A central question in the domain of atmospheric electricity has been,"What processes maintain the persistent global vertical electric field?" This electric field, a natural force that acts on electrically charged objects, was first described in the mid-18th century. It pushes ions and charged aerosols in the atmosphere, driving a downward electric current, the fair-weather current. Near thunderstorms, a larger current flows upwards. People have estimated the averaged total thunderstorm and fair-weather currents and found them to be comparable, perhaps with fair-weather current somewhat larger. Nobody has succeeded in making accurate estimates of both currents on short time scales. For instance, when there is a burst of global thunderstorm activity, therefore thunderstorm current, how does the fair-weather current respond? A quantitative answer to this question would provide essential information for improving understanding of how this system works.

The primary scientific objective of this project is to investigate the relation between global thunderstorm-driven currents and the fair-weather current on time scales as fast as 1000 seconds. The past difficulty to doing this was in measuring the varying global thunderstorm current. Recent global lightning detection networks have made it possible to locate and size thunderstorms, and so thunderstorm current, on a time-scale comparable to the lifetime of a thunderstorm cell, tens of minutes.

Near Earth's surface, air currents, pollutants, precipitation, cloud cover, dust, aerosols, and orography can perturb the fair-weather current density. However, throughout the middle and upper atmosphere and away from these factors, earlier observations have suggested that the unperturbed fair-weather current density (fair-weather current per unit area) is approximately the same regardless of geographic location or altitude. Thus, an important measurement objective of our project was to collect fair-weather current density measurements from at least two widely separated locations at the same time, high in the stratosphere. We succeeded in acquiring middle atmosphere measurements of fair-weather current density in June 2021 over an interval of several days.

Making measurements from a stratospheric balloon over several days requires instruments, a power system, a control computer, a communications system, timing and position information, a payload structure, terminate and ballast systems, and launch equipment (Fig 1). After developing several payloads, two test flights were undertaken in July and September 2019. These provided launch practice and a study of the returned data identified problems requiring hardware and software changes. From the test flights, it became clear that a better campaign time was near the summer solstice, but after upper level winds had shifted to easterly. The pandemic prevented field activity in 2020, so our efforts turned to using lightning location and precipitation data to estimate thunderstorm area, hence thunderstorm current, and comparing these estimates with ground-based electric field measurements, a proxy for fair-weather current (Fig 2). Storm area is estimated by summing active grid region areas, where an active region is defined as having hourly precipitation or lightning rates above selected thresholds. Hourly lists of electric field data and storm area enable the study of trends in return current or sources on this time scale.

Once we were permitted to travel and work in close contact again, a successful campaign was begun in June 2021, during which two instrumented balloons were launched a day apart, collecting electric field and conductivity data continuously for several days (Fig 3). Because we planned to obtain measurements above the northeastern Pacific ocean, thunderstorms were not expected near the balloon trajectories. The collected data was sufficient to calculate fair-weather vertical current density (Fig 4).

Findings from this project include (1) measurement of vertical electric field and conductivities were comparable with what others have found, validating our measurements; (2) under-appreciated factors associated with the probe-payload geometry and operation limit the measurement of atmospheric conductivity; (3) current density measurements from different middle atmosphere locations did not correlate as well as expected (Fig 4); (4) ground-based vertical electric field measurements, a proxy for fair-weather vertical current, correlates well with thunderstorm area estimates (Fig 2). Progress on evaluating the relation between fair-weather and thunderstorm currents was limited by the unexpected finding that fair-weather current density seemed to vary with position, and thunderstorm area estimates depend on grid choice. An appropriate grid is yet to be identified.

At UW, one undergraduate and three graduate students were trained in some or all aspects of research activities through this project. At NWRA/DigiPen, eight undergraduate and two graduate students were similarly trained.

Last Modified: 03/10/2023
Modified by: Jeremy N Thomas