Unusually low density regions in the compressed slow wind: Solar wind transients of small coronal hole origin

¹ State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, PR China
e-mail: liuyong@spaceweather.ac.cn
² College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, PR China
³ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
⁴ Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, Shandong, PR China
⁵ Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
⁶ School of Earth and Space Sciences, Peking University, 100871 Beijing, PR China
⁷ Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire, USA

Received: 14 May 2019
Accepted: 2 January 2020

Abstract

We report on two small solar wind transients embedded in the corotating interaction region, characterized by surprisingly lower proton density compared with their surrounding regions. In addition to lower density, these two small solar wind transients showed other interesting features like higher proton temperature, higher alpha-proton ratios, and lower charge states (C⁺⁶/C⁺⁵ and O⁺⁷/O⁺⁶). A synthesized picture for event One combining the observations by STEREO B, ACE, and Wind showed that this small solar transient has an independent magnetic field. Back-mapping links the origin of the small solar transient to a small coronal hole on the surface of the Sun. Considering these special features and the back-mapping, we conclude that such small solar wind transients may have originated from a small coronal hole at low latitudes.