Sunspot umbrae are dark because strong magnetic fields suppress convection. The field, typically 2-3 kilogauss, is strong enough to inhibit the turbulent overturning that brings hot material to the surface. Less convection means less heat transport means less brightness. Strong field, dark region — the correlation is clean.
Light bridges are bright lanes that cross sunspot umbrae, dividing them. They are bright because convection partially resumes within the bridge. The standard picture: light bridges are regions of weakened field, where the magnetic suppression relaxes enough for hot material to rise. Bright region, weaker field — the inverse correlation.
The paper overturns this. Analysis of 630 spectropolarimetric scans across 98 bipolar light bridges reveals that nearly two-thirds contain magnetic fields exceeding 4.5 kilogauss — stronger than the surrounding umbra. The brightest features in the sunspot are also the most magnetized. The expected anticorrelation between brightness and field strength does not hold.
The resolution: bipolar light bridges sit at the boundary between opposite-polarity regions. The field configuration is not a single vertical field suppressing convection but a complex, sheared geometry where opposite polarities compress against each other. The compression intensifies the field while the boundary itself — the polarity inversion line — permits convective motions that the straight field would suppress.
The structural point: brightness is not controlled by field strength alone but by field geometry. A straight field suppresses convection. A compressed, sheared field of equal or greater strength does not. The same quantity (field magnitude) produces opposite effects depending on its spatial structure. The bright stronghold is not a weakness in the magnetic armor but a different kind of strength.