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| Titel |
Scaled experimental investigation of the moderation of auroral cyclotron emissions by background plasma |
| VerfasserIn |
S. L. McConville, D. C. Speirs, K. M. Gillespie, A. D. R. Phelps, A. W. Cross, M. E. Koepke, C. G. Whyte, K. Matheson, C. W. Robertson, R. A. Cairns, I. Vorgul, R. Bingham, B. J. Kellett, K. Ronald |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067566
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| Zusammenfassung |
| Scaled laboratory experiments have been conducted at Strathclyde University [1,2] to further
the understanding of the naturally occurring generation of Auroral Kilometric Radiation
(AKR) in the Earth’s polar magnetosphere. At an altitude of around 3200km there exists a
region of partial plasma depletion (the auroral density cavity), through which electrons
descend towards the Earth’s atmosphere and are subject to magnetic compression. Due to
conservation of the magnetic moment these electrons sacrifice parallel velocity
for perpendicular velocity resulting in a horseshoe shaped distribution in velocity
space which is unstable to the cyclotron maser instability [3,4]. The radiation is
emitted at frequencies extending down to the local electron cyclotron frequency
with a peak in emission at ~300kHz. The wave propagation is in the X-mode with
powers ~109W corresponding to radiation efficiencies of 1% of the precipitated
electron kinetic energy [5]. The background plasma frequency within the auroral
density cavity is approximately 9kHz corresponding to an electron plasma density
~106m-3.
Previous laboratory experiments at Strathclyde have studied cyclotron radiation emission
from electron beams which have horseshoe shaped velocity distributions. Radiation
measurements showed emissions in X-like modes with powers ~20kW and efficiencies
~1-2%, coinciding with both theoretical and numerical predictions [6-9] and magnetospheric
studies.
To enhance the experimental reproduction of the magnetospheric environment a Penning
trap was designed and incorporated into the existing apparatus [10]. The trap was placed in
the wave generation region where the magnetic field would be maintained at ~0.21T. The
trap allowed a background plasma to be generated and its characteristics were studied using
a plasma probe. The plasma had a significant impact on the radiation generated,
introducing increasingly sporadic behaviour with increasing density. The power and
efficiency of the radiation generated was lower than with no plasma present. Plasma
diagnostics established the plasma frequency on the order of 150-300MHz and
electron density ranging from ~1014-1015m-3, whilst the cyclotron frequency of the
electrons within the Penning trap was 5.87GHz giving fce/fpe ~19-40, comparable to
the auroral density cavity. Numerical simulations coinciding with this part of the
experimental research program are currently being carried out using the VORPAL code.
Details of these simulations will be presented in a separate paper [Speirs et al] at this
meeting.
McConville SL et al 2008, Plasma Phys. Control. Fusion, 50, 074010
Ronald et al 2011, Plasma Phys. Control. Fusion, 53, 074015
Bingham R and Cairns RA, 2002, Phys. Scr., T98, 160-162
Ergun RE et al, 1998, Geophys. Res. Lett., 25, 2061
Gurnett DA et al, 1974, J. Geophys. Res., 79, 4227-4238
Cairns RA et al, 2011, Phys. Plasmas, 18, 022902
Gillespie KM et al, 2008, Plasma Phys. Control. Fusion, 50, 124038
Speirs et al 2010, Phys. Plasmas, 17, 056501
Vorgul et al 2011, Phys. Plasmas, 18, 056501
McConville SL et al 2011, Plasma Phys. Control. Fusion, 53, 124020 |
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