Upper atmospheres and ionospheres of planets and satellites

The upper atmospheres of the planets and their satellites are more directly exposed to sunlight and solar wind particles than the surface or the deeper atmospheric layers. At the altitudes where the associated energy is deposited, the atmospheres may become ionized and are referred to as ionospheres. The details of the photon and particle interactions with the upper atmosphere depend strongly on whether the object has anintrinsic magnetic field that may channel the precipitating particles into the atmosphere or drive the atmospheric gas out to space. Important implications of these interactions include atmospheric loss over diverse timescales, photochemistry and the formation of aerosols, which affect the evolution, composition and remote sensing of the planets (satellites). The upper atmosphere connects the planet (satellite) bulk composition to the near-planet (-satellite) environment. Understanding the relevant physics and chemistry provides insight to the past and future conditions of these objects, which is critical for understanding their evolution. This chapter introduces the basic concepts of upper atmospheres and ionospheres in our solar system, and discusses aspects of their neutral and ion composition, wind dynamics and energy budget. This knowledge is key to putting in context the observations of upper atmospheres and haze on exoplanets, and to devise a theory that explains exoplanet demographics.Comment: Invited Revie

Ancient hydrothermal seafloor deposits in Eridania basin on Mars

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Jupiter's Double-arc Aurora as a Signature of Magnetic Reconnection: Simultaneous Observations from HST and JunoJupiter's Double-arc Aurora as a Signature of Magnetic Reconnection: Simultaneous Observations from HST and Juno

Jupiter’s powerful auroral emission is usually divided into the polar, main, and equatorward components. The driver of Jupiter’s main aurora is a central question for the community. Previous investigations reveal many distinct substructures on the main auroral oval, which are indicators of fundamentally different magnetospheric processes. Understanding these substructures could provide key constraints for uncovering the driver of Jupiter’s main aurora emission. In this study, we show the evolution of a double- auroral arc on the dawnside from observations by the Hubble Space Telescope (HST). Simultaneous in situ observations from the Juno spacecraft provide direct evidence of magnetic reconnection and magnetic dipolarization. By analyzing the datasets from Juno and HST, we suggest that the evolution of the double-arc structure is likely a consequence of the non-steady progress of magnetic reconnection

Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter

Io leaves a magnetic footprint on Jupiter's upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates(1-3). The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiter's magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiter's ionosphere(4-6). The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Io's magnetic footprint extends well beyond the immediate vicinity of Io's flux-tube interaction with Jupiter, and much farther than predicted theoretically(4-6); the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiter's magnetic field despite their thin atmospheres.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62861/1/415997a.pd

Long Exposure Chandra X‐Ray Observation of Jupiter's Auroral Emissions During Juno Plasmasheet Encounters in September 2021

On 15 September 2021, Chandra carried out a 40‐hr (∼4 jovian rotations) observation as part of its longest planetary campaign to study the drivers of jovian X‐ray aurora that may be linked to ultra‐low frequency (ULF) wave activity. During this time, Juno's orbit had taken the spacecraft into Jupiter's dusk magnetosphere. Here is believed to be the most probable location of ULF waves propagating along jovian magnetic field lines that drive the X‐ray auroral emissions. This is the first time that this region has been observed by an orbiter since Galileo >20 years ago, and never before has there been contemporaneous in situ and X‐ray observations. A 1D solar wind propagation model identifies a compression event near the midpoint of the 40‐hr observation window. The influence of a compression is confirmed when comparing the measured magnetic field in the dusk lobes of the magnetotail from Juno MAG data against a baseline lobe field model. Data from the Juno Waves instrument also show activation of broadband kilometric (bKOM) emissions during the arrival of the shock, a feature that has previously been observed during compression events. Therefore this is the first time we can fully analyze the morphological variability during the evolution of a shock. Wavelet transforms and Rayleigh testing are used to search for statistically significant quasi‐periodic pulsations (QPPs) of the X‐ray emissions in the data set, and find significant QPPs with periods of 25–26 min for the northern auroral X‐rays

Identifying the Variety of Jovian X-Ray Auroral Structures: Tying the Morphology of X-Ray Emissions to Associated Magnetospheric Dynamics

We define the spatial clustering of X-rays within Jupiter's northern auroral regions by classifying their distributions into “X-ray auroral structures.” Using data from Chandra during Juno's main mission observations (24 May 2016 to 8 September 2019), we define five X-ray structures based on their ionospheric location and calculate the distribution of auroral photons. The morphology and ionospheric location of these structures allow us to explore the possibility of numerous X-ray auroral magnetospheric drivers. We compare these distributions to Hubble Space Telescope (HST) and Juno (Waves and MAG) data, and a 1D solar wind propagation model to infer the state of Jupiter's magnetosphere. Our results suggest that the five sub-classes of “X-ray structures” fall under two broad morphologies: fully polar and low latitude emissions. Visibility modeling of each structure suggests the non-uniformity of the photon distributions across the Chandra intervals are likely associated with the switching on/off of magnetospheric drivers as opposed to geometrical effects. The combination of ultraviolet (UV) and X-ray morphological structures is a powerful tool to elucidate the behavior of both electrons and ions and their link to solar wind/magnetospheric conditions in the absence of an upstream solar monitor. Although much work is still needed to progress the use of X-ray morphology as a diagnostic tool, we set the foundations for future studies to continue this vital research

A pulsating auroral X-ray hot spot on Jupiter

Jupiter's X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planet's polar regions(1-3). Here we report high-spatial-resolution observations that demonstrate that most of Jupiter's northern auroral X-rays come from a 'hot spot' located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared(4-7) and ultraviolet(8) emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft(9,10). These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62624/1/4151000a.pd

Intervals of Intense Energetic Electron Beams Over Jupiter's Poles

Juno's Jupiter Energetic particle Detector Instrument often detects energetic electron beams over Jupiter's polar regions. In this paper, we document a subset of intense magnetic field‐aligned beams of energetic electrons moving away from Jupiter at high magnetic latitudes both north and south of the planet. The number fluxes of these beams are often dominated by electrons with energies above about 1 MeV. These very narrow beams can create broad angular responses in the Jupiter Energetic particle Detector Instrument with unique signatures in the detector count rates, probably because of >10 MeV electrons. We use these signatures to identify the most intense beams. These beams occur primarily above the swirl region of the polar cap aurora. This polar region is described as being of low brightness and high absorption and the most magnetically “open” at Jupiter