Large ball lightning might explain some UFO reports. Other types of UFO might exist of course, even though many reports are undoubtedly hoaxes or misperceptions. It is well-known that large electric currents exist in the ionosphere. Suppose that a large specimen of ball lightning 10m in radius was created by such a current. It could weigh several tons, fall to the earth and burn a spot on the ground. It might live for several hours, but eventually its density would decrease and it would rise or hover. Jets of escaping plasma could disturb the earth beneath a ball, propel it to high velocity or impart rotation, which could result in an erratic or zigzag path.
The polar regions of a large ball would have less current density and hence might be lost, leaving a disk or lens-shaped object. A large ball, like lightning, would have a bright silvery-metallic appearance by day, and shine brightly by night. Irregularities in the field could produce light or dark spots on a ball; the color could vary, depending on the effective surface temperature. A row of evenly spaced lights might arise from a circular form of bead lightning, which probably consists of a standing longitudinal electric current wave. Intense heat, light and ultraviolet light would be radiated, as well as possibly radio interference or microwaves.
The current in a 10m ball at ground atmospheric density would be about 108 amperes. The resulting large external magnetic field could disturb a compass up to a kilometer away, or open magnetically activated circuit breakers in nearby cars or electrical equipment. A large ball might appear to follow a car or airplane due to magnetic attraction to the iron, but large eddy currents induced in the metal would repel, preventing it from getting too close. Rotation of a large ball through its own magnetic field could produce a radial potential difference of thousands of volts, possibly giving rise to electrical effects such as beams, corona or electric shock. Perturbations by air currents or plasma jets could cause a large ball's magnetic axis to wobble about the direction of the earth's magnetic field, with a period of a few seconds. The net linear magnetic force on a ball is proportional to the gradient of the earth's field and is negligible.
A humming might be produced by radial oscillations of the ball. Pulsing or beeping could result from a periodic transfer of energy between different modes, or by atmospheric effects. Gases, such as ozone or nitrogen oxides, could be given off, producing a pungent smell causing nearby observers to become ill, incoherent or unconscious. This might explain odd stories related by witnesses.