21 research outputs found
Properties of AKR from an interferometer analysis of the ISEE-1 and -2 plasma wave data
Both ISEE 1 and 2 are equipped with identical wide-band analog plasma wave receivers which makes long baseline interferometry possible. A cross-correlation of the received signals from each spacecraft during periods of intense Auroral Kilometric Radiation (AKR) provides information on the source size of AKR. From the magnitude of the cross-correlation coefficient, measured over baselines from 40 Km to 8000 Km, an upper limit to the statistical source size can be determined. Thirty-five AKR events at 125 kHz are being analyzed. Preliminary results indicate that the apparent source sizes are smaller than .1 Re
Direction finding measurements of auroral kilometric radiation
Direction finding measurements with plasma wave experiments onboard the Hawkeye-1 and IMP-8 satellites were used to locate the source region of auroral kilometric radiation. The radiation exhibits peak intensities between about 100 kHz and 300 kHz, and emits intense sporadic bursts lasting for between one half hour to several hours. The total power emitted in this frequency range exceeds 10 to the 9th power watts at peak intensity. The occurrence of the radiation is known to be closely associated with bright auroral arcs which occur in the local evening auroral regions
Direction-finding measurements of type 3 radio bursts out of the ecliptic plane
Direction-finding measurements with the plasma wave experiments on the HAWKEYE 1 and IMP 8 satellites are used to find the source locations of type 3 solar radio bursts in heliocentric latitude and longitude in a frequency range from 31.1 kHz to 500 kHz. Using an empirical model for the emission frequency as a function of radial distance from the sun the three-dimensional trajectory of the type 3 radio source can be determined from direction-finding measurements at different frequencies. Since the electrons which produce these radio emissions follow the magnetic field lines from the sun these measurements provide information on the three-dimensional structure of the magnetic field in the solar wind. The source locations projected into the ecliptic plane follow an Archimedian spiral. Perpendicular to the ecliptic plane the source locations usually follow a constant heliocentric latitude. With direction-finding measurements of this type it is also possible to determine the source size from the modulation factor of the received signals
Dependence of CMI Growth Rates on Electron Velocity Distributions and Perturbation by Solitary Waves
We calculate growth rates and corresponding gains for RX and LO mode
radiation associated with the cyclotron maser instability for parameterized
horseshoe electron velocity distributions. The velocity distribution function
was modeled to closely fit the electron distribution functions observed in the
auroral cavity. We systematically varied the model parameters as well as the
propagation direction to study the dependence of growth rates on model
parameters. The growth rate depends strongly on loss cone opening angle, which
must be less than for significant CMI growth. The growth rate is
sharply peaked for perpendicular radiation (), with a
full-width at half-maximum , in good agreement with observed k-vector
orientations and numerical simulations. The fractional bandwidth varied between
10 and 10, depending most strongly on propagation direction. This
range encompasses nearly all observed fractional AKR burst bandwidths. We find
excellent agreement between the computed RX mode emergent intensities and
observed AKR intensities assuming convective growth length 20-40 km
and group speed 0.15. The only computed LO mode growth rates compatible
observed LO mode radiation levels occurred for number densities more than 100
times the average energetic electron densities measured in auroral cavities.
This implies that LO mode radiation is not produced directly by the CMI
mechanism but more likely results from mode conversion of RX mode radiation. We
find that perturbation of the model velocity distribution by large ion solitary
waves (ion holes) can enhance the growth rate by a factor of 2-4. This will
result in a gain enhancement more than 40 dB depending on the convective growth
length within the structure. Similar enhancements may be caused by EMIC waves.Comment: 21 pages, 11 figures. J. Geophys. Res. 2007 (accepted
Striated AKR Emission: A Remote Tracer of Ion Solitary Structures
We describe the statistical properties of narrowband drifting auroral
kilometric radiation ('striated' AKR) based on observations from the Cluster
wideband receiver during 2002-2005. We show that the observed characteristics,
including frequency drift rate and direction, narrow bandwidth, observed
intensity, and beaming angular sizes are all consistent with triggering by
upward traveling ion solitary structures (`ion holes'). We calculate the
expected perturbation of a horseshoe electron distribution function by an ion
hole by integrating the resonance condition for a cyclotron maser instability
(CMI) using the perturbed velocity distribution. We find that the CMI growth
rate can be strongly enhanced as the horseshoe velocity distribution contracts
inside the passing ion hole, resulting in a power gain increase greater than
100 dB. The gain curve is sharply peaked just above the R-mode cut-off
frequency, with an effective bandwidth ~50 Hz, consistent with the observed
bandwidth of striated AKR emission. Ion holes are observed in situ in the
acceleration region moving upward with spatial scales and speeds consistent
with the observed bandwidth and slopes of SAKR bursts. Hence, we suggest that
SAKR bursts are a remote sensor of ion holes and can be used to determine the
frequency of occurrence, locations in the acceleration region, and lifetimes of
these structures.Comment: 10 pages, 10 figures. J .Geophys. Res. (in press
Stakeholder Salience for Small Businesses : A Social Proximity Perspective
This paper advances stakeholder salience theory from the viewpoint of small businesses. It is argued that the stakeholder salience process for small businesses is influenced by their local embeddedness, captured by the idea of social proximity, and characterised by multiple relationships that the owner-manager and stakeholders share beyond the business context. It is further stated that the ethics of care is a valuable ethical lens through which to understand social proximity in small businesses. The contribution of the study conceptualises how the perceived social proximity between local stakeholders and small business owner-managers influences managerial considerations of the legitimacy, power and urgency of stakeholders and their claims. Specifically, the paradoxical nature of close relationships in the salience process is acknowledged and discussed.Peer reviewe
Elemental Analysis of Copper-Based Metal Standards by ICP-MS, ICP-OES and pXRF for Roman Metallurgy
Copper-based metals were used extensively in Roman times for tools, weapons, and jewelry but not many studies have been done on the use of particular alloys. This study uses inductively-coupled plasma mass spectroscopy (ICP-MS), inductively-coupled plasma optical emission spectroscopy (ICP-OES), and portable X-Ray Fluorescence (pXRF) analysis to create a database of major and minor elements for the analysis of Roman period copper-based metal artifacts. Over 60 standard-reference materials were selected for analysis to establish calibration curves for elements that will allow quantitative comparison of archaeological samples. These copper alloys could contain tin, lead, zinc, antimony, and iron. The data gathered may be used to determine patterns of elemental composition based on the type of object and its use. It can also be used to view the evolution of Roman technology in regards to elemental variations in the alloys. ICP-MS and ICP-OES were used for their high level of accuracy. The pXRF was utilized although it is less precise because it can analyze artifacts nondestructively. The ICP-OES measured most of the major elements, including copper, zinc and tin. The ICP-MS analyzed trace elements like iron, silver, gold and antimony. All of these elements but antimony were measured by the pXRF