Inverse Bremsstrahlung in Shocked Astrophysical Plasmas

There has recently been interest in the role of inverse bremsstrahlung, the emission of photons by fast suprathermal ions in collisions with ambient electrons possessing relatively low velocities, in tenuous plasmas in various astrophysical contexts. This follows a long hiatus in the application of suprathermal ion bremsstrahlung to astrophysical models since the early 1970s. The potential importance of inverse bremsstrahlung relative to normal bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying velocity distributions of the interacting species. In this paper, we identify the conditions under which the inverse bremsstrahlung emissivity is significant relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We determine that, since both observational and theoretical evidence favors electron temperatures almost comparable to, and certainly not very deficient relative to proton temperatures in shocked plasmas, these environments generally render inverse bremsstrahlung at best a minor contributor to the overall emission. Hence inverse bremsstrahlung can be safely neglected in most models invoking shock acceleration in discrete sources such as supernova remnants. However, on scales > 100pc distant from these sources, Coulomb collisional losses can deplete the cosmic ray electrons, rendering inverse bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly detectable.Comment: 13 pages, including 2 figures, using apjgalley format; to appear in the January 10, 2000 issue, of the Astrophysical Journa

The Biggest Snowball Fight in Earth History: Stratigraphy, Facies Analysis, and Geochronology of the Pocatello Formation

The Snowball Earth Hypothesis details a time in Earth’s history (the Cryogenian period) where the entire planet was encapsulated by kilometer thick ice sheets for two, multi-million-year glaciations. The first, known as the Sturtian, lasted from 717 – 660 million years ago while the second, known as the Marinoan, lasted form approximately 650 – 635 million years ago. Snowball Earth was caused by a few processes that sort of built upon each other: Rodinia began splitting apart ~740 million years ago which allowed for increased rates of silicate weathering. High rates of silicate weathering resulted in CO2 drawdown which in turn caused the global temperature to drop. The gradual drop in temperature allowed for polar ice to advance and, once the ice reached a critical latitude, the earth experienced a runaway ice albedo effect – meaning ice advanced uncontrollably all the way to the equator – resulting in a completely ice-covered planet. Evidence for Snowball Earth comes in the form of paleomagnetic data that show that Rodinia was rifting at this time, a global distribution and synchroneity of Snowball Earth deposits, carbon isotope ratios, and cap carbonates (a unique lithology present only in Snowball Earth deposits). In Pocatello, Idaho, geologists have identified deposits (the Pocatello Formation) that fall within the age range of Snowball Earth and that exhibit similar lithologic characteristics to other known Snowball Earth deposits. The goal of this study is to determine whether the Pocatello Formation was deposited during Snowball Earth, and if it is, which glaciation(s)/deglaciation(s) it was deposited during. To accomplish this goal, we conducted field work to characterize the different lithologies of the Pocatello Formation and then we used a radiometric dating technique called chemical abrasion – isotope dilution – thermal ionization mass-spectrometry to find out exactly how old the Pocatello Formaton is. Our results show that the Pocatello Formation is host to Sturtian glacial deposits, Marinoan glacial deposits, and Marinoan deglacial deposits. These findings will allow for: the revision to local stratigraphy and age boundaries, correlation of the Pocatello Formation with other Snowball Earth deposits worldwide, and further insight into the Cryogenian period

The Role of Social Media for Knowledge Sharing and Collaboration in Distributed Teams

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The Role of Social Media for Knowledge Sharing and Collaboration in Distributed Teams

Social media are providing a medium through which individuals are reshaping how they do many things – finding romantic partners, providing social support, grieving for loved ones, even buying mundane items like toothpaste. They are also reshaping organizations – the way they function, the relationships they contain, and the ways organizations interact with external stakeholders. In this paper, we consider the changes that social media have introduced to organizational knowledge-sharing practices. We believe the social and technical affordances of social media create new challenges for organizations and necessitate research examining the ways in which: (1) technological affordances impact knowledge sharing within and between teams, (2) interpersonal and relationship development processes shape knowledge sharing among distributed team members and their individual social networks, and (3) organizational social networks influence knowledge sharing among distributed team members

Using Gamma-Ray Burst Prompt Emission to Probe Relativistic Shock Acceleration

It is widely accepted that the prompt transient signal in the 10 keV - 10 GeV band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the ultra-relativistic expansion. The detailed understanding of the dissipation and accompanying acceleration at these shocks is a currently topical subject. This paper explores the relationship between GRB prompt emission spectra and the electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The focus is on the array of possible high-energy power-law indices in accelerated populations, highlighting how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. It is emphasized that diffusive shock acceleration theory generates no canonical spectrum at relativistic MHD discontinuities. This diversity is commensurate with the significant range of spectral indices discerned in prompt burst emission. Such system diagnostics are now being enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM) provides key diagnostics on the lower energy portions of the particle population, the focus here is on constraints in the non-thermal, power-law regime of the particle distribution that are provided by the Large Area Telescope (LAT).Comment: 15 pages, 2 figures. Accepted for publication in Advances of Space Researc

Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.Comment: 62 pages, AASTeX format, including 1 table and 11 figures, accepted for publication in The Astrophysical Journal (Vol 513, March 1, 1999