The Interstellar Boundary Explorer (IBEX) has been observing the ebb and flow of energetic neutral atoms within our solar system for the past decade. How has the flux of these particles changed in that time?
Global maps of the ENA flux at five energies, for four time periods. Click to enlarge. [Desai et al. 2019]
Studying the Solar System Bubble
Our solar system is shielded from the interstellar medium by a bubble of solar-wind plasma called the heliosphere. We can study the properties of the heliosphere by monitoring the flux of energetic neutral atoms (ENAs), which form when speedy solar-wind ions steal electrons from incoming neutral interstellar atoms and lose their positive charge.
No longer confined to move along the magnetic field of the solar wind, these neutral atoms can return to the inner solar system, where they are detected by Earth-orbiting spacecraft like IBEX. Over the past decade, IBEX has given us a three-dimensional view of the ENAs within our solar system. The solar wind has certainly changed a lot in that time — how have the ENAs responded?
Projected global maps of the spectral index for four energy ranges over the nine years of IBEX observations. The spectral index clearly increases in the northern hemisphere in the highest energy bands. The ribbon feature has been masked. Click to enlarge. [Desai et al. 2019]
Mapping the Heliosphere
A team led by Mihir Desai (Southwest Research Institute and University of Texas at San Antonio) analyzed nine years of IBEX observations to learn how the flux of ENAs has changed over time. They focused on the globally distributed flux, which represents the primary ENA population, as opposed to the so-called “ribbon feature”, a strange, arc-like feature thought to trace ENAs produced through a different mechanism.
By comparing the flux of ENAs in adjacent energy bands, the authors constructed maps of the ENA spectral indices — a measure of how the ENA flux is distributed over different energies — over the whole sky. They found that the evolution of the spectral indices over the years depends on both the energy of the ENAs and their latitude. Particularly striking was the increase in the spectral indices in the northern hemisphere at the highest energies (1.7–4.3 keV), which was caused by a larger decrease in the flux of the highest energy ENAs.
By comparing the evolution of the ENA spectral indices to the speed of the solar wind during the same time period, the authors find that the behavior of the ENA flux is linked to changes in the solar wind speed with a delay of 2–3 years. This finding solidifies the connection between the solar wind parameters and the ENA flux.
Reconstructed solar wind speed from 1985 to 2018. The solar wind speed exhibits more latitudinal variation during solar minimum (e.g. 2009) than solar maximum (e.g. 2014). [Desai et al. 2019]
Continued Evolution
How will the ENA flux evolve in the future? Based on the observed solar wind speeds and the expected time delay, the authors predict that the ENA spectral indices will continue to evolve.
More specifically, because the northern hemisphere solar wind speed increased from 2014 to 2018, the authors expect the ENA spectral index to decrease in the northern hemisphere, especially at the higher energies that they’ve found to be more responsive to solar wind changes. In the southern hemisphere, they expect the spectral indices to rise and fall with the more variable solar wind speed at those latitudes.
IBEX has enough fuel to support its continued operations for several decades — so we can expect to learn much more about the ENAs in our solar system in the future!
Citation
“Temporal Evolution of the Latitude and Energy Dependence of the Energetic Neutral Atom Spectral Indices Measured by the Interstellar Boundary Explorer (IBEX) Over the First Nine Years,” M. I. Desai et al 2019 ApJ 875 2. doi:10.3847/1538-4357/ab0f37