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Interstellar conditions deduced from interstellar neutral helium observed by IBEX and global heliosphere modeling
(IOP Publishing, 2023) Swaczyna, Paweł; Bzowski, Maciej; Heerikhuisen, Jacob; Kubiak, Marzena A.; Rahmanifard, Fatemeh; Zirnstein, Eric J.; Fuselier, Stephen A.; Galli, André; McComas, David J.; Möbius, Eberhard; Schwadron, Nathan A.
In situ observations of interstellar neutral (ISN) helium atoms by the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission are used to determine the velocity and temperature of the pristine very local interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere, known as the primary population, originate in the pristine VLISM. As the primary atoms travel through the outer heliosheath, they charge exchange with He+ ions in slowed and compressed plasma, creating the secondary population. With more than 2.4 million ISN helium atoms being sampled by IBEX during ISN seasons 2009-2020, we compare the observations with the predictions of a parameterized model of ISN helium transport in the heliosphere. We account for the filtration of ISN helium atoms at the heliospheric boundaries by charge-exchange and elastic collisions. We examine the sensitivity of the ISN helium fluxes to the interstellar conditions described by the pristine VLISM velocity, temperature, magnetic field, and composition. We show that comprehensive modeling of the filtration processes is critical for interpreting ISN helium observations, as the change in the derived VLISM conditions exceeds the statistical uncertainties when accounting for these effects. The pristine VLISM parameters found by this analysis are the flow speed (26.6 km s−1), inflow direction in ecliptic coordinates (255.°7, 5.°04), temperature (7350 K), and B − V plane inclination to the ecliptic plane (53.°7). The derived pristine VLISM He+ density is 9.7 × 10−3 cm−3. Additionally, we show a strong correlation between the interstellar plasma density and magnetic field strength deduced from these observations.
Relative in-flight response of IBEX-Lo to interstellar neutral helium atoms
(IOP Publishing, 2023) Swaczyna, Paweł ; Bzowski, Maciej; Fuselier, Stephen A.; Galli, André; Heerikhuisen, Jacob; Kubiak, Marzena A.; McComas, David J.; Möbius, Eberhard; Rahmanifard, Fatemeh; Schwadron, Nathan A.
The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission measures interstellar neutral (ISN) helium atoms. The detection of helium atoms is made through negative hydrogen (H−) ions sputtered by helium atoms from the IBEX-Lo’s conversion surface. The energy spectrum of ions sputtered by ISN helium atoms is broad and overlaps the four lowest IBEX-Lo electrostatic analyzer (ESA) steps. Consequently, the energy response function for helium atoms does not correspond to the nominal energy step transmission. Moreover, laboratory calibration is incomplete because it is difficult to produce narrow-energy neutral atom beams that are expected for ISN helium atoms. Here, we analyze the ISN helium observations in ESA steps 1–4 to derive the relative in-flight response of IBEX-Lo to helium atoms. We compare the ratios of the observed count rates as a function of the mean ISN helium atom energy estimated using the Warsaw Test Particle Model (WTPM). The WTPM uses a global heliosphere model to calculate charge exchange gains and losses to estimate the secondary ISN helium population. We find that the modeled mean energies of ISN helium atoms, unlike their modeled fluxes, are not very sensitive to the very local interstellar medium parameters. The obtained relative responses supplement the laboratory calibration and enable more detailed quantitative studies of the ISN helium signal. A similar procedure that we applied to the IBEX-Lo observations may be used to complement laboratory calibration of the next-generation IMAP-Lo instrument on the Interstellar Mapping and Acceleration Probe (IMAP) mission.
Visual illusion susceptibility following mild traumatic brain injury in adolescents
(The University of Waikato, 2026) Cliff, Emily; Starkey, Nicola J.
Following Mild Traumatic Brain Injury (mTBI), clinical symptoms often resolve before full physiological recovery is complete, highlighting the need for objective indicators of ongoing neural disruption. Visual illusions, which rely on contextual visual processing, may provide a sensitive behavioural marker of subtle perceptual changes following concussion. While reduced susceptibility to visual illusions has been demonstrated in adults following mTBI, little is known about whether similar effects are observed in adolescents. The present study examined susceptibility to visual illusions in adolescents with a history of mTBI and in a TBI-free comparison group. A secondary aim was to explore associations between illusion performance, age, and post-concussive symptom burden within the mTBI group. Adolescents with a history of mTBI and age-matched TBI-free comparison participants completed two visual illusion tasks (Ebbinghaus and Müller–Lyer), each comprising context (illusion) and no-context trials. Post-concussion symptoms were assessed using developmentally appropriate versions of the Post-Concussion Symptom Inventory (PCSI). No significant group differences were observed on context trials of either illusion task. Adolescents with mTBI showed significantly lower accuracy on no-context trials of the Ebbinghaus task than comparison participants. Age was not significantly associated with illusion susceptibility in either group. Within the mTBI group, post-concussion symptom severity was not significantly related to performance on Müller–Lyer or Ebbinghaus context trials for either parent-reported or adolescent self-reported symptoms. These findings suggest that visual integration accuracy appears comparable to typical performance in adolescents assessed at a relatively long interval following mTBI, which may reflect recovery or resolution of any earlier disruption. However, whether visual integration was affected during the acute or subacute stages of injury remains unclear. Future research is needed to determine whether visual-illusion paradigms provide a useful, objective complement to symptom-based assessment for detecting subtle perceptual differences following paediatric mTBI.
The effect of adipose tissue on transdermal monochromatic light presented to the human fetus using Monte Carlo simulations
(Springer Nature, 2025) Isaac, Zac; Heerikhuisen, Jacob; Reid, Vincent
The extent to which transdermal light sources illuminate the uterine environment is unknown. Recent work indicates the human fetus responds to external visual stimuli, such as laser diodes, and initial modeling suggests the fetus may not develop in a completely dark environment as previously assumed. Development of the human visual system begins within the womb, and there is motivation in fields such as developmental psychology, transabdominal oximetry, and photoacoustics to explore the extent to which light penetrates maternal abdominal tissue and understand how transdermal stimuli appear to the fetus. We develop and adapt a Monte Carlo model utilising third trimester histological properties of maternal tissue to simulate transdermal monochromatic collimated light sources, with particular focus on the 650 nm wavelength used in experimental applications. We determine approximate levels of third trimester uterine illumination from such stimuli, ranging from being comparable to an overcast night to a full moon in clear conditions. We further discuss the scope for multiple stimuli to be visibly distinct in utero for experimental applications. This modeling provides quantitative guidance on the interactions between transdermal monochromatic collimated source laser diode stimuli and maternal tissue to practitioners and researchers in the fields of fetal vision, ultrasound, and developmental psychology.
The effect of angular scattering imposed by charge exchange and elastic collisions on interstellar neutral hydrogen atoms
(American Astronomical Society, 2023-12-14) Rahmanifard, Fatemeh; Swaczyna, Paweł; Zirnstein, Eric J.; Heerikhuisen, Jacob; Galli, André; Sokół, Justyna M.; Schwadron, Nathan A.; Möbius, Eberhard; McComas, David J.; Fuselier, Stephen A.
Angular scattering (AS) in charge exchange and elastic collisions between interstellar ions and neutral (ISN) atoms has been assumed to be negligible in previous studies. Here, we investigate the momentum transfer associated with the AS of H atoms using Monte Carlo calculations to simulate their transport through the outer heliosheath. We consider two cases where charge exchange and elastic collisions between ISN H atoms and protons occur with and without AS in the outer heliosheath. We show that considering AS decelerates and heats primary ISN H, reducing the effect of selective charge exchange in the outer heliosheath. Secondary ISN H atoms, on the other hand, are not significantly affected by AS. We then simulate the transport of ISN H atoms inside the heliosphere to simulate count rates observed in the lowest energy bin of IBEX-Lo. We study the effect of radiation pressure on the ISN H measurements for the cases with and without AS and compare them with our previous findings. We find an effective radiation parameter (μeff, which represents force associated with radiation pressure relative to gravity) for the years 2009–2018 based on the longitudinal shift of the ISN H signal. The two cases with and without AS reproduce the longitudinal shift in accordance with variations in solar activity, in agreement with our previous results, and they result in similar values for the μeff, which is ∼22%–23% larger than the value found based on solar Lyα profile observations.