Exospheric atomic hydrogen (H) resonantly scatters solar Lyman‐α (121.567 nm) radiation, observed as the geocorona. Measurements of scattered solar photons allow one to probe time‐varying ...three‐dimensional distributions of exospheric H atoms. The Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission images the magnetosphere in energetic neutral atom (ENA) fluxes and additionally carries Lyman‐α detectors (LADs) to investigate exospheric atomic hydrogen. Knowledge of exospheric properties is essential for the interpretation of magnetospheric images in ENA fluxes produced in charge exchange between energetic ions and H atoms. We describe the process of obtaining exospheric distributions and present, as an example, a global H number density distribution for 11 June 2008. The TWINS LAD experimental data are especially sensitive to atomic hydrogen at geocentric distances from 3 to 8 Earth radii. The distribution reveals asymmetries from day to night, dawn to dusk, and north to south. We discuss the available data sets and their coverage, limitations, and promise for a study of exospheric response to seasonal, solar, and geomagnetic variations.
Key Points
Obtaining exospheric H distributions from TWINS LAD observations
Global distribution of exospheric hydrogen from 3 to 8 Earth radii
Exosphere asymmetries
Summary
Phenotypic plasticity, which confers a fitness advantage under heterogeneous and novel environments, has been commonly suggested to contribute to the success of invasive plants in their ...introduced range. For example, plasticity in response to changes in light availability could facilitate invasiveness by allowing plants to both rapidly establish in unshaded, disturbed habitats, and tolerate shaded or crowded environments. The plastic responses of invasive plants to shade were mostly studied in morphological traits. However, plasticity in physiological traits might provide more rapid and reversible responses and thus be more effective in environments with rapid temporal variations. Here, we compared plasticity in a range of morphological and physiological traits that provide shade avoidance or tolerance between two native and six introduced populations of Impatiens glandulifera. In a common garden, we subjected second‐generation siblings of native and invasive plants to two light availability treatments and measured their morphological, physiological and performance responses. Impatiens glandulifera from invasive populations exhibited greater phenotypic plasticity in response to light availability in four out of 12 measured traits. Moreover, this greater plasticity was mostly limited to physiological traits associated with photosynthetic acclimation. These results suggest high phenotypic plasticity in response to light availability could have facilitated I. glandulifera's spread in both disturbed habitats and woodlands or under intense light competition. The results of this study highlight the importance of considering physiological traits when studying the role of plasticity in the success of invasive plants.
The dominant neutral constituent in Earth's upper exosphere, atomic hydrogen (H), resonantly scatters solar Lyman‐alpha (121.567 nm) radiation, observed as the geocorona. We report here observations ...of an exospheric response to geomagnetic storms obtained using measurements of the geocorona by Lyman‐alpha detectors on the Two Wide‐angle Imaging Neutral‐atom Spectrometers mission. We introduce a new parameter, NH, the number of H atoms in the spherical shell from a geocentric distance of 3 to 8 Earth radii, to quantitatively characterize in a simplified way global exospheric conditions. Five geomagnetic storms observed during three months in the second half of 2011 are accompanied by abrupt temporary increases, spikes, of NH from 6% to 17%, lasting not longer than a day. These increases seem to show some correlation with the minimum Dst index reached during the peak of each storm.
Key Points
First observations of exosphere variations during geomagnetic eventsCorrelation of exospheric density with time‐integrated main‐phase Dst indexLimited duration of exospheric density enhancements
The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local ...interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo-electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock-heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude λ, latitude β) = (221°, 39°), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon's nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.
IBEX : THE FIRST FIVE YEARS (2009-2013) McComas, D J; Allegrini, F; BZOWSKI, M ...
The Astrophysical journal. Supplement series,
08/2014, Letnik:
213, Številka:
2
Journal Article
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The Interstellar Boundary Explorer (IBEX) returned its first five years of scientific observations from 2009 to 2013. In this study, we examine, validate, initially analyze, and provide to the broad ...scientific community this complete set of energetic neutral atom (ENA) observations for the first time. IBEX measures the fluxes of ENAs reaching 1 AU from sources in the outer heliosphere and most likely the very nearby interstellar space beyond the heliopause. The data, maps, and documentation provided in this study represent the fourth major release of the IBEX data, incorporate important improvements, and should be used for future studies and as the citable reference for the current version of the IBEX data. In this study, we also examine five years of time evolution in the outer heliosphere and the resulting ENA emissions. These observations show a complicated variation with a general decrease in ENA fluxes from 2009 to 2012 over most regions of the sky, consistent with a 2-4 year recycle time for the previously decreasing solar wind flux. In contrast, the heliotail fluxes continue to decrease, again consistent with a significantly more distant source in the downwind direction. Finally, the Ribbon shows the most complicated time variations, with a leveling off in the southern hemisphere and continued decline in the northern one; these may be consistent with the Ribbon source being significantly farther away in the north than in the south. Together, the observations and results shown in this study expose the intricacies of our heliosphere's interaction with the local interstellar medium.
The Spacecraft Interaction Plasma Software package (SPIS), a three‐dimension particle in cell (PIC) code, was used to model the Parker Solar Probe (PSP) spacecraft and FIELDS instrument and their ...interactions with the Solar wind. Our SPIS modeling relied on material properties of new spacecraft materials that we had obtained in previous work. The model was used to find the floating potentials of the spacecraft and FIELDS antennas at different distances from the Sun (from 1AU to 0.046AU). We find the following results: At greater distances from the Sun, the shadowed spacecraft charges negatively while the illuminated Thermal Protection System (TPS) charges positive due to the high resistance of the TPS Alumina shield at low temperatures. As the spacecraft approaches the Sun, the temperature of the TPS increases, the resistance between it and the spacecraft drops, and its photoemission increases, driving the spacecraft more positive. At the same time, an electrostatic barrier forms near the illuminated surface of the TPS and reflects the photoelectrons back leading to negative charging of some surfaces. The FIELDS antennas and shield also see this barrier forming but on a smaller scale. The FIELDS antennas charge positively at all distances modeled when no current bias is applied. Current biasing of the antennas affects their floating potential.
Plain Language Summary
Measuring the electric field in a space plasma is important for understanding how plasma flows are driven, charge particles are accelerated and heated, and electromagnetic waves propagate. Measuring the voltage difference between two spatially separated electrodes immersed in a space plasma is one way to estimate the electric field that is present in the plasma. Interpretation of these voltage differences is complicated by the fact that the electrodes often float at a significant voltage relative to the nearby plasma so as to achieve current balance between the electrode and the charged particle environment around it. Different surfaces will float to different potentials depending upon their surface materials, their location relative to other surfaces, their orientation with respect to the incident Sun's light and solar wind flows, and numerical modeling is required to predict how all these factors influence what is observed.
Key Points
We predict the floating potentials of the Parker Solar Probe spacecraft and FIELDS antennas using the Spacecraft Interaction Plasma Software package (SPIS) particle‐in‐cell model
We used laboratory evaluation of photoelectron and secondary electron properties of exposed surfaces
We predict electrostatic barriers forming near the illuminated surfaces of the FIELDS antennas and shields
This research shows Part II of the Spacecraft Interaction Plasma Software (SPIS) used to model the parker solar probe (PSP) FIELDS instrument and its interactions with the Solar Wind. Flight data ...were used to run the PSP model and compared with models using past predicted parameters. The effect of voltage biasing between the antenna, its shield, and the spacecraft on the current balance of each surface was investigated at first perihelion (0.16AU). The model data were reduced to I–V curves to find current saturations (analysis results 52 µA vs. flight results 54–72 µA), and sheath resistances (analysis results of 307 kΩ vs. flight results of 51 kΩ). The recommended bias current to ensure optimal sensitivity of the FIELDS antenna was between −52 and −22 µA, which corresponded to a differential potential with respect to the spacecraft between −5 and 5 V. The analysis also shows that plasma sheath of the FIELDS antenna and the plasma sheath of the FIELDS shield interacted between each other with an impedance of ∼220 kΩ.
Plain Language Summary
Measuring the electric field in a space plasma is important for understanding how plasma flows are driven, charge particles are accelerated and heated, and how electromagnetic waves propagate. Measuring the voltage difference between two spatially separated electrodes immersed in a space plasma is one way to estimate the electric field that is present in the plasma. Interpretation of these voltage differences is complicated by the fact that the electrodes often float at a significant voltage relative to the nearby plasma so as to achieve current balance between the electrode and the charged particle environment around it. Different surfaces will float to different potentials depending upon their surface materials, their location relative to other surfaces, their orientation relative to the Sun's light and solar wind flows, and numerical modeling is required to accurately predict how all these factors influence what is observed. Comparison between such numerical models and in situ measurements of potentials and currents, allows one to better understand how the instrument works, and how to operate it better to produce the highest quality electric field estimates.
Key Points
We predict the floating potentials of the parker solar probe FIELDS antennas
We analyze the model antenna I–V curves to determine the optimal current and voltage biases for maximum sensor sensitivity
We compare the theoretical predictions with mission flight data and find qualitative agreement, but some quantitative differences
This study provides, for the first time, complete and validated observations from the first three years (2009-2011) of the Interstellar Boundary Explorer (IBEX) mission. Energetic neutral atom (ENA) ...fluxes are corrected for both the time-variable cosmic ray background and for orbit-by-orbit variations in their probability of surviving en route from the outer heliosphere in to 1 AU where IBEX observes them. In addition to showing all six six-month maps, we introduce new annual ram and anti-ram maps, which can be produced without the need for algorithm-dependent Compton-Getting corrections. Together, the ENA maps, data, and supporting documentation presented here support the full release of these data to the broader scientific community and provide the citable reference for them. In addition, we show that heliospheric ENA emissions have been decreasing over the epoch from 2009 to 2011 with the IBEX Ribbon decreasing by the largest fraction and only the heliotail (which is offset from the down wind direction by the interstellar magnetic field) showing essentially no reduction and actually some increase. Finally, we show how the much more complete observations provided here strongly indicate a quite direct and latitude-dependent solar wind source of the Ribbon.
The first all‐sky maps of Energetic Neutral Atoms (ENAs) from the Interstellar Boundary Explorer (IBEX) exhibited smoothly varying, globally distributed flux and a narrow “ribbon” of enhanced ENA ...emissions. In this study we compare the second set of sky maps to the first in order to assess the possibility of temporal changes over the 6 months between views of each portion of the sky. While the large‐scale structure is generally stable between the two sets of maps, there are some remarkable changes that show that the heliosphere is also evolving over this short timescale. In particular, we find that (1) the overall ENA emissions coming from the outer heliosphere appear to be slightly lower in the second set of maps compared to the first, (2) both the north and south poles have significantly lower (∼10–15%) ENA emissions in the second set of maps compared to the first across the energy range from 0.5 to 6 keV, and (3) the “knot” in the northern portion of the ribbon in the first maps is less bright and appears to have spread and/or dissipated by the time the second set was acquired. Finally, the spatial distribution of fluxes in the southernmost portion of the ribbon has evolved slightly, perhaps moving as much as 6° (one map pixel) equatorward on average. The observed large‐scale stability and these systematic changes at smaller spatial scales provide important new information about the outer heliosphere and its global interaction with the galaxy and help inform possible mechanisms for producing the IBEX ribbon.
Organ regeneration in mammals is hypothesized to require a functional pool of stem or progenitor cells, but the role of these cells in lung regeneration is unknown. Whereas postnatal regeneration of ...alveolar tissue has been attributed to type II alveolar epithelial cells (AECII), we reasoned that bronchioalveolar stem cells (BASCs) have the potential to contribute substantially to this process. To test this hypothesis, unilateral pneumonectomy (PNX) was performed on adult female C57/BL6 mice to stimulate compensatory lung regrowth. The density of BASCs and AECII, and morphometric and physiological measurements, were recorded on days 1, 3, 7, 14, 28, and 45 after surgery. Vital capacity was restored by day 7 after PNX. BASC numbers increased by day 3, peaked to 220% of controls (P<0.05) by day 14, and then returned to baseline after active lung regrowth was complete, whereas AECII cell densities increased to 124% of baseline (N/S). Proliferation studies revealed significant BrdU uptake in BASCs and AECII within the first 7 days after PNX. Quantitative analysis using a systems biology model was used to evaluate the potential contribution of BASCs and AECII. The model demonstrated that BASC proliferation and differentiation contributes between 0 and 25% of compensatory alveolar epithelial (type I and II cell) regrowth, demonstrating that regeneration requires a substantial contribution from AECII. The observed cell kinetic profiles can be reconciled using a dual-compartment (BASC and AECII) proliferation model assuming a linear hierarchy of BASCs, AECII, and AECI cells to achieve lung regrowth.
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