First Analysis of Ground-Level Enhancement (GLE) 72 on 10 September 2017: Spectral and Anisotropy Characteristics

Using data obtained with neutron monitors and space-borne instruments, we analyzed the second ground-level enhancement (GLE) of Solar Cycle 24, namely the event of 10 September 2017 (GLE 72), and derived the spectral and angular characteristics of associated GLE particles. We employed a new neutron-monitor yield function and a recently proposed model based on an optimization procedure. The method consists of simulating particle propagation in a model magnetosphere in order to derive the cutoff rigidity and neutron-monitor asymptotic directions. Subsequently, the rigidity spectrum and anisotropy of GLE particles are obtained in their dynamical evolution during the event on the basis of an inverse-problem solution. The derived angular distribution and spectra are discussed briefly.     

Axial rotation of near-earth asteroids: The influence of the YORP effect

The distribution of axial rotation velocities of near-Earth asteroids (NEAs) substantially differs from that of the Main-Belt asteroids by an excess of both quickly and slowly rotating objects. Among the possible causes of this difference is the influence of the solar radiation—the so-called YORP effect—that arises from the absorption of solar energy and its reemission in the thermal range by a rotating body of irregular shape. It is known that the magnitude of this effect depends on the asteroid size and the quantity of received solar energy (the insolation). Analysis of the observational data showed that the mean diameter of NEAs decreases from the middle of the distribution to the edges, i.e., the excess of both slowly (ω ≤ 2 rev/day) and quickly (ω = 8–11 rev/day) rotating objects is formed due to the asteroids with sizes smaller than those in the middle of the distribution, which agrees well with the influence of the YORP effect. Moreover, the dependence of the axial rotation velocity of NEAs on the relative insolation shows that, for the NEAs referred to, both excesses are found in orbits where, on average, they receive 8–10% more solar energy than the NEAs in the middle of the distribution. This result also agrees with the character of the influence of the YORP effect and can be considered as an additional argument in its support. Thus, the study showed that one can infer that the currently available observational data suggest the possible influence of the YORP effect on the axial rotation of the near-Earth asteroids having sizes of D ～ 2 km and less. This is the first attempt to find the influence of the YORP effect on the axial rotation of the NEA family as a whole.     

Complex crater formation: Insights from combining observations of shock pressure distribution with numerical models at the West Clearwater Lake impact structure

Large impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock‐metamorphosed quartz‐bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block‐model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35–40 km and has since experienced up to ~2 km of differential erosion.     

An interdisciplinary study of weathering effects in ordinary chondrites from the Acfer region,Algeria

Abstract— Weathering effects on meteorite finds from the Acfer region were studied by various analytical techniques and in dependence on the depth of sampling. In thin sections of weathered meteorites, weathering effects usually decrease from the outside to the interior of the meteorite. The results of evolved gas analysis indicate that variation in weathering between surface and core is not significant in respect to the formation of Fe-oxyhydroxides. The secondary alteration effects in the noble gases are distributed unevenly throughout the specimens, as seen in the nonsystematic differences observed for the heavy noble gases. Chemical analyses show significant enrichment of Ba and Sr in the outer parts of the weathered samples due to element contamination through aqueous solution. Iron, Ni, and Co are partly flushed from the system as the metal oxidation proceeds. Oxygen isotopes show increases in δ¹⁸O and δ¹⁷O with increasing terrestrial age. For a set of H3 chondrites, the degree of weathering determined from the water content was correlated with terrestrial ages and is discussed with respect to possible weathering mechanisms.     

Spectral Trends of Solar Bursts at Sub-THz Frequencies

Previous sub-THz studies were derived from single-event observations. We here analyze for the first time spectral trends for a larger collection of sub-THz bursts. The collection consists of a set of 16 moderate to small impulsive solar radio bursts observed at 0.2 and 0.4 THz by the Solar Submillimeter-wave Telescope (SST) in 2012?–?2014 at El Leoncito, in the Argentinean Andes. The peak burst spectra included data from new solar patrol radio telescopes (45 and 90 GHz), and were completed with microwave data obtained by the Radio Solar Telescope Network, when available. We critically evaluate errors and uncertainties in sub-THz flux estimates caused by calibration techniques and the corrections for atmospheric transmission, and introduce a new method to obtain a uniform flux scale criterion for all events. The sub-THz bursts were searched during reported GOES soft X-ray events of class C or larger, for periods common to SST observations. Seven out of 16 events exhibit spectral maxima in the range 5?–?40 GHz with fluxes decaying at sub-THz frequencies (three of them associated to GOES class X, and four to class M). Nine out of 16 events exhibited the sub-THz spectral component. In five of these events, the sub-THz emission fluxes increased with a separate frequency from that of the microwave spectral component (two classified as X and three as M), and four events have only been detected at sub-THz frequencies (three classified as M and one as C). The results suggest that the THz component might be present throughout, with the minimum turnover frequency increasing as a function of the energy of the emitting electrons. The peculiar nature of many sub-THz burst events requires further investigations of bursts that are examined from SST observations alone to better understand these phenomena.     

Effects of Steady Flow on Magnetoacoustic-Gravity Surface Waves: I. The Weak Field Case

Magnetoacoustic gravity (MAG) waves have been studied for some time. In this article, we investigate the effect that a shear flow at a tangential discontinuity embedded in a gravitationally stratified and magnetised plasma has on MAG surface waves. The dispersion relation found is algebraically analogous to the relation of the non-flow cases obtained by Miles and Roberts (Solar Phys. 141, 205, 1992), except for the introduction of a Doppler-shifted frequency for the eigenvalue. This feature, however, introduces rather interesting physics, including the asymmetric presence of forward- and backward-propagating surface waves. We find that increasing the equilibrium flow speed leads to a shift in the permitted regions of propagation for surface waves. For most wave number combinations this leads to the fast mode being completely removed, as well as more limited phase speed regimes for slow-mode propagation. We also find that upon increasing the flow, the phase speeds of the backward propagating waves are increased. Eventually, at high enough flow speeds, the wave’s direction of propagation is reversed and is in the positive direction. However, the phase speed of the forward-propagating wave remains mainly the same. For strong enough flows we find that the Kelvin–Helmholtz instability can also occur when the forward- and backward-propagating modes couple.     

Does Magnetic Flux Submerge at Flux Cancelation Sites?

Simultaneous measurements of the magnetic fields in the photosphere and chromosphere were used to investigate if magnetic flux is submerging at sites between adjacent opposite polarity magnetic network elements in which the flux is observed to decrease or `cancel'. These data were compared with chromospheric and coronal intensity images to establish the timing of the emission structures associated with these magnetic structures as a function of height. We found that most of the cancelation sites show either that the bipole is observed longer in the photosphere than in the chromosphere and corona (44%) or that the timing difference of the disappearance of the bipole between these levels of the atmosphere is unresolved. The magnetic axis lengths of the structures associated with the cancelation sites are on average slightly smaller in the chromosphere than the photosphere. These observations suggest that magnetic flux is retracting below the surface for most, if not all, of the cancelation sites studied.     

A new look at the origin of the 6.67 hr period X-ray pulsar 1E 161348-5055

The point X-ray source 1E 161348-5055 is observed to display pulsations with the period 6.67?hr and $|\dot{P}| \leq1.6 \times10^{-9}\,{\rm s\,s^{-1}}$ . It is associated with the supernova remnant RCW?103 and is widely believed to be a ～2000?yr old neutron star. Observations give no evidence for the star to be a member of a binary system. Nevertheless, it resembles an accretion-powered pulsar with the magnetospheric radius ～3000?km and the mass-accretion rate $\sim 10^{14}\,{\rm g\,s^{-1}}$ . This situation could be described in terms of accretion from a (residual) fossil disk established from the material falling back towards the star after its birth. However, current fall-back accretion scenarios encounter major difficulties explaining an extremely long spin period of the young neutron star. We show that the problems can be avoided if the accreting material is magnetized. The star in this case is surrounded by a fossil magnetic slab in which the material is confined by the magnetic field of the accretion flow itself. We find that the surface magnetic field of the neutron star within this scenario is ～10¹²?G and that a presence of $\gtrsim10^{-7}\,{\rm M_{\odot}}$ magnetic slab would be sufficient to explain the origin and current state of the pulsar.