RECOVERY SIGNATURES OF LIGHTNING-ASSOCIATED VLF PERTURBATIONS AS A MEASURE OF THE LOWER IONOSPHERE

A new model of the physical processes associated with subionospheric V LF signal perturbations caused by lightning-induced electron precipita tion (LEP) bursts is developed to diagnose the state of the lower iono sphere (e.g., electron number density and rate coefficients for variou s chemical reactions) on the basis of measurements of VLF recovery sig natures. The model accounts for the energy spectrum of the electron bu rsts precipitated by lightning-generated whistlers, the chemical relax ation of enhanced secondary ionization in the nighttime D region due t o LEP bursts, and quantitatively treats the resultant effects on propa gation of the VLF signal in the Earth-ionosphere waveguide. Applicatio n of the model to experimental data obtained for the VLF propagation p ath from NPM station (Hawaii) to Palmer station (Antarctica) indicates that effective electron detachment rate gamma, enhanced secondary ion ization profile (e.g., energy content of LEP bursts), as well as the a mbient electron density distribution, may be estimated using observed subionospheric VLF recovery signatures. The effective detachment rate was identified as similar to 10(-18)N S-1, where N is total number den sity of neutrals. Model indicates in particular that the attachment-de tachment processes play the dominant role in recovery of subionospheri c VLF signal perturbations on timescales similar to 100 s, and that th e observed perturbations of the NPM-Palmer signal correspond to the LE P bursts consisting of relatively soft (< 250 keV) electrons.