|dc.description.abstract||Thunderstorms and their lightning discharges are of great interest to many areas
of geophysics and atmospheric electricity. A thunderstorm is an electric generator;
it can produce both electrostatic and quasi-electrostatic fields in the overhead
atmospheric D region. The D region is the lower part of the ionosphere that extends
from about 40-90 km altitude where the electrons and ions are sufficient
enough to affect the propagation of radio waves. In contrast to the electrostatic
field, the quasi-electrostatic fields can be much stronger in magnitude, but shorter
in duration, and can trigger halos. A halo is one type of the transient luminous
events (TLEs) and typically appears within 1-2 ms after an intense cloud to ground
lightning discharge. It looks like a relatively homogeneous glow in the shape of a
pancake that is centered around 75-80 km altitude with a horizontal extent of tens
of kilometers and vertical thickness of several kilometers.
The goals of this dissertation research are to investigate the electrical effects of
thunderstorm electrostatic and quasi-electrostatic fields on the nighttime lower
ionosphere, and their covert relation to the formation of atmospheric halos. This
work entails numerical and theoretical modeling analyses, and comparison of current
theory and simulation results with the actual observations. For the first part of this study we have demonstrated that, under steady state conditions,
electrostatic fields of <0.4Eҝ values (not strong enough to produce TLEs)
can be established in the lower ionosphere due to underlying thunderstorms. We
utilized the simplified nighttime ion chemistry model described in the work of Liu
 to investigate how these fields affect the lower ionosphere ion density profile.
The three-body electron attachment, through which electrons can be converted to
negative ions, is the only process whose rate constant depends on the field values
within the above-mentioned limit. As a result of the variation of the rate constant
with the electric field, the nighttime steady state electron density profile can be
reduced by ∼40% or enhanced by a factor of ∼6.
We have improved our model in order to self-consistently calculate the steady state
conductivity of the lower ionosphere above a thunderstorm. The new model takes
into account the heating effects of thunderstorm electrostatic fields on the free electrons.
The modeling results indicate that under steady state condition, although
the electron density is generally increased, the nighttime lower ionospheric conductivity
can be reduced by up to 1-2 orders of magnitude because electron mobility is
significantly reduced due to the electron heating effect. Because of this reduction,
it is found that for a typical ionospheric density profile, the resulting changes in
the reflection heights of ELF and VLF waves are 5 and 2 km, respectively.
In the second part of this dissertation, a one-dimensional plasma discharge fluid
model is developed to study the response of the nighttime lower ionosphere to
the quasi-electrostatic field produced by cloud-to-ground lightning flashes. When
the quasi-electrostatic field reaches and exceeds about Eҝ, a halo can be triggered
in the lower ionosphere. The modeling results indicate that the ionospheric
perturbation is determined by the ambient ionospheric density profile, the charge moment change, and charge transfer time. Tenuous ambient profiles result in larger
changes in the ionospheric electron density. Cloud-to-ground lightning discharges,
with larger charge moment changes and shorter charge transfer times, result in a
larger change in the ionospheric electron density.
In particular, the enhancement in the lower ionospheric electron density due to
impulsive negative cloud-to-ground lightning flashes has been investigated. It is
found that the enhancement can reach up to about 3 orders of magnitude above
∼70 km altitude in a few seconds. Below ∼75 km altitude, this enhancement recovers
in a few seconds due to the fast electron attachment process. The recovery
time of the electron enhancement above ∼75 km altitude is controlled by a slower
recombination process; it depends on the ambient density profile and can last for
tens of minutes to hours.
Finally, the modeling results of the lower ionosphere recovery time are analyzed
to investigate the role of halos in producing early VLF events with long recovery
time. It is found that these events can be explained when sufficient ionization is
produced around ∼80 km altitude. Such ionization can be produced by the impact
of impulsive negative cloud-to-ground lightning flashes with a relatively large
charge moment change on a tenuous ionospheric density profile.||en_US