Autumn is Aurora Season

by Dr.Tony Phillips (Spaceweather.com)
Sept. 4, 2016

Summer is ending in the northern hemisphere.  That’s good news for sky watchers because autumn is “aurora season.” Autumn is special in part because lengthening nights and crisp pleasant evenings tempt stargazers outside; they see things they ordinarily wouldn’t. But there’s more to it than that: autumn really does produce a surplus of geomagnetic storms–almost twice the annual average.

see captionIn fact, both spring and autumn are good aurora seasons. Winter and summer are poor. This is a puzzle for researchers because auroras are triggered by solar activity. The Sun doesn’t know what season it is on Earth–so how could one season yield more auroras than another?

Left: Geomagnetic activity from 1875 to 1927, from “Semiannual Variation of Geomagnetic Activity” by C.T. Russell and R.L. McPherron, JGR, 78(1), 92, 1973. See also this analysis by NASA solar physicist David Hathaway.

To understand the answer, we must first understand what causes auroras themselves.

Auroras appear during geomagnetic storms–that is, when Earth’s magnetic field is vibrating in response to a solar wind gust. Such gusts pose no danger to people on the ground because our magnetic field forms a bubble around Earth called the magnetosphere, which protects us. The magnetosphere is filled with electrons and protons. “When a solar wind gust hits the magnetosphere, the impact knocks loose some of those trapped particles,” explains space physicist Tony Lui of Johns Hopkins University. “They rain down on Earth’s atmosphere and cause the air to glow where they hit–like the picture tube of a color TV.”

Below: Still frames from a digital movie show how solar wind gusts rattle Earth’s magnetosphere and trigger auroras. Click to view the 750 kb Quicktime animation created by Digital Radiance, Inc.

see caption
Some solar wind gusts (“coronal mass ejections”) are caused by explosions near sunspots, others are caused by holes in the Sun’s atmosphere (“coronal holes”) that spew solar wind streams into interplanetary space. These gusts sweep past Earth year-round, which returns us to the original question: why do auroras appear more often during spring and autumn?

The answer probably involves the Sun’s magnetic field near Earth. The Sun is a huge magnet, and all the planets in the solar system orbit within the Sun’s cavernous magnetosphere. Earth’s magnetosphere, which spans about 50,000 km from side to side, is tiny compared to the Sun’s.

The outer boundary of Earth’s magnetosphere is called the magnetopause–that’s where Earth’s magnetic field bumps into the Sun’s and fends off the solar wind. Earth’s magnetic field points north at the magnetopause. If the Sun’s magnetic field tilts south near the magnetopause, it can partially cancel Earth’s magnetic field at the point of contact.

see caption“At such times the two fields (Earth’s and the Sun’s) link up,” says Christopher Russell, a Professor of Geophysics and Space Physics at UCLA. “You can then follow a magnetic field line from Earth directly into the solar wind.” Researchers call the north-south component of the Sun’s nearby magnetic field “Bz” (pronounced “Bee-sub-Zee”). Negative (south-pointing) Bz‘s open a door through which energy from the solar wind can reach Earth’s inner magnetosphere. Positive (north-pointing) Bz‘s close the door.

Above: Coronal holes spewing solar windappear as dark areas in ultraviolet and x-ray images of the Sun.

In the early 1970’s Russell and colleague R. L. McPherron recognized a connection between Bz and Earth’s changing seasons. “It’s a matter of geometry,” explains Russell. Bz is the component of the Sun’s magnetic field near Earth which is parallel to Earth’s magnetic axis. As viewed from the Sun, Earth’s tilted axis seem to wobble slowly back and forth with a one-year period. The wobbling motion is what makes Bz wax and wane in synch with the seasons.

In fact, Bz is always fluttering back and forth between north and south as tangled knots of solar magnetic field drift by Earth. What Russell and McPherron realized is that the average size of the flutter is greatest in spring and fall. When Bz turns south during one of those two seasons, it really turns south and “opens the door wide” for the solar wind.

see captionLeft: A solar wind gust triggered these bright auroras in Finland on Sept. 7, 2002. Photo credit: Martti Tenhunen. [more]

Mystery solved? Not yet. In a Geophysical Research Letter (28, 2353-2356, June15, 2001), Lyatsky et al argued that Bz and other known effects account for less than one-third of the seasonal ups-and-downs of geomagnetic storms. “This is an area of active research,” remarks Lui. “We still don’t have all the answers because it’s a complicated problem.”

But not too complicated to enjoy. Dark nights, bright stars, an occasional meteor–and the promise of Northern Lights. Perhaps scientists haven’t figured out why auroras prefer autumn, but it’s easy to understand why sky watchers do….

What lies inside Jupiter?

July 5, 2016: Jupiter’s swirling clouds can be seen through any department store telescope. With no more effort than it takes to bend over an eyepiece, you can witness storm systems bigger than Earth navigating ruddy belts that stretch hundreds of thousands of kilometers around Jupiter’s vast equator. It’s fascinating.

It’s also vexing. According to many researchers, the really interesting things–from the roots of monster storms to stores of exotic matter–are located at depth. The clouds themselves hide the greatest mysteries from view.

NASA’s Juno probe, which went into orbit on July 4,2016, could change all that. The goal of the mission is to answer the question, What lies inside Jupiter?

juno crop for ICYMI 160701

“Our knowledge of Jupiter is truly skin deep,” says Juno’s principal investigator, Scott Bolton of the SouthWest Research Institute in San Antonio, TX. “Even the Galileo probe, which dived into the clouds in 1995, penetrated no more than about 0.2% of Jupiter’s radius.”

There are many basic things researchers would like to know—like how far down does the Great Red Spot go? How much water does Jupiter hold? And what is the exotic material near the planet’s core?

Juno will lift the veil without actually diving through the clouds. Bolton explains how: “Swooping as low as 5000 km above the cloudtops, Juno will spend a full year orbiting nearer to Jupiter than any previous spacecraft. The probe’s flight path will cover all latitudes and longitudes, allowing us to fully map Jupiter’s gravitational field and thus figure out how the interior is layered.”

Jupiter is made primarily of hydrogen, but only the outer layers may be in gaseous form. Deep inside Jupiter, researchers believe, high temperatures and crushing pressures transform the gas into an exotic form of matter known as liquid metallic hydrogen–a liquid form of hydrogen akin to the slippery mercury in an old-fashioned thermometer. Jupiter’s powerful magnetic field almost certainly springs from dynamo action inside this vast realm of electrically conducting fluid.

“Juno’s magnetometers will precisely map Jupiter’s magnetic field,” says Bolton. “This will tell us a great deal about the planet’s inner magnetic dynamo and the role liquid metallic hydrogen plays in it.”

diagram of Jupiter's interior.

Juno will also probe Jupiter’s atmosphere using a set of microwave radiometers.

“Our sensors can measure the temperature and water content at depths where the pressure is 50 times greater than what the Galileo probe experienced,” says Bolton.

Jupiter’s water content is of particular interest. There are two leading theories of Jupiter’s origin: One holds that Jupiter formed more or less where it is today, while the other suggests Jupiter formed at greater distances from the sun, later migrating to its current location. (Imagine the havoc a giant planet migrating through the solar system could cause.) The two theories predict different amounts of water in Jupiter’s interior, so Juno should be able to distinguish between them—or rule out both.

Finally, Juno will get a grand view of the most powerful Northern Lights in the Solar System.

“Juno’s polar orbit is ideal for studying Jupiter’s auroras,” explains Bolton. “They are really strong, and we don’t fully understand how they are created.”

Auroras on JupiterUnlike Earth, which lights up in response to solar activity, Jupiter makes its own auroras. The power source is the giant planet’s own rotation. Although Jupiter is ten times wider than Earth, it manages to spin around 2.5 times as fast as our little planet. As any freshman engineering student knows, if you spin a magnet—and Jupiter is a very big magnet—you’ve got an electric generator. Induced electric fields accelerate particles toward Jupiter’s poles where the aurora action takes place. Remarkably, many of the particles that rain down on Jupiter’s poles appear to be ejecta from volcanoes on Io. How this complicated system actually works is a puzzle.

It’s a puzzle that members of the public will witness at close range thanks to JunoCam—a public outreach instrument modeled on the descent camera for Mars rover Curiosity. When Juno swoops low over the cloudtops, JunoCam will go to work, snapping pictures better than the best Hubble images of Jupiter.

“JunoCam will show us what you would see if you were an astronaut orbiting Jupiter,” says Bolton. “I am looking forward to that.”