On the origin of microscale magnetic holes in the solar wind

Magnetic holes are sudden changes in the magnetic field intensity \B\ from typical interplanetary values (similar to 10 nT) to less than 1 nT in a mat ter of seconds. The intensity then recovers within seconds or up to similar to 30 min later. These \B\ dropouts can be seen daily. Less often observed , but even more dramatic, are magnetic field depletions that last for up to several hours. We use selected periods of magnetic flux dropouts observed with various sensors of the Advanced Composition Explorer (ACE), which has a unique combination of magnetic field, plasma, and composition experiments , to establish the origin of these peculiar objects. We conclude that these microscale magnetic holes very likely develop in the heliosphere and are n ot of direct solar origin. We also suggest a possible formation mechanism a ssociated with magnetic reconnection close to the Sun.