Search for: All records

Abstract The outer heliosphere is profoundly influenced by nonthermal energetic pickup ions (PUIs), which dominate the internal pressure of the solar wind beyond ~10 au, surpassing both solar wind and magnetic pressures. PUIs are formed mostly through charge exchange between interstellar neutral atoms and solar wind ions. This study examines the apparent heating of PUIs in the distant supersonic solar wind before reaching the heliospheric termination shock. New Horizons’ SWAP observations reveal an unexpected PUI temperature change between 2015 and 2020, with a notable bump in PUI temperature. Concurrent observations from the ACE and Wind spacecraft at 1 au indicate a ~50% increase in solar wind dynamic pressure at the end of 2014. Our simulation suggests that the bump observed in the PUI temperature by New Horizons is largely associated with the enhanced solar wind dynamic pressure observed at 1 au. Additional PUI temperature enhancements imply the involvement of other heating mechanisms. Analysis of New Horizons data reveals a correlation between shocks and PUI heating during the declining phase of the solar cycle. Using a PUI-mediated plasma model, we explore shock structures and PUI heating, finding that shocks preferentially heat PUIs over the thermal solar wind in the outer heliosphere. We also show that the broad shock thickness observed by New Horizons is due to the large diffusion coefficient associated with PUIs. Shocks and compression regions in the distant supersonic solar wind lead to elevated PUI temperatures and thus they can increase the production of energetic neutral atoms with large energy. 

Abstract H₂O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H₂O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H₂O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers. 

X-ray self-heating is a common by-product of X-ray Free Electron Laser (XFEL) techniques that can affect targets, optics, and other irradiated materials. Diagnosis of heating and induced changes in samples may be performed using the x-ray beam itself as a probe. However, the relationship between conditions created by and inferred from x-ray irradiation is unclear and may be highly dependent on the material system under consideration. Here, we report on a simple case study of a titanium foil irradiated, heated, and probed by a MHz XFEL pulse train at 18.1 keV delivered by the European XFEL using measured x-ray diffraction to determine temperature and finite element analysis to interpret the experimental data. We find a complex relationship between apparent temperatures and sample temperature distributions that must be accounted for to adequately interpret the data, including beam averaging effects, multivalued temperatures due to sample phase transitions, and jumps and gaps in the observable temperature near phase transformations. The results have implications for studies employing x-ray probing of systems with large temperature gradients, particularly where these gradients are produced by the beam itself. Finally, this study shows the potential complexity of studying nonlinear sample behavior, such as phase transformations, where biasing effects of temperature gradients can become paramount, precluding clear observation of true transformation conditions. 

Abstract The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune–Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft—in a class equivalent to the NASA/ESA/ASI Cassini spacecraft—would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moon's gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ∼37 minutes to the 10 bar pressure level in Neptune's atmosphere just before Odyssey's orbit-insertion engine burn. Odyssey's mission would end by conducting a Cassini-like “Grand Finale,” passing inside the rings and ultimately taking a final great plunge into Neptune's atmosphere. 

Abstract We detail the sensitivity of the proposed liquid xenon DARWIN observatory to solar neutrinos via elastic electron scattering. We find that DARWIN will have the potential to measure the fluxes of five solar neutrino components: pp , $$^7$$ 7 Be, $$^{13}$$ 13 N, $$^{15}$$ 15 O and pep . The precision of the $$^{13}$$ 13 N, $$^{15}$$ 15 O and pep components is hindered by the double-beta decay of $$^{136}$$ 136 Xe and, thus, would benefit from a depleted target. A high-statistics observation of pp neutrinos would allow us to infer the values of the electroweak mixing angle, $$\sin ^2\theta _w$$ sin 2 θ w , and the electron-type neutrino survival probability, $$P_{ee}$$ P ee , in the electron recoil energy region from a few keV up to 200 keV for the first time, with relative precision of 5% and 4%, respectively, with 10 live years of data and a 30 tonne fiducial volume. An observation of pp and $$^7$$ 7 Be neutrinos would constrain the neutrino-inferred solar luminosity down to 0.2%. A combination of all flux measurements would distinguish between the high- (GS98) and low-metallicity (AGS09) solar models with 2.1–2.5 $$\sigma $$ σ significance, independent of external measurements from other experiments or a measurement of $$^8$$ 8 B neutrinos through coherent elastic neutrino-nucleus scattering in DARWIN. Finally, we demonstrate that with a depleted target DARWIN may be sensitive to the neutrino capture process of $$^{131}$$ 131 Xe.