Earth was braced for a big magnetic storm on October 25, 2003. Two days earlier a class X5 flare erupted on the Sun, and a large "Coronal Mass Ejection" seemed on its way. A headline in The Washington Post that day predicted "solar activity that can disrupt communications could last into Sunday."

On Monday, October 27, the predicted big storm hadn't materialized. Can you guess why?

[The question above was originally presented on October 27. The next two days the Sun erupted again twice, causing a pair of the most intense magnetic storms in years--but that is a different story, see end of the reply.]

Answer (as written October 27):

Yogi Berra supposedly said, "It's hard to make predictions, especially about the future." The Sun did erupt, it had big sunspots--but the interplanetary magnetic field refused to cooperate.

To understand what happened, you need know a bit about magnetic field lines ("lines of force"). You have probably seen plots of such lines near Earth, fanning out from near the southern magnetic pole and converging again near the northern one. Such imaginary lines, always following the direction in which a freely suspended magnetic needle would point, are widely used to map and visualize magnetic fields.

But in space, they have a more active role. A piece of wood splits easily along its grain, but not in the perpendicular direction. Similarly, field lines are the "grain" of magnetic fields in space. Ions and electrons in such fields slide easily along them, like beads on a wire--but not across them. And where such particles go, their energy also goes.

Other magnetic field lines, connected to the Sun rather than to Earth, fill interplanetary space. On the scale of the solar system, these may curve and bend, but the Earth is so small on that scale, that near it they essentially follow a single direction. If you wait, that direction of course changes, because (one reason) the Sun rotates and its rotation brings different lines into the region.

When the Sun sends a blast into space--as it did in the second half of October 2003--what happens at Earth depends critically on the direction of local interplanetary field lines. The closer interplanetary lines match the direction of field lines above the Earth's magnetic poles, the stronger is the linkage between Earth and interplanetary space. If they slant southwards, they usually manage to "interconnect" with magnetic field lines of Earth, creating continuous bundles of lines which connect interplanetary space to Earth. Such a "pipeline" allows energy from the blast to penetrate the Earth's immediate neighborhood, and a big magnetic storm is quite possible, with disrupted communications, and with "northern lights" in areas which rarely get to view them.

If however they slant northwards, magnetic field lines either fail to interconnect or (when sideways slants also exist) do so inefficiently. The blast then tends to slide around Earth without much linkage. The Earth's magnetic environment may get compressed or agitated, but not much energy gets into it.

Guess, which of these two possibilities happened on October 25, 2003? The direction of interplanetary field lines was almost due north, and the field was quite strong--not a usual condition. The magnetic linkage between Earth and interplanetary space was quite weak, and the days following the Sun's blast were magnetically quiet.

Other things being equal, whether interplanetary field lines slant north or south is a 50:50 proposition. Of course, the Sun is still active, and big sunspots are still out there. They will also come back as the Sun rotates. Next time the Sun lets loose a blast, the dice may roll differently, and we could get a big magnetic storm: but it is hard to make predictions, especially about the future.

Magnetic storm index Dst for the twin intense magnetic storms of the
end of October 2003. Anything below -50 nT (nanotesla) is considered a
sizable magnetic storm.

Postscript: About one solar rotation later, on November 20, the active area on the Sun again faced Earth and an even bigger storm occurred. It reached -430 nT and produced "northern light" above large parts of Europe. Credit: British Antarctic Survey

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Note added Friday 31 October 2003:

Read again that last paragraph! That "next time" happened just one day later, in the middle of Tuesday the 28th: a huge solar flare, accompanied by a burst of high energy ions, and a large coronal mass ejection (CME) observed by SOHO. Those who watched the film of the CME may have noted a "snowstorm," really radiation-caused "static" interfering with the image.

The blast took only 19 hours to reach Earth, near a record. The interplanetary magnetic field varied from slightly northward to slightly southward, but between 17 hours Universal Time (UT, Greenwich time) on the 29th and 2 UT on the 30th it was strongly southward. Earth then experienced one of the most intense magnetic storms, with the equatorial "Dst disturbance" briefly reaching 300 nT (nanotesla). That is only about 1% of the magnetic intensity at the surface of Earth--but out in space, it's enough to completely change the magnetic environment.

The recovery from this storm was remarkably fast, too, but the break did not last long, because a second eruption followed the first, about a day later--and a second storm, just as intense. Red aurora was seen around 7:30 pm above the Washington, DC area, and made the front page of "The Washington Post." This time the recovery was rapid, too, but who know what comes next? The Sun remains very active.

In case of a further disturbance, you might like to follow it using the world-wide web, which posts space data almost immediately. For Dst plots which track magnetic storms, go to: http://www.antarctica.ac.uk/SatelliteRisks/lastweek.html which gives you a choice of weekly, monthly or current data--the latter citing the value of Dst (note the color bar on the right, showing how severely disturbed the Earth environment is). The graph of the last week is shown here--black line gives observations, blue one, short-term predictions based on data. Incidentally, Dst is roughly proportional to the energy provided by the storm.

For magnetic data, go to the site of the ACE spacecraft which hangs in space between us and the Sun, at: http://sec.noaa.gov/ace/ACErtsw_home.html. That is an introductory page, click then on "dynamic plots" and choose what to look at--magnetic and plasma data, or perhaps high energy particles (click on SIS).

Also, a NOAA site gives an estimated map of the polar aurora in the northern hemisphere, at:
http://www.sec.noaa.gov/today.html then hit "Space Weather Now" If you see your location lit up, you might want go outside, to some dark location, and look towards north. Remember, though, you still need luck (auroral displays change rapidly) as well as a cloudless sky.