March 2010: The Ring of Fire Captured by the SDO Observatory

A Spectacular Solar Event

In March 2010, a few weeks after its launch, NASA's Solar Dynamics Observatory (SDO) recorded one of its first striking images: a solar prominence in the shape of a ring, visible in extreme ultraviolet (304 Å). This structure, often called a "ring of fire" by visual analogy, corresponds to a vast magnetic arc charged with hot plasma extending nearly 300,000 kilometers, or about 25 times the Earth's diameter. Its annular morphology results from the three-dimensional projection of a plasma flow confined by coronal magnetic field lines.

Source: NASA SDO Science Publications and Solar Physics Journal (2011).

Physics of Coronal Rings

These structures, called coronal loops, are formed by plasma following the lines of the solar magnetic field. The plasma temperature in these regions can reach \(1-3 \times 10^6\) kelvins. The equation governing magnetic equilibrium is: \( \nabla p = \frac{1}{\mu_0} (\nabla \times \mathbf{B}) \times \mathbf{B} \) where \(p\) is the plasma pressure and \(\mathbf{B}\) is the magnetic field.

The observed prominence had temperatures ranging from \(5 \times 10^4 \ \mathrm{K}\) (transition plasma) to over \(10^6 \ \mathrm{K}\) (coronal plasma). Typical electron densities reach \(10^9 - 10^{11} \ \mathrm{cm^{-3}}\) - extremely dense for astrophysical plasma, and ejection speeds can exceed \(500 \ \mathrm{km \ s^{-1}}\) - about 0.17% of the speed of light and comparable to a fast solar wind or moderate stellar ejection.

N.B.: Electron density represents the number of free electrons per unit volume.

Mechanisms Involved

The "ring of fire" is the visual manifestation of magnetic reconnection: the magnetic field lines suddenly rearrange, releasing energy and propelling plasma. The likely scenario is that of an unstable magnetic flux rope (kink or torus instability) that rises and carries dense plasma from the chromosphere, visible in the He II 304 Å band.

What is Magnetic Reconnection?

Magnetic reconnection is a fundamental process in plasma physics where magnetic field lines break and reconnect, converting magnetic energy into kinetic and thermal energy. This phenomenon explains:

• Solar flares
• Coronal mass ejections
• Heating of the solar corona

A typical example is observed during the solar "ring of fire," where magnetic reconnection leads to the formation of bright coronal loops visible in extreme ultraviolet, a signature of plasma heated to several million kelvins.

Comparative Table between a Ring of Fire and a Prominence

Sources: NASA/SDO and NASA ADS.

SDO Has Far Exceeded Its Initial Mission

Launched in February 2010, the Solar Dynamics Observatory (SDO) has far exceeded its initial 5-year mission. In 2025, the spacecraft continues its observations of the Sun, although several of its instruments show signs of wear after more than 15 years in geosynchronous orbit. The ultraviolet imaging sensors (AIA) and the magnetic field measurement instrument (HMI) continue to provide valuable data, but with regular calibration adjustments to compensate for detector degradation and optical contamination.

The data collected by SDO since its launch now constitute one of the most complete solar archives ever established, covering more than a complete solar cycle.

• Over 500 million high-resolution images of the Sun
• Complete coverage of an entire solar cycle (24th cycle)
• Prediction of 95% of major eruptions with 72 hours advance notice

In 2025, NASA has reduced the pace of its high-cadence observation campaigns to optimize the lifespan of the still-active systems while preparing the transition to a new satellite.

Solar-C: The Successor to SDO

This successor, named Solar-C (or Solar-C Extreme Ultraviolet Observatory, SCEO), is a joint project between NASA, JAXA, and ESA, scheduled for launch at the end of the 2020s. It will benefit from more sensitive detectors in the extreme ultraviolet and increased temporal resolution, allowing the tracking of solar magnetic processes at unprecedented spatio-temporal scales. Solar-C will also continue the mission of studying the dynamics of the solar corona and its interactions with the solar wind, ensuring scientific continuity between SDO and future generations of solar missions.