Investigating the IBEX Ribbon Structure a Solar Cycle Apart

A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by the Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding of the heliosphere boundaries and the physical processes occurring at the interstellar interface. The Ribbon signal is intertwined with that of a globally distributed flux (GDF) that spans the entire sky. To a certain extent, Ribbon separation methods enabled examining its evolution independent of the underlying GDF. Observations over a full solar cycle revealed the Ribbon's evolving nature, with intensity variations closely tracking those of the solar wind (SW) structure after a few years delay accounting for the SW-ENA recycling process. In this work, we examine the Ribbon structure, namely, its ENA fluxes, angular extent, width, and circularity properties for two years, 2009 and 2019, representative of the declining phases of two adjacent solar cycles. We find that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south of it down to ~ 25{\deg} (for energies below 1.7 keV) and not at mid and high ecliptic latitudes; (ii) The Ribbon width exhibits significant variability as a function of azimuthal angle; (iii) Circularity analysis suggests that the 2019 Ribbon exhibits a statistically consistent radius with that in 2009. The Ribbon's partial recovery is aligned with the consensus of a heliosphere with its closest point being southward of the nose region. The large variability of the Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely indicative of small-scale processes within the Ribbon.Comment: 5 figure

Whence the interstellar magnetic field shaping the heliosphere?

Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation, B (magpol), aligned with Galactic coordinates l = 42 degrees, b = 49 degrees. Further analysis shows the ISMF is more orderly for "downfield" stars located over 90 degrees from B (magpol). The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates lambda, beta approximate to 219 degrees +/- 15 degrees, 43 degrees +/- 9 degrees (Galactic coordinates l, b approximate to 40 degrees, 56 degrees, +/- 17 degrees). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, "Filament A," extends over 300 degrees of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55 degrees long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5 degrees of the filament and within 10 degrees of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB

Simulating hydrogen energetic neutral atom flux measurements for NASA's IBEX mission

The heliosphere is a “comet-like” bubble of plasma reaching from ∼102 to over 103 astronomical units in size. It is created by the outflow of solar wind (SW) plasma and its interaction with the partially-ionized local interstellar medium (LISM). Due to its large size, it is unfeasible to take in situ measurements at the edges of this interaction. Therefore it is necessary to develop sensing techniques to remotely probe the heliosphere and its boundaries. The NASA-funded Interstellar Boundary EXplorer (IBEX) mission is aimed at improving our understanding of the heliospheric interface. Launched in 2008 October, IBEX measures fluxes of energetic neutral atoms (ENAs) that are created through the SW–LISM interaction, as well as interstellar neutral atoms that permeate the heliospheric boundary. Out of the neutral atom species that IBEX can detect, hydrogen (H) atoms are the most abundant in interstellar space and the heliosphere. Hydrogen ENAs, in particular, are created when relatively energetic protons from the heliospheric plasma charge-exchange with interstellar H atoms. Due to their high energies, and thus large mean free paths, H ENAs can propagate large distances before ionizing (i.e., on the order of the size of the heliosphere), and can be detected by IBEX. The purpose of this study is to simulate H ENA flux measurements at 1 AU and relate these to the IBEX mission. Three goals of this study that are of particular interest to IBEX are: (1) to simulate H ENA fluxes measured in the solar (inertial) and IBEX spacecraft frames of reference in order to better understand IBEX measurements made in different frames of reference; (2) to study the effects of pickup ions, i.e., non-thermalized ions, on H ENA fluxes, and determine how IBEX observations can reveal the properties of PUIs in the distant heliosphere; (3) to analyze the effects of a time-dependent solar cycle on IBEX H ENA measurements, particularly the “ribbon” of enhanced flux encircling the sky. The simulations are performed by post-processing a pre-simulated, “background” heliosphere containing plasma and neutral H properties (e.g., density, temperature, velocity) produced from a three-dimensional magnetohydrodynamic/kinetic simulation of the SW–LISM interaction

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices <1.8. The ENA spectral index over the poles shows a periodic evolution over the solar cycle 24. The surface area with flatter ENA spectra (<1.8) around the ecliptic south pole increases slightly from 2009–2011 and then decreased gradually from 2012–2014. The PCH completely disappears in 2016 and then starts to appear again starting in 2017, gradually growing until 2019. This evolution shows a clear correlation with the change in the PCH area observed at the Sun once the delay in the ENA observation time is included. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the ENA spectrum near the south pole in 2014 and 2017. The rapid evolution in 2014 is related to a rapid closing of PCHs in 2012 and that in 2017 is related to a rapid opening of PCHs in late 2014. These results also agree qualitatively with the evolution of the ENA spectral index from simulations using a simple time-dependent heliospheric flow model