Formation of hot channels in pre-CME coronal flux ropes and their role in the

onset of eruptions

Yuhong Fan and Piyali ChatterjeeHigh Altitude Observatory, National Center for Atmospheric Research

Simulation set up• Numerically solve the following MHD equations, assuming a perfect gas

with adiabatic index g = 1.1 for the coronal plasma:

• Solve the total energy equation in conservative form and include thermal conduction along magnetic field lines

• The domain is resolved by a non-uniform grid of 432x192x240

• Initially the corona is a static isothermal atmosphere at 1MK with a pre-existing potential arcade field, with cs=135 km/s, va0=1951km/s.

• At the lower boundary, we impose (kinematically) the emergence of a twisted torus for t=0 to t=tstp after which the emergence is stopped and the field lines are rigidly anchored subsequently

€

Spherical coronal simulationdomain :

r∈ Rs, 5.496Rs[ ],

θ ∈ 5π /12, 7π /12[ ],

φ∈ −π /9.6, π /9.6[ ]

Fan (2012)

Hyperbolic flux tube (HFT) (e.g. Titov 2002, Savcheva et al. 2012)

• Strongest current layers form along QSLs with very large squashing factor Q, which measures the “squashing” of the flux tube cross-section as it is mapped from one foot point to the other (Titov et al. 2002; Titov 2007; Pariat & Demoulin 2012).

€

Orange surfaces : iso - surfaces of J/B = 1/(10 gridsize),Pink surface : iso - surface of T =1.2 MK

AIA 171 AIA 193

Reversed AIA images from Regnier et al. (2011)

AIA 211

Berger (2012)Regnier et al. (2011)

U-shape or horns

Observation of a hot channel prior to and during a CME: “driver” of the eruption (Zhang et al. 2012, Cheng et al. 2013)

Summary

• A sigmoid-shaped vertical current layer forms along the QSL underlying the pre-eruption coronal flux rope

• Reconnections in the current layer effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically

• As a result of such tether-cutting reconnections a central hot, low-density channel containing reconnected, twisted flux threading under the flux rope axis form on top of the central current layer.

• When viewed in the line of sight roughly aligned with the hot channel, the current layer appears as a high-density vertical column with upward extensions as a “U” shaped dense shell (or “horns”) enclosing a central hot, low-density void.