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Correlation Between Halo Coronal Mass Ejections and Solar Surface Activity? A&A 397, 1057–1067 (2003) Astronomy DOI: 10.1051/0004-6361:20021463 & c ESO 2003 Astrophysics Correlation between halo coronal mass ejections and solar surface activity? Guiping Zhou1;2, Jingxiu Wang1, and Zhuoliang Cao2 1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China e-mail: [email protected]; [email protected] 2 Department of Physics, AnHui University, HeFei 230039, PR China e-mail: [email protected] Received 22 May 2002 / Accepted 17 September 2002 Abstract. We survey all the coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) aboard the Solar & Heliospheric Observatory (SOHO) in the interval from 1997 to 2001, and select 197 frontside halo CMEs whose associated near-surface activity could be clearly identified from SOHO EUV Imaging Telescope (EIT) and other space-borne and ground-based observations. A statistical analysis has been made with the emphasis on the correlation between CMEs and solar surface activity. We have found in our sample that all the CMEs were accompanied by local brightening in the CME source regions in EIT and/or Hα images. However, if we only classify the events with a brightening increase more than 50% above the quiet background as flares, as measured from EIT data, approximately 88% of the earth-directed CMEs are associated with flares and more than 94% are associated with eruptive filaments. With regard to the locations of CME source regions, there are about 79% CMEs initiating from active regions, while only 21% originate outside active regions. We evaluate the symmetry between CMEs and associated solar surface activity in spatial distributions. We find that in about half the events the surface activity is asymmetric with respect to the CME. For the flares having GOES X-ray records in this study, allowing for the uncertainties specified by the data cadence, 59% of the CME initiations seem to precede the flare onset in the CME source regions, while 41% are preceded by flare onsets. The statistical correlation seems to suggest that CMEs are intrinsically related to surface activity. Key words. Sun: coronal mass ejections (CMEs) – Sun: flares – Sun: filaments 1. Introduction made in this direction. The study of the association of CMEs with other forms of solar activity has a multi-fold importance. CMEs are the most spectacular form of solar activity. They of- Firstly, such association will help us to understand the physi- ten consist of very large structures containing plasma and mag- cal links between the very large-scale activity (e.g. CME) and netic fields that are expelled from the Sun into the heliosphere. the rather small-scale phenomena (e.g. flares and filament erup- Most of the ejected material comes from the low corona, al- tions). Secondly, we need to understand the common physics though cooler, denser material, probably of chromospheric ori- behind the varied forms of solar activity, to learn how the differ- gin, can also be ejected (Webb 2000). Compared with solar ence in spatial scale specifies the physical properties. Thirdly, flares and filaments, CMEs are relatively new to astronomers. in the narrow sense of CME studies, without knowledge of the In fact, their analogues have just started to be identified in other associated surface activity, the CME initiation will not be prop- astrophysical subjects (Bond et al. 2001). Since CMEs were erly understood. first detected in the early 1970, the solar community has tried to understand whether or not this newly discovered phenomenon Before the SOHO mission, Munro et al. (1979) carried out is intrinsically related to the other well-known solar activities, the first detailed surveys associating about 75 major Skylab such as flares, filament eruptions and specific types of radio CMEs viewed in the middle corona with near-surface activity bursts. Both case studies and statistical approaches have been from 1973–1974, in combination with reports of associated ac- tivity from the Solar Geophysical Data (SGD) archives. They Send offprint requests to: G. p. Zhou, found that 75% of the CMEs observed were associated with e-mail: [email protected] other forms of solar activity, 40% of the CMEs observed were ? Table A1 is only available in electronic form at associated with flares, 50% with eruptive prominences solely http://www.edpsciences.org (without flares), and more than 70% of CMEs with eruptive Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021463 1058 Guiping Zhou et al.: Correlation between halo CMEs and solar surface activity prominences (with and without flares). Webb & Hundhausen to search for correlations and avoid sweeping and unwarranted (1987) examined Solar Maximum Mission (SMM) data during conclusions. the activity maximum in 1980 and found that about two thirds Halo CMEs are those that originate from the disk and ap- of the CMEs observed were associated with other forms of so- pear as expanding, circular brightening that completely sur- lar activity. Among them, 68% of the observed CMEs were rounds or spans a large angle outside the coronagraph occulter, found to be associated with erupting prominences, 37% with which blocks the bright solar photospheric light. The observa- Hα flares, and 47% with X-ray events. Cyr & Webb (1991) tions suggest that these CMEs are spherically symmetric struc- performed similar studies with SMM data from 1984 to 1986. tures moving outward either toward or away from Earth that Slightly less than half of the CMEs were found to have associ- are detected after expanding to a size exceeding the diameter ations with other forms of solar activity. The results were that of the coronagraph’s occulter (Webb 2000). The first halo CME 76% were associated with erupting prominences, 26% with Hα was reported by Howard et al. (1982), using observations made flares, and 74% with X-ray events. with the Solwind coronagraph on the P78-1 spacecraft. Several After the SOHO mission, the LASCO coronagraphs and halo-CME events have been reported by LASCO (Plunkett the EUV Imaging Telescope (EIT) aboard SOHO provided the et al. 1998; Thompson et al. 1998). Halo CMEs are particu- opportunity for CME studies. SOHO allows for solar obser- larly useful for studying CME coronal sources, since most of vations uninterrupted by a day-night cycle. Yohkoh SXT has their source regions are expected to be near the center of the so- a higher time cadence and continuity of operation than previ- lar disk for those directed toward the Earth (Sterling 2000). The ous space-borne X-ray telescopes. The combined observations study of halo CMEs would help us to establish a correlation be- from LASCO with SXT and EIT for imaging the rather hotter tween disk active phenomena and CMEs observed outside the and cooler corona, respectively, give us unprecedented oppor- solar limb, to understand their initiation mechanism and, even- tunities to study CME coronal sources (Sterling 2000). Hudson tually, to forecast when the eruptions will occur (Zhang et al. et al. (1998) and Webb et al. (2000) both examined halo-type 2001b). Before the SOHO mission, halo-type CMEs had been CMEs and their associated solar activity observed during the rarely reported in coronagraph observations of the Sun (e.g. first half of 1997, and found that about half of the halo events Howard et al. 1982). However, with the increased field of view were associated with frontside activity, as expected. Gilbert and sensitivity of LASCO, such CMEs have been detected at et al. (2000) analyzed 54 Hα events from February 1996 to June a rate of 1–3 per month, or about 11% the rate of all LASCO 1998 to study the relationship between prominence activity and CMEs (Cyr et al. 2000; Webb 2000). To date, LASCO has ob- CMEs. They found that 94% of eruptive prominences (EPs) served from solar minimum well into the maximum phase of had an associated CME and 76% of the CMEs were associated the current (the 23rd) solar cycle and has detected CMEs at a with eruptive prominences in their study. Subramanian & Dere rate slightly higher than but comparable to the earlier observa- (2001) examined 32 CMEs from January 1996 to May 1998. tions (Howard et al. 1997; Cyr et al. 1997, 2000; Webb 2000). They also investigated the relationship between CMEs and Thanks to the great success of SOHO and Yohkoh, the disk prominence eruptions, and found that 44% of the CME-related observations of CME initiation become possible, particularily transients were associated with eruptions of prominences em- for the earth-directed halo CMEs. To avoid the ambiguity in bedded in active regions, and 15% with eruptions of promi- locating CME source regions, as argued by Feyman & Martin nences outside active regions. (1995), only frontside halo CMEs are selected in this statistical There are also some case studies of the association of study. A CME with span angle greater than 130◦ is referred to CMEs and surface activity. Zhang et al. (2001a) studied the as a halo CME in our approach. There were 519 halo CMEs temporal relationship between coronal mass ejections and identified from 1997 to 2001, an interval roughly covering the flares. They concluded that the impulsive acceleration phase of Solar Minimum to Maximum in this solar cycle. The statistics CMEs coincided well with the rise phase of the accompanying are made with the emphasis on the correlation between CMEs X-ray flares. While the duration of the rise phase of flares can and surface activity in terms of flares and filament eruptions. be a few minutes to tens of minutes, the corresponding CMEs In the next section, we introduce the data base and event se- always showed an apparent speed increment along with that lection.
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