Global upper-atmospheric heating on Jupiter by the polar aurorae

The study addresses the long-standing question of why Jupiter’s upper atmosphere (thermosphere/ionosphere) is much hotter than can be explained by solar heating alone. A prevailing hypothesis is that auroral processes deposit large amounts of energy at high latitudes which are then redistributed globally. However, many global circulation models predict that strong Coriolis forces and rapid rotation confine auroral energy near the poles, implying that other heat sources (e.g., upward-propagating gravity or acoustic waves from the lower atmosphere) might be responsible. Distinguishing among these mechanisms requires high-resolution, planet-wide temperature gradients in the upper atmosphere. The authors present near-infrared spectroscopy with 2° spatial resolution from pole to equator to determine whether temperatures decrease monotonically from auroral regions toward the equator, as expected for auroral redistribution, or show localized low-latitude peaks indicative of wave heating. They find a steady decrease from auroral to equatorial latitudes and observe a transient, planetary-scale hot structure consistent with auroral energy propagation during a solar wind compression, supporting auroral redistribution as the dominant heating source.

Prior work has highlighted the Jovian thermospheric temperature anomaly since the 1970s and explored magnetosphere–ionosphere–thermosphere coupling as a potential energy source. Multiple global circulation models generally predict trapping of auroral energy at high latitudes due to Coriolis forces on a rapidly rotating planet, challenging the redistribution hypothesis. Alternative mechanisms considered include dissipation of gravity and acoustic waves from the lower atmosphere, which would produce distinct low-latitude temperature enhancements. Earlier global H3+ temperature maps lacked sufficient spatial resolution (only a few pixels spanning 45–90° of latitude), making it difficult to assess connections between auroral and low-latitude regions and suggesting equatorial temperatures similar to auroral values. Recent magnetic field models (from Juno) enable precise mapping of auroral ovals and satellite footprints, while observational studies have documented auroral precipitation and H3+ behavior (including anticorrelation between H3+ temperature and density) and radiative cooling effects. Models also suggest that transient solar wind compressions can drive equatorward heat propagation from the main auroral oval, potentially raising local temperatures by 50–175 K. This study builds upon these foundations by providing high-resolution, near-global maps to directly assess temperature gradients and investigate solar wind–driven heating events.

Observations: Jupiter was observed for approximately five hours on 14 April 2016 (04:53–10:22 UTC) and 25 January 2017 (11:36–16:28 UTC) using the 10-m Keck II telescope’s NIRSPEC near-infrared echelle spectrograph (spectral resolving power λ/δλ ≈ 25,000). The spectral slit (24″ × 0.432″; 0.144″/pixel along slit) was aligned roughly north–south along the rotation axis. As Jupiter rotated, spectral images were acquired to build longitudinal coverage. On 14 April, 115 spectral images (30 s each, six 5 s integrations), with average 2.4 min between images (≈1.4° longitude rotation). On 25 January, 80 images (60 s each, six 10 s integrations), average 3.4 min between images (≈2.3° rotation). Seeing was 0.61″ and 0.81″, respectively. Absolute calibration: Standard reduction steps included sky subtraction (to remove terrestrial atmospheric emissions), flat-fielding, dark-current subtraction, and flux calibration using standard stars (HR2250 on 14 April; HR3314 on 25 January). Spatial mapping: Multiple guider images per spectrum determined slit placement; spectra were assigned to longitude × latitude bins accounting for slit width, seeing, and tracking. Data were organized into five bin sizes (10°×10°, 8°×8°, 6°×6°, 4°×4°, 2°×2°), forming 4D arrays (longitude × latitude × spectra × overlap). Overlapping spectra in each bin were combined by taking the median per spectral channel to mitigate outliers, trading spatial resolution for SNR when necessary. H3+ retrievals: Two bright H3+ ro-vibrational lines, R(3,0) at 3.41277 μm and Q(1,0) at 3.9529 μm, with consistently high SNR across latitudes, were fitted using the MPFIT least-squares routine. Non-H3+ lines were removed where detected. Line ratios yielded column-averaged H3+ temperature; absolute line strengths provided line-of-sight column-integrated H3+ density, corrected by cos(emission angle) for viewing geometry. Total H3+ radiance (radiative cooling rate) was computed by summing modeled emission over wavelengths. Assumptions and interpretation: H3+ is assumed in quasi-local thermodynamic equilibrium with the upper atmosphere (altitudes ≈600–1,000 km), so derived H3+ temperatures represent the ambient upper-atmospheric temperatures near the H3+ density peak. Column-integrated densities are known lower bounds (underestimated by ≥20%) due to vertical gradients. Uncertainty-limited binning: For each date and parameter (temperature, density, radiance), 15 maps (five bin sizes) were generated with propagated uncertainties. Final maps were built by populating the smallest available bins meeting uncertainty thresholds: temperature and radiance ≤5% uncertainty; density ≤15%, then progressively larger bins if thresholds were unmet. Magnetic context: Using a contemporary Jovian magnetic field model, footprints of the main auroral oval (mapping to ~30 RJ in the equatorial plane) and of Io (5.9 RJ) and Amalthea (2.54 RJ) were overlaid to relate thermospheric structures to magnetospheric mapping. Solar wind context and event analysis: A solar wind propagation model estimated dynamic pressure at Jupiter near observation times. On 25 January, dynamic pressures were found to be >10× quiet levels and ~3× those on 14 April, with arrival-time uncertainties of ±1 day (April) and ±1.5 days (January), indicating a magnetospheric compression likely during the January observations. Propagating hot feature kinematics: On 25 January, a high-temperature structure equatorward of the main oval was analyzed by measuring its latitude separation from the main oval versus longitude (with longitude increasing in time due to rotation). Equatorward velocities between 180° and 260° longitude were estimated in 20° steps, with ~33 minutes between steps, yielding a median 620 m/s (range 500–1,500 m/s).

• Near-global, high-resolution H3+-derived upper-atmospheric temperature maps show a monotonic decrease from auroral to equatorial latitudes: typical temperatures drop from ~1,000 K at auroral latitudes to ~600 K near the equator on both observing dates. - H3+ column densities, enhanced by auroral particle precipitation, fall off sharply near the main auroral oval, indicating direct auroral influence ends within a few degrees of the oval. However, equatorward of the oval the temperatures remain elevated and do not drop sharply, implying heat transport away from the aurora via meridional (equatorward) winds. - During 25 January 2017, planet-wide temperatures and main-oval H3+ densities were higher than on 14 April 2016. Solar wind modeling indicates dynamic pressures >10× quiet conditions within a day of 25 January and ~3× higher than on 14 April, consistent with a global heating event due to magnetospheric compression. - A planetary-scale, high-temperature structure was observed on 25 January equatorward of the main oval, spanning ~160° in longitude. The feature consisted of relatively cool (~800 K) atmosphere surrounded by hotter (~1,000 K) auroral and sub-auroral atmosphere. Kinematic analysis indicates equatorward propagation with median velocity ~620 m/s (min 500 m/s, max 1,500 m/s), comparable to terrestrial large-scale traveling ionospheric disturbances and faster than previously reported at Saturn or modeled for Jupiter. - An anticorrelation between H3+ temperatures and densities was found in the vicinity of the main oval, consistent with higher-energy particle precipitation producing H3+ at lower, cooler altitudes and/or efficient H3+ radiative cooling of the atmosphere. - Between the equator and 30°N, median H3+ column-integrated densities were ~4×10^15 m^-2 (14 April) and ~2×10^15 m^-2 (25 January); the lower January value is consistent with a lower solar activity level (F10.7 of 82.5 SFU vs. 111.8 SFU on April 14) and with retrievals being lower bounds due to vertical gradients. - H3+ radiance correlates positively with both temperature and density, indicating the role of H3+ in radiative cooling of the thermosphere.

The observed temperature gradients—steady, planet-wide decreases from auroral regions toward the equator—provide strong, direct evidence that auroral energy is redistributed across Jupiter’s upper atmosphere, addressing the core question of the thermospheric energy budget. These gradients are inconsistent with dominant wave heating, which would appear as localized low-latitude temperature peaks, and instead support equatorward heat transport from the auroral zones. The sharp density cutoff near the main oval coupled with sustained elevated temperatures equatorward suggests that dynamics (meridional winds) transport heat more effectively than previously predicted by many global circulation models that trap energy at high latitudes. The January 2017 observations captured a global heating episode likely driven by solar wind compression of the magnetosphere, consistent with model expectations that such events can drive heat away from the main oval. The detection of a large-scale, hot, equatorward-propagating structure provides a plausible manifestation of this process, with propagation speeds similar to terrestrial traveling ionospheric disturbances and greater than those at Saturn. Anticorrelation between H3+ temperature and density near the main oval highlights the interplay between particle precipitation depth and radiative cooling, potentially explaining local relative coolness despite recent heating. These results imply that processes not fully captured in many models—such as transient forcing, enhanced coupling, or mechanisms that disrupt polar confinement—permit efficient meridional heat transport on Jupiter.

This study delivers the first planet-wide, high-resolution maps of Jupiter’s upper-atmospheric temperatures that unambiguously exhibit a monotonic decrease from auroral latitudes to the equator, demonstrating that redistributed auroral energy is the dominant heat source of the Jovian thermosphere. The capture of a planetary-scale, hot, equatorward-propagating structure during a solar wind compression event further supports magnetospheric forcing as a key driver of global heating and transport. These findings resolve a major aspect of the Jovian thermospheric energy puzzle and challenge models that confine auroral heating to the poles. Future work should: (1) identify and model the specific dynamical processes that overcome polar confinement (e.g., transient forcing, wave–mean flow interactions, ion–neutral coupling); (2) expand temporal coverage to capture variability across different solar wind conditions; (3) couple improved thermospheric models with updated magnetic mapping and ionospheric electrodynamics; and (4) obtain multi-wavelength and multi-instrument observations to constrain vertical structure and energy pathways.

• Temporal coverage is limited to two observation dates, providing snapshots rather than continuous evolution, and solar wind arrival times at Jupiter carry uncertainties (±1 day on 14 April; ±1.5 days on 25 January). - Retrievals are based on column-integrated H3+ emissions and convolve vertical structure; column densities are known lower bounds (underestimated by ≥20%) due to vertical gradients, and temperatures are weighted toward the H3+ density peak altitude. - Spatial mapping uses uncertainty-limited binning; in regions of low SNR, larger bins were required, reducing spatial resolution, and some areas lack data coverage. - Interpretation assumes quasi-local thermodynamic equilibrium between H3+ and the neutral upper atmosphere and relies on magnetic field models to place auroral ovals and satellite footprints. - The mechanism enabling efficient meridional heat transport remains unresolved; absence of known sub-auroral current systems is assumed, but unrecognized processes could contribute. - The unusual affiliation of temperature–density anticorrelation near the main oval could reflect competing effects of precipitation depth and radiative cooling, complicating local temperature interpretation.