Magnetic field structure of interplanetary magnetic clouds at 1 AU

TLDR

Interplanetary magnetic clouds are frequent at 1 AU, with diameters of 0.2–0.4 AU, enhanced magnetic fields of 15–30 nT, and lower plasma temperature and density than the surrounding solar wind. The authors created a least‑squares fitting program that applies cylindrical force‑free field solutions to in‑situ magnetic data to estimate cloud size, peak field strength, axis orientation, and spacecraft closest approach. Assuming a constant force‑free parameter α, the magnetic field inside a cloud follows a cylindrically symmetric solution where the axial component is proportional to the zeroth‑order Bessel function, the tangential component to the first‑order Bessel function, and the radial component is zero. Analysis of 18 clouds at 1 AU shows the cloud axes are typically within 15° of the ecliptic plane and about 100° from the Sun’s direction in that plane, with orientations ranging up to 80° from the ecliptic, and additional statistical properties are reported.

Abstract

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 ▽² B = −α² B , 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.