Chromosphere

The chromosphere is a distinct layer of the Sun’s atmosphere located above the photosphere and beneath the corona. It derives its name from the Greek words chroma meaning “colour” and sphaira meaning “sphere”, referring to its reddish appearance visible during a total solar eclipse. The chromosphere plays a vital role in solar physics as it marks the transition between the relatively cool photosphere and the extremely hot corona, exhibiting complex dynamics and energetic phenomena that influence the solar system.

Structure and Composition

The chromosphere extends approximately 2,000 to 3,000 kilometres above the photosphere, although its height can vary depending on solar activity. It consists primarily of hydrogen, with smaller quantities of helium, calcium, and other trace elements. The temperature within this layer increases dramatically from about 4,500 K at its base to nearly 25,000 K near its upper boundary. This steep temperature gradient is one of the most striking features of the solar atmosphere and is believed to be driven by magnetic and wave-related energy transfer mechanisms.
At the lower boundary, the chromosphere appears relatively faint compared to the bright photosphere beneath it. However, when observed through special filters such as H-alpha (656.3 nm wavelength), it reveals a complex texture of fine structures including fibrils, spicules, and plages. These features are constantly in motion and serve as indicators of intense magnetic and plasma activity.

Observation and Visibility

The chromosphere is normally invisible to the naked eye because the overwhelming brightness of the photosphere masks it. During a total solar eclipse, when the Moon completely obscures the photosphere, a thin reddish rim surrounding the dark solar disk becomes visible—this is the chromosphere. The reddish hue arises from the emission of hydrogen alpha light, produced when electrons in hydrogen atoms transition from the third to the second energy level.
Astronomers use spectroscopic techniques to study the chromosphere in detail. Narrow-band filters centred on the H-alpha line allow for the observation of solar prominences, flares, and other dynamic phenomena originating from this layer. Instruments such as the Solar Dynamics Observatory (SDO) and ground-based solar telescopes, like the Swedish Solar Telescope, provide continuous high-resolution monitoring of chromospheric activity.

Dynamic Features and Phenomena

The chromosphere is characterised by an abundance of transient and small-scale structures that change rapidly over time. Key features include:

• Spicules: Jet-like eruptions of gas that shoot upwards from the chromosphere into the corona at speeds of 20–30 km/s. They last only a few minutes but are abundant across the Sun’s surface, playing a significant role in energy transport.
• Plages: Bright regions surrounding sunspots, associated with strong magnetic fields. These are prominent in H-alpha images and indicate localised heating.
• Filaments and Prominences: Long, thread-like structures of cooler, denser plasma suspended above the solar surface by magnetic fields. When observed against the solar disk they appear dark (filaments), but when seen extending beyond the limb they appear bright (prominences).
• Fibrils and Mottles: Smaller, fine-scale filaments that form a dynamic mesh-like pattern across the chromosphere, tracing the Sun’s magnetic field lines.

Solar flares often originate in the chromosphere, where magnetic energy stored in the Sun’s field lines is suddenly released. This can cause rapid heating, particle acceleration, and the emission of electromagnetic radiation across the spectrum, from radio waves to X-rays.

Temperature Gradient and Energy Transfer

Unlike the photosphere, which cools with increasing altitude, the chromosphere displays a reverse temperature gradient. This paradoxical heating is not yet fully understood but is thought to result from magnetic wave dissipation and reconnection processes. Alfvén waves—oscillations of magnetic field lines carrying energy upward—are believed to deposit energy into the chromosphere, contributing to its rising temperature.
The energy transfer in the chromosphere is critical for understanding the heating of the solar corona. The so-called coronal heating problem—why the corona is millions of degrees hotter than the solar surface—may be partly explained by the processes occurring within the chromosphere, acting as an intermediary energy transfer zone.

Importance in Solar and Space Studies

The chromosphere serves as a laboratory for studying plasma physics under extreme conditions. Its dynamic behaviour reflects the Sun’s magnetic activity cycle, which influences space weather and the near-Earth environment. Variations in chromospheric activity are linked to the occurrence of solar flares and coronal mass ejections, both of which can affect satellite operations, radio communications, and power grids on Earth.
Furthermore, understanding chromospheric dynamics aids in developing models of stellar atmospheres. Many stars exhibit chromospheres similar to the Sun’s, and comparing these layers across stellar types helps astronomers infer magnetic activity and age-related changes in stars.

Historical Context and Scientific Progress

The chromosphere was first identified during total solar eclipses in the 19th century. In 1868, French astronomer Pierre Janssen and English scientist Norman Lockyer independently observed a bright yellow spectral line in the chromosphere, later identified as due to helium—a new element at that time. This discovery not only confirmed the existence of the chromosphere but also led to the identification of helium before it was found on Earth.
Over the 20th and 21st centuries, technological advances in spectroscopy, imaging, and computational modelling have expanded scientific understanding of this solar layer. The advent of space-based solar observatories has allowed continuous monitoring free from atmospheric interference, leading to detailed maps of chromospheric magnetic structures and their evolution.

Role in the Solar Atmosphere

The chromosphere forms an essential interface between the dense photosphere and the rarefied corona. Energy, matter, and magnetic fields are continuously exchanged across these layers. The region known as the transition region marks the upper boundary of the chromosphere, where temperatures rise abruptly to over a million Kelvin, signifying the onset of the corona.
In this way, the chromosphere acts as a bridge in both physical and energetic terms, mediating the processes that power the outer solar atmosphere. Its study remains central to understanding solar variability and its far-reaching effects across the solar system.