The Solar Wind as a Possible Source of Fast Temporal Variations of the Heliospheric Ribbon

Abstract

We present a possible source of pickup ions (PUIs) the ribbon observed by the Interstellar Boundary EXplorer (IBEX). We suggest that a gyrating solar wind and PUIs in the ramp and in the near downstream region of the termination shock (TS) could provide a significant source of energetic neutral atoms (ENAs) in the ribbon. A fraction of the solar wind and PUIs are reflected and energized during the first contact with the TS. Some of the solar wind may be reflected propagating toward the Sun but most of the solar wind ions form a gyrating beamlike distribution that persists until it is fully thermalized further downstream. Depending on the strength of the shock, these gyrating distributions can exist for many gyration periods until they are scattered/thermalized due to wave-particle interactions at the TS and downstream in the heliosheath. During this time, ENAs can be produced by charge exchange of interstellar neutral atoms with the gyrating ions. In order to determine the flux of energetic ions, we estimate the solar wind flux at the TS using pressure estimates inferred from in situ measurements. Assuming an average path length in the radial direction of the order of a few AU before the distribution of gyrating ions is thermalized, one can explain a significant fraction of the intensity of ENAs in the ribbon observed by IBEX. With a localized source and such a short integration path, this model would also allow fast time variations of the ENA flux.

SYMMETRY OF THEIBEXRIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

“…Variability of spectra over the ENA sky maps indicates the spectra follow the general latitudinal ordering of the solar wind (Funsten et al 2009a;Dayeh et al 2012;McComas et al 2012b;, are influenced by heliospheric pickup ion populations (Livadiotis et al 2012), and may be composed of multiple source ion populations (Desai et al 2014). Temporal variation is also consistently observed in the sky maps, with a general reduction in ENA flux over time ) that appears to be driven by reduction in the solar wind over the current solar cycle (McComas et al 2012b;Kucharek et al 2013). An energy-dependent temporal variation and a north-south temporal asymmetry of ENA flux is systematically observed at the ecliptic poles (Reisenfeld et al 2012;Allegrini et al 2012), which are viewed continuously throughout the IBEX mission and thus serve as a statistically robust temporal baseline.…”

Heliosheath Processes and the Structure of the Heliopause: Modeling Energetic Particles, Cosmic Rays, and Magnetic Fields

“…Right panel: Full sky map for ENAs at 1.1 keV from IBEX-Hi with the directions of the two Voyager spacecraft and the upwind direction (nose) indicated [64]. termination shock, the inner and outer heliosheath, all the way to the nearby edge of the local interstellar cloud [25,26,66,67,68,69,70,71]. All models involve charge exchange between ions and neutral atoms.…”

Low Energy Neutral Atoms From the Heliosheath

“…Some of these pickup ions charge exchange in the inner heliosheath, creating a population of ENAs with energy that is higher than energies of populations created in processes 1 and 2 in Figure 1 (Malama et al 2006). Finally, the highest energy ENAs (process 4 in Figure 1) are produced by solar wind (and pickup ions) that are accelerated at the termination shock and by turbulence in the inner heliosheath before charge exchanging in the inner heliosheath (Chalov & Fahr 2000;Chalov et al 2003;Fahr et al 2011;Siewert et al 2012;Kucharek et al 2013). The energy of these ENAs extends well beyond the 6 keV upper limit of the IBEX ENA cameras.…”