Solar Corona

The outermost layer of the Sun's atmosphere — a region of hot, tenuous plasma structured by the solar magnetic field. Extends from just above the chromosphere out to the Alfvén surface (~10–20 solar radii, 7–14 million km), where it merges into the solar-wind. The direct subject of the parker-solar-probe mission and the source of CMEs.

Normally invisible due to glare from the solar disk; visible to the naked eye only during a total solar eclipse, or with a coronagraph (invented by Bernard Lyot, 1930).

The Coronal Heating Problem

The corona's most perplexing feature: it is 200× hotter than the Sun's surface.

This violates naive expectation — temperature should decrease with distance from the heat source. The second law of thermodynamics prevents direct heat flow from the cooler photosphere to the hotter corona. Some other energy transport mechanism must operate.

Power required: ~1 kW per square meter of chromospheric surface — 1/40,000 of the Sun's total light output. Small in relative terms, but the mechanism has resisted explanation for over 50 years.

Historical note: The corona's unusual spectrum led 19th-century astronomers to propose it contained a new element, "coronium." In 1940, Bengt Edlén (building on Walter Grotrian's 1939 work) correctly identified the spectral lines as highly ionized iron — Fe XIV (Fe¹³⁺) — confirming the extreme temperatures.

Heating Theories

Wave heating — Magneto-acoustic and Alfvén waves generated by turbulent granulation at the photosphere carry energy upward. Alfvén waves carry sufficient energy but historically were thought not to dissipate it quickly enough. 2003 computer simulations by Bogdan et al. showed Alfvén waves can transmute into other wave modes at the coronal base, providing a viable dissipation pathway. First direct observation of coronal waves: 1997, SOHO (1 mHz magneto-acoustic waves carrying ~10% of required energy). Recent SDO and Hinode observations confirmed ubiquitous Alfvénic oscillations in active regions, coronal holes, and quiet sun.

Magnetic reconnection and nanoflares — The Sun's surface is covered with millions of small magnetized regions (the "magnetic carpet"), constantly churned by granulation. The coronal magnetic field must continuously reconnect to match these surface motions, releasing bursts of heat. Eugene Parker proposed in the 1980s that millions of "nanoflares" — individually tiny but collectively sufficient — could heat the corona. Still controversial: UV/EUV telescopes observe micro-flares as small brightenings but count too few to fully account for the required energy.

2012 discovery: High-resolution X-ray imaging from the Hi-C sounding rocket revealed tightly wound coronal braids. Reconnection of these braids could heat active-region corona to up to 4 million K; MHD waves are thought to dominate heating of the quiescent corona (~1.5 MK).

Type II spicules (2010): A newly discovered class of fast jets (~100 km/s, shorter lifespans) inject heated plasma directly into the corona, observed with SDO and Hinode — another candidate energy pathway.

Parker Solar Probe

parker-solar-probe was specifically designed to resolve the coronal heating mystery by flying *inside* the corona for the first time — sampling particles and magnetic fields in situ at distances as close as 3.9 million miles from the solar surface. Its December 2021 transit through the corona was the first direct sampling of this region and continues to return data on the heating mechanisms.

Coronal Structures

• Coronal loops — Arched magnetic field lines anchored at the photosphere; sites of intense local heating
• Helmet streamers — Large plasma-filled structures capping magnetic field closed loops; associated with the slow solar wind
• Coronal holes — Regions of open magnetic field; source of the fast solar wind
• Prominences — Dense cool plasma suspended by magnetic fields above the chromosphere