Explosive spread F caused by lightning-induced electromagnetic effects C. P. LIAo, J. P. FREIDBERG and M. C. LEE Plasma Fusion Center and Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. Absrract-The lightning-produced electromagnetic effects may produce significant modifications in the ionospheric plasmas. An outstanding phenomenon investigated in this paper is the so-called “explosive spread F”, whose close link with lightning has been identified (W~~MA~ R. F. and zyxwvutsrqponmlkjihgfed KUDEKI E., 1984, G’eop/~y~. Res. htt. 11, 1165). The parametric instability excited by the lightning-induced whistler waves is proposed as a potent~l source mechanism causing the explosive spread F. Some observed striking features of this phenomenon can be reasonably explained by the proposed mechanism. 1. INTRODUCTION It has been noted that ionospheric plasmas can be modified by lightning, especially, at the equator. Well documented observations show that gravity waves with scale lengths of several hundred kilometers pro- duced by lightning can induce large-scale ionospheric plasma perturbations. During the late evening, these plasma ~rturbations seed the Rayle~gh-Taylor insta- bility at the bottomside of the equatorial ionospheric F peak. The instability, subsequently, creates iono- spheric plasma bubbles full of intense density tluc- tuations (irregularities). These ionospheric irregu- larities give rise to plumes of radar echoes and spread echoes in the ionograms known as the radio charac- teristics of the “equatorial spread F” phenomenon (see, e.g., RCITTGER. 1982, and refs therein). Another ionospheric plasma phenomenon, termed “explosive spread F”, was recently identified to be also caused by lightning. It was discovered in the radar experiments carried out in Peru by W~DMAN and LA Hoz (1976) to investigate the equatorial spread F with the Jicamarca radar at the frequency of 50 MHz. Enhanced radar echoes from the ionospheric F region were recorded during lightning events (WOODMAN and KUDEKI, 1984). A close correlation has been unam- biguously established between the occurrence of light- ning and the appearance of enhanced radar echoes. The radar echoes have a large aspect sensitivity, namely, strong echoes are obtained only when the radar beams orthogonally toward the Earth’s mag- netic field in the ionospheric F region. In other words, what scatters back the 50MHz radar signal are the lightning-indu~d field-Alfred plasma density striations having a scale length of 3 m, which is half of the radar wavelength. “Explosive spread F”, there- fore, refers to the ionospheric plasma process which can generate short-scale, e.g., meter-scale density striations. The phenomenon of “explosive spread F” is not related to the equatorial spread F, because they can occur “independently” with characteristically differ- ent time scales. While the explosive spread F has been observed in a few seconds after the occurrence of lightning, it takes tens of minutes for the lightning- produced gravity waves to trigger the equatorial spread F. This is because the gravity waves have to propagate upward to reach the ionospheric F region and seed the Rayleigh-Taylor instability, which requires 10 min or longer to be excited. The short time-scale of the “explosive spread F” naturally leads to the speculation that this phenomenon may result from the lighting-induced electromagnetic effects. WO~DMAN and KUDEKI (1984) and KELLEY et al. (1984) suggested that the lj~tning-produced tran- sient E field can penetrate into the ionosphere and become the potential source driving the “explosive spread F”. KELLEY et ul. (1984) proposed that the transient E field may generate a Hall current in a time-scale less than the ion gyro-period, and that this “transient Hall current” excites the modified two- stream instability analyzed by MCBRIDE et al. (1972). This Hall current results from the E x B drift of elec- trons, where E represents the lightning-induced tran- sient electric field and B the static earth’s magnetic field. It is a transient current since it lasts for a time period shorter than the ion gyro-period ( -0.04 s). Beyond this period, the E x B drift of ions together with that of electrons will not yield a cross-field cur- rent (i.e., the Hall current).