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Celestial Lights Down to Earth

by Lady Michelle Jennifer Santos, Founder and Publisher 

April 19, 2012 (TSR) – As our official adieu to winter and welcome of spring, we want to treat our subscribers with some amazing time-lapse photography of Aurora Borealis (a.k.a. Northern Lights) from Norway.

City of Tromsø. Copyright: Ole Christian Salomonsen (Click image to enlarge)

We found some awe-inspiring images in vimeo.com, captured  by Norwegian amateur photographer Ole Christian Salomonsen from Tromsø, Norway, who received praise from NASA last autumn for his images as well as being the only finalist for Astronomy Photographer of the year 2011. This is his latest video he produced, released yesterday:

 

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth’s magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.

Salomonsen showing his shots at the galleries of Polaria in Tromsø, Norway

Time-lapse photography is a technique whereby the frequency at which film frames are captured (the frame rate) is much lower than that used to view the sequence. When played at normal speed, time appears to be moving faster and thus lapsing. For example, an image of a scene may be captured once every second, then played back at 30 frames per second. The result is an apparent 30-times speed increase. Time-lapse photography can be considered the opposite of high speed photography or slow motion.

The technique has been used to photograph crowds, traffic, and even television. The effect of photographing a subject that changes imperceptibly slowly, creates a smooth impression of motion. A subject that changes quickly is transformed into an onslaught of activity.

Here is Salomonsen’s first video project he released in 2011:

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A BRIEF COSMOLOGY LESSON ON AURORAS

An aurora (plural: aurorae or auroras) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth’s magnetic field into the atmosphere. Aurora is classified as diffuse or discrete aurora. Most aurorae occur in a band known as the auroral zone which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth’s magnetic dipole. During ageomagnetic storm, the auroral zone will expand to lower latitudes. The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurora are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed only in the night sky because they are not as bright as the sunlit sky. Aurorae occur occasionally poleward of the auroral zone as diffuse patches or arcs (polar cap arcs) which are generally invisible to the naked eye.

Some of Salomonsen images exhibited in the gallery of Polaria in Tromsø, Norway earlier this year. "The Sun, The Moon and The Northern Lights" (middle) was selected as the featured front cover of The Digital Photographer Magazine in 2011 (Issue Number 107). Photo Credit: Truls Melbye Tiller.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the “Dance of the Spirits“. In Europe, in the Middle Ages, the auroras were commonly believed a sign from God.

Aurora Australis

Its southern counterpart, the aurora australis (or the southern lights), has almost identical features to the aurora borealis and changes simultaneously with changes in the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand and Australia.

The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday‘s [1791 – 1867] work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cutby), rather than along, the lines of the magnetic field, an electric current is said to be induced into that conductor and electrons will flow within it. The amount of current flow is dependent upon a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process (“the dynamo effect“), any and all conductors, solid or otherwise are so affected including plasmas or other fluids.

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969; Zmuda and Armstrong, 1973) which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest (“region 2″) detours, leaving again through field lines closer to the equator and closing through the “partial ring current” carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so such currents require a driving voltage, which some dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms.

Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoctes. It is not well understood why geomagnetic storms are tied to Earth’s seasons while polar activity is not. But it is known that during spring and autumn, the interplanetary magnetic field and that of Earth link up. At the magnetopause, Earth’s magnetic field points north.

Aurorae occur on other planets. Similar to the Earth’s aurora, they are visible close to the planet’s magnetic poles.

Solar Wind and the Magnetosphere

Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction. At some times, they form “quiet arcs”; at others (“active aurora”), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth’s magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.

The similarity to curtains is often enhanced by folds called “striations”. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a “corona” of diverging rays, an effect of perspective.

Although it was first mentioned by Ancient Greek explorer/geographer PytheasHiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named “auroral electrojets” (see also Birkeland currents).

The Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree heat of the Sun’s outermost layer, the corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cm3 and magnetic field intensity around 2–5 nT (nanoteslas; Earth’s surface field is typically 30,000–50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger.

Earth magnetosphere

The IMF originates on the Sun, related to the field of sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge (“limb”) of the visible Sun.

Earth’s magnetosphere is formed by the impact of the solar wind on the Earth’s magnetic field. It forms an obstacle to the solar wind, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re), forming a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of the magnetosphere abreast of Earth, is typically 190,000 km (30 Re), and on the night side a long “magnetotail” of stretched field lines extends to great distances (> 200 Re).

The magnetosphere is full of trapped plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with increases in solar wind density and speed, with increase in the southward component of the IMF and with increases in turbulence in the solar wind flow. The flow pattern of magnetospheric plasma is from the magnetotail toward the Earth, around the Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to the Earth’s magnetic field, some magnetospheric plasma travel down along the Earth’s magnetic field lines and lose energy to the atmosphere in the auroral zones. Magnetospheric electrons which are accelerated downward by field-aligned electric fields are responsible for the bright aurora features. The un-accelerated electrons and ions are responsible for the dim glow of the diffuse aurora.

How do auroras work?

The aurora borealis (the Northern Lights) and the aurora australis (the Southern Lights), both surrounding the north magnetic pole (aurora borealis) and south magnetic pole (aurora australis), occur when highly charged electrons from the solar wind interact with elements in the earth’s atmosphere. Solar winds stream away from the sun at speeds of about 1 million miles per hour. When they reach the earth, some 40 hours after leaving the sun, they follow the lines of magnetic force generated by the earth’s core and flow through the magnetosphere, a teardrop-shaped area of highly charged electrical and magnetic fields.

As the electrons enter the earth’s upper atmosphere, they will encounter atoms of oxygen and nitrogen at altitudes from 20 to 200 miles above the earth’s surface. The color of the aurora depends on which atom is struck, and the altitude of the meeting.

  • Green – oxygen, up to 150 miles in altitude
  • Red – oxygen, above 150 miles in altitude
  • Blue – nitrogen, up to 60 miles in altitude
  • Purple/violet – nitrogen, above 60 miles in altitude

Basically, the colors you see are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth’s magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule:

 

Oxygen emissions
Green or brownish-red, depending on the amount of energy absorbed.
Nitrogen emissions
Blue or red. Blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an excited state.

Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.

All of the magnetic and electrical forces react with one another in constantly shifting combinations. These shifts and flows can be seen as the auroras “dance,” moving along with the atmospheric currents that can reach 20,000,000 amperes at 50,000 volts. (In contrast, the circuit breakers in your home will disengage when current flow exceeds 15-30 amperes at 120 volts.)

The auroras generally occur along the “auroral ovals,” which center on the magnetic poles (not the geographic poles) and roughly correspond with the Arctic and Antarctic circles. There are times, though, when the lights are farther south, usually when there are a lot of sunspots. Sunspot activity follows an 11-year cycle. The next peak will occur in 2011 and 2012, so opportunities to see auroras outside their normal range should be good.

There are many stories about sounds associated with auroras, but there are no recordings of auroral sounds. Scientists can’t agree on what would produce sounds during the aurora.

For our aspiring photographers and those who wish to have a new hobby: How time-lapse works

The frame rate of time-lapse movie photography can be varied to virtually any degree, from a rate approaching a normal frame rate (between 24 and 30 frames per second) to only one frame a day, a week, or more, depending on subject.

The term “time-lapse” can also apply to how long the shutter of the camera is open during the exposure of EACH frame of film (or video), and has also been applied to the use of long-shutter openings used in still photography in some older photography circles. In movies, both kinds of time-lapse can be used together, depending on the sophistication of the camera system being used. A night shot of stars moving as the Earth rotates requires both forms. A long exposure of each frame is necessary to enable the dim light of the stars to register on the film. Lapses in time between frames provide the rapid movement when the film is viewed at normal speed.

Film is often projected at 24 frame/s, meaning 24 images appear on the screen every second. Under normal circumstances, a film camera will record images at 24 frame/s. Since the projection speed and the recording speed are the same, the images onscreen appear to move at normal speed.

Even if the film camera is set to record at a slower speed, it will still be projected at 24 frame/s. Thus the image on screen will appear to move faster.

The change in speed of the onscreen image can be calculated by dividing the projection speed by the camera speed.

So a film recorded at 12 frames per second will appear to move twice as fast. Shooting at camera speeds between 8 and 22 frames per second usually falls into the undercranked fast motion category, with images shot at slower speeds more closely falling into the realm of time-lapse, although these distinctions of terminology have not been entirely established in all movie production circles.

If you wish to know more, you can check out this page for tips.

 

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