Plasma wave signatures detected by Venera and by PVO have
been linked to lightning [see reviews of Russell, 1991;
Strangeway, 1994]. At Earth it is known that lightning in the
atmosphere generates electromagnetic plasma waves that propagate
upwards into the magnetosphere. The group velocity of these waves
is frequency-dependent and they are referred to as whistlers
because when they are impulsively generated, they arrive at a
distance with a frequency-dispersion like that of a whistle.
However, the signals detected by Venera could have been those of
electrostatic discharges through the spacecraft and the PVO signals
could have been locally generated plasma waves instead of whistler
waves propagating upward from an atmospheric source region. The
search for lightning is of interest because it is not clear that
convectively driven storms will generate lightning in the
atmosphere of the planet. The clouds at Venus consist of sulfuric
acid droplets. Electrostatic charges could build up on the
droplets, but droplet size is also relevant and it is uncertain
whether the droplets are large enough to support the charge
separation required for generation of lightning by the mechanisms
that apply to discharges at earth.
Gurnett et al. [1991] seized the flyby opportunity to look
for signals associated with lightning in the atmosphere of Venus.
The Galileo plasma wave detector [ Gurnett et al., 1992]
measures electric fields up to 5.6 Mhz (=10
Hz), thus being
sensitive to signals well above the frequency range of locally
generated plasma waves in a band that can propagate upward from the
atmosphere through the ionosphere. Nine impulsive events were
detected. The plasma wave team ruled out such a high level of
false signals and concluded that there was no reasonable hypothesis
other than lightning-generated whistler noise that could explain
the observations. This important support for the existence of
lightning on Venus underscores the importance of further work on
the mechanisms by which it is generated.