The southern auroral oval. Image from the Space Shuttle Discovery. Credit: NASA, Astronaut Overmeyer and Dr. Hallinan.
 
 

Then, rising with Aurora's light,
The Muse invoked, sit down to write;
Blot out, correct, insert, refine,
Enlarge, diminish, interline.

--- Jonathan Swift

On July 16 and then again on August 9, 2000, the European Space Agency (ESA) launched a small squadron of four spacecraft called Cluster II from the Baikanor Cosmodrome in Kazakhstan. The individual satellites were given the names Rumba, Salsa, Samba, and Tango.

The Cluster II spacecraft were originally scheduled to spend two years investigating the interaction between the Sun and our own world. However, ESA extended the mission until 2009 and then again until 2012. The four probes were designed to explore the layers and boundaries that exist within Earth's magnetosphere, as well as monitor the magnetic poles and the magnetotail. They constantly cross the intersection that delineates Earth's magnetic bubble from the solar wind.

Discovering the interaction between the solar wind and the magnetosphere is Cluster's primary goal. The identical spacecraft are flying a tetrahedral formation in a highly elliptical orbit. Cluster is currently collecting information about how charged particles from the Sun enter the Earth's atmosphere through magnetic "funnels" at each pole.

Earth's complex, electrically active magnetotail (or plasma tail) extends for millions of kilometers, always pointing away from the Sun. The solar wind is captured by the magnetosphere, and along with ions ejected by the Earth, collects in a plasma sheet within the magnetotail that is held together by our planet's magnetic field.

Between 1902 and 1903, Kristian Birkeland's Norwegian Aurora Polaris Expedition found electric currents from the Aurora Borealis flowing parallel to the polychromatic displays. Since electric currents must complete a circuit, and because the aurora seemed to be influenced by energetic events beyond the atmosphere, he wrote that those currents probably flow down from space at one end of the auroral arc and back out to space at the other.

Toroids of charged particles, otherwise known as "plasma," surround Earth, confined by its magnetic field. They are commonly called "Van Allen radiation belts" after Dr. James Van Allen, whose Explorer I and Explorer 3 missions confirmed Birkeland's theoretical conclusions. Similar structures exist around other planets.

Plasma contains charged particles. They can be negatively charged electrons, positively charged ions, or dust particles that have an excess of either positive or negative charge. Neon signs are plasma. Lightning is plasma. Earth’s magnetosphere, the solar wind, and the Sun itself are plasma. Nebulae in space, referred to most often as "hot gas and dust," are plasma. Dark interstellar clouds, mostly composed of hydrogen, are plasma because they exhibit magnetic fields and radiate in radio wavelengths.

Solar ions travel along Earth's magnetic field into the poles, causing atmospheric molecules to emit light: red from oxygen at high altitudes, green from oxygen lower down, and blue from nitrogen. Strong electromagnetic disturbances are observed when a bright aurora is seen.

Recently, ESA announced that Cluster has successfully measured the acceleration of ions as they enter Earth's environment. Although ESA mission specialists speculate that the aurora are created by so-called "magnetic reconnection" phenomena, previous Picture of the Day articles have demonstrated why that explanation is incorrect.

Conventional theories state that Earth's magnetosphere deforms like a teardrop because it is bombarded by the Sun's solar wind. The field is compressed on the sunward side and stretched on the dark side. Magnetic field lines are said to "flap like a flag waving in the wind."

When magnetic field lines cross and "reconnect" (through an unexplained mechanism), they are said to "detonate" with bursts of heat, light and electricity. The power from those "magnetic explosions" is thought to flow down into Earth's poles, energizing auroral colors.

Magnetic field lines cannot stretch, compress, or flap because they are no more "real" than lines of latitude and longitude: they are character symbols. A circuit schematic is no more a circuit board than magnetic field lines are a magnetic field. To say that magnetic field lines can cross, or flap, or break and reconnect is tantamount to saying that weather diagrams can produce rain.

"The knowledge gained since 1896, in radioactivity has favoured the view to which I gave expression in that year, namely, that magnetic disturbances on the earth, and aurora borealis, are due to corpuscular rays emitted by the Sun."

--- Kristian Birkeland "The Norwegian Aurora Polaris Expedition 1902-1903, Volume 1, Part 1"

Stephen Smith

Hat tip to James Parker