An Artificial Particle Precipitation Technique Using ...

AFRL-VS-HA-TR-2007-1021

An Artificial Particle Precipitation Technique Using HAARP-Generated VLF Waves

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M. J. Kosch T. Pedersen J. Bortnik R. Esposito D. Gallagher R. Marshall M. McCarrick R. Friedel U. Inan

2 November 2006

Approved for Public Release; Distribution Unlimited

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An Artificial Particle Precipitation Technique Using HAARP Generated VLF Waves.

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.AUTHORS

M.J. Kosch* T. Pedersen, J. Bortnik", R. Esposito, D. Gallagher* Marshall"*, M. McCarrick%, R. Friedel%% and U. Inan"*

4287

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Air Force Research Laboratory /VSBXI 29 Randolph Road Hanscom AFB, MA 01731-3010

AFRL-VS-HA-TR-2007-1021

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AFRL/VSBXI 11. SPONSOR/MONITOR'*; RFPHRT

20100322149

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Approved for public release; distribution unlimited.

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'AFRL/VSB, Lancaster Univ., Lancaster, UK., & Univ. of Kwazulu-Natal, Durban, South Africa, UCLA, Los Angeles, CA NASA Marshall Space Flight Center, AL ""STAR Lab., Stanford Univ.,Palo Alto, CA /oBAE Systems, Washington, DC "'"Los Alamos Nat'l Lab, NM 14. ABSTRACT, A new ground-based experimental technique is described, which is designed to optimize artificial precipitation of magnetospheric electrons at high latitudes, due to man-made VLF waves. High-latitude ionospheric modification facilities may be used to modulate the Hall conductance, which modulates the electrojet current, and in turn radiates VLF whistler waves. Assuming the VLF waves are ducted along the magnetic field lines, the frequency of the VLF wave is modulated in time such that Doppler-shifted cyclotron resonance between the wave and gyrating electrons is maintained along a large fraction of the magnetic field line for a selected particle energy. The electron travel time is a function of particle energy, which is selected to be at the loss cone, and magnetic field strength. The VLF wave travel time is a function of frequency, which we calculate, the electron gyro-frequency, which is a function of magnetic field strength, and the plasma frequency, which we model. The frequency-time modulated VLF wave patterns have been successfully implemented at the HAARP ionospheric modification facility in Alaska, USA. Two initial attempts have failed to produce an unambiguous signature of particle precipitation to date.

15. SUBJECT TERMS

Wave-particle interactions Optical emissions

VLF propagation Ionospheric heating

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Particle precipitation

19a. NAME OF RESPONSIBLE PERSON Todrl R. Pedersen 19B. TELEPHONE NUMBER (Include area code)

UNL Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std Z39.18

Acknowledgements The HAARP program is a Department of Defense project managed jointly by the U.S. Air Force and U.S. Navy. This work was supported by AFOSR task 231 IAS. M.J.K acknowledges support by the National Research Council Research Associateship program.

in

IV

1.

Introduction

A large number of spacecraft now orbiting the planet provide services such as communications, navigation, weather monitoring and images, all now considered essential for modern living. The operational lifetime of spacecraft is partly determined by the radiation dose it receives in space, which affects the lifetime and performance of the electronics. On occasion, relativistic "killer" electrons, whether geomagnetic or manmade in origin, may cause premature spacecraft failure [e.g., Baker et al., 1998]. Given the enormous and everincreasing investment in satellite assets, it is reasonable to investigate methods of reducing the radiation associated with degradation and failure of spacecraft. The problem is particularly acute in the belts of trapped radiation where spacecraft can spend much of their time. This radiation consists of energetic protons and electrons. The proton radiation belt contains significant fluxes in excess of 10 MeV [Walt, 1994, p.77], and occupies the region between -1.5 and 7

RE.

The trapped, energetic electrons (>1 MeV) exhibit a two-zone

structure [Walt, 1994, p.82]: an inner radiation belt (3

RE)

RE)

which is relatively stable, and an

which can vary by a factor of > 1000 on timescales ranging from minutes to

hours [Blake et al., 1992; Li et al., 1993; Baker et al., 1994; Selesnick and Blake, 1997; Onsager et al., 2002], and respond in an apparently unpredictable way to geomagnetic activity [Reeves et al., 2003].. Work done by Abel and Thome [1998] and Inan et al. [2003] show that ground-based Very Low Frequency (VLF) transmissions significantly shorten the lifetime of energetic particles, especially in the inner radiation belt. It is well known that lightning strikes radiate VLF electromagnetic waves in the kilo-Hertz range, which propagate in the Earth-ionosphere waveguide, and partially leak through the ionosphere. This wave energy may become ducted in enhanced electron density structures and propagate along magnetic field lines through the magnetosphere to the opposite hemisphere. Artificially generated VLF waves may do the same [Carlson et al., 1985; Inan et al., 2004]. The frequency dependent propagation of so-called whistlers has been extensively used as a natural ground-based diagnostic of magnetospheric plasma [e.g. Carpenter, 1963; Sazhin et al., 1992]. Artificially injected VLF waves have also been used to study waveparticle interactions in the near-Earth magnetosphere [Carlson et al., 1985; Helliwell, 1988].

Naturally occurring whistler-mode waves, e.g. hiss or chorus, which originate in the plasma itself, have been associated with the optical aurora, i.e. particle precipitation, at L = 4 from ground-based observations [Hansen and Scourfield, 1990; Hansen et al., 1988, 1986; Scourfield et al., 1984; Helliwell et al., 1980]. Artificially injected VLF waves from the ground have been observed to amplify or trigger stronger secondary VLF emissions, which were correlated to fluxes of energetic particles (1° for single encounters with the VLF wave. If this process brings the electrons' pitch angles into the bounce loss cone, they will precipitate in the hemisphere where the VLF wave originated. Imhof et al. [1983, 1985] estimated the VLF-induced precipitating energy flux to be -10^ erg.cm"3.s"'. Assuming 100% conversion efficiency, this translates to a photon flux of 2.2 Rayleigh (R) at 557.7 nm, which is within the detectable range of modern optical imagers (threshold -0.5 R). If we assume a more realistic 4% conversion efficiency [Rees et al., 1963] then the photon flux becomes 0.09 R at 557.7 nm, which is still detectable by suitable background subtraction and/or integration. The primary reason that past ground-based VLF transmission experiments have not resulted in detectable optical emissions is that the imaging technology was not sensitive enough, having a typical threshold of 100-1000 R at the time of the Siple experiments. Sensitive photometers have existed for a long time, but the location of the VLF ducts is not known in advance, which severely limits the usefulness narrow field-ofview optics, and using wide field-of-view optics admits a large background signal which swamps the desired signal. The typical diameter of VLF ducts is -30 km [Helliwell et al., 1980], which is a small area compared to the -500 km visible radius. Given the optical technological advancements, the possibility now presents itself to repeat the early groundbased VLF experiments at Siple and elsewhere using all-sky imagers and attempt to detect the precipitation optically near the VLF source, something that has not been reported before.

It is well known that high power radio waves transmitted from the ground cause large electron temperature enhancements (-2500 K) in the D-layer [Kero et al., 2000], thereby modulating the Hall conductance. If the ionospheric polar electrojet flows through this heated region, as is the case for high latitude ionospheric modification facilities (e.g. EISCAT, L = 6.2, and HAARP, L= 4.8), it can be similarly modulated to radiate observable coherent whistler-mode VLF waves of up to 1-3 pT in strength and up to ~7 kHz [Stubbe et al., 1982]. The VLF signal strength falls off rapidly for 6 kHz [Rietveld et al., 1989] and is a function of the electron temperature and density time constants to reach a new equilibrium as well as reflected pump-wave self-interference. Pumping with X-mode polarisation produces a much stronger signal strength than does O-mode [Stubbe et. al., 1982]. VLF signals produced by electrojet modulation at HAARP have been detected by satellite [Platino et al., 2004] and are known to propagate, on occasion, to the opposite hemisphere and back, presumably by ducting [Inan et al., 2004]. Although whistler waves will propagate in the magnetosphere without ducting [Carpenter and Sulic, 1988], the experiment described below relies on VLF ducting to reach the equatorial plane. Only here is cyclotron resonance possible because of the relatively low maximum VLF frequency which can be achieved by modulating the electrojet (