[ITAL]Yohkoh[/ITAL] SXT Observations of X-Ray “Dimming” Associated with a Halo Coronal Mass Ejection

Abstract

A sudden depletion or intensity "dimming" of the X-ray corona sometimes accompanies a solar eruptive flare or coronal mass ejection (CME). We have identified a dimming that occurred just prior to a "halo" CME, observed on 1997 April 7 using the Soft X-ray Telescope (SXT) on Yohkoh. Halo CMEs are prime candidates for "space weather" effects. The dimming occurred in compact regions near a flare of 14 UT on April 7, over a projected area of about 10 20 cm Ϫ2 , and indicate that a mass of a few times 10 14 g was ejected. This is a lower limit imposed by the obscuration of the dimming volume by the brightness of the accompanying flare and other factors. Most of the mass deficit comes from two regions close to the ends of a preflare S-shaped active-region structure, and the resulting dimmings in these regions persisted for more than three days following the flare. A cusp-shaped loop-not apparent prior to the flare-dominates the emission in the flare decay phase, and has a mass comparable to that lost in the dimming regions. Our findings are consistent with the source of the CME being a flux rope that erupted, leaving behind the dimming regions. The cusp-shaped loop probably represents magnetic fields reconfigured or reconnected by the eruption. We do not see an X-ray analog of the wavelike disturbance evident in SOHO EUV images.

Large-scale Globally Propagating Coronal Waves

“…Dimmings are most clearly seen in the EUV range (Thompson et al, , 2000a, but have also been reported in SXRs (Sterling and Hudson, 1997). In He i, brightenings that are associated with coronal dimmings can be detected (Vršnak et al, 2002b;de Toma et al, 2005).…”

Peculiar Stationary Euv Wave Fronts in the Eruption on 2011 May 11

“…It is noticed that this stationary front is located at the boundary of the core dimmings. Since the core dimmings are generally believed to be due to the evacuation of plasma associated with the erupting flux rope (Sterling & Hudson 1997;Jiang et al 2003), this stationary front might be formed at the interface between the flux rope (near the footpoint) and the envelope magnetic field (which is more like potential). In this sense, the current shell model proposed by Delannée et al (2008) may provide a sound explanation for front F 1 .…”

“…Since the coronal dimmings can be observed in different wavelengths, they are mainly due to the depletion of plasma density. Several authors quantitatively calculated the dimmings due to the plasma density depletion using Yohkoh/SXT (Sterling & Hudson 1997), SOHO/EIT (Zhukov & Auchère 2004) and STEREO data (Aschwanden 2009).…”

Space Storms and Space Weather Hazards

“…Since a CME involves the ejection of mass into interplanetary space, it is reasonable to look for a decrease in the coronal emission at and/or near the site of the eruption. For example, Sterling and Hudson (1997) noted a sudden depletion (or dimming) of the corona as seen by the broad-band soft X-ray telescope on Yohkoh after the CME on April 7, 1997. They argued that roughly 10 14 g had been ejected, that the CME involved the ejection of a magnetic flux rope and that the dimming could be viewed as being a "transient coronal hole" (Hudson and Webb, 1997; see also Rust and Hildner, 1976).…”

“…This seems rather low for a coronal magnetosonic speed, implying either a high density, or low field strength. It is interesting to note that Sterling and Hudson (1997) were unable to see a wave in their broad-band X-ray images from Yohkoh. One possible reason is that the SXT instrument measures much higher temperatures than EIT: a second was poor data coverage.…”