The Sun’s atmosphere consists of three main layers: the photosphere, the chromosphere, and the corona. The photosphere is the visible surface, radiating most of the Sun’s light and heat at about 5,800 Kelvin (K). Above this, the chromosphere (up to 20,000 K) and the corona (one to two million K) are significantly hotter but radiate far less light. This temperature inversion creates a profound observational challenge. Specialized techniques are required to overcome the sheer brilliance of the photosphere and reveal these faint, superheated layers.
The Fundamental Barrier to Observation
Observing the chromosphere and corona is complicated by the overwhelming brightness of the photosphere. The photosphere emits light across the entire visible spectrum, acting like a powerful veil that drowns out the light from the layers beyond.
The corona, in particular, is extremely tenuous and only about one-millionth as bright as the photosphere. Although the gas is millions of degrees hotter, its extremely low density means it produces very little light compared to the Sun’s visible surface. This contrast makes the corona invisible to the naked eye except during a total solar eclipse. The technical difficulty lies in suppressing the glare of the 5,800 K surface to detect the faint emissions from the much hotter, less dense plasma.
Ground-Based Chromosphere Observation
To view the chromosphere from the ground, observers must employ narrow-band filtering. Since the chromosphere is primarily hydrogen, it is most clearly observed by isolating the Hydrogen-alpha (H-alpha) wavelength, a deep red color at 656.3 nanometers. This filtering works because the chromosphere emits strongly at this wavelength, significantly reducing the continuous light from the bright photosphere.
This filtration is achieved using a specialized optical device called an etalon, often a Fabry-Pérot interferometer. The etalon uses two highly flat, partially reflective glass plates to allow only an extremely narrow sliver of light, typically less than 0.1 nanometers wide, to pass through via destructive interference. This minute bandpass isolates the desired H-alpha light, revealing dynamic features like spicules and prominences.
Another method involves observing the Calcium K-line, a violet wavelength near 393.4 nanometers. While H-alpha shows the magnetic structures and motions of hydrogen plasma, the Calcium K-line highlights the magnetic network higher in the chromosphere. Specialized telescopes often use a mechanical tilting mechanism or pressure tuning to adjust the filter’s precise wavelength, allowing scientists to perform Doppler-shift studies that reveal plasma velocity.
Specialized Techniques for Coronal Study
Unlike the chromosphere, the corona requires blocking the light from the solar disk entirely. The primary instrument for this task is the coronagraph, invented by Bernard Lyot in 1931. A coronagraph uses an internal occulting disk placed within the telescope to create an artificial solar eclipse, casting a shadow on the camera sensor.
The key challenge for coronagraphs is managing diffracted light, which scatters around the occulter’s edge and can overwhelm the faint coronal emission. Lyot solved this by incorporating the Lyot stop, an aperture placed further down the optical path to block scattered light. Modern ground-based coronagraphs, often situated at high-altitude observatories, study the inner and middle corona but must contend with the brightness of the Earth’s atmosphere, which scatters sunlight and contributes background noise.
The natural world offers the clearest view of the corona during a total solar eclipse. During totality, the Moon acts as a perfect occulting disk, cleanly blocking the photospheric light. Although short-lived and geographically limited, total solar eclipses provide a unique opportunity to study the finest structures of the innermost corona.
Space-Based Instrumentation
For observations of the solar atmosphere, instrumentation must be placed in space. Since our atmosphere strongly absorbs high-energy radiation, it is impossible to observe the hottest parts of the corona and transition region from Earth. Satellites like the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO) study the Sun in extreme ultraviolet (EUV) and X-ray wavelengths.
The high-temperature corona emits strongly in EUV and X-ray light, allowing space-based instruments to image these layers directly. For example, the SDO’s Atmospheric Imaging Assembly (AIA) captures full-disk images in different EUV wavelengths. Each wavelength corresponds to plasma at a specific temperature, providing a detailed, multi-layered view of the Sun’s magnetic activity up to the million-degree corona.
Space-based observatories also utilize sophisticated coronagraphs, such as the Large Angle and Spectrometric Coronagraph (LASCO) on SOHO, to monitor the outer corona continuously. These coronagraphs use external occulters positioned far in front of the telescope optics to block the Sun’s direct light. This design minimizes internal light scattering and allows for uninterrupted, long-term monitoring of vast coronal structures, including Coronal Mass Ejections (CMEs).