High‐energetic charged particles, such as solar protons, and phenomena such as Forbush decreases are eligible candidates to affect the global electric circuit. These effects have been studied by analyzing disturbances of the potential gradient in ground‐based measurements in fair weather regions. In this paper, we investigate deviations in the potential gradient diurnal curve, during solar proton events, and Forbush decreases, from the mean values obtained in fair weather conditions. In each situation, we select only events which are not accompanied or followed by the other. Using the superposed epoch analysis, in order to enhance the visualization of small effects, we study the potential gradient data recorded between January 2010 and December 2019 at two stations located at low and middle‐latitudes, and at two different altitudes: the Complejo Astronómico El Leoncito (CASLEO), Argentina: 31.78°S, 2550 m a.s.l., and the Geophysical Observatory in Świder (SWIDER), Poland: 52.12°N, 100 m a.s.l., respectively. For intense solar proton events (>100 MeV) we found a statistically significant increase of the potential gradient after solar proton events recorded at CASLEO and no such deviation in the potential gradient recorded at SWIDER. For Forbush decrease events (greater than 4%), no significant deviation of the potential gradient after the start of the event was found in both stations, however for very intense Forbush decreases (>7%) we found an increase of the potential gradient recorded at CASLEO.
This work presents the first simultaneous X-ray measurement and high-speed video observation of the propagation of a lightning leader producing X-rays. As a result, the three-dimensional leader distance from the X-ray measurement and, for the first time, the conditions of the preexisting channel during the leader propagation were observed. Although four leaders in this seven-stroke flash followed the same path to ground, X-rays were only observed during the leader before the return stroke with the highest peak current. The fact that the other three leaders following the same path to ground did not produce detectable X-rays confirms the hypothesis that leader line charge density is an important factor that determines X-ray production. The fact that X-rays was recorded only when the leader tip was at a certain portion of the lightning channel confirms that the orientation of the leader plays an important role in the detection of X-rays. Plain Language SummaryIt was known that lightning can produce X-rays. However, in this study, thanks to the use of a high-speed video camera it was possible to determine when lightning produces X-rays, how far it was, how it was oriented when the detection of X-rays, and what the conditions of the preexisting channel were during the leader propagation. The observations of the present work allow for new insights, confirmation of some hypotheses, and comparison with past studies. The results presented help to understand why X-rays are sometimes detected and sometimes not. It is shown that the amount of charge transferred by the discharge plays a crucial role. This study also confirms that the orientation of the descending leader plays an important role in the detection of X-rays.
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