282 research outputs found
Suprathermal electron distributions in the solar transition region
Suprathermal tails are a common feature of solar wind electron velocity
distributions, and are expected in the solar corona. From the corona,
suprathermal electrons can propagate through the steep temperature gradient of
the transition region towards the chromosphere, and lead to non-Maxwellian
electron velocity distribution functions (VDFs) with pronounced suprathermal
tails. We calculate the evolution of a coronal electron distribution through
the transition region in order to quantify the suprathermal electron population
there. A kinetic model for electrons is used which is based on solving the
Boltzmann-Vlasov equation for electrons including Coulomb collisions with both
ions and electrons. Initial and chromospheric boundary conditions are
Maxwellian VDFs with densities and temperatures based on a background fluid
model. The coronal boundary condition has been adopted from earlier studies of
suprathermal electron formation in coronal loops. The model results show the
presence of strong suprathermal tails in transition region electron VDFs,
starting at energies of a few 10 eV. Above electron energies of 600 eV,
electrons can traverse the transition region essentially collision-free. The
presence of strong suprathermal tails in transition region electron VDFs shows
that the assumption of local thermodynamic equilibrium is not justified there.
This has a significant impact on ionization dynamics, as is shown in a
companion paper
Synthetic IRIS spectra of the solar transition region: Effect of high-energy tails
The solar transition region satisfies the conditions for presence of
non-Maxwellian electron energy distributions with high-energy tails at energies
corresponding to the ionization potentials of many ions emitting in the EUV and
UV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IV
spectra in the far ultra-violet (FUV) channel of the Interface Region Imaging
Spectrograph (IRIS). Ionization, recombination, and excitation rates are
obtained by integration of the cross-sections or their approximations over the
model electron distributions considering particle propagation from the hotter
corona. The ionization rates are significantly affected by the presence of
high-energy tails. This leads to the peaks of the relative abundance of
individual ions to be broadened with pronounced low-temperature shoulders. As a
result, the contribution functions of individual lines observable by IRIS also
exhibit low-temperature shoulders, or their peaks are shifted to temperatures
an order of magnitude lower than for the Maxwellian distribution. The
integrated emergent spectra can show enhancements of Si IV compared toO IV by
more than a factor of two. The high-energy particles can have significant
impact on the emergent spectra and their presence needs to be considered even
in situations without strong local acceleration
On ion-cyclotron-resonance heating of the corona and solar wind
This paper concisely summarizes and critically reviews recent work by the authors on models of the heating of the solar corona by resonance of ions with high-frequency waves (up to the proton cyclotron frequency). The quasi-linear theory of pitch angle diffusion is presented in connection with relevant solar wind proton observations. Hybrid fluid-kinetic model equations, which include wave-particle interactions and collisions, are derived. Numerical solutions are discussed, representative of the inner corona and near-Sun solar wind. A semi-kinetic model for reduced velocity distributions is presented, yielding kinetic results for heavy ions in the solar corona. It is concluded that a self-consistent treatment of particle distributions and wave spectra is required, in order to adequately describe coronal physics and to obtain agreement with observations
Probabilistic Phase Space Trajectory Description for Anomalous Polymer Dynamics
It has been recently shown that the phase space trajectories for the
anomalous dynamics of a tagged monomer of a polymer --- for single polymeric
systems such as phantom Rouse, self-avoiding Rouse, Zimm, reptation, and
translocation through a narrow pore in a membrane; as well as for
many-polymeric system such as polymer melts in the entangled regime --- is
robustly described by the Generalized Langevin Equation (GLE). Here I show that
the probability distribution of phase space trajectories for all these
classical anomalous dynamics for single polymers is that of a fractional
Brownian motion (fBm), while the dynamics for polymer melts between the
entangled regime and the eventual diffusive regime exhibits small, but
systematic deviations from that of a fBm.Comment: 8 pages, two figures, 3 eps figure files, minor changes,
supplementary material included moved to the appendix, references expanded,
to appear in J. Phys.: Condens. Matte
Coronal ion-cyclotron beam instabilities within the multi-fluid description
Spectroscopic observations and theoretical models suggest resonant
wave-particle interactions, involving high-frequency ion-cyclotron waves, as
the principal mechanism for heating and accelerating ions in the open coronal
holes. However, the mechanism responsible for the generation of the
ion-cyclotron waves remains unclear. One possible scenario is that ion beams
originating from small-scale reconnection events can drive micro-instabilities
that constitute a possible source for the excitation of ion-cyclotron waves. In
order to study ion beam-driven electromagnetic instabilities, the multi-fluid
model in the low-beta coronal plasma is used. While neglecting the electron
inertia this model allows one to take into account ion-cyclotron wave effects
that are absent from the one-fluid MHD model. Realistic models of density and
temperature as well as a 2-D analytical magnetic field model are used to define
the background plasma in the open-field funnel region of a polar coronal hole.
Considering the WKB approximation, a Fourier plane-wave linear mode analysis is
employed in order to derive the dispersion relation. Ray-tracing theory is used
to compute the ray path of the unstable wave as well as the evolution of the
growth rate of the wave while propagating in the coronal funnel. We demonstrate
that, in typical coronal holes conditions and assuming realistic values of the
beam velocity, the free energy provided by the ion beam propagating parallel
the ambient field can drive micro-instabilities through resonant ion-cyclotron
excitation.Comment: 8 pages, 6 figures, submitted to A&
Non-Equilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind \textit{(Invited Review)}
We review the presence and signatures of the non-equilibrium processes, both
non-Maxwellian distributions and non-equilibrium ionization, in the solar
transition region, corona, solar wind, and flares. Basic properties of the
non-Maxwellian distributions are described together with their influence on the
heat flux as well as on the rates of individual collisional processes and the
resulting optically thin synthetic spectra. Constraints on the presence of
high-energy electrons from observations are reviewed, including positive
detection of non-Maxwellian distributions in the solar corona, transition
region, flares, and wind. Occurrence of non-equilibrium ionization is reviewed
as well, especially in connection to hydrodynamic and generalized
collisional-radiative modelling. Predicted spectroscopic signatures of
non-equilibrium ionization depending on the assumed plasma conditions are
summarized. Finally, we discuss the future remote-sensing instrumentation that
can be used for detection of these non-equilibrium phenomena in various
spectral ranges.Comment: Solar Physics, accepte
Amplitude and Frequency Spectrum of Thermal Fluctuations of A Translocating RNA Molecule
Using a combination of theory and computer simulations, we study the
translocation of an RNA molecule, pulled through a solid-state nanopore by an
optical tweezer, as a method to determine its secondary structure. The
resolution with which the elements of the secondary structure can be determined
is limited by thermal fluctuations. We present a detailed study of these
thermal fluctuations, including the frequency spectrum, and show that these
rule out single-nucleotide resolution under the experimental conditions which
we simulated. Two possible ways to improve this resolution are strong
stretching of the RNA with a back-pulling voltage across the membrane, and
stiffening of the translocated part of the RNA by biochemical means.Comment: Significantly expanded compared to previous version, 13 pages, 4
figures, to appear in J. Phys.: Condens. Matte
Interferometric imaging with LOFAR remote baselines of the fine structures of a solar type-IIIb radio burst
Context. Solar radio bursts originate mainly from high energy electrons accelerated in solar eruptions like solar flares, jets, and coronal mass ejections. A sub-category of solar radio bursts with short time duration may be used as a proxy to understand wave generation and propagation within the corona.Aims. Complete case studies of the source size, position, and kinematics of short term bursts are very rare due to instrumental limitations. A comprehensive multi-frequency spectroscopic and imaging study was carried out of a clear example of a solar type IIIb-III pair.Methods. In this work, the source of the radio burst was imaged with the interferometric mode, using the remote baselines of the LOw Frequency ARray (LOFAR). A detailed analysis of the fine structures in the spectrum and of the radio source motion with imaging was conducted.Results. The study shows how the fundamental and harmonic components have a significantly different source motion. The apparent source of the fundamental emission at 26.56 MHz displaces away from the solar disk center at about four times the speed of light, while the apparent source of the harmonic emission at the same frequency shows a speed of <0.02 c. The source size of the harmonic emission observed in this case is smaller than that in previous studies, indicating the importance of the use of remote baselines.Peer reviewe
Exploring the Circular Polarisation of Low-Frequency Solar Radio Bursts with LOFAR
The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March-April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long-lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin, but instead they indicate the existence of multiple, possibly unrelated acceleration processes originating from solar minimum active regions.Peer reviewe
- …