Stereoscopic Flares and CMEs: Difference between revisions

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{{Infobox Nugget
|name = Stereoscopic Flares and CMEs
|title = Nugget Details
|number = 114
|first_author = Nariaki Nitta
|second_author =
|publish_date = 12 November 2009
|next_nugget = TBD
|previous_nugget = [[]]
}}


== Introduction ==
Recent RHESSI Science Nuggets (e.g., 12 August 2009, 31 August 2009) discussed interesting observations that may give a clue to the long-standing problem the relation between CMEs and flares. The observations given in these nuggets primarily come from the time when the Sun was still active. Wishing another solar maximum to come again in our life time, this science nugget takes up a similar subject on the basis of STEREO observations of flares during 2007-2009. In retrospect, December 2006 was the last active period with four X-class flares. We had 11 M-class and 84 C-class flares between January 2007 and October 2009. We have reconfirmed that flares, if not X-class, are not highly correlated with CMEs, and found that one of the CME-related signatures, namely large-scale coronal dimming, is a good indicator of flare-associated CME. A summary of this study is given here with all the associated movies and plots. The summary table, which is occasionally updated, includes all the M- and C-class flares between March 2007 and July 2009 as well as many isolated B- and A-class flares or even non flares.
== CME formation close to the Sun ==
We were made aware that a spectacular CME can be launched without appreciable signatures in the corona by a striking example. But if the CME is somehow associated with a flare, we can expect the first CME manifestation to be seen close to the Sun, say within 0.5 solar radius above the solar surface. The C1 telescope of SOHO LASCO delivered such observations (see "On the Temporal Relationship between Coronal Mass Ejections and Flares"), but it stopped functioning in June 1998. Now each STEREO spacecraft carries a coronagraph (COR1) whose starting height is 1.3 solar radius from the center. By combining data from COR1 and EUVI (which is an instrument like SOHO EIT), we can easily confirm the occurrence of a CME in association with the given flare. The increasing separation of the STEREO spacecraft reduces the dependence of CME visibility due to Thomson scattering. Even if one CME is largely out of the plane of the sky as viewed by one coronagraph, it can be a limb CME as viewed by another coronagraph. We used the FESTIVAL software as included in SolarSoft. Two examples are given below (click to run the Quicktime movies).
<figure>
== Decoupling of flares from CMEs -- dependence of ARs ==
By comparing these EUVI-COR1 movies with COR2 movies available with, e.g., the NRL SECCHI movie maker, it is found that only a small number of M- and C-class flares were unambiguously associated with CMEs. Examples are: 2007/06/03 09 UT, 2007/12/31 00 UT, 2008/03/25 18 UT. One can argue that experts can identify more CMEs, but are those CMEs labeled "Very Poor" (see, for example, the CDAW LASCO CME catalog) real CMEs? It would be interesting to compare this with events picked up by automated CME detection schemes such as CACTus. We note that the poor association of these M- and C-class flares with CMEs is largely affected by the fact that they occurred in a small number of active regions that happened to be CME-poor. They are AR 0960 (June 2007), AR 0963 (July 2007), AR 0978 (December 2007), and AR 1024 (July 2009). These regions had relatively large (200-600 millionth) sunspot areas, and high flux near sheared polarity inversion (as indicated by MDI data), which may indeed be used to forecast major flares ("A Characteristic Magnetic Field Pattern Associated with All Major Solar Flares and Its Use in Flare Forecasting"). One needs to be careful when saying large flares and CMEs interchangeably. They may indeed be synonymous for extremely big events, but the same is not true for more frequent events. The CME productive regions (such as AR 0956) or those that produced at least one decent CME did not have particularly large spot areas (when large CMEs occurred) or sheared polarity inversion line. Flares in these regions were less than the C-class. In fact, the examples shown above with the Quicktime movies occurred in regions with the sunspot area of 10 millionth and without a sunspot.
== Dimming and waves ==
We have analyzed EUVI data to associate the presence or absence of CME with coronal dimming and EUV waves. In order to observe these phenomena, we should use two kinds of difference images, that is running difference (one image before in the sequence is subtracted) and base difference (the first or pre-event image is subtracted). The EUV wave was first identified in running difference images, but base difference images capture long-lasting dimming better. There are technical issues of how to correct for solar differential rotation, so there is a limit to the span of base difference images. There are several publications that show both sets such as "On the nature of EIT waves, EUV dimmings and their link to CMEs". The following example (click to run the movie) shows both (left: running difference, right: base difference).
<figure>
The impression from repeated viewing of the movies is usually consistent with more quantitative analysis as to the magnitude of dimming that may be correlated with the energy of the associated CME. Even though the CME may start small and become larger-scale as it propagates ("Why are CMEs large-scale coronal events: nature or nurture?"), its associated dimming is expected to be large. Therefore we study the light curves in summed pixels or macro pixels. Here we summed 40x40 EUVI full resoluton pixels (about 1.6 arcseconds). An example is shown here. If there are more than 10 macropixels which start to dim around the flare onset and go down below 90% of the pre-event value, at least in two wavelength channels, then a subsantial CME will be seen. This is a rule of thumb. There were a number of events with clear wave-like propagations. Some of them are associated with only minor (diffuse, narrow, slow) CMEs if dimming does not meet the above criterion.
== Conclusions ==
The main purpose of this nugget is to remind the reader that flares can be poorly correlated with CMEs depending on the source regions. Large sunspot areas and strong shear (as found in longitudinal magnetograms) may not necessarily signal CMEs, although they may be linked to major flares. Coronal dimming in EUV images could partly be due to temperature effect, but if it is large and deep enough to be clearly identified, it is a good indicator of the CME and CME size. A homework for the RHESSI workers is to determine whether it is possible to distinguish flares associated with major CMEs from those not associated, on the basis of hard X-ray images.

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