Thunderstorm III 4-Megahertz Burst-Mode Data Acquisition System

Experimental results from many platforms indicate strong coupling between lightning and the ionosphere and show the necessity of high-frequency sampling in future experiments. We describe the lightning-triggered, burst- mode data acquisition algorithm and circuit used in the NASA sounding rocket 36.111, which is scheduled for launch in 1995. Burst-mode acquisition increases the time resolution by almost two orders of magnitude over that of continuous sampling, but requires a smart controller to select important data. The increase in time resolution will yield sounding-rocket electromagnetic-wave data valid up to the plasma frequency for the first time in the lightning environment. The instrument samples the vector electric field and one component of the magnetic field at 4 MHz with 10-bit, resolution, yielding a burst rate of 320 Mbits/s. Burst-mode instruments use ground commands, precursors, or recorded preburst data to ensure the proper data are recorded. N the early years of space research, the electromagnetic waves launched by lightning strokes were found to disperse in time and to create an audio-frequency radio wave, which whistled when con- verted to sound waves. These whistlers bounce between the Earth's hemispheres along magnetic field lines and are an important loss mechanism for the radiation belts. The study of whistlers has re- vealed much about the plasma around the earth, including the dis- tribution of ionization in the ionosphere.1 Research in the last two decades, including a number of curious ground-based, rocket, satel- lite, and Space Shuttle observations during severe storm conditions, showed that lightning may be an important source of ionospheric disturbance as well. At this time, known interactions between light- ning and the ionospheric plasma are virtually unexplained by exist- ing theories. In situ rocket and satellite measurements performed in the past have been hampered by telemetry bandwidth limitations, and hence, crucial information is absent. A summary of these ex- perimental results is given. The phenomenon of explosive spread F (an event in which the radar backscatter signal due to electron density irregularities from approximately 250 km increases to sizeable levels in tens of mil- liseconds and decays in hundreds of milliseconds) was first reported by Woodman and La Hoz. 2 The NASA rocket flight Thunder Hi (33.022) measured large-amplitude transient electric fields owing to lightning in the ionosphere above an active thunderstorm3 with amplitudes sufficient to drive plasma instabilities in the F region of the ionosphere.4 Woodman and Kudeki5 subsequently conducted a radar experiment that showed explosive spread F is highly correlated with lightning activity. A class of early/fast Trimpi events (a type of transient perturbation in the propagation of subionospheric vlf signals) was reported that cannot be explained by the whistler-based theory of resonant pitch- angle scattering and precipitation of radiation-bel t particles. One theory is that plasma in the mesosphere is heated, resulting in a perturbation to the Earth-ionosphere waveguide, causing the Trimpi event.6'7 The Thunderstorm II sounding rocket (27.120) measured large (10-40 mV/m), long-duration (1 ms) electromagnetic (EM) pulses with electric-field components parallel to the Earth's magnetic field