Year: 2025
Type: article
Abstract: Interstellar neutral (ISN) atoms enable studies of the physical conditions in the local interstellar medium surrounding the heliosphere. ISN helium, which is the most abundant species at 1 au, is directly observed by space missions, such as Interstellar Boundary Explorer (IBEX). However, some of these atoms are ionized by solar UV radiation before reaching 1 au, producing pickup ions (PUIs). A recent analysis of IBEX data suggests that the helium photoionization rates predicted by models are underestimated by up to 40%. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons enables the study of PUIs, giving complementary insight into the other side of the ionization process. Our goal is to verify this increased helium ionization by determining the ionization rate of ISN helium in the heliosphere based on SWAP observations of helium PUIs. For this purpose, we analyze SWAP data collected between 2012 and 2022, at distances 22–54 au from the Sun. We develop a new method for fitting model distribution functions to the observational data using the maximum likelihood method. Our approach accounts for the spacecraft’s rotation and the SWAP response function, which depends on both energy and inflow direction. We estimate SWAP’s efficiency for helium relative to that for hydrogen, and determine the ISN helium ionization rate. We find that the photoionization rate obtained from the SWAP observations is ∼43% larger than the rates predicted by models, confirming the IBEX results.
Source: The Astrophysical Journal
Authors: Małgorzata Antonik, Paweł Swaczyna, David J. McComas, Heather A. Elliott, Maciej Bzowski
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G5306384885, G563090337, G6539802142, G753524653, THE PLANNED PERIOD OF PERFORMANCE IS NOVEMBER 1 2017 THROUGH OCTOBER 31 2019. THE FOLLOWING ACTIVITIES WILL BE PERFORMED DURING THE EXTENDED MISSION BY PRINCETON. A. PROVIDE PI LEADERSHIP OF THE IBEX SCIENCE EFFORT THROUGHOUT THE EXTENDED MISSION B. MANAGE THE PRINCETON SCIENCE EFFORT THROUGHOUT THE EXTENDED MISSION C. PREPARE AND SUBMIT QUARTERLY TECHNICAL PROGRESS REPORTS D. SUPPORT TEAM TELECONS 1 TO 2 TIMES PER MONTH E. ATTEND SCIENCE TEAM MEETINGS AND SCIENTIFIC CONFERENCES AS CALLED BY THE PI F. DEVELOP ALGORITHMS AND SOFTWARE AS REQUEST BY THE PI IN SUPPORT OF IBEX DATA ANALYSIS OR THEORY EFFORTS G. ANALYZE AND PUBLISH IBEX INVESTIGATION SCIENCE DATA H. RECEIVE AND ANALYZE PAYLOAD SCIENCE DATA; WORK WITH THE IBEX TEAM TO DEVELOP MODELS ALGORITHMS ETC. TO ANALYZE THE SCIENCE DATA; PUBLISH IBEX SCIENCE RESULTS; SUPPORT THE ARCHIVAL OF SCIENCE DATA
OpenAlex ID: W4416977943