Physical Nature of Coronal Shocks: Flare Waves and Moreton Waves Revealed

1. New inferences on the physical nature and the causes of coronal shocks Alexander Warmuth Astrophysikalisches Institut Potsdam

2. 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?

3. 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 Ha, 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?

4. Flare wave event Moreton wave of 2 May 1998 Above: Ha difference movie (13:38 - 13:47 UT) Left: Moreton fronts (black) and EIT fronts (white) Kanzelhöhe Solar Observatory

5. The physical nature of flare waves • all signatures follow closely associated kinematical curves • one common physical disturbance

6. 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

7. 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)

8. 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

9. 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

10. The fast-mode MHD shock Geometry of the disturbance type II source Passage of the fast-mode MHD shock through the corona (C) and its signatures in the transition region (TR) and chromosphere (Ch). magnetic field lines filament agent causing HeI forerunner HeI patch r & T enhancement HeI intensity profile Ha line center intensity profile Ha blue wing intensity profile Doppler velocity profile Ha red wing intensity profile

11. 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

12. Flares Characteristics • 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 g ~ 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

13. Extrapolated wave onset times Comparison with HXR burst

14. Extrapolated wave source points Off-set of starting location

15. Flares Relation 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

16. Small-scale ejecta Ha and SXR Upper row:Bright Ha 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)

17. Small-scale ejecta Characteristics • Morphology/types of ejecta: • Ha: 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

18. Small-scale ejecta Relation 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

19. CMEs Characteristics • 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

20. CMEs Relation 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

21. Current status Association: favors flares & CMEs Timing: favors flares Spatial aspects: favors small-scale ejecta No conclusive results on wave initiation mechanism • 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 search for events with TRACE & RHESSI coverage

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

23. 23 July 2002 - Ha Moreton 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

24. 23 July 2002 - TRACE 195 Å Overview EL: erupting loop/bubble 00:22 - 00:27 UT v ~ 170 km/s W: small wavefront 00:27 - 00:30 UT v ~ 150 km/s BL: moving/brightening loop 00:28 - 00:30 - 00:34 UT vmax ~ 120 km/s NR: depression of coronal structures 00:24 - 00:30 (max) red contours: RHESSI 6-12 keV blue contours: RHESSI 50-100 KeV W BL NR EL

25. 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

26. 23 July 2002 - CME Timing & 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

27. 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 Ha • 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?