A Physics Student’s Guide to Extra-Terrestrial Auroras

What causes auroras? 

Auroras are a phenomenon caused when highly energetic particles are released by the Sun in what is called the solar wind. When these particles approach Earth they are attracted towards the Earth’s magnetic poles and will become concentrated there. They interact with the atmosphere and excite the gas molecules, causing them to emit light in a variety of colours. But how does this phenomenon affect the other planets in our solar system?

Mercury

Mercury, being so small and so close to the Sun, does not have an atmosphere. Even though it is constantly blasted with solar radiation, with no atmosphere to interact with the particles cannot cause any auroras.

Venus

Venus does not have its own magnetic field, however that does not mean it cannot have auroras. There is a layer of Venus’ atmosphere called the ionosphere which is a layer that is composed of mostly charged particles. When solar wind reaches this layer, it is thought that it can induce a magnetic field. This was not thought possible until 2012, when ESA’s Venus Express spacecraft measured plasma in the planet’s magnetotail (see https://en.wikipedia.org/wiki/Magnetosphere) in a process called magnetic reconnection. This helped to explain dim lights seen on the planet and supports the theory.

Mars

Like Venus, Mars does not have its own magnetic field; yet it still forms auroras. Instead of the energetic particles being attracted towards the poles, on Mars there are no poles so the particles will hit wherever. It is thought that due to Mars’ carbon dioxide-heavy atmosphere, the aurorae would appear green to the observer.

Below is an image from NASA’s MAVEN spacecraft, showing the distribution of aurorae in the northern hemisphere of Mars in 2014. This is a much more widespread distribution of events than Earth.

aurora mars

Jupiter

As the largest planet, it is no surprise that Jupiter has the strongest magnetic fields and in turn some of the most impressive aurorae in the solar system. However, unlike the other planets, it is believed that they are not only governed by the solar wind. Using Japan’s Hisaki satellite, the Hubble Space Telescope and the Juno spacecraft, scientists have found that the moon Io may have an unexpected effect on the formation of aurorae. Io is the most active volcanic object in the solar system and the sulphur gas emitted is driven towards Jupiter’s polar region, amplifying the auroras.

jupiter 1.jpg

Aurora Jupiter

Saturn

Saturn experiences strong aurorae, however these can only be seen in ultraviolet light. The picture below was taken by the Hubble Space Telescope in 2004.

saturn.jpg

The picture below was taken by Cassini.

Aurora Cassini

Uranus

Spotting an aurora on Uranus is a rare event. The Hubble Space Telescope has only caught the event twice, both in 2011. Scientists tracked two large solar wind bursts to monitor their effect on the planet. It is so difficult to catch the auroras as the magnetic field of the planet is angled at 59 degrees from the axis of rotation and therefore the auroras form far away from the north and south poles.

Aurora uranusu

Neptune

The auroras on Neptune are estimated to only be half the strength of those on Earth. Like Uranus, the rotation is not inline with the magnetic field making detection difficult.

Links:

https://www.space.com/15270-auroras-uranus-hubble-telescope-photos.html

https://www.nasa.gov/mission_pages/hubble/science/uranus-aurora.html

https://saturn.jpl.nasa.gov/resources/4313/

http://hubblesite.org/newscenter/archive/releases/solar-system/jupiter/2000/38/results/100/

https://www.nasa.gov/image-feature/goddard/maven-captures-aurora-on-mars

 

 

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A Match On the Moon: Hubble’s Mind-Blowing Sensitivity

A while ago in an astrophysics class we were discussing the absolute and apparent magnitude scales for categorizing the brightness of the stars in the night sky. An interesting question came up with regards to the sensitivity of the Hubble Space Telescope; would Hubble be able to detect a match on the surface of the moon if it were an Earth distance away?


The Hubble Space Telescope can detect objects as faint as 31st magnitude [1], and for this to seem significant we have to discuss the magnitude system.

Image result for hipparchus
Hipparchus

(source; en.wikipedia.org/wiki/Hipparchus)


The Stellar Magnitude System
The first trace of such a magnitude system being used is around 130 BC by the Greek astronomer Hipparchus. He ranked the stars in order of their apparent brightness by naming the brighter ones “of the first magnitude” and the dimmest he could see “of the sixth magnitude”. This system was then picked up by Ptolemy around 140 AD to use with his own catalog of data and as astronomy became more and more popular the system was more generally used. 


It was Galileo and his invention of the telescope that first revolutionised this cataloging system. Using his telescope, he discovered that there are many stars that are unseen to the naked eye. He therefore added further orders of magnitude to the scale and it became apparent that there was no dimmest magnitude. 


In 1856, a British astronomer Norman Robert Pogson decided that this system needed to be organised and the magnitudes given formal values. He defined a 1st magnitude star as one that is around 100 times brighter than a 6th magnitude star. This created the logarithmic scale used today, with the star Vega being a reference point. 

To calculate the apparent magnitude of an object:

where f is the flux of the object.  (For more about flux read: Flux ¦ Cosmos ).
Bear in mind that the apparent magnitude is the brightness that we see here on Earth. The true brightness of the star will be different depending on how far away you are. Therefore a scale called the absolute magnitude scale was created which categorised the brightness of stars from a distance of 10 parsec away (10 parsec = 3.09 x 1017metres)


Match on the Moon


The question introduced in the astrophysics class was “Could the Hubble Space Telescope detect a match on the surface of the moon provided it had a luminosity L of 7 x 10-10erg/s?”


The relationship between flux and luminosity is:

where d is the distance between the moon and Earth (384,400km). 


Plugging in values you obtain a value of f of 3.78×10^(-32) erg/s⋅cm².

Using the equation mentioned previously we can use this flux to calculate the apparent magnitude:


So how does this compare to the Hubble Space Telescope?


The HST can detect up to the 31st magnitude and remembering that the larger magnitudes are fainter this means that the magnitude of the match is within the detection range. 


As both are around the same magnitude, the match on the moon is an excellent way of putting into perspective the staggering capabilities of our space technology.



Future


The James Webb Space Telescope, the successor to the HST is said to be able to see “10 to 100 times fainter than Hubble can see” [2].