New inferences on the physical nature and the causes

of coronal shocks

Alexander Warmuth

Astrophysikalisches Institut Potsdam

Motivation

coronal shocks:

• have important consequences: role in acceleration of particles, SEP events, ...

• can be used to probe corona: Alfven speed, magnetic field strength, ...

• give information on flare/CME processes

consider here: signatures of propagating shocks in low corona

• metric type II bursts: long discussion on cause (flare-launched blast wave vs. CME-associated piston-driven shock)

• flare waves (a.k.a. Moreton waves): not much discussion until discovery of EIT waves

• relation type II bursts - flare waves?

A multiwavelength study of flare waves

use advantages of flare waves to study nature & origin of shocks

• imaging observations good kinematics & spatial information • no dependence on coronal density model• back-extrapolation of shock initiation time & location comparison with possible causes

study of 12 flare wave events

• imaging observations in H, He I, EIT, SXT, Nobeyama 17 GHz• radiospectral data • study association, morphology, kinematics & evolution of waves• study associated phenomena (flares, CMEs, ejecta, ...)

12 additional “class 2” events

• some signatures of flare waves, but no nice coherent wavefronts• low-amplitude limit of phenomenon?

Flare wave eventMoreton wave of 2 May 1998

Above: Hdifference movie (13:38 - 13:47 UT)

Left: Moreton fronts (black) and EIT fronts (white)

Kanzelhöhe Solar Observatory

The physical nature of flare waves

• all signatures follow closely associated kinematical curves

one common physical disturbance

The physical nature of flare waves

• all signatures follow closely associated kinematical curves

one common physical disturbance

• morphology of the signatures, down-up swing of chromosphere

wave-like disturbance

The physical nature of flare waves

• all signatures follow closely associated kinematical curves

one common physical disturbance

• morphology of the signatures, down-up swing of chromosphere

wave-like disturbance

• waves travel perpendicular to field lines, are compressive, initial speeds of nearly 1000 km/s

fast-mode MHD wave, waves are (at least initially) shocked (Mms ~ 2-4)

The physical nature of flare waves

• all signatures follow closely associated kinematical curves

one common physical disturbance

• morphology of the signatures, down-up swing of chromosphere

wave-like disturbance

• waves travel perpendicular to field lines, are compressive, initial speeds of nearly 1000 km/s

fast-mode MHD wave, waves are (at least initially) shocked (Mms ~ 2-4)

• deceleration, perturbation broadening and weakening

shock formed from large-amplitude simple wave; eventually shock decays to ordinary fast-mode wave

The physical nature of flare waves

• all signatures follow closely associated kinematical curves

one common physical disturbance

• morphology of the signatures, down-up swing of chromosphere

wave-like disturbance

• waves travel perpendicular to field lines, are compressive, initial speeds of nearly 1000 km/s

fast-mode MHD wave, waves are (at least initially) shocked (Mms ~ 2-4)

• deceleration, perturbation broadening and weakening

shock formed from large-amplitude simple wave; eventually shock decays to ordinary fast-mode wave

• 100% association with metric type II bursts, correlations in timing & kinematics

flare waves and metric type II bursts are signatures of the same underlying disturbance

Passage of thefast-mode MHD shock through the corona (C) and its signatures in the transition region (TR) and chromosphere (Ch).

The fast-mode MHD shockGeometry of the disturbance

type II source

HeI patch

magnetic field lines

agent causing HeI forerunner

HeI intensity profileT enhancement

H line center intensity profile

filament

H blue wing intensity profileDoppler velocity profile

H red wing intensity profile

What launches the waves?Possible triggers of the fast-mode shock

• Flares: may launch disturbance via pressure-pulse mechanism (classical blast wave scenario)

• Small-scale ejecta (sprays, erupting loops or plasmoids, ...):

may act as temporary piston which creates initially driven shock which later continues propagation as free blast wave

• CMEs:

may either create a piston-driven shock or launch a blast wave

FlaresCharacteristics

Spatial characteristics:

• flares often near the dominating spot, invariably at periphery of the sunspot group

Energetics:

• flare importances: C8.6 - X4.9 (mean: X1.4; median: M8.3) no importance threshold• GOES SXR rise times (begin-max): 5 - 22 min (mean: 8.8 min) less than average• GOES SXR max. temperature: 13-28 MK (mean: 20 MK)• comparatively hard power-law photon spectra (mean ~ 3)• wave-associated flares have higher SXR impulsiveness• class 2-associated flares are less impulsive, only slightly cooler

Flares seem to form distinct class, but rather wide range in characteristics

Extrapolated wave onset timesComparison with HXR burst

Extrapolated wave source pointsOff-set of starting location

FlaresRelation with waves

Temporal relation:

• extrapolated wave onset times near begin/initial rise of HXR bursts

Spatial relation:

• wave source points clearly dislocated from flare center

Energetics:

• no significant correlations between flare energetics and wave parameters

Small-scale ejectaHand SXR

Upper row: Bright H flare ejecta in the event of 2 May 1998 (Kanzelhöhe Solar Observatory)Lower row: Ejected SXR blob/loop in the event of 18 Aug 1998 (Yohkoh/SXT)

Small-scale ejectaCharacteristics

Morphology/types of ejecta:

• H: bright ejecta (sprays) in impulsive phase, dark ejecta in later phase• SXR: erupting loops and blobs (plasmoids), jets

Spatial characteristics:

• originate in or near flare, propagate away from AR/main spot

Kinematics:

• maximum speeds 40-1500 km/s (mean 600 km/s)

inhomogeneous group, wide range of characteristics

Small-scale ejectaRelation with waves

Association:

• in ~85% of events some kind of ejecta present

Temporal relation:

• in ~75% of events starting times of ejecta agree roughly with wave initiation times

Spatial relation:

• rough agreement between ejecta and wave starting points• direction of ejecta agree with wave direction in all events

Kinematics:

• in majority of events (66%) ejecta significantly slower than wave

• in only < 50% of events ejecta which may be accounted for wave generation • no precise timing/kinematics for ejecta due to observational constraints

CMEsCharacteristics

Spatial characteristics:

• angular widths: 45° - 360° (mean: 177°), 25% halo CMEs wider than average

Kinematics:

• linear CME speeds: 227 - 1200 km/s (mean: 683 km/s) faster than average

CMEs are more energetic than the average, but wide range in parameters

CMEsRelation with waves

Association:

• high ( > 90%, possibly 100%)

Temporal relation:

• most CMEs start well before flare/wave, but onset times are inaccurate

Spatial relation:

• at time when wave becomes observable: - mean distance wave-starting point: 100 Mm - mean CME height above photosphere: 1,9 Rs

can such a large-scale structure drive/launch small & sharp disturbances? Kinematics:

• in most events CMEs slower than waves (78%) or type II bursts (88%)• no significant correlations between CME kinematics and wave parameters• CMEs associated with class 2 events even more energetic

Current status

What is needed:

• direct observation of initial disturbance and of the transformation to the more familiar flare wave signatures• better data on kinematics of ejecta• better data on flare energetics

need for high-cadence and high-resolution data

Association: favors flares & CMEsTiming: favors flares

Spatial aspects: favors small-scale ejecta

No conclusive results on wave initiation mechanism

search for events with TRACE & RHESSI coverage

The X4.8 flare of 23 July 2002First wave event with TRACE & RHESSI coverage

W

W

NR

23 July 2002 - HMoreton wave

atypical Moreton wave:

• protracted activity near flare (in region NR) before wave initiation

• diffuse & irregular morphology („class 1.5 event“)

• difficulty in determining kinematics & starting time/location

23 July 2002 - TRACE 195 Å Overview

EL

BL

W

EL: erupting loop/bubble00:22 - 00:27 UTv ~ 170 km/s

W: small wavefront00:27 - 00:30 UTv ~ 150 km/s

BL: moving/brightening loop00:28 - 00:30 - 00:34 UTvmax ~ 120 km/s

NR: depression of coronal structures00:24 - 00:30 (max)

red contours: RHESSI 6-12 keV

blue contours: RHESSI 50-100 KeV

NR

23 July 2002 - TRACE 195 Å Evolution in region NR

• erupting loop EL

• further erupting/opening loops

• depression of coronal structures in NR

• small wave at N edge of FOV

00:23:30 - 00:34:13 UT

23 July 2002 - CMETiming & Kinematics

by courtesy of the Catholic University of America

• energetic CME: halo, speed 1726 km/s, IP type II burst• starting time 00:11UT rough agreement with flare• but: only 2 measurements (both at R > 20 Rs) uncertainty in timing & kinematics of early phase

23 July 2002 - Summary

• 00:22:12: EUV loop/bubble starts to erupt• 00:24:22: coronal structures in NR start being pushed down• 00:26:15: abrupt increase in HXR emission• 00:26:45: BR begins to brighten in H• 00:27:18: small wave in EUV starts• 00:28:00: type II burst starts• 00:28:45: BR has transformed into (patchy) Moreton front

• perturbation probably initiated in the range 00:24 - 00:27 UT

• perturbation originates from/above region BR/DM

• wave initiation more gradual than in typical Moreton event different generation mechanisms?

• motions & restructuring of coronal magentic fields is prevalent cause or effect of wave/shock?