When observing the sun through the MgI B2 line, we are looking at the absolute base of the solar atmosphere—specifically within the transition zone from the high photosphere to the low chromosphere (the temperature minimum) at an altitude of approximately 600 to 700 kilometers above the visible solar surface. At this specific height, the gas pressure is still relatively high, causing the churning gas to powerfully push the magnetic fields aside and tightly compress them along the edges of the giant supergranulation cells. As a result, a MgI B2 image reveals supergranulation as a sharp, thin, and geometric 'spiderweb' of bright magnetic lines, interspersed with relatively quiet, dark cell centers where the normal, smaller granulation of the photosphere still faintly shimmers through. This yields a pure and undistorted view that lays bare the exact roots and foundation of solar magnetism.
Comparing MgI B2 with CaII H.
When we compare these images with observations in the CaII H line, we ascend several floors up into the active chromosphere, reaching an altitude of 1,000 to 1,800 kilometers. Because the gas pressure drops exponentially at this great height, the crushing force that compressed the magnetic fields below completely vanishes. Consequently, the sharp, thin lines from the magnesium image transform into broad, fluffy, and cloud-like structures in the calcium image; the magnetic fields flare out in a funnel-like shape, forming the so-called magnetic canopy that partially drapes over the dark cell centers.
Furthermore, the entire texture of the sun changes completely: the crisp, geometric appearance of the magnesium line gives way to a chaotic and 'hairy' landscape in the calcium line, filled with fibrils (magnetic gas streams shooting upward like blades of grass) and bright flashes from acoustic shock waves. Finally, due to intense heating higher up in the chromosphere, active regions around sunspots light up as gigantic, brilliant magnetic clouds (plages) in the calcium line, whereas those very same regions remain highly compact and sharply bounded in the lower magnesium line.
Concretely, we see the emergence of supergranulation in Mg I B2, which then further develops and expands in CaII H. I dived into my archives and reprocessed my images from December 28, 2025. The results clearly showcase the sharp, crisp definition of the supergranulation in MgI B2 versus the fluffy, chaotic magnetic field boundaries in CaII H. According to solar physics papers, the phenomena we observe in MgI B2 can only be explained if we assume they occur in an NLTE (Non-Local Thermodynamic Equilibrium) environment. This stands in contrast to LTE (Local Thermodynamic Equilibrium), which is typically much easier to calculate and model.
Setup & Processing Details:
Equipment: SOLEX by James R with a 2nd Gen Slit, mounted on an AZ-EQ6, using an ASI678MM camera.
Capture & Initial Processing: Captured in SharpCap and processed via Inti.
MgI B2 Image: Created from a stack of 3 exposures using AstroSurface, with final editing done in IMPPG and Photoshop CS4.
CaII H Image: A single-exposure capture, processed and enhanced using Photoshop CS4 and Topaz DeNoise AI.
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| Reversed images |
