The Earth-Sun Interaction

(R.W. Service The Cremation of Sam McGee,1907)

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The Northern Lights are a true wonder of Canadian skies. The dance of the aurora brings together two "partners" - the churning, boiling active sun 149 million kilometers "above our heads" and the magnetic field created in the Earth's spinning, molten core 6000 km beneath us. As lovely as the Northern Lights are there is also a sinister, destructive side to the aurora.

The Aurora in History and Legend

Inuit and Dene legends explain the aurora as lamps lighted by the spirits of departed souls guiding the way of new arrivals into the heavens. The crackling sound that many report hearing during intense displays of aurora were interpreted as the voices of the spirits speaking to the people on Earth. In the presence of the aurora children were cautioned to whisper - if you startled the spirits above they may snatch you away!

While all northern people have legends and stories of the aurora there have been reports of displays of the northern lights as far south as the Caribbean. Here are a few examples of auroral events noted by historians:

• 60 AD Roman soldiers march 60 miles to Rome after seeing the northern night sky a blaze of red and assumed that Rome was under attack. When they arrived all was normal! Possibly they saw a vary rare auroral display.

• 992 Arab explorer Ahmed ibn-Fadlan described a "red cloud looking like fire was near me; from it came noise and voices, and in it one could see people and horses..."

• 1661 an English pamphlet describes "Miraculous Sights seen in the Air ... by diverse persons of credit standing on London Bridge between 7 and 8 of the clock at night. ... They saw armies that clashed and abruptly vanished ..."

The Great Superstorm of 1859

In late summer 1859 a one-in-500 year storm was raging - not on Earth but on the surface of the Sun. By August 26, 1859 a huge sunspot group had formed in the low equatorial region of the Sun. On August 28 the first blast of the storm reached Earth and wreaked havoc with telegraphic systems around the world. That evening one of the greatest auroral displays in history was observed as far south as Cuba. The southern lights (aurora australis) were observed in Australia and New Zealand. Various reports spoke of a crimson red aurora so bright that one could read a newspaper by it. The storm raged for a week and it was during this storm that Richard Carrington observed the flare noted in Chapter 7.3.

Figure 7.28 shows a video clip of a less violent storm seen on the Sun in late October, 2003. The bright white regions are intense knots of UV light being emitted by flares on the Sun's surface. At the end of the clip, near the middle you will see a bright new flare occur. Had this been a storm of the magnitude of the 1859 event the economic impact on Earth would more than rival that of Hurricane Katrina with damage in tens of billions of dollars. Orbiting satellites would be rendered temporarily inoperable, the GPS system would be down and massive power failures (and collateral damage) would take place world wide. A much smaller storm in March 1989 knocked out the Quebec power grid for over 8 hours and cost Quebec Hydro more than 1 billion dollars. The 1859 and 1989 events are examples of geomagnetic storms and are part of space weather.

Figure 7.28 A series of intense flares on the Solar surface October 2003 imaged in UV-light. (Video courtesy NASA)

What causes Magnetic Storms and Aurora?

The aurora are a direct result of the interaction between:

• the sun's magnetic field
• the solar wind and coronal mass ejections
• the earth's magnetic field.

In a literal sense, the aurora tells us about what happens on the surface of the sun and the center of the earth!

Figure 7.29 is a video clip of a Coronal Mass Ejection taken with the SOHO spacecraft using a coronagraph. The central disk of the sun is blocked out in this image enabling the fainter detail of the corona to be seen. Solar flares explosively propel matter outward from the Sun at speeds in excess of 1000 km/s. If this gas collides with Earth the interaction with the Earth's magnetic field produces a geomagnetic storm. Earth's magnetic field provides us with a critical first defense against the solar wind and coronal mass ejections! The magnetic field forms a region called the magnetosphere that surrounds the earth and both deflects and traps particles from the solar wind. Figure 7.30 illustrates this. The pink "bow wave" is the solar wind being deflected around the magnetosphere. The earth's magnetic field traps some of the particles from the solar wind. These particles contribute to some of the activity that we associate with aurora but are not the main cause.
		Figure 7.29 Coronal Mass Ejection (NASA/SOHO image)

Gusts in the solar wind distort and buffet the magnetosphere and radiation belts. The solar wind acts like an electric current flowing across the magnetic field of the magnetosphere and this produces a dynamo effect - very strong electrical currents and a strong voltage potential are produced. The gusts also inject charged particles into the magnetosphere and these are accelerated down into the Earth's atmosphere producing the aurora that we see.

The charged particles follow the magnetic field of the Earth and enter the atmosphere in a place called the auroral oval as shown in Figure 7.31. During intense auroral "storms" the oval swells and sinks to lower latitudes.

	A typical auroral arc has a thickness of only a few kilometers, but can stretch vertically from its base at 100 km above the Earth to about 500 km above the Earth's surface. The entire arc can easily stretch thousands of kilometers and will fold back and forth as the magnetic field geometry changes in response to gusts in the solar wind. The interaction between the solar wind and Earth's magnetic field also produces extremely low frequency radio waves. These are radio waves that have frequencies comparable to acoustic sound waves and can be heard with radio receivers specially designed to detect such waves.
	sample of low frequency radio waves produced by aurora
Figure 7.30 The magnetosphere (Image courtesy NASA)
	During a bright auroral display the energy generated is enormous! Typical currents are tens of millions of amperes while the voltages are 40 000 to 50 000 volts. This means that a bright display rivals the power consumption of North America! Aurora also create havoc with power systems, radio communications and satellites! Aurora, if not understood and compensated for, would also create extreme corrosion problems in northern pipelines.
Figure 7.31 The Auroral Oval

Watching Aurora

Figure 7.32 Aurora near Sherwood Park, Alberta, February 11, 2004.

What produces the magnificent hues of a bright aurora? Figure 7.32 shows a bright aurora with the typical greenish colour produced by Oxygen atoms excited by particles from the solar wind at an altitude of about 100 km. The colour and brightness of aurora depend on three key factors:

1. intensity of particle flux from the solar wind hitting the atmosphere (affects brightness)
2. energy of particles (affects depth of penetration)
3. density of the atmosphere

Table 7.3 summarizes the different colours of the aurora and how they are produced. If you want to learn about how to photograph aurora follow the Take a Closer Look link below.

THEMIS-Canada

But What Produces the Colours?

Our atmosphere is 79% Nitrogen and 21% Oxygen so it comes as no surprise that Nitrogen and Oxygen are the primary agents for the aurora. What is surprising, however, is just how these atoms (and their molecular forms) actually produce the aurora. The key is that high energy electrons "beam" down into the upper atmosphere where the gas density is so low that the average time between atomic collisions is from a second or so to a few minutes. When this happens the atoms are able to produce line transitions that cannot be produced on Earth - we just don't have good enough vacuums to do this! Astronomers call these transitions forbidden line transitions or auroral line transitions. The following table summarizes this:

1. On September 4, 2011 a ground based telescope monitors a huge solar flare and coronal mass ejection. A geomagnetic storm watch is issued. When would expect to see the effects of this storm? What would be some of the things that you might notice?
2. You are science journalist for a national newspaper chain. Write a short paragraph that would describe and explain the following image:
   
   

Themis is the Greek goddess of "good counsel". Theis also means "law of nature".