MHD modeling of

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MHD modeling of coronal disturbances related to
CME lift-off J. Pomoell1, R. Vainio1, S.
Pohjolainen2 1Department of Physics, University
of Helsinki 2Tuorla Observatory, University of
Turku
jens.pomoell_at_helsinki.fi
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Introduction

• Solar flares and coronal mass ejections (CMEs)
  capable of launching global large-amplitude
  coronal disturbances and shocks
• Observed directly in Ha (Moreton waves), EUV (EIT
  waves), soft X-rays, He I and radio

STEREO AHEAD EUVI 195 Å May 19, 2007

• Play a role in the acceleration of electrons and
  ions to high energies, exact mechanisms unclear
• Observed in-situ and as various EM signatures

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Type II radio bursts

• Plasma emission (FH) caused by shock-accelerated
  e-
• , knowing
  gives

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Questions Aims

• Current consensus Interplanetary type IIs
  generated by CME driven shocks. But what about
  coronal type IIs, generated by blast waves
  (flares) or driven waves (CMEs)?

or ?

• What about high-frequency type IIs?

• We address such issues by performing MHD
  simulations of CME lift-off
• Look for features that might be of importance
  when interpreting observations

(Pohjolainen, Pomoell, Vainio AA 490, 2008)
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MHD Model

• 2D model, gravitationally stratified corona
  including a dense loop
• Superimpose flux rope structure with higher
  density

• Alfvén speed increases in the higher corona, low
  in the loop

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Eruption dynamics
Density Speed

• When the flux rope starts to rise, a perturbation
  is formed around the flux rope, and steepens to a
  shock
• Below the loop, the shock remains weak, but
  strengthens and slows down quickly when entering
  the loop
• As the flux rope decelerates, the displaced loop
  and shock escape from the driver
• The shock escapes quickly after exiting the loop

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Dynamic spectrum

• Assuming radio type II emission is produced at
  the leading edge of the shock, we plot frequency
  vs. time

• Qualitative similarities

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Driven or blast wave?

• In a simulation without dense loops, the shock
  also escapes from the flux rope

Density Temperature

• The skirt of the shock sweeps the solar surface
  followed by another wave
• EIT waves?

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Summary of results

• Depending on the variations of the Alfvén speed
  in the low corona, the erupting CME can at times
  acts as the driver of the shock, while at other
  times the shock may propagate freely
• Difficult to determine whether coronal waves
  caused by flare or CME, low-cadence observations
  may be misleading
• Correlation between speed and location of type II
  bursts and ejecta can be very complex
• Possible that fragmented, high-frequency type IIs
  due to CME driven shocks propagating through
  dense coronal loops

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Conclusion

• By performing numerical simulations side by side
  with analysis of observations, the physics
  involved in the coronal phenomena can more
  readily be extracted than by solely analyzing the
  observational data
• All approaches needed in order to understand
  these dynamical processes