Thomas Howson
Gather.town id
SW03
Poster Title
Coronal oscillations, the Kelvin-Helmholtz instability and magnetic reconnection
Institution
University of St Andrews
Abstract (short summary)
In recent years, many studies have highlighted the potential for the Kelvin-Helmholtz instability to form in transversely oscillating coronal flux tubes. The development of the instability may enhance the rate of wave heating by initiating an energy cascade to small spatial scales. As the instability develops, large gradients form in both the perturbed velocity and magnetic fields, leading to increased viscous and Ohmic dissipation and, consequently, enhanced wave heating. The compressive flows that form as a result of the instability force misaligned magnetic field lines together and may therefore trigger magnetic reconnection.
We present results of numerical MHD simulations of driven, transverse waves in a simple geometry. The instability is triggered by a velocity shear that forms across a resonant layer of field lines. We discuss the implications of the KHI on magnetic reconnection rates and explore the effects of field line length and non-potential equilibrium fields. In the latter case, we discuss whether the instability will enhance the rate of reconnection of the background magnetic field.
We present results of numerical MHD simulations of driven, transverse waves in a simple geometry. The instability is triggered by a velocity shear that forms across a resonant layer of field lines. We discuss the implications of the KHI on magnetic reconnection rates and explore the effects of field line length and non-potential equilibrium fields. In the latter case, we discuss whether the instability will enhance the rate of reconnection of the background magnetic field.
Plain text (extended) Summary
Slide 1: Summary - Will the Kelvin-Helmholtz instability (KHI) lead to reconnection in the corona? Yes, but we find that it does not extract significant energy from the background field.
Fig. 1a - Schematic of the role of the potential role of wave energy in coronal heating - energy can be transferred to small scales by a variety of mechanisms; phase mixing, resonant absorption, wave-induced turbulence. Fig. 1b – Cartoon of model setup. How we model waves in cylindrical structures using a simplified Cartesian geometry.
Slide 2: Summary - KHIleads to small scales in the magnetic and velocity fields. Growth rate is reduced for short or sheared field lines.
Fig. 2a shows deformation of the density profile during the growth phase of the instability. Fig. 2b shows lower growth rates for shorter and sheared field lines. Fig. 2c shows the effects of the reduced growth rates on the density profile e.g. smaller vortices when growth rate is reduced.
Slide 3: Summary – We get magnetic reconnection when the field line integral of the parallel component of the electric field (Xi) is non-zero. The parallel electric field is largest around KH vortices. Connectivity change greatest with high KH growth rates.
Fig. 3a shows contours of Xi over KH vortices. The value of Xi is largest at the edge of vortices. Fig. 3b shows how field lines are able to reconnection across the velocity shear layer during the growth of the instability. The reconnection is greatest when the growth rate of the KHI is not suppressed.
Slide 4: Summary - Reconnection rate is greatest during the initial phase of instability and when the growth rate is largest (long unsheared field lines). The slide describes the method for calculating the reconnection rate in the turbulent-like plasma.
Fig 4 shows the reconnection rate for different field line lengths and for different amounts of magnetic shear.
Fig. 1a - Schematic of the role of the potential role of wave energy in coronal heating - energy can be transferred to small scales by a variety of mechanisms; phase mixing, resonant absorption, wave-induced turbulence. Fig. 1b – Cartoon of model setup. How we model waves in cylindrical structures using a simplified Cartesian geometry.
Slide 2: Summary - KHIleads to small scales in the magnetic and velocity fields. Growth rate is reduced for short or sheared field lines.
Fig. 2a shows deformation of the density profile during the growth phase of the instability. Fig. 2b shows lower growth rates for shorter and sheared field lines. Fig. 2c shows the effects of the reduced growth rates on the density profile e.g. smaller vortices when growth rate is reduced.
Slide 3: Summary – We get magnetic reconnection when the field line integral of the parallel component of the electric field (Xi) is non-zero. The parallel electric field is largest around KH vortices. Connectivity change greatest with high KH growth rates.
Fig. 3a shows contours of Xi over KH vortices. The value of Xi is largest at the edge of vortices. Fig. 3b shows how field lines are able to reconnection across the velocity shear layer during the growth of the instability. The reconnection is greatest when the growth rate of the KHI is not suppressed.
Slide 4: Summary - Reconnection rate is greatest during the initial phase of instability and when the growth rate is largest (long unsheared field lines). The slide describes the method for calculating the reconnection rate in the turbulent-like plasma.
Fig 4 shows the reconnection rate for different field line lengths and for different amounts of magnetic shear.
URL
http://www-solar.mcs.st-and.ac.uk/~tah2/
Poster file