Eric J. Zirnstein Ph.D.

Associate Research Scholar
Email Address: 
ejz AT
Office Location: 
127 Peyton Hall
PDF icon CV

Ph.D., Physics, University of Alabama in Huntsville (2014)

M.S., Physics, University of Alabama in Huntsville (2012)

B.S., Physics, University of Alabama in Huntsville (2010)


Eric Zirnstein is a research scientist specializing in the simulation and analysis of 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 pickup ions, 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.

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
  • Simulation of spacecraft measurements

Selected Publications

(See more at Google Scholar)

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

Simulation of the IBEX Ribbon

(Left) Model of the IBEX ribbon at (a) 1.11 and (c) 2.73 keV, when all guiding center transport terms are included. (Right) IBEX observations at (b) 1.11 and (d) 2.73 keV, after removal of the globally distributed flux.

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

ENA Production Rate in the Heliosheath

Simulated ENA flux production (ΔJ/Δr) in a meridional cut of the heliosphere, in units log10[(cm3 s sr keV)−1]. Integrating the flux as a function of radial distance from the Sun yields the ENA flux at 1 au, i.e., fluxes accumulate at 1 au exactly at the same time in 2009.5 (top), 2012.5 (middle), and 2015.5 (bottom), for 0.71 keV (left) and 4.29 keV (right). Note the different color bar ranges for the different ENA energies.

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

Energy-Dependent Source of the IBEX Ribbon

Isocontours of the ribbon ENA production rate outside the heliopause (HP) denoted by five colors distinguishing the ENA energies. The background color represents the magnetic field magnitude, with some ISMF lines (black curves). Suprathermal ions outside the HP become neutralized by charge- exchange (blue circles) and form ENAs that may travel back inward toward IBEX (gray lines). The majority of ribbon ENAs originate near B· r ~ 0 (colored contours, only shown for acos(B· r) > 85°). Due to the curvature of the ISMF and the finite temperature of parent ENAs, the ribbon source region is broad; however, the line of sight integrated flux decreases farther from B· r = 0.

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.