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Traveling 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:
- Occurrence peaks at about 09 and 14 MLT
- This is because the perturbation amplitude increases during
the first four minutes or so of the event!
- Occurrence peaks at about 73 degrees invariat latitude
- Morning side vortices seem to occur on lower latitudes than
the afternoon ones, and thus the Scandinavian magnetometers do
not see the 14 MLT sector vortices!
- Tailward propagation speeds are 0.1-0.3 degrees/s, i.e.,
- 3-10 km/s in the ionosphere
- 130-400 km/s at the magnetopause
- The calculated equivalent currents flow opposite to the measured
plasma drift (Lühr and Blawert, 1991)
- This means that the magnetic deflections on the ground are
caused by Hall currents
- Vortices are separated by 1000-2000 km
- FAC current densities can be several 10^-6 A/m^2 in the ionosphere
- Total FAC amounting to a few hundred Amperes
- Some dynamic features of the dayside aurora may be related
to TCVs (Oguti et al., 1988).
- The high current densities observed provide possibilities
for several plasma instabilities that could energize electrons
- Conjugate vortices in the two hemispheres have opposite senses
Although the ionospheric features of TCVs are well documented,
the processes taking place on the other end of the FACs are more
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:
- Flux transfer events (FTE)
- Currently not considered an important factor for several reasons:
- It is still unclear what kind of ionospheric signatures FTEs
should create (for example if the observed FAC pair can be produced)
- IMF Bz seems not to control the TCV event occurrence
- There is no poleward motion observed for the TCVs as reconnection
- FTEs should be associated with mass motion at the ionospheric
foot point of the reconnected flux tube, whose velocity should
not be larger than the speed of sound. This contradicts with the
observed tailward velocities of TCVs.
- Impulsive plasma penetration events
- The implications of these events are not very well understood
- Solar wind pressure pulses
- Some TCV events have been correlated with measured pressure pulses
- For example, one AMPTE Lithium release in the solar wind caused a TCV event
- However, many events seem NOT to be related with important pressure variations!
- Some models for the pressure pulse response in the magnetosphere/ionosphere
system indicate formation of two pairs of vortices, one on either
side of noon
- However, all observed features cannot be explained
- Rapid changes of the IMF
- No observational backing yet
Note also that multiple convection vortex systems related to
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
sudden impulses (SI). (Note, however, that
Korotova and Sibeck (1994) do not agree on the latter point!)
- Friis-Christensen, E., M. A. McHenry, C. R. Clauer, and S.
Vennerstrom, Ionospheric traveling convection vortices observed
near the polar cleft: A triggered response to sudden changes in
the solar wind, Geophys. Res. Lett. 15, 253-256,
- Glassmeier, K.-H., Traveling magnetospheric convection twin-vortices:
observations and theory, Ann. Geophysicae, 10, 547-565,
- Glassmeier, K.-H., M. Hönisch, and J. Untiedt, Ground-based
and satellite observations of traveling magnetospheric convection
twin-vortices, J. Geophys. Res., 94, 2520-2528,
- Korotova, G. I., D. G. Sibeck, Generation of ULF magnetic
pulsations in response to sudden variations in solar wind
dynamic pressure, in Solar Wind Sources of Magnetospheric
Ultra-Low-Frequency Waves, Geophysical Monograph 81,
- Lühr, H. and W. Blawert, Ground signatures of travelling
convection vortices, in Solar Wind Sources of Magnetospheric
Ultra-Low-Frequency Waves, Geophysical Monograph 81,
- McHenry, M. A., C. R. Clauer, and E. Friis-Christensen, Relationship
of solar wind parameters to continuous, dayside, high-latitude
traveling ionospheric convection vortices, J. Geophys. Res.,
95, 15007-15022, 1990.
- Oguti, T., T. Yamamoto, K. Hayashi, S. Kokubun, A. Egeland,
and J. A. Holtet, Dayside auroral activity and related magnetic
impulses in the polar cusp region, J. Geomagn. Geoelectr.,
40, 387-408, 1988.
- Yahnin, A. and T. Moretto, Travelling convection vortices in the
ionosphere map to the central plasma sheet,
Ann. Geophysicae, 14, 1025-1031, 1996.
- Yahnin, A. G., V. G. Vorobjev, T. Bösinger, R. Rasinkangas,
D.G. Sibeck, and P. T. Newell, On the source region of the travelling
convection vortices, Geophys. Res. Lett., accepted, 1996.