229 research outputs found
An elliptic expansion of the potential field source surface model
Context. The potential field source surface model is frequently used as a
basis for further scientific investigations where a comprehensive coronal
magnetic field is of importance. Its parameters, especially the position and
shape of the source surface, are crucial for the interpretation of the state of
the interplanetary medium. Improvements have been suggested that introduce one
or more additional free parameters to the model, for example, the current sheet
source surface (CSSS) model.
Aims. Relaxing the spherical constraint of the source surface and allowing it
to be elliptical gives modelers the option of deforming it to more accurately
match the physical environment of the specific period or location to be
analyzed.
Methods. A numerical solver is presented that solves Laplace's equation on a
three-dimensional grid using finite differences. The solver is capable of
working on structured spherical grids that can be deformed to create elliptical
source surfaces.
Results. The configurations of the coronal magnetic field are presented using
this new solver. Three-dimensional renderings are complemented by
Carrington-like synoptic maps of the magnetic configuration at different
heights in the solar corona. Differences in the magnetic configuration computed
by the spherical and elliptical models are illustrated.Comment: 11 pages, 7 figure
Possible in situ Tests of the Evolution of Elemental and Isotopic Abundances in the Solar Convection Zone
Helioseismology has shown that the chemical composition of the Sun has
changed over its lifetime. The surface abundance of helium and heavy elements
is believed to have decreased by up to 10% relative to their initial values.
However, this reduction is too small to be tested by direct observations of the
photospheric chemical composition. Here, we compare the predicted variations in
the solar photospheric composition with precise measurements of abundances in
meteorites and the solar wind composition. Although elemental composition
ratios can vary by roughly a percent (e. g. for Ca/Mg and Ca/Fe) over the Sun's
lifetime, their measurements are rife with uncertainties related to
uncertainties in the interpretation of meteoritic measurements, photospheric
determinations, and the complex fractionation processes occurring between the
upper photosphere and lower chromosphere and the corona. On the other hand,
isotopic ratios can be measured much more accurately and are not expected to be
affected as much by extrasolar processes, although more work is required to
quantify their effect. As the isotopic ratios evolve in the Sun proportionally
to the mass ratios of the isotopes, light elements yield the highest variations
in isotopic ratios. They are predicted to reach as high as 0.6% for
O/O and are only slightly lower in the cases of
Mg/Mg and Si/Si. Such a value should be well within
the sensitivity of new missions such as Genesis.Comment: 10 pages, 6 figures, accepted for publication in Journal of
geophysical Research-Space Physic
Recommended from our members
Evolution of the Suprathermal Proton Population at Interplanetary Shocks
We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a "hump" profile peaking around similar to 10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (5-10 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v(-5) is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.NASA-HGI grant [NNX16AF73G]; NASA/LWS grant [NNX15AD03G]; NASA [NNH17ZDA001N-LWS]; NASA under ACE grant [NNX10AT75G]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Applications of Abundance Data and Requirements for Cosmochemical Modeling
Understanding the evolution of the universe from Big Bang to its present state requires an understanding of the evolution of the abundances of the elements and isotopes in galaxies, stars, the interstellar medium, the Sun and the heliosphere, planets and meteorites. Processes that change the state of the universe include Big Bang nucleosynthesis, star formation and stellar nucleosynthesis, galactic chemical evolution, propagation of cosmic rays, spallation, ionization and particle transport of interstellar material, formation of the solar system, solar wind emission and its fractionation (FIP/FIT effect), mixing processes in stellar interiors, condensation of material and subsequent geochemical fractionation. Here, we attempt to compile some major issues in cosmochemistry that can be addressed with a better knowledge of the respective element or isotope abundances. Present and future missions such as Genesis, Stardust, Interstellar Pathfinder, and Interstellar Probe, improvements of remote sensing instrumentation and experiments on extraterrestrial material such as meteorites, presolar grains, and lunar or returned planetary or cometary samples will result in an improved database of elemental and isotopic abundances. This includes the primordial abundances of D, ^3He, ^4He, and ^7Li, abundances of the heavier elements in stars and galaxies, the composition of the interstellar medium, solar wind and comets as well as the (highly) volatile elements in the solar system such as helium, nitrogen, oxygen or xenon
Unusual composition of the solar wind in the 2–3 May 1998 CME observed with SWICS on ACE
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95193/1/grl11810.pd
Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona
We present a detailed analysis of oxygen ion velocity distributions in the
extended solar corona, based on observations made with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. Polar coronal holes at
solar minimum are known to exhibit broad line widths and unusual intensity
ratios of the O VI 1032, 1037 emission line doublet. The traditional
interpretation of these features has been that oxygen ions have a strong
temperature anisotropy, with the temperature perpendicular to the magnetic
field being much larger than the temperature parallel to the field. However,
recent work by Raouafi and Solanki suggested that it may be possible to model
the observations using an isotropic velocity distribution. In this paper we
analyze an expanded data set to show that the original interpretation of an
anisotropic distribution is the only one that is fully consistent with the
observations. It is necessary to search the full range of ion plasma parameters
to determine the values with the highest probability of agreement with the UVCS
data. The derived ion outflow speeds and perpendicular kinetic temperatures are
consistent with earlier results, and there continues to be strong evidence for
preferential ion heating and acceleration with respect to hydrogen. At
heliocentric heights above 2.1 solar radii, every UVCS data point is more
consistent with an anisotropic distribution than with an isotropic
distribution. At heights above 3 solar radii, the exact probability of isotropy
depends on the electron density chosen to simulate the line-of-sight
distribution of O VI emissivity. (abridged abstract)Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 679; May
20, 2008
Flat Proton Spectra in Large Solar Energetic Particle Events
We present solar energetic particle events observed at 1 AU from the Sun for which the proton energy spectra at energies between ~50 keV to ~1 MeV flatten during a period of at least ~12 hours prior to the passage of the associated interplanetary shock. The flattening of the proton energy spectra occurs when the source of the particles (presumably the traveling interplanetary shock) is still downwind from the spacecraft and particle intensities are still continuously increasing. The arrival of the shock at the spacecraft is then characterized by a steepening of the spectra, where low-energy proton intensities show a more pronounced enhancement than the high-energy proton intensities. We discuss the mechanisms that may result in this flattening of the spectra in terms of current models presented in the literature
Solar energetic electron events measured by MESSENGER and Solar Orbiter. Peak intensity and energy spectrum radial dependences: statistical analysis
Context/Aims: We present a list of 61 solar energetic electron (SEE) events
measured by the MESSENGER mission and the radial dependences of the electron
peak intensity and the peak-intensity energy spectrum. The analysis comprises
the period from 2010 to 2015, when MESSENGER heliocentric distance varied
between 0.31 and 0.47 au. We also show the radial dependencies for a shorter
list of 12 SEE events measured in February and March 2022 by spacecraft near 1
au and by Solar Orbiter around its first close perihelion at 0.32 au.
Results: Due to the elevated background intensity level of the particle
instrument on board MESSENGER, the SEE events measured by this mission are
necessarily large and intense; most of them accompanied by a CME-driven shock,
being widespread in heliolongitude, and displaying relativistic (1 MeV)
electron intensity enhancements. The two main conclusions derived from the
analysis of the large SEE events measured by MESSENGER, which are generally
supported by Solar Orbiter's data results, are: (1) There is a wide variability
in the radial dependence of the electron peak intensity between 0.3 au
and 1 au, but the peak intensities of the energetic electrons decrease
with radial distance from the Sun in 27 out of 28 events. On average and within
the uncertainties, we find a radial dependence consistent with . (2)
The electron spectral index found in the energy range around 200 keV
(200) of the backward-scattered population near 0.3 au measured by
MESSENGER is harder in 19 out of 20 (15 out of 18) events by a median factor of
20% (10%) when comparing to the anti-sunward propagating beam
(backward-scattered population) near 1 au.Comment: 20 pages, 13 figure
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