Ellerman Bombs: Photospheric Reconnection Flame Wings

What an Ellerman Bomb Actually Is

An Ellerman bomb is a compact, impulsive brightening rooted in the Sun's photosphere—the visible surface layer—where magnetic energy is dumped into a tiny volume of gas over a few minutes. Ferdinand Ellerman first described them in 1917 from Mount Wilson spectroheliograph plates (W. M. Mitchell had noted them in 1909), and because they showed up only in hydrogen lines he dubbed them solar hydrogen bombs. Russian observers later called them moustaches for their spectral shape.

The defining trait is spectral, not spatial. In the Hα line at 656.3 nm, an EB lights up the two wings of the line profile (roughly ±1 Å, or ±1 Å ≈ ±45 km/s from line center) into bright emission, while the core stays in dark absorption. The same happens in Hβ, Hγ, and Ca II lines. That distinctive wings-bright, core-dark profile is the fingerprint that lets astronomers separate EBs from ordinary faculae or flare ribbons.

• Site: intergranular lanes in emerging-flux active regions
• Size: about one arcsecond (≲1,000 km) or smaller
• Lifetime: minutes, often flickering in sub-minute bursts

The Reconnection Mechanism

Ellerman bombs are the low-atmosphere face of magnetic reconnection. In a growing active region, new magnetic flux rises through the surface as compact bipoles. Where an emerging polarity is driven against pre-existing opposite polarity in the crowded intergranular lanes, the field lines are forced together and a thin current sheet forms. Oppositely directed field annihilates there, converting stored magnetic energy into heat, bulk flows, and radiation.

The governing quantity is the magnetic energy density, u_B = B²/8π (Gaussian units). With photospheric field strengths of B ≈ 100–1,000 G, even a sub-arcsecond flux tube stores enough energy to power an EB when it cancels. Reconnection converts this at a rate set by the inflow speed relative to the Alfvén speed v_A = B/√(4πρ); in the dense photosphere v_A is only a few km/s, which is why EBs are cooler and slower than coronal reconnection. Observationally, EBs sit squarely on sites of measured magnetic flux cancellation, tying the brightening directly to the disappearance of opposite-polarity flux.

Characteristic Numbers and a Worked Estimate

Typical EB parameters, assembled from high-resolution studies: linear size ~500–1,000 km, lifetime 2–15 minutes, temperature enhancement of 600–3,000 K above the local temperature-minimum value (some models push to 4,000–5,000 K), all concentrated near a height of 500–700 km where the temperature minimum and the 170 nm UV continuum form. Total radiated energy runs 10^25–10^27 erg, with rare giants near 3×10^28 erg.

A quick sanity check on the magnetic reservoir: take B = 500 G over a flux tube of radius r = 250 km and depth d = 500 km. The magnetic energy is roughly

• E ≈ (B²/8π) × V = (500²/8π) erg/cm³ × (π r² d)
• V = π(2.5×10^7 cm)²(5×10^7 cm) ≈ 10^23 cm³
• E ≈ (2.5×10^5/25) × 10^23 ≈ 10^27 erg

That single flux tube, if a modest fraction of its field reconnects, comfortably supplies a typical EB—confirming the energetics are self-consistent.

How They Are Observed and Where They Hide

EBs are detected in the wings of chromospheric lines—most often Hα scanned ±1 Å off center, or in the wings of Ca II 8542 Å and 854.2/396.8 nm. Crucially, they are not visible in the Hα core: dense chromospheric fibrils form a canopy of high-opacity gas above the photosphere that shades the core, so only the deep-forming wing emission escapes. This is why the naive image of an EB in Hα-core looks blank while the wing image blazes.

Ground-based instruments with adaptive optics—the Swedish 1-m Solar Telescope (SST/CRISP), GST at Big Bear, DKIST—resolve their sub-arcsecond structure and often reveal flame-like jets flickering upward, direct morphological evidence of photospheric reconnection (Watanabe et al. 2011). Space-based IRIS and SDO/AIA add UV-continuum (170/160 nm) counterparts. Bidirectional Doppler signatures in the wings mark the reconnection outflows.

Ellerman Bombs versus UV Bursts and Quiet-Sun Cousins

EBs are easily confused with two neighbors. UV bursts (Peter et al. 2014, discovered with IRIS in Si IV 1394/1403 Å) are also reconnection brightenings, but they form higher and hotter—in the upper chromosphere and transition region, reaching tens of thousands of kelvin—so they glow in Si IV while a pure EB does not. Some events show both signatures stacked along one current sheet; many do not, precisely because the reconnection height differs.

• Ellerman bomb: photospheric, Hα-wing, 600–3,000 K, active regions.
• UV burst: chromospheric/TR, Si IV, 10^4–10^5 K, hotter reconnection.
• Quiet-Sun EB (QSEB): same physics, far weaker, ubiquitous—about 500,000 present on the Sun at any instant, best seen in the Hβ wing.

Distinguishing them matters because each traces reconnection at a different atmospheric layer, mapping how energy is released from surface to corona.

Significance and Open Questions

Ellerman bombs are a natural laboratory for reconnection in a partially ionized, high-density plasma—a regime very different from the fully ionized corona, and one where neutral–ion collisions (ambipolar diffusion) may set the reconnection rate. They are also a leading candidate contributor to chromospheric heating during flux emergence, and they trace the fundamental process of magnetic flux cancellation that reshapes active-region topology.

Open questions remain sharp. How hot do they really get? Estimates span 600 K to over 10,000 K depending on the modeling assumptions, and whether Hα wing emission alone requires such heating is debated. Do EBs and UV bursts share one current sheet or two? How much do the ubiquitous QSEBs contribute to the Sun's total energy budget? DKIST, with its 4-meter aperture resolving structures below 30 km, and future high-cadence spectropolarimetry are now measuring the magnetic field inside reconnecting flame wings for the first time—turning a century-old curiosity into a precision test of reconnection physics.

Ellerman bombs versus their close cousins in the low solar atmosphere

Property	Ellerman bomb	UV burst	Quiet-Sun EB (QSEB)
Reconnection height	Photosphere, ~500 km	Upper chromosphere / TR	Photosphere, ~400–600 km
Best diagnostic	Hα / Ca II wings	Si IV 1394/1403 Å (IRIS)	Hβ / Hα wings
Temperature excess	600–3,000 K	10,000–80,000 K	~600–2,000 K
Location	Emerging active regions	Active-region canopy	Everywhere (network, quiet Sun)
Energy	10^25–10^27 erg	10^26–10^28 erg	~10^22–10^24 erg
Number at once	Tens per active region	Fewer, active-region bound	~500,000 across the Sun