We have used a unique constellation of Earth‐orbiting spacecraft and ground‐based measurements in order to study a relatively isolated magnetospheric substorm event on August 27, 2001. Global ...ultraviolet images of the northern auroral region established the substorm expansion phase onset at 0408:19 (±1 min) UT. Concurrent measurements from the GOES‐8, POLAR, LANL, and CLUSTER spacecraft allow us to construct a timeline which is consistent with magnetic reconnection on the closed field lines of the central plasma sheet near XGSM ∼ −18 RE some 7 minutes prior to the near‐earth and auroral region times of substorm expansion phase onset. This suggests that magnetic reconnection (i.e., the substorm neutral line) in this case formed in the mid‐tail region substantially before current disruption, field dipolarization near geostationary orbit, or auroral substorm onsets occurred. Thus, the magnetic reconnection process is interpreted as the causative driver of dissipation in this well‐observed case.
Deformation is a near ubiquitous process that is observed within nearly all naturally forming rocks. Electron backscatter diffraction (EBSD) is a technique that enables slip‐systems (a form of ...plastic deformation) to be inferred from intracrystalline misorientations at a comparable scale to the representative CPO analysis (≥300 crystals for the nakhlites). Extensive laboratory and studies on naturally occurring samples have identified preferential mantle condition extrinsic parameters for specific slip‐system signatures within olivine and clinopyroxene. Intracrystalline misorientation patterns for olivine and augite (high Ca‐clinopyroxene) for 16 different Martian nakhlite meteorites (21 sections) were analyzed and assessed against these known parameters. Investigation of high and low deformation regions within the nakhlites revealed a shift in intracrystalline misorientation patterns for 10 of the 21 sections. Interpreted as both shock (high deformations) and emplacement (low deformation) signatures, the observed variations in deformation patterns for the two main regimes of deformation indicate heterogeneous sampling of the nakhlite source crater. Our findings indicate that shock deformation is prevalent throughout the nakhlites, and that great care needs to be taken when interpreting intracrystalline misorientations of crystals within apparent lower deformation regions.
Plain Language Summary
Clinopyroxene and olivine are important minerals for studying igneous processes on Mars and Earth (from the surface to the upper mantle). Here, clinopyroxene and olivine intracrystalline misorientation patterns—deformational movement within a crystal—were investigated in a group of meteorites from Mars using the specialist microscopic technique of electron backscatter diffraction (EBSD). The nakhlites are mafic rocks representing the largest collection of rocks from a singular—but as yet unknown—location on Mars. Combined intracrystalline misorientation patterns for both olivine and clinopyroxene reveal nine different shock deformation signatures for the nakhlites indicating that they were sourced from multiple locations within the ejection crater. Nonshock related deformation can also be observed but tends to be masked by the dominance of shock deformation features even in low deformation regions.
Key Points
Electron backscatter diffraction reveals shock‐derived deformation to mask nonshock deformation even in low deformation regions in the nakhlites
Intracrystalline misorientation patterns indicate nine distinct derived deformation signatures for 16 nakhlites
Intracrystalline misorientation patterns are identified within low deformation regions interpreted as emplacement deformation
The Martian nakhlite meteorites, which represent multiple events that belong to a single magma source region represent a key opportunity to study the evolution of Martian petrogenesis. Here 16 of the ...26 identified nakhlite specimens are studied using coupled electron backscatter diffraction (EBSD) and emplacement end‐member calculations. EBSD was used to determine shape preferred orientation of contained augite (high Ca‐clinopyroxene) phenocrysts by considering their crystallographic preferred orientation (CPO). Parameters derived from EBSD, and energy dispersive X‐ray spectroscopy spectra were used in basic emplacement models to assess their dominant mechanism against three end‐member scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO analyses indicate low intensity weak‐moderate CPO. In all samples, a consistent foliation within the axes of augite are observed typically coupled with a weaker lineation CPO in one of the other crystallographic axes. These CPO results agree best with crystal settling being the dominant emplacement mechanism for the nakhlites. Modeled crystal settling results identify two distinguishable groups outside of the model's resolution indicating the presence of secondary emplacement mechanisms. Comparison of the two identified groups against CPO, geochemical, and age parameters indicate random variability between individual meteorites. Therefore, coupled CPO and emplacement modeling results identify an overarching characteristic of a dominant crystal settling emplacement mechanism for the nakhlite source volcano despite exhibiting random variation with each discharge through time.
Plain Language Summary
A group of Martian meteorites known as the nakhlites was investigated to better understand volcanism on Mars by using the specialist microscopy technique of electron backscatter diffraction and basic modeling of known igneous processes. The presented data suggest that the magma that was discharged from the nakhlite source volcano varied randomly through time but solidified into rocks via a consistent dominant mechanism of crystal settling. These findings show Martian volcanism is dynamic, where individual eruptions are variable, yet retain overarching volcanic source characteristics that can be identified from large studies of multiple samples.
Key Points
S‐type to LS‐type crystallographic preferred orientation (CPO) identified in the nakhlites
CPO indicate nakhlite formation in environments characteristic of crystal settling
CPO and emplacement modeling suggest random variation in discharge through time with overarching consistent emplacement
The dynamics of high-energy electron fluxes (with energies over 40 keV) is analyzed in 13 events of magnetic field dipolization observed by the Cluster satellites in the near-tail of the Earth ...magnetosphere. In all of the events, the observed energetic electron fluxes are enhanced simultaneously with initial dipolization. Good correlation (correlation coefficient >0.6) is observed between the dynamics of the energetic electron fluxes with energies up to 90 keV and the
B
Z
component of the magnetic field. Electron fluxes with higher energies display a decline of correlation with the magnetic field. The increase in electron fluxes with energies up to 90 keV during dipolization development is shown to be mainly due to the mechanism of betatron acceleration. The dynamics of electron fluxes with higher energies is poorly described by the betatron scenario and requires consideration of other, probably nonadiabatic, mechanisms.
The energetic electrons and ions embedded in Earthward‐moving plasmoid structures have been observed. These plasmoids are associated with a rotational local Bz component (bi‐polar) signature. ...Energetic electrons are found to be confined in a smaller spatial region than ions inside the plasmoid. Energetic ions and electrons seem to be a good indicator for the structure boundary. The fleet of Cluster spacecraft cross the plasmoid structure in a “first entry, last out” order (Note: when spacecraft cross a planar discontinuity, e.g. magnetopause, they will be in “first entry, first out” order). This documents the fact that the plasmoid has a non‐planar nested structure. The large separation distance (around 1 RE) of the Cluster satellites in October 2002 is an advantage to provide constraints on the size and shape of the plasmoid structure of interest. In addition, the plasmoid (with closed field lines) should preserve the ion composition information where it is formed. The ion composition observed in the plasmoid shows significantly lower O and He than in the ambient plasma. This implies few heavy ions are involved in the reconnection process where the plasmoid is formed. Multiple flux ropes/plasmoids observation presented in this paper can be interpreted as strong evidence for multiple X‐lines.
We use multipoint observation data by Cluster during time periods when the interspacecraft separation distance was between 1 and 1.5 Earth radii in order to study the physical processes related to ...diffuse ions at <200 keV/e. For our analysis we use data from the Research with Adaptive Particle Imaging Detectors (RAPID) experiment onboard Cluster SC1 and SC3. We determine spatial ion density gradients by using proton intensities in the 27.7–159.7 keV energy range and helium intensities in the 137.8–235.1 keV energy range as a function of distance from the bow shock along the magnetic field. Our results show that the diffuse ions are subject to diffusive transport and the ion partial densities decrease exponentially with increasing distance from the bow shock. By complementing RAPID data with Cluster Ion Spectrometry measurements at lower energies (from 10 to 32 keV) from the same upstream ion event we find that the e‐folding distance of energetic ion density increases almost linearly with energy. This effect is also seen in the hardening of the particle spectra with increasing distance from the bow shock. We determine the spatial diffusion mean free path and the diffusion coefficient as a function of ion energy by assuming that upstream diffusion is balanced by downstream convection.
A case study is presented to determine the source of the energetic electron layer frequently observed along the high-latitude magnetopause. Measurements by the Cluster spacecraft show bursts of ...field-aligned electrons occurring during time periods with high potential for dayside reconnection. These properties are compared with the expected signatures from several sources including escape from the exterior cusp, acceleration in a reconnection region, and release from the dayside trapping region through reconnection. The observed properties are most consistent with the electrons being released from the magnetosphere due to reconnection. In this model the electrons would flow along the newly reconnected IMF draped along the magnetopause and propagate along the high-latitude magnetopause. These observations demonstrate an active source for populating the energetic particle layer frequently observed along and just outside the high-latitude magnetopause.
Recent evidence suggests that the human gastric microbiota is much more diverse than previously thought. The aim of this study was to assess the potential for isolating lactobacilli from the human ...stomach. Lactobacilli were selectively cultured from gastric biopsies from 12 patients undergoing routine endoscopy. Lactobacilli were present in four of 12 biopsies. We isolated, in total 10 different strains representing five species (Lactobacillus gasseri, L. fermentum, L. vaginalis, L. reuteri and L. salivarius). The 10 isolates varied greatly in their ability to inhibit the growth of two Gram-positive bacteria and two Gram-negative bacteria. Furthermore, the acid and bile resistance profiles of the 10 isolates spanned a wide range. Five different Lactobacillus species were cultured from human gastric biopsies for the first time. Diverse Lactobacillus species are more prevalent in the human stomach than previously recognized, representing an untapped source of bacteria with beneficial probiotic and/or biotechnological properties.
Nakhlite meteorites are ~1.4 to 1.3 Ga old igneous rocks, aqueously altered on Mars ~630 Ma ago. We test the theory that water-rock interaction was impact driven. Electron backscatter diffraction ...demonstrates that the meteorites Miller Range 03346 and Lafayette were heterogeneously deformed, leading to localized regions of brecciation, plastic deformation, and mechanical twinning of augite. Numerical modeling shows that the pattern of deformation is consistent with shock-generated compressive and tensile stresses. Mesostasis within shocked areas was aqueously altered to phyllosilicates, carbonates, and oxides, suggesting a genetic link between the two processes. We propose that an impact ~630 Ma ago simultaneously deformed the nakhlite parent rocks and generated liquid water by melting of permafrost. Ensuing water-rock interaction focused on shocked mesostasis with a high density of reactive sites. The nakhlite source location must have two spatially correlated craters, one ~630 Ma old and another, ejecting the meteorites, ~11 Ma ago.
Fortunate positioning of Cluster and TC‐1 in the plasma sheet (PS) of the Earth's magnetotail has allowed studies of the current sheet (CS) structure and particle dynamics in mesoscale and microscale ...in both sides of the near‐Earth reconnection, which took place between 03:42 and 03:55 UT on 22 September 2004. The distinctive feature of this event was the presence of a strong negative BY field forming a “bell‐like” spatial profile with the maximum absolute value near the neutral plane. The magnitude of this BY field was almost two times larger than the interplanetary magnetic field (IMF) and therefore could not be explained solely by the IMF penetration into the magnetotail. We propose a possible intrinsic mechanism of the BY field enhancement near the neutral plane based on peculiarities of the nonadiabatic ion interaction with the thin CS. An analysis of test particle trajectories shows that in the presence of a guide field with the “bell‐like” spatial profile, a pronounced north‐south asymmetry appears in the refraction/reflection properties of nonadiabatic ions from the CS. In a region tailward of the reconnection (BZ < 0), this asymmetry results in an increase of the density of the keV ions ejected into the northern PS and moving tailward. These ions can carry the tailward current which may be responsible for the strong negative BY near the neutral plane, i.e., self‐consistent enhancement of a BY field could occur near the neutral plane.
Key Points
A possible mechanism of guide BY enhancement near the neutral plane is proposed
PDF
