Pick-up-generated ion cyclotron waves around Io

¹ Space Research Insitute, Austrian Academy of Sciences, Graz, Austria
² Department of Earth, Planetary and Space Sciences, UCLA, Los Angeles, CA, USA
³ Dept. of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
⁴ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
⁵ Max Planck Institute for Solar System Research, Göttingen, Germany
⁶ Geophysics, Stanford Universtity, Palo Alto, CA, USA
⁷ Division of Space and Plasma Physics, KTH, Stockholm, Sweden
⁸ Institute of Physics, University Graz, Graz, Austria

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 July 2025
Accepted: 25 January 2026

Abstract

Context. Io, the innermost Galilean moon of Jupiter, is the main source of plasma in the Jovian magnetosphere. The neutral gas coming from the moon will get ionized through ultra-violet radiation and electron impacts. The newly created ions then get picked up by the Jovian magnetic field and start gyrating, thereby creating a ring-beam distribution in velocity space. This type of distribution is unstable with respect to the generation of ion cyclotron waves.

Aims. The aim of this study is to characterize the escaping gas from Io’s atmosphere into the Jovian magnetosphere.

Methods. The Galileo magnetometer data have been investigated for the five Io flybys that have magnetometer data available. The ion cyclotron waves can be measured with magnetometers and through spectral analysis the specific pick-up ions can be determined. Assuming that the energy of the ions in the ring-beam distribution is fully transferred to the cyclotron waves, the pick-up ion densities can be estimated for all these species.

Results. We found evidence of sulfur-bearing ions SO₃⁺, SO₂⁺, SO⁺, and S⁺, as well as either H₂S⁺ or ³⁴S⁺ (which have the same mass-to-charge ratio and cannot be discerned), and for non-sulfur-bearing ions: ³⁵Cl⁺, ³⁷Cl⁺, K⁺, and Si⁺. We also present a first plausible detection of Io-genic phosphorous through the detection of P⁺ cyclotron waves.

Conclusions. The main pick-up densities are related to SO₂⁺ and SO⁺, varying with distance from Io between ∼10⁸ and ∼10⁶ m⁻³, with the other ions exhibiting a similar variation, but their pick-up densities are lower by an order of magnitude.