What’s happening in Jupiter’s upper atmosphere down at the equator?

The northern and southern lights of Jupiter are a vibrant and dynamic phenomena, generated by a complex array of mechanisms that create the most powerful aurora in the solar system. There are many aspects of the Jovian aurora which remain to be discovered – something Juno will reveal during it’s time at Jupiter – but the main auroral emission, which forms an irregular oval around the pole is thought to be well understood.

To investigate the aurora, many models have been created and most require the upper atomosphere of Jupiter’s equator to be rotating around with the planet. However, there is this other model which suggests that the upper atmosphere of Jupiter’s equatorial region is not rotating around with the planet. This model was developed to explain a strange phemonena observed in the ultraviolet, known as the bulge. When studying Jupiter with an instrument capable of observing ultraviolet light, you can observe the bulge as a bright spot fixed in a position near to Jupiter’s equator. The bulge was first discovered by Voyager on its journey out of the solar system, as it travelled past the gas giants. This is a really unusual feature, the origins of which remains a mystery… One model suggests that the bulge is caused by supersonic jets in Jupiter’s upper atmosphere colliding and causing a brightening. They think that the two jets zoom down from Jupiter’s auroral regions and collide at the equator causing Eastward and Westward jets to emerge from the collision site, which causes brightening at the bulge position.

Until now no studies have measured the velocity of equatorial ionosphere, so there was no evidence to suggest which model was correct. Our team at the Radio Space Plasma Physics group in the Department of Physics & Astronomy have recently published some work which reveals the motions of the upper atompshere at Jupiter’s equator.

To discover what the motions of Jupiter’s upper atomsphere are in the equatorial region we measured the velocity of a charged molecule called H₃⁺. Jupiter looks especially excellent in the mid-infrared, where emission mainly comes from a H₃⁺. This particular charged molecule is created from neutral hydrogen through a fast chain reaction that begins with ionisation. In the polar regions, fast electrons that have traveled from further out in Jupiter’s magnetic field, stream down the magnetic field lines and collide with hydrogen. This causes ionisation where the an electron is knocked out the neutral hydrogen molecule creating a charge molecule, or ion, called H₂⁺. This quickly reacts with some more neutral hydrogen to make H₃⁺.

Things go down a little different at the equator, where H₃⁺ is made in a slightly different way. The ionisation of the neutral hydrogen is caused by extreme ultra-violet radiation, that has travelled to Jupiter from the Sun. After ionisation the H₂⁺ quickly reacts with neutral hydrogen and makes the equatorial H₃⁺. Radiation from the sun ionises the Earth’s upper atmosphere too but unfortunately doesn’t create H₃⁺ due to different conditions in the atmosphere.

To observe H₃⁺ at Jupiter we use the NASA Infrared Telescope Facility (IRTF) at the Mauna Kea observatories in Hawaii. Unfortunately I didn’t get to visit Hawaii this time (but I have been before…), the data was collected by my supervisor and colleagues in 1998, 2007 and 2012. This telescope has an instrument, known as a spectrometer and called CSHELL, which can split up the wavelengths of light, allowing us to focus in on the mid-infrared and observe the wavelength at which H₃⁺ gives out light.

An image of Jupiter taken in the infrared wavelengths using the NSF cam which used to be at IRTF until it blew up in a liquid nitrogen related incident (no one was hurt)! The north and south aurora are both visible in this image but the north aurora is better displayed due to the configuration of the Earth and Jupiter relative to each other. This image also shows the disk emission and the H3+ at the equator. Credit: J Connerney for collecting the iamges and T Stallard for processing the image.

The tricky thing is the jets might exist higher up in Jupiter’s atmosphere than the H₃⁺ population, and it is uncertain the exact altitude at which the peak H₃⁺ emission comes from. Jupiter’s atomsphere is strongly coupled and so if there is some super strong winds high up, we might expect that to influence the layers of atomspheres below. So in this investigation measured the velocity of the H3+ ions at the equator to see if the supersonic jets exist and if there is any influence from the bulge.

The great thing about H₃⁺ is not only does it give us temperatures of Jupiter’s upper atmosphere and amazing spectral images, it also allows the velocity of the H₃⁺ ions to be calculated. This allows us to actually investigate the flows of H₃⁺ in the upper atomsphere at Jupiter’s equator to see if the supersonic jets exist and work out if there is any effect from the bulge.

Even though you may claim not the know what Doppler shift is, you actually do! It’s that effect where an ambulance zooms past you and the pitch of the siren changes. We use this exact same principal to study the velocity of the H₃⁺ ions. As they move around in Jupiter’s upper atmosphere the wavelength of the light they emit changes: when they move towards the observer the light is shifted towards the blue end of the spectrum and when they move away from the observer the light is shifted towards the red end of the spectrum. Using this we calculated the speed of the ions at Jupiter’s equator.

We found that the H₃⁺ ions were rotating around with the planet and couldn’t find any evidence of flows relating to the supersonic jets or the bulge. We also did this thing where we added up a lot of measurements to get better signal, which was a kind of average of the velocities. This showed us that the general trend of the H3+ ions was to go round with the planet. This was the first time anyone has measured the velocity of H3+ ions at the equator. Our results support the our current understanding of how the aurora is made, so the aurora modelling scientists can stay happy.Even though we didn’t find any evidence of the jets they could still be a possibility, but not at the H₃⁺ altitude, potentially existing at higher altiudes.

This is just the beginning and there’s so much more to learn about the equatorial bulge of Jupiter, which my colleagues are already investigating. With Juno orbiting Jupiter we will be able to probe the Jovian equator further. Using the ultraviolet instrument, Juno will be able to gather more information about the bulge, potentially unlocking the mysterious behind the generation of this strange feature. Juno will measure the magnetic field strength and collect plasma data with its instruments MAG and JADE. This will help us infer what is happening in Jupiter’s upper atmosphere due to the strong link between the magnetic field lines and the charged upper atomosphere of Jupiter and give us an overview of the system as a whole.

This work was recently published in Icarus: Measurements of the rotation rate of the jovian mid-to-low latitude ionosphere.