Radiation Belt Precipitation due

to Manmade VLF

Transmissions: Satellite

ObservationsRory J Gamble1, Craig J Rodger1, Mark A Clilverd2,

Neil R Thomson1, Simon L Stewart1, Robert J McCormick1,

Michel Parrot3, Jean-André Sauvaud4, Jean-Jacques Berthelier5.

1 - Department of Physics, University of Otago, Dunedin, New Zealand

2 - Physical Sciences Division, British Antarctic Survey (NERC), Cambridge, United Kingdom

3 - Laboratoire de Physique et Chimie de l'Environnement, Orleans, France

4 - Centre d'Étude Spatiale des Rayonnements, Toulouse, France

5 - Centre d'Études des Environnements Terrestre et Planétaires, Saint Maur des Fosses, France

Introduction/Motivation- We know that inner belt losses are dominated by waves, but don't know relative importance of different types.

- Manmade VLF transmissions may dominate losses in the inner radiation belts [Abel and Thorne, 1998]

- Particle enhancements not well-tied to VLF wave observations.

- Occurrence frequency of drift-loss cone enhancements above transmitters is unknown.

- DEMETER satellite can be used to study trapped electron population characteristics.

- Ground-based Radiation Belt Remediation.

History

Imhof et al. (1973):- VLF transmitter interactions first observed as narrow peaks in satellite electron spectrometer data.- Energy of peak flux followed first-order resonance relationship for a single VLF transmitter.

Imhof, W. L. et al. (1973), Dynamic Variations in Intensity and Energy Spectra of Electrons in the Inner Radiation Belt. J. Geophys Res. 78 (22).

History

Datlowe and Imhof (1990):- Determined that resonances observed by the S81-1 satellite are probably due to NWC and NAA.- Quantitatively showed that interaction is by first order cyclotron resonance.

History

Figure 1. Datlowe, D. (2006), Differences between transmitter precipitation peaks and storm injection peaks in low altitude energetic electron spectra, J. Geophys Res., 111, A12202, doi:10.1029/2006JA011957

DEMETER SatelliteDetection of Electro-Magnetic Emissions Transmitted from

Earthquake Regions

- Developed by CNES (France)- Instruments including:

ICE (Electric field detector)IDP (Energetic particle detector)

- Data for invariant latitudes below ~65°, L~1-7- Low Earth orbit: 710km altitude- Sun-synchronous polar orbit (see next slide)

http://smsc.cnes.fr/DEMETER/index.htm

DEMETER Orbit Configuration

DEMETER - ICE-Records one component of E field vector-We use VLF power spectrum data (type '1132')

- 1024 channel FFT- 19.5Hz– 20kHz (19.5Hz /channel)- Averaged spectrum given every 4s

DEMETER – Nighttime ICE

World map ice, showing nwc, at night time.

DEMETER – Daytime ICE

Received power ~1200 times lower over transmitter during local daytime.

DEMETER - IDP- Records energetic electron fluxes spectra- Very good quality:

- 17.8 keV channel resolution- 73keV – 2.35MeV (128 channels)

- Complete spectrum every 4s- Looks perpendicularly to the orbital plane of satellite (never illuminated by the sun)- Measures particle fluxes inside (or just outside) the drift loss cone.

DEMETER – Daytime IDP

DEMETER – Nighttime IDP

NWC VLF Transmitter

- US Naval VLF transmitter (Northwest Cape, Australia)- Well located to influence inner belt energetic particles:

- higher latitude transmitters interact with energies lower than IDP instrument can detect.

- immediately east of the SAMA- L=1.45, inner radiation belt

- One of the world’s most powerful: 1000kW radiated power- Frequency: 19.8kHz, within DEMETER’s 20kHz resolution- Signal strength is recorded by OmniPAL reciever in Dunedin (AARDDVARK network).

NWC

Dunedin

NWC VLF Transmitter

NWC VLF Transmitter

Single Half-Orbit: West of NWC

Single Half-Orbit: West of NWC

Pcolors of ICE and IDP as a function of time

Single Half-Orbit: East of NWC

Single Half-Orbit: East of NWC

Pcolors of ICE and IDP as a function of time, including wisp

Wisp Characteristics

Wisp Characteristics

Wisp Characteristics

Wisp Occurence

- Data from 12 August – 26 September 2005 period- Half orbits which pass within +/- 25º longitude of the Tx are selected for closer inspection.- These half orbits are visually inspected for the presence of wisps (blind inspection)- Half orbits are sorted into day/night and east/west orbits

Day Night

East 46 (0) 43 (40)

West 46 (0) 45 (3)

Over 95% of night, eastern orbits showed wisps!

NWC Dependence

- Apart from short periods of weekly daytime routine maintenance, NWC is rarely down- Continuous recordings of NWC signal strength show that NWC was offline for 15 days (13 – 28 June 2005)…

NWC Dependence

- Apart from short periods of weekly routine maintenance, NWC is rarely down- Continuous recordings of NWC signal strength show that NWC was offline for 15 days (13 – 28 June 2005).- All suitable orbital passes from this period were examined.

Result: No wisps seen while NWC was offline.

Wisp Characteristics

Lower L

Middle L

Upper L

Lower E (keV)

Middle E (keV)

Upper E (keV)

Peak Flux Enhancement

Mean 1.62 1.71 1.84 105 181 280 410 x bg

Median 1.61 1.71 1.85 94 169 281 111 x bg

The L range, energy range and maximum flux enhancement (w.r.t. a reference background spectrum) was recorded for each wisp.

Lower energy bound limited by instrument

Upper energy can be as large as 453 keV

Peak flux can be as large as 3600 times background levels

Variation with L of the first-order equatorial cyclotron resonant energy with a 19.8 kHz wave (black), and the plasmaspheric electron number density used in this calculation (dashed gray).

Compiled by C. J. Rodger using Chang and Inan [1983], Carpenter and Anderson [1992], Mahaian and Brace [1969]

NPM Hawaii

NPM Hawaii

- L=1.17, inner radiation belt- Also potentially well positioned for study:

- more equatorial than NWC- further east of NWC

- Less powerful: 500kW radiated power- Expect higher interaction energy

No evidence of wisps was found due to NPM

Conclusions

- DEMETER is used to study enhancements in DLC electron population due to NWC - NWC operation directly associated with DLC enhancements:

- Interaction relies on night time ionosphere- Very common: ~95% of suitable orbits

showed clear interaction.- Also using DEMETER to characterise stormtime processes

January 2005 Storm Period

January 2005 Storm Period

Storm Period Movie

Acknowledgements

Craig RodgerNeil ThomsonMark ClilverdSimon Stewart & Robert McCormick

Polar Environments Research ThemeCNES/DEMETER