10 research outputs found
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
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
Signatures of the non-Maxwellian -distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)
We investigated the possibility of diagnosing the degree of departure from
the Maxwellian distribution using the Fe XVII - Fe XVIII spectra originating in
plasmas in collisional ionization equilibrium, such as in the cores of solar
active regions or microflares. The original collision strengths for excitation
are integrated over the non-Maxwellian electron -distributions
characterized by a high-energy tail. Synthetic X-ray emission line spectra were
calculated for a range of temperatures and . We focus on the 6-24 A
spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray
Spectrometer MaGIXS. We find that many line intensity ratios are sensitive to
both and . Best diagnostic options are provided if a ratio
involving both Fe XVII and Fe XVIII is combined with another ratio involving
lines formed within a single ion. The sensitivity of such diagnostics to
is typically a few tens of per cent. Much larger sensitivity, of about
a factor of two to three, can be obtained if the Fe XVIII 93.93 A line observed
by SDO/AIA is used in conjuction with the X-ray lines. We conclude that the
MaGIXS instrument is well-suited for detection of departures from the
Maxwellian distribution, especially in active region cores.Comment: Astronomy & Astrophysics, accepte
Shock-reflected electrons and X-ray line spectra
Aims. The aim of this paper is to try to explain the physical origin of the non-thermal electron distribution that is able to form the enhanced intensities of satellite lines in the X-ray line spectra observed during the impulsive phases of some solar flares.
Methods. Synthetic X-ray line spectra of the distributions composed of the distribution of shock reflected electrons and the background Maxwellian distribution are calculated in the approximation of non-Maxwellian ionization, recombination, excitation and de-excitation rates. The distribution of shock reflected electrons is determined analytically.
Results. We found that the distribution of electrons reflected at the nearly-perpendicular shock resembles, at its high-energy part, the so called n-distribution. Therefore it could be able to explain the enhanced intensities of Si xi
Magnetic energy powers the corona: how we can understand its 3D storage & release
The coronal magnetic field is the prime driver behind many as-yet unsolved mysteries: solar eruptions, coronal heating, and the solar wind, to name a few. It is, however, still poorly observed and understood. We highlight key questions related to magnetic energy
storage, release, and transport in the solar corona, and their relationship to these important problems. We advocate for new and multi-point co-optimized measurements, sensitive to
magnetic field and other plasma parameters, spanning from optical to γ-ray wavelengths, to bring closure to these long-standing and fundamental questions. We discuss how our approach can fully describe the 3D magnetic field, embedded plasma, particle energization, and their joint evolution to achieve these objectives
Magnetic Energy Powers the Corona: How We Can Understand its 3D Storage & Release
Synopsis The coronal magnetic field is the prime driver behind many as-yet unsolved mysteries: solar eruptions, coronal heating, and the solar wind, to name a few. It is, however, still poorly observed and understood. We highlight key questions related to magnetic energy storage, release, and transport in the solar corona, and their relationship to these important problems. We advocate for new and multi-point co-optimized measurements, sensitive to magnetic field and other plasma parameters, spanning from optical to γ-ray wavelengths, to bring closure to these long-standing and fundamental questions. We discuss how our approach can fully describe the 3D magnetic field, embedded plasma, particle energization, and their joint evolution to achieve these objectives. Magnetic Energy Powers the Corona: How We Can Understand its 3D Storage & Releas