstract] |
Feb 12 Eureka! New Insights Into Hydrogen (water) at the Lunar Poles Rich Miller, Department of Physics, UAHuntsville [Abstract] |
Feb 19 TBA Dragana Tankosic (NASA MSFC)) |
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Feb 26 "TBA" Mike Newchurch (Atmospheric Sciences, UAH)
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Mar 5 "TBA" Nick Pogorelov, Department of Physics and CSPAR, UAHuntsville, UAH
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Mar 12 "TBA" |
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Mar 19 "TBA" Nazirah Jehta, CSPAR, UAHuntsville
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Mar 26 "TBA" |
April 2 "TBA" |
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April 9 "TBA" Amy Winebarger (NASA MSFC)
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April 16 "TBA"TBA |
April 23 "TBA"TBA |
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April 30 Last Seminar |
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Abstract
X-ray observations of galaxy clusters: the gas mass fraction and dynamical state
Max Bonamente, Department of Physics, UAHuntsville
We have observed a large sample of X-ray clusters with Chandra to measure the gas mass fraction, and check its consistency with the cosmological ratio of baryons to total matter. We find agreement with the cosmological value of Omega_b/Omega_M at an overdensity radius of r500, contrary to several previous studies. We also investigate the dynamical state of cluster Abell 1835, detected out to its virial radius. We find interesting results that indicate departure from hydrostatic equilibrium, and hint at the presence of an additional phase in the intra-cluster medium.
Gamma-ray Bursts:connecting the macro with the micro
Michael Burgess, Department of Physics, UAHuntsville
Gamma-ray bursts (GRBs) are the brightest and most relativistic events in the universe. Believed to be the death of super-massive stars or the coalescing of two compact objects, these event accelerate particles in their beamed outflows to bulk Lorentz factors of up to ~1000. This bulk energy is converted into radiation either kinetically or via the violent reconnection of magnetic fields which accelerate charged particles. However, our knowledge of GRBs comes entirely from the brief (.001 - several hundreds seconds) flashes that are detected by gamma-ray observatories and their associated energy spectra. Without knowledge of the processes that occur within their outflows we have been left to fit their spectra with empirical functions that describe the shape of the radiation spectrum but tells us little about the physics behind them. My research has been focused on developing physical fitting functions that eliminate the need for empirical functions and allow for direct testing of theoretical models on the data. Still, being able to test theories has opened up even more mysteries for GRBs.
Ground-based Ionospheric Observations from Equatorial Africa
Linda Krause, NASA MSFC
This talk presents the installation, commissioning, and select results of the long-term deployment of a Digisonde DPS-4 in Ilorin, Nigeria. Ionosondes are used to derive the ionospheric plasma density as a function of altitude by measuring the time-of-flight of a radio pulse that is "reflected" by a layer whose plasma frequency is (approximately) equal to that of the pulse's radio frequency. Because there is a monotonic relationship between the plasma frequency and density, we can derive these density versus altitude profiles by sweeping through the radio pulse frequencies and systematically measuring the time-of-flight as a function of that frequency. Since the radio wave propagation speed is dependent on the refractive index, which in turn is dependent on the plasma density, the inversion is not simple! We will cover the physics associated with this sensing technique, as well as a discussion of the exciting trip to Nigeria. This site was selected because of its unique geographical properties that help us understand one more piece of the following puzzle: What triggers equatorial ionospheric plasma instabilities?
On the spectral hardening of >$300 keV X-ray emission in solar flares
Gang Li, Physics Department, UAH
It has been noted for a long time that the spectra of observed continuum emissions in many solar flares are consistent with double power laws with a hardening at energies $\sim > $ 300 keV. It is now largely believed that at least in electron-dominated events the hardening in photon spectrum reflects an intrinsic hardening in the source electron spectrum. In this Letter, we point out that a power law spectrum of electron with a hardening at high energies can be explained by diffusive shock acceleration of electrons at a termination shock with a finite width. Our suggestion is based on an early analytical work by Drury et al., where the steady state transport equation at a shock with a tanh profile was solved for a $p$-independent diffusion coefficient. Numerical simulations with a $p$-dependent diffusion coefficient show hardenings in the accelerated electron spectrum which are comparable with observations. One necessary condition for our proposed scenario to work is that high energy electrons resonate with the inertial range of the MHD turbulence and low energy electrons resonate with the dissipation range of the MHD turbulence at the acceleration site, and the spectrum of the dissipation range $\sim k^{-2.7}$. A $\sim k^{-2.7}$ dissipation range spectrum is consistent with recent solar wind observations.
Eureka! New Insights into Hydrogen (Water) at the Lunar Poles
Rich Miller, Department of Physics, UAH
Water, water everywhere. The search for enhanced hydrogen abundances on the Moon continues to be a focus of multiple investigations from lunar orbit. Of particular interest are permanently shadowed regions (PSRs) at the poles since these cold traps are potential reservoirs of volatile compounds including water ice. Orbital neutron spectroscopy is a powerful remote sensing technique for these investigations in part because of distinctive hydrogen signatures and the potential for probing stratigraphy distributions. I will discuss recent analyses including the development of robust statistical methods, illustrate ongoing experimental challenges, and present evidence for enhanced hydrogen deposits at the lunar poles. Of particular note, I will highlight our identification of water ice at the surface of Shackleton Crater at the Moon's South Pole. Hypotheses outlining why Shackleton Crater appears to be unique will also be presented.
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