102 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
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
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
Implementation and validation of the GEANT4/AtRIS code to model the radiation environment at Mars
A new GEANT4 particle transport model -- the Atmospheric Radiation
Interaction Simulator (AtRIS, Banjac et al. 2018a. J. Geophys. Res.) -- has
been recently developed in order to model the interaction of radiation with
planets. The upcoming instrumentational advancements in the exoplanetary
science, in particular transit spectroscopy capabilities of missions like JWST
and E-ELT, have motivated the development of a particle transport code with a
focus on providing the necessary flexibility in planet specification
(atmosphere and soil geometry and composition, tidal locking, oceans, clouds,
etc.) for the modeling of radiation environment for exoplanets. Since there are
no factors limiting the applicability of AtRIS to Mars and Venus, AtRIS' unique
flexibility opens possibilities for new studies. Following the successful
validation against Earth measurements Banjac et al. 2018, J. Geophys. Res.,
this work applies AtRIS with a specific implementation of the Martian
atmospheric and regolith structure to model the radiation environment at Mars.
We benchmark these first modeling results based on different GEANT4 physics
lists with the energetic particle spectra recently measured by the Radiation
Assessment Detector (RAD) on the surface of Mars. The good agreement between
AtRIS and the actual measurement provides one of the first and sound
validations of AtRIS and the preferred physics list which could be recommended
for predicting the radiation field of other conceivable (exo)planets with an
atmospheric environment similar to Mars
Dependence of the Martian radiation environment on atmospheric depth: Modeling and measurement
The energetic particle environment on the Martian surface is influenced by
solar and heliospheric modulation and changes in the local atmospheric pressure
(or column depth). The Radiation Assessment Detector (RAD) on board the Mars
Science Laboratory rover Curiosity on the surface of Mars has been measuring
this effect for over four Earth years (about two Martian years). The
anticorrelation between the recorded surface Galactic Cosmic Ray-induced dose
rates and pressure changes has been investigated by Rafkin et al. (2014) and
the long-term solar modulation has also been empirically analyzed and modeled
by Guo et al. (2015). This paper employs the newly updated HZETRN2015 code to
model the Martian atmospheric shielding effect on the accumulated dose rates
and the change of this effect under different solar modulation and atmospheric
conditions. The modeled results are compared with the most up-to-date (from 14
August 2012 to 29 June 2016) observations of the RAD instrument on the surface
of Mars. Both model and measurements agree reasonably well and show the
atmospheric shielding effect under weak solar modulation conditions and the
decline of this effect as solar modulation becomes stronger. This result is
important for better risk estimations of future human explorations to Mars
under different heliospheric and Martian atmospheric conditions
Measurements of Forbush decreases at Mars: both by MSL on ground and by MAVEN in orbit
The Radiation Assessment Detector (RAD), on board Mars Science Laboratory's
(MSL) Curiosity rover, has been measuring ground level particle fluxes along
with the radiation dose rate at the surface of Mars since August 2012. Similar
to neutron monitors at Earth, RAD sees many Forbush decreases (FDs) in the
galactic cosmic ray (GCR) induced surface fluxes and dose rates. These FDs are
associated with coronal mass ejections (CMEs) and/or stream/corotating
interaction regions (SIRs/CIRs). Orbiting above the Martian atmosphere, the
Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has also been
monitoring space weather conditions at Mars since September 2014. The
penetrating particle flux channels in the Solar Energetic Particle (SEP)
instrument onboard MAVEN can also be employed to detect FDs. For the first
time, we study the statistics and properties of a list of FDs observed in-situ
at Mars, seen both on the surface by MSL/RAD and in orbit detected by the
MAVEN/SEP instrument. Such a list of FDs can be used for studying
interplanetary CME (ICME) propagation and SIR evolution through the inner
heliosphere. The magnitudes of different FDs can be well-fitted by a power-law
distribution. The systematic difference between the magnitudes of the FDs
within and outside the Martian atmosphere may be mostly attributed to the
energy-dependent modulation of the GCR particles by both the pass-by ICMEs/SIRs
and the Martian atmosphere
Modelling two Energetic Storm Particle Events Observed by Solar Orbiter Using the Combined EUHFORIA and iPATH Models
By coupling the EUropean Heliospheric FORcasting Information Asset (EUHFORIA)
and the improved Particle Acceleration and Transport in the Heliosphere (iPATH)
model, two energetic storm particle (ESP) events, originating from the same
active region (AR 13088) and observed by Solar Orbiter (SolO) on August 31 2022
and September 05 2022, are modelled. While both events originated from the same
active region, they exhibited notable differences, including: 1) the August ESP
event lasted for 7 hours, while the September event persisted for 16 hours; 2)
The time intensity profiles for the September event showed a clear cross-over
upstream of the shock where the intensity of higher energy protons exceeds
those of lower energy protons, leading to positive (``reverse'') spectral
indices prior to the shock passage. For both events, our simulations replicate
the observed duration of the shock sheath, depending on the deceleration
history of the CME. Imposing different choices of escaping length scale, which
is related to the decay of upstream turbulence, the modelled time intensity
profiles prior to the shock arrival also agree with observations. In
particular, the cross-over of this time profile in the September event is well
reproduced. We show that a ``reverse'' upstream spectrum is the result of the
interplay between two length scales. One characterizes the decay of upstream
shock accelerated particles, which are controlled by the energy-dependent
diffusion coefficient, and the other characterizes the decay of upstream
turbulence power, which is related to the process of how streaming protons
upstream of the shock excite Alfv\'{e}n waves. Simulations taking into account
real-time background solar wind, the dynamics of the CME propagation, and
upstream turbulence at the shock front are necessary to thoroughly understand
the ESP phase of large SEP events.Comment: Accepted by A&A. 16 pages, 11 figure
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