Day the solar wind almost disappeared: Magnetic field fluctuations, wave refraction and dissipation

On May 11, 1999, the Advanced Composition Explorer (ACE) spacecraft observed a rarefied parcel of solar wind. This has come to be known as "The Day the Solar Wind Disappeared." Little if any change is seen in the large-scale interplanetary magnetic field during this time, but the magnetic field fluctuations are depressed and significantly more transverse to the mean field. The high Alfven speed resulting from the constant field intensity and low ion density enhances wave refraction, and we examine this as a possible explanation for the fluctuation properties. The solar wind possesses a very low proton beta, thereby separating the cyclotron and ion inertial length scales and permitting a test of possible dissipation dynamics. We find that the test favors the ion inertial scale theories.