Travel(l)ing Convection Vortex (TCV) is a name given to a specific type of daytime magnetic impulsive events observed by high-latitude ground based magnetometer arrays. Since the most likely source for TCVs is the solar wind, they are considered to be important processes in transfering energy from the solar wind into the magnetosphere. Two important early studies of the phenomenon are those by Friis-Christensen et al. (1988) and Glassmeier et al. (1989).

A TCV seen by a single magnetic station shows an isolated magnetic field variation with a bipolar structure in the H-component, i.e., a negative-positive (NP) or positive-negative (PN) deflection. The associated D-component shows a single positive or negative excursion (making the H-component variation the negative time derivative of the D-component variation). When several, longitudinally separated stations are used, this signature is seen to propagate tailward (westward in the morning sector): hence a "traveling" vortex. Furthermore, when a large, both longitudinally and latitudinally distributed grid of stations is used to calculate the 2D ground-equivalent ionospheric current system, a twin-vortex structure emerges. It has been shown that these oppositely rotating cells of current vortices are created by a pair of upward and downward flowing field-aligned currents (FAC). This can also been shown theoretically using basic plasma physics equations.

The main characteristics of TCVs as observed from the ground are:

Although the ionospheric features of TCVs are well documented, the processes taking place on the other end of the FACs are more complicated. The most likely source region for TCVs is was thought to be the magnetopause, as one could assume from the high magnetic latitudes they are observed. However, some recent results suggest that the source could be inside the plasma sheet (Yahnin and Moretto, 1996; Yahnin et al., 1996). The high degree of conjugacy of the events suggest a source close to the equatorial plane, while the MLT distribution implies generation close to the subsolar magnetopause. Several processes have been suggested:

Note also that multiple convection vortex systems related to Pc 5 pulsations (and most likely to Kelvin-Helmholtz instability in the LLBL) are considered to be different events from the TCVs (McHenry et al., 1990). Another important distinction is that TCV events are not related to the much larger scale twin-vortex structures accompanying sudden impulses (see SI & SSC). (Note, however, that Korotova and Sibeck (1994) do not agree on the latter point!)

References