From Earth to Ganymede: Auroras, Magnetic Fields and the JUICE Mission

Overview

A concise overview of how auroras arise from interactions between energetic particles and atmospheres, the distinct green and red emissions on Earth, and how similar processes occur on Ganymede, Jupiter's largest moon. The talk then highlights the JUICE mission, which will investigate Ganymede's magnetosphere and its potential subsurface ocean, offering insights into how planetary bodies interact with their giant neighbors.

Introduction

The lecture invites the audience on a conceptual journey from Earth to Jupiter, focusing first on aurora at Earth and then on aurora within the Jovian system, particularly around Ganymede. The speaker emphasizes that aurorae are dynamical, moving light displays that have captivated humanity for centuries, with folklore and scientific inquiry intertwined in their history.

What Is an Aurora

Auroras result when energetic particles, usually electrons or ions from the space environment, collide with atoms and molecules in a planet’s upper atmosphere. The excited species relax by emitting photons spanning from X-rays to infrared. For a true aurora, the driving energy must come from outside the atmosphere, signaling a tracer of plasma interaction with the body, whether Earth, a moon, or a planet with an atmosphere.

Earth’s Auroral Emissions

The two dominant colors are green and red, produced by emission lines of atomic oxygen. The green line sits at 557.7 nanometers, typically emitted around 100 kilometers altitude, while the red line at 630 nanometers emerges above about 200 kilometers. A violet line from nitrogen can appear at the lower boundary of the aurora. The transcript explains how spectra decompose into lines from O, N, and molecular species, revealing atmospheric composition and energy processes. The audience learns that emissions provide a spectral fingerprint of atmospheric constituents.

Observational Context and Altitude

Green emissions originate at lower altitudes than red, because the latter has a longer radiative lifetime and requires lower collision rates to emit photons. Observations from the International Space Station illustrate the spatial separation: green below red, with altitude ranges tied to atmospheric density and outdoor conditions such as cloud cover that can obscure optical aurora displays.

Solar Wind, Magnetosphere, and Space Weather

The aurora is driven by the solar wind interacting with Earth’s magnetic field. The magnetosphere redirects plasma so auroral ovals form at high latitudes. The talk links auroras to space weather and the practical impacts on communications and power systems, citing the 1989 Quebec blackout as a historical example of severe space weather effects. The solar cycle, about 11 years, modulates the likelihood of large eruptions that can push the auroral oval toward mid-latitudes, occasionally bringing auroras to London and other temperate regions.

Aurora as a Remote Sensing Tool

Auroral observations are a powerful diagnostic for the incoming particle flux and atmospheric responses. Spectroscopic measurements provide quantitative information about the energy input and atmospheric composition. In practical terms, aurora studies inform us about space weather, which can perturb satellite operations and ground-based electrical grids. The talk frames aurora as both a visual wonder and a scientific instrument for studying planetary environments.

Ganymede and the Jupiter System

The discussion then shifts to Jupiter’s moons, focusing on Ganymede. Ganymede is unique because it generates its own global magnetic field, a result that has been inferred from ultraviolet observations and magnetometer data from the Galileo mission. The presence of a magnetic field implies a differentiated interior with a possible subsurface ocean, a topic reinforced by oscillations in auroral footprints tied to magnetic interactions with the moon.

Subsurface Ocean and Induction

If a salty subsurface ocean exists, the changing magnetic environment around Ganymede can induce electric currents within the ocean, generating its own magnetic signature. Demonstrations rooted in Faraday’s law illustrate how a changing magnetic field can induce an electric field and currents in a conducting medium, providing a conceptual framework for how magnetic measurements can reveal subsurface oceans without drilling through ice.

The JUICE Mission and Future Prospects

The Jupiter Icy Moon Explorer (JUICE) is highlighted as Europe’s flagship mission to the Jovian system, with Earth and Moon gravity assists used to reach Jupiter by 2031. JUICE will explore Ganymede and its magnetosphere, employing a suite of instruments including a UV spectrograph and a magnetometer to constrain the moon’s interior structure and the salinity of any subsurface ocean. The presentation also notes the NASA Europa Clipper mission, scheduled for 2030, as a complementary effort to study other icy moons. The combined observations aim to illuminate habitability and the interplay between Jupiter and its moons, potentially revealing environments that could harbor life in subsurface oceans.

Takeaways

In summary, aurorae provide a fingerprint of atmospheric processes and space environment interactions. The Earth aurora serves as a familiar anchor, while Ganymede offers a natural laboratory for studying magnetospheric physics and the possibility of a subterranean ocean. JUICE represents a major step toward understanding how icy moons and giant planets shape each other, and how space weather can influence planetary habitability far from the Sun.