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We provide extensive evidence that runaway electron acceleration and subsequent bremsstrahlung X-ray emission are a common feature in negative electrical discharges with voltages as low as 100 kV, indicating that all negative lightning could potentially produce runaway electrons. Centimeter long streamer corona discharges produce bursts of X-ray radiation, emitted by a source highly compact in space and time, leading to photon pileup. Median photon burst energies vary between 33 and 96 keV in 100 kV discharges. Statistical analysis of 5,000+ discharges shows that X-rays are observed in as many as 60% of the triggers, depending on the configuration. X-ray detection is more frequent when streamers are not followed by a spark, the detector is oriented perpendicular to the gap, and a thicker anode is used. In an 8-cm-long gap, X-rays are produced when runaway electrons hit the anode, and the electron acceleration is not necessarily correlated with streamer collisions. Plain Language Summary Small electrical sparks prolifically emit X-rays. These discharges are produced in the lab by electrical energy sources 100 to 1,000 times weaker than what is available in thunderstorms and lightning. Therefore, this is strong evidence that all lightning should produce X-rays and energetic electrons.

The programmable combined receiver/digital signal processor platform presented in this article is designed for digital downsampling and processing of general waveform inputs with a 66 MHz initial sampling rate and multi‐input synchronized sampling. Systems based on this platform are capable of fully autonomous low‐power operation, can be programmed to preprocess and filter the data for preselection and reduction, and may output to a diverse array of transmission or telemetry media. We describe three versions of this system, one for deployment on sounding rockets and two for ground‐based applications. The rocket system was flown on the Correlation of High‐Frequency and Auroral Roar Measurements (CHARM)‐II mission launched from Poker Flat Research Range, Alaska, in 2010. It measured auroral “roar” signals at 2.60 MHz. The ground‐based systems have been deployed at Sondrestrom, Greenland, and South Pole Station, Antarctica. The Greenland system synchronously samples signals from three spaced antennas providing direction finding of 0–5 MHz waves. It has successfully measured auroral signals and man‐made broadcast signals. The South Pole system synchronously samples signals from two crossed antennas, providing polarization information. It has successfully measured the polarization of auroral kilometric radiation‐like signals as well as auroral hiss. Further systems are in development for future rocket missions and for installation in Antarctic Automatic Geophysical Observatories.