A team of 41 astronomers headed by Shyeh Tjing (Cleo) Loi published a paper in the May 2015 Geophysical Research Letters announcing the first real time imaging of plasma density ducts in Earth’s upper atmosphere.
What are these plasma density ducts? How did Loi and her team image them?
What is a Plasma?
Most people are familiar with solids, liquids, and gasses as the three phases of matter. Think of plasma as a fourth phase of matter.
A plasma is a gas in which heat, light, or other energy has stripped the atoms of one or more electrons. A plasma is therefore a hot gas consisting of free electrons intermixed with atomic ions. (An ion is an atom that either is missing or has extra electrons.)
Plasma is rare near Earth’s surface, so few people are familiar with matter in this phase. However most of the matter in the universe is plasma because the Sun and other stars are made of plasma.
Most plasma on Earth is in the upper atmosphere. The ionosphere is an upper atmospheric layer containing ionized atoms. The plasmasphere is above the ionosphere, as Earth’s atmosphere transitions into interplanetary space.
Since the middle of the 20th century, scientists have thought that the plasmasphere contains cylinder shaped variations in the density of the plasma. Ducts having higher density alternate with regions having lower density. Science believed that these cylindrical density ducts followed Earth’s magnetic field lines.
There was indirect evidence that these plasma density ducts exist, but Loi’s recent work provides the first direct imaging evidence.
The Radio Telescope Loi Used
Although Loi’s work provides the first direct images of plasma density ducts, she did not image them optically. Hence there are no pictures in visible light that the human eye can detect. That is why the illustration of the density ducts above is an artist’s conception – rather than a photograph. Loi imaged the density ducts with the Murchison Widefield Array (MWA) radio telescope in western Australia, so the images are at radio wavelengths.
The Murchison Widefield Array is an array of radio telescopes with three capabilities crucial to Loi’s work. First as the name implies, astronomers designed this radio telescope to image a very large area of the sky. Most research telescopes have very small fields of view, usually much smaller than the Moon in the sky.
With the MWA, Loi imaged a region of the sky extending 30 degrees north-south and 50 degrees east-west. This extremely large field of view allowed her to image structures in Earth’s upper atmosphere that are too large for the fields of view of most telescopes.
The MWA also has high time resolution capability that allowed Loi to image rapid changes in the plasma density structure.
Finally, Loi was able to divide the radio telescopes in the array into two groups to allow her to measure the distance to the plasma density ducts, just as using both eyes allows us to judge distances.
On 15 October, 2013 Loi used the MWA to take a series of two minute snapshots of the same region of the sky nearly directly overhead. The plasma density variations act like lenses for the radio waves. By refracting (bending) the radio waves, the density variations slightly change the apparent positions of sources of celestial radio waves. Loi’s team looked at how the apparent positions of the cataloged celestial objects in the field of view changed in each of the snapshots to deduce the plasma density at different position. They imaged the plasma density ducts for each of the 46 snapshots. Comparing the snapshots told Loi how the plasma density ducts changed over the 1.5 hours she observed them.
Loi observed elongated cylinders with higher plasma density alternating with regions having a lower density plasma. These elongated cylinders of increased plasma density are the plasma density ducts.
Altitude of the Plasma Density Ducts
To measure the height of the ducts, Loi divided the MWA into two smaller arrays at different locations and used an effect astronomers call parallax. Humans use the parallax effect with two eyes to judge distances.
To understand parallax, hold your thumb in front of your face. Alternately open and close each eye and notice how the position of your thumb appears to change compared to the background.
Move your thumb closer and notice how the apparent distance your thumb moves increases. You could calculate the length of your arm by knowing the distance between your eyes and the amount your thumb appears to move, but in this case it is much easier to use a meter stick.
When a direct measurement is not possible, however, the parallax effect allows astronomers to measure distances. Loi used the two sub-arrays like two eyes. The parallax between these sub-arrays allowed her to calculate the distance to the plasma ducts.
Loi found an average altitude of 570+40 km for the plasma ducts. During the hour and a half that she made the observations the average altitude decreased from720+90 to 470+40 km. The altitude of the plasma ducts also varied with latitude.
The altitude increased from about 400 km at the southern end of the field to 1000 km at the northern end. Because the MWA is in the southern hemisphere the ducts’ altitudes are higher near the equator than the poles. This slope follows Earth’s magnetic field lines. These altitude bridge the top of the ionosphere into the plasmasphere.
Plasma Density Ducts: Confirmed
Loi’s work directly confirms the existence of plasma density ducts that scientists had long suspected exist in Earth’s upper atmosphere. It is remarkable that Loi was an undergraduate student at the University of Sydney when she did this work for her senior thesis. The Astronomical Society of Australia awarded Loi the 2015 Bok Prize.