Earth-affecting solar transients: a review of progresses in solar cycle 24

Abstract

This review article summarizes the advancement in the studies of Earth-affecting solar transients in the last decade that encompasses most of solar cycle 24. It is a part of the effort of the International Study of Earth-affecting Solar Transients (ISEST) project, sponsored by the SCOSTEP/VarSITI program (2014–2018). The Sun-Earth is an integrated physical system in which the space environment of the Earth sustains continuous influence from mass, magnetic field, and radiation energy output of the Sun in varying timescales from minutes to millennium. This article addresses short timescale events, from minutes to days that directly cause transient disturbances in the Earth’s space environment and generate intense adverse effects on advanced technological systems of human society. Such transient events largely fall into the following four types: (1) solar flares, (2) coronal mass ejections (CMEs) including their interplanetary counterparts ICMEs, (3) solar energetic particle (SEP) events, and (4) stream interaction regions (SIRs) including corotating interaction regions (CIRs). In the last decade, the unprecedented multi-viewpoint observations of the Sun from space, enabled by STEREO Ahead/Behind spacecraft in combination with a suite of observatories along the Sun-Earth lines, have provided much more accurate and global measurements of the size, speed, propagation direction, and morphology of CMEs in both 3D and over a large volume in the heliosphere. Many CMEs, fast ones, in particular, can be clearly characterized as a two-front (shock front plus ejecta front) and three-part (bright ejecta front, dark cavity, and bright core) structure. Drag-based kinematic models of CMEs are developed to interpret CME propagation in the heliosphere and are applied to predict their arrival times at 1 AU in an efficient manner. Several advanced MHD models have been developed to simulate realistic CME events from the initiation on the Sun until their arrival at 1 AU. Much progress has been made on detailed kinematic and dynamic behaviors of CMEs, including non-radial motion, rotation and deformation of CMEs, CME-CME interaction, and stealth CMEs and problematic ICMEs. The knowledge about SEPs has also been significantly improved. An outlook of how to address critical issues related to Earth-affecting solar transients concludes this article.

A Coronal Mass Ejection and Magnetic Ejecta Observed In Situ by STEREO-A and Wind at 55° Angular Separation

“…The magnetic structure of coronal mass ejections (CMEs) is primarily known from direct in situ measurements of their magnetic field in the heliosphere and through reconstruction and fitting of magnetic field measurements, both in the photosphere and heliosphere (e.g., see the recent review by Zhang et al 2021). Multispacecraft measurements of CMEs, while rare, have been central in revealing that CMEs are global structures (Burlaga et al 1982) that can often be understood as near-force-free flux ropes (Burlaga 1988).…”

Solar activity and space weather

“…The altered properties of CMEs resulted space weather that is milder than what is expected from the reduced activity alone. The same applies to space weather caused by CIRs [4,29,30]. Instead of looking at the intense geomagnetic storms, if one starts with the magnetic clouds (MCs) and examines their space weather impact, the reduction in geoeffectiveness (as measured by Dst) is again clear [23,30].…”

A Coronal Mass Ejection and Magnetic Ejecta Observed In Situ by STEREO-A and Wind at 55$^\circ$ Angular Separation

“…The magnetic structure of coronal mass ejections (CMEs) is primarily known from direct in situ measurements of their magnetic field in the heliosphere and through reconstruction and fitting of magnetic field measurements, both in the photosphere and heliosphere (e.g., see recent review by Zhang et al 2021). Multi-spacecraft measurements of CMEs, while rare, have been central in revealing that CMEs are global structures (Burlaga et al 1982) that can often be understood as near-force-free flux ropes (Burlaga 1988).…”