Interplanetary medium

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 nucl−1) to galactic cosmic-ray energies (∼500 MeV nucl−1). 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.

Abstract: Magnetic clouds are defined as regions;with a radial dimension roughly-equal0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component of B> nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty-five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface-associated clouds may have been swept up by corotating streams. 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 outsidemore » 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.« less

Abstract: Interplanetary magnetic clouds emerge as a feature of the solar wind at 1 AU, exhibiting enhanced field strength and lower plasma temperature and density than the surrounding plasma. A least-squares program has been developed which fits magnetic field data within a cloud, while estimating such cloud properties as its size, maximum field strength, and axis inclination. The results obtained from a study of 12 clouds observed at 1 AU point to a probable cloud axis direction within 15 deg of the ecliptic plane and about 100 deg from the sun's direction, when projected into the ecliptic plane. A wide variety of orientations is observed; some extend to 80 deg from the ecliptic.

Abstract: The COBE Diffuse Infrared Background Experiment (DIRBE) was designed to search for the cosmic infrared background (CIB) radiation. For an observer confined to the inner solar system, scattered light and thermal emission from the interplanetary dust (IPD) are major contributors to the diffuse sky brightness at most infrared wavelengths. Accurate removal of this zodiacal light foreground is a necessary step toward a direct measurement of the CIB. The zodiacal light foreground contribution in each of the 10 DIRBE wavelength bands ranging from 1.25 to 240 μm is distinguished by its apparent seasonal variation over the whole sky. This contribution has been extracted by fitting the brightness calculated from a parameterized physical model to the time variation of the all-sky DIRBE measurements over 10 months of liquid He cooled observations. The model brightness is evaluated as the integral along the line of sight of the product of a source function and a three-dimensional dust density distribution function. The dust density distribution is composed of multiple components: a smooth cloud, three asteroidal dust bands, and a circumsolar ring near 1 AU. By using a directly measurable quantity that relates only to the IPD cloud, we exclude other contributors to the sky brightness from the IPD model. High-quality maps of the infrared sky with the zodiacal foreground removed have been generated using the IPD model described here. Imperfections in the model reveal themselves as low-level systematic artifacts in the residual maps that correlate with components of the IPD. The most evident of these artifacts are located near the ecliptic plane in the mid-IR and are less than 2% of the zodiacal foreground brightness. Uncertainties associated with the model are discussed, including implications for the CIB search.