Energetic neutral atoms (ENAs) are made from the interaction between solar wind particles and atoms from the interstellar medium. Some of the ENAs happen to get knocked in a straight line in just the right way so that they travel in through the Solar System toward the IBEX spacecraft. This is how the scientists can map the boundary—they know the direction of travel of each particle since they did not change direction between the heliopause and the IBEX spacecraft.
IBEX's sensors, called IBEX-Hi and IBEX-Lo, sort the ENA particles and keep track of the direction of travel of all of the particles, the time they entered the sensor, the mass of the particles, and the amount of energy each particle has. The only piece of data that IBEX cannot determine is the distance that the particles have traveled, but this is not necessarily an important piece of information when making the maps.
As IBEX spins once every 15 seconds, particles continually enter the sensors. As IBEX spins and orbits Earth, and as Earth orbits the Sun, IBEX's sensors are able to sweep across the entire sky every six months. Over the course of days, weeks, and months, IBEX counts and sorts particles. The maps that have been created show the numbers of ENAs detected at different energy levels coming from all areas of the sky. The scientists assign colors to indicate the numbers of ENAs detected, and the locations on the map show the direction from which those particles came. Red indicates the highest number of ENAs measured by the spacecraft. Yellow and green indicate lower numbers of ENAs, and blue and purple show the lowest number of ENAs. The maps appear oval for the same reason that two-dimensional maps of spherical Earth look oval.