Electron Density Fluctuations In A Disturbed Ionospheric Environment

ABSTRACT Electron density fluctuations in the earth's ionosphere can adversely affect SDIO systems which involve electromagnetic wave propagation, e.g., laser beams. This is particularly true for severely disturbed ionospheric conditions produced by high altitude nuclear explosions (HANEs). In this paper we briefly describe, in general terms, the HANE environment, and the various plasma instabilities which could generate small-scale (£ 10's m) electron density irregularities. As an example, we present an analysis of a single instability, the lower-hybrid-drift instability, which is likely to be excited in the ionosphere following a high altitude burst. Both the linear and nonlinear behavior of this instability are discussed. We describe how these results can be applied to potential SDIO laser systems in a HANE environment. 1. INTRODUCTION It is known from the FISHBOWL test series conducted by the United States in the early 1960's that high altitude nuclear explosions (HANEs) produce an extremely disturbed ionospheric environment. SDI systems must be designed to operate in such an environment; therefore, it is crucial to determine both the physical characteristics of the HANE environment, and the A environ­ mental impact' on SDI systems. One potential SDI system under investigation involves the propagation of intense laser beams over distances of 1000's km. For the purposes of discussion we will considerwavelengths on the order of I/j (X 10-6 m)and beam diameters £ 10 m. Some of the questions that need to be answered are (1) can the beam be accurately targeted over such long distances, and (2) can the beam maintain a sufficiently high energy density to be useful. In this paper we will be concerned with the former issue.The primary problem regarding targeting of the laser beam is the possibility of beam diffraction and/or refraction as it propa­ gates through regions of different electron density. The HANE plasma environment is characterized by regions of very high electron density (ne £ 1010 cm~^), and intense electron density fluctuations over an enormous range of scale sizes (10's cm - 100's km). The type of interaction between the laser beam and background plasma fluctuations depends upon the ratiowhere r^ is the beam radius and X is the scale size (or wavelength) associated with the electron density irregularities. Beam refraction can be important when r^/X « 1. This is likely to occur because HANEs generate intense plasma striations with scale sizes - 1000's km along the geomagne­ tic field, and £ few km transverse to it. Although the index of refraction (n = 1 - topi/to2 where Wpe is the electron plasma frequency *\nd w is the laser frequency) remains near unity because oopS/w^ « 1, the cumulative effect of many small deflections (~ 100's) over great distances (~ 1000's km) may be sufficient to cause a ^target' to be missed. In the opposite limit, when r^/X £ 1, the electron density fluctuations can cause filamentation of the beam and/or phase changes across the beam front.To date, the bulk of HANE research on electron density irregularities has focussed on understanding and modeling the evolution of large scale plasma striations (X ~ km's) because they can severely degrade military communication systems. In contrast to this, the purpose of the present paper is to investigate small scale irregularities in the HANE environment, i.e., X £ 10r s m. The ultimate goal of this study is to determine the power spectra of electron density fluctuations for these scale sizes. This information can then be used in laser propagation studies to assess the impact of small scale irregularities on beam propaga­ tion. The overall plan to achieve this goal is the following: (1) define an appropriate HANE environment at several characteristic times [e.g., early time (t ~ 1 min), late time (t - 30 min)]; (2) identify plasma instabilities which can generate density fluctuations in the range £ 10's m; (3) determine the nonlinear saturation mechanisms of these instabilities; and (4) estimate the power spectrum of the density fluctuations, i.e., |&n/nj2 vs. k. We will present a general discussion of these issues, and describe in some detail a specific instability, the lower-hybrid-drift instability, as a case study.