Observation of Electron Conics by Juno: Implications for Radio Generation and Acceleration Processes

• Published in: Geophysical research letters 2018-09, Vol.45 (18), p.9408-9416
• Main Authors: Louarn, P., Allegrini, F., McComas, D. J., Valek, P. W., Kurth, W. S., André, N., Bagenal, F., Bolton, S., Ebert, R. W., Imai, M., Levin, S., Szalay, J. R., Wilson, R. J.
• Format: Article
• Language: English
• Subjects: Acceleration; Alfven waves; Astrophysics; Cones; Conics; Cyclotrons; Earth; Earth and Planetary Astrophysics; Electric fields; Emissions; Emitters; Emitters (electron); Fluxes; Free energy; Growth rate; Instability; Instruments; Interactions; Ionosphere; Jupiter; Jupiter probes; LF electric fields; Magnetic field; Magnetic fields; Magnetohydrodynamics; Particle acceleration; Pitch (inclination); Radio; Radio emission; Radio waves; Sciences of the Universe; Space missions; Stability; Turbulence; Wave amplification; Wave generation; Wave propagation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078973

Using Juno plasma, electric and magnetic field observations (from JADE, Waves, and MAG instruments), we show that electron conic distributions are commonly observed in Jovian radio sources. The conics are characterized by maximum fluxes at oblique pitch angles, ~20°–30° from the B field, both in the upward and downward directions. They constitute an efficient source of free energy for the cyclotron maser instability. Growth rates of ~3 to 7 × 104 s−1 are obtained for hectometric waves, leading to amplification by e10 with propagation paths of 50–100 km. We show that stochastic acceleration due to interactions with a low‐frequency electric field turbulence located a few 104 km above the ionosphere may form the observed conics. A possible source of turbulence could be inertial Alfvén waves, suggesting a connection between the auroral acceleration and generation of coherent radio emissions. Plain Language Summary Jupiter, as many astrophysical magnetized objects, is a powerful emitter of nonthermal radio emissions. The coherent process required for their generation is likely the cyclotron maser instability (CMI). However, the exact conditions of wave amplification are not known precisely at Jupiter. With Juno mission, for the first time, it is possible to explore the auroral regions of Jupiter, where the particles are accelerated and the nonthermal emissions produced. With several crossing of the radio sources, the free energy used by the CMI can now be identified. It corresponds to conic‐like distributions, characterized by an accumulation of particles just outside the loss cones. Applying the CMI theory, large growth rates are obtained, showing that the conics probably play a central role in the wave generation source. The formation of the conics could be due to an interaction with a low‐frequency Alfvénic turbulence. This suggests a close relationship between the radio wave generation and the particle acceleration, as at Earth, the details of the scenario being, nevertheless, slightly different. Key Points Electron conics are observed by Juno in Jovian radio sources, and their role in the wave amplification is analyzed The observed conics may very efficiently drive the cyclotron maser, from decametric to kilometric wavelength ranges The formation of conics is modeled by a stochastic acceleration due to a low‐frequency parallel electric field turbulence