MHD modeling of coronal disturbances related to

CME lift-off

J. Pomoell1, R. Vainio1, S. Pohjolainen2

1Department of Physics, University of Helsinki2Tuorla Observatory, University of Turku

jens.pomoell@helsinki.fi

Introduction

Solar flares and coronal mass ejections (CMEs) capable of launching global large-amplitude coronal disturbances and shocks

Observed directly in Hα (Moreton waves), EUV (EIT waves), soft X-rays, He I and radio

Play a role in the acceleration of electrons and ions to high energies, exact mechanisms unclear

Observed in-situ and as various EM signatures

ST

ER

EO

AH

EA

D

EU

VI

195

ÅM

ay 1

9, 2

007

Type II radio bursts

Plasma emission (F+H) caused by shock-accelerated e-

, knowing gives

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)?

What about high-frequency type IIs?

(Pohjolainen, Pomoell, Vainio: A&A 490, 2008)

We address such issues by performing MHD simulations of CME lift-off

Look for features that might be of importance when interpreting observations

or ?

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

Eruption dynamics

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

Density Speed

Dynamic spectrum

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

Qualitative similarities

Driven or blast wave?

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

The skirt of the shock sweeps the solar surface followed by another wave

EIT waves?

Density Temperature

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

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