The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun

Abstract

It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two Electronic supplementary material The online version of this article (http://dx.doi.org/10.1007/s11207-008-9255-z) contains supplementary material, which is available to authorized users. . We study the morphology of the dimmings and how they recover. We find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings. We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J. 560, 425, 2001) of interchange reconnections between "open" magnetic field and small coronal loops is a strong candidate for the mechanism facilitating the recovery of the dimmings. This process disperses the concentration of "open" magnetic field (forming the dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic connectivity to the Sun.

Sequential Coronal Mass Ejections from AR8038 in May 1997

“…However, since we have only one view of a 3D density depletion structure, both the EUV dimming structure and the connection between EUV dimming and CME fluxrope are not clearly understood. Attempts to compare the magnetic flux in the EUV dimming regions with the corresponding in-situ ICME magnetic fluxropes have shown some, albeit moderate, success (Webb et al, 2000;Qiu et al, 2007;Attrill et al, 2008). Using a sequence of wavelet-enhanced EIT images and EIT difference images, we found that the CMEs on 5 and 16 May have also EIT double dimmings similar to the 12 May case (see attached animations eit19970505difmv.gif, eit19970512mv.gif and eit19970516difmv.gif).…”

“…Symmetric double dimmings are rarely observed but attracted a lot of attention and research effort because they well represent the common interpretation, i.e., that the two dimming regions are the footprints of an expanding fluxrope still connected to the Sun and experiencing significant density depletion Webb et al, 2000;Yurchyshyn et al, 2006;Liu et al, 2007;Attrill et al, 2008). However, since we have only one view of a 3D density depletion structure, both the EUV dimming structure and the connection between EUV dimming and CME fluxrope are not clearly understood.…”

CORONAL MASS EJECTION INDUCED OUTFLOWS OBSERVED WITHHINODE/EIS

“…The gradual recovery of the EUV brightness is a typical feature of coronal dimmings (Reinard & Biesecker 2008;Attrill et al 2008), which is correlated with the overlying corona closure and heating by the confined plasma in the closed magnetic loops (McIntosh et al 2007). Attrill et al (2008) proposed a model to interpret the maintained outflows during the recovery phase in which interchange reconnections are continually occurring between emerging flux in the dimming region and the opening field lines caused by CME.…”

Coronal Mass Ejections: Models and Their Observational Basis

“…On the other side, as the gas pressure in the cavity decreases during the CME eruption, the pressure gradient is expected to drive outflow from the transition region and even chromosphere Jin et al, 2009). This process, in addition to the interchange reconnection between the stretched magnetic field and small coronal loops (Attrill et al, 2008), refills the depleted corona, making the coronal dimming recover to normal in tens of hours.…”