Certain areas on the surface of Mars also contain strong, localized magnetic anomalies where it appears that a bar magnets have been buried below the surface [Albee, 1998]. At these locations, an ionospheric dynamo effect may occur as atmospheric thermal tides drag ionospheric plasma across magnetic field lines, inducing electric fields and currents. This localized phenomenon is likely very complex and could significantly influence the electric field created by the global electric circuit at these locations. Due to the lack of a substantial, intrinsic, global magnetic field at Mars, the ionospheric dynamo generator probably does not significantly influence the global electric circuit of Mars outside of these surface magnetic anomalies.
We have already noted how the atmospheric conductivity varies with time. Changes in the atmospheric conductivity will cause corresponding temporal changes in the global electric field. Also, temporal changes in the global electric field will occur due to changes in the number of current sources. Thomas and Gierasch [1985] reported seeing dust devils on smooth terrain in Viking images. If dust devils only occur on smooth terrain, then the number of dust devils can vary on a daily cycle depending upon how much smooth terrain is illuminated by the Sun causing convective atmospheric motion. A variation in the number of dust devils can affect the global electric circuit in the same way that the number of thunderstorms influence the global electric circuit of Earth, i.e., the Carnegie curve [Mauchly, 1921].
Ryan and Lucich [1983] noted that dust devils are most common in the northern hemisphere during the spring and summer months. Therefore, we would expect that the number of current sources would decrease in the northern hemisphere winter and autumn. However, it is precisely during these months that global scale dust storms can develop in the southern hemisphere. These storms can encompass half of the globe and raise dust up to 30 km in altitude [Haberle, 1981]. If the dust dynamics and charging mechanisms are the same as those we have assumed for dust devils, then these global storms may move a vast amount of charge high into the Martian atmosphere. Therefore, perhaps the global electric circuit is dominated by meter to kilometer scale dust devils during the northern hemisphere spring and summer and by global scale dust storms during the northern hemisphere winter.
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Electrical Discharge on the Martian Surface by Joseph C. Kolecki and Geoffery A. Landis
Artist's conception of an electrical storm on Mars (Image courtesy of Michael Kaiser, NASA Goddard Space Center). Click on image to enlarge it.
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