Jupiter Radii Research Articles

Revealing the Local Time Structure of the Alfvén Radius and Travel Times in Jupiter's Magnetosphere

AbstractJovian magnetospheric plasma is coupled to the ionosphere through Alfvén waves. Alfvén waves enable the transport of angular momentum and energy between the planet and magnetospheric plasma, a process that ultimately generates Jupiter's bright auroral emissions. However, past the Alfvén radius, the location where the radial velocity is greater than the Alfvén velocity, magnetospheric plasma is decoupled from the planet. Alfvén waves launched by magnetospheric plasma do not reach the ionosphere, angular momentum cannot be transported from the planet, and auroral emissions should diminish. Knowledge of Jupiter's Alfvén radius location is critical for interpreting drivers of auroral emissions, in situ data, and applications of numerical models. Previous studies that determined the location of the Alfvén radius assumed an azimuthally symmetric magnetosphere and local‐time independent magnetic field. Here, we employ a statistical description of the magnetic field that includes local time effects. We find a minimum Alfvén radius of 30 (Jupiter radii) at 6 LT, with plasma decoupled from the planet in the post‐dusk through dawn sector. Furthermore, no Alfvén radius exists within 60 between 8 and 20 LT. At distances greater than 50 , the Alfvén travel time is such that magnetospheric plasma moves substantially in the magnetosphere before angular momentum can be efficiently transferred from the atmosphere. Therefore, the angular momentum supplied may no longer be sufficient for the local conditions. Our results highlight the importance of local time considerations and offer new interpretations for local time dependent auroral features, such as the polar collar.

The metal-poor atmosphere of a potential sub-Neptune progenitor

Young transiting exoplanets offer a unique opportunity to characterize the atmospheres of freshly formed and evolving planets. We present the transmission spectrum of V1298 Tau b, a 23-Myr-old warm Jupiter-sized (0.91 ± 0.05 RJ, where RJ is the radius of Jupiter) planet orbiting a pre-main-sequence star. We detect a mostly clear primordial atmosphere with an exceptionally large atmospheric scale height, and a water vapour absorption at a 5σ level of significance, from which we estimate a planetary mass upper limit (23 Earth masses, 0.12 g cm−3 at a 3σ level). This is one of the lowest-density planets discovered so far. We retrieve a low atmospheric metallicity (logZ=−0.7−0.7+0.8solar\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\log{Z}=-0.{7}_{-0.7}^{+0.8}\\,{\\mathrm{solar}}$$\\end{document}), consistent with solar/sub-solar values. Our findings challenge the expected mass–metallicity relation from core-accretion theory. Our observations can instead be explained by in situ formation via pebble accretion together with ongoing evolutionary mechanisms. We do not detect methane, which hints at a hotter-than-expected interior from just the formation entropy of this planet. Our observations suggest that V1298 Tau b is likely to evolve into a sub-Neptune.

Properties of Electrons Accelerated by the Ganymede‐Magnetosphere Interaction: Survey of Juno High‐Latitude Observations

AbstractThe encounter between the Jovian co‐rotating plasma and Ganymede gives rise to electromagnetic waves that propagate along the magnetic field lines and accelerate particles by resonant or non‐resonant wave‐particle interaction. They ultimately precipitate into Jupiter's atmosphere and trigger auroral emissions. In this study, we use Juno/JADE, Juno/UVS data, and magnetic field line tracing to characterize the properties of electrons accelerated by the Ganymede‐magnetosphere interaction in the far‐field region. We show that the precipitating energy flux exhibits an exponential decay as a function of downtail distance from the moon, with an e‐folding value of 29°, consistent with previous UV observations from the Hubble Space Telescope (HST). We characterize the electron energy distributions and show that two distributions exist. Electrons creating the Main Alfvén Wing (MAW) spot and the auroral tail always have broadband distribution and a mean characteristic energy of 2.2 keV while in the region connected to the Transhemispheric Electron Beam (TEB) spot the electrons are distributed non‐monotonically, with a higher characteristic energy above 10 keV. Based on the observation of bidirectional electron beams, we suggest that Juno was located within the acceleration region during the 11 observations reported. We thus estimate that the acceleration region is extended, at least, between an altitude of 0.5 and 1.3 Jupiter radius above the 1‐bar surface. Finally, we estimate the size of the interaction region in the Ganymede orbital plane using far‐field measurements. These observations provide important insights for the study of particle acceleration processes involved in moon‐magnetosphere interactions.

CHEOPS observations of KELT-20 b/MASCARA-2 b: An aligned orbit and signs of variability from a reflective day side

Context. Occultations are windows of opportunity to indirectly peek into the dayside atmosphere of exoplanets. High-precision transit events provide information on the spin-orbit alignment of exoplanets around fast-rotating hosts. Aims. We aim to precisely measure the planetary radius and geometric albedo of the ultra-hot Jupiter (UHJ) KELT-20 b along with the spin-orbit alignment of the system. Methods. We obtained optical high-precision transits and occultations of KELT-20 b using CHEOPS observations in conjunction with simultaneous TESS observations. We interpreted the occultation measurements together with archival infrared observations to measure the planetary geometric albedo and dayside temperatures. We further used the host star’s gravity-darkened nature to measure the system’s obliquity. Results. We present a time-averaged precise occultation depth of 82 ± 6 ppm measured with seven CHEOPS visits and 131−7+8 from the analysis of all available TESS photometry. Using these measurements, we precisely constrain the geometric albedo of KELT-20 b to 0.26 ± 0.04 and the brightness temperature of the dayside hemisphere to 2566−80+77 K. Assuming Lambertian scattering law, we constrain the Bond albedo to 0.36−0.05+0.04 along with a minimal heat transfer to the night side (ϵ = 0.14−0.10+0.13). Furthermore, using five transit observations we provide stricter constraints of 3 9 ± 1 1 deg on the sky-projected obliquity of the system. Conclusions. The aligned orbit of KELT-20 b is in contrast to previous CHEOPS studies that have found strongly inclined orbits for planets orbiting other A-type stars. The comparably high planetary geometric albedo of KELT-20 b corroborates a known trend of strongly irradiated planets being more reflective. Finally, we tentatively detect signs of temporal variability in the occultation depths, which might indicate variable cloud cover advecting onto the planetary day side.

Juno Plasma Wave Observations at Europa

AbstractJuno passed by Europa at an altitude of 355 km on 29 September, day 272, 2022. As one of Juno's in situ science instruments, the Waves instrument obtained observations of plasma waves that are essential contributors to Europa's interaction with its environment. Juno observed chorus, a band at the upper hybrid frequency providing the local plasma density, and electrostatic solitary structures in the wake. In addition, impulses due to micron‐sized dust impacts on Juno were recorded with a local maximum very close to Europa. The peak electron density near Europa was ∼330 cm−3 while the surrounding magnetospheric density was in the range of 50–150 cm−3. There was a significant separation between the Europa flyby and Juno's crossing of Jupiter's magnetic equator, enabling a unique identification of effects associated with the moon as opposed to magnetospheric phenomena normally occurring at the magnetic equator near 10 Jovian radii.

Plasma Wave and Particle Dynamics During Interchange Events in the Jovian Magnetosphere Using Juno Observations

AbstractInterchange instability is known to drive fast radial transport of particles in Jupiter's inner magnetosphere. Magnetic flux tubes associated with the interchange instability often coincide with changes in particle distributions and plasma waves, but further investigations are required to understand their detailed characteristics. We analyze representative interchange events observed by Juno, which exhibit intriguing features of particle distributions and plasma waves, including Z‐mode and whistler‐mode waves. These events occurred at an equatorial radial distance of ∼9 Jovian radii on the nightside, with Z‐mode waves observed at mid‐latitude and whistler‐mode waves near the equator. We calculate the linear growth rate of whistler‐mode and Z‐mode waves based on the observed plasma parameters and electron distributions and find that both waves can be locally generated within the interchanged flux tube. Our findings are important for understanding particle transport and generation of plasma waves in the magnetospheres of Jupiter and other planetary systems.

Eight Exoplanet Candidates in SAO Survey

Here we present eight new candidates for exoplanets detected by the transit method at the Special Astrophysical Observatory of the Russian Academy of Sciences. Photometric observations were performed with a 50-cm robotic telescope during the second half of 2020. We detected transits with depths of $\Delta m = 0.056-0.173^m$ and periods $P = 18.8^h-8.3^d$ in the light curves of stars with magnitudes of $m = 14.3-18.8^m$. All considered stars are classified as dwarfs with radii of $R_* = 0.4-0.6 R_{sun}$ (with the uncertainty for one star up to $1.1 R_{sun}$). We estimated the candidate radii (all are greater than 1.4 times the Jovian radius), semi-major axes of their orbits ($0.012-0.035 AU$), and other orbital parameters by modelling. We report the light curves with transits for two stars obtained in 2022 based on individual observations.

Study of Nonlinearity and Statistics of Photons in BTA High-Resolution Fiber-Optic Spectrograph Images

Here we present eight new candidates for exoplanets detected by the transit method at the Special Astrophysical Observatory of the Russian Academy of Sciences. Photometric observations were performed with a 50-cm robotic telescope during the second half of 2020. We detected transits with depths of $\Delta m = 0.056-0.173^m$ and periods $P = 18.8^h-8.3^d$ in the light curves of stars with magnitudes of $m = 14.3-18.8^m$. All considered stars are classified as dwarfs with radii of $R_* = 0.4-0.6 R_{sun}$ (with the uncertainty for one star up to $1.1 R_{sun}$). We estimated the candidate radii (all are greater than 1.4 times the Jovian radius), semi-major axes of their orbits ($0.012-0.035 AU$), and other orbital parameters by modelling. We report the light curves with transits for two stars obtained in 2022 based on individual observations.

Confirmation and characterisation of three giant planets detected by TESS from the FIES/NOT and Tull/McDonald spectrographs

We report the confirmation and characterisation of TOI-1820 b, TOI-2025 b, and TOI-2158 b, three Jupiter-sized planets on short-period orbits around G-type stars detected by TESS. Through our ground-based efforts using the FIES and Tull spectrographs, we have confirmed these planets and characterised their orbits, and find periods of around 4.9 d, 8.9 d, and 8.6 d for TOI-1820 b, TOI-2025 b, and TOI-2158 b, respectively. The sizes of the planets range from 0.96 to 1.14 Jupiter radii, and their masses are in the range from 0.8 to 4.4 Jupiter masses. For two of the systems, namely TOI-2025 and TOI-2158, we see a long-term trend in the radial velocities, indicating the presence of an outer companion in each of the two systems. For TOI-2025 we furthermore find the star to be well aligned with the orbit, with a projected obliquity of 9−31+33°. As these planets are all found in relatively bright systems (V ~ 10.9–11.6 mag), they are well suited for further studies, which could help shed light on the formation and migration of hot and warm Jupiters.

Dynamo Simulations of Jupiter's Magnetic Field: The Role of Stable Stratification and a Dilute Core

AbstractUnderstanding Jupiter's present‐day interior structure and dynamics is key to constraining planetary accretion models. In particular, the extent of stable stratification (i.e., non‐convective regions) in the planet strongly influences long‐term cooling processes, and may record primordial heavy element gradients from early in a planet's formation. Because the Galileo entry probe measured a subsolar helium abundance, Jupiter interior models often invoke an outer stably stratified region due to helium rain. Additionally, Juno gravity data suggest a deeper, potentially stratified dilute core extending halfway through the planet. However, fits to Jupiter's gravitational data are non‐unique, and outstanding uncertainty over the equations of state for hydrogen and helium remain. Here, we use high‐resolution numerical magnetohydrodynamic simulations of Jupiter's magnetic field to place constraints on the extent of stable stratification within the planet. We find that compared to traditional interior models, an upper stably stratified layer between 0.9 and 0.95 Jupiter radii (RJ) helps to explain both Jupiter's dipolar magnetic field and zonal winds. In contrast, an extended dilute core that is entirely stably stratified (no convective layers) yields significantly worse fits to both. However, our models with extended deep stratification still generate dipolar magnetic fields if an upper stratified region is also present. Overall, we find that a planet with a dilute core i.e., strongly stably stratified is increasingly challenging to reconcile with Jupiter's magnetic field and winds. Thus if a dilute core is present, alternative modalities such as a fully convective dilute core, a complex multilayered interior structure, or double diffusive convection may be required.

The Internal Structure and Dynamics of Jupiter Unveiled by a High‐Resolution Magnetic Field and Secular Variation Model

AbstractUnique information about the dynamo process acting at Jupiter can be inferred by modeling and interpreting its magnetic field. Using the fluxgate magnetometer measurements acquired during the 4 years of the Juno mission, we derive a magnetic field model which describes simultaneously the main field and the secular variation (SV) up to spherical harmonic degrees 16 and 8, respectively. Apart from the Earth's, this is the first time another planetary magnetic field along with its time variation is described to such a high degree. We use properties of the power spectrum of the static field to infer the upper boundary of the dynamo convective region at 0.830 ± 0.022 Jupiter radius. The SV and correlation times are relatively comparable to the Earth's and indicate that the field is dominated by advection. The field and SV morphologies suggest zonal as well as non‐zonal deep fluid motions.

Investigating the Occurrence of Magnetic Reconnection at Jupiter's Dawn Magnetopause During the Juno Era

AbstractIn this study, observations from the Jovian auroral distributions experiment, Jupiter energetic particle detector instrument, and Magnetic field investigation instruments on Juno are used to identify signatures of magnetic reconnection at Jupiter's dawn magnetopause and relate these signatures to the local plasma environment. Magnetopause crossings occurred between 73–114 Jovian radii and 4.3–6.2 magnetic local time at low latitudes. Reconnection signatures include plasma energization and ion velocity enhancements resembling reconnection jets. We test for diamagnetic suppression which considers the magnetic shear and plasma beta (β) before and after a magnetopause crossing. Findings show that a large majority of these dawn magnetopause crossings at Jupiter have a low likelihood for local magnetic reconnection (are diamagnetically suppressed) because of high delta β values and/or low magnetic shear angles across the magnetopause boundary. These crossings exhibit no evidence of local reconnection while crossings that are not diamagnetically suppressed show multiple signatures of reconnection, adhering to the Swisdak relation.

TESS Hunt for Young and Maturing Exoplanets (THYME). VI. An 11 Myr Giant Planet Transiting a Very-low-mass Star in Lower Centaurus Crux

Abstract Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15 R ⊕ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05 R J (9.5 R ⊕) planet transiting a very-low-mass star (0.170 ± 0.015 M ⊙) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5 M J. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5 R ⊕ planets.

Why hot Jupiters can be large but not too large

ABSTRACT Tidal heating is often used to interpret ‘radius anomaly’ of hot Jupiters (i.e. radii of a large fraction of hot Jupiters are in excess of 1.2 Jupiter radius which cannot be interpreted by the standard theory of planetary evolution). In this paper, we find that tidal heating induces another phenomenon ‘runaway inflation’ (i.e. planet inflation becomes unstable and out of control when tidal heating rate is above its critical value). With sufficiently strong tidal heating, luminosity initially increases with inflation, but across its peak it decreases with inflation such that heating is stronger than cooling and runaway inflation occurs. In this mechanism, the opacity near radiative-convective boundary (RCB) scales approximately as temperature to the fourth power and heat cannot efficiently radiate away from planet interior, which induces runaway inflation (similar to a tight lid on a boiling pot). Based on this mechanism, we find that radii of hot Jupiters cannot exceed 2.2RJ, which is in good agreement with the observations. We also give an upper limit for orbital eccentricity of hot Jupiters. Moreover, by comparison to the observations we infer that tidal heating locates near RCB.

An empirical model of the current sheet in Jupiter's magnetosphere

Based on the Magnetometer data from the Galileo and Juno mission, we construct an analytical ring current model that provides the azimuthal current (Jφ) distribution in radial (ρ), height (z), and local time (LT) dimensions. After binning the in-situ magnetic field data into 2 RJ ​× ​2 RJ ​× ​3 ​h (RJ is the radius of Jupiter) bins, the three-dimensional Jφ is calculated by differencing of grid values from the Maxwell equation. The modeled Jφ is obtained by fitting it to a lognormal distribution in ρ and a Gaussian distribution in z. The local time dependence is included by fitting the coefficients of the lognormal using a harmonic expansion. The results show a local time asymmetry of the ring current. Jφ is weaker and the current sheet is compressed at the noon and dusk sectors, comparing with the situation at the midnight and dawn sectors. The ring current peaks at ∼1.5MA/RJ2(2.94×10−1nA/m2) near midnight. Furthermore, the current sheet is thickest near ρ ​= ​10 RJ. The thickness decrease with ρ when ρ ​< ​35 RJ and increase slightly again when ρ ​> ​35 RJ. This empirical model of Jupiter's current sheet is more realistic than previous models.

Internal-current Lorentz-force Heating of Astrophysical Objects

Abstract Two forms of ohmic heating of astrophysical secondaries have received particular attention: unipolar-generator heating with currents running between the primary and secondary, and magnetic induction heating due to the primary’s time-varying field. Neither appears to cause significant dissipation in the contemporary solar system. But these discussions have overlooked heating derived from the spatial variation of the primary’s field across the interior of the secondary. This leads to Lorentz-force-driven currents around paths entirely internal to the secondary, with resulting ohmic heating. We examine three ways to drive such currents, by the cross product of (1) the secondary’s azimuthal orbital velocity with the nonaxially symmetric field of the primary, (2) the radial velocity (due to nonzero eccentricity) of the secondary with the primary’s field, or (3) the out-of-plane velocity (due to nonzero inclination) with the primary’s field. The first of these operates even for a spin-locked secondary whose orbit has zero eccentricity, in strong contrast to tidal dissipation. We show that Jupiter’s moon Io today could dissipate about 600 GW (more than likely current radiogenic heating) in the outer 100 m of its metallic core by this mechanism. Had Io ever been at 3 Jovian radii instead of its current 5.9, it could have been dissipating 15,000 GW. Ohmic dissipation provides a mechanism that could operate in any solar system to drive inward migration of secondaries that then necessarily comes to a halt upon reaching a sufficiently close distance to the primary.

Simultaneous UV Images and High‐Latitude Particle and Field Measurements During an Auroral Dawn Storm at Jupiter

AbstractWe present multi‐instrument Juno observations on day‐of‐year 86, 2017 that link particles and fields in Jupiter's polar magnetosphere to transient UV emissions in Jupiter's northern auroral region known as dawn storms. Juno ranged from 42°N to 51°N in magnetic latitude and 5.8–7.8 Jovian radii (1 RJ = 71,492 km) during this period. These dawn storm emissions consisted of two separate, elongated structures which extended into the nightside, rotated with the planet, had enhanced brightness (up to at least 1.4 megaRayleigh) and high color ratios. The color ratio is a proxy for the atmospheric penetration depth and therefore the energy of the electrons that produce the UV emissions. Juno observed electrons and ions on magnetic field lines mapping to these emissions. The electrons were primarily field‐aligned, bidirectional, and, at times, exhibited sudden intensity decreases below ∼10 keV coincident with intensity enhancements up to energies of ∼1,000 keV, consistent with the high color ratio observations. The more energetic electron distributions had characteristic energies of ∼160–280 keV and downward energy fluxes (∼70–135 mW m−2) that were a significant fraction needed to produce the UV emissions for this event. Magnetic field perturbations up to ∼0.7% of the local magnetic field showing evidence of upward and downward field‐aligned currents, whistler mode waves, and broadband kilometric radio emissions were also observed along Juno's trajectory during this time frame. These high‐latitude observations show similarities to those in the equatorial magnetosphere associated with dynamics processes such as interchange events, plasma injections, and/or tail reconnection.

Statistics on Jupiter’s Current Sheet With Juno Data: Geometry, Magnetic Fields and Energetic Particles

AbstractJupiter’s magnetosphere contains a current sheet of huge size near its equator. The current sheet not only mediates the global mass and energy cycles of Jupiter’s magnetosphere, but also provides a site for many localized dynamic processes, such as reconnection and wave‐particle interaction. To correctly evaluate its role in these processes, a statistical description of the current sheet is required. To this end, here we conduct statistics on Jupiter’s current sheet, by using four‐year Juno data obtained in the 20–100 Jupiter radius, 0–6 local time magnetosphere. The statistics show the thickness of the current sheet is comparable with the gyro‐radius of ions dominating the plasma pressure. Magnetic fields in the current sheet decrease in power‐law with increasing radial distances. At given energy, the flux of energetic electrons and protons increases with decreasing radial distances. On the other hand, at given radial distances, the flux decreases in power‐law with increasing energy. The flux also varies with the distances to the current sheet center. The corresponding relationship can be described by a Gaussian function peaking at the current sheet center. In addition, the statistics show the flux of oxygen‐ and sulfur‐group ions is comparable with the flux of protons at the same energy and radial distances, indicating the non‐negligible role of heavy ions in the current sheet dynamics. From these results, a statistical model is constructed, providing us with a starting point of understanding the dynamics of the whole Jupiter’s magnetosphere.

Could Low-Altitude Reconnection Power Jupiter’s Polar Aurorae?

Magnetic reconnection events less than 2 Jovian radii above the planet’s cloud tops could explain why Juno has yet to observe a source for Jupiter’s polar aurore.

Jupiter’s X-ray aurora during UV dawn storms and injections as observed byXMM–Newton, Hubble, andHisaki

ABSTRACTWe present results from a multiwavelength observation of Jupiter’s northern aurorae, carried out simultaneously by XMM–Newton, the Hubble Space Telescope (HST), and the Hisaki satellite in 2019 September. HST images captured dawn storms and injection events in the far-ultraviolet aurora several times during the observation period. Magnetic reconnection occurring in the middle magnetosphere caused by internal drivers is thought to start the production of those features. The field lines then dipolarize, which injects hot magnetospheric plasma from the reconnection site to enter the inner magnetosphere. Hisaki observed an impulsive brightening in the dawnside Io plasma torus (IPT) during the final appearance of the dawn storms and injection events, which is evidence that a large-scale plasma injection penetrated the central IPT between 6 and 9RJ (Jupiter radii). The extreme ultraviolet aurora brightened and XMM–Newton detected an increase in the hard X-ray aurora count rate, suggesting an increase in electron precipitation. The dawn storms and injections did not change the brightness of the soft X-ray aurora and they did not ‘switch-on’ its commonly observed quasi-periodic pulsations. Spectral analysis of the X-ray aurora suggests that the precipitating ions responsible for the soft X-ray aurora were iogenic and that a power-law continuum was needed to fit the hard X-ray part of the spectra. The spectra coincident with the dawn storms and injections required two power-law continua to get good fits.