496 research outputs found
Disparity among low first ionization potential elements
The elemental composition of the solar wind differs from the solar
photospheric composition. Elements with low first ionization potential (FIP)
appear enhanced compared to O in the solar wind relative to the respective
photospheric abundances. This so-called FIP effect is different in the slow
solar wind and the coronal hole wind. However, under the same plasma
conditions, for elements with similar FIPs such as Mg, Si, and Fe, comparable
enhancements are expected. We scrutinize the assumption that the FIP effect is
always similar for different low FIP elements, namely Mg, Si, and Fe. We
investigate the dependency of the FIP effect of low FIP elements on the O7+/O6+
charge state ratio depending on time and solar wind type. We order the observed
FIP ratios with respect to the O7+/O6+ charge state ratio into bins and analyze
separately the respective distributions of the FIP ratio of Mg, Si, and Fe for
each O7+/O6+ charge state ratio bin. We observe that the FIP effect shows the
same qualitative yearly behavior for Mg and Si, while Fe shows significant
differences during the solar activity maximum and its declining phase. In each
year, the FIP effect for Mg and Si always increases with increasing O7+/O6+
charge state ratio, but for high O7+/O6+ charge state ratios the FIP effect for
Fe shows a qualitatively different behavior. During the years 2001-2006,
instead of increasing with the O7+/O6+ charge state ratio, the Fe FIP ratio
exhibits a broad peak. Also, the FIP distribution per O7+/O6+ charge state bin
is significantly broader for Fe than for Mg and Si. These observations support
the conclusion that the elemental fractionation is only partly determined by
FIP. In particular, the qualitative difference behavior with increasing O7+/O6+
charge state ratio between Fe on the one hand and Mg and Si on the other hand
is not yet well explained by models of fractionation
An elliptic expansion of the potential field source surface model
Context. The potential field source surface model is frequently used as a
basis for further scientific investigations where a comprehensive coronal
magnetic field is of importance. Its parameters, especially the position and
shape of the source surface, are crucial for the interpretation of the state of
the interplanetary medium. Improvements have been suggested that introduce one
or more additional free parameters to the model, for example, the current sheet
source surface (CSSS) model.
Aims. Relaxing the spherical constraint of the source surface and allowing it
to be elliptical gives modelers the option of deforming it to more accurately
match the physical environment of the specific period or location to be
analyzed.
Methods. A numerical solver is presented that solves Laplace's equation on a
three-dimensional grid using finite differences. The solver is capable of
working on structured spherical grids that can be deformed to create elliptical
source surfaces.
Results. The configurations of the coronal magnetic field are presented using
this new solver. Three-dimensional renderings are complemented by
Carrington-like synoptic maps of the magnetic configuration at different
heights in the solar corona. Differences in the magnetic configuration computed
by the spherical and elliptical models are illustrated.Comment: 11 pages, 7 figure
Evolution of an equatorial coronal hole structure and the released coronal hole wind stream: Carrington rotations 2039 to 2050
The Sun is a highly dynamic environment that exhibits dynamic behavior on
many different timescales. In particular, coronal holes exhibit temporal and
spatial variability. Signatures of these coronal dynamics are inherited by the
coronal hole wind streams that originate in these regions and can effect the
Earth's magnetosphere. Both the cause of the observed variabilities and how
these translate to fluctuations in the in situ observed solar wind is not yet
fully understood. During solar activity minimum the structure of the magnetic
field typically remains stable over several Carrington rotations (CRs). But how
stable is the solar magnetic field? Here, we address this question by analyzing
the evolution of a coronal hole structure and the corresponding coronal hole
wind stream emitted from this source region over 12 consecutive CRs in 2006. To
this end, we link in situ observations of Solar Wind Ion Composition
Spectrometer (SWICS) onboard the Advanced Composition Explorer (ACE) with
synoptic maps of Michelson Doppler imager (MDI) on the Solar and Heliospheric
Observatory (SOHO) at the photospheric level through a combination of ballistic
back-mapping and a potential field source surface (PFSS) approach. Together,
these track the evolution of the open field line region that is identified as
the source region of a recurring coronal hole wind stream.
We find that the shape of the open field line region and to some extent also
the solar wind properties are influenced by surrounding more dynamic closed
loop regions. We show that the freeze-in order can change within a coronal hole
wind stream on small timescales and illustrate a mechanism that can cause
changes in the freeze-in order. The inferred minimal temperature profile is
variable even within coronal hole wind and is in particular most variable in
the outer corona
An easy-to-use function to assess deep space radiation in human brains
Health risks from radiation exposure in space are an important factor for astronauts' safety as they venture on long-duration missions to the Moon or Mars. It is important to assess the radiation level inside the human brain to evaluate the possible hazardous effects on the central nervous system especially during solar energetic particle (SEP) events. We use a realistic model of the head/brain structure and calculate the radiation deposit therein by realistic SEP events, also under various shielding scenarios. We then determine the relation between the radiation dose deposited in different parts of the brain and the properties of the SEP events and obtain some simple and ready-to-use functions which can be used to quickly and reliably forecast the event dose in the brain. Such a novel tool can be used from fast nowcasting of the consequences of SEP events to optimization of shielding systems and other mitigation strategies of astronauts in space
Possible in situ Tests of the Evolution of Elemental and Isotopic Abundances in the Solar Convection Zone
Helioseismology has shown that the chemical composition of the Sun has
changed over its lifetime. The surface abundance of helium and heavy elements
is believed to have decreased by up to 10% relative to their initial values.
However, this reduction is too small to be tested by direct observations of the
photospheric chemical composition. Here, we compare the predicted variations in
the solar photospheric composition with precise measurements of abundances in
meteorites and the solar wind composition. Although elemental composition
ratios can vary by roughly a percent (e. g. for Ca/Mg and Ca/Fe) over the Sun's
lifetime, their measurements are rife with uncertainties related to
uncertainties in the interpretation of meteoritic measurements, photospheric
determinations, and the complex fractionation processes occurring between the
upper photosphere and lower chromosphere and the corona. On the other hand,
isotopic ratios can be measured much more accurately and are not expected to be
affected as much by extrasolar processes, although more work is required to
quantify their effect. As the isotopic ratios evolve in the Sun proportionally
to the mass ratios of the isotopes, light elements yield the highest variations
in isotopic ratios. They are predicted to reach as high as 0.6% for
O/O and are only slightly lower in the cases of
Mg/Mg and Si/Si. Such a value should be well within
the sensitivity of new missions such as Genesis.Comment: 10 pages, 6 figures, accepted for publication in Journal of
geophysical Research-Space Physic
Scope and limitations of ad hoc neural network reconstructions of solar wind parameters
Solar wind properties are determined by the conditions of their solar source
region and transport history. Solar wind parameters, such as proton speed,
proton density, proton temperature, magnetic field strength, and the charge
state composition of oxygen, are used as proxies to investigate the solar
source region of the solar wind. The transport and conditions in the solar
source region affect several solar wind parameters simultaneously. The observed
redundancy could be caused by a set of hidden variables. We test this
assumption by determining how well a function of four of the selected solar
wind parameters can model the fifth solar wind parameter. If such a function
provided a perfect model, then this solar wind parameter would be uniquely
determined from hidden variables of the other four parameters. We used a neural
network as a function approximator to model unknown relations between the
considered solar wind parameters. This approach is applied to solar wind data
from the Advanced Composition Explorer (ACE). The neural network
reconstructions are evaluated in comparison to observations. Within the limits
defined by the measurement uncertainties, the proton density and proton
temperature can be reconstructed well. We also found that the reconstruction is
most difficult for solar wind streams preceding and following stream
interfaces. For all considered solar wind parameters, but in particular the
proton density, temperature, and the oxygen charge-state ratio, parameter
reconstruction is hindered by measurement uncertainties. The reconstruction
accuracy of sector reversal plasma is noticeably lower than that of streamer
belt or coronal hole plasma. The fact that the oxygen charge-state ratio, a
non-transport-affected property, is difficult to reconstruct may imply that
recovering source-specific information from the transport-affected proton
plasma properties is challenging
A generalized approach to model the spectra and radiation dose rate of solar particle events on the surface of Mars
For future human missions to Mars, it is important to study the surface
radiation environment during extreme and elevated conditions. In the long term,
it is mainly Galactic Cosmic Rays (GCRs) modulated by solar activity that
contributes to the radiation on the surface of Mars, but intense solar
energetic particle (SEP) events may induce acute health effects. Such events
may enhance the radiation level significantly and should be detected as
immediately as possible to prevent severe damage to humans and equipment.
However, the energetic particle environment on the Martian surface is
significantly different from that in deep space due to the influence of the
Martian atmosphere. Depending on the intensity and shape of the original solar
particle spectra as well as particle types, the surface spectra may induce
entirely different radiation effects. In order to give immediate and accurate
alerts while avoiding unnecessary ones, it is important to model and well
understand the atmospheric effect on the incoming SEPs including both protons
and helium ions. In this paper, we have developed a generalized approach to
quickly model the surface response of any given incoming proton/helium ion
spectra and have applied it to a set of historical large solar events thus
providing insights into the possible variety of surface radiation environments
that may be induced during SEP events. Based on the statistical study of more
than 30 significant solar events, we have obtained an empirical model for
estimating the surface dose rate directly from the intensities of a power-law
SEP spectra
Ready functions for calculating the Martian radiation environment
It is extremely important to understand and model the Martian radiation
environment in preparation for future human missions to Mars, especially during
extreme and elevated conditions such as an intense solar energetic particle
(SEP) event. Such events may enhance the radiation level drastically and should
be forecasted as soon as possible to prevent severe damage to humans and
equipment. Besides, the omnipresent galactic cosmic rays (GCRs) also contribute
significantly to the radiation in space and on the surface of Mars and may
cause long-term damages to current and future missions. Based on GEANT4 Monte
Carlo simulations with the Martian atmospheric and regolith environment setup,
we have calculated and obtained some ready-to-go functions which can be used to
quickly convert any given SEP or GCR proton/helium ion spectra to the radiation
dose on the surface of Mars and also at different depth of the atmosphere. We
implement these functions to the RADMAREE tool under the Europlanet project
which can be easily accessed by the public
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