Abstract
We present a statistical analysis of coronal mass ejections (CMEs) imaged by the Heliospheric Imager (HI) instruments on board NASA’s twin-spacecraft STEREO mission between April 2007 and August 2017 for STEREO-A and between April 2007 and September 2014 for STEREO-B. The analysis exploits a catalogue that was generated within the FP7 HELCATS project. Here, we focus on the observational characteristics of CMEs imaged in the heliosphere by the inner (HI-1) cameras, while following papers will present analyses of CME propagation through the entire HI fields of view. More specifically, in this paper we present distributions of the basic observational parameters – namely occurrence frequency, central position angle (PA) and PA span – derived from nearly 2000 detections of CMEs in the heliosphere by HI-1 on STEREO-A or STEREO-B from the minimum between Solar Cycles 23 and 24 to the maximum of Cycle 24; STEREO-A analysis includes a further 158 CME detections from the descending phase of Cycle 24, by which time communication with STEREO-B had been lost. We compare heliospheric CME characteristics with properties of CMEs observed at coronal altitudes, and with sunspot number. As expected, heliospheric CME rates correlate with sunspot number, and are not inconsistent with coronal rates once instrumental factors/differences in cataloguing philosophy are considered. As well as being more abundant, heliospheric CMEs, like their coronal counterparts, tend to be wider during solar maximum. Our results confirm previous coronagraph analyses suggesting that CME launch sites do not simply migrate to higher latitudes with increasing solar activity. At solar minimum, CMEs tend to be launched from equatorial latitudes, while at maximum, CMEs appear to be launched over a much wider latitude range; this has implications for understanding the CME/solar source association. Our analysis provides some supporting evidence for the systematic dragging of CMEs to lower latitude as they propagate outwards.
Highlights
The results of numerous statistical studies of the coronal properties of coronal mass ejections (CMEs) have appeared in the literature since their discovery in coronagraph imagery of the early 1970s
Of less relevance to the results presented in this paper, it is worth pointing out that Yashiro et al (2004) identified a clear increase in the average speed of CMEs observed in the corona over the ascending phase of Solar Cycle 23; a statistical analysis of the kinematic properties of CMEs detected in the heliosphere will be the subject of a follow-on to this paper
We note that in WP3 of the HELCATS project, the majority of the HICAT catalogue entries for CMEs that are identified as being good or fair are augmented with estimates of the CME kinematic properties – launch time, 3D propagation direction, and radial speed – derived from application of the fixed phi fitting (FPF), harmonic mean fitting (HMF), and self-similar expansion Fitting (SSEF) techniques to their time-elongation profiles manually extracted from combined Heliospheric Imager (HI)-1/HI-2 time-elongation maps (J-maps); this endeavour is briefly discussed by Möstl et al (2017), but a more detailed description will be given in a subsequent paper (Barnes et al, 2018)
Summary
The results of numerous statistical studies of the coronal properties of coronal mass ejections (CMEs) have appeared in the literature since their discovery in coronagraph imagery of the early 1970s. Whereas Cremades and Bothmer (2004) and Wang et al (2011) discussed equatorward drag in relation to potential CME source regions, a number of studies of equatorward drag were based on coronagraph observations alone, with no reference to CME source regions; this is discussed below It is worth noting here some results of a study undertaken by Michalek and Yashiro (2013), who investigated the relationship between CMEs and active regions through the analysis of almost 700 CMEs observed near the peak of Solar Cycle 23 (from 2001 to 2004). Longerterm statistical studies of parameters such as CME propagation direction and angular width would enable a better understanding of these results, in terms of their solar cycle variation Many of these works document statistical analyses of the morphology and kinematic properties of CMEs based on visible-light coronagraph observations. These works have provided important information on the physics of CME onsets and the early propagation
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