Coronal Mass Ejections

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A coronal mass ejection(CME) is an ejection of the corona of the Sun. They are almost exclusively observed by using a white light coronagraph (an attachment for a telescope that is designed to block out the direct light from a star so that nearby objects that would be hidden in the star’s bright glare can be resolved). Each CME consists of a plasma primarily made up of electron and protons. Small quantities of heavier elements such as helium, oxygen, and iron are ejected along with the coronal magnetic field.

Most CMEs originate from active regions (groupings of sunspots associated with frequent flares). These regions have closed magnetic field lines, where the magnetic field strength is large enough to allow the containment of the plasma; the CME must open these field lines at least partially to escape from the sun. However, CMEs can also be initiated in quiet sun regions (although in many cases the quiet region was recently active). During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, CMEs originate from active regions whose latitudinal distribution is more homogeneous. CMEs range in speed from about 20 km/s up to 3,200 km/s with an average speed of 489 km/s. The average mass based on coronagraph images is 1.6×10¹⁵g. The frequency of ejections depends on the phase of the solar cycle: from about one every other day near solar minimum to 5–6 per day near solar maximum. Current knowledge of CME kinematics indicates that the CME starts with an initial pre-acceleration phase characterized by a slow rising motion, followed by a period of rapid acceleration away from the Sun until a near-constant velocity is reached. Some CMEs, usually the very slowest ones, lack this three-stage evolution, instead accelerating slowly and continuously throughout their flight. Even for CMEs with a well-defined acceleration stage, the pre-acceleration stage is often absent or unobservable.

A CME can impact the Earth in a couple of ways. When the ejection reaches the Earth it may disrupt the Earth’s magnetosphere by compressing it on the day side and extending the night-side magnetic tail. When the magnetosphere reconnects on the night side, it creates trillions of watts of power which are directed back into the upper atmosphere. This process can cause particularly strong aurora( aurora borealis or aurora australis). CME events, along with solar flares, can disrupt radio transmissions, cause power outages, and damage satellites and electrical transmission lines.

This is the NASA page about coronal mass ejections. Here on Universe Today we have a great article about A 3D view of a CME. Astronomy Cast offers a good episode about auroras.

References:
NASA Cosmicopia
NASA Solar Science

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