Odd auroral arc crosses rather than circling the North Pole

From the Marshall Space Flight Center Space Science News page.

May 28, 1998: For something as dynamic as the aurora borealis, acting up is normal. But even it can behave in ways that are simply odd.

A transpolar arc is barely visible in this UVI image of the aurora on Jan. 11, 1997, at 07:50:32 UT. Credit: NASA/Marshall Space Flight Center.

"It's an interesting problem that I came upon last year," explained Dr. James Spann, a solar-terrestrial physicist at NASA's Marshall Space Flight Center. Spann is also a co-investigator on the Ultraviolet Imager, a special camera aboard the Polar spacecraft. With UVI, scientists can observe the aurora borealis on the day and night side while the spacecraft is over the northern hemisphere. Today he describes unusual transpolar auroral arcs at the annual spring meeting of the American Geophysical Union in Boston. Colleagues working with him are Glynn Germany of the University of Alabama in Huntsville and George Parks of Washington University.

The aurora acts like a TV picture tube painting out images on the Earth's upper atmosphere. The electron gun in this picture tube is the magnetosphere, the immense baggy region of space where the Earth's magnetic field traps charged particles. When the magnetosphere is energized, electrons and ions are squirted from the tail region towards the magnetic poles.

Observing the aurora thus becomes a way of indirectly observing what is happening deep in the magnetosphere.

It was during a geomagnetic storm during in January 10-11, 1997, that the aurora behaved oddly.

"We observed polar arcs form from both the dusk and dawn sides and co-exist in the polar cap at the same time," Spann said. "The models we have of the magnetosphere don't explain all the features that we observed."

More than an hour later, at 09:25:36 UT, the circular aurora does not link with a faint transpolar arc. Credit: NASA/Marshall Space Flight Center.

The first such an event - called the Theta aurora - was recorded by the imager aboard the Dynamics Explorer 1 satellite in 1985. In this event, named after the Greek letter it resembled, the aurora developed a distinct bar that moved from one side of the polar region to the other.

In the event Spann saw, the aurora had two weak crossbars, one that spanned the polar region, and one that extends into it but fails to make a clear connection on the other side.

"The other thing that's very interesting is on one of these features we see energy propagating towards the dayside along the arc," Spann said. "It's not clear that we've seen that before."

It is especially curious because the polar region encircled by the aurora is normally open to space. Since the aurora is the footprint of the magnetosphere - or the image drawn by an every changing picture tube - the question arises, What is going on?

"For these polar arcs to even exist there has to be some specific conditions in the magnetosphere to let the field lines penetrate the Earth's surface through the center of the polar region," Spann said. "We are seeing a record of a dynamic event in the magnetosphere."

Just to keep him and his colleagues guessing, the aurora acted up again the next day.

"A large magnetic cloud came by in the solar wind, we saw electron precipitation at the poles, and then we saw the onset of a substorm," he said. "We saw faint remnants of a transpolar arc which then brightened. So, this storm's onset feeds the transpolar arc."

An hour later, events took a different turn.

"Then another substorm kicks in, but it's confined to the oval and ignores the transpolar arc," Spann said. "With two substorms separated by an hour, something has changed in the magnetosphere and caused these things - the storm area and the transpolar arc - not to talk to each other."

Spann said the event will take some time to unravel, but provides further evidence that far more happens deep in the magnetosphere than we suspect, or perhaps can suspect.