S. Grzedzielski

WARM BREEZE FROM THE STARBOARD BOW: A NEW POPULATION OF NEUTRAL HELIUM IN THE HELIOSPHERE

We investigate the signals from neutral helium atoms observed in situ from Earth orbit in 2010 by the Interstellar Boundary Explorer (IBEX). The full helium 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 helium that we call the Warm Breeze. The Warm Breeze is approximately 2 times 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 ~19° from the inflow direction of interstellar gas. The Warm Breeze seems to be a long-term, perhaps permanent feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere. This effect 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. We discuss possible sources for the Warm Breeze, including (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 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. Whatever the nature of the Warm Breeze, its discovery exposes a critical new feature of our heliospheric environment.

Model Predictions and Remote Observations of the Hydrogen Density Profile in the Distant Heliosphere

Abstract Observations of the hydrogen Ly-α glow by the deep space ultraviolet photometers and spectrometers on board Pioneer 10 and Voyager 2 have been utilized to infer the presence of a nearby solar wind shock. It is a general feature of the available solar shock models that the inflowing interstellar hydrogen will be depleted by charge exchange scattering as it traverses the heliosphere at its transport rate of about four AU per year. This depletion occurs primarily beyond the solar shock in the hot subsonic plasma. The presence of a solar shock will lead to look direction dependent incorrect estimates of the true local interstellar density. Prior estimates of the interstellar hydrogen density must accordingly be reevaluated. The observational and theoretical basis for our current understanding of the heliospheric neutral density profile is presented.

Assessment of Energetic Neutral He Atom Intensities Expected from the IBEX Ribbon

Full sky maps of energetic neutral atoms (ENA) obtained with the Interstellar Boundary Explorer revealed a bright, arc-like Ribbon. We compare possible, though as yet undetected, He ENA emission in two models of the Ribbon origin. The models were originally developed for hydrogen ENA. In the first one, ENA are produced outside the heliopause from the ionized neutral solar wind in the direction where the local interstellar magnetic field is perpendicular to the line of sight. The second model proposes production at the contact layer between the Local Interstellar Cloud (LIC) and the Local Bubble (LB). The models are redesigned to helium using relevant interactions between atoms and ions. Resulting intensities are compared with possible emission of helium ENA from the heliosheath. In the first model, the expected intensity is ~0.014 (cm2 s sr keV)–1, i.e., of the order of the He emission from the heliosheath, whereas in the second, the LIC/LB contact layer model, the intensity is ~(2-7) (cm2 s sr keV)–1, i.e., a few hundred times larger. If the IBEX Ribbon needs a source population of He ENA leaving the heliosphere, it should not be visible in He ENA fluxes mainly because of the insufficient supply of the parent He ENA originating from the neutralized solar wind α-particles. We conclude that full-sky measurements of He ENA could give promising prospects for probing the Local Interstellar Medium at the distance of a few thousand AU and create the possibility of distinction between the above mentioned models of Ribbon origin.

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.

Imaging the heliosheath using HSTOF energetic neutral atoms and Voyager 1 ion data

Context. Voyager 1 and 2 LECP instruments measure the distributions of the heliosheath ions of energies ≥40 keV. This threshold energy is an order of magnitude higher than the maximum energy (6 keV) of the energetic neutral atoms (ENA) to be measured by the forthcoming IBEX mission. On the other hand, the energy range of SOHO/CELIAS/HSTOF ENA measurements is 58−88 keV for H and 28−58 keV/n for He atoms, which overlaps with the energy range of Voyagers LECP. This offers a unique opportunity to combine HSTOF ENA measurements at 1 AU with LECP ion measurements in the heliosheath and obtain information about the large-scale structure of the heliosphere. Aims. We use energetic neutral atoms data at 1 AU and a Voyager 1 spectrum in the heliosheath to estimate the average column density of neutral hydrogen in selected sectors of the forward heliosheath. Methods. We reanalyzed the energetic neutral hydrogen and helium data from HSTOF to identify the contribution to the neutral atoms flux originating in directions close to the apex of the Sun’s motion relative to the local interstellar medium (LISM). We combine the data from HSTOF with the parent ion spectrum in the heliosheath measured by Voyager 1 to derive the background neutral hydrogen column density NH in the heliosheath and estimate the thickness L of the heliosheath within ±45 ◦ from the apex direction and in two 90 ◦ wide flank sectors. Results. In the forward sector of the heliosheath NH = (0.63 ± 0.19) × 10 14 cm −2 , corresponding to the thickness L = (42 ± 12 AU)/nH/(0. 1c m −3 )w herenH = average H density in the heliosheath. This is within the range of values predicted by theoretical models, but suggests that the heliosheath is thinner than expected. The hydrogen column densities in the flank sectors are not symmetric relative to the apex, but the difference is within the statistical uncertainty. The H/He ratio measured by HSTOF is lower than the value following from Voyager 1 heliosheath spectra.

Trapped Radiation in the Outer Heliosphere

Abstract Time evolution of the electromagnetic radiation trapped in the outer heliosphere and interacting with the solar wind of fluctuating intensity is studied within linear theory in the geometrical optics approximation. A net energy transfer from moving solar wind fluctuations to trapped radiation is found, resulting in the increase of the radiation frequency at the rate of ~1 kHz/year. We discuss the implications of this result for understanding the origin of the 2-3 kHz signals observed by the Voyager missions.

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 on board the SOHO satellite has been measuring weak fluxes of He atoms of 28–58 keV/n (helium energetic neutral atoms, He ENAs). The probable source region is the inner heliosheath. Aims. We aim to understand the emission mechanism of He ENAs based on knowledge of the heliosheath spatial extent and plasma content resulting from Voyager 1 and 2 measurements in the period after termination shock crossings. Methods. He ENAs 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 ENAs form a higher energy part of general heliospheric He ENA fluxes and can be explained by the proposed mechanism to within 2σ 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.

Expected Beams of Energetic Neutral Atoms in the Outer Heliosphere

Abstract Estimates of the fluxes of energetic neutral atoms (ENA) in the distant heliosphere are calculated taking into account all important interactions between solar wind plasmas and neutral gases in the heliosphere. Effects of charge-exchange, double charge-exchange (for helium component), electron impact ionization, photoionization, recombination in thermal and non-thermal plasmas are calculated for various conditions over the solar cycle. The resulting neutral beams of energetic atoms emerging from the heliosphere may carry into the local interstellar medium (LISM) up to % 0.1-0.3 of the total solar wind energy, depending mainly on the location of the termination shock (“small” or “large” heliosphere), spatial direction and rather weakly on adopted plasma and gas parameters (densities, temperatures). The free energy thus transferred to LISM may significantly affect the state of the interstellar plasma in the immediate vicinity of the heliosphere.

On the Asymmetry of the Distribution of the Anomalous Cosmic-Ray Component in the Heliosphere

An analysis based on a fluid type equation for the spatial diffusion of the anomalous component of cosmic rays in the solar wind is presented. The source distributions of the high-energy particles are related to the pick-up ion flux intensities at the position of the heliospheric shock. Due to the strong asymmetric distributions of the different species of pick-up ions in the heliosphere and a possible aspherical termination shock of the solar wind, the source distributions are expected to exhibit element specific upwind-downwind asymmetries. An analytical treatment of the problem, using boundary conditions derived from observations by the Pioneer and Voyager satellites, leads to an estimate of the asymmetries of the anomalous component in the heliosphere. The investigation is performed in two stages: the problem is solved for an axially symmetric heliosphere in the first instance, and a latitudinal variation of the solar wind velocity as well as in the hydrodynamic diffusion coefficient is incorporated to model the effects of a heliospheric magnetic field in the second. Subject headings: acceleration of particles — cosmic rays — shock waves — solar wind

Energetic Ions and the Observations of the Heliosheath by means of ENA

Voyager observations show that the inner heliosheath is characterized by high intensity of the energetic (E>28 keV) ions. Due to neutralization of these ions, the heliosheath is an important source of the energetic neutral atoms, which in turn can be observed in the inner solar system. The fluxes of the energetic hydrogen (58–88 keV) and helium (28–58 keV/n) atoms from the heliosheath have been measured by the instrument HSTOF on board SOHO. By combining these observations with the Voyager post‐shock ion data it is possible to estimate the parameters of the inner heliosheath. We discuss recent developments related to this approach.Since Voyager ion data are restricted to the “nose” part of the heliosheath, they cannot be used to interpret the ENA observations of the flanks or the tail region. We use a simple model of the heliosphere to study the effects of the transport and loss mechanisms shaping the ion density distribution in the different regions.