Kavli Affiliate: Melania Nynka
| First 5 Authors: Kaya Mori, Charles Hailey, Gabriel Bridges, Shifra Mandel, Amani Garvin
| Summary:
Electrons accelerated on Earth by a rich variety of wave scattering or
stochastic processes generate hard non-thermal X-ray bremsstrahlung up to >~ 1
MeV and power Earth’s various types of aurorae. Although Jupiter’s magnetic
field is an order of magnitude larger than Earth’s, space-based telescopes have
previously detected X-rays only up to ~7 keV. On the basis of theoretical
models of the Jovian auroral X-ray production, X-ray emission in the ~2-7 keV
band has been interpreted as thermal (arising from electrons characterized by a
Maxwell-Boltzmann distribution) bremsstrahlung. Here we report the observation
of hard X-rays in the 8-20 keV band from the Jovian aurorae, obtained with the
NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope
0.60+/-0.22 – a spectral signature of non-thermal, hard X-ray bremsstrahlung.
We determine the electron flux and spectral shape in the keV to MeV energy
range using coeval in situ measurements by the Juno spacecraft’s JADE and JEDI
instruments. Jovian electron spectra of the form we observe have previously
been interpreted to arise in stochastic acceleration, rather than coherent
acceleration by electric fields. We reproduce the X-ray spectral shape and
approximate flux observed by NuSTAR, and explain the non-detection of hard
X-rays by Ulysses, by simulating the non-thermal population of electrons
undergoing precipitating electron energy loss, secondary electron generation
and bremsstrahlung emission in a model Jovian atmosphere. The results highlight
the similarities between the processes generating hard X-ray auroras on Earth
and Jupiter, which may be occurring on Saturn, too.
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