atmosphere and is heated to the background temperature. The vapors then continue to expand diffusively. The diffusive expansion of the exhaust vapors, along with the transport via neutral winds, controls the spreading of the reactive exhaust vapors, and, consequently, controls the size and location of the ionospheric "hole". The ionosphere recovers by photoionization of the neutrals and by plasma transport into the modified region. External electric fields can transport plasma across magnetic field lines. The depleted region can be filled in by plasma flowing along geomagnetic field lines. This is the only recovery mechanism during the night. When the ionosphere is sunlit, the sun's extreme ultraviolet rays directly re-ionize the modified region. Consequently, the ionospheric recovery is slower at night than during the daytime. The reduction in plasma concentration affects the thermal properties of the upper atmosphere. In the modified region, the cooling of electrons onto ions is reduced. This causes an increase (by as much as 2000 K) in the electron temperature. This temperature change influences the plasma transport and the chemical reaction rates. Chemical reactions resulting from an exhaust release produces excited molecules. These molecules can radiate airglow at IR, visible, and UV wavelengths. The intensity of this airglow can be 20 K Rayleighs or greater. The plasma gradients in the ionospheric hole may induce internal electric fields. These fields can cause the region to become unstable, breaking up into irregularities. These irregularities are most likely to form for releases near the earth's magnetic equator. Here, where the geomagnetic field lines are horizontal, the earth's gravity produces naturally occurring plasma irregularities. Depending on the amount and the geometry of an exhaust release, instabilities may be triggered or damped by the vapor injection. Radiowave propagation will be altered in the disturbed ionosphere. Propagation via refraction in the bottomside ionosphere can suffer the effects of focusing, defocusing or multipath fading in the vicinity of an ionospheric hole. Communication between satellite and ground-based locations can be degraded by artificially stimulated plasma irregularities. Theoretical studies are necessary to predict the changes in radiowave propagation in an ionosphere modified by the injection of rocket exhaust.
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