During the 2022 New Mexico monsoon season, we deployed two X-ray scintillation detectors, coupled with a 180 MHz data acquisition system to detect X-rays from natural lightning at the Langmuir Lab mountain-top facility, located at 3.3 km above mean sea level. Data acquisition was triggered by an electric field antenna calibrated to pick up lightning within a few km of the X-ray detectors. We report the energies of over 240 individual photons, ranging between 13 keV and 3.8 MeV, as registered by the LaBr3(Ce) scintillation detector. These detections were associated with four lightning flashes. Particularly, four stepped leaders and seven dart leaders produced energetic radiation. The reported photon energies allowed us to confirm that the X-ray energy distribution of natural stepped and dart leaders follows a power-law distribution with exponent ranging between 1.09 and 1.96, with stepped leaders having a harder spectrum. Characterization of the associated leaders and return strokes was done with four different electric field sensing antennas, which can measure a wide-range of time scales, from the static storm field to the fast change associated with dart leaders.

The dissonant development of positive and negative lightning leaders is a central question in atmospheric electricity. It is also the likely root cause of other reported asymmetries between positive and negative lightning flashes, including the ones regarding: stroke multiplicity, recoil activity, leader velocities, and emission of energetic radiation. In an effort to contrast lightning leaders of different polarities, we highlight the staggering differences between two rocket‐triggered lightning flashes. The flash beginning with upward positive leaders exhibits an initial continuous current stage followed by multiple sequences of dart leaders and return strokes. On the other, in its opposite‐polarity counterpart, the upward development of negative leaders is by itself the entire flash. As a result, the flash with negative leaders is faster, briefer, transfers less charge to the ground, has lower currents, and smaller spatial extent. We conclude by presenting a discussion on the three fundamental leader propagation modes.

When the electric field below a thunderstorm or other electrified cloud is around 10 kV/m, it is sometimes possible to initiate (“trigger”) an upward‐propagating lightning‐leader by launching a rocket that uncoils a wire from the ground. The triggered leader propagates upward from the tip of the wire lifted by the rocket. When the channel is hot enough, a flash is visible. Triggering is common when the leader carries positive charge, but not when it carries negative charge. This article is about four flashes consisting of triggered negative leaders that branched into low‐altitude regions of positive cloud charge over Langmuir Laboratory in central New Mexico. Measurements of current and the locations of leader channels are available for three of the four flashes. Some current pulses at the ground for Flash 2 originated at negative leader steps more than 3 km away, which is a greater distance than has been reported from video measurements. Flashes 3 and 4 propagated only into thunderstorm lower positive charge, and the average lightning‐charge densities inside the volumes occupied by these two flashes are remarkably close. Our best estimate of density for Flashes 3 and 4 lies between −4.2 and −1.8 C/km 3 , which is compatible with the large spread in cloud‐charge densities derived from instruments carried on airplanes or balloons into low positive regions in thunderstorms.

Data and instructions on how to produce the figures in an article titled "Thunderstorm lower positive charge and triggered negative lightning-leaders" to be submitted to the Journal of Geophysical Research, Atmospheres. Triggered negative leaders propagated into thunderstorm lower positive charge. Current measured at the ground where the triggering wire was attached and an estimate of the volume occupied by the branched leaders give an average charge density in the lower positive region.

In 1995, a series of four balloon flights with an X‐ray spectrometer and an electric field meter were conducted to examine if strong electric fields could accelerate, and perhaps multiply, cosmic ray secondary electrons and produce bremsstrahlung X‐rays. X‐ray intensities between 10 and 1000 times that of normal background were observed in conjunction with strong electric fields. Both negative and positive polarity electric fields (as referenced to the vertical field) produced X‐rays, which lasted for time scales on the order of tens of seconds. It was also observed that the increased X‐ray intensity would return to near background levels after lightning reduced the local electric field. The observations indicate that X‐rays observed above background are most likely produced by a runaway electron process occurring in the strong static electric field present in thunderstorms. The production of runaway electrons can occur over long periods of time without causing an electrical breakdown. This may provide a leakage current that limits the large scale electric field to values near the runaway threshold, especially in regions where the thunderstorm charging rate is low.

A color photograph has been obtained of a negative lightning leader in clear air at 10.3 km altitude. The individual leader steps are resolved as relatively straight segments of at least ~200 m in length, between sharp kinks (nodes) in the channel. Each node is accompanied by a group of streamers of ~100 m in length. One node has an unconnected secondary leader with streamers at both ends. Lightning Mapping Array observations show that the leader was part of an intracloud (IC) flash. The observation shows that steps of negative leaders near 10 km altitude are an order of magnitude longer than values reported in the literature for negative leaders near sea level. Since negative leaders propagate at comparable velocities at low and high altitudes, stepping occurs at a lower rate in IC flashes, which can explain why RF emissions from IC flashes are more intermittent than those from cloud‐to‐ground flashes.

A triggered lightning flash that transferred negative charge to ground in central New Mexico produced more than three levels of branching above the main channel to ground in a 1 km vertical field of view. A high‐speed video recording shows that the main channel had about 50 brief luminous pulses, many of which were superimposed on a slowly changing persistent luminosity. In contrast, superposition was rare in the uppermost visible branches because luminous pulses first appeared on preexisting dark channels before merging into a luminous channel. This observation suggests that luminous pulses in triggered and natural lightning originate only on dark branches and that the complexity of the main channel to ground is the result of multiple mergers of dark branches with pulses into luminous branches without pulses. This suggestion is contrary to an earlier conclusion that there are two kinds of luminous pulses. We also observe behavior characteristic of electromagnetic waves on transmission lines: when a downward propagating luminous pulse reaches a junction with another initially dark branch, it travels both upward and downward along that branch. Upon reaching the ground the downward propagating wave produces a bright reflection which also splits at the junctions, producing luminosity for a short distance upward in one direction while propagating much farther upward along the path charged by the downward propagating wave. However, when a downward moving luminous pulse reaches a junction with an initially luminous branch, splitting is not evident, probably due to the greater conductivity of the luminous channel.

On 3 August 2010 an extensive lightning flash was triggered over Langmuir Laboratory in New Mexico. The upward positive leader propagated into the storm's midlevel negative charge region, extending over a horizontal area of 13 × 13 km and 7.5 km altitude. The storm had a normal‐polarity tripolar charge structure with upper positive charge over midlevel negative charge. Lightning Mapping Array (LMA) observations were used to estimate positive leader velocities along various branches, which were in the range of 1–3 × 104 m s−1, slower than in other studies. The upward positive leader initiated at 3.4 km altitude, but was mapped only above 4.0 km altitude after the onset of retrograde negative breakdown, indicating a change in leader propagation and VHF emissions. The observations suggest that both positive and negative breakdown produce VHF emissions that can be located by time‐of‐arrival systems, and that not all VHF emissions occurring along positive leader channels are associated with retrograde negative breakdown.

An intracloud lightning flash in central New Mexico began with the initiation of a negative stepped leader at an altitude of 8.2 km above sea level. As this leader propagated eastward and upward, at 9.1 km above sea level it passed about 200 m to the north of a balloon-borne, electric field-change instrument (Esonde). After the first leader stopped, a second negative stepped leader began near the point of origin of the first leader, but it propagated away from the Esonde. From the changes in the electric vectors and the locations of impulsive radio frequency sources detected by a lightning-mapping array (LMA), we conclude the following: (1) The first negative stepped leader was not preceded by any significant charge rearrangement due to positive leaders. (2) Each step of the first negative leader had both a forward-going wave and a step recoil wave that propagated simultaneously backwards away from the leader tip along the existing channel. The presence of a step recoil wave during each step leads to an explanation for the existence of stepping. (3) After the first (nearby) leader stopped, step recoil waves from the second (distant) leader may have found their way onto the channel formed by the first leader. (4) After the second leader stopped, waves carrying negative charge propagated along the channel of the first leader, producing strong K changes in the electric field at the Esonde and providing a good record of the wavefront shapes.

We present analysis of thunderstorm data collected with a liquid nitrogen-cooled germanium spectrometer with energies between 13 keV-2.6 MeV that was deployed at Langmuir Lab on South Baldy Peak in New Mexico for June through August 2005. The motivation was to search for gamma ray emissions from radioactive chlorine-39 and chlorine-38, as suggested by Greenfield et al. (2003). Based on the observations, we place an upper limit on the rate of chlorine production through such a process (6.8 x 10(-17) chlorine atoms per argon atom). This rate is sufficiently low to suggest that the anomalous gamma ray count increases observed by Greenfield et al. (2003) were not caused by radioactive chlorine.