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

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

Inner Belt Variability (Baker et al., Geophys Res. Lett., 2007)

“inner” belt

“outer” belt

Radiation Belt fluxes for various energy thresholds (Starfish corrected) (Singley and Vette, 1972)

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)

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

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

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)

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

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)

--- Random noise ~ 5o K --- No effect seen on three nights of observation --- some kind of systematic error not understood Po

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Time interval of shift in Faraday rotation angle

Real part of cross spectrum, bins 480-580, July 25, 2006

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)