Summary
Scientists have decoded why Jupiter and Saturn have two different ‘invisible’ auroras—one in ultraviolet (UV) and one in infrared (IR)—that don’t always match. The secret lies in their radically different response times: one flashes in an instant, while the other relies on slower chemistry, acting like a glowing ember.
Quick Facts
Jupiter and Saturn have auroras in ultraviolet (UV) and infrared (IR) light, invisible to the human eye.
UV auroras are created instantly when electrons strike hydrogen molecules, taking less than a hundredth of a second.
IR auroras come from the H3+ ion, which is created through a multi-step chemical process that can take from 10 seconds to several hours.
The IR aurora's slow response acts like a 'memory' of recent auroral activity.
This time difference explains why simultaneous images of UV and IR auroras can look surprisingly different.
The Discovery: Solving an Auroral Puzzle
For years, astronomers have observed the magnificent auroras on Jupiter and Saturn using telescopes that can see in ultraviolet (UV) and infrared (IR) light. But they noticed a puzzle: sometimes the UV and IR pictures would show auroras in the same place, but other times they looked completely different. Why would two types of auroras, happening at the same time, not match? Researchers led by Chihiro Tao realized the answer wasn’t in *where* the aurora was, but *when*. They built a detailed computer model to simulate the physics behind each type of emission. The model revealed that the UV aurora is like a flash of lightning—a direct, instantaneous result of an electron hitting a hydrogen molecule. The IR aurora, however, is a much more complex and slower process, giving it a unique character.
The observed differences between UV and IR emissions can be understood by the differences in their time scales.
— Chihiro Tao, Lead Researcher, ISAS/JAXA
The Science Explained Simply
Think of the two auroras like this: the UV aurora is a sprinter, while the IR aurora is a glowing ember.
The UV Sprinter: When a high-energy electron from Jupiter’s magnetosphere zips into the atmosphere, it smacks into a hydrogen molecule (H₂). This collision gives the H₂ a jolt of energy, and it releases that energy almost instantly as a flash of UV light. The whole process, from impact to flash, takes less than 0.01 seconds. It’s a direct, immediate reaction.
The IR Ember: The IR aurora starts the same way, but the electron impact is so hard it knocks an electron off the H₂, creating an H₂⁺ ion. This ion then finds another H₂ molecule and combines with it to form a new, crucial ion: H₃⁺. This chemical creation takes time. Once formed, the H₃⁺ gets heated by the surrounding atmosphere and starts to glow in infrared. Because it depends on this chemical chain, the IR aurora takes anywhere from 10 to 10,000 seconds to build up and fade away, like an ember that glows long after the initial fire has died down.
The ion chemistry, present in the IR but absent in the UV emission process, could play a key role.
— Tao, Badman, and Fujimoto
The Aurora Connection
This ‘two-speed’ system is incredibly useful for scientists. At NorthernLightsIceland.com, we know that Earth’s auroras are a direct window into the space weather hitting our planet. Jupiter’s dual auroras offer an even more detailed view. By comparing the fast UV aurora with the slow IR aurora, scientists can tell what kind of electron precipitation is happening. A sudden, short-lived UV flare with a weak IR response might mean a quick burst of electrons. But a steadily glowing IR aurora suggests a long, sustained shower of energy that has had time to build up the H₃⁺ ion population. It’s like having two different instruments to measure the same storm. This helps us understand the complex magnetic fields of giant planets and how they channel high-energy particles into their atmospheres, creating auroras far grander than our own.
A Peek Inside the Research
The researchers didn’t fly a probe into Jupiter’s aurora. Instead, they used powerful computer simulations to model every step of the process. Their model included the physics of how electrons travel through Jupiter’s hydrogen atmosphere, calculating the rates of different types of collisions. They then added a detailed ion chemistry model to track the creation and destruction of the H₃⁺ ion at different altitudes. Finally, they calculated the resulting UV and IR light emissions. To test their model, they applied it to real-life observations. For example, they simulated the Io footprint aurora—a spot of aurora caused by Jupiter’s moon Io. Their model correctly predicted that the IR glow from this fast-moving spot would be weaker than the main aurora, simply because the spot doesn’t stay in one place long enough for the H₃⁺ ’ember’ to get fully lit. This confirmed that time scales are the key to the puzzle.
Comparative UV-IR studies tell us more about the underlying mechanisms that produce the auroral features.
— Research Team
Key Takeaways
UV and IR auroras are driven by different physical processes, giving us two ways to study a planet's atmosphere.
The UV aurora is a direct snapshot of incoming electron energy, like a real-time activity monitor.
The IR aurora reflects both current activity and the recent history of energy input due to its reliance on ion chemistry.
The energy of the incoming electrons changes where the aurora forms in the atmosphere, which can make the IR aurora brighter or dimmer.
This research provides a powerful tool to diagnose the type of 'space weather' hitting Jupiter and Saturn from their magnetospheres.
Sources & Further Reading
Frequently Asked Questions
Q: Why can’t we see these auroras with our own eyes?
A: These auroras shine in ultraviolet (UV) and infrared (IR) light, which are wavelengths outside the range of human vision. We need special telescopes and cameras to capture images of them and translate them into colors we can see.
Q: What is H3+ and why is it so important?
A: H3+ is an ion made of three hydrogen atoms. It’s one of the most common ions in the universe and plays a huge role in the chemistry of gas giant atmospheres and interstellar clouds. On Jupiter and Saturn, it’s a key atmospheric coolant, radiating heat away into space as infrared light.
Q: Does Earth’s aurora have different time scales too?
A: Yes, but in a different way. Earth’s aurora is created by electrons hitting nitrogen and oxygen. The green light from oxygen is relatively fast (about 1 second), while the red light from oxygen at higher altitudes is much slower (taking up to 2 minutes to glow). So the principle of different colors having different ‘lag times’ is universal!
Q: So is the IR aurora just a ‘delayed’ version of the UV?
A: It’s more than just delayed. Because it takes time to build up and fade away, the IR aurora smooths out rapid changes. While the UV aurora might flicker wildly during a magnetic storm, the IR aurora will show a slower, more gradual brightening and dimming, reflecting the average energy over the last several minutes or hours.
