A comparison of CME-associated atmospheric waves observed in coronal (Fe XII 195A) and

chromospheric ( He I 10830A) lines

Holly R. Gilbert, Thomas E. Holzer, Barbara J. Thompson, and John T. Burkepile

(2004, ApJ, 607, 540)

Okamoto Takenori

Abstract & ConclusionSimultaneous observations -- chromospheric waves (He I) -- coronal waves (EIT)

Main goal -- to begin an investigation into whether both coronal and chromospheric waves are mechanical (MHD waves)

Conclusion -- chromospheric waves are imprints of mechanical waves (=coronal waves)

Observations

Instruments coronal waves -- SOHO/EIT (195A) 10-30 min, 2”.6 /pix chromospheric waves -- Mauna Loa Solar Observatory -- Chromospheric Helium Imaging Photometer (CHIP) - He I (10830A) 3 min, 7” /pix -- Ha (6563A) - Polarimeter for Inner Coronal Studies (PICS) CME -- SOHO/LASCO C2 and C3

Observations (2000 November 25)

CME (17:30)

He wave(18:24)

EIT wave(18:35)

GOES peak (X1.9, 18:44)

CME(19:30)

another filamenteruption (18:38)

filament eruption(17:13)

CME

773 km/s (17:30)671 km/s (19:30)

direction

17:30

19:30 (halo)

Event 1 (2000 November 25)

EIT wave18:35 – 47 – 73 UT

He wave18:33 – 40 – 43 – 46 – 49 UT

Wave front tracings

He wave and EIT wave -- cospatialHe-wave speed ~ 300 km/s

the correction of wave front motion + spreading +recombination time

(< the spatial resolution of CHIP instrument)

Observations (2001 January 20)

-- He wave (18:37 UT) -- two ribbon flare (18:40 UT) -- CME (19:30 UT) -- GOES 18:33 – 18:47 (peak M1.2) – 18:59 UT -- He I brightenings & EIT dimmings (18:46 UT)

CME

839 km/s

19:30

direction

Event 2 (2001 January 20)

EIT waveHe wave

18:43 – 47 – 50 – 53 – 59 UT

Wave speed ~ 219 km/s(western part ~ 429 km/s)

cospatial

Formation of He I 10830A

He I absorption line -- formed in the upper chromosphere -- absorption of photospheric continuum radiation at 10830 A -- depends on the column density

-- collision process -- photoionization-recombination mechanism

Possible Physical Explanation

Chromospheric waves -- not mechanical waves (MHD waves) propagating horizontally through the upper atmosphere -- but “imprints” of MHD waves travelling in the corona

The bright front associated with the compressive wave travellingin the corona leads to an enhanced chromospheric absorption of thePhotospheric continuum at 10830A ~ cospatial dark front in the He I.

Vrsnak et al. (2002) concluded the He I disturbances consist of two main parts -- a forerunner -- a main dip corresponding Ha disturbance

the cause of the main dip -- a sudden pressure jump in the corona (shock wave) - density/temperature increase (behind the shock wave) - collisional processes are enhanced - He I absorption increase

Photoionization-recombination process may cause a He I enhancement in the diffuse component of the perturbation.

In these events -- These waves have not necessarily steepened into shocks. -- Photoionization-recombination mechanism is the main cause of increased He I absorption.

It is difficult to distinguish between the two proposed picturesWithout performing complicated radiative transfer calculations.

Dimming & Brightening

EIT dimming ~ He I brightening (cospatial)

Dimming -- a decrease in density of the overlying corona -- caused by the opening of magnetic field (occurring during the explosive CMEs)

The expansion of dimming regions mimic the propagation of a wave. -- waves are observed propagating to much greater distances from their source region than the outward expansion of EIT dimmings or He I brightenings.