Energy and momentum transport from the Sun (solar wind) to the near Earth system.
When the IMF connect with the Earths magnetic field on the dayside in a process
called reconnection, the solar wind electric fiels is coupled to
the Earths magnetosphere. This process results in efficient transport
of solar wind energy to the magnetosphere and increases the magnetospheric
plasma convection. This process which is called dayside reconnection
is a measure of energy transport in the Solar-Magnetosphere system.
Subsequent reconnection of the
lobe magnetic field in the magnetotail transports energy into the closed
magnetic field region. Plasma is then transported across the boundary of
open-closed field lines in the nightside magnetosphere. This plasma flow
is a measure of the efficiency of energy transport
inside the magnetosphere and is called the nightside reconnection rate. To estimate this energy transport two quantaties
must be defined: 1) The exact location and orientation of the open-closed boundary
and 2) The plasma flow across this bounadry.
Global images from the
IMAGE FUV system guide us to identify ionospheric signatures of the open-closed
field line boundary observed by the two EISCAT radars at Tromso (VHF) and Svalbard (ESR).
Continuous radar and optical monitoring of the open-closed field line boundary
is used to determine the location, orientation and velocity of the open-closed
boundary and the ion flow velocity perpendicular to this boundary.
The magnetotail reconnection electric field is found to be a bursty
process that oscillates between between 0 mV/m and 1 mV/m with ~10-15 min periods.
These ULF oscillations are mainly caused by the motion of the open-closed boundary.
In situ measurements earthward of the reconnection site in the magnetotail by
Geotail show similar oscillations in the duskward electric field. We also find
that bursts of increased magnetotail reconnection not necessarily have any
associated auroral signatures. Finally we find that the reconnection rate
correlates poorly with the solar wind electric field. This indicate that the
magnetotail reconnection is not directly driven, but is an internal magnetospheric
process. Estimates of a coupling efficiency between the solar wind electric
field and magnetotail reconnection only seems to be relevant as average on
long time scales. The oscillation mode at 1 mHz corresponds to the internal
cavity mode with additional lower frequencies, 0.5 and 0.8 mHz, that might be
modulated by solar wind pressure variations.
N. Østgaard, J. Moen, S. B. Mende, H. U. Frey, T. J. Immel, P. Gallop,
K. Oksavik, M. Fujimoto. Estimates of magnetotail reconnection rate based on
IMAGE FUV and EISCAT measurements Ann. Geophys. (Eleventh International EISCAT Workshop),
23 (1), 123-134, 2005.