November 11, 2013 - The Astrophysical Journal published two research papers by SPA graduate students, Eric Zirnstein and Udara Senanayake. Eric developed a computational code that allows him to calculate the amount of energetic neutral atom (ENA) flux seen at earth on the basis of 3D data for the distribution of neutral hydrogen and protons from simulations of the global heliosphere. For this paper, he adapted his analysis software to allow for the motion of the IBEX spacecraft which measures ENAs. By computing fluxes both in the Sun and spacecraft frames, Eric was able to quantify the so-called "Compton-Getting" correction technique that the IBEX team use to transform their spacecraft data into data as would be measured in the frame of the Sun. Eric showed that while the correction works well when the ENA spectrum is close to a powerlaw, more complex spectral features are not always accounted for. Since this paper involves an analysis the methods used by the IBEX team, the Principal Investigator of the IBEX mission, Dave McComas, and another senior team member, Nathan Schwadron, participated as co-authors, along with Eric's advisor Prof. Jacob Heerikhuisen.

Working with his advisor, Prof. Vladimir Florinski, Udara investigated the acceleration of cosmic particles known as anomalous cosmic rays (ACRs) at a massive space plasma shock wave produced by the solar wind expansion into the surrounding galactic gas cloud. Using computer models, Udara showed that the shape of the shock critically determines the spatial distribution and energy features of the accelerated particles. The work will help explain the puzzling observations made by Voyager 1 and 2 space probes that shows a large deficit of ACRs along their trajectories, contrary to expectations. The new paper shows that ACR helium ions are primarily concentrated in the so called tail region of the heliosphere, away from the regions of space sampled by the Voyagers.


E. J. Zirnstein, J. Heerikhuisen, D. J. McComas, and N. A. Schwadron, Simulating the Compton-Getting effect for hydrogen fluix measurements: implications for IBEX-Hi and -Lo observations, Astrophys. J., 778, 112 (2013).

U. K. Senanayake and V. Florinski, Is the acceleration of anomalous cosmic rays affected by the geometry of the termination shock?, Astrophys. J., 778, 122 (2013).