HELIOSPHERIC NEUTRAL ATOM SPECTRA BETWEEN 0.01 AND 6 keV FROMIBEX

Abstract

Since 2008 December, the Interstellar Boundary Explorer (IBEX) has been making detailed observations of neutrals from the boundaries of the heliosphere using two neutral atom cameras with overlapping energy ranges. The unexpected, yet defining feature discovered by IBEX is a Ribbon that extends over the energy range from about 0.2 to 6 keV. This Ribbon is superposed on a more uniform, globally distributed heliospheric neutral population. With some important exceptions, the focus of early IBEX studies has been on neutral atoms with energies greater than ∼0.5 keV. With nearly three years of science observations, enough low-energy neutral atom measurements have been accumulated to extend IBEX observations to energies less than ∼0.5 keV. Using the energy overlap of the sensors to identify and remove backgrounds, energy spectra over the entire IBEX energy range are produced. However, contributions by interstellar neutrals to the energy spectrum below 0.2 keV may not be completely removed. Compared with spectra at higher energies, neutral atom spectra at lower energies do not vary much from location to location in the sky, including in the direction of the IBEX Ribbon. Neutral fluxes are used to show that low energy ions contribute approximately the same thermal pressure as higher energy ions in the heliosheath. However, contributions to the dynamic pressure are very high unless there is, for example, turbulence in the heliosheath with fluctuations of the order of 50-100 km s −1 .

Low Energy Neutral Atoms From the Heliosheath

“…In February (August), ENA measurements are made when the spacecraft is in the solar wind (in Earth's magnetosphere, shown by the shaded regions). measurement of the same flux over their common energy range (e.g., McComas et al 2009b;Fuselier et al 2012) and observations of the same features (such as the ribbon) in the sky maps (McComas et al 2012b) are powerful evidence for detection of heliospheric ENAs without significant interference from near-Earth backgrounds, at least for ribbon-level fluxes near 1 keV.…”

“…IBEX observes a power law or a Kappa distribution over this energy range with a power law index of ∼-1.5 to −3 (Funsten et al 2009b;McComas et al 2009b). The extension of this power law to energies less than 0.5 keV is more difficult to explain on the basis of bulk flow observations in the inner heliosheath (Fuselier et al 2012). Although the plasma in the inner heliosheath is slowed and heated across the termination shock, on average, it is still moving away from the Sun with average radial bulk velocity of ∼100 km s −1 (Richardson & Wang 2011).…”

“…In contrast, a local, nearEarth background can have large variations over any timescale, including those shorter than an IBEX orbit. For example, ions accelerated at Earth's bow shock enter the IBEX-Lo sensor and create a background and are present intermittently over a wide range of timescales from minutes to hours (e.g., Fuselier et al 2012). Short time variable backgrounds such as the example presented here are removed from the IBEX heliospheric ENA data set by using only those intervals when the flux is the lowest during a given orbit (McComas et al 2012b).…”

“…Short time variable backgrounds such as the example presented here are removed from the IBEX heliospheric ENA data set by using only those intervals when the flux is the lowest during a given orbit (McComas et al 2012b). Although these IBEX-Hi and -Lo data culls differ for the two sensors (Fuselier et al 2012), the selection of the lowest fluxes during a given orbit is a very efficient method for removing background that varies on timescales of less than about one week.…”

“…A very powerful method for distinguishing signal from background is to compare ENA fluxes from two time periods separated by 6 months, after the appropriate frame transformation from the frame moving with the Earth to the inertial frame fixed with the Sun. Fuselier et al (2012) attempted such a comparison with limited success, especially with IBEX-Lo at low energies. The primary difficulty with this attempt was that there were significant backgrounds at low energies associated with ENA measurements when IBEX was in Earth's magnetotail.…”