1,049 research outputs found
Does the spacecraft trajectory strongly affect the detection of magnetic clouds?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections
(ICMEs) where a magnetic flux rope is detected. Is the difference between MCs
and ICMEs without detected flux rope intrinsic or rather due to an
observational bias? As the spacecraft has no relationship with the MC
trajectory, the frequency distribution of MCs versus the spacecraft distance to
the MCs axis is expected to be approximately flat. However, Lepping and Wu
(2010) confirmed that it is a strongly decreasing function of the estimated
impact parameter. Is a flux rope more frequently undetected for larger impact
parameter? In order to answer the questions above, we explore the parameter
space of flux rope models, especially the aspect ratio, boundary shape, and
current distribution. The proposed models are analyzed as MCs by fitting a
circular linear force-free field to the magnetic field computed along simulated
crossings.
We find that the distribution of the twist within the flux rope, the
non-detection due to too low field rotation angle or magnitude are only weakly
affecting the expected frequency distribution of MCs versus impact parameter.
However, the estimated impact parameter is increasingly biased to lower values
as the flux-rope cross section is more elongated orthogonally to the crossing
trajectory. The observed distribution of MCs is a natural consequence of a
flux-rope cross section flattened in average by a factor 2 to 3 depending on
the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s,
present an almost uniform distribution of MCs vs. impact parameter, which is
consistent with round shaped flux ropes, in contrast with the slower ones. We
conclude that either most of the non-MC ICMEs are encountered outside their
flux rope or near the leg region, or they do not contain any
Transfer reactions in the sudden limit of the pairing-rotor model
The transfer of multiple pairs of particles in heavy-ion reactions is studied in the sudden limit of the macroscopic pairing-rotor model
Characterization of the Turbulent Magnetic Integral Length in the Solar Wind: From 0.3 to 5 Astronomical Units
The solar wind is a structured and complex system, in which the fields vary
strongly over a wide range of spatial and temporal scales. As an example, the
turbulent activity in the wind affects the evolution in the heliosphere of the
integral turbulent scale or correlation length [{\lambda}], usually associated
with the breakpoint in the turbulent-energy spectrum that separates the
inertial range from the injection range. This large variability of the fields
demands a statistical description of the solar wind. In this work, we study the
probability distribution function (PDF) of the magnetic autocorrelation lengths
observed in the solar wind at different distances from the Sun. We use
observations from Helios, ACE, and Ulysses spacecraft. We distinguish between
the usual solar wind and one of its transient components (Interplanetary
Coronal Mass Ejections, ICMEs), and study also solar wind samples with low and
high proton beta [\beta_p ]. We find that in the last 3 regimes the PDF of
{\lambda} is a log-normal function, consistent with the multiplicative and
non-linear processes that take place in the solar wind, the initial {\lambda}
(before the Alfv\'enic point) being larger in ICMEs
Role of break-up processes in fusion enhancement of drip-line nuclei at energies below the Coulomb barrier
We carry out realistic coupled-channels calculations for
Be + Pb reaction in order to discuss the effects of break-up
of the projectile nucleus on sub-barrier fusion.
We discretize in energy the particle continuum states, which are associated
with the break-up process, and construct the coupling form factors to these
states on a microscopic basis.
The incoming boundary condition is employed in solving coupled-channels
equations, which enables us to define the flux for complete fusion inside the
Coulomb barrier. It is shown that complete fusion cross sections are
significantly enhanced due to the couplings to the continuum states compared
with the no coupling case at energies below the Coulomb barrier, while they are
hindered at above barrier energies.Comment: RevTex, 3 pages, 5 figure
Superposed epoch study of ICME sub-structures near Earth and their effects on galactic cosmic rays
Interplanetary coronal mass ejections (ICMEs) are the interplanetary
manifestations of solar eruptions. The overtaken solar wind forms a sheath of
compressed plasma at the front of ICMEs. Magnetic clouds (MCs) are a subset of
ICMEs with specific properties (e.g. the presence of a flux rope). When ICMEs
pass near Earth, ground observations indicate that the flux of galactic cosmic
rays (GCRs) decreases. The main aims of this paper are to find: common plasma
and magnetic properties of different ICME sub-structures, and which ICME
properties affect the flux of GCRs near Earth. We use a superposed epoch method
applied to a large set of ICMEs observed \insitu\ by the spacecraft ACE,
between 1998 and 2006. We also apply a superposed epoch analysis on GCRs time
series observed with the McMurdo neutron monitors. We find that slow MCs at 1
AU have on average more massive sheaths. We conclude that it is because they
are more effectively slowed down by drag during their travel from the Sun. Slow
MCs also have a more symmetric magnetic field and sheaths expanding similarly
as their following MC, while in contrast, fast MCs have an asymmetric magnetic
profile and a compressing sheath in compression. In all types of MCs, we find
that the proton density and the temperature, as well as the magnetic
fluctuations can diffuse within the front of the MC due to 3D reconnection.
Finally, we derive a quantitative model which describes the decrease of cosmic
rays as a function of the amount of magnetic fluctuations and field strength.
The obtained typical profiles of sheath/MC/GCR properties corresponding to
slow, mid, and fast ICMEs, can be used for forecasting/modelling these events,
and to better understand the transport of energetic particles in ICMEs. They
are also useful for improving future operative space weather activities.Comment: 13 pages, 6 figures, paper accepted in A&
Expansion of magnetic clouds in the outer heliosphere
A large amount of magnetized plasma is frequently ejected from the Sun as
coronal mass ejections (CMEs). Some of these ejections are detected in the
solar wind as magnetic clouds (MCs) that have flux rope signatures. Magnetic
clouds are structures that typically expand in the inner heliosphere. We derive
the expansion properties of MCs in the outer heliosphere from one to five
astronomical units to compare them with those in the inner heliosphere. We
analyze MCs observed by the Ulysses spacecraft using insitu magnetic field and
plasma measurements. The MC boundaries are defined in the MC frame after
defining the MC axis with a minimum variance method applied only to the flux
rope structure. As in the inner heliosphere, a large fraction of the velocity
profile within MCs is close to a linear function of time. This is indicative
of} a self-similar expansion and a MC size that locally follows a power-law of
the solar distance with an exponent called zeta. We derive the value of zeta
from the insitu velocity data. We analyze separately the non-perturbed MCs
(cases showing a linear velocity profile almost for the full event), and
perturbed MCs (cases showing a strongly distorted velocity profile). We find
that non-perturbed MCs expand with a similar non-dimensional expansion rate
(zeta=1.05+-0.34), i.e. slightly faster than at the solar distance and in the
inner heliosphere (zeta=0.91+-0.23). The subset of perturbed MCs expands, as in
the inner heliosphere, at a significantly lower rate and with a larger
dispersion (zeta=0.28+-0.52) as expected from the temporal evolution found in
numerical simulations. This local measure of the expansion also agrees with the
distribution with distance of MC size,mean magnetic field, and plasma
parameters. The MCs interacting with a strong field region, e.g. another MC,
have the most variable expansion rate (ranging from compression to
over-expansion)
Progressive transformation of a flux rope to an ICME
The solar wind conditions at one astronomical unit (AU) can be strongly
disturbed by the interplanetary coronal mass ejections (ICMEs). A subset,
called magnetic clouds (MCs), is formed by twisted flux ropes that transport an
important amount of magnetic flux and helicity which is released in CMEs. At 1
AU from the Sun, the magnetic structure of MCs is generally modeled neglecting
their expansion during the spacecraft crossing. However, in some cases, MCs
present a significant expansion. We present here an analysis of the huge and
significantly expanding MC observed by the Wind spacecraft during 9 and 10
November, 2004. After determining an approximated orientation for the flux rope
using the minimum variance method, we precise the orientation of the cloud axis
relating its front and rear magnetic discontinuities using a direct method.
This method takes into account the conservation of the azimuthal magnetic flux
between the in- and out-bound branches, and is valid for a finite impact
parameter (i.e., not necessarily a small distance between the spacecraft
trajectory and the cloud axis). Moreover, using the direct method, we find that
the ICME is formed by a flux rope (MC) followed by an extended coherent
magnetic region. These observations are interpreted considering the existence
of a previous larger flux rope, which partially reconnected with its
environment in the front. These findings imply that the ejected flux rope is
progressively peeled by reconnection and transformed to the observed ICME (with
a remnant flux rope in the front part).Comment: Solar Physics (in press
Spinodal Instabilities in Nuclear Matter in a Stochastic Relativistic Mean-Field Approach
Spinodal instabilities and early growth of baryon density fluctuations in
symmetric nuclear matter are investigated in the basis of stochastic extension
of relativistic mean-field approach in the semi-classical approximation.
Calculations are compared with the results of non-relativistic calculations
based on Skyrme-type effective interactions under similar conditions. A
qualitative difference appears in the unstable response of the system: the
system exhibits most unstable behavior at higher baryon densities around
in the relativistic approach while most unstable
behavior occurs at lower baryon densities around in
the non-relativistic calculationsComment: 18 pages, 7 figure
Interplanetary Magnetic Field Guiding Relativistic Particles
The origin and the propagation of relativistic solar particles (0.5 to few Ge V) in the interplanetary medium remains a debated topic. These relativistic particles, detected at the Earth by neutron monitors have been previously accelerated close to the Sun and are guided by the interplanetary magnetic field (IMF) lines, connecting the acceleration site and the Earth. Usually, the nominal Parker spiral is considered for ensuring the magnetic connection to the Earth. However, in most GLEs the IMF is highly disturbed, and the active regions associated to the GLEs are not always located close to the solar footprint of the nominal Parker spiral. A possible explanation is that relativistic particles are propagating in transient magnetic structures, such as Interplanetary Coronal Mass Ejections (ICMEs). In order to check this interpretation, we studied in detail the interplanetary medium where the particles propagate for 10 GLEs of the last solar cycle. Using the magnetic field and the plasma parameter measurements (ACE/MAG and ACE/SWEPAM), we found widely different IMF configurations. In an independent approach we develop and apply an improved method of the velocity dispersion analysis to energetic protons measured by SoHO/ERNE. We determined the effective path length and the solar release time of protons from these data and also combined them with the neutron monitor data. We found that in most of the GLEs, protons propagate in transient magnetic structures. Moreover, the comparison between the interplanetary magnetic structure and the interplanetary length suggest that the timing of particle arrival at Earth is dominantly determined by the type of IMF in which high energetic particles are propagating. Finally we find that these energetic protons are not significantly scattered during their transport to Earth
The role of alpha particles in the emission of plasma waves inside solar ejecta
The enhancement of the resonant instability of right-hand polarized electromagnetic ion cyclotron waves by alpha particles for physical parameters corresponding to coronal mass ejections is studied. We focus on the effects of alpha thermal anisotropy and relative He++/H+ abundance on growth and absorption rates. The first parameter governs directly wave emission, while the second modifies also the wave speed and indirectly enhances the wave excitation
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