STEREO observations of interplanetary coronal mass ejections and prominence deflection during solar minimum period

Abstract

Abstract. In this paper we study the occurrence rate and solar origin of interplanetary coronal mass ejections (ICMEs) using data from the two Solar TErrestrial RElation Observatory (STEREO) and the Wind spacecraft. We perform a statistical survey of ICMEs during the late declining phase of solar cycle 23. Observations by multiple, well-separated spacecraft show that even at the time of extremely weak solar activity a considerable number of ICMEs were present in the interplanetary medium. Soon after the beginning of the STEREO science mission in January 2007 the number of ICMEs declined to less than one ICME per month, but in late 2008 the ICME rate clearly increased at each spacecraft although no apparent increase in the number of coronal mass ejections (CMEs) occurred. We suggest that the near-ecliptic ICME rate can increase due to CMEs that have been guided towards the equator from their high-latitude source regions by the magnetic fields in the polar coronal holes. We consider two case studies to highlight the effects of the polar magnetic fields and CME deflection taking advantage of STEREO observations when the two spacecraft were in the quadrature configuration (i.e. separated by about 90 degrees). We study in detail the solar and interplanetary consequences of two CMEs that both originated from high-latitude source regions on 2 November 2008. The first CME was slow (radial speed 298 km/s) and associated with a huge polar crown prominence eruption. The CME was guided by polar coronal hole fields to the equator and it produced a clear flux rope ICME in the near-ecliptic solar wind. The second CME (radial speed 438 km/s) originated from an active region 11007 at latitude 35° N. This CME propagated clearly north of the first CME and no interplanetary consequences were identified. The two case studies suggest that slow and elongated CMEs have difficulties overcoming the straining effect of the overlying field and as a consequence they are guided by the polar coronal fields and cause in-situ effects close to the ecliptic plane. The 3-D propagation directions and CME widths obtained by using the forward modelling technique were consistent with the solar and in-situ observations.

Solar wind ion trends and signatures: STEREO PLASTIC observations approaching solar minimum

“…In the latter part of the survey period, CMEs originating from high latitude active regions and polar crown filament eruptions were deflected toward the ecliptic, as reported by Kilpua et al (2009b). They suggest such events may be a significant contribution to the ICMEs observed by STEREO.…”

“…The more nominal charge states we observe for the other ICMEs reported by Kilpua et al (2009b) and Jian (2009) may be related to their general weak (modest magnetic field strength) nature and/or their solar origins. Reinard (2005Reinard ( , 2008 produced a comprehensive study of eight years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of in-ecliptic ICMEs signatures using ACE data, particularly compositional signatures.…”

“…This in turn leads to a less tilted heliospheric current sheet. See Kilpua et al (2009b) In Fig. 3, the data set of solar wind iron average charge states is presented as a scatter plot against the corresponding proton bulk speed.…”

“…3 are taken from the ICME list compiled by Jian (2009) and from the ICMEs identified by Kilpua et al (2009b). The Kilpua et al events are all associated with field strength >5 nT, have an organized magnetic field structure (although not necessarily magnetic cloud), and a duration of at least 3 h. These ICMEs are grouped by the authors according to whether they are compressed in the leading edge of a high-speed stream, or surrounded by slow solar wind flows.…”

Coronal mass ejections and their sheath regions in interplanetary space

“…For instance, high latitude CMEs frequently deflect towards lower latitudes, in particular near solar minimum (e.g., Cremades et al 2006;Kilpua et al 2009), and the variations in the preferred latitudes of the source regions of CMEs, namely streamer belt, active regions and polar crown filaments, also affect the fraction of CMEs that intercept the ecliptic (e.g., Hundhausen et al 1984;Kilpua et al 2011).…”