Estimates of magnetotail reconnection rate based on IMAGE FUV and EISCAT measurements

Dayside merging between the interplanetary and terrestrial magnetic fields couples the solar wind electric field to the Earth's magnetosphere, increases the magnetospheric convection and results in efficient transport of solar wind en- ergy into the magnetosphere. Subsequent reconnection of the lobe magnetic field in the magnetotail transports energy into the closed magnetic field region. Combining global imag- ing and ground-based radar measurements, we estimate the reconnection rate in the magnetotail during two days of an EISCAT campaign in November-December 2000. Global images from the IMAGE FUV system guide us to identify ionospheric signatures of the open-closed field line bound- ary observed by the two EISCAT radars in Tromso (VHF) and on Svalbard (ESR). Continuous radar and optical mon- itoring of the open-closed field line boundary is used to de- termine 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 0 mV/m and 1 mV/m with 10-15 min periods. These ULF oscilla- tions are mainly due to the motion of the open-closed bound- ary. 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 do not necessarily have any asso- ciated auroral signatures. Finally, we find that the reconnec- tion rate correlates poorly with the solar wind electric field. This indicates that the magnetotail reconnection is not di- rectly driven, but is an internal magnetospheric process. Es- timates of a coupling efficiency between the solar wind elec- tric field and magnetotail reconnection only seem to be rel- evant as averages over long time intervals. 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.

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