Astronomy
Extreme Ultraviolet Spectroscopy of the Solar Corona
Quick fact
By analyzing extreme ultraviolet (EUV) light from the corona, scientists have measured temperatures spanning 1 to 10 million K—far hotter than the Sun's visible surface at ~5,500°C. These observations rely on spectral lines from highly ionized elements like iron that emit only in the EUV range.
Why this is interesting
The Sun's outer atmosphere, the corona, is millions of degrees hotter than its surface—yet it's invisible to the naked eye. How do we know its temperature, or even that it exists?
Read the full explanation
Understanding Extreme Ultraviolet Spectroscopy of the Solar Corona
The solar corona is a tenuous plasma of ions and electrons surrounding the Sun, visible only during a total solar eclipse. It emits most brightly in extreme ultraviolet (EUV) and X-rays, wavelengths that are completely absorbed by Earth's atmosphere, so we must observe from space. Every chemical element, when ionized, emits light at specific wavelengths determined by electron transitions between energy levels. In the corona, temperatures reach millions of degrees, which strips atoms of many electrons. The resulting ions have far more tightly bound electrons, so the photon energies (and therefore frequencies) are much higher—placing the emission in the EUV range around 10–124 nm. Each spectral line acts as a unique fingerprint. For example, iron atoms that have lost 12 electrons (Fe XII) produce a strong line at 19.5 nm, while Fe XVIII (13 electrons lost) emits at 9.4 nm. Because different ion stages have different excitation energies, the relative strength of these lines tells us the temperature of the plasma. Similarly, the density can be inferred from pairs of lines whose intensity ratio is sensitive to how crowded the ions are by electron collisions. This is much like how a doctor reads a thermometer and a stethoscope to assess a patient's condition, but here the diagnostic tool is the spectrum.
A deeper explanation
The mechanism behind EUV spectroscopy is rooted in atomic physics and plasma thermodynamics. When a coronal plasma is at thermal equilibrium, the distribution of ions among charge states follows the Saha ionization equation, which depends on temperature. By observing the emission-line intensity ratios of two different ions of the same element (e.g., Fe XIV/Fe XV), we can derive the electron temperature. The intensity of a line also depends on the electron density, because collisions excite the upper energy level. Some emission lines are so-called 'density-sensitive' pairs (e.g., Si IX and Si X lines) where the ratio of two transitions from similar upper levels responds to the frequency of collisions. Furthermore, the Doppler effect applies to EUV lines: if plasma moves toward or away from us, the line's wavelength shifts. This allows measurement of line-of-sight velocities, revealing outflows, oscillations, and flows along magnetic field lines. Line broadening goes beyond thermal effects—turbulence and waves add non-thermal broadening, providing clues to the mechanisms heating the corona. These techniques are crucial because the corona is optically thin: we see the integrated emission along the line of sight, but with spectroscopic imaging we can map temperatures and densities across the corona. Instruments like the EUV Imaging Spectrometer (EIS) on Hinode and the Interface Region Imaging Spectrograph (IRIS) are dedicated to this task. Understanding EUV spectroscopy lets us probe fundamental questions: Why is the corona so hot? What accelerates the solar wind? How are coronal mass ejections initiated? The spectral lines are our remote sensors, revealing the hidden properties of million-degree plasma.