280 research outputs found
A new 3D solar wind speed and density model based on interplanetary scintillation
The solar wind (SW) is an outflow of the solar coronal plasma, expanding
supersonically throughout the heliosphere. SW particles interact by charge
exchange with interstellar neutral atoms and on one hand, they modify the
distribution of this gas in interplanetary space, and on the other hand they
are seed population for heliospheric pickup ions and energetic neutral atoms
(ENAs). The heliolatitudinal profiles of the SW speed and density evolve during
the cycle of solar activity. A model of evolution of the SW speed and density
is needed to interpret observations of ENAs, pickup ions, the heliospheric
backscatter glow, etc. We derive the Warsaw Heliospheric Ionization Model 3DSW
(WawHelIon 3DSW) based on interplanetary scintillation (IPS) tomography maps of
the SW speed. We take the IPS tomography data from 1985 until 2020, compiled by
\citet{tokumaru_etal:21a}. We derive a novel statistical method of filtering
these data against outliers, we present a flexible analytic formula for the
latitudinal profiles of the SW speed based on Legendre polynomials of varying
order with additional restraining conditions at the poles, fit this formula to
the yearly filtered data, and calculate the yearly SW density profiles using
the latitudinally invariant SW energy flux, observed in the ecliptic plane.
Despite application of refined IPS data set, a more sophisticated data
filtering method, and a more flexible analytic model, the present results
mostly agree with those obtained previously, which demonstrates the robustness
of IPS studies of the SW structure.Comment: Submitted to ApJ
The flow direction of interstellar neutral H from SOHO/SWAN
Interstellar neutral hydrogen flows into the heliosphere as a mixture of the
primary and secondary populations from two somewhat different directions due to
splitting occurring in the magnetized outer heliosheath. The direction of
inflow of interstellar neutral H observed in the inner heliosphere, confronted
with that of the unperturbed flow of interstellar neutral helium, is important
for understanding the geometry of the distortion of the heliosphere from axial
symmetry. It is also needed for facilitating remote-sensing studies of the
solar wind structure based on observations of the helioglow, such as those
presently performed by SOHO/SWAN, and in a near future by IMAP/GLOWS. In the
past, the only means to measure the flow direction of interstellar hydrogen
were spectroscopic observations of the helioglow. Here, we propose a new method
to determine this parameter based on a long series of photometric observations
of the helioglow. The method is based on purely geometric considerations and
does not depend on any model and absolute calibration of the measurements. We
apply this method to sky maps of the helioglow available from the SOHO/SWAN
experiment and derive the mean flow longitude of interstellar hydrogen. We
obtain 253.1\degr \pm 2.8\degr, which is in perfect agreement with the
previously obtained results based on spectroscopic observations.Comment: Accepted for Ap
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Strongly Localized Donor Level in Oxygen Doped Gallium Nitride
A classification in terms of localization of donor defects in GaN is performed by Raman spectroscopy under large hydrostatic pressure. We observe a significant decrease of free carrier concentration in highly O doped GaN epitaxial films at 22 GPa, indicating the presence of a strongly localized donor defect at large pressure. Monitoring the phonon plasmon coupled mode, we find similarities with results on highly n-type bulk crystals. We refine the model of localized defects in GaN and transfer it to the AlGaN system
High Electron Mobility in AlGaN/GaN Heterostructures Grown on Bulk GaN Substrates
Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities
High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates
Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities
A search for point sources of EeV photons
Measurements of air showers made using the hybrid technique developed with
the fluorescence and surface detectors of the Pierre Auger Observatory allow a
sensitive search for point sources of EeV photons anywhere in the exposed sky.
A multivariate analysis reduces the background of hadronic cosmic rays. The
search is sensitive to a declination band from -85{\deg} to +20{\deg}, in an
energy range from 10^17.3 eV to 10^18.5 eV. No photon point source has been
detected. An upper limit on the photon flux has been derived for every
direction. The mean value of the energy flux limit that results from this,
assuming a photon spectral index of -2, is 0.06 eV cm^-2 s^-1, and no celestial
direction exceeds 0.25 eV cm^-2 s^-1. These upper limits constrain scenarios in
which EeV cosmic ray protons are emitted by non-transient sources in the
Galaxy.Comment: 28 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Reconstruction of inclined air showers detected with the Pierre Auger Observatory
We describe the method devised to reconstruct inclined cosmic-ray air showers
with zenith angles greater than detected with the surface array of
the Pierre Auger Observatory. The measured signals at the ground level are
fitted to muon density distributions predicted with atmospheric cascade models
to obtain the relative shower size as an overall normalization parameter. The
method is evaluated using simulated showers to test its performance. The energy
of the cosmic rays is calibrated using a sub-sample of events reconstructed
with both the fluorescence and surface array techniques. The reconstruction
method described here provides the basis of complementary analyses including an
independent measurement of the energy spectrum of ultra-high energy cosmic rays
using very inclined events collected by the Pierre Auger Observatory.Comment: 27 pages, 19 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (JCAP
Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory
The azimuthal asymmetry in the risetime of signals in Auger surface detector
stations is a source of information on shower development. The azimuthal
asymmetry is due to a combination of the longitudinal evolution of the shower
and geometrical effects related to the angles of incidence of the particles
into the detectors. The magnitude of the effect depends upon the zenith angle
and state of development of the shower and thus provides a novel observable,
, sensitive to the mass composition of cosmic rays
above eV. By comparing measurements with predictions from
shower simulations, we find for both of our adopted models of hadronic physics
(QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass
increases slowly with energy, as has been inferred from other studies. However,
the mass estimates are dependent on the shower model and on the range of
distance from the shower core selected. Thus the method has uncovered further
deficiencies in our understanding of shower modelling that must be resolved
before the mass composition can be inferred from .Comment: Replaced with published version. Added journal reference and DO
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