A solar flare is a sudden, intense burst of radiation caused by the release of magnetic energy associated with sunspots. These flares emit across the electromagnetic spectrum, including X-rays , which are particularly useful for studying the hot plasma involved.
The energy range 3.8–10 keV (kilo-electron volts) is crucial because it contains:
- Emission lines from highly ionized atoms like calcium (Ca), iron (Fe), and nickel (Ni).
- A steeply decreasing continuum , meaning that the intensity drops rapidly as energy increases.
This part of the X-ray spectrum helps scientists infer properties of the hottest parts of the solar flare plasma , such as temperature and elemental abundances.
Thermal vs. Nonthermal Components in Solar Flare X-ray Emission
During a solar flare impulsive phase , two main types of X-ray emission are observed:
- Thermal Emission:
- Comes from very hot (millions of degrees) plasma.
- Emits a continuum plus discrete emission lines.
- Seen near or above the loop apex (the top of magnetic loops connecting flare footpoints).
- Steeply decreasing with increasing energy.
- Nonthermal Emission:
- Dominates at higher energies (> ~20 keV).
- Less steeply decreasing — follows a power-law dependence on energy (intensity ∝ E⁻ᵞ).
- Caused by bremsstrahlung radiation from nonthermal electrons accelerated during the flare.
- These electrons travel downward along magnetic loops and stop in the chromosphere or lower corona .
- Observed at loop footpoints via RHESSI and Yohkoh images.
Key Features in the 3.8–10 keV Range
Two prominent features are observed in the thermal component of the X-ray spectrum:
1. The Fe Line Feature (~6.6–6.9 keV)
This feature is actually a complex of overlapping emission lines , mostly from iron ions :
- Fe XXV:
- Transitions from 1s² → 1s2l (where l = s, p).
- Excited at temperatures around Te ≈ 12 MK (million Kelvin) .
- Dielectronic satellites from Fe XIX–XXIV:
- Satellites are spectral lines from recombination processes.
- Also contribute to the 6.6 keV region.
- Fluorescence lines from Fe II:
- Low-ionization state, centered around 6.6 keV.
- Formed when photons excite neutral or low-ionization Fe atoms.
- Fe XXVI (Lyman series):
- Transitions from 1s → 2p.
- Centered around 6.9 keV .
- Excited at much higher temperatures, around Te ≈ 30 MK .