We present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low-activity comet at a heliocentric distance of 2.8 AU on April 19th, 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualisation method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well-known partial rings of cometary pickup ions at high-activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy-angle dispersion and manifest as partial rings in velocity space.
Surface charging properties of a non-conducting surface that has a deep cavity and is in contact with the solar wind plasma are investigated by means of the particle-in-cell plasma simulations. The modeled topography is intended with a portion of irregular surfaces found on solid planetary bodies. The simulations have revealed unconventional charging features in that the cavity bottom is charged up to positive values even without any electron emission processes such as photoemission, provided that the surface location is accessible to a portion of incoming solar wind ions. The major driver of the positive charging is identified as drifting ions of the solar wind plasma, and an uncommon current ordering where ion currents exceed electron currents is established at the innermost part of the deep cavity. This also implies that the cavity bottom surface may have a positive potential of several hundred volts, corresponding to the kinetic energy of the ions. The present study also clarifies the role of photoelectrons in developing the distinctive charging environment inside the cavity. The photoemitted electrons can no longer trigger positive charging at the cavity bottom, but rather exhibit the effect of relaxing positive potentials caused by the solar wind ions. The identified charging process, which are primarily due to the solar wind ions, are localized at the depths of the cavity and may be one possible scenario for generating intense electric fields inside the cavity.
We present a statistical study of Jupiter’s disk X-ray emissions using 19 years of Chandra X-Ray Observatory (CXO) observations. Previous work has suggested that these emissions are consistent with solar X-rays elastically scattered from Jupiter’s upper atmosphere. We showcase a new Pulse Invariant (PI) filtering method that minimises instrumental effects which may produce unphysical trends in photon counts across the nearly-two-decade span of the observations. We compare the CXO results with solar X-ray flux data from the Geostationary Operational Environmental Satellites (GOES) X-ray Sensor (XRS) for the wavelength band 1-8 Å (long channel), to quantify the correlation between solar activity and jovian disk counts. We find a statistically significant Pearson’s Correlation Coefficient (PCC) of 0.9, which confirms that emitted jovian disk X-rays are predominantly governed by solar activity. We also utilise the high spatial resolution of the High Resolution Camera Instrument (HRC-I) on board the CXO to map the disk photons to their positions on Jupiter’s surface. Voronoi tessellation diagrams were constructed with the JRM09 (Juno Reference Model through Perijove 9) internal field model overlaid to identify any spatial preference of equatorial photons. After accounting for area and scattering across the curved surface of the planet, we find a preference of jovian disk emission at 2-3.5 Gauss surface magnetic field strength. This suggests that a portion of the disk X-rays may be linked to processes other than solar scattering: the spatial preference associated with magnetic field strength may imply increased precipitation from the radiation belts, as previously postulated.
The magnetometer of the InSight mission operated on the martian surface from November 2018 until May 2022. Previously, satellites have provided information on the martian magnetic field environment from orbit, however, the degree to which external fields penetrate to and interact with the surface could not be studied prior to the InSight landing. Here, we present an overview of the complete surface magnetic field data from InSight sols 14 to 1241 that display different external magnetic field phenomena, transient and periodic. Periodic observations range from short period waves (100s-1000s of seconds), diurnal variations, ~26 sol Carrington rotations, to seasonal fluctuations. Transient events are observed in response to space weather and dust movement. We find that ionospheric variations are the dominant contribution as seen from the surface, while contributions from the undisturbed IMF are more subtle. We discuss limitations associated with a single point measurement and opportunities that future missions could enable. Including magnetometers on future missions at a variety of locations for long-duration continuous observations will be of great value in understanding a range of external field phenomena and will enable further investigations in different crustal magnetic field settings.
We investigate the magnetic fabrics of Impact melt breccia at the Dhala impact structure to understand its emplacement mechanism. Our results show that the pseudo-single domains of Ti-poor magnetite and Ti-hematite are the prime magnetic carriers in the impact melt breccia. The magnetic fabrics from most sites reveal a general westward flow of impact melt breccia (IMB), with magnetic lineations of individual specimens trending between NW and SW. This indicates the emplacement of IMB in a semi-molten state with temperatures below c. 1500°C, which is the melting point of Ti-magnetite. Occurrence of poorly sorted clasts implies that IMB was emplaced as surficial flow rather than aerial. The variation in the dips of magnetic fabrics among individual specimens from a site resembles a pyroclastic flow rather than a ground-hugging volatile- and melt-rich flow. We, therefore, suggest that the IMB at Dhala was ballistically ejected and then moved in a semi-molten state as surficial pyroclastic-like flow with temperatures below c. 1500°C. Most flow vectors aligned between NW-SW, may represent a dominant westward excavation flow of the IMB (rather than radially outward flow), which may be activated by an east-to-west directed impactor striking at an impact angle below 50°.