Summary
By the end of this article, you will understand how a giant, straight aurora can appear at the same time as a small, swirling one, and what this rare event tells us about the invisible power grid in Earth’s magnetosphere.
Quick Facts
A global-scale aurora (the Transpolar Arc) and a local one (the Spiral) appeared simultaneously.
This happened during the late recovery phase of a geomagnetic substorm.
The power source for the spiral was about three orders of magnitude (1,000 times) weaker than the arc's.
The source of both auroras in the magnetotail was a long, stretched-out region, even though the spiral looked like a small spot in the sky.
Scientists needed two different supercomputer simulations to replicate the event.
The Discovery: An Unexpected Cosmic Duo
The Story begins on January 10, 1997. As Earth was recovering from a magnetic substorm, satellite images from the Polar UVI instrument captured something unusual. A massive, faint ribbon of light, a Transpolar Arc (TPA), stretched across the entire north pole. At the same time, a ground camera in Svalbard, Norway, spotted a small, bright, whirlpool-like aurora, known as an auroral spiral. This was a puzzle; these two types of aurora are usually driven by very different conditions. Using modern global MHD (magnetohydrodynamic) simulations, scientists re-created the event. Their models confirmed the Surprise: both could exist at once, but the spiral was a ghost, powered by an electrical current about 1,000 times weaker than the arc.
A global-scale transpolar arc and local-scale auroral spiral can appear simultaneously.
— Nowada et al., Key Points
The Science Explained Simply
The key concept is Field-Aligned Currents (FACs). Think of them as invisible electrical wires connecting Earth’s distant magnetotail to our upper atmosphere, carrying particles that create auroras. To Build a Fence around this idea: it’s NOT that the spiral is just a smaller version of the arc. The TPA is like a huge, stable power line, drawing steady energy from a vast region of the magnetotail. The auroral spiral, however, is like a tiny, flickering, twisted wire formed by a much weaker and more localized process. The research suggests the spiral’s source region had lower plasma density and a stronger magnetic field, which physics predicts would create a weaker current, explaining the huge power difference.
The magnetotail field-aligned current (FAC) intensity of the auroral spiral was about 3 orders of magnitude weaker than that of the TPA.
— Nowada et al., Key Points
The Aurora Connection
These two coexisting auroras act as visual reporters for the complex state of Earth’s magnetic environment. They show us that the magnetosphere isn’t just ‘on’ or ‘off’. Even during a ‘recovery’ phase, it’s a dynamic place. The TPA tells us about large-scale, slow changes in the entire magnetotail, likely related to the orientation of the solar wind’s magnetic field. The spiral, on the other hand, hints at smaller, faster processes, possibly linked to plasma waves rippling through the magnetic field lines. Observing them together provides a more complete weather report of our planet’s shield against the solar wind, revealing both the calm, large-scale fronts and the small, local eddies.
A Peek Inside the Research
This discovery relied on combining three types of Knowledge and Tools. First, historical satellite data from Polar UVI provided the global picture. Second, two powerful but different global MHD simulation codes, BATS-R-US and REPPU, were used to model the physics of the magnetosphere and ionosphere. These simulations were the only way to estimate the strength of the invisible currents. Finally, ground-based magnetometer data from the IMAGE network provided ‘ground truth’, confirming the direction of the current associated with the spiral. This synergy—linking space observations, theoretical models, and ground measurements—is how scientists unravel the complex processes that drive space weather.
A new solar wind-magnetosphere-ionosphere coupling system with minimal substorm effects is required to explain weak spiral FAC formation.
— Nowada et al., Key Points
Key Takeaways
Earth's magnetosphere can support large, stable energy flows and small, weak instabilities at the same time.
An auroral spiral can be formed by surprisingly weak field-aligned currents (FACs).
The shape of an aurora in the sky (e.g., a spot) can map to a very different shape in space (e.g., a long tail).
Computer simulations are essential tools for understanding the complex physics behind what satellites observe.
ULF (Ultra-Low-Frequency) waves in the magnetosphere might play a role in creating auroral spirals.
Sources & Further Reading
Frequently Asked Questions
Q: Why was the spiral’s current so much weaker?
A: The simulations showed the spiral’s source in the magnetotail was in a region with lower plasma density and a stronger magnetic field. Physics equations show that these conditions naturally produce a much weaker electrical current compared to the TPA’s source region.
Q: Could you see both auroras from the ground at the same time?
A: It would be extremely difficult. The auroral spiral is a small, local feature you might see if you were right underneath it. The Transpolar Arc is enormous and faint, stretching across the entire polar cap, making it very hard to see its full structure from one location.
Q: What is a geomagnetic substorm?
A: A substorm is a brief but intense disturbance in Earth’s magnetosphere that releases a huge amount of energy. This energy release causes the auroras to brighten dramatically and expand, creating the brilliant displays many people are familiar with. This event was observed after the main part of the substorm was over.
