Measuring low-current discharges from grounded rods under high background electric fields

This work presents the development of an inductive current sensor for measuring pulsating corona discharges observed under high background electric field. Laboratory and field experiments in different setups are used to corroborate the discharge features. The first location is on the roof of a regular building, and the second is on a flat area. Due to the enhancement of the E-field caused by lightning strikes in the vicinity of a grounded rod with a sharp tip, positive and negative pulses before or after the strikes are observed. When lightning activity is far from the structure, but the electric field remains high, pulses are also observed. Finally, one prototype of this sensor was modified to work as a pulse detector with a higher current threshold, allowing detections of leader currents above 0.5 A. The sensor performance is validated in the laboratory and investigated in the field, in association with lightning events.Peer ReviewedPostprint (published version

Meteorological factors in the production of gigantic jets by tropical thunderstorms in Colombia

Gigantic jets are electric discharges that on rare occasions can be seen at night shooting out of the top of tropical thunderclouds, reaching the ionosphere (90 km). Using sensitive camera systems and detection software, we recorded 70 events over northern Colombia and adjacent seas, most of them captured between 2016 and 2022. This is the first study to compare the meteorological background conditions for thunderstorms that produced gigantic jets in 48 nights against 83 reference cases with monitored thunderstorms that did not produce jets, using vertical profiles from ERA5 reanalysis near the event location. From the vertical profiles, various meteorological parameters are calculated, grouped by metrics of the low level convective parcel, instability, humidity, warm cloud and mixed phase parameters, and vertical wind shear, not limited to default levels. We report statistically significant differences and effect sizes (Cohen's d) for gigantic jet producing environments compared to null environments. Gigantic jets are produced in conditions with reduced low level temperatures in combination with warmer mid levels. This causes a lower cloud base and higher -10 °C isotherm altitude, thus a greater warm cloud depth, as well as reduced updraft and downdraft buoyancy. Over northern Colombia the non-GJ producing storms tend to grow in an environment that supports more vigorous, multicellular convection by enhanced low-level storm-relative winds and stronger downdrafts. Over western Colombia, the non-GJ cases tend to have a lower equilibrium level while having favorable warm cloud parameters. No evidence is found for hypotheses that upper level vertical wind shear enables gigantic jet production, nor are overshooting tops larger. The findings can be used for forecasting gigantic jets and their climatologically optimal regions on Earth. We speculate that the environmental conditions shift the droplet size distribution towards larger drops at the cost of cloud droplets, with enhanced droplet shattering ice multiplication processes as they freeze. Depending on convective evolution, low rime accretion rates could briefly expand the negative charge region downward by inverse polarity charging while the upper positive charge concentration may weaken at the same time, which could lead to a temporary negatively imbalanced electric potential distribution in the cloud needed for gigantic jet emission.Peer ReviewedPostprint (published version

Experiments with a tethered drone to investigate induced charges on a vertically arranged conductor during fair weather

To better understand the phenomena associated with electrostatic charges and potential distribution over vertical conductors, this work describes preliminary results obtained with a tethered drone that is used to lay vertically a 100-m long-stranded copper wire. Several flights were conducted in fair weather conditions. In the first configuration, a conductive spherical shell with a spool for the wire is placed in a Teflon holder and used for ensuring that the charges inducted on the conductive body do not produce any corona from the bottom of the system. A field mill is placed below the sphere at a fixed distance, and its measurement can be used as a reference to the charge distribution over the sphere during the flights. The second configuration consists of grounding the bottom sphere, taking the null potential up to the tip of the wire that is deployed. In this setup, several approaches for measuring currents were explored.Peer ReviewedPostprint (published version

Corona discharges from grounded rods under high ambient electric field and lightning activity

We assess results of corona pulses detected under high background electric field in the laboratory and in the field in two different experimental sites. The first location is on the roof of a regular building, and the second is on a flat area. Due to the enhancement of the E-field caused by lightning strikes in the vicinity of a grounded rod with a sharp tip, we observed positive and negative pulses before or after the strikes. When lightning activity is far from the structure, but the electric field remains high, pulses are still observed.Postprint (published version

Tropical TGF paradox: a perspective from TRMM precipitation radar

The Terrestrial Gamma-ray Flash (TGF) to lightning ratio, computed over the three tropical chimneys, presents a paradox: African thunderstorms produce the most lightning but yield the lowest fraction of TGFs when compared to American and Southeast Asian thunderstorms. To understand the physical insights into this asymmetry, Tropical Rainfall Measuring Mission Precipitation Radar measurements are used to depict the vertical precipitation structure and infer the vertical electrical charge fraction distribution of the observed thunderstorms in the three regions and the thunderstorms during TGF occurrences detected by the AGILE, Fermi-GBM, and RHESSI sensors. Regional differences show that African thunderstorms are taller, smaller, and have a higher concentration of denser ice particles (graupel and or hail) above the freezing level in addition to having more lightning flashes per thunderstorm. The overall TGF-related thunderstorms are taller, more intense (0.5–1.5 dBZ) and present similar radar reflectivity decay with height independent of the region. The two dimensional precipitation vertical distribution diagrams indicate that TGF thunderstorms develop to a mature stage. Independent of the region, thunderstorms show a midlevel negative charge center varying from 4.6 to 8.1 km in height and an upper level positive charge center ranging from 7.4 to 14.6 km. TGF thunderstorms have thicker positive inferred charge layer and present larger vertical distances between the opposite charging layers in comparison with the overall thunderstorm population, here defined as Climatology. African TGF thunderstorms have higher fraction of positive charges near the negative layer, helping to produce more and shorter lightning discharges.Peer ReviewedPostprint (author's final draft

Upward propagation of gigantic jets revealed by 3D radio and optical mapping

Occasionally, lightning will exit the top of a thunderstorm and connect to the lower edge of space, forming a gigantic jet. Here, we report on observations of a negative gigantic jet that transferred an extraordinary amount of charge between the troposphere and ionosphere (~300 C). It occurred in unusual circumstances, emerging from an area of weak convection. As the discharge ascended from the cloud top, tens of very high frequency (VHF) radio sources were detected from 22 to 45 km altitude, while simultaneous optical emissions (777.4 nm OI emitted from lightning leaders) remained near cloud top (15 to 20 km altitude). This implies that the high-altitude VHF sources were produced by streamers and the streamer discharge activity can extend all the way from near cloud top to the ionosphere. The simultaneous three-dimensional radio and optical data indicate that VHF lightning networks detect emissions from streamer corona rather than the leader channel, which has broad implications to lightning physics beyond that of gigantic jets.Peer ReviewedPostprint (published version