Direct observations of X-rays produced by upward positive lightning

Area of Science:

• Atmospheric Physics
• Electromagnetism
• High-Energy Physics

Background:

• X-ray emissions have been documented in downward cloud-to-ground and rocket-triggered lightning, primarily during leader phases.
• Previous observations linked X-rays to downward negative and upward negative flashes.

Purpose of the Study:

• To investigate X-ray emissions during upward positive lightning flashes.
• To correlate X-ray detection with leader propagation characteristics in upward positive lightning.

Main Methods:

• Simultaneous recording of electric current, electric field strength, and X-ray flux from upward positive lightning flashes.
• Utilizing a high-speed camera to capture leader development.
• Analysis of time-synchronized data.

Main Results:

• X-ray emissions were detected during the initial phase of upward negative leader propagation in upward positive lightning.
• X-ray pulse frequency and energy decreased over time, disappearing within the first millisecond.
• X-ray emission correlated with current derivative and electric field changes, supporting cold electron runaway theory.

Conclusions:

• This study provides the first evidence of X-ray detection during upward positive lightning.
• The findings confirm the association of X-ray emissions with the initial steps of upward negative leaders.
• Understanding these phenomena is crucial for mitigating damage to structures and aircraft from upward lightning.