1. Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
2. Jet Propulsion Laboratory, Pasadena, California 91109, USA
3. University of Michigan, Ann Arbor, Michigan 48109, USA
4. Institut d'Astrophysique de Paris–CNRS, 98 bis Blvd Arago, Paris 75014, France
5. NASA Goddard Space Flight Center, Greenbelt, Maryland 2077, USA
6. Université de Liège, 5 Avenue de Cointe, B4000 Liège, Belgium
7. Southwest Research Institute, Culebra Road, San Antonio, Texas 78228, USA
8. University of Colorado, Boulder, Colorado 80303, USA

Correspondence to: J. T. Clarke¹ Correspondence and requests for materials should be addressed to J.T.C. (e-mail: Email: jclarke@bu.edu).

Abstract

Io leaves a magnetic footprint on Jupiter's upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates^{1, 2, 3}. The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiter's magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiter's ionosphere^{4, 5, 6}. The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Io's magnetic footprint extends well beyond the immediate vicinity of Io's flux-tube interaction with Jupiter, and much farther than predicted theoretically^{4, 5, 6}; the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiter's magnetic field despite their thin atmospheres.