Space Science Seminar Series: Bridging Kinetic and MHD Models

Wednesday, May 21, 2025 - 3:30 p.m. to 4:30 p.m.

Morse Hall, rm 301 -OR- Zoom

Speaker: 
Dr. Harry Arnold – Johns Hopkins Applied Physics Laboratory (JHU/APL) 

Full title: 
Overstretched Thin Current Sheets and the Onset of Magnetotail Reconnection: Bridging Kinetic and MHD Models 

Abstract: 
Magnetic reconnection in Earth’s magnetotail requires the current sheet (CS) to thin down to ion thermal gyroradius scales or smaller to enable ion demagnetization and dissipation. However, conventional isotropic plasma models fail to reproduce such thin sheets at distances where X-lines are typically observed. Recent particle-in-cell (PIC) simulations have revealed a class of equilibria—overstretched thin current sheets (OTCSs)—formed in weakly anisotropic ion plasmas, where the current density contours extend well beyond the magnetic field lines. These structures remain stable despite their extreme aspect ratios and exhibit force balances significantly modified by weak ion anisotropy and agyrotropy, which contribute comparably to the pressure gradient.

2-D PIC simulations of OTCSs help explain the observed locations of X-lines in Earth's magnetotail. These simulations reproduce the onset of reconnection within realistic tail equilibria under conditions of weak external driving. The transition to magnetic topology change—the hallmark of reconnection—is consistently preceded by divergent plasma flows in both electron and ion populations, indicating that ion tearing initiates reconnection.

In this talk, we also discuss ongoing efforts to incorporate ion scale thin current sheets into global MHD models. These developments aim to include key kinetic physics into global models that will enable more realistic reconnection x-line formation.