Registration of X‐rays at 2500 m altitude in association with lightning flashes and thunderstorms

Abstract

Electric fields and high-energy radiation of natural lightning measured at close range from a mountaintop tower are discussed. In none of the 12 negative cloud-to-ground upward flashes were X-rays observed. Also no energetic radiation was found in one negative upward leader at close range (20 m). In the first of two consecutive negative cloud-to-ground flashes, X-rays were detected during the last 1.75 ms of the leader. During the time of energetic radiation in the flash an intense burst of intracloud VHF sources was located by the interferometers. The X-ray production is attributed to the high electric field runaway electron mechanism during leader stepping. Even though the second flash struck closer than the previous one, no X-rays were detected. The absence of energetic radiation is attributed to being outside of the beam of X-ray photons from the leader tip or to the stepping process not allowing sufficiently intense electric fields ahead of the leader tip. High-speed video of downward negative leaders at the time when X-rays are commonly detected on the ground revealed the increase of speed and luminosity of the leader. Both phenomena allow higher electric fields at the leader front favoring energetic radiation. Background radiation was also measured during thunderstorms. The count rate of a particular day is presented and discussed. The increases in the radiation count rate are more coincident with radar reflectivity levels above~30 dBZ than with the total lightning activity close to the site. The increases of dose are attributed to radon daughter-ion precipitation.

Abrupt Elongation (Stepping) of Negative and Positive Leaders Culminating in an Intense Corona Streamer Burst: Observations in Long Sparks and Implications for Lightning

“…It is important to note, however, that the positive polarity results are likely to be biased due to the breakdown voltage for positive polarity being lower than for negative polarity. In natural lightning, Yoshida et al (2008) presented evidence from their observations in winter storms in Japan that X-rays can be produced by both negative and positive upward leaders, while Montanyà et al (2014) reported nondetection of X-rays from upward leaders of either polarity initiated from a mountain-top tower in Spain. Mallick et al (2012) noted that for a given leader, not every step is accompanied by a detectable X-ray burst.…”

Global distribution of winter lightning: a threat to wind turbines and aircraft

“…In order to produce a stable leader, the field needs to increase at the tip of turbine (Bazelyan and Raizer, 1998). This increase of the electric field can be produced thanks to the fast neutralization of charge produced in a CG (e.g., Warner et al, 2012 andMontanyà et al, 2014b) or an IC flash. Because of the slow ion mobility of the space charge at the tip of the turbine, the electric field is not screened and it is increased.…”

Characterizing Upward Lightning With and Without a Terrestrial Gamma Ray Flash

“…Multiple brief (∼1 μs) bursts of lower‐energy X‐rays (up to one or at most a few MeV) are often seen associated with descending negative stepped or dart leaders (e.g., Dwyer et al, ; Montanyà et al, ; Moore et al, ; Saleh et al, ; Schaal et al, ) in both natural and triggered lightning. These are interpreted as due to cold runaway , in which a very high field exists in a small region at the leader tip during stepping, accelerating every available free electron to semirelativistic energies.…”