Radiation astronomy/Bands

The image on the right is from a location within the auroral oval where it frequently appears directly overhead.

The image shows the bands form of auroras. Credit: Jan Curtis.
This is a band aurora that occurred over Manitoba, Canada. Credit: Euan Mearns. {{fairuse}}
This image shows bands over Iceland. Credit: Theo Schacht. {{free media}}

Auroras come in several different and sometimes overlapping forms:

  1. bands (continuous but irregular lower borders with kinks and folds, third down on the left),
  2. drapes (long-rayed band with folds, sixth down on the left is a white aurora),

"Those long arches that extend roughly east-west (actually magnetic east-west) from horizon to horizon are called arcs. If of nonuniform curvature, these forms are called bands. No really meaningful difference exists between arcs and bands, except that the more convoluted form, the band, is often brighter than the arc, and the appearance of bands usually signifies that the overall display is becoming more active."[1]

"Arcs and bands are thin ribbons set on edge parallel to the ground. The thickness of an arc or band may be as little as 100 meters (100 yds). The lower edge is typically 80 to 120 km (50 to 75 miles) above the earth and the upper edge is usually 10 to 100 km above that. Off out to the east or west, arcs and bands appear to meet the horizon. Still roughly 100 km above the surface, the aurora there is more than 600 miles distant. Hence, the aurora seen to meet the eastern horizon at Fairbanks is actually nearly directly overhead at Whitehorse, and visa versa. Similarly, an arc or band seen from Fairbanks to be 20 degrees above the north horizon is directly overhead Fort Yukon."[1]

An aurora occurring as bands can have several bands in close proximity following a similar path as with the bands in the right image.

The auroral oval is a permanent structure over the polar regions. Bands should occur around the polar regions as specific co-parallel contributors to the oval structure. In support of this, the image on the right shows bands over northern Canada. The second image down on the right shows bands over Iceland.

Planetary sciencesEdit

 
Split auroral ring is shown. Credit: Unknown, or unstated.
 
North and South Auroras Aren't Mirrored: This series of near-simultaneous auroras were observed between 11:24 am and 12:10 pm Universal Time. Credit: NASA.

Usually, planets exhibit auroral rings as seen from space around their magnetic poles.

In the example on the right, the auroral ring is actually split into two portions, perhaps with a black aurora between the rings.

"This series of near-simultaneous auroras [on the left] were observed between 11:24 am and 12:10 pm Universal Time (6:24am and 7:10am ET) on October 23, 2002. Observations were made of the northern (left) and southern (right) hemispheres by IMAGE and Polar satellites, respectively. White dots indicate the geographic poles. Analysis of the spacecraft images showed how the auroras shift depending on the "tilt" of the Earth's magnetic field toward the sun and conditions in the solar wind. The "12" at the top indicates noon (the direction toward the sun), and "0" at the bottom indicates midnight, (the direction away from the sun). Likewise, the "6" indicates dawn or morning side of the Earth, while "18" indicates dusk or evening side of the Earth, thus placing the auroras on a 24 hour clock face."[2]

"Looking at the auroras from space, they look like almost circular bands of light around the North and South Poles."[2]

"From spacecraft observations made in October, 2002, scientists noticed that these circular bands of aurora shift in opposite directions to each other depending on the orientation of the sun's magnetic field, which travels toward the Earth with the solar wind flow. They also noted that the auroras shift in opposite directions to each other depending on how far the Earth's northern magnetic pole is leaning toward the sun."[2]

"What was most surprising was that both the northern and southern auroral ovals were leaning toward the dawn (morning) side of the Earth for this event."[2]

"This is the first analysis to use simultaneous observations of the whole aurora in both the northern and southern hemispheres to track their locations."[3]

Theoretical aurorasEdit

 
The Aurora Borealis, or Northern Lights, shines above Bear Lake. Credit: United States Air Force photo by Senior Airman Joshua Strang.
 
This is an aurora forecast based on the OVATION model for 13 May 2015. Credit: NOAA Space Weather.
 
This is NOAA's WSA-Enlil Solar Wind Prediction graphic, from 21:00 UTC (5 p.m. ET) on May 12, 2015. Credit: NOAA, with notations by Scott Sutherland.

Def. an "atmospheric phenomenon created by charged particles from the sun striking the upper atmosphere, creating coloured lights in the sky"[4] is called an aurora.

On the lower right is an aurora forecast based on the OVATION model for 13 May 2015.

"If you're living in northern regions of Atlantic Canada, Quebec or Ontario, and anywhere across the Prairies, northeastern British Columbia and northward from there, there was a very good chance you saw something last night, as shown [on the lower right]."[5]

"The Sun has been fairly quiet lately, with a few spits and spots worth mentioning, but amid that relative quiet, a strong wind has been blowing. A few days ago, a large coronal hole rotated into view on the face of the Sun. This region - the dark "open eye" of the winky face the Sun is making in the image above - is where the Sun's magnetic field lines have opened up, allowing charged solar particles to stream away from the surface at very high speed."[5]

The image on the left is NOAA's WSA-Enlil Solar Wind Prediction graphic for the effects of the solar wind stream from the coronal hole.

"The wide band of yellow-orange-red, with Earth's dot is right smack dab in the middle of it, is that fast stream of solar particles - a region of the solar wind called a "coronal hole high speed stream" (CH HSS). The reason why the particles of the solar wind are moving so quickly in this region is because they're traveling through space that has been 'swept clear' of other particles by a band of denser solar plasma that swept past us earlier in the week."[5]

"When those fast-moving solar particles interact with Earth's magnetic field, it can have the same effect as when a dense cloud of plasma (a coronal mass ejection, or CME) washes over us - it can cause a geomagnetic storm, resulting in a heightened auroral activity."[5]

"The CME shown in the image resulted from a large dark filament of solar material blasting away from the Sun's western limb. Since this solar plasma was launched out into space well ahead of Earth, it is not expected to affect us."[5]

VisualsEdit

 
This yellow aurora near the horizon has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals. Credit: Unknown, or unstated. {{fairuse}}
 
These two rays in the foreground are distinctive from the background aurora. Credit: Unknown, or unstated. {{fairuse}}
 
The image shows rays from an unexpected aurora. Credit: Anderson. {{fairuse}}
 
This is a white and aqua aurora over Finland. Credit: Unknown, or unstated. {{fairuse}}

The image on the right shows a yellow aurora near the horizon that has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals.

The second image down on the right shows two distinctive rays in the foreground that terminate in yellow over Queenstown, New Zealand, in July 2012.

"This aurora [on the left] was a bit of a surprise. For starters, on this Friday morning in August 2002, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In the [left] image, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon. Large sunspot groups indicate that activity from an active Sun is relatively likely, possibly causing other streams of energetic particles to cascade onto the Earth and so causing more auroras."[6]

"The ray structure often seen in arcs and bands marks out the orientation of the magnetic field, nearly vertical at high latitude. The vertical extent of arcs and bands is also along this direction. Though the rays appear to converge upward, they are, in reality, essentially parallel shafts of light."[1]

"If rayed aurora is directly overhead, the point to which the rays appear to converge is the magnetic zenith. A line from that point to the observer marks out the local direction of the earth's magnetic field."[1]

"Standing in the aurora like pickets in a fence, the rays sometimes move sideways across the arcs and bands at high speeds. Sometimes one even sees them appear to move past each other both to the left and the right."[7]

"Rays line up along the direction of the earth's magnetic field, which points nearly vertically and somewhat to the northeast over Alaska and western Canada. To recognize the cross-sectional shapes of the rays, one needs to see them directly overhead in the sky. When they are in that position, they don't look like rays anymore; one reason why it took so long to discover their true shapes."[7]

"Not until very sensitive, high-speed television cameras were aimed at the bottoms of rays overhead was the mystery resolved. [The] rays were tightly wound up spirals only a kilometer or two across. Their form is difficult to recognize with the naked eye because the curled up shapes develop so quickly--sometimes in a second or so--and they often move very rapidly."[7]

"With a television camera capable of taking 30 pictures each second, it was possible to record the development of the spiral-shaped rays and measure their motion. Sometimes they move across the sky at speeds one hundred times that of a jet aircraft. To the observer on the ground, they do not appear to move quite that fast because the rays are so far away."[7]

This is a white aurora at the lower center and an aqua aurora in the upper part of the image on the lowest right.

VioletsEdit

 
The aurora borealis imaged shows blue, violet, and purple colors. Credit: Ragnar Sigurdsson.
 
This multicolored aurora has a strong violet band above the pink band. Credit: Black Swamp Storm Intercept Team.

The aurora borealis imaged on the right shows blue, violet, and purple colors with the Milky Way in the background.

The second aurora on the right contains an intense violet band above the pink band.

CyansEdit

 
This aurora borealis is a greenish-blue or cyan. Credit: beautiful-portals.tumblr.com.
 
This aurora contains a band of aqua-blue. Credit: Unknown, or unstated.

The aurora borealis on the right is probably the usual green aurora but appears greenish-blue or cyan. This cyan aurora, partially corroborated by the second image down on the right is the only total cyan aurora found so far.

SaturnEdit

 
This is Saturn imaged with the Stockholm Infrared Camera (SIRCA) in the H2O band. Credit: M. Gålfalk, G. Olofsson and H.-G. Florén, Nordic Optical Telescope.

At the right is Saturn imaged by the Stockholm Infrared Camera (SIRCA) in the H2O infrared band to show the presence of water vapor. The image is cut off near the top due to the presence of Saturn's rings.

The Sun's emission in the lowest UV bands, the UVA, UVB, and UVC bands, are of interest, as these are the UV bands commonly encountered from artificial sources on Earth. The shorter bands of UVC, as well as even more energetic radiation as produced by the Sun, generate the ozone in the ozone layer when single oxygen atoms produced by UV photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking UVB and part of UVC, since the shortest wavelengths of UVC (and those even shorter) are blocked by ordinary air.

SatellitesEdit

 
This auroral image was taken from the International Space Station. Credit: NASA.
 
The image is taken from the ISS and shows aurora above the ocean off Australia. Credit: Unknown, or unstated.
 
This view of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb. Credit: NASA.
 
NASA and the CSA created a network of satellites for THEMIS. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab.

Both images on the right was shot from the International Space Station on or about 13 July 2012 and in 2014, respectively going down the page. Note that lights are blurred across the image rather than top to bottom in the first image.

"This view [on the left] of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb. Calculated to be at altitudes ranging from 80 - 120 km (approx. 50-80 miles), the auroral light shown is due to the "excitation" of atomic oxygen in the upper atmosphere by charged particles (electrons) streaming down from the magnetosphere above."[2]

To study macroscale interactions during substorms, NASA and the Canadian Space Agency (CSA) created a network of satellites shown in the image on the lower left for “Time History of Events and Macroscale Interactions during Substorms" (THEMIS).

See alsoEdit

ReferencesEdit

  1. 1.0 1.1 1.2 1.3 T. Neil Davis (18 December 1979). Auroral Arcs and Bands. Fairbanks, Alaska USA: Geophysical Institute, University of Alaska Fairbanks. http://www2.gi.alaska.edu/ScienceForum/ASF3/362.html. Retrieved 2015-12-01. 
  2. 2.0 2.1 2.2 2.3 2.4 Rob Gutro (4 April 2005). Earth's Auroras Don't Mirror. Washington, DC USA: NASA. http://www.nasa.gov/vision/earth/lookingatearth/dueling_auroras.html#.VljKkMbvu3U. Retrieved 2015-11-27. 
  3. Timothy J. Stubbs (4 April 2005). Earth's Auroras Don't Mirror. Washington, DC USA: NASA. http://www.nasa.gov/vision/earth/lookingatearth/dueling_auroras.html#.VljKkMbvu3U. Retrieved 2015-11-27. 
  4. Emperorbma (29 November 2004). aurora. San Francisco, California: Wikimedia Foundation, Inc. https://en.wiktionary.org/wiki/aurora. Retrieved 2015-11-18. 
  5. 5.0 5.1 5.2 5.3 5.4 Scott Sutherland (13 May 2015). Solar wind sparks amazing auroras in Canada, Round 2 tonight. The Weather Network. http://www.theweathernetwork.com/news/articles/with-the-solar-wind-blowing-watch-out-for-auroras-tonight/51022/. Retrieved 2015-11-18. 
  6. Robert Nemiroff; Jerry Bonnell (19 November 2005). Rays from an Unexpected Aurora. Washington, DC USA: NASA. http://www.astronet.ru/db/xware/msg/1209585. Retrieved 2015-11-30. 
  7. 7.0 7.1 7.2 7.3 T. Neil Davis (30 March 1981). Auroral Rays. Fairbanks, Alaska USA: Geophysical Institute, University of Alaska Fairbanks. http://www2.gi.alaska.edu/ScienceForum/ASF3/362.html. Retrieved 2015-12-01. 

External linksEdit

{{Geology resources}}