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High-cadence Radio Observations of an EIT Wave

High-cadence Radio Observations of an EIT Wave. S. M. White and B. J. Thompson 2005, ApJ, 620, L63. OKAMOTO Joten/Takenori (D1). Abstract. Radio observations of the 1997 September 24 EIT wave The radio spectrum is consistent with optically thin coronal emission.

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High-cadence Radio Observations of an EIT Wave

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  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

  3. 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

  4. 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

  5. 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)

  6. EIT wave on 1997 September 24 (Thompson et al. 2000) sharp front SOHO/EIT flare site 2:33 UT 2:49 UT 3:03 UT

  7. Moreton wave on 1997 September 24 (Thompson et al. 2000) 3:03 3:23 50 49 47 Moreton wave: 500 km/s EIT wave: 300 km/s 45 EIT Ha (Ha) (EIT) (Ha) (Ha) 02:45 UT 02:47 UT 02:49 UT 02:50 UT

  8. 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)

  9. Radio wave event (1999 September 24) 100” No wave Brightness temperature 5000 K -> 1000K The evolution is very similar to that seen in Ha. The exact time of EIT is uncertain. (2 min) At 02:51:00 UT -> identical in position, shape If so, estimate EM and temperature.

  10. Radio wave The radio wave speed is 835 km/s. The onset time of the flare is 02:47 UT, later than 02:43 UT made by Thompson et al. (2000) on the basis of Ha. (but, Ha: 30 s, 40” uncertainty)

  11. 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 ∝ν^2) • Density enhancements, not temperature effects • Temperature increase -> radio flux decrease (if density is not changed)

  12. Discussion 2 • The EIT wave shows no deceleration over the almost 5 minutes. • Moreton wave -> (deceleration) -> 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: 02:49 ~02:53 UT • 500 km/s Moreton wave between 02:47 and 02:50 UT • error of order 3 minutes (Ha) would be needed if the radio wave is colocated with the Moreton wave.

  13. Conclusion • Radio observations of 1997 September 24 EIT wave  830 km/s (> 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.

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