The mapping of thundercloud electric fields at middle and subauroral latitudes is investigated analytically as a three‐dimensional boundary value problem. The electrical conductivity is represented by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70‐km altitude. The geomagnetic field lines are assumed to be straight and vertical below 150‐km altitude. Electric field strength at great heights depends sensitively on conductivity and thundercloud models used in the calculations. Sample calculations using representative nighttime profiles show that ‘giant’ thunderclouds can produce transverse electric fields of tens of microvolts per meter in the equatorial plane of the midlatitude magnetosphere. In the daytime, corresponding electric fields are about an order of magnitude less. These results suggest that giant thunderclouds may be an important source of localized electric fields that can form field‐aligned electron density irregularities in the ionosphere and the magnetosphere.

In a recent paper we discussed the penetration of dc thundercloud electric fields into the ionosphere and magnetosphere (Park and Dejnakarintra, 1973). In the present paper we extend the analysis to the time‐varying case and consider how electric field variations due to lightning are transmitted to ionospheric heights. We represent the thundercloud by a vertical dipole whose moment recovers exponentially with time following periodic lightning discharges. Electrical conductivity profiles are approximated by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70‐km altitude. The source wave form is Fourier‐analyzed, and electric fields at each harmonic frequency up to 10 Hz are mapped upward separately. The total electric field at any point in space is then obtained by adding all frequency components together. It is found that electric fields are transmitted upward more efficiently as the frequency increases. This compensates for the decrease with frequency in the amplitudes of the source spectrum, so that the electric field spectrum becomes fairly flat at large heights. In the time domain the recovery time following a lightning discharge decreases rapidly with altitude. At the 100‐km level the electric field wave form appears as a sharp spike of ∼100‐ms duration. The peak amplitude of the spike may be several hundred microvolts per meter across the geomagnetic field at night. In the daytime the corresponding amplitude is about an order of magnitude smaller because of larger conductivities. These fields should be investigated as a possible excitation source of geomagnetic pulsations in the magnetosphere.

No abstract

No abstract

The transient electric fields that would be generated by sudden changes in the atmospheric conductivity profile are estimated using a theoretical model. The model assumes the existence of a horizontal electric field with a spatial extent of ≥20‐km at 150‐km altitude and involves the solution of the boundary value problem in two dimensions. The geomagnetic field lines below 150‐km altitude are assumed to be vertical, but the anisotropy of the medium above ∼70‐km altitude is fully accounted for. The computed electric field strength depends sensitively on the size of the source field as well as the conductivitity profile of the atmosphere. In the steady state, a small‐scale (∼20‐km) source field produces relatively weak dc electric fields in the troposphere, whereas a large scale (∼100‐km) source produces stronger observed fields. On the other hand, during a transient the peak electric fields produced in the case of a small‐scale source field can be considerably larger than those due to large‐scale source distributions. The more rapid the increase in the slope of the conductivity profile with altitude, the more efficient should be the downward mapping of both transient and steady state fields. Since the transient electric fields are oscillatory and die out at a rate depending on the local conductivity, the ac electric fields at the ground level are found to last longer than those at higher altitudes.