Deconstructing EIT Waves

Marco Velli

Jet Propulsion Laboratory, Caltech andDipartimento di Astronomia e Scienza dello Spazio,

Università di Firenze

Thompson et al. 1999

“EIT waves” large scale propagating disturbances and associated with brightenings and dimmings found with running and fixed difference images using extreme ultraviolet telescope, occurring in connection with CMEs and flares (Moses et al. 97, Thompson et al.98)

Summary of a somewhat bewildering set of observations

Summary of models

Open questions, relative observations, and future diagnostic use of global MHD perturbations of the solar corona

EIT wave and associated brightening/dimming (fixed difference

images)

EIT wave progression (running difference images)

Basic observational characteristics of EIT waves

Propagate with nearly circular symmetry in the simple corona

In complex corona, avoid active regions, neutral lines, stop near coronal holes

Typical velocities 170-350 km/s (271 km/s av) Klassen et al. 00

Moreton waves - (Moreton 1961; Athay & Moreton 1961; Ramsey & Smith 1966) seen in Ha images. Observed to propagate out to large distances from the flare sites, with evidence of chromospheric depression seen in the wings of the Ha bandpass. The inferred speeds in the range 330 - 4200 km/s (Smith & Harvey 1971); such high speeds are generally accepted as evidence of a coronal, rather than a chromospheric origin for this phenomenon.

Explanation given in classic paper by Uchida (1968) which analyzes the WKB propagation of fast modes in a spherically symmetric stratified geometry with a radial magnetic field.

Natural to think of EIT waves as a coronal counterpart of Moreton waves. Driven by a “blast”

An aside: polar and Friedrichs diagrams: hereVs=0.9Va

Shock fronts from a point “supersonic” objectCs=0.8 Va hereIn general slow cusps even smaller and aligned with Va. SuchModes should lose coherence very very quickly unless supportedby some “effective surface”

Warmuth et al. 2001: At least in some cases Halpha and EIT wave cospatiality is established (1997 November 3 and 1998 May 2), favoring blast-wave fast mode shock scenario

(Khan & Aurass, 2002)

Wu et al, 2001:3D, time-dependent, numerical MHD modelPressure pulse

Running difference images EIT 195Å

BUT: a very slow fast mode (high beta or low B corona)

Sturrock - Shibata 2 ribbon flare and CMEunified scenario

Chen argues in favor of a combined model explaining both Moreton and EIT waves in terms of erupting CME or piston model

Numerical MHD simulations. Erupting CME induces piston-driven shock (coronal Moreton wave?) But the “EIT wave” corresponds to a density front moving behind the shock front. The “EIT wave” results from the combination of the stretching of closed field lines covering the erupting flux and the propagation of the consequent Alfvén wave down the field lines overarching the rope. (Chen et al. 2002; Chen, Fang & Shibata 2005)

The “slow” EIT wave propagation is obtained by combining the piston-propagation time with the fast (Alfvén) propagation time along longer and longer loops:

Scenario more specifically ties propagation of EIT wave to dimmings and reconnection and onset of the CME (Delannee’ 2000, Delannee’ et al 2007 Zhukov and

Auchere 2004) “These observations suggest that EIT wave can be regarded as a bimodal phenomenon. The wave mode represents a wave-like propagating disturbance. Its characteristic features are propagation of a bright front to large distances from dimming sites and quasi-circular appearance. The eruptive mode is the propagation of a dimming and of an EIT wave as a result of successive opening of magnetic field lines during the CME lift-off. It can be identified by noting the expansion of a dimming and the appearance of another dimming ahead of a bright front.”

dimmings

EC

front

dimmings

EC

front

EIT wave front rotation

Podladchkova and Berghmans 2005

Rotation of brightening in counter clockwise sense` ~

Rotation of EIT wave fronts Podladchikova and Berghmans 2005,Attrill et al. 2007

12th May 1997 event

7th April 1997 event

Attrill et al. 07

ACW rotation

12th May 1997 ACW event

CW rotation

7th April 1997 CW event Reverse “S” Sigmoid

Negative Helicity

Forward “S” Sigmoid Positive Helicity

Attrill et al 2007

Amari et al. 2003

CME flux rope eruption

Evolution in two phases:First a twisted flux rope is created, slow and almost quasi-static;second a disruption, which is confined for a small initial helicity and global for a large initial helicity.

Following the evolution of flux rope AND waves in such geometries is computationally difficult.

Kink in itself tendentially slow/alfvenic

Extended unfolding wave source, might conceivably explain rotation of wave front

Attrill model: multiple small scale reconnection events

Deep core dimmings and widespread transient dimmings -> evacuated plasma -> mass of CME (Zhukov & Auchére 2004).

Summing up for origin of EIT Waves

A) Wave Theories FAST+ avoids regions of large Alfven speed, whether they be active regions or coronal holes. Halpha correspondence (Vrshnak paper)- why such low speeds? There may be selection effects (17 minute cadence, 1000 secs., 1000km/sec wave travels over 1 solar radius) SLOW+ perhaps the overall speed of the perturbation- it is hard to reconcile with the coherent structure required for the propagation of such a front in the widely varying coronal topology. ALFVEN--- no density perturbation, collimation and extremely anisotropic propagation However, an extended source (erupting loop) might help out. BLAST SOLITON

B) CME pseudo-wave or reconnection wave + ways to make a speed which is lower than the sound speed

- how do you explain correlation of front rotation with helicity?

Why is the EIT wave circular or quasi circular, and not ribbon like (as in two ribbon flares) or even more structured if it depends on reconnection with peripheral magnetic fields? (Or is this precisely what the Zhukov Auchere and Delannee and Attrill analyses show?)

STEREO: Higher cadence (to start with, accurate determinationof front propagation speeds).

Accurate wave identification/measurement allows coronal diagnostics.

Propagation of EIT waves and their effects on coronal structures (Ofman and Thompson 2002)

EIT waves require a CME Cliver et al 2005

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