197 research outputs found
The Specific Acceleration Rate in Loop-structured Solar Flares -- Implications for Electron Acceleration Models
We analyze electron flux maps based on RHESSI hard X-ray imaging spectroscopy
data for a number of extended coronal loop flare events. For each event, we
determine the variation of the characteristic loop length with electron
energy , and we fit this observed behavior with models that incorporate an
extended acceleration region and an exterior "propagation" region, and which
may include collisional modification of the accelerated electron spectrum
inside the acceleration region. The models are characterized by two parameters:
the plasma density in, and the longitudinal extent of, the
acceleration region. Determination of the best-fit values of these parameters
permits inference of the volume that encompasses the acceleration region and of
the total number of particles within it. It is then straightforward to compute
values for the emission filling factor and for the {\it specific acceleration
rate} (electrons s per ambient electron above a chosen reference
energy). For the 24 events studied, the range of inferred filling factors is
consistent with a value of unity. The inferred mean value of the specific
acceleration rate above keV is s, with a
1 spread of about a half-order-of-magnitude above and below this value.
We compare these values with the predictions of several models, including
acceleration by large-scale, weak (sub-Dreicer) fields, by strong
(super-Dreicer) electric fields in a reconnecting current sheet, and by
stochastic acceleration processes
Properties of the Acceleration Regions in Several Loop-structured Solar Flares
Using {\em RHESSI} hard X-ray imaging spectroscopy observations, we analyze
electron flux maps for a number of extended coronal loop flares. For each
event, we fit a collisional model with an extended acceleration region to the
observed variation of loop length with electron energy , resulting in
estimates of the plasma density in, and longitudinal extent of, the
acceleration region. These quantities in turn allow inference of the number of
particles within the acceleration region and hence the filling factor --
the ratio of the emitting volume to the volume that encompasses the emitting
region(s). We obtain values of that lie mostly between 0.1 and 1.0; the
(geometric) mean value is , somewhat less than, but
nevertheless consistent with, unity. Further, coupling information on the
number of particles in the acceleration region with information on the total
rate of acceleration of particles above a certain reference energy (obtained
from spatially-integrated hard X-ray data) also allows inference of the
specific acceleration rate (electron s per ambient electron above the
chosen reference energy). We obtain a (geometric) mean value of the specific
acceleration rate keV)
electrons s per ambient electron; this value has implications both for
the global electrodynamics associated with replenishment of the acceleration
region and for the nature of the particle acceleration process
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
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
Genesis and impulsive evolution of the 2017 September 10 coronal mass ejection
The X8.2 event of 10 September 2017 provides unique observations to study the
genesis, magnetic morphology and impulsive dynamics of a very fast CME.
Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot ( MK) bright rim around a quickly expanding cavity, embedded inside a much
larger CME shell ( MK). The CME shell develops from a dense set
of large AR loops (0.5 ), and seamlessly evolves into the CME
front observed in LASCO C2. The strong lateral overexpansion of the CME shell
acts as a piston initiating the fast EUV wave. The hot cavity rim is
demonstrated to be a manifestation of the dominantly poloidal flux and
frozen-in plasma added to the rising flux rope by magnetic reconnection in the
current sheet beneath. The same structure is later observed as the core of the
white light CME, challenging the traditional interpretation of the CME
three-part morphology. The large amount of added magnetic flux suggested by
these observations explains the extreme accelerations of the radial and lateral
expansion of the CME shell and cavity, all reaching values of km
s. The acceleration peaks occur simultaneously with the first RHESSI
keV hard X-ray burst of the associated flare, further underlining the
importance of the reconnection process for the impulsive CME evolution.
Finally, the much higher radial propagation speed of the flux rope in relation
to the CME shell causes a distinct deformation of the white light CME front and
shock.Comment: Accepted for publication in the Astrophysical Journa
Relationship between Hard and Soft X-ray Emission Components of a Solar Flare
X-ray observations of solar flares routinely reveal an impulsive high-energy
and a gradual low-energy emission component, whose relationship is one of the
key issues of solar flare study. The gradual and impulsive emission components
are believed to be associated with, respectively, the thermal and nonthermal
components identified in spectral fitting. In this paper, a prominent about 50
second hard X-ray (HXR) pulse of a simple GOES class C7.5 flare on 20 February
2002 is used to study the association between high energy, non-thermal and
impulsive evolution, and low energy, thermal and gradual evolution. We use
regularized methods to obtain time derivatives of photon fluxes to quantify the
time evolution as a function of photon energy, obtaining a break energy between
impulsive and gradual behavior. These break energies are consistent with a
constant value of about 11 keV in agreement with those found spectroscopically
between thermal and non-thermal components, but the relative errors of the
former are greater than 15% and much greater than the a few percent errors
found from the spectral fitting. These errors only weakly depend on assuming an
underlying spectral model for the photons, pointing to the current data being
inadequate to reduce the uncertainties rather than there being a problem
associated with an assumed model. The time derivative method is used to test
for the presence of a 'pivot energy' in this flare. Although these pivot
energies are marginally consistent with a constant value of about 9 keV, its
values in the HXR rise phase appear to be lower than those in the decay phase
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
Three-dimensional Reconstruction of Coronal Mass Ejections by CORAR Technique through Different Stereoscopic Angle of STEREO Twin Spacecraft
Recently, we developed the Correlation-Aided Reconstruction (CORAR) method to
reconstruct solar wind inhomogeneous structures, or transients, using dual-view
white-light images (Li et al. 2020; Li et al. 2018). This method is proved to
be useful for studying the morphological and dynamical properties of transients
like blobs and coronal mass ejection (CME), but the accuracy of reconstruction
may be affected by the separation angle between the two spacecraft (Lyu et al.
2020). Based on the dual-view CME events from the Heliospheric Imager CME Join
Catalogue (HIJoinCAT) in the HELCATS (Heliospheric Cataloguing, Analysis and
Techniques Service) project, we study the quality of the CME reconstruction by
the CORAR method under different STEREO stereoscopic angles. We find that when
the separation angle of spacecraft is around 150{\deg}, most CME events can be
well reconstructed. If the collinear effect is considered, the optimal
separation angle should locate between 120{\deg} and 150{\deg}. Compared with
the CME direction given in the Heliospheric Imager Geometrical Catalogue
(HIGeoCAT) from HELCATS, the CME parameters obtained by the CORAR method are
reasonable. However, the CORAR-obtained directions have deviations towards the
meridian plane in longitude, and towards the equatorial plane in latitude. An
empirical formula is proposed to correct these deviations. This study provides
the basis for the spacecraft configuration of our recently proposed Solar Ring
mission concept (Wang et al. 2020b).Comment: 18 pages, 9 figure
Primary and albedo protons detected by the Lunar Lander Neutron and Dosimetry experiment on the lunar farside
The Lunar Lander Neutron and Dosimetry (LND) Experiment aboard the Chang’E-4 Lander on the lunar far-side measures energetic charged and neutral particles and monitors the corresponding radiation levels. During solar quiet times, galactic cosmic rays (GCRs) are the dominating component of charged particles on the lunar surface. Moreover, the interaction of GCRs with the lunar regolith also results in upward-directed albedo protons which are measured by the LND. In this work, we used calibrated LND data to study the GCR primary and albedo protons. We calculate the averaged GCR proton spectrum in the range of 9–368 MeV and the averaged albedo proton flux between 64.7 and 76.7 MeV from June 2019 (the seventh lunar day after Chang’E-4’s landing) to July 2020 (the 20th lunar day). We compare the primary proton measurements of LND with the Electron Proton Helium INstrument (EPHIN) on SOHO. The comparison shows a reasonable agreement of the GCR proton spectra among different instruments and illustrates the capability of LND. Likewise, the albedo proton measurements of LND are also comparable with measurements by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) during solar minimum. Our measurements confirm predictions from the Radiation Environment and Dose at the Moon (REDMoon) model. Finally, we provide the ratio of albedo protons to primary protons for measurements in the energy range of 64.7–76.7 MeV which confirm simulations over a broader energy range
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