The IBEX ribbon is a narrow, arc-shaped area with more energetic neutral atoms found by NASA's IBEX mission in 2009. It is an important feature at the edge of the heliosphere, which is the area where the Sun's influence is strongest.

Discovery

The IBEX spacecraft, launched in 2008, was created to measure the amount of ENAs coming from the edge of the heliosphere. In its first full map of the sky, IBEX found a surprising, bright, and narrow pattern of ENA emissions, now called the IBEX ribbon. This discovery was not expected by earlier models and showed that the solar wind and the local interstellar magnetic field interact in a complicated way.

Origin theories

Scientists have proposed several ideas to explain the IBEX ribbon.

The first idea, introduced by McComas et al. (2009) and Schwadron et al. (2009), suggests that the ribbon forms through a series of chemical reactions. In this model, a ring-shaped group of ions in the magnetic field of the local interstellar space creates energetic neutral atoms (ENAs), assuming these ions do not spread out evenly.

A second idea, proposed by Schwadron and McComas (2013), states that the ribbon forms when newly ionized atoms are temporarily trapped in a special area of the local interstellar medium. These ions mostly come from neutral atoms in the solar wind and from ions that have entered the solar system beyond the solar wind termination shock.

A third idea, introduced by Fahr et al. (2011) and Siewert et al. (2013), places the source of the ribbon’s ENAs inside the heliosphere. These ENAs are believed to come from ions that have cooled in a specific way near the solar wind termination shock and from ions that have been accelerated by shocks beyond it.

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

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

As of 2014, none of these ideas has been fully accepted, and each has challenges in explaining all observed features.

Schwadron and McComas (2019) later explained the ribbon as being formed mainly by atoms from the solar wind that are not directly from the sun, as well as by atoms from a broader and shifted structure in the heliosheath.

Xu and Liu (2023) proposed that magnetic turbulence in the very local interstellar medium plays a key role in forming the IBEX ribbon through a process called mirror diffusion. In this model, magnetic turbulence creates areas where particles bounce between magnetic fields. These areas, rather than the overall magnetic field, strongly influence the movement of ions. The effectiveness of this process depends on the angle at which ions move relative to the magnetic field.

The width of the IBEX ribbon (about 20° at 1 keV) depends on two factors: the range of angles where the mirror effect works best and the movement of magnetic field lines caused by specific types of magnetic waves. The researchers found that for the ribbon to appear consistently across the sky, the turbulence in the very local interstellar medium must begin at a scale smaller than about 500 astronomical units. This model explains how ions retain their initial movement patterns through mirror diffusion, allowing them to return to the heliosphere as ENAs after neutralization, creating the observed ribbon.

Temporal variability

IBEX observations over more than 10 years show that the intensity and shape of the ribbon change over the solar cycle. Both the ribbon and the globally distributed flux (GDF) react to changes in the solar cycle, 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’s structure. For energies below 1.7 keV, the ribbon’s intensity 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 changed depending on the viewing angle around the map center, with different patterns observed in 2009 and 2019. However, analysis showed the ribbon’s radius remained statistically similar between the two years. The partial recovery matched models suggesting the heliosphere’s closest point is south of the nose region. The changing width patterns may indicate small-scale processes in the ribbon’s source area.

At low latitudes, where solar wind speeds average below 500 km/s, most observed energetic neutral atoms have energies below 2 keV. This explains why major intensity changes 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 showed the ribbon to be very round, with its center located at sky coordinates (219.2° ± 1.3°, 39.9° ± 2.3°). This center is 50° away from the direction where the solar wind faces the interstellar space, and it is believed to match the direction of the local interstellar magnetic field. The ribbon remained very consistent in shape across all observed energy levels (0.7-4.3 keV), with a measurement called δC ≤ 0.014, which suggests the interstellar magnetic field in that area is very uniform over large distances.

The study also found small details in the ribbon’s structure: it was slightly stretched in one direction (about 0.3 times its width) perpendicular to the line connecting the ribbon’s center and the solar wind’s direction. The brightness of the ribbon was uneven, with more intensity on the inside edge. At higher energy levels (4.3 keV), the ribbon appeared slightly larger and shifted compared to lower energy levels. These features help scientists test models explaining how the ribbon forms and how it interacts with the space around our solar system. However, the exact reasons for these features are still unknown.

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 energetic neutral atoms (ENA). IMAP was launched on September 24, 2025.