Between Local Interstellar Magnetic and Dynamic Pressure Balance of Heliospheric Boundaries Measured with the IBEX Ribbon—A New Paradigm

Abstract

Abstract The Interstellar Boundary Explorer (IBEX) mission discovered a global structure observed in energetic neutral atoms (ENA) emissions—the IBEX “ribbon.” We study the ribbon spatially retained by Alfvén waves. The IBEX ribbon width is used to derive Alfvén speeds in the modified local interstellar medium (LISM), which, averaged across the ribbon, is v A = 42.7 ± 3.0 km s−1. The increase of Alfvén speeds from polar regions to near the nose yields the compression ratio from the pristine LISM into the modified LISM as a function of angle from the ENA pressure maximum in the globally distributed flux. Averaged across the ribbon, the average compression ratio is r c = 1.63 ± 0.22. The compression ratios and Alfvén speeds across the ribbon provide for extrapolations into the pristine LISM, resulting in an average LISM Alfvén speed, v A−LISM = 31.7 ± 3.3 km s−1. With the LISM flow speed of u LISM = 25.82 ± 0.33 km s−1 derived from previous IBEX studies, we find that the ratio of the ram flow energy density to the magnetic energy density in the LISM is u LISM 2 / v A − LISM 2 = 0.66 ± 0.14 . This indicates that the heliosphere is between the extremes of dynamic and magnetic pressure balance. The complex and time-varying structure of the heliosphere is strongly influenced both by interactions with the interstellar magnetic field and the interstellar flow, representing a new paradigm that will be explored as we connect IBEX observations with those made in the future by the Interstellar Mapping and Acceleration Probe (IMAP).