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.
Highlights
Earth-affecting solar transients refer to a broad range of energetic and/or eruptive events occurring on the Sun that have direct effects on the space environment near the Earth and cause adverse space weather impact on advanced technological systems of human society
This paves the way for future, real-time Sun-to-Earth simulations with magnetized Coronal Mass Ejection (CME) initiated based on magnetograms, EUV images, and early coronagraphic images
It is unclear whether simulations with magnetized CMEs initiated at 0.1 AU using multi-viewpoints coronagraphic measurements will perform worse than simulations with magnetized CMEs initiated at the solar surface in term of space weather forecasting capabilities
Summary
The solar corona is structured by open and closed magnetic field regions that transition at a certain distance as open field into interplanetary space (see Fig. 29). Other solar features that can cause strong geomagnetic events are stream interaction regions (SIRs) which are related to coronal holes, known as areas on the Sun with predominantly open magnetic field. Approaching the descending phase of a solar activity cycle, polar CHs extends to lower latitudes that change the solar wind in the ecliptic (i.e., encompassing the planets) quite dramatically. This evolutionary process is revealed in EUV data from the clear change of morphology (area, shape) and location of CHs. Wang and Sheeley Jr 1990 describe the magnetic structure of CHs and find that there is a close relation between the flux tube expansion and the underlying photospheric magnetic field. In the case of CIRs, the cause of the disturbances is often Alfvenic waves in the HSS (Tsurutani et al 2018)
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