VLF scattering from red sprites: Vertical columns of ionization in the Earth-ionosphere waveguide


  • Craig J. Rodger,

  • Neil R. Thomson,

  • James R. Wait


Red sprites were discovered by chance in 1989 when a low-light TV system was pointed above an active thunderstorm. Optically, they are observed as clusters of short-lived (∼50 ms) pinkish-red luminous events, sometimes stretching from ∼40 to ∼85 km altitude. Red sprites are associated with ionization in the Earth-ionosphere waveguide, which can persist for up to ∼100 s at higher altitudes. The structured ionization associated with red sprites has been observed through “VLF sprites,” perturbations of the phase and/or amplitude of subionospheric VLF transmissions, which can be used to study the electrical properties of the red sprite plasma. Previous theoretical studies have simplified the problem by assuming the red sprite associated ionizations were columns of infinite length, or inside a “flat-Earth” waveguide. In this paper we present an approach for examining the scattering of VLF transmissions by a finite-length column of ionization inside the Earth-ionosphere waveguide, including the effect of realistic curvature of the waveguide. These expressions are applied to the detection and study of red sprites by assuming that the associated ionization can be treated as a (cluster of) truncated columns in the Earthionosphere waveguide. An example calculation is presented, examining the farthest distance from the red sprite from which it might be detected as a VLF sprite.