Ultra slow electron holes in collisionless plasmas: stability at high ion temperature
Numerical simulations recover ultra slow electron holes (EH) of electron-acoustic genre propagating stably well below the ion acoustic speed where the ion response disallows any known pure electron perturbation. The reason of stability of EH at high ion temperature ($T_{i}> T_{e}$) is traced to the loss of neutralizing cold ion response. In a background of cold ions, $θ=T_e/T_i\gg 1$, they have an ion compression that accelerates to jump over a forbidden velocity gap and settle on the high velocity tail of the electron distribution $f_e$, confirming to a recently identified limit of the nonlinear dispersion relation. For $θ=T_e/T_i \le 1$, however, the warm ions begin to supplement the electron response transforming the ion compression to decompression at the hole location and triggering multiplicity of the scales in trapped electron population which prompts an immediate generalization of the basic EH theory.