The IBEX ribbon is a narrow, curved structure where more energetic neutral atoms (ENA) are found. It was discovered in 2009 by NASA’s Interstellar Boundary Explorer (IBEX) mission. This ribbon is an important feature at the edge of the heliosphere, which is the area of space where the Sun’s influence is strongest.

Discovery

The IBEX spacecraft was launched in 2008. It was designed to measure the amount of ENAs coming from the edge of the heliosphere. In its first all-sky map, IBEX found a surprising, bright, and narrow pattern of ENA emissions, now called the IBEX ribbon. Scientists did not expect this feature. It showed a complex interaction between the solar wind and the magnetic field in the space around our solar system.

Origin theories

Scientists have proposed several ideas to explain the IBEX ribbon.

The first idea, suggested by McComas et al. (2009) and Schwadron et al. (2009), says the ribbon forms through a series of charge-exchange processes. In this model, pick-up ions in the local interstellar magnetic field create a ring-shaped distribution. This leads to the production of energetic neutral atoms (ENAs) if the ions do not spread out evenly.

A second idea by Schwadron and McComas (2013) states that the ribbon forms when newly ionized atoms are temporarily held in a special area of the local interstellar medium. These ions mainly come from neutralized solar wind and pick-up ions beyond the solar wind termination shock.

A third idea by Fahr et al. (2011) and Siewert et al. (2013) suggests that the ribbon’s ENAs originate from pick-up ions inside the heliosphere. These ions are linked to cooled cosmic rays and shock-accelerated ions near the solar wind termination shock.

A fourth idea by Grzedzielski et al. (2010) claims that ENAs form when neutral hydrogen atoms at the edge of the local interstellar cloud interact with hot protons in the Local Bubble through charge exchange.

A fifth idea by Fichtner et al. (2014) proposes that the ribbon results from uneven areas in the local interstellar medium. These areas, called "H-waves," move along the magnetic field. When these waves meet the heliopause, they create regions with more ENA production, forming the ribbon.

As of 2014, no theory has been widely accepted. Each idea struggles to fully explain all observed features.

Schwadron and McComas (2019) later explained the ribbon as being formed mainly from secondary solar wind atoms and a broader structure made from secondary suprathermal atoms in the heliosheath.

Xu and Liu (2023) proposed that turbulence in the very local interstellar medium, called magnetohydrodynamic (MHD) turbulence, helps create the ribbon through a process called mirror diffusion. In this model, magnetic mirrors from turbulence interact with pick-up ions near areas where the magnetic field is perpendicular to the line of sight. These mirrors, not the average magnetic field, dominate the mirroring effect because of their strong magnetic field gradients. The mirroring effect works best for pick-up ions with pitch angles below a certain value.

The width of the IBEX ribbon (about 20° at 1 keV) depends on two factors: the range of pitch angles where turbulent mirroring is effective and the movement of magnetic field lines caused by Alfvénic modes. The researchers found that for the ribbon to appear consistent across the sky, the turbulence in the very local interstellar medium must have an injection scale smaller than about 500 astronomical units. The model explains how pick-up ions keep their original pitch angles through mirror diffusion, allowing them to return to the heliosphere as ENAs after neutralization, forming the observed ribbon.

Temporal variability

IBEX observations over more than 10 years show that the intensity and shape of the ribbon change as the sun's activity cycle progresses. Both the ribbon and the globally distributed flux (GDF) react to these changes, but they do so at different times. The GDF responds faster than the ribbon by a few years.

A study comparing IBEX data from 2009 and 2019 found important changes in the ribbon over time. At energies below 1.7 keV, the ribbon's brightness increased near the nose direction and up to 25° southward, but not at mid and high ecliptic latitudes, which were in similar stages of solar cycles 23 and 24. The ribbon's width varied depending on the viewing angle around the map center, with different patterns observed in 2009 and 2019. However, analysis showed that the ribbon's radius remained statistically similar between the two periods. The partial recovery matched models that suggest the heliosphere's closest point is located southward of the nose region. The changing width patterns may indicate small-scale processes happening in the ribbon's source area.

At low latitudes, where solar wind speeds are usually below 500 km/s, most observed energetic neutral atoms have energies below 2 keV. This explains why major changes in intensity occurred mainly at these lower energy levels. The recovery was first seen in southern regions where the heliosphere's boundaries are closest to the sun, matching models that predict different response times based on how far particles travel.

Connection to the interstellar magnetic field

The 2013 study found the ribbon to be "extraordinarily circular," with its center located at sky coordinates (219.2° ± 1.3°, 39.9° ± 2.3°). This center is positioned 50° away from the direction the solar wind faces and is believed to match the direction of the local interstellar magnetic field. The ribbon showed high consistency in shape across all observed energy levels (0.7-4.3 keV), with a measurement of δC ≤ 0.014, indicating the interstellar magnetic field in this area is very uniform over large distances.

The study also found small structural details: the ribbon was slightly stretched in one direction (eccentricity ~0.3), generally perpendicular to the line connecting the ribbon’s center and the solar wind-facing direction. The brightness of the ribbon was uneven, being higher on the inside part. At higher energy levels (4.3 keV), the ribbon appeared slightly larger and shifted compared to lower energy levels. These features help scientists better understand how the ribbon forms and its connection to the interaction between the heliosphere and the surrounding space, though the exact reasons for these features are not yet fully understood.

Future observations

As IBEX continues its mission, future studies will compare its findings with data from NASA’s Interstellar Mapping and Acceleration Probe (IMAP). IMAP is expected to provide more detailed measurements of ENA particles. IMAP launched on September 24, 2025.