19 research outputs found
The TWINS-LAD mission: Observations of terrestrial Lyman-? fluxes
International audienceThe TWINS project (Two Wide-angle Imaging Neutral-atom Spectrometers) is mainly devoted to measure high energy neutral atoms (ENAs) originating via charge exchange of protons with geocoronal hydrogen atoms in the plasmasphere and magnetosphere. In order to unfold the local ion density along the line-of-sight (LOS) from the integrated ENA flux measurements, a good knowledge of the geocoronal hydrogen density distribution is needed. Therefore, two Lyman-? detectors (LADs) - designed and calibrated by the authors - were added to the TWINS package. These detectors register line-integrated Lyman-? resonance emission intensities which then can be used to get the actual local hydrogen densities with the help of a numerical inversion routine
The TWINS exospheric neutral H-density distribution under solar minimum conditions
Terrestrial exospheric atomic hydrogen (H) resonantly
scatters solar Lyman-α (121.567 nm) radiation, observed as the glow of the
H-geocorona. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS)
satellites are equiped with two Lyman-α line-of-sight Detectors (LADs) each.
Since during the past solar minimum conditions the relevant solar control
parameters practically did not vary, we are using LAD data between June and
September 2008 to create a time averaged hydrogen geocorona model
representative for these solar minimum conditions. In this averaged model we
assume that the H-geocorona is longitudinally symmetric with respect to the
earth-sun line. We find a 3-dimensional H-density distribution in the range
from 3 to 8 earth radii which with some caution can also be extrapolated to
larger distances. For lower geocentric distances than 3 earth radii a best
fitting r-dependent Chamberlain (1963)-like model is adapted. Main findings
are larger than conventionally expected H-densities at heights above 5 <I>R</I><sub>E</sub> and a pronounced day-to-night side H-density asymmetry. The
H-geocorona presented here should serve as a reference H-atmosphere for the
earth during solar minimum conditions
Soft Xâray and ENA Imaging of the Earthâs Dayside Magnetosphere
The LEXI and SMILE missions will provide soft Xâray images of the Earth's magnetosheath and cusps after their anticipated launch in 2023 and 2024, respectively. The IBEX mission showed the potential of an Energetic Neutral Atom (ENA) instrument to image dayside magnetosheath and cusps, albeit over the long hours required to raster an image with a single pixel imager. Thus, it is timely to discuss the two imaging techniques and relevant science topics. We simulate soft Xâray and lowâENA images that might be observed by a virtual spacecraft during two interesting solar wind scenarios: a southward turning of the interplanetary magnetic field and a sudden enhancement of the solar wind dynamic pressure. We employ the OpenGGCM global magnetohydrodynamics model and a simple exospheric neutral density model for these calculations. Both the magnetosheath and the cusps generate strong soft Xârays and ENA signals that can be used to extract the locations and motions of the bow shock and magnetopause. Magnetopause erosion corresponds closely to the enhancement of dayside reconnection rate obtained from the OpenGGCM model, indicating that images can be used to understand globalâscale magnetopause reconnection. When dayside imagers are installed with highâENA innerâmagnetosphere and FUV/UV aurora imagers, we can trace the solar wind energy flow from the bow shock to the magnetosphere and then to the ionosphere in a selfâstanding manner without relying upon other observatories. Soft Xâray and/or ENA imagers can also unveil the dayside exosphere density structure and its response to space weather
Effect of plant litter addition on element leaching in young sandy soils.
The knowledge about element leaching and biogeochemical cycles during initial stages of soil development is very limited. Therefore, we studied the effects of parent material characteristics and plant litter addition on element leaching from young sandy soils in a microcosm experiment. Our objective was to evaluate the function of young soils as a source and/or sink for nutrients during initial pedogenesis and to identify main processes which are involved in the initial development of biogeochemical cycles. The main research questions were: (1) How do differences in parent material characteristics affect nutrient leaching?; and (2) How is nutrient leaching of young soils influenced by litter addition of different plant functional groups (e. g., legume and grass species)? Combined treatments of two minimally weathered parent materials (pure sand and loamy sand) with plant litter of two plant species (Lotus corniculatus L. and Calamagrostis epigejos L.) were investigated in a soil column experiment. In addition, control columns with parent material or plant litter only were included. Carbonate weathering as a main source for calcium leaching was induced by the moderately acidic irrigation solution used in the experiment. It was 7.5 fold greater for the loamy sand parent material compared to the pure sand despite lower carbonate contents in the loamy sand. Leaching of K was very low for both parent materials but greater for the loamy sand parent material, likely due to transfer processes from fixed to exchangeable potassium forms in the clay minerals of the loamy sand. Plant litter addition generally increased leaching losses. Carbonate dissolution was intensified by both plant litter types, especially by L. corniculatus, very likely due to H+ released during nitrification of N released from plant litter and an increase in partial pressure of CO2 from microbial respiration. In contrast, K was largely retained in the soils, probably due to fixation by clay minerals and microbial immobilization. Only the pure sand treated with L. corniculatus litter leached K, resulting in 4-6 fold greater leaching losses compared to all other treatments. Nitrogen released from L. corniculatus litter was almost completely nitrified and was nearly doubled as compared to that from C. epigejos, resulting in greater N leaching. The results of our study allow identifying the general function and processes of vegetation patches in young ecosystems formed as a result of initial parent material characteristics and invading vegetation with respect to litter decomposition, soil solution composition, nutrient retention and leaching, and effects on the soil mineral phase. These patterns are not mere additive effects of parent materials plus plant litter, but reflect differences in biogeochemical process intensities and could result in an increasing heterogeneity of soil properties, nutrient availability, and element leaching fluxes with time
Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum
The Lyman-α Detectors (LAD) on board the two
TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant
emission glow of the H-geocorona from very different orbital positions.
Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar
Lyman-α (121.567 nm) radiation. During the past solar minimum,
relevant solar parameters that influence these emissions were quite stable.
Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between
June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data
are combined to produce two datasets containing (summer) solstice and
(combined spring and fall) equinox emissions. In the range from 3 to 10
Earth
radii (RE), a three-dimensional (3-D) mathematical model is used that allows
for density asymmetries in longitude and latitude. At lower geocentric
distances (< 3 RE), a best fitting r-dependent (Chamberlain,
1963)-like model is adapted to enable extrapolation of our information to
lower heights. We find that dawn and dusk H-geocoronal densities differ by up
to a factor of 1.3 with higher densities on the dawn side. Also, noon
densities are greater by up to a factor of 2 compared to the dawn and dusk
densities. The density profiles are aligned well with the EarthâSun line and
there are clear density depletions over both poles that show additional
seasonal effects. These solstice and equinox empirical fits can be used to
determine H-geocoronal densities for any day of the year for solar minimum
conditions
Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission
Circumterrestrial Lyman-α column brightness observations above 3
Earth radii (<i>R</i><sub>e</sub>) have been used to derive separate 3-D neutral
hydrogen density models of the Earth's exosphere for solar minimum (2008,
2010) and near-solar-maximum (2012) conditions. The data
used were measured by Lyman-α detectors (LAD1/2) onboard each of
the TWINS satellites from very different orbital positions with respect to
the exosphere. Exospheric H atoms resonantly scatter the near-line-center
solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions
above 3<i>R</i><sub>e</sub> along a line of sight (LOS), the scattered
LOS-column intensity is proportional to the LOS H-column density. We found
significant differences in the density distribution of the terrestrial
exosphere under different solar conditions. Under solar maximum conditions
we found higher H densities and a larger spatial extension compared to solar
minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity,
significantly lower densities were found. Differences in shape and
orientation of the exosphere under different solar conditions exist.
Above 3 <i>R</i><sub>e</sub>, independent of solar activity, increased
H densities appear on the Earth's nightside shifted towards dawn. With
increasing distance (as measured at 8<i>R</i><sub>e</sub>) this feature is
shifted westward/duskward by between â4 and â5° with
respect to midnight. Thus, at larger geocentric distance the exosphere
seems to be aligned with the aberrated Earthâsolar-wind line, defined by the
solar wind velocity and the orbital velocity of the Earth. The results
presented in this paper are valid for geocentric distances between
3 and 8<i>R</i><sub>e</sub>
3-D-geocoronal hydrogen density derived from TWINS Ly-α-data
Based on Ly-α-line-of-sight measurements
taken with two Ly-α detectors onboard of the
satellite TWINS1 (Two Wide-angle Imaging Neutral-atom Spectrometers)
density profiles of the exospheric, neutral geocoronal hydrogen were derived
for the time period between summer solstice and fall equinox 2008. With the
help of specifically developed inversion programs from Ly-α
line of sight intensities the three-dimensional density
structure of the geocoronal hydrogen at geocentric distances r>3 RE could be derived for the period mentioned characterized by very
low solar 10.7 cm radiofluxes of â65â70 [10−22 W mâ2 Hzâ1]. The time-variable,
solar "line-centered"-Ly-α-flux was extracted on the basis of
daily (terrestrial) NGDC 10.7 cm radioflux data using the models from
Barth et al. (1990) and Vidal-Madjar (1975).
The results for the geocoronal H-densities are compared here
both with theoretical calculations based on a Monte-Carlo model by Hodges
(1994) and with density profiles obtained with the Geocoronal Imager (GEO)
by Ăstgaard and Mende (2003). In our results we find a remarkably more
pronounced day-/night-side asymmetry which clearly hints to the existence of
a hydrogen geotail (i.e. a tail structure with comparatively higher hydrogen
densities on the night side of the earth for geocenctric distances >4 RE), and a only weakly pronounced polar depletion.
These unexpected features we try to explain by new models in the near future. The derived 3-D-H-density
structures are able to explain the line-of-sight (LOS) dependent Ly-α intensity variations for all LOS seen up to now
with TWINS-LAD. The presented results are valid for the region with
geocentric distances 3 RE<r<7 RE
and are based on the reasonable assumption of an optically thin H-exosphere
with respect to resonant Ly-α-scattering allowing the use of single
scattering calculations
Neutralized solar wind ahead of the Earth's magnetopause as contribution to non-thermal exospheric hydrogen
In a most recent paper by Qin and Waldrop (2016), it had been found that the
scale height of hydrogen in the upper exosphere of the Earth, especially
during solar minimum conditions, appears to be surprisingly large. This
indicates that during minimum conditions when exobasic temperatures should be
small, large exospheric H-scale heights predominate. They thus seem to
indicate the presence of a non-thermal hydrogen component in the upper
exosphere. In the following parts of the paper we shall investigate what
fraction of such expected hot hydrogen atoms could have their origin from
protons of the shocked solar wind ahead of the magnetopause converted into
energetic neutral atoms (ENAs) via charge-exchange
processes with normal
atmospheric, i.e., exospheric hydrogen atoms that in the first step evaporate
from the exobase into the magnetosheath plasma region. We shall show that, dependent on the sunward location of the magnetopause,
the density of these types of non-thermal hydrogen atoms (H-ENAs) becomes
progressively comparable with the density of exobasic hydrogen with increasing
altitude. At
low exobasic heights, however, their contribution is negligible. At the end
of this paper, we finally study the question of whether the H-ENA population
could even be understood as a self-consistency phenomenon of the H-ENA
population, especially during solar activity minimum conditions, i.e., H-ENAs
leaving the exosphere being replaced by H-ENAs injected into the exosphere