Masafumi Imai

A new view of Jupiter's auroral radio spectrum

Junos first perijove science observations were carried out on 27 August 2016. The 90° orbit inclination and 4163 km periapsis altitude provide the first opportunity to explore Jupiters polar magnetosphere. A radio and plasma wave instrument on Juno called Waves provided a new view of Jupiters auroral radio emissions from near 10 kHz to ~30 MHz. This frequency range covers the classically named decametric, hectometric, and broadband kilometric radio emissions, and Juno observations showed much of this entire spectrum to consist of V-shaped emissions in frequency-time space with intensified vertices located very close to the electron cyclotron frequency. The proximity of the radio emissions to the cyclotron frequency along with loss cone features in the energetic electron distribution strongly suggests that Juno passed very close to, if not through, one or more of the cyclotron maser instability sources thought to be responsible for Jupiters auroral radio emissions.

Generation of the Jovian hectometric radiation: First lessons from Juno

Using Juno plasma and wave and magnetic observations (JADE and Waves and MAG instruments), the generation mechanism of the Jovian hectometric radio emission is analyzed. It is shown that suitable conditions for the cyclotron maser instability (CMI) are observed in the regions of the radio sources. Pronounced loss cone in the electron distributions are likely the source of free energy for the instability. The theory reveals that sufficient growth rates are obtained from the distribution functions that are measured by the JADE-Electron instrument. The CMI would be driven by upgoing electron populations at 5–10 keV and 10–30° pitch angle, the amplified waves propagating at 82°–87° from the B field, a fraction of a percent above the gyrofrequency. Typical e-folding times of 10−4 s are obtained, leading to an amplification path of ~1000 km. Overall, this scenario for generation of the Jovian hectometric waves differs significantly from the case of the auroral kilometric radiation at Earth.

Io‐Jupiter decametric arcs observed by Juno/Waves compared to ExPRES simulations

We compare observations from the Juno/Waves radio experiment with simulations of radio « arcs » in the time-frequency plane resulting from the Io-Jupiter interaction, performed with the ExPRES code. We identify the hemisphere of origin of the observed arcs directly from simulations, and confirm this identification through comparison with Juno, Nancay and Wind observations. The occurrence and shape of observed arcs are well modeled, at low latitudes with their usual shapes as seen from Earth, as well as at high latitudes with longer, bowl-shaped, arcs observed for the first time. Predicted emission is actually observed only when the radio beaming angle θ = (k,B) ≥ 70° ± 5°, providing new constraints on the generation of the decameter emission by the Cyclotron Maser Instability. Further improvements of ExPRES are outlined, that will then be applied to Juno and Earth-based observations of radio emissions induced by other Galilean satellites or associated to the main auroral oval.

Plasma waves in Jupiter's high‐latitude regions: Observations from the Juno spacecraft

The Juno Waves instrument detected a new broadband plasma wave emission (~50 Hz to 40 kHz) on 27 August 2016 as the spacecraft passed over the low-altitude polar regions of Jupiter. We investigated the characteristics of this emission and found similarities to whistler mode auroral hiss observed at Earth, including a funnel-shaped frequency-time feature. The electron cyclotron frequency is much higher than both the emission frequency and local plasma frequency, which is assumed to be ~20–40 kHz. The E/cB ratio was about three near the start of the event and then decreased to one for the rest of the period. A correlation of the electric field spectral density with the flux of an upgoing 20 to 800 keV electron beam was found, with a correlation coefficient of 0.59. We conclude that the emission is propagating in the whistler mode and is driven by the energetic upgoing electron beam.

Latitudinal beaming of Jovian decametric radio emissions as viewed from Juno and the Nançay Decameter Array

Two well-defined Jovian decametric radio arcs were observed at latitudinal separations of 11 ∘ –16 ∘ from the Juno spacecraft near Jupiter and the Nancay Decameter Array (NDA) at Earth on 17 May and 25 August 2016. These discrete arcs are from the so-called A source covering both Io-related and non-Io-related emissions. By measuring the wave arrival time at two distant observers with propagation time correction, the remaining delay times are 92.8 ± 1.3 min for the first arc and 116.0 ± 1.2 min for the second arc. This implies that both radio sources are not controlled by the orbital motion of Io but Jupiters rotation itself. The geometrical information for Juno and NDA and the loss cone-driven electron cyclotron maser instability theory provide these radio sources that are located at about 173 ∘ ± 10 ∘ in system III longitude projected onto Jupiters north surface and imply resonant electron energy ranges from 0.5 to 11 keV.

Statistical study of latitudinal beaming of Jupiter's decametric radio emissions using Juno†

Synoptic decametric (DAM) radio observations at Jupiter were made in a broad Jovicentric latitudinal range of −21∘ to +15∘ by the Juno polar orbiting spacecraft from 21 June to 10 December, 2016. We investigated the occurrence probability of non-Io-related DAM. At 19.5 MHz, as Junos latitude varies from +15∘ to −21∘, a peak of non-Io-B occurrence probability at 175∘ System III central meridian longitude (CML) gradually shifts in longitude to 140∘ CML. Also, another peak occurs at 110∘ CML between −15∘ and −9∘, merging into the bottom edge of the former peak. This J-shaped feature is similarly seen at 16.5 MHz. Using the Jovian magnetic field models, the fixed hollow cone model can reasonably account for the J-shaped structure for radio sources traced along active magnetic flux tubes onto Jupiters surface projected at about 135∘–149∘ System III longitude. Moreover, these non-Io-B spectral profiles extend from 13.5 to 23.5 MHz.

Prevalent lightning sferics at 600 megahertz near Jupiter’s poles

Lightning has been detected on Jupiter by all visiting spacecraft through night-side optical imaging and whistler (lightning-generated radio waves) signatures1–6. Jovian lightning is thought to be generated in the mixed-phase (liquid–ice) region of convective water clouds through a charge-separation process between condensed liquid water and water-ice particles, similar to that of terrestrial (cloud-to-cloud) lightning7–9. Unlike terrestrial lightning, which emits broadly over the radio spectrum up to gigahertz frequencies10,11, lightning on Jupiter has been detected only at kilohertz frequencies, despite a search for signals in the megahertz range12. Strong ionospheric attenuation or a lightning discharge much slower than that on Earth have been suggested as possible explanations for this discrepancy13,14. Here we report observations of Jovian lightning sferics (broadband electromagnetic impulses) at 600 megahertz from the Microwave Radiometer15 onboard the Juno spacecraft. These detections imply that Jovian lightning discharges are not distinct from terrestrial lightning, as previously thought. In the first eight orbits of Juno, we detected 377 lightning sferics from pole to pole. We found lightning to be prevalent in the polar regions, absent near the equator, and most frequent in the northern hemisphere, at latitudes higher than 40 degrees north. Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet16,17, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles9,16,18. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.Observations of broadband emission from lightning on Jupiter at 600 megahertz show a lightning discharge mechanism similar to that of terrestrial lightning and indicate increased moist convection near Jupiter’s poles.

Jupiter Lightning‐Induced Whistler and Sferic Events With Waves and MWR During Juno Perijoves

During the Juno perijove explorations from 27 August 2016 through 1 September 2017, strong electromagnetic impulses induced by Jupiter lightning were detected by the Microwave Radiometer (MWR) instrument in the form of 600-MHz sferics and recorded by the Waves instrument in the form of Jovian low-dispersion whistlers discovered in waveform snapshots below 20 kHz. We found 71 overlapping events including sferics, while Waves waveforms were available. Eleven of these also included whistler detections by Waves. By measuring the separation distances between the MWR boresight and the whistler exit point, we estimated the distance whistlers propagate below the ionosphere before exiting to the magnetosphere, called the coupling distance, to be typically one to several thousand of kilometers with a possibility of no subionospheric propagation, which gives a new constraint on the atmospheric whistler propagation. Plain Language Summary Lightning at Jupiter produces a strong electromagnetic impulse, which can escape the Jovian atmosphere and enter the inner magnetosphere. Among the lightning, microwave-frequency sferics come from lightning spots, and audio-frequency whistlers propagate away from the spots below the ionosphere. If certain plasma conditions are met, these whistlers can leak into the magnetosphere. Estimates of whistler propagation distances at the planet have not been previously performed. Since the arrival at Jupiter on 5 July 2016, the Juno spacecraft has provided the opportunity to monitor the two kinds of lightning activity with two onboard instruments during its closest approach to Jupiter. This opportunity happens every 53.6 day in the eccentric, polar orbit of Juno. Using data collected during Juno’s closest approaches to Jupiter, the whistler propagation distance was estimated to be approximately one to several thousand kilometers, which may be comparable to the terrestrial equivalent. This new approach provides the benefit of understanding multidimensional structures of lightning at Jupiter.

Direction-finding measurements of Jovian low-frequency radio components by Juno near Perijove 1: JUNO DF STUDY FOR BKOM, ECR, AND NKOM

With the aid of the radio and plasma wave (Waves) instrument onboard the Juno spacecraft, the first scientific close encounter to Jupiter (Perijove 1) of Juno led to an opportunity to perform direction finding measurements of the intense Jovian broadband kilometric (bKOM) radiation at 10 to 142 kHz, two escaping continuum radiation (ECR) events at 9 to 22 kHz, and two narrowband kilometric (nKOM) radiation events at 45–112 kHz. We conclude that the northern bKOM radio sources are localized on M-shell=50–60 field lines where M-shell is similar to L-shell for non-dipolar fields. The beam cone half-angle varies from 40∘ to 55∘. By intersecting the wave k vector with the Jovian centrifugal equator, two ECR sources are located inside and outside of 11–12 RJ, and two nKOM sources are found between 11 and 20 RJ. These source frequencies and locations can be used for plasma diagnostics in Jupiters inner magnetosphere.