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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.

Shock-associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure and the sum is higher than the pressure of the material outside of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alfven speed.

Abstract. The magnetic eld experiment o n A CE provides continuous measurements of the local magnetic eld in the interplanetary medium. These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particles distributions. The experiment consists of a pair of twin, boommounted, triaxial uxgate sensors which are located 165 inches = 4.19 meters from the center of the spacecraft on opposing solar panels. The electronics and digital processing unit DPU is mounted on the top deck of the spacecraft. The two triaxial sensors provide a balanced, fully redundant v ector instrument and permit some enhanced assessment of the spacecraft's magnetic eld. The instrument provides data for Browse and high-level products with between 3 and 6 vector s ,1 resolution for continuous coverage of the interplanetary magnetic eld. Two highresolution snapshot bu ers each hold 297 seconds of 24 vector s ,1 data while onboard Fast Fourier Transforms extend the continuous data to 12 Hz resolution. Real-time observations with 1 second resolution are provided continuously to the Space Environmental Center SEC of the National Oceanographic and Atmospheric Association NOAA for near-instantaneous, world-wide dissemination in service to space weather studies. 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 interplanetary medium in support of the fundamental goals of the ACE mission and cooperative studies with other ACE investigators using the combined ACE dataset as well as other ISTP spacecraft involved in the general program of Sun-Earth Connections.

Magnetic clouds observed at 1 AU are modeled as cylindrically symmetric, constant alpha force-free magnetic fields. The model satisfactorily explains the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft in terms of the possible orientations of the axis of a magnetic cloud. The model also explains why the magnetic field strength is observed to be higher inside a magnetic cloud than near its boundaries. However, the model predicts that the magnetic field strength profile should be symmetric with respect to the axis of the magnetic cloud, whereas observations show that this is not generally the case. INTRODUCTIONA magnetic cloud was identified by Burlaga et al. [1981] as an interplanetary structure with a dimension of the order of 0.25 AU in which the magnetic field strength is higher than average, the magnetic field direction rotates monotonically through a large angle, the temperature is low, and the plasma beta is significantly lower than 1. Observations concerning the nature, origin, and evolution of magnetic clouds were reviewed by Burlaga [1984]. Burlaga et al. A simple solution for a cylindrically symmetric force-free field with constant alpha was found by Lundquist [ 1950], and many subsequent studies have shown that constant alpha force-free fields are very special and important configurations. Taylor [1974, 1976, 1986] conjectured that in a plasma with a finite resistivity, however small, confined to a volume bounded by perfectly conducting walls, the helicity over the whole volume is constant, and the system evolves by turbulent relaxation to a constant alpha force-free configuration. This concept has been applied to astrophysical jets (Konigl and Choudhur [1985]; but see Turner [1986]) and to solar magnetic fields [Heyvaerts and Priest, 1984].By minimizing the magnetic energy subject to the constraint of constant magnetic helicity, Woltjer [1958] has shown that a force-free field with constant alpha represents the state of lowest magnetic energy which a closed ideal magnetohydrodynamic (MHD) system may attain. He claimed that such a system is stable, and he suggested without proof that it represents the natural end configuration of a system with dissipation. The stability of constant alpha force-free configurations was discussed from a different point of view by Chandrasekhar and Woltjer [1958]. On the other hand, it is not clear that a constant alpha force-free configuration is stable. Voslamber and Callebaut [1962] showed that a force-free field with constant alpha is unstable to kink-type perturbations. However, the characteristic wavelength is several times the cylinder radius, and the growth time of the instability is much larger than the transit time to 1 AU in the case of a magnetic cloud.Force-free fields with nonreversed axial field are believed to be MHD unstable because of a minimum in the q profile [Taylor, 1976]. 7217

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Abstract. The Advanced Composition Explorer was launched August 25, 1997 carrying six high resolution spectrometers that measure the elemental, isotopic, and ionic charge state composition of nuclei from H to Ni (1 Z 28) from solar wind energies (1 keV/nuc) to galactic cosmic ray energies (500 MeV/nuc). Data from these instruments is being used to measure and compare the elemental and isotopic composition of the solar corona, the nearby interstellar medium, and the Galaxy, and to study particle acceleration processes that occur in a wide range of environments. ACE also carries three instruments that provide the heliospheric context for ion composition studies by monitoring the state of the interplanetary medium. From its orbit about the Sun-Earth libration point 1.5 million km sunward of Earth, ACE also provides real-time solar wind measurements to NOAA for use in forecasting space weather. This paper provides an introduction to the ACE mission, including overviews of the scientific goals and objectives, the instrument payload, and the spacecraft and ground systems.