Abstract. We present a model addressing the possible electrification of Martian dust storms based on the effective electrical charging of an individual dust grain. An upper charge bound on a grain can be determined based on the grain capacitance in the lowpressure Martian atmosphere. It is assumed that treiboelectric and inductive processes, like that presumed operating in terrestrial dust storms, can electrify the grain to significant levels. A collection of such grains charged in a dust cloud of many tens of kilometers in size can yield a substantial electric field moment. Given various grain charge and dust storm sizes, the electric moment will be determined along with estimates of electrical discharge and emitted radio power based upon known models. We also suggest the possibility that remote detection of discharge-related VLF emission propagating in the surface/ionosphere waveguide can be used to determine subsurface conductivity. However, to date, there has been no report of orbiter or lander optical images of lightning-like discharges. Further, there is no report of lightning-induced interference on radio 3795

Terrestrial dust devils are known to be triboelectric generators: electric charge is generated and exchanged via the mixing and colliding of moving grains. Static electric fields from the structures have been previously reported, this in excess of 1/2 kV/m at a couple hundred meters from the devil. In this work, we report on AC magnetic measurements made during the passage of an intense ∼10‐m wide, couple hundred‐meter tall dust devil in the Nevada desert. The devil‐related magnetic activity appears in two forms: Impulsive extremely‐low‐frequency (ELF: 30–300 Hz) static discharges to the instrument as the sensor became immersed directly into the devil's electrified dust and ultra‐low‐frequency (ULF: 3–30 Hz), continuous emissions that were sensed remotely as the storm approached and receded from the mobile station. The latter is a new finding not previously reported. Such measurements not only prove that individual dust grains within the devil are charged, but that the grains are transported in bulk to form an extended, coherent radiation source.

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The Altus Cumulus Electrification Study (ACES) is an unmanned aerial vehicle (UAV)based project that investigated thunderstorms in the vicinity of the Florida Everglades in August 2002. ACES was conducted to investigate storm electrical activity and its relationship to storm morphology, and to validate satellite-based lightning measurements. In addition, as part of the NASA sponsored UAV-based science demonstration program, this project provided a scientifically useful demonstration of the utility and promise of UAV platforms for Earth science and applications observations. ACES employed the Altus II aircraft, built by General Atomics-Aeronautical Systems, Inc. Key science objectives simultaneously addressed by ACES are to: (1) investigate lightning-storm relationships, (2) study storm electrical budgets, and (3) provide Lightning Imaging Sensor validation. The ACES payload included electrical, magnetic, and optical sensors to remotely characterize the lightning activity and the electrical environment within and around thunderstorms. ACES contributed important electrical and optical measurements not available from other sources. Also, the high altitude vantage point of the UAV observing platform (up to 55,000 feet) provided "cloud-top" perspective. By taking advantage of its slow flight speed (70 to 100 knots), long endurance, and high altitude flight, the Altus was flown near, and when possible, over (but never into) thunderstorms for long periods of time that allowed investigations to be conducted over entire storm life cycles. An innovative real time weather system was used to identify and vector the aircraft to selected thunderstorms and safely fly around these storms, while, at the same time monitor the weather near our base of operations. In addition, concurrent ground-based observations that included radar (Miami and Key West WSR88D, NASA NPOL), satellite imagery, and lightning (NALDN and Los Alamos EDOT) enable the UAV measurements to be more completely interpreted and evaluated in the context of the thunderstorm structure, evolution, and environment.