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Using a 3D MHD simulation, we model the quasi-static evolution and the onsetof eruption of a coronal flux rope. The simulation begins with a twisted fluxrope emerging at the lower boundary and pushing into a pre-existing coronalpotential arcade field. At a chosen time the emergence is stopped with thelower boundary taken to be rigid. Then the coronal flux rope settles into aquasi-static rise phase during which an underlying, central sigmoid-shapedcurrent layer forms along the so called hyperbolic flux tube (HFT), ageneralization of the X-line configuration. Reconnections in the dissipatingcurrent layer effectively add twisted flux to the flux rope and thus allow itto rise quasi-statically, even though the magnetic energy is decreasing as thesystem relaxes. We examine the thermal features produced by the current layerformation and the associated "tether-cutting" reconnections as a result ofheating and field aligned thermal conduction. It is found that a central hot,low-density channel containing reconnected, twisted flux threading under theflux rope axis forms on top of the central current layer. When viewed in theline of sight roughly aligned with the hot channel (which is roughly along theneutral line), the central current layer appears as a high-density verticalcolumn with upward extensions as a "U" shaped dense shell enclosing a centralhot, low-density void. Such thermal features have been observed within coronalprominence cavities. Our MHD simulation suggests that they are the signaturesof the development of the HFT topology and the associated tether-cuttingreconnections, and that the central void grows and rises with thereconnections, until the flux rope reaches the critical height for the onset ofthe torus instability and dynamic eruption ensues.

Thermal signatures of tether-cutting reconnections in pre-eruption coronal flux ropes: hot central voids in coronal cavities