Biography

Bishwas Shrestha is a Postdoctoral Researcher interested in the pickup ion (PUI) dynamics at shocks and energetic neutral atom (ENA) emission from the solar wind-local interstellar medium interaction. His Ph.D. thesis was focused on using global ENA models to extract information about the heliospheric termination shock from ENAs observed by NASA's Interstellar Boundary Explorer (IBEX) spacecraft. He has developed a method to estimate the strength of the heliospheric termination shock in several directions of the sky using IBEX observations. At Princeton, Bishwas is working on analyzing the hi-cadence in-situ measurement of PUIs in the outer heliosphere from the Solar Wind Around Pluto (SWAP) instrument onboard New Horizons. He is also working on the hi-resolution ENA observations from IBEX to understand the time-dependent evolution of our heliosphere. In addition, Bishwas is leading the development of the Level 3 algorithms for the Solar Wind and Pickup Ion (SWAPI) instrument being developed at Princeton University by the Space Physics Group.

Research Interests

• Solar wind-local interstellar medium interaction

• Pickup ion acceleration at the heliospheric termination shock, interplanetary shock, and Planetary bow shock

• Simulation and Analysis of Global Hydrogen Energetic Neutral Atom (ENA)

• Space Plasma simulations

Selected Publications

Suprathermal H⁺ Pickup Ion Tails in the Outer Heliosphere, Shrestha et al. 2024, ApJ 960, 35

This study provides a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H⁺ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. The shock compression ratio varies between approximately 1.4 and 3.2. The H⁺ PUI density and temperature show a gradual increase across the shock, while the H⁺ solar wind density shows erratic behavior without a distinct downstream compression. The H⁺ PUI cooling index variation across the shock displays different characteristics in each shock. This study demonstrates, for the first time, the variation of the number density of downstream H⁺ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H⁺ PUI tail densities for stronger shocks.

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations, Shrestha et al. 2023, ApJ

Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices <1.8. The ENA spectral index over the poles shows a periodic evolution over the solar cycle 24. The surface area with flatter ENA spectra (<1.8) around the ecliptic south pole increases slightly from 2009–2011 and then decreased gradually from 2012–2014. The PCH completely disappears in 2016 and then starts to appear again starting in 2017, gradually growing until 2019. This evolution shows a clear correlation with the change in the PCH area observed at the Sun once the delay in the ENA observation time is included. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the ENA spectrum near the south pole in 2014 and 2017. The rapid evolution in 2014 is related to a rapid closing of PCHs in 2012 and that in 2017 is related to a rapid opening of PCHs in late 2014. These results also agree qualitatively with the evolution of the ENA spectral index from simulations using a simple time-dependent heliospheric flow model.

Strength of the Termination Shock Inferred from the Globally Distributed Energetic Neutral Atom Flux from IBEX, Shrestha et al. 2021, ApJS

In this study, we estimate the heliospheric termination shock (HTS) compression ratio at multiple directions in the sky from a quantitative comparison of the observed and simulated inner heliosheath (IHS) energetic neutral atom (ENA) fluxes. We use a 3D steady-state simulation of the heliosphere to simulate the ENA fluxes by postprocessing the MHD plasma using a multi-Maxwellian distribution for protons in the IHS. The simulated ENA fluxes are compared with time exposure–averaged IBEX-Hi data for the first 3 yr of the mission. The quantitative comparison is performed by calculating the fractional difference in the spectral slope between the observed and simulated ENA fluxes for a range of compression ratios, where the simulated ENA spectrum is varied as a function of downstream pickup ion temperature as a function of compression ratio. The estimated compression ratio in a particular direction is determined by the minimum value of the fractional difference in spectral slope. Our study shows that the compression ratio estimated by this method is in close agreement with the large-scale compression ratio observed by Voyager 2 in its travel direction. Also, the compression ratio in other directions near the ecliptic plane is similar to the compression ratio at the Voyager 2 direction. The weakest shock compression is found to be on the port side of the heliosphere at direction (27°, 15°). This is the first study to estimate the HTS compression ratio at multiple directions in the sky from IBEX data.

Energetic Neutral Atom Flux from the Inner Heliosheath and Its Connection to Termination Shock Properties, Shrestha et al. 2020, ApJ

We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEXHi instrument over the time period 2009–2015. The statistical comparison is performed by calculating the chisquare value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (i) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (ii) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.