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. 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 time scales from minutes to millennium. This article addresses short time scale events, from minutes to days that directly cause transient disturbances in the Earth 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 3-D and over a large volume in the heliosphere. 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.

Earth-affecting Solar Transients: A Review of Progresses in Solar Cycle 24

We report on a remarkable finding that the halo coronal mass ejections (CMEs) in cycle 24 are more abundant than in cycle 23, although the sunspot number in cycle 24 has dropped by about 40%. We also find that the distribution of halo-CME source locations is different in cycle 24: the longitude distribution of halos is much flatter with the number of halos originating at central meridian distance >/=60 degrees twice as large as that in cycle 23. On the other hand, the average speed and the associated soft X-ray flare size are the same in the two cycles, suggesting that the ambient medium into which the CMEs are ejected is significantly different. We suggest that both the higher abundance and larger central meridian longitudes of halo CMEs can be explained as a consequence of the diminished total pressure in the heliosphere in cycle 24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more than usual making them appear as halos.

The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24

The Wind spacecraft is a critical element in NASA's Heliophysics System Observatory (HSO) – a fleet of spacecraft created to understand the dynamics of the sun-Earth system – owing to the co...

A Quarter Century of Wind Spacecraft Discoveries

ICMEs (interplanetary coronal mass ejections), the heliospheric counterparts of what is observed with coronagraphs at the Sun as CMEs, have been the subject of intense interest since their close association with geomagnetic storms was established in the 1980s. These major interplanetary plasma and magnetic field transients, often preceded and accompanied by solar energetic particles (SEPs), interact with planetary magnetospheres, ionospheres, and upper atmospheres in now fairly well-understood ways, although their details and context affect their overall impacts. The term ICME as it is used here refers to the complete solar-wind plasma and field disturbance, including the leading shock (if present), the compressed, deflected solar-wind plasma and the field behind the shock (“sheath”), and the coronal ejecta (the “driver”) – often called a magnetic cloud. Many uncertainties remain in understanding both the relationship to what is observed at the Sun and the variety of local outcomes suggested by in-situ observations. This impacts our abilities to interpret events and to forecast effects based on solar observations. Here, we briefly consider what is known about ICMEs and their evolution en route from the Sun from the combination of available observations and interpretive models that have been developed up to now. The included references are only representative of the large body of work that has been published on this subject. Our aim is to provide the reader with an updated synthesis of research results in this still active area of heliophysics at the dawn of the Parker Solar Probe (PSP) and Solar Orbiter (SO) mission era.

ICME Evolution in the Inner Heliosphere

We present quantitative evidence that interplanetary type II radio bursts and sustained gamma-ray emission (SGRE) events from the Sun are closely related. Out of about 30 SGRE events reported in Share et al. we consider 13 events that had a duration exceeding 5 hr to exclude any are-impulsive phase gamma-rays. The SGRE duration also has a linear relation with the ending frequency of the bursts. The synchronism between the ending times of SGRE and the type II emission strongly supports the idea that the same shock accelerates electrons to produce type II bursts and protons (>300 MeV) that propagate from the shock to the solar surface to produce SGRE via pion decay. The acceleration of high-energy particles is confirmed by the associated solar energetic particle (SEP) events detected at Earth and/or at the Solar Terrestrial Relations Observatory spacecraft. Furthermore, the presence of >300 MeV protons is corroborated by the fact that the underlying coronal mass ejections (CMEs) had properties identical to those associated with ground-level enhancement events: they had speeds of >2000 km s1 and all were full-halo CMEs. Many SEP events did not have detectable flux at Earth in the >300 MeV energy channels, presumably because of poor magnetic connectivity.

P.A. Mӓkelӓ

Papers

Sun-to-earth propagation of the 2015 June 21 coronal mass ejection revealed by optical, EUV, and radio observations

A catalog of prominence eruptions detected automatically in the SDO/AIA 304 Å images