Investigation of the Geomagnetic Tail by the GEOTAIL Satellite

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1. Global structure of the distant tail GEOTAIL observations have confirmed that the magnetotail extends to 200 Re and beyond. It flaps violently north-south as well as east-west with amplitudes comparable to its average radius of about 20 Re, but there is no indication that it is filamentary. Plasma sheet is clearly distinguishable from the lobe or the magnetosheath by high temperature (around 1 keV) and low density (around 0.1/cc). Its magnetic structure and internal convection profile are governed by the north-south polarity of the interplanetary magnetic field.2. Distant tail under the southward IMF Under the southward IMF,magnetic field lines convect toward the neutral sheet where they are reconnected. The probability that open field lines are reconnected earthward of 150 Re is about 75%. The reconnection rate is the highest at the tail axis and decreases toward the flanks by a factor of 2 to 3.3. Plasma heating and relaxation at the plasma-sheet lobe boundary At the plasma sheet boundary, the slow-mode shock is formed in the convective flow where the cold ions of the lobe become heated. Very energetic ions flow upstream from the shock. The mixed population of cold and hot particles in this boundary region is a rich source of plasma waves whose non-linear development is seen.4. Distant tail under northward IMF Under the northward IMF,the open field lines which are presumably produced in the cusp region of the dayside magnetopause sweep across the tail. The convection is parallel to the neutral sheet which is highly twisted in the direction of IMF and has opposite directions (dawn to dusk vs dusk to dawn) on opposite sides of the neutral sheet. The source of the hot plasma in the plasma sheet which is located in the open field lines poses a new problem.