Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 1 Aurora From Wikipedia, the free encyclopedia An aurora, sometimes referred to as a polar lights or north- ern lights, is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctic) regions.[1] Au- roras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (ther- mosphere/exosphere), where their energy is lost. The resulting ion- ization and excitation of atmospheric constituents emits light of varying color and complexity. The form of the aurora, occur- ring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating parti- cles. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton Images of auroras from around the world, auroras are usually observed at including those with rarer red and blue lower latitudes.[2] lights Wikipedia Reader 2 May 1 2017 Contents 1 Occurrence of terrestrial auroras 1.1 Images 1.2 Visual forms and colors 1.3 Other auroral radiation 1.4 Aurora noise 2 Causes of auroras 2.1 Auroral particles 2.2 Auroras and the atmosphere 2.3 Auroras and the ionosphere 3 Interaction of the solar wind with Earth 3.1 Magnetosphere 4 Auroral particle acceleration 5 Auroral events of historical significance 6 Historical theories, superstition and mythology 7 Non-terrestrial auroras 8 See also 9 Notes 10 References 11 Further reading 12 External links Occurrence of terrestrial auroras Most auroras occur in a band known as the auroral zone,[3] which is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora is called the auroral oval, a band displaced towards the nightside of the Earth. Early evidence for a geomagnetic connection comes from Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 3 the statistics of auroral observations. Elias Loomis (1860), and later Hermann Fritz (1881) [4] and S. Tromholt (1882)[5] in more detail, established that the aurora appeared mainly in the auroral zone. Day-to-day positions of the auroral ovals are posted on the internet.[6] In northern latitudes, the effect is known as the Kp map of North America Aurora Borealis or the Northern Lights. The for- mer term was coined by Galileo in 1619, from the Roman goddess of the dawn and the Greek name for the north wind.[7] The southern counterpart, the Aurora Australis or the Southern Lights, has features that are almost identical to the Aurora Borealis and changes simultaneously with changes in the northern auroral zone.[8] The Aurora Australis is visible from high south- ern latitudes in Antarctica, Chile, Argentina, New Zealand, and Australia. Kp map of Eurasia A geomagnetic storm causes the auroral ovals (north and south) to expand, and bring the aurora to lower latitudes. It was hardly ever seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras (“auroral oval”)[3] is slight- These NOAA maps of North ly different, being centered about 3–5 degrees America and Eurasia show nightward of the magnetic pole, so that auroral the local midnight equator- arcs reach furthest toward the equator when the ward boundary of the aurora magnetic pole in question is in between the ob- at different levels of geo- server and the Sun. The aurora can be seen best magnetic activity; a Kp=3 at this time, which is called magnetic midnight. corresponds to low levels of geomagnetic activity, while Kp=9 represents high levels Wikipedia Reader 4 May 1 2017 Auroras seen within the auroral oval may be directly over- head, but from farther away they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs,[9] which can be sub-visual. Auroras are occasionally seen in latitudes below the auroral zone, when a geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most com- mon during the peak of the eleven-year sunspot cycle or during the three years after the peak.[10][11] An aurora may appear overhead as a “corona” of rays, radiating from a distant and apparent central location, which results from perspective. An electron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to the local geomagnetic field vector B. This angle is known as the “pitch angle” of the particle. The distance, or radius, of the electron from the field line at any time is known as its Larmor radius. The pitch angle increases as the electron travels to a region of greater field strength nearer to the atmosphere. Thus it is possible for some particles to return, or mirror, if the angle becomes 90 degrees before entering the atmosphere to collide with the denser molecules there. Other particles that do not mir- ror enter the atmosphere and contribute to the auroral dis- play over a range of altitudes. Other types of auroras have been observed from space, e.g.”poleward arcs” stretching sunward across the polar cap, the related “theta aurora”,[12] and “dayside arcs” near noon. These are relatively infrequent and poorly understood. There are other interesting effects such as flickering aurora, “black aurora” and sub-visual red arcs. In addition to all these, a weak glow (often deep red) observed around the two polar cusps, the field lines separat- ing the ones that close through the Earth from those that are Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 5 swept into the tail and close remotely. Images The altitudes where auroral emissions occur were re- vealed by Carl Størmer and his colleagues who used cameras to triangulate more than 12,000 auroras.[13] They discovered that most of the light is produced be- tween 90 and 150 km above the ground, while extending at times to more than 1000 km. Images of auroras are significantly more common today than in the past due to the increase in use of digital cameras that have high enough sensitivities.[14] Film and digital exposure to au- roral displays is fraught with difficulties. Due to the differ- ent color spectrum present, and the temporal changes occurring during the exposure, the results are somewhat unpredictable. Different layers of the film emulsion re- spond differently to lower light levels, and choice of film can be very important. Longer exposures superimpose rapidly changing features, and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess. David Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the im- ages in the laboratory to recreate the visual display more accurately.[15] For scientific research, proxies are often used, such as ultra-violet, and color-correction to simu- late the appearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highly useful tool for aurora hunters.[16] Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by major websites.[17] It is possible to take excellent imag- es with standard film (using ISO ratings between 100 and Wikipedia Reader 6 May 1 2017 400) and a single-lens reflex camera with full aperture, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora’s brightness.[18] Early work on the imaging of the auroras was done in 1949 by the University of Saskatchewan using the SCR-270 radar. Estonia 18.03.2015 Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 7 Estonia 17.03.2015 Estonia 14.12.2015 Wikipedia Reader 8 May 1 2017 Estonia 07.10.2015 Estonia 18.03.2015 Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 9 Estonia 18.03.2015 Estonia 07.10.2015 Wikipedia Reader 10 May 1 2017 Estonia 18.03.2015 Diffuse aurora observed by DE-1 satellite from high Earth orbit Wikipedia Reader https://en.wikipedia.org/wiki/Aurora Selected by Julie Madsen - Entry 10 11 Aurora during a geomagnetic storm that was most likely caused by a coronal mass ejection from the Sun on 24 May 2010. Taken from the Visual forms and colors ISS Auroras frequently appear either as a diffuse glow or as “curtains” that extend approximately in the east-west direction. At some times, they form “quiet arcs”; at oth- ers they evolve and change constantly. These are called “active aurora”. The most distinctive and brightest are the curtain-like auroral arcs. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field, consistent with auroras being shaped by Earth’s magnetic field. In-situ particle measurements confirm Wikipedia Reader 12 May 1 2017 that auroral electrons are guided by the geomagnetic field, and spiral around them while moving toward Earth. The similarity of an auroral display to curtains is often enhanced by folds within the arcs.