Warm Breeze From the Starboard Bow: A New Population of Neutral Helium in the Heliosphere

Abstract

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 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 ∼19 • from the inflow direction of interstellar gas. The Warm Breeze seems a longterm, 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 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. Whatever the nature of the Warm Breeze, its discovery exposes a critical new feature of our heliospheric environment.Subject headings: keywords population was seen as an excess of the observed signal over the one-component fit to the NIS He inflow (Fig. 1) during the early orbits of the IBEX NIS gas observation seasons.Also, the one-component Maxwellian inflow could not explain a portion (not shown) of the observed signal with elevated wings on both sides of the signal from the primary population of NIS gas. In this paper we investigate these signals and their implications in more detail. Observations Data collectionIBEX is a spin-stabilized spacecraft following a highly elliptical orbit around the Earth. The boresight of the IBEX-Lo instrument is perpendicular to the spin axis (H lond et al. 2012), which is adjusted at the beginning of each IBEX orbit to maintain it within ∼7 • from the Sun. Interstellar atoms can be detected only when the IBEX-Lo aperture is looking into the flow, which happens during the first quarter of each year. The precise orientation of the IBEX spin axis...

INTERSTELLAR NEUTRAL HELIUM IN THE HELIOSPHERE FROM IBEX OBSERVATIONS. II. THE WARSAW TEST PARTICLE MODEL (WTPM)

“…( ) y Thus, averaging over spin-angle bins does not introduce any significant additional error. In all, the calculation load in this aspect is reduced by ∼30% in comparison with the model (2) approach used by Bzowski et al (2012) and Kubiak et al (2014) and, additionally, the accuracy is higher. We have verified that using lower-order polynomials does not always provide a sufficient accuracy, while using a higher order method would not necessarily bring better results, but certainly would increase the calculation load in comparison with method (2).…”

“…It was also used by Bzowski et al (2014) to re-analyze observations from GAS/Ulysses, including the first analysis of the data from the last Ulysses orbit in 2007, which had previously not been analyzed. This analysis brought a flow vector similar to the original analysis by Witte (2004), but a temperature higher by at least ∼1000 K. It was also used by Bzowski et al (2013a) and Park et al (2014) to analyze the abundance of Ne/O ratio in the LIC based on IBEX-Lo measurements, and by Kubiak et al (2014) to discover the additional ISN He population detected by IBEX-Lo dubbed the Warm Breeze, which is very likely the secondary heliospheric population of ISN He. The WTPM model was also used by Kubiak et al (2013) to predict possibilities of detection of the ISN D flux by IBEX-Lo, subsequently found in the IBEX-Lo signal by Rodríguez Moreno et al (2013Moreno et al ( , 2014.…”

“…Taking this simulation as baseline, we compared results of three methods, simple and easy to implement, to obtain the simulations averaged over 6°b ins: (1) tabulating the flux with a 6°step at the center of the bin (thick dots in Figure 9), (2) arithmetic averaging of the flux sampled every 1°(the method used by Bzowski et al 2012 andKubiak et al 2014), and (3) integrating a polynomial representation of the flux, sampled every 1 5, according to the formula from Equation (39).…”

Ibex Observations of Secondary Interstellar Helium and Oxygen Distributions

“…Third, the green pixels between λ=160°and 200°r epresent the secondary ISN He population, which is also called "Warm Breeze" Kubiak et al 2014Kubiak et al , 2016. Because the logarithmic representation does not show the directional distribution of this population in detail, we compare the latitude profiles of the count rates along the latitudinal swaths between λ=160°and 300°in the He maps.…”

“…Bzowski et al (2012) hypothesizedthe secondary population of the ISN He atoms to describe the differences between simulations and observations. Kubiak et al (2014) discovered what they termed the "Warm Breeze," which is slower and warmer than the primary ISN He population, and its flow direction differs by about 19°from the ISN gas flow. Kubiak et al (2016) revisited the inflow direction of the Warm Breeze and found that it is almost co-planar with the inflow directions of ISN H and He and the direction to the center of IBEX Ribbon.…”