Associating ground magnetometer observations with current or voltage generators

A circuit analogy for magnetosphere-ionosphere current systems has two extremes for driversof ionospheric currents: ionospheric elec tric fields/voltages constant while current/conductivity vary—the“voltage generator”—and current constant while electric field/conductivity vary—the “current generator.”Statistical studies of ground magnetometer observations associated with dayside Transient High LatitudeCurrent Systems (THLCS) driven by similar mechanisms find contradictory results using this paradigm:some studies associate THLCS with voltage generators, others with current generators. We argue that mostof this contradiction arises from two assumptions used to interpret ground magnetometer observations:(1) measurements made at fixed position relative to the THLCS field-aligned current and (2) negligibleauroral precipitation contributions to ionospheric conductivity. We use observations and simulations toillustrate how these two assumptions substantially alter expectations for magnetic perturbations associatedwith either a current or a voltage generator. Our results demonstrate that before interpreting groundmagnetometer observations of THLCS in the context of current/voltage generators, the location of a groundmagnetometer station relative to the THLCS field-aligned current and the location of any auroral zoneconductivity enhancements need to be taken into account.