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  #30a.   Aurora above Chicago

A somewhat personal account, 6 November 2001

  (Files in red–history)


26H. Birkeland, 1895

27. Aurora from Space

28. Aurora Origin

28a. Plus and Minus

29. Low Polar Orbit

30. Magnetic Storms

30.a Chicago Aurora

31. Space Weather

32. Magnetic planets

33. Cosmic Rays

34. Energetic Particles
      A concrete example sometimes helps put a concept into focus. As author and curator of this web site, a constant flow of e-mail messages arrives at my computer. This one was waiting when I came home from a late lecture on Monday, 5 November 2001:

           Funny Looking sky

    Subject: Funny looking sky.
    Date: Mon, 5 Nov 2001 21:13:14 -0600
    From: Ingrid

    Hello Dr.

      My son and I were looking up tonight, and observed something I had never seen. The sky had wide swaths of color, primarily red/orange and green. We live just north of Chicago, and I didn't think the Aurora Borealis isvisible this far south. My guess was the Van Allen belts, but preliminary research tells me that was a bad guess. Are the V belts visible? If not, what were we looking at? If you get this, and could either answer or direct us somewhere, I'd appreciate it.

    Thanks a million!


    Hello, Ingrid (and son)

  Yes, it could be the aurora--especially if what you saw was in the northern sky. There does exist an unusually high activity on the Sun, and a coronal mass ejection occurred two days ago. For details, see

  I had to wait a long time for that page to come up, so perhaps many people are accessing it... and maybe something interesting is indeed happening.
  Aurora north of Chicago, even overhead, is not unheard of. See the illustration on my web page on space weather [copied on the right]

  Is it the Van Allen belt? Yes and no. In a magnetic storm, a large number of protons and electrons (also oxygen ions) is energized and pushed earthward from the long tail of the Earth's magnetic field. The electrons, and the electric currents that accompany them, produce the aurora. The protons and electrons left over at the end are what populates the outer radiation belt. The inner belt, discovered by Van Allen, is something else again.

  The above page is one of a set of about 60 on the radiation belts etc. Their home page is reached from the bottom of that page, and they also contain one with auroral pictures. Feel free to study the whole set--you'll learn a lot about space, but it's also gonna take you a long, long time.

Here is what NOAA site predicted that day, including the plot from the satellite GOES-10, showing the spike of the X-ray flare:

3-day Solar-Geophysical Forecast issued Nov 5 at 22:00 UTC

  Solar Activity Forecast: Solar activity is expected to remain at moderate to high. Region 9684 remains capable of producing another major flare. Regions 9687 and 9690 are also capable of producing M-class events and have a slight chance of producing isolated major flare activity.

Geophysical Activity Forecast:

  The geomagnetic field is expected to be active to major storm levels on the first day of the forecast period. A coronal mass ejection (CME) from the X1 event on 04 November is expected to impact the geomagnetic field early on 06 November. Major storming is expected with isolated severe storming possible at higher latitudes. Conditions are expected to decrease to unsettled to minor storming on the second day and quiet to active on the third. However additional CME's may have been produced by several long duration flares that occurred after the X1 event. It is nearly impossible to detect these events as the LASCO imagery has been degraded by the current proton storm. If there are subsequent CME's, the geomagnetic storming could continue into the second and third day of the forecast period.

        Further e-mail, next morning

Subject: Re: Colors in the sky
Date Tue, 6 Nov 2001 08:47:27 -0600
From: Jim

Dr. Stern, Thank you!
  I'll sit down with my son (and wife) and look at the links tonight. I appreciate the time and effort you put into the response. We don't see too may heavenly phenomena in Chicago; heck, we hardly see stars. Thanks for clarifying! Son's name is Joe, he's 11, and inquisitive. It was a neat family moment; the 3 of us out front, him in a Boy Scout uniform, seeing a colored sky!


Subject: Re: Colors in the sky
Date: Tue, 06 Nov 2001 10:19:27 -0500

Hello, Jim

  Thanks for your letter--but I should caution you. It also could be a vapor release from a high altitude rocket, or from the space shuttle (if it is in orbit now--I do not know). An aurora is predominantly green, and it is likely to be constantly changing--see

  A high altitude release of barium (green) or lithium (red) will be in the eastern sky, usually within an hour or so of sunset. It will not change shape appreciably. The vapors are released to trace winds and electric fields in the high atmosphere, and to be visible they must be illuminated by sunlight, while the observer below already sees a dark sky. That is why they are seen after sunset. See also

        Belay that!

Subject: Aurora
Date: Tue, 06 Nov 2001 11:13:48 -0500

Hello, Jim, Ingrid and Joe

  Belay my last message--what you saw probably WAS an aurora. I just got a call from my son Allon, in Purcelville, Virginia, telling me about the aurora he saw around midnight, last night. He took photos of it (which he may post on the web) and said (probably based on web information) that it was seen all the way to Texas. It started red (high altitude aurora from relatively low energy electrons) and then changed to green, the usual auroral color, emitted at about 60 miles.

  I myself went out to the porch after reading your message, around 11 pm, trying to see if an aurora was visible. But the sky in Greenbelt is not as dark as in rural Virginia, and trees here obscure the sky near the northern horizon, so I saw nothing.

        Reply from Jim:

Subject: Re: Colors in the sky
Date Tue, 6 Nov 2001 09:36:37 -0600

  Thanks again Dr. Stern. For simplicity sake, I'm going to have to call it an aurora. It was mostly green, and fairly easy to understand. I can't wait to share w Joe!

        Reply from Ingrid:

Subject: Re: Aurora
Date: Tue, 6 Nov 2001 10:39:10 -0600

  Thank you for all your detailed information. I went out to walk the dog with a friend at 8:45 and experienced something that I never have before. The sky had streams of bright white lights coming from different directions with a blue/green hue in the background and with huge pockets of an orange glow in different areas. It sure was something to look at. Simply beautiful. Thanks again for all the info - Ingrid

        Further comment:

Dear Ingrid

  So much in life depends on the right timing! The way you timed taking your dog for a walk was near-perfect. I am attaching here a plot of the magnetic observatory in Kakioka, Japan, which posts its observations on the web every day at

(cited on and copied below)

  Pay attention only to the top plot, giving the strength of the northward magnetic component H, the force which pulls the magnetic needle towards a north-south orientation. The horizontal axis of the graph is in UT or universal time, the time of Greenwich, England (a suburb of London where the observatory of the Royal Astronomer used to be: now the building is a museum). If London is 5 hours ahead of us, then 0 hours (=midnight) at Greenwich means 7 pm in Greenbelt and 6 pm in Chicago.

  At 2 hours UT (8 pm Chicago time), a brief upward spike is observed. That is a "sudden commencement" marking the sudden compression of the Earth's magnetic field as it is hit by a fast-moving cloud of interplanetary gas, at the start of a magnetic disturbance. This interpretation was first proposed in 1930 by Sidney Chapman and Vincent Ferraro. The front of the cloud (actually, a shock front) moves past the Earth and the pressure spike relaxes again.

  But events have been set in motion in the Earth's magnetic tail, and ions and electrons are streaming from there into the near-Earth magnetic field, reinforcing the outer radiation belt. As a result, the northward magnetic intensity H starts dropping. The drop is not big--it bottoms out at about 250 nanotesla, less than 1% of the total northward intensity (compare to the vertical scale on the plot!). However, a lot of energy is involved, because this disturbance stretches over a region in space several times as wide as the entire Earth.

  The drop gets steep about 0.5 hours after the initial spike. Then at 8:45 Chicago time, quarter of an hour later, you take a stroll outdoors and are amazed by the colorful display in the sky. Note the time on the graph: the northward component is dropping very fast, so the radiation belt is being pumped up at a maximum rate.

At midnight Eastern time, or 5 am UT, my son watches the aurora in Virginia. His pictures (one of which is shown here) are on

though regretfully, they are all long time exposures and therefore blur-out the more intricate structure. The big initial push is over, but the magnetic disturbance is still near its peak (more accurately, bottom). Even at 0 UT the next day, the magnetic field is not yet back to normal, though it is getting close.

Cheers               David

Postscript:     No news as yet from space, except that the star-tracker on the WIND spacecraft has stopped working. The experimenters have reason to hope it recovers. (PS: It did recover)

Questions from Users:
            ***     Why has the aurora been so frequent lately?
                  ***     Can Polar Aurora be seen in Atlanta, Georgia?

Next Stop: #31  Space Weather

Last updated 25 November 2001
Re-formatted 3-13-2006