Interplanetary Field Enhancements: An Overview

The phenomenon, dubbed the Interplanetary Field Enhancement, occurs in the solar wind near and inside of 1 AU and is attributed to collisional dust production and subsequent solar wind pickup.  The duration and strength of these events appears to depend on the heliocentric distance of the detection, the largest event was recorded by the PVO spacecraft in orbit about Venus in 1982.  It lasted 11 hours and was over 20 million km in radial extent.  While no such large structure has been seen since by PVO or Venus Express since that time observations at 1 AU by STEREO, and the flotilla of spacecraft near the L-1 Lagrangian point have continued to see smaller events.  These are now attributed to collisions of asteroidal debris, small rocks destroying each other when they collide at a mean velocity of 20 km/s at 1 AU.  Such a speed of collision with a 1 kg rock will completely destroy a 10⁶ kg target.  Even if the number of small asteroids were constant with heliocentric distance the increased orbital speeds inside 1 AU should greatly increase the destructive power of collisions so that larger events should occur at closer distances to the Sun.  We review the statistics available from Pioneer Venus and Venus Express and compare them with 1 AU data to test this hypothesis.