58 research outputs found
Heavy coronal ions in the heliosphere. II. Expected fluxes of energetic neutral He atoms from the heliosheath
Aims. A model of heliosheath density and energy spectra of alpha-particles
and He+ ions carried by the solar wind is developed. Neutralization of
heliosheath He+ ions, mainly by charge exchange (CX) with neutral interstellar
H and He atoms, gives rise to ~0.2 - ~100 keV fluxes of energetic neutral He
atoms (He ENA). Such fluxes, if observed, would give information about plasmas
in the heliosheath and heliospheric tail. Methods. Helium ions crossing the
termination shock (TS) constitute suprathermal (test) particles convected by
(locally also diffusing through) hydrodynamically calculated background plasma
flows (three versions of flows are employed). The He ions proceed from the TS
towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations
of the evolution of alpha- and He+ particle densities and energy spectra
include binary interactions with background plasma and interstellar atoms,
adiabatic heating (cooling) resulting from flow compression (rarefaction), and
Coulomb scattering on background plasma. Results. Neutralization of
suprathermal He ions leads to the emergence of He ENA fluxes with energy
spectra modified by the Compton-Getting effect at emission and ENA loss during
flight to the Sun. Energy-integrated He ENA intensities are in the range ~0.05
- ~50 cm^-2 s^-1 sr^-1 depending on spectra at the TS (assumed
kappa-distributions), background plasma model, and look direction. The
tail/apex intensity ratio varies between ~1.8 and ~800 depending on model
assumptions. Energy spectra are broad with maxima in the ~0.2 - ~3 keV range
depending on the look direction and model. Conclusions. Expected heliosheath He
ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft.
Data could offer insight into the heliosheath structure and improve
understanding of the post-TS solar wind plasmas. HT direction and extent could
be assessed.Comment: 11 pages, 8 figures. Accepted Astronomy & Astrophysic
Solar wind He pickup ions as source of tens-of-keV/n neutral He atoms observed by the HSTOF/SOHO detector
Context. Since 1996, during periods of low solar activity, the HSTOF
instrument onboard the SOHO satellite has been measuring weak fluxes of He
atoms of 28-58 keV/n (helium energetic neutral atoms - He ENA). The probable
source region is the inner heliosheath.
Aims. We aim to understand the emission mechanism of He ENA based on
knowledge of heliosheath spatial extent and plasma content resulting from
Voyager 1 & 2 measurements in the period after termination shock crossings.
Methods. He ENA are generated by charge-exchange neutralization of energetic
helium ions on interstellar neutral H and He. Energy spectra of helium ions in
the heliosheath are calculated by following the evolution of their velocity
distribution functions when carried by and undergoing binary interactions with
plasma constituents of a background flow whose particle populations are modeled
to approximately render post-termination-shock Voyager data.
Results. The observed HSTOF He ENA form a higher energy part of general
heliospheric He ENA fluxes and can be explained by the proposed mechanism to
within 2{\sigma} error. The main factor determining the level of emission (and
its uncertainty) is the energy spectrum of He^+ pickup ions in post-termination
shock plasmas.Comment: 6 pages, 2 figures, v2: version accepted for publication in Astronomy
and Astrophysic
Solar ions in the heliosheath: a possible new source of heavy neutral atoms
We show that multiply ionized coronal C, N, O, Mg, Si, S ions carried by the
solar wind and neutralized by consecutive electron captures from neutral
interstellar atoms constitute an important new source of neutral atoms in the
inner heliosheath, with energies up to ~ 1 keV/n. In the model we developed,
the heavy ions are treated as test particles carried by hydrodynamic plasma
flow (with a Monte-Carlo description of interstellar neutrals) and undergoing
all relevant atomic processes determining the evolution of all charge-states of
considered species (radiative and dielectronic recombination, charge exchange,
photo-, and electron impact ionization). The total strength of the source is
from ~10^6 g/s for S to ~10^8 g/s for O, deposited as neutrals below the
heliopause. These atoms should provide, as they drift to supersonic wind
region, important sources of PUIs and eventually ACRs, especially for species
that are excluded from entering the heliosphere because of their ionization in
the LISM. The expected corresponding ENA fluxes at 1 AU are in the range 10^-4
- 10^0 at./(cm^2 s sr), depending on the species and direction (Table 2).Comment: Submitted for IGGP Astrophysics Conference, March 2006; 6 page
Imprints of a heliospheric bowshock on interstellar oxygen populations
International audienceIt is well known that the nearby interstellar medium represents a partially ionized gas composed by different chemical species, amongst them hydrogen, helium and oxygen as the most abundant elements. While the passage of the interstellar protons and H-atoms over the solar system has been satisfactorily well modelled meanwhile, the entrance into the heliosphere of the other interstellar chemical species needs some additional care. Here we especially follow with Boltzmann-kinetic treatments the phase-space history of interstellar oxygen ions and atoms at their passage over the outer heliospheric bowshock and at their further approach towards the inner heliosphere. We describe the nose region of the bowshock as a mild MHD shock of a nearly perpendicular type. While the O-atoms pass over this shock with no velocity-space imprints, by contrast the O+-ions run over the shock structure with a typical overshoot velocity and on the downstream region are then picked up by the frozen-in magnetic fields that comove with the plasma bulk flow, i.e. the proton bulk flow. This leads to a strongly non-relaxated O-ion velocity distribution which substantially differs both from the local O-atom and proton distribution functions. Due to a strong charge exchange coupling between O-ions/atoms and H-atoms/ions the distributions downstream from the bowshock undergo permanent changes which we estimate in this study here. Even though relaxation processes operate by Coulomb collisions, wave-particle interactions and elastic atom-atom collisions, the bowshock imprints on the O- ion and atoms distribution functions may be conserved throughout the whole heliospheric interface, and, as we are going to suggest in this paper, in fact as such may be used as tracers to the physics and the nature of the bowshock. One of the main conclusions is that the population of O-atoms created in the vicinity of the bow-shock should carry a signature of the effective shock strength that could be detectable if the LIC magnetic field is about 2 microgauss or stronger
A possible generation mechanism for the IBEX ribbon from outside the heliosphere
The brightest and most surprising feature in the first all-sky maps of
Energetic Neutral Atoms (ENA) emissions (0.2-6 keV) produced by the
Interstellar Boundary Explorer (IBEX) is an almost circular ribbon of a
~140{\deg} opening angle, centered at (l,b) = (33{\deg}, 55{\deg}), covering
the part of the celestial sphere with the lowest column densities of the Local
Interstellar Cloud (LIC). We propose a novel interpretation of the IBEX results
based on the idea of ENA produced by charge-exchange between the neutral H
atoms at the nearby edge of the LIC and the hot protons of the Local Bubble
(LB). These ENAs can reach the Sun's vicinity because of very low column
density of the intervening LIC material. We show that a plane-parallel or
slightly curved interface layer of contact between the LIC H atoms (n_H = 0.2
cm^-3, T = 6000-7000 K) and the LB protons (n_p = 0.005 cm^-3, T ~ 10^6 K),
together with indirect contribution coming from multiply-scattered ENAs from
the LB, may be able to explain both the shape of the ribbon and the observed
intensities provided that the edge is < (500-2000) AU away, the LIC proton
density is (correspondingly) < (0.04-0.01) cm^-3, and the LB contains ~1% of
non-thermal protons over the IBEX energy range. If this model is correct, then
IBEX, for the first time, has imaged in ENAs a celestial object from beyond the
confines of the heliosphere and can directly diagnose the plasma conditions in
the LB.Comment: Accepted by Ap.J.Lett
WHY ARE THE MAGNETIC FIELD DIRECTIONS MEASURED BY VOYAGER 1 ON BOTH SIDES OF THE HELIOPAUSE SO SIMILAR?
ABSTRACT The solar wind carves a cavity in the interstellar plasma bounded by a surface, called the heliopause (HP), that separates the plasma and magnetic field of solar origin from those of interstellar origin. It is now generally accepted that in 2012 August Voyager 1 (V1) crossed that boundary. Unexpectedly, the magnetic fields on both sides of the HP, although theoretically independent of each other, were found to be similar in direction. This delayed the identification of the boundary as the HP and led to many alternative explanations. Here, we show that the Voyager 1 observations can be readily explained and, after the Interstellar Boundary Explorer (IBEX) discovery of the ribbon, could even have been predicted. Our explanation relies on the fact that the Voyager 1 and undisturbed interstellar field directions (which we assume to be given by the IBEX ribbon center (RC)) share the same heliolatitude (∼34. • 5) and are not far separated in longitude (difference ∼27 • ). Our result confirms that Voyager 1 has indeed crossed the HP and offers the first independent confirmation that the IBEX RC is in fact the direction of the undisturbed interstellar magnetic field. For Voyager 2, we predict that the difference between the inner and outer magnetic field directions at the HP will be significantly larger than that observed by Voyager 1 (∼30 • instead of ∼20 • ), and that the outer field direction will be close to the RC
Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere
We investigate the signals from neutral He atoms observed from Earth orbit in
2010 by IBEX. The full He signal observed during the 2010 observation season
can be explained as a superposition of pristine neutral interstellar He gas and
an additional population of neutral He that we call the Warm Breeze. The Warm
Breeze is approximately two-fold slower and 2.5 times warmer than the primary
interstellar He population, and its density in front of the heliosphere is ~7%
that of the neutral interstellar helium. The inflow direction of the Warm
Breeze differs by ~19deg from the inflow direction of interstellar gas. The
Warm Breeze seems a long-term feature of the heliospheric environment. It has
not been detected earlier because it is strongly ionized inside the
heliosphere, which brings it below the threshold of detection via pickup ion
and heliospheric backscatter glow observations, as well as by the direct
sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the
secondary population of interstellar helium, created via charge exchange and
perhaps elastic scattering of neutral interstellar He atoms on interstellar He+
ions in the outer heliosheath, or (2) a gust of interstellar He originating
from a hypothetic wave train in the Local Interstellar Cloud. A secondary
population is expected from models, but the characteristics of the Warm Breeze
do not fully conform to modeling results. If, nevertheless, this is the
explanation, IBEX-Lo observations of the Warm Breeze provide key insights into
the physical state of plasma in the outer heliosheath. If the second hypothesis
is true, the source is likely to be located within a few thousand of AU from
the Sun, which is the propagation range of possible gusts of interstellar
neutral helium with the Warm Breeze characteristics against dissipation via
elastic scattering in the Local Cloud.Comment: submitted to ApJ
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