The Dependence of Electron-only Magnetic Reconnection on Plasma Beta and Magnetic Shear: Theory and Observations

We investigate the onset conditions for electron-only magnetic reconnection through an analysis of 86 reconnection events observed by the Magnetospheric Multiscale mission in magnetosheath and magnetospheric current sheets. Our study presents the first observational evidence that the occurrence of electron-only reconnection depends on both the magnetic-field shear angle ( θ ) across the current sheet and the cross-sheet electron beta jump, quantified either by the inflow electron beta difference (Δ β _e ) or by a modified electron beta jump ( ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }$ ) defined by the cross-sheet electron pressure difference normalized by the average of the inflow magnetic pressure in the two inflow regions. We extend the diamagnetic-drift suppression mechanism from standard reconnection to electron-only reconnection by deriving relations applicable to electron-scale current sheets. Using scaling estimates, the electron-only reconnection suppression relation is derived as ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }\gtrsim 2(L/{d}_{{\rmé}}})\tan (\theta /2)$ , while a natural extension of the ion-coupled reconnection suppression criterion is ${\rm{\Delta }}{\beta }_{{\rmé}}}\gtrsim 2(L/{d}_{{\rmé}}})\tan (\theta /2)$ . At low Δ β _e and ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }$ values, reconnection is detected across a large range of magnetic shear (from low to high), whereas high Δ β _e and ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }$ conditions restrict reconnection to large shear configurations: shear angles as small as θ ≈ 5 ^∘ occur when Δ β _e (or ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }$ ) ≲0.1. However, only events exceeding θ ≳ 70 ^∘ are found when Δ β _e ≳ 1.0; this apparent restriction weakens when employing ${\rm{\Delta }}{\beta }_{{\rmé}}}^{* }$ , as the corresponding data points shift toward smaller values of the modified electron beta jump. These findings quantitatively support theoretical predictions of reconnection suppression in high- β _e plasmas at low shears, attributed to X-line drift induced by pressure gradients across electron-scale current sheets.