The Juno Waves instrument can be used to accurately determine the electron density inside Io’s orbit, the inner Io torus. These observations have revealed a local peak in the electron density just inside M=5 and at centrifugal latitudes above about 10º that is likely the ’cold torus’ as identified in Earth-based observations of S+ emissions. This peak or ’finger’ is separated from the more dense Io torus by a local minimum or ’trough’ at M ≥ 5. The electron densities are inferred by identifying characteristic frequencies of the plasma such as the low-frequency cutoff of Z-mode radiation at fL=0 and the low-frequency cutoff of ordinary mode radiation at fpe that depend on the electron density. The ’finger’ density ranges from about 0.2 to 65 cm-3 and decreases with increasing centrifugal latitude. The ’trough’ densities range from 0.05 to ~10 cm-3. This pattern of a density ’trough’ followed by the ’finger’ closer to Jupiter is found on repeated passes through the inner Io torus over a range of centrifugal latitudes. Using a simple model for the electron densities measured above about 10º centrifugal latitude, we’ve estimated the scale height of the ’finger’ densities as about 1.17 RJ with respect to the centrifugal equator, which is somewhat surprising given the expected cold temperature of the cold torus. The larger scale height suggests a population of light ions, such as protons, are elevated off the centrifugal equator. This is confirmed by a multi-species diffusive equilibrium model.

Juno’s highly eccentric polar orbit takes it to perijove distances of ∼ 1.06 RJ on each orbit. For the first perijove, this occurred just north of the jovigraphic equator, but has precessed north by about a degree per orbit over the mission. Minimum altitudes vary from ∼3200 to 8000 km through the mission. The Waves instrument observes a number of plasma wave modes in and near the non-auroral ionosphere that provide information on the local electron number density, including electron plasma oscillations that occur at the electron plasma frequency fpe and whistler-mode hiss which has an upper frequency limit of fpe in Jupiter’s strongly magnetized inner magnetosphere. The electron plasma frequency provides the electron number density. Over the ∼59 perijoves analyzed to date, peak densities range from ∼100 to 80,000 cm-3. More recent perijoves reveal topside ionospheric peaks at latitudes greater than about 40°. The density profiles can be highly variable from one perijove to the next. And, there can be deviations from simple smooth increases and decreases with altitude within individual ionospheric passes. Spatial variations may be responsible for some of the variability, perhaps related to Jupiter’s complex, higher order magnetic field. We show the variation in ionospheric density profiles and the distribution of peak densities as a function of latitude and System III longitude as well as other geometric parameters. In addition to the complex magnetic field, possible factors affecting ionospheric density variations investigated here are ionospheric dynamos analogous to those at Earth and precipitation of energetic particles.

Magnetic reconnection has been commonly reported between the solar wind IMF and the magnetic field of Earth and other planets. This article is the first report of a reconnection event in the tail region of Ganymede, the only known moon with an intrinsic magnetic field. We show strong evidence of Juno passing very close to an X-line at Ganymede's tail. We report the observation of distinctive electron Bernstein mode waves with unique characteristics particular to a reconnection site. We detect a clear reversal of a magnetic field component. Electron densities and pitch angle distributions also support the findings. Finally, from the time sequence of the observations by the different instruments on Juno, we reconstruct a likely trajectory of Juno around the reconnection site.

Heliophysics data analysis often involves combining diverse science measurements, many of them captured as time series. Although there are now only a few commonly used data file formats, the diversity in mechanisms for automated access to and aggregation of such data holdings can make analysis that requires inter-comparison of data from multiple data providers difficult. The Heliophysics Application Programmer’s Interface (HAPI) is a recently developed standard for accessing distributed time-series data to increase interoperability. The HAPI specification is based on the common elements of existing data services, and it standardizes the two main parts of a data service: the request interface and the response data structures. The interface is based on the REpresentational State Transfer (REST) or RESTful architecture style, and the HAPI specification defines five required REST endpoints. Data are returned via a streaming format that hides file boundaries; the metadata is detailed enough for the content to be scientifically useful, e.g., plotted with appropriate axes layout, units, and labels. Multiple mature HAPI-related open-source projects offer server-side implementation tools and client-side libraries for reading HAPI data in multiple languages (IDL, Java, MATLAB, and Python). Multiple data providers in the US and Europe have added HAPI access alongside their existing interfaces. Based on this experience, data can be served via HAPI with little or no information loss compared to similar existing web interfaces. Finally, HAPI has been recommended as a COSPAR standard for time series data delivery.

The MASER (Measuring, Analysing and Simulating Radio Emissions) project provides a comprehensive infrastructure dedicated to low frequency radio emissions (typically < 50 to 100 MHz). The four main radio sources observed in this frequency are the Earth, the Sun, Jupiter and Saturn. They are observed either from ground (down to 10 MHz) or from space (down to a few kHz). Ground observatories are more sensitive than space observatories and capture high resolution data streams (up to a few TB per day for modern instruments). Conversely, space-borne instruments can observe below the ionospheric cut-off (10 MHz) and can be placed closer to the studied object. Several tools have been developed in the last decade for sharing space physcis data. Data visualization tools developed by the CDPP (http://cdpp.eu, Centre de Données de la Physique des Plasmas, in Toulouse, France) and the University of Iowa (Autoplot, http://autoplot.org) are available to display and analyse space physics time series and spectrograms. A planetary radio emission simulation software is developed in LESIA (ExPRES: Exoplanetary and Planetary Radio Emission Simulator). The VESPA (Virtual European Solar and Planetary Access) provides a search interface that allows to discover data of interest for scientific users, and is based on IVOA standards (astronomical International Virtual Observatory Alliance). The University of Iowa also develops Das2server that allows to distribute data with adjustable temporal resolution. MASER is making use of all these tools and standards to distribute datasets from space and ground radio instruments available from the Observatoire de Paris, the Station de Radioastronomie de Nançay and the CDPP deep archive. These datasets include Cassini/RPWS, STEREO/Waves, WIND/Waves, Ulysses/URAP, ISEE3/SBH, Voyager/PRA, Nançay Decameter Array (Routine, NewRoutine, JunoN), RadioJove archive, swedish Viking mission, Interball/POLRAD… MASER also includes a Python software library for reading raw data. This work is supported by CDPP, CNES, PADC and Europlanet-2020-RI. The Europlanet 2020 Research Infrastructure project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654208.