On 19 and 20 August 2024 the Jupiter Icy Moons Explorer (Juice) executed during its cruise phase towards Jupiter a combined Lunar Earth Gravity Assist (LEGA) maneuver. These close flybys of the Moon and the Earth provided so far the best opportunity to test the behavior, performance, and calibration of the Submillimetre Wave Instrument (SWI) onboard Juice. This paper shows typical data taken during the Earth Gravity Assist and the following few days. Data quality and problems resulting from unexpected behavior of the hardware are discussed.
Using a coupled EUHFORIA–Gorgon-Space modelling chain, we simulate the magnetosphere–ionosphere coupled response to two coronal mass ejections (CMEs) that impacted Earth on 23 and 28 June 2015. The first CME (CME1), launched on 21 June, triggered a major geomagnetic storm (Kp = 8), driving strong magnetosphere–ionosphere coupling characterised by a cross-polar cap potential of 160 kV and intense field-aligned currents exceeding 23 MA, key indicators of energy transfer from the solar wind into the high-latitude ionosphere. In contrast, the faster CME2 (launched on 25 June) produced only a weak disturbance (Kp ≈ 2) due to the absence of a prolonged southward interplanetary magnetic field (IMF), resulting in minimal magnetospheric driving and ionospheric feedback. These results validate an end-to-end space-weather forecasting framework and demonstrate that CME geoeffectiveness is governed primarily by the IMF orientation and reconnection-driven magnetosphere–ionosphere energy coupling, rather than by kinematic properties such as speed alone.
During the JUICE Lunar-Earth Gravity Assist (LEGA) period in August 2024, the JUICE ultraviolet imaging spectrograph (JUICE-UVS) performed a series of observations of the Earth and Moon, detecting reflected sunlight at the Moon and emissions of atmospheric species including hydrogen, oxygen, and nitrogen at the Earth. These observations provided the first opportunity for in-flight calibration of the instrument response to extended planetary targets. They were used to refine the wavelength calibration across the full instrument bandpass, confirm accurate knowledge of the pointing of the UVS field-of-view relative to the spacecraft, and validate previous measurements of the UV effective area determined from observations of UV-bright stars. The observations performed also demonstrate the range of scientific analyses to be performed during the science phase of the mission and are useful for the development and testing of relevant mapping tools and procedures. The JUICE-UVS LEGA data confirm that the instrument is in good health and well suited to its goals of characterizing the surfaces and atmospheres of Jupiter's icy moons, mapping and monitoring Jupiter's aurora and upper atmosphere, and studying the Jupiter-Io connection.
In many studies of the electrodynamics of the coupled ionosphere-thermosphere (IT) system at high latitudes, the ionosphere is represented as a two-dimensional spherical shell and the height-integrated ionospheric Ohm's law is used to understand IT electrodynamic coupling. Thermospheric winds play a central role in IT electrodynamics, but they are generally ignored in existing empirical models and assimilative methods. While the primary issue is a lack of comprehensive wind measurements, there is also a gap in the literature on how to represent the thermospheric winds – which often exhibit strong variations with altitude – in a height-integrated description of high-latitude IT electrodynamics, and what the associated sources of error might be. Here we highlight that there is in general no single suitable definition of the neutral wind term in high-latitude, height-integrated IT electrodynamics. Instead, two neutral wind terms weighted by Hall and Pedersen conductivities appear in the height-integrated Ohm's law. Using altitude profiles of neutral winds and ionospheric conductivities respectively derived from sounding rocket chemical release experiments near Poker Flat, Alaska, and Poker Flat Incoherent Scatter Radar (PFISR) measurements, we find magnitude differences of order 10–100 m s−1 between the two neutral wind terms. The difference in magnitude increases with increasing geomagnetic activity. We show that a commonly used expression for Joule heating in terms of height-integrated quantities is a lower bound of the actual height-integrated Joule heating. We demonstrate how during geomagnetically quiet periods both the magnitude and direction of the neutral wind may influence total Joule heating, while during active periods the neutral wind influences total Joule heating primarily via its orientation relative to the plasma convection. We also find that measurements of the thermospheric wind at altitudes of ∼100–120 km are a more accurate estimate of the thermospheric wind terms in expressions of height-integrated, high-latitude electrodynamics than simply assuming the neutral winds are zero in Earth's corotating frame of reference. This points to the possible utility of, for example, Fabry–Perot interferometers that measure 557.7-nm (green-line) emissions around this altitude range.
The VISIONS-2 sounding rockets performed in-situ measurements of the active dayside auroral region. Numerous broadband dispersed signatures up to keV energies are visible in the electron electrostatic analyser data, typical of Alfvénic precipitation. In order to characterize the region where the particles are accelerated, we estimate source altitudes based on different fits of the observed energy–time dispersions. Additionally, a method based on pitch-angle–time dispersions is developed, which relaxes the assumption that all electron energies are released simultaneously. Both approaches are found to yield similar source altitudes. For most of the analysed dispersed precipitation structures, these are found to lie between 1000 and 3000 km, and increase in height for larger electron energies. Variations across events suggest differences in the plasma and/or wave conditions in the acceleration region. Finally, a comparison with previous observational studies and theoretical predictions is performed, and our estimated source altitudes are found to be generally consistent with some inertial Alfvén wave velocity profiles, particularly those associated with relatively small O+ scale heights. Overall, the results presented here provide further detail about the Alfvénic auroral acceleration region on the dayside. The developed method also opens the possibility of inferring the plasma density profiles and essential wave parameters above the spacecraft.
