☀️ Magnetic Reconnection — Solar Flares & Substorms

Bx = B0·tanh(y/δ) is the classic Harris sheet: it smoothly reverses sign across the mid-plane (positive above, negative below) while staying nearly uniform far from y = 0, which is exactly the reversed-field geometry that sets up a current sheet and X-point. It is a standard analytic stand-in for the field configuration that MHD simulations of reconnection produce numerically.

Resistivity η enters the Lundquist number S = L·v_A/η on the bottom, so turning η up lowers S. The code's Sweet-Parker rate function returns drive/√S, so a lower S directly raises the target reconnection rate, which also widens the diffusion-region half-width δ ∝ 1/√S used to draw the current sheet and set the field profile.

The magnetic energy bar decreases at a rate proportional to the current reconnection rate (energy converted ≈ recRate × 0.9 × dt each frame), while the kinetic energy bar is computed directly from the live velocity of every plasma particle on screen. Together they show magnetic energy being converted into the kinetic energy of the outflow jets as the simulation runs.

Clicking or tapping near the mid-plane (within 0.35 simulation units of y = 0) manually nudges the reconnection rate upward by a small amount that fades with distance from the click's x-position. It is a quick way to simulate a local disturbance triggering faster reconnection, similar to how a perturbation can seed the onset of a real reconnection event.

Field lines mode traces magnetic streamlines with RK2 integration from the left and right edges, colouring the upper (blue) and lower (orange) reversed regions separately so you can see them bend into the X-point. Current |J| mode instead renders a heat map of |∇×B|, computed from the sech²(y/δ) derivative of the tanh profile, which peaks exactly on the thin current sheet at y = 0.