AbstractToday the extreme space weather events of early August 1972 are discussed as benchmarks for Sun‐Earth transit times of solar ejecta (14.6 hr) and for solar energetic particle fluxes (10 MeV ion flux >70,000 cm−2·s−1·sr−1). Although the magnetic storm index, Dst, dipped to only −125 nT, the magnetopause was observed within 5.2 RE and the plasmapause within 2 RE. Widespread electric‐ and communication‐grid disturbances plagued North America late on 4 August. There was an additional effect, long buried in the Vietnam War archives that add credence to the severity of the storm impact: a nearly instantaneous, unintended detonation of dozens of sea mines south of Hai Phong, North Vietnam on 4 August 1972. The U.S. Navy attributed the dramatic event to magnetic perturbations of solar storms. Herein we discuss how such a finding is broadly consistent with terrestrial effects and technological impacts of the 4 August 1972 event and the propagation of major eruptive activity from the Sun to the Earth. We also provide insight into the solar, geophysical, and military circumstances of this extraordinary situation. In our view this storm deserves a scientific revisit as a grand challenge for the space weather community, as it provides space‐age terrestrial observations of what was likely a Carrington‐class storm.

The Sun produces the most powerful explosions in the solar system, solar flares, that can also be accompanied by large eruptions of magnetised plasma, coronal mass ejections (CMEs). These processes can accelerate electron beams up to relativistic energies through magnetic reconnection processes during solar flares and CME-driven shocks. Energetic electron beams can in turn generate radio bursts through the plasma emission mechanism. CME shocks, in particular, are usually associated with type II solar radio bursts. However, on a few occasions, type II bursts have been reported to occur either in the absence of CMEs or shown to be more likely related with the flaring process. It is currently an open question how a shock generating type II bursts forms without the occurrence of a CME eruption. Here, we aim to determine the physical mechanism responsible for a type II burst which occurs in the absence a CME. By using radio imaging from the Nan{\c c}ay Radioheliograph, combined with observations from the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory spacecraft, we investigate the origin of a type II radio burst that appears to have no temporal association with a white-light CME. We identify a typical type II radio burst with band-split structure that is associated with a C-class solar flare. The type II burst source is located above the flaring active region and ahead of disturbed coronal loops observed in extreme ultraviolet images. The type II is also preceded by type III radio bursts, some of which are in fact J-bursts indicating that accelerated electron beams do not all escape along open field lines. The type II sources show single-frequency movement towards the flaring active region. The type II is located ahead of a faint extreme-ultraviolet (EUV) front propagating through the corona.

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 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. They occur near the Sun on timescales of minutes to hours, and the resulting effects on the Earth can take place in minutes to days. These transient events are commonly categorized into four different types: (1) solar flares, (2) coronal mass ejections (CMEs) and their interplanetary counterparts, Interplanetary CMEs (ICMEs), (3) solar energetic particle (SEP) events, and (4) stream interaction regions (SIRS) including corotating interaction regions (CIRs). These four types of Earth-affecting transient events differ in their observational appearances, physical origin or processes, as well as the geoeffectiveness in their own unique ways (Table 1 ). Table 1 Four types of Earth-affecting solar transients and their key physical processes and geoeffectiveness Earth-affecting solar transients Key physical processes Effects on near-earth space environment Effects on technological system and life Solar flares Magnetic reconnection; ideal MHD instability; particle acceleration; plasma heating Disturbances in the ionosphere; heating and expansion of upper atmosphere High-frequency radio communication; satellite drag (Earth climate from long-term variation of solar irradiance) CMEs and ICMEs Ideal MHD instability; flux rope formation; shock formation; particle acceleration; aerodynamic drag; CME-CME interaction; magnetic reconnection Geomagnetic storms; substorms; disturbances in the ionosphere; ionosphere scintillations; radiation belt storms GPS systems and navigation; satellite communication; high-frequency radio communication; electric power transmission; satellite degradation and failure (single event upset; dielectric material charging and discharging; surface charging); radiation hazards to astronauts; radiation hazards to aircraft crew and passengers SEPs Particle acceleration; injection; propagation; turbulence Particle radiation storms Satellite degradation and failure; high-frequency radio communication; radiation hazards to astronauts; atmospheric chemistry SIRs/CIRs Stream interaction; particle acceleration Substorms; They are emitted from the Sun in association with solar flares and Coronal Mass Ejection (CME)-driven shock waves. SEP radiation storms may have durations from a period of hours to days or even weeks and have a large range of energy spectrum profiles. These events pose a threat to modern technology strongly relying on spacecraft and are a serious radiation hazard to humans in space, and additionally of concern for avionics and commercial aviation in extreme circumstances (Malandraki and Crosby 2018a , b ). This section is divided into subsections, devoted to the progress of SEP research from both the observational as well as the theoretical and modeling perspective. Observationally, our understanding of CMEs has progressed significantly over the next four decades: a well-defined magnetic structure ejected along with the coronal plasma. While the vast majority of CMEs near the Sun do not drive a shock, the small number of shock-driving CMEs have the most intense consequences in the heliosphere. Although we treated SEP events as a transient, they are closely related to CMEs via their shocks. SEPs not related to shocks are generally the weak and short-lived impulsive events thought to be accelerated in the flare reconnection region.

We have investigated the magnetic connectivity of coronal mass ejections (CMEs) to the Sun using Ulysses observations of suprathermal electrons at various distances between 1 and 5.2 AU. Drawing on ideas concerning the eruption and evolution of CMEs, we had anticipated that there might be a tendency for CMEs to contain progressively more open field lines, as reconnection back at the Sun either opened or completely disconnected previously closed field lines threading the CMEs. Our results, however, did not yield any discernible trend. By combining the potential contribution of CMEs to the heliospheric flux with the observed buildup of flux during the course of the solar cycle, we also derive a lower limit for the reconnection rate of CMEs that is sufficient to avoid the "flux catastrophe" paradox. This rate is well below our threshold of detectability. Subject headings: solar wind - Sun: activity - Sun: corona - Sun: coronal mass ejections (CMEs) - On-line material: color figure Sun: magnetic fields

As an active star, the Sun frequently ejects large systems of magnetized plasma called Interplanetary Coronal Mass Ejections (ICMEs). When these ICMEs reach Earth, they cause geomagnetic storms, and, depending on their intensity, can wreak havoc on modern society [3]. In this project, we designed, executed, and diagnosed a proof-of-principle experiment aimed at studying ICMEs in a laboratory setting.

The Sun is the star located at the centre of the Solar System. It is a massive sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is the main source of energy for life on Earth. The Sun has been an object of veneration in many cultures and a The Sun's magnetic field leads to many effects that are collectively called solar activity. Solar flares and coronal mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at the photospheric surface. Both coronal mass ejections and high-speed streams of solar wind carry plasma and the interplanetary magnetic field outward into the Solar System. The effects of solar activity on Earth include auroras at moderate to high latitudes and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Changes in solar irradiance over the 11-year solar cycle have been correlated with changes in sunspot number. The solar cycle influences space weather conditions, including those surrounding Earth. For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few… The first satellites designed for long-term observation of the Sun from interplanetary space were Pioneer 6, 7, 8, and 9, launched by NASA between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long time, transmitting data until May 1983. In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.