synchrotron radiation at 50 mhz: a tool for monitoring ... · synchrotron radiation at 50 mhz: a...
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Synchrotron Radiation at 50 MHz: A Tool for Monitoring Radiation Belts?
Jim LaBelle
Department of Physics and Astronomy Dartmouth College, Hanover, NH, USA
Acknowledgements: Ron Woodman, Koki Chau, Don Farley
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http://space-env.esa.int/index.php/ ESA-ESTEC-Space-Environment-TEC-EES.html
http://seesproxy.tksc.jaxa.jp/fw /dfw/SEES/Japanese/Reports/ reports_mds-1_j.shtml
Apr 1999 Apr 2000 Apr 2001 Apr 2002
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Inner Belt Variability (Baker et al., Geophys Res. Lett., 2007)
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“inner” belt
“outer” belt
Radiation Belt fluxes for various energy thresholds (Starfish corrected) (Singley and Vette, 1972)
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Summary---Inner Belt Electrons largely steady-state compared to outer belt (slow time constant associated with radial diffusion) Occasional injection events affect L<2, especially --trapping of solar energetic electrons --magnetospheric compression-induced radial transport solar cycle variation Need for baseline (e.g., in case of artificial injection)
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Angle gets smaller with increasing energy:
Relativistic synchrotron radiation:
Smaller angle shorter pulses at observer broader spectrum
-- beamed perpendicular to magnetic field -- approximately linearly polarized EW at equator -- gamma>4 (>2 MeV) radiation up to >50 MHz
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Schwinger (1949) formula:
nβ sinα sinψ 1-βcosα cosψ zn =
where: Pn = power per electron n = harmonic number β = v/c ωce = electron gyrofrequency α = pitch angle ψ = observation angle
e2 n2ωce2 β sin2α βcosα - cosψ
8π2ε0c (1-βcosα cosψ)4 β sinα sinψ [Jn(zn)]2 + Jn2(zn) Pn =
Dyce and Nakada (1959), Peterson and Hower (1963), Peng et al. (1974) Matthews, LaBelle
Pn ~ 10-30 W Hz-1ster-1
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Power = Pn h Δh A λ flux h2 L c
2
Pn ~ 10-30 W Hz-1 ster-1 (power per electron) A = L2 = (300 m)2 (Jicamarca) λ = 6 m = 600 cm (50 MHz) Δh ~ 2000x105 cm (.3 RE) flux ~ 105 electrons/cm2s (AE-8 model, Starfish corrected) c = 3x1010 cm/s
Power per Hz ~ 2x10-22 W/Hz ~ 10oK
Observable? (Detailed modelling by Dyce & Nakada, others, get ~13oK)
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Galactic background ~ 4000oK
--- high sample rate/long integration ~106 spectra --- Dicke radiometer scheme (temperature controlled noise source) --- Polarization discrimination: synchrotron radiation linearly polarized (EW plane) galactic background unpolarized use Jicamarca pencil beam---look 3o off mag zenith desired signal rotates 180o in ~ 20 minutes across dawn undesired signal does not rotate.
Both experience Faraday rotation through ionosphere
Angle for which daytime Faraday rotation differs from nightime by 180o
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Peak about 20 min after atmospheric nuclear test at Johnston Atoll Brightness Temp ~ 4500oK Exponential decay
Drift Echo
(from Ochs et al., 1963)
Jicamarca observation of synchrotron radiation from Starfish nuclear test (1963)
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--- Random noise ~ 5o K --- No effect seen on three nights of observation --- some kind of systematic error not understood Po
wer
in T
empe
ratu
re U
nits
Time interval of shift in Faraday rotation angle
Real part of cross spectrum, bins 480-580, July 25, 2006
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Summary---Detection Scheme: -- Separate EW from NS antennas -- Detect with Dicke Radiometer -- Sample for approximately 1-2 hrs across dawn -- Discriminate component of signal which Faraday rotates (requires extremely low cross-talk between EW and NS signals)