Extreme Rarefaction of Solar Wind: A Study on Origin and Characteristics Using Ulysses Observations

From the Ulysses observation of the solar wind between the heliocentric distance r _h of ∼1.0 and ∼5.4 au during 1990–2009, we identified 53 intervals when the solar wind exhibited extreme rarefaction, ∼2 orders of magnitude decreases in the solar wind proton density N _p from their ambient values. These extremely low-density solar wind (ELDSW) events, characterized by an average (median) N _p of ∼0.28 ± 0.09 (∼0.30) cm ^−3 , ram pressure of ∼0.07 ± 0.04 (∼0.07 nPa) and mass flux of ∼166 ± 84 (∼159) 10 ^−22 kg cm ^−2 s ^−1 all normalized to 1 au, have an average (median) duration of ∼6.0 ± 3.5 days (∼5.5 days), and radial extent of ∼1.9 ± 1.1 au (∼1.9 au). A clear hemispheric asymmetry is noted in their solar/interplanetary origin, with 70% being identified in the south hemisphere, and 30% in the north hemisphere of the heliosphere. About 23% of the events were encountered between r _h of 2.25 and 4 au, and 77% at r _h > 4 au, indicating that these are not intrinsic properties of the Sun/solar corona but are created by the evolution of the solar wind with increasing radial distance from the Sun. The majority (49%) of the events occurred during magnetic clouds, 34% in solar wind high-speed stream (HSS) tails, 11% during the proper HSSs, and 6% during interplanetary sheaths. The identification of ELDSWs will have important consequences for their interaction with the magnetospheres of Jupiter and Saturn.