Properties of slow magnetic clouds

Abstract

Slow (V SW <400 km s −1) magnetic clouds have been analyzed to determine their characteristics and geoeffectiveness. It is found that slow clouds have mean magnetic field strengths of ∼13 nT , peak B S ∼9 nT , and dawn–dusk electric fields of E SW ∼2.5 mV m −1 . The clouds are small in spatial size, typically ∼0.18 AU . The slowest events may have been accelerated to their speeds by interaction with the slow solar wind. Slow clouds are surprisingly geoeffective. Five out of 27 events caused major (D ST ⩽−100 nT) magnetic storms. Likewise, these geoeffective clouds may have been decelerated to V SW <400 km s −1 . The issue of interplanetary acceleration/deceleration will be examined with SOHO coronal mass ejection data in the near future.