Highcadence Radio Observations of an EIT Wave

1
High-cadence Radio Observations of an EIT Wave

• S. M. White and B. J. Thompson
• 2005, ApJ, 620, L63

OKAMOTO Joten/Takenori (D1)
2
Abstract

• Radio observations of the 1997 September 24 EIT
  wave
• The radio spectrum is consistent with optically
  thin coronal emission.
• No deceleration is observed during the 4 minutes
  from the flare onset

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EIT wave

• EIT stands for EUV Imaging Telescope.
• In 1997, a flare-associated wave was
• Observed with SOHO/EIT.
• Associated with
• -- Moreton waves in Ha
• -- Coronal mass ejections (CME)
• -- Type II radio bursts
• Fe
  XII 195Å
• Plasma
  at 1.5 MK

green/original monochromatic/running difference
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Uchida model
In 1968, Uchida proposed fast-mode MHD
shock-wave model to explain Moreton waves.
chromosphere Moreton wave (a shock wave
depresses solar chromosphere) corona ???
EIT wave ? (what is a coronal Moreton
wave ?)
model
corona
chromosphere
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EIT wave ( the other waves)

• EIT wave
• The wave is launched by the impulsive phase of
  the flare
• It arises in association with CME following the
  flare
• What is an EIT wave?
• Fast-mode shock wave
• Propagating fronts in soft X-ray images (e.g.
  Narukage et al. 2002)
• Dark features in He I 10830A (e.g. Gilbert et al.
  2004)
• Coronal Moreton wave or not (e.g. Thompson et al.
  2000, Eto et al. 2002)
• Deceleration of wave front over time (e.g.
  Warmuth et al. 2001)

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EIT wave on 1997 September 24 (Thompson et al.
2000)
sharp front
SOHO/EIT
flare site
233 UT
249 UT
303 UT
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Moreton wave on 1997 September 24 (Thompson et
al. 2000)
303
323
50
49
47
Moreton wave 500 km/s EIT wave 300 km/s
45
EIT
Ha
(EIT)
(Ha)
(Ha)
(Ha)
0245 UT
0247 UT
0249 UT
0250 UT
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Radio wave

• Difficult to detect
• Two events reported
• A radio feature moving in the same direction as
  an EIT wave, but not coincident with it (Aurass
  et al. 2002)
• A feature moving with a Moreton wave, but no EIT
  data (Warmuth et al. 2004)

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Radio wave event (1999 September 24)
100
No wave
Brightness temperature 5000 K -gt 1000K
The evolution is very similar to that seen in Ha.
The exact time of EIT is uncertain. (2 min)
At 025100 UT -gt identical in position, shape
If so, estimate EM and temperature.
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Radio wave
The radio wave speed is 835 km/s. The onset time
of the flare is 0247 UT, later than 0243 UT
made by Thompson et al. (2000) on the basis of
Ha. (but, Ha 30 s, 40 uncertainty)
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Discussion 1

• Production of radio emission
• thermal free-free emission
• Emission region (spectra 17 GHz and 34 GHz)
• optically thin coronal emission (flat spectrum)
• optically thick chromospheric emission (I ??2)
• Density enhancements, not temperature effects
• Temperature increase -gt radio flux decrease (if
  density is not changed)

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Discussion 2

• The EIT wave shows no deceleration over the
  almost 5 minutes.
• Moreton wave -gt (deceleration) -gt EIT wave ?
• All Moreton waves appear to decrease in velocity
  (Warmuth et al. 2004)
• 800 km/s in radio for up to 300 s from the flare
  onset 0249 0253 UT
• 500 km/s Moreton wave between 0247 and 0250 UT
• error of order 3 minutes (Ha) would be needed if
  the radio wave is colocated with the Moreton wave.

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Conclusion

• Radio observations of 1997 September 24 EIT wave
  ? 830 km/s (gt EIT wave speed 300 km/s)
• we cannot measure fast EIT-wave speed because of
  the short cadence 12 min
• Radio wave appeared at some distance from the
  flare site.
• Since radio data are sensitive to density
  enhancements rather than temperature changes, the
  wave phenomenon observed by EIT is not a
  temperature effect.
• No deceleration is observed while it is visible
  in the radio images.
• The issue is critical since it is required to
  explain why EIT waves have much slower than the
  Moreton waves, if they are the same disturbance.