Category: News

PI Claudio Mazzoleni and Co-PI Kendra Wright (Physics) have been awarded a $30,000 grant/student fellowship from the National Aeronautics and Space Administration for “Radiative Properties of Free Tropospheric Aerosols in the North Atlantic at the Pico Mountain Observatory.”

From Tech Today.

How would negative energy density affect a classic Friedmann cosmology? Although never measured and possibly unphysical, the evolution of a universe containing a significant cosmological abundance of any of a number of hypothetical stable negative energy components is explored. These negative energy (Omega < 0) forms include negative phantom energy (w<-1), negative cosmological constant w=-1, negative domain walls w=-2/3, negative cosmic strings (w=-1/3), negative mass w=0, negative radiation (w=1/3), and negative ultra light (w > 1/3). Assuming that such universe components generate pressures as perfect fluids, the attractive or repulsive nature of each negative energy component is reviewed.

The Friedmann equations can only be balanced when negative energies are coupled to a greater magnitude of positive energy or positive curvature, and minimal cases of both of these are reviewed. The future and fate of such universes in terms of curvature, temperature, acceleration, and energy density are reviewed including endings categorized as a Big Crunch, Big Void, or Big Rip and further qualified as “Warped”, “Curved”, or “Flat”, “Hot” versus “Cold”, “Accelerating” versus “Decelerating” versus “Coasting”. A universe that ends by contracting to zero energy density is termed a Big Poof. Which contracting universes “bounce” in expansion and which expanding universes “turnover” and contract are also reviewed.

I am working with Dr. Petra Huentemeyer on the HAWC (short for High Altitude Water Cherenkov) experiment, a very high energy (VHE) gamma-ray observatory currently being built in Mexico. In contrast to optical or radio waves, gamma-ray photons cannot penetrate the Earth’s atmosphere. Instead they collide with particles in the atmosphere and create showers of secondary particles through electromagnetic and hadronic interactions. Once completed the HAWC experiment will consist of 300 water Cherenkov detectors (WCDs), that will measure these secondary particles as they sweep through the array. The directions of the primary gamma-ray photons are reconstructed using the time between the signals in each WCD. Thus timing calibration, which is what I am currently working on, is crucial for good angular resolution. As of now, more than one third of the array is finished and operational. At the moment, I am doing a preliminary analysis of data collected with this sub-array. Figure 1 shows a picture of HAWC on July 26, 2013. My research is focusing on pulsar wind nebulae (PWNe), the largest class of galactic VHE gamma-ray sources. PWNe produce electromagnetic radiation in a very broad energy range from radio to VHE gamma rays, and thereby provide an excellent laboratory to study the physical processes at very high energy. Figure 2 shows the crab nebula at different wavelengths. Using data collected with the complete HAWC array, I will reconstruct the energy spectrum of gamma rays emitted by PWNe. I will combine my analysis results with data from other experiments that take measurements at lower energies, to reveal the physics processes occurring in PWNe.

by Hao Zhou

Recipients of Summer Undergraduate Research Fellowships (SURFs) will present end-of-summer project updates Monday and Tuesday (Aug. 12 and 13), from 9 to 11 a.m., in Fisher 139.

From Tech Today.

SURF award recipients in physics include:

Joseph Charnawskas
Advisor: Raymond Shaw
The Effects of the Gravitational Force on Water Particles in a Turbulent Flow

Mick Small
Advisor: Yoke Khin Yap
Photovoltaic Responses of Quantum Dot Sensitized ZnO Nanowires

Angela Small (Honor’s Institute)
Advisor: Jacek Borysow
Analysis of Artificial Breath Samples Using Raman Spectroscopy for Medical Diagnosis

Kevin Rocheleau (Honor’s Institute)
Advisor: Petra Huentemeyer
Analysis and Modeling of Diffuse Gamma-Ray Emission from the Cygnus Region using FERMI and HAWC Data

Minimalist model of ice microphysics in mixed-phase stratiform clouds

Fan Yang

Advisor: Raymond Shaw

Mixed-phase clouds, which can exist days even weeks, are frequently observed in the Arctic region where they play an important role in the radiation balance. Observations also show that ice particles precipitate from these clouds nearly all time. The question of whether persistent ice crystal precipitation from supercooled layer clouds can be explained by time-dependent, stochastic ice nucleation is explored using an approximate, analytical model, and a large-eddy simulation (LES) cloud model. The updraft velocity in the cloud defines an accumulation zone, where small ice particles cannot fall out until they are large enough, which will increase the residence time of ice particles in the cloud. Ice particles reach a quasi-steady state between growth by vapor deposition and fall speed at cloud base. The analytical model predicts that ice water content (w_i) has a 2.5 power law relationship with ice number concentration (n_i). w_i and n_i from a LES cloud model with stochastic ice nucleation also confirm the 2.5 power law relationship. The prefactor of the power law is proportional to the ice nucleation rate, and therefore provides a quantitative link to observations of ice microphysical properties. Figure 1 shows LES results for two ice nucleation rates: Blue points correspond to low ice nucleation rate and red points to high ice nucleation rate. It’s clearly to see that points follow the 2.5 power law as our analytical model expected. The intercept shift predicted by the minimalist model is 1.05, which is very close to the best fitted line shift in Figure 1, 5.77-4.75=1.03. This provides a compelling link between ice microphysical properties and the ice nucleation rate within the cloud, which may be used in future analysis of cloud observation.

Reference

Yang, F., M. Ovchinnikov, R.A. Shaw (2013), Minimalist model of ice microphysics in mixed-phase stratiform clouds, Geophys. Res. Lett. doi: 10.1002/grl.50700 (accepted)