Abstract. The magnetic field experiment on WIND will provide data for studies of a broad range of scales of structures and fluctuation characteristics of the interplanetary magnetic field throughout the mission, and, where appropriate, relate them to the statics and dynamics of the magnetosphere. The basic instrument of the Magnetic Field Investigation (MFI) is a boom-mounted dual triaxial fluxgate magnetometer and associated electronics. The dual configuration provides redundancy and also permits accurate removal of the dipolar portion of the spacecraft magnetic field. The instrument provides (1) near real-time data at nominally one vector per 92 s as key parameter data for broad dissemination, (2) rapid data at 10.9 vectors s -1 for standard analysis, and (3) occasionally, snapshot (SS) memory data and Fast Fourier Transform data (FFT), both based on 44 vectors s -I. These measurements will be precise (0.025%), accurate, ultra-sensitive (0.008 nT/step quantization), and where the sensor noise level is < 0.006 nT r.m.s, for 0-10 Hz. The digital processing unit utilizes a 12-bit microprocessor controlled analogue-to-digital converter. The instrument features a very wide dynamic range of measurement capability, from :E4 nT up to • 536 nT per axis in eight discrete ranges. (The upper range permits complete testing in the Earth's field.) In the FTT mode power spectral density elements are transmitted to the ground as fast as once every 23 s (high rate), and 2.7 rain of SS memory time series data, triggered automatically by pre-set command, requires typically about 5.1 hours for transmission. Standard data products are expected to be the following vector field averages: 0.0227-s (detail data from SS), 0.092 s ('detail' in standard mode), 3 s, 1 rain, and 1 hour, in both GSE and GSM coordinates, as well as the FFT spectral elements. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the solar wind: (1) as a collisionless plasma laboratory, at all time scales, macro, meso and micro, but concentrating on the kinetic scale, the highest time resolution of the instrument (=0.022 s), (2) as a consequence of solar energy and mass output, (3) as ~n external source of plasma that can couple mass, momentum, and energy to the Earth's magnetosphere, and (4) as it is modified as a consequence of its imbedded field interacting

Interplanetary magnetic clouds, although not dominant, are a relatively common feature of the solar wind at 1 AU. Their diameters at 1 AU fall in the range of 0.2–0.4 AU, and they have enhanced field strength (B ≃ 15–30 nT at 1 AU), and lower plasma temperature and density than the surrounding plasma. The internal field is a magnetic force‐free configuration, and therefore the current density (J) is proportional to B everywhere: J = α B, giving ▽×B = α B. If α is constant throughout the cloud (Burlaga, 1988), then ▽²B = −α²B, which has a cylindrically symmetric field solution that is consistent with observations: the axial field is proportional to the zeroth‐order Bessel function of r, where r is the perpendicular distance from the cloud's axis, the tangential component is proportional to the first‐order Bessel function, and the radial component is zero. We have developed a least squares program that fits magnetic field data within a cloud to these functions and which estimates various properties of the cloud, such as its size, maximum B, and inclination of its axis, as well as closest approach distance of the spacecraft. Results of a study of 18 clouds observed at 1 AU indicate that the most probable direction of the cloud's axis is within 15° of the ecliptic plane and ≃100° from the Sun's direction when it is projected into the ecliptic plane. A broad range of orientations is observed with some extending to 80° from the ecliptic. Other statistical properties are presented, and three cases are discussed in detail.

[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.

Abstract. Using an observed relation between speeds of CMEs near the Sun and in the solar wind, we determine an "effective" acceleration acting on the CMEs.We found a linear relation between this effective acceleration and the initial speed of the CMEs. The acceleration is similar to that of the slow solar wind in magnitude. The average solar wind speed naturally divides CMEs into fast and slow ones. Based on the relation between the acceleration and initial speed, we derive an empirical model to predict the arrival of CMEs at 1 AU.

Magnetic reconnection is the process by which magnetic field lines of opposite polarity reconfigure to a lower-energy state, with the release of magnetic energy to the surroundings. Reconnection at the Earth's dayside magnetopause and in the magnetotail allows the solar wind into the magnetosphere. It begins in a small 'diffusion region', where a kink in the newly reconnected lines produces jets of plasma away from the region. Although plasma jets from reconnection have previously been reported, the physical processes that underlie jet formation have remained poorly understood because of the scarcity of in situ observations of the minuscule diffusion region. Theoretically, both resistive and collisionless processes can initiate reconnection, but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key processes predicted by models of collisionless reconnection. The data therefore demonstrate that collisionless reconnection occurs in the magnetotail.

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, space and astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnection in the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (R(E)), even though the associated current sheets span many tens or hundreds of R(E). Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390R(E) (or 2.5 x 10(6) km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.

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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