Passive Remote Sensing of Artificial Relativistic Electron Beams in the Middle Atmosphere

Two methods of passive remote sensing of mildly (E 5 MeV) relativistic electron beams as they propagate through the Earth’s upper and middle atmosphere are presented. Utilization of bremsstrahlung emissions as a diagnostic indicator of beam characteristic energy and particle flux is compared and contrasted with that of the optical emission technique. A new MeV aurora1 electron model has been developed to compute line emission rates of O(lD) -+ O(3P) (X = 630.0-636.4 nm doublet), O(rS) + O(iD) (X = 557.7 nm), Nz(B3C,f) 3 N~(X2C~) (A = 391.4 nm and 427.8 nm from the N.$(lN) band), and Ns(C311,) 4 Nz(B3111,) (X = 337.1 nm from the Nz(2P) band). The 427.8 nm, .391.4 nm, and 337.1 nm lines are strong in intensity,.with production rates several orders of magnitude greater in than those of the 0 lines examined here. It is shown that the production of 337.1 nm is insensitive to compositional change and has a quenching height lower in altitude than the propagation depth of a 5 MeV electron beam, and thus the signature may be suitable as an indicator of electron flux for beams of comparable energy. The ratio of 427.8 nm to 391.4 nm emissions was found to be relatively ‘insensitive to compositional changes, and the ratio varies with altitude at lower altitudes, suggesting that it may suitable for inference of characteristic beam energy for MeV electron beams. Advantages and disadvantages associated with both the bremsstrahlung and the optical techniques are presented. L. Habash Krause is with Boston College, Institute for Scientific Research, and is supported by the-Air Force Research Laboratorv, Battlesnace Environment Division (VSB). 29 Randolph Rd.“,’ Hanscom Air Force Base, MA, Oi731.” Email: [email protected]. T. Neubert is with the Danish Meteorological Institute, Solar-Terrestrial Physics Division, Lyngbyvej 100, 2100 Conenhasen 0, Denmark. Email: [email protected]. B. E. Gilchrist is with the University of Michigan, Space Physics Research Laboratory, 2455 Hayward, Ann Arbor, MI, 48109. E-mail: [email protected]. Copyright (c) 1999 by Boston College Institute for Scientific Research. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Nomenclature Ad = absolute area of planar detector, cm2 E = primary electron energy, eV & = loss rate, ith state, /cm3/s f = density scaling relative to MSIS-E-90, unitless nk = atmospheric neutral number density, Icth species, /cm3 pi = production rate, ith state, /cm3/s p” e = elastic backscatter coefficient, lath species, /s 4 = electron impact ionization rate, /cm3/s/eV q+ = electron cascade /cm3/s/eV xd = detector-source horizontal displacement, km W = secondary electron energy, eV Zb& = altitude corresponding to beam penetration depth, km zinj = altitude, beam injection, km zsat = altitude, LEO diagnostic satellite, km 4 = hemispherical electron flux, /cm2/s/eV X = emission wavelength, nm (p) = electron flux average pitch angle, rad ck a = inelastic absorption cross section, Icth species, cm2 uk e = elastic scattering cross section, kth species, cm2 a = excitation production cross section, cm2 ed = detector view angle from the vertical, rad AR = solid angle subtended by effective detector area, sterad E = cross section empirical parameter, unitless INTRODUCTION AND BACKGROUND With the technology on the horizon to potentially launch a relativistic (E N 5 MeV) electron beam from an orbiting spacecraft or suborbital sounding rocket [l] comes the challenge of determining the appropriate diagnostic method by which investigators may characterize the propagation of such a beam during an active experiment in space. There are several candidate methods under consideration, including in situ beam electron flux measurements, active remote sensing of beam-induced ionization of the atmosphere (e.g., with incoherent scatter radar), and passive remote sensing of radiative emissions associated with