The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM stops at the heliopause, outside of which the interstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether". Composition and physical characteristics The interplanetary medium includes interplanetary dust, cosmic rays, and hot plasma from the solar wind. The temperature of the interplanetary medium varies. For dust particles within the asteroid belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU. The density of the interplanetary medium is very low, decreasing in inverse proportion to the square of the distance from the Sun. It is variable, and may be affected by magnetic fields and events such as coronal mass ejections. At about 5 particles per cubic centimeter in the vicinity of the Earth, it may rise to as high as 100 particles/cm. Since the interplanetary medium is a plasma, or gas of ions, the interplanetary medium has the characteristics of a plasma, rather than a simple gas. For example, it carries the Sun's magnetic field with it, is highly electrically conductive (resulting in the heliospheric current sheet), forms plasma double layers where it comes into contact with a planetary magnetosphere or at the heliopause, and exhibits filamentation (such as in aurorae). The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's tesla magnetic dipole field would reduce with the cube of the distance to about tesla. But satellite observations show that it is about 100 times greater at around tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field induces electric currents which in turn generate magnetic fields, and in this respect it behaves like an MHD dynamo. Extent of the interplanetary medium The outer edge of the heliosphere is the boundary between the flow of the solar wind and the interstellar medium. This boundary is known as the heliopause and is believed to be a fairly sharp transition of the order of