Distance to the IBEX Ribbon beyond the Heliopause

Abstract Our understanding of the solar–interstellar interaction greatly improved with measurements of energetic neutral atoms (ENAs) from the outer heliosphere by NASA’s Interstellar Boundary Explorer (IBEX). The IBEX mission measurements showed us the solar–interstellar interaction was far more complex than we originally thought. One such measurement was the Ribbon: a narrow, enhanced band of ENA emissions wrapped across the sky. We now generally understand its origin being outside the heliopause. This was confirmed, among other methods, using the astronomical method of parallax and temporal cross correlations with the solar wind output from the Sun. Here, we perform cross correlations of Ribbon ENAs with the solar wind to derive the distance to the Ribbon, and associated uncertainties, in two different sections of the sky (centered ∼30° below and ∼30° above the ecliptic plane). Parameters that introduce uncertainties include the neutral H density in the very local interstellar medium, the speed of the parent neutral particles that travel across the heliopause, and the cross correlation. We derive the distance to the Ribbon in these two spatially averaged sections, yielding distances of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>d</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 90 ± 56 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>d</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 192 ± 42 au for the southern and northern sections of the Ribbon, respectively, or an average of 155 ± 34 au—a rough value one may use for the Ribbon source distance in the forward hemisphere of the heliosphere. Quantifying uncertainties is important for understanding the accuracy of these calculations as they can be a significant fraction of the calculated distance to the Ribbon itself.