[1] Examination of Geotail measurements in the near-tail (X > À30 R E ) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of $1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hV x i = 431 km/s, and 38 moving tailward, hV x i = À451 km/s. We refer to these two populations as ''BBF-type'' and ''plasmoid-type'' flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal ÁB z perturbations, i.e., Ç for the ''BBF'' events and ± for the ''plasmoid'' events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within $1-2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 R E and 20 nT and 4.4 R E and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X-Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events.

A best‐fit ellipse and hyperbola have been calculated to represent several hundred magnetopause and bow‐shock positions observed by six Imp spacecraft. Average geocentric distances to the magnetopause and bow shock near the ecliptic plane are 11.0 and 14.6 RE in the sunward direction, 15.1 and 22.8 RE in the dawn meridian, and 15.8 and 27.6 RE in the dusk meridian. The bow‐shock hyperbola is oriented in a direction consistent with that expected when the aberration of a radial solar wind is considered. Observed magnetopause crossings agree well with theoretical predictions in the noon meridian plane but fall outside the theoretical boundaries in the dawn‐dusk meridian planes. Imp 4 plasma data are used to demonstrate that the solar‐wind momentum flux is the prime factor controlling the orbit‐to‐orbit changes in the boundary positions. Data suggest that the interplanetary‐field orientation also affects the distance to the magnetopause boundary, more earthward crossings corresponding to southward fields. Six unusual bow‐shock locations up to 22 RE beyond the average position are found to be due to an enhanced standoff distance associated with a low Alfvén Mach number. The possibility that the solar wind may have become sub‐Alfvénic on July 31, 1967, is suggested.

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Abstract. For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15 Re behind the Earth while the solar wind remained very quiet (V=330 km s -•, n=14-21 cm -3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n=19 cm-3), cool (Tp=40 eV), rapidly flowing (V=310 km s -1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s-l), lower but substantial densities (n=3 cm -3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.

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Fifty orbits of Explorer 34 data have been used to study 0.01–0.05 Hz transverse waves in the interplanetary medium region between the bow shock and the spacecraft apogee of 34 RE. It is concluded that the waves are associated with the earth's bow shock since they only occur when projection of the interplanetary field observed at the spacecraft intersects the shock. The waves are observed 18.5% of the time when a total of 134 days of interplanetary data is considered, but more than 90% of the time when the field has the proper orientation with respect to the bow shock. On the basis of this result it is suggested that these waves with 20–100 second periods are a permanent feature of the solar wind‐earth interaction. The transverse component of the waves is typically several gammas in amplitude in 4–8 gamma fields. The disturbance vector in the XY plane generally exhibits the same sense of rotation in a coordinate system where the field is oriented along the positive z axis. Attenuation of wave amplitudes with distance from the bow shock is estimated to be only a factor of 2 when the spacecraft is 15 RE from the bow shock. The absence of waves at particular field orientations, even though the field line intersects the shock, is interpreted as a propagation effect. This observation is the basis for calculations that yield an average velocity in the plasma frame of 2.7 ± 0.4 times the solar wind velocity. Whistler propagation and local generation by two‐stream instability are discussed as alternate theoretical explanations for the presence of the waves. It is suggested that the data favor the latter mechanism.

[1] An analytical approximation is developed for the shape of the nightside tail current sheet, representing it as a function of the Earth's dipole tilt angle, solar wind ram pressure, and the interplanetary magnetic field (IMF). The model is based on 5-min average magnetometer data of the Geotail and Polar spacecraft, spanning the periods 1994-2002, and 1999-2001, respectively. All the magnetospheric data were tagged by concurrent values of the solar wind dynamic pressure and IMF B y and B z components, averaged over 30-min intervals immediately preceding the magnetospheric observations. Warping and twisting parameters were calculated by minimizing the number of mismatches between the observed and predicted orientation of the magnetic field on both sides of the model current sheet. The model is valid within the nightside magnetosphere in the range of tailward distances À50 R E X GSM 0. Variations of the solar wind pressure P change the shape of the deformed current sheet in such a way that an increase of P results in a decrease of the magnetotail ''hinging distance'' R H , but increases the magnitude of its transverse warping. The IMF B z component affects the magnitude of the seasonal/diurnal motion of the current sheet in the north-south direction, and it also controls the degree of the IMF B yrelated twisting, which becomes much larger during the periods with northward IMF B z .

Over 3000 hours of Imp 6 magnetic field data obtained between 20 and 33 R in the geomagn'etic tail have been used in a staEbistical study of the tail configuration.A distribution of 2. 5-min averages of B as a function of position across the tail rlveals that more flux crosses the equatorial plane near the dawn and dusk flanks (B = 3.5y) than near midnight (B = 1 . 8y) .The tiil field projected in the solar magnetospheric equatorial plane deviates fro~ the x axis due to flaring and solar wind aberration by an angle ex = -0.9 Y.s M -2.7, where YSM is in earth radii and ex is ~n degrees.After removing these effects, the B component of the tail field is found to depenJ' on interplanetary sector structure.During an 'away' sector the B component of the tail field is on average O. 5~ greater than that during a 'toward' sector, a result that is true in both tail lobes and is independent of location across the tail. This effect means the average field reversal between northern gnd southern lobes 0& the tail _ is more often 17~ rather than the 180 that is generally supposed.