Barring the Earth's Halo
(Image of the north pole in ultraviolet taken by the satellite "Polar BEAR" on
1/11/86 0614 UT. This global picture of the aurora shows a very unusual
and debated configuration, called the "theta-aurora")
Presented bilingually. Click to toggle :
Scientific Presentation
Plain English
(Based on a paper in Science, 24 November 95)
The Earth wears a halo around each magnetic pole. The halo is visible to
the naked eye (well, at least to an astronaut above the poles) near winter
solstice, and
can be observed any time in ultraviolet (uv). This ring, called the auroral
oval, divides the Earth's magnetic field lines into two types:
- "closed" field lines which come out of the Earth and return back
into it;
- "open" field lines, near the magnetic poles, which come out of
the Earth and connect up to the solar wind.
At the interface between open and closed
field lines dramatic plasma physics processes occur, including the ring of
light forming the auroral oval (
Story 4
).
In 1982, Professor Frank of the University of Iowa, using ultraviolet
images taken from the NASA satellite DE-1, reported a dramatic effect:
inexplicably, a bar moved out from one side of the oval into the center of
the polar cap, bifurcating the oval. Many scientists, including the
present authors, were skeptical whether this effect was real. It is known
that when the interplanetary magnetic field points in the same direction
as the Earth's field (northward) the oval shrinks because fewer field
lines are connected to the solar wind
(explained in
Story 3).
As the area of open field lines shrinks, the image can look like a theta,
even though the open field line region is not bifurcated
(especially since the imagers are not particularly sensitive). In fact
there is no doubt that some cases of "theta-aurora" studied by scientists
really are of this more mundane type. Actually even Frank and co-workers
assumed that their theta-aurora occurred for northward IMF, since most really
high-latitude arcs do. Even textbooks began to say so!
We were all wrong.
In 1986 it was decided to fly a global auroral imager on the cheap.
A satellite that had been hanging for 25 years from the roof of the
Smithsonian (then called an "Oscar" satellite) was taken down, refurbished,
and equipped with a uv imager, and placed into low-Earth orbit. The image at
the top of this page, and the next two
images all come from this satellite, renamed "Polar BEAR"
These are among a series of images taken by Polar BEAR which are
unambiguous: thetas are real.
Another new set of measurements which are complementary and prove to be key
in understanding the puzzle come from the Air Force
DMSP series of satellites (which are mainly designed so that the military can
have its own private weather forecasting, but they let space scientists study
"space weather"). These satellites measure the charged particles from space
which cause aurora. The particle data from these Air Force satellites proves
that the theta in the images really do have the weird open/closed/open
configuration. (The first "spectrogram" shown below is of the particles
measured by a DMSP satellite at the same time as the image at the top of the
page.) These "spectrograms" show that the isolated arcs really do consist of
plasma from the closed field lines of the magnetotail surrounded by polar
rain.
The arrow shows the theta in the particle data; it corresponds to the bar
in the image at the top of this page. This particle data shows that the
bar is on closed field lines, surrounded by open.
Notice how much the charged particles creating the "bar" in
the center resemble those found to the left (the theta particles look like
those from the poleward portion of the nightside auroral oval)
Polar rain is the hottest part of the solar wind
(see story 3
). Therefore
we now know that a theta really is an open-closed-open configuration,
with plasma from the nightside of the Earth surrounded by field lines going
out into the solar wind!
How does it happen? It turns out that this weird situation occurs during
a re-configuration of the near-Earth space regions (the magnetosphere)
which occurs when the interplanetary magnetic field flips sign from pointing
in the same direction as the Earth's magnetic field (which is northward)
to pointing opposite to the Earth field (southward).
When the interplanetary field turns southward, flux starts opening up
to the solar wind on the dayside. The solar wind then drags these
field lines towards midnight, but they do so by convecting around the
polar cap boundary. Since the northward IMF sun-aligned arcs do not connect
up to the site where flux is being opened up, it is possible for newly
open flux to push closed arcs into the polar cap.
