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Concept
Solar particle event
Summary
In solar physics, a solar particle event (SPE), also known as a solar energetic particle (SEP) event or solar radiation storm, is a solar phenomenon which occurs when particles emitted by the Sun, mostly protons, become accelerated either in the Sun's atmosphere during a solar flare or in interplanetary space by a coronal mass ejection shock. Other nuclei such as helium and HZE ions may also be accelerated during the event. These particles can penetrate the Earth's magnetic field and cause partial ionization of the ionosphere. Energetic protons are a significant radiation hazard to spacecraft and astronauts.
SPEs occur when charged particles in the Sun's atmosphere are accelerated to extremely high velocities. These charged particles, referred to as solar energetic particles, can escape into interplanetary space where they follow the interplanetary magnetic field.
When solar energetic particles interact with the Earth's magnetosphere, they are guided by the Earth's magnetic field towards the north and south poles where they can penetrate into the upper atmosphere.
The physical mechanism behind the acceleration of solar energetic particles leading up to SPEs is currently debated. However, SPEs can generally be divided into two classes
Gradual SPEs are thought to involve the acceleration of particles by shocks driven by coronal mass ejections in the upper corona. They are associated with type II radio bursts and are characterized by elemental abundances, charge states, and temperatures similar to that of the ambient corona. These events produce the highest particle intensities near Earth.
Impulsive SPEs are thought to involve the acceleration of particles mostly by processes associated with magnetic reconnection and wave-particle interactions at the locations of solar flares. They are associated with short-duration flare emissions at low altitudes and type III radio bursts. They are less intense near Earth than gradual events.
An additional hybrid class has been identified which involves characteristics of both gradual and impulsive events.
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Coronal mass ejection
A coronal mass ejection (CME) is a significant ejection of magnetic field and accompanying plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established. If a CME enters interplanetary space, it is referred to as an interplanetary coronal mass ejection (ICME).
Coronal hole
A coronal hole is a temporary region of relatively cool, less dense plasma in the solar corona where the Sun's magnetic field extends into interplanetary space as an open field. Compared to the corona's usual closed magnetic field that arches between regions of opposite magnetic polarity, the open magnetic field of a coronal hole allows solar wind to escape into space at a much quicker rate. This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.