Biography

Eric Zirnstein is a research scientist specializing in the simulation and analysis of pickup ion (PUI) dynamics and hydrogen energetic neutral atom (ENA) emission from the solar wind-very local interstellar medium interaction. For his PhD thesis, he developed a numerical code that computes hydrogen ENA fluxes at 1 AU for NASA’s Interstellar Boundary Explorer (IBEX) mission by post-processing 3D magnetohydrodynamic-plasma/kinetic-neutral simulation results of the heliosphere and generating ENAs via charge-exchange between the simulated plasma and neutral populations. He developed both time-independent and -dependent algorithms, employing MPI and distributed/shared memory algorithms in C that run on multi-node computer clusters, and performs simulation/data analysis and visualization in IDL and Matlab. His current research continues the simulation and analysis of observations made by the IBEX mission, including the relationship between ENA flux observations and the structure of the heliosphere, the properties of non-thermal PUIs at shocks and turbulence, and their evolution with the solar cycle, as well as the interstellar magnetic field draped around the heliosphere. He also analyzes New Horizon’s Solar Wind Around Pluto (SWAP) instrument measurements of pickup ions at the Pluto plasma environment and in the solar wind. He is lead data analyst for the Solar Wind And Pickup Ion (SWAPI) instrument and science team member for the IMAP-Lo, IMAP-Hi, and IMAP-Ultra ENA imagers, all in development for the Interstellar Mapping and Acceleration Probe (IMAP) mission to be launched in 2025.

Research Interests

• Interaction of the solar wind with the local interstellar medium
• Energetic neutral atoms from the heliosphere
• Pickup ion dynamics in the solar wind and interstellar medium
• Particle acceleration at shocks and turbulence
• Simulation of spacecraft measurements

Selected Publications

See full publication list at the Publications Page, Google Scholar, Web of Science, or ORCID.

Oblique and rippled heliosphere structures from the Interstellar Boundary Explorer (2022), Zirnstein et al., NatAs

Past analysis has shown that the heliosphere structure can be deduced from correlations between long-scale solar wind pressure evolution and energetic neutral atom emissions. However, this required spatial and temporal averaging that smoothed out small or dynamic features of the heliosphere. In late 2014, the solar wind dynamic pressure increased by roughly 50% over a period of 6 months, causing a time and directional-dependent rise in around 2–6 keV energetic neutral atom fluxes from the heliosphere observed by the Interstellar Boundary Explorer. Here, we use the 2014 pressure enhancement to provide a simultaneous derivation of the three-dimensional heliospheric termination shock (HTS) and heliopause (HP) distances at high resolution from Interstellar Boundary Explorer measurements. The analysis reveals rippled HTS and HP surfaces that are oblique with respect to the local interstellar medium upwind direction, with significant asymmetries in the heliosphere structure compared to steady-state heliosphere models. We estimate that the heliosphere boundaries contain roughly ten astronomical unit-sized spatial variations, with slightly larger variations on the HTS surface than the HP and a large-scale, southwards-directed obliquity of the surfaces in the meridional plane. Comparisons of the derived HTS and HP distances with Voyager observations indicate substantial differences in the heliosphere boundaries in the northern versus southern hemispheres and their motion over time.

Heliosheath Proton Distribution in the Plasma Reference Frame (2021), Zirnstein et al., ApJS

Properties of the inner heliosheath (IHS) plasma are inferred from energetic neutral atom (ENA) observations by ~1 au spacecraft. However, the Compton–Getting effect due to the plasma velocity relative to the spacecraft is rarely taken into account, even though the plasma speed is a significant fraction of the ENA speed. In this study, we transform Interstellar Boundary Explorer (IBEX) ENA spectra to the IHS plasma frame using flow profiles from a 3D heliosphere simulation. We find that proton spectra in the plasma frame are steeper by ~30% to 5% at ~0.5 to 6 keV, respectively, compared to ENAs in the spacecraft frame. While radial plasma flows contribute most to the Compton–Getting effect, transverse flows at mid/high latitudes and the heliosphere flanks account for up to ~30% of the frame transformation for IBEX-Hi at ~0.7 keV and up to ~60% for IBEX-Lo at ~0.1 keV. We determine that the majority of IHS proton fluxes derived from IBEX-Hi measurements in 2009–2016are statistically consistent with power-law distributions, with mean proton index ~2.1 and standard deviation ~0.4. We find significantly fewer spectral breaks in IBEX observations compared to early analyses, which we determine were a product of the “ion gun” background prevalent in ~2009-2012 before corrections made by McComas et al. in subsequent data releases. We recommend that future analyses of the IHS plasma utilizing ENA measurements take into account the Compton–Getting effect including radial and transverse flows, particularly IBEX and Interstellar Mapping and Acceleration Probe measurements below ~10 keV.

In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary Shock (2018), Zirnstein et al., PhRvL

Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed that PUIs should be preferentially heated at quasi-perpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H+) PUIs at an interplanetary shock by the New Horizons’ Solar Wind Around Pluto (SWAP) instrument at ~34 au from the Sun. At this shock, H+ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H+ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H+ SWIs lose ~85% of their energy flux across the shock and H+ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H+ PUIs is approximately six times that of H+ SWIs. We find that H+ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.

The Role of Pickup Ion Dynamics Outside of the Heliopause in the Limit of Weak Pitch Angle Scattering: Implications for the Source of the IBEX Ribbon (2018), Zirnstein et al., ApJ

We present a new model of the Interstellar Boundary Explorer (IBEX) ribbon based on the secondary energetic neutral atom (ENA) mechanism, under the assumption that there is negligible pitch angle scattering of pickup ions (PUIs) outside the heliopause. Using the results of an MHD-plasma/kinetic-neutral simulation of the heliosphere, we generate PUIs in the outer heliosheath, solve their transport using guiding center theory, and compute ribbon ENA fluxes at 1 au. We implement several aspects of the PUI dynamics, including (1) parallel motion along the local interstellar magnetic field (ISMF), (2) advective transport with the interstellar plasma, (3) the mirror force acting on PUIs propagating along the ISMF, and (4) betatron acceleration of PUIs as they are advected within an increasing magnetic field toward the heliopause. We find that ENA fluxes at 1 au are reduced when PUIs are allowed to move along the ISMF, and ENA fluxes are reduced even more by the inclusion of the mirror force, which pushes particles away from IBEX lines of sight. Inclusion of advection and betatron acceleration do not result in any significant change in the ribbon. Interestingly, the mirror force reduces the ENA fluxes from the inner edge of the ribbon more than those from its outer edge, effectively reducing the ribbon’s width by ~6° and increasing its radius projected on the sky. This is caused by the asymmetric draping of the ISMF around the heliopause, such that ENAs from the ribbon’s inner edge originate closer to the heliopause, where the mirror force is strongest.

Structure of the Heliotail from Interstellar Boundary Explorer Observations: Implications for the 11-year Solar Cycle and Pickup Ions in the Heliosheath (2017), Zirnstein et al., ApJ

Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) from the heliotail show a multi-lobe structure of ENA fluxes as a function of energy between ~0.71 and 4.29 keV. Below ~2 keV, there is a single structure of enhanced ENA fluxes centered near the downwind direction. Above ~2 keV, this structure separates into two lobes, one north and one south of the solar equatorial plane. ENA flux from these two lobes can be interpreted as originating from the fast solar wind (SW) propagating through the inner heliosheath (IHS). Alternatively, a recently published model of the heliosphere suggests that the heliotail may split into a “croissant- like” shape, and that such a geometry could be responsible for the heliotail ENA feature. Here we present results from a time-dependent simulation of the heliosphere that produces a comet-like heliotail, and show that the 11-year solar cycle leads to the formation of ENA lobes with properties remarkably similar to those observed by IBEX. The ENA energy at which the north and south lobes appear suggests that the pickup ion (PUI) temperature in the slow SW of the IHS is ~10⁷ K. Moreover, we demonstrate that the extinction of PUIs by charge-exchange is an essential process required to create the observed global ENA structure. While the shape and locations of the ENA lobes as a function of energy are well reproduced by PUIs that cross the termination shock, the results appear to be sensitive to the form of the distribution of PUIs injected in the IHS.

Local Interstellar Magnetic Field Determined from the Interstellar Boundary Explorer Ribbon (2016), Zirnstein et al., ApJL

The solar wind emanating from the Sun interacts with the local interstellar medium (LISM), forming the heliosphere. Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASAʼs Interstellar Boundary Explorer (IBEX). IBEX observations show the existence of a “ribbon” of intense ENA emission projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field (ISMF) vector. Here we show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93±0.08μG) and direction (227.28°±0.69°, 34.62°±0.45° in ecliptic longitude and latitude) of the pristine ISMF far (~1000 AU) from the Sun. We find that the ISMF vector is offset from the ribbon center by ~8.3° toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF. Our results yield draped ISMF properties close to that observed by Voyager 1, the only spacecraft to directly measure the ISMF close to the heliosphere, and give predictions of the pristine ISMF that Voyager 1 has yet to sample.