Cite abstracts as Author(s) (2007), Title, Eos Trans. AGU,
88(52), Fall Meet. Suppl., Abstract xxxxx-xx

HR: 1340h
AN: AE23A-0907
TI: Daytime Early VLF Perturbations Exhibiting Long Recoveries and Wide-angle
Scattering
AU: * Cotts, B R
EM: bcotts@stanford.edu
AF: Stanford University, 350 Serra Mall, Packard Building, Stanford, CA 94305,
United States
AU: Inan, U S
EM: inan@stanford.edu
AF: Stanford University, 350 Serra Mall, Packard Building, Stanford, CA 94305,
United States
AU: Lehtinen, N G
EM: nleht@stanford.edu
AF: Stanford University, 350 Serra Mall, Packard Building, Stanford, CA 94305,
United States
AB: Recent observations of perturbations on Very Low Frequency (VLF) transmitter
signals at remote sites show evidence of direct coupling of lightning to the
ionosphere in the form of Early VLF events during daylight hours and exhibit
different characteristics than their nighttime counterparts. The daytime events
are coincident with recorded NLDN lightning flashes and separately recorded
radio atmospherics, and are so far observed in two distinct situations. The
first set of observations are perturbations on relatively short paths where the
distance between the transmitter and receiver is ~400 km, with the causative
lightning (and hence the likely ionospheric disturbance) within 20-50 km of the
receiver. The second type of daytime Early VLF events occur on long-distance
paths (>1000 km), with the causative lightning several hundred kilometers
distant from the receiving site. The identified events exhibit highly
directional scattering patterns including wide-angle scattering of up to 180
degrees indicating either a hard or a highly structured/periodic scatterer. In
contrast to nighttime Early VLF events in which VLF backscatter occurs only very
rarely (<5%), three of the six identified daytime disturbances coincident with
NLDN-identified lightning flashes exhibit wide-angle scattering. Most of the
daytime Early VLF events also exhibit unusually long recoveries (>9 minutes).
These newly identified events imply the existence of a new type of scattering
source which is more likely to be observable during daylight hours. It is also
possible that the nature of the underlying ionospheric disturbances that cause
Early VLF events are similar during daytime or nighttime and that the different
scattering properties (i.e., forward scattering at night versus wide angle
during daytime) are due to the different ambient characteristics of the
Earth-ionosphere waveguide (e.g., much lower VLF reflection height) during
daytime.
DE: 2435 Ionospheric disturbances
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3360 Remote sensing
DE: 6934 Ionospheric propagation (0689, 2487, 3285, 4275, 4455)
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting