ISSI International Research Team project on “Slow solar wind sources and acceleration mechanisms in the corona” Team Leaders: Lucia Abbo (INAF/OATo, Italy) and Leon Ofman (CUA/NASA, USA) Date: 28 March 2013 Research Domain: Solar and Heliospheric Physics Abstract: The main goal of this ISSI Team project is to merge observational and numerical modeling knowledge and expertise in order to investigate the sources of the Slow Solar Wind, the physical mechanisms at the base of its acceleration and the key role of the topology of the coronal magnetic field . Results from the ESA-NASA SOlar and Heliospheric Observatory (SOHO), -combined with theoretical modeling, have helped to investigate many aspects of the solar wind. Fundamental physical properties of the coronal plasma have been derived from SOHO/UVCS data and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the slow solar wind (SSW). One of these was the discovery that quiescent streamers have a marked oxygen ion depletion in their cores with respect to their bright lateral branches. Furthermore, the elemental abundances (e.g. for Fe and O) vary among different coronal structures with respect to the photospheric values (this is the so called First Ionization Potential effect). Recently, it has been discovered that a core dimming is also apparent in the Mg X 62.5 nm line, even if less evident than for O VI 103.2 nm. Moreover, it has been found that a significant amount of slow solar wind originates outside the streamer/coronal hole boundaries, where the wind velocity depends on the magnetic field topology of flux tubes. Hence, it is fundamental to establish whether the physical parameters of the solar wind, such as outflow speed and energy deposition in the extended corona, are indeed modulated by the degree of expansion of the magnetic flux tubes channelling the wind through the corona. SOHO/LASCO white light coronagraphic observations provide critical evidence linking the solar-minimum streamer belt with the SSW: several small CMEs in the form of slow “streamer blowouts” were observed and they were considered as tracers of SSW. More recently, STEREO COR1/2 and HI data provided the velocity of small-scale propagating transients without projection effects, demonstrating the capability of STEREO instruments to monitor solar ejecta out to and beyond 1 AU. Recent results from the Hinode and SDO missions show persistent outflows at the edges of active regions whose contribution to the SSW is not completely understood. It remains to be determined what fraction of the SSW comes from the coronal hole/streamer interface and what part is produced by transient eruptions in coronal streamers. The Slow Solar Wind origin is now one of the outstanding questions arising in the post SOHO era and forms a major objective for planned future missions such as the Solar Orbiter and Solar Probe Plus. From a theoretical point of view, the ejection of material may be caused by loss of confinement due to pressure- driven instabilities as the heated plasma accumulates or due to current-driven instabilities (tearing and or kink-type instabilities) in the sheared field of the streamer. On small scales, 2.5-D magneto-hydrodynamic (MHD) models can account for the plasmoid blowouts observed in helmet streamers: these are due to magnetic reconnection at the current sheet above the cusp and are accelerated by a Kelvin-Helmholtz instability triggered by the reconnection itself. Multi-fluid 2.5-D models of the SSW have been developed to simulate streamers that contain heavy ions (in addition to protons and electrons), since traditional single fluid MHD models do not account for different properties and dynamics of various ions. Nevertheless, only very few multi-fluid studies of the SSW have attempted to reproduce the streamer observations by considering heavy ions. Due to their intrinsic complexity, so far kinetic models of the solar wind have been largely devoted to exploring the kinetic physics involved rather than offering direct detailed comparisons with observations. Hence we will mainly adopt a multi-fluid approach as the modeling component of our project. Today, there is an urgent need to integrate different expertise in the solar scientific community. On one hand, many coronal and solar wind models have ad-hoc and idealized boundary conditions at the Sun that coronal observations can otherwise help to constrain. On the other hand, it is often challenging to analyze coronal observations due to projection effects in the optically thin corona. In the latter case, numerical modelling can help to guide the determination of the 3D coronal structure. Unique and innovative results derived from this approach based on a synergy between observations and numerical modeling will provide an estimate of contributions to the SSW from different sources such as streamer boundary and streamer cusp and will quantify their role in the overall mass and energy budget of the SSW. 1 Scientific rationale of the project The solar wind is a continuous stream of charged particles (mainly electrons and protons, but also heavy ions) -14 ejected from the Sun at an average mass loss rate of (2 – 3)×10 Msun /year. Results from Ulysses and from the SOlar and Heliospheric Observatory (SOHO), two ESA-NASA space missions, combined with theoretical modeling, have helped in the understanding of many aspects of the solar wind. However, the heating and acceleration mechanism of the solar wind remains poorly understood. Ulysses with in situ measurements,, has clarified its 3-dimensional structure, demonstrating that there exists two principal components: a “slow” ( vslow ~ 300 – 400 km/s at 1AU) and a “fast” ( vfast ~700 – 800 km/s at 1AU) solar wind component, ejected during the minimum phase of the solar activity cycle from the low and high latitude regions, respectively (McComas et al., 2000). The processes that lead to the different fast and slow solar wind acceleration and properties have not yet been completely identified. It has been suggested that the geometry of magnetic field lines in the solar atmosphere determined at several heliocentric heights, could play a role in the fast wind acceleration (e.g. Munro & Jackson, 1977). There are also distinct pointers to other acceleration mechanisms, such as ion cyclotron resonance (Isenberg, 2001) or Alfvén wave acceleration mechanisms (see, Ofman 2010 for a review). During solar minimum, the low latitude Slow Solar Wind (SSW) at 1 AU can be traced back to source regions near equatorial coronal streamers. At least four possible source regions of SSW have been proposed concerning coronal streamers (Fig.1): (1) mixing of plasma inside the streamer/coronal hole boundaries, (2) plasma leakage from just one side in the core, just below the cusp, (3) quasi-steady flow from the legs of streamers, and (4) parcels of plasma escaping from the core, carrying loops of magnetic flux (e.g. see Suess et al. 2009). Crucial results on the origin of the solar wind have been derived not only from in- situ measurements, but also from remote sensing techniques . During the last 17 years two coronagraphs, the Large Angle and Spectrometric Coronagraph ( LASCO ; Brueckner et al., 1995) and the Ultraviolet Coronagraph Spectrometer ( UVCS ; Kohl et al., 1995) onboard SOHO, have acquired data of the extended corona. LASCO white light observations provided evidence Fig.1: Four possible sources of slow wind (Suess et al. 2009). linking the solar-minimum streamer belt with the SSW. For example, Sheeley et al. (1997) used LASCO C2 to observe several small eruptions in the form of slow “streamer blowouts” which are widely believed to be tracers of SSW. Mierla et al. (2007) have obtained SSW outflow speeds of 10-20 km/s at 1.3 R sun using LASCO C1 data. UVCS spectroscopic observations have significantly contributed to our understanding of streamers since the beginning of the SOHO mission. The main results derived from UVCS observations of coronal streamers and the SSW wind can be summarized as follows (see also Antonucci 2006 and Kohl et al. 2006 for reviews of UVCS results): • Streamer core oxygen depletion: Previous studies of streamers at solar minimum based on UVCS data have revealed an abundance anomaly (e.g. Noci et al. 1997, Marocchi et al. 2001, Uzzo et al. 2003) that might be related to the origin of the low-speed streams. Quiescent streamers show depletion in O VI emission in the core with respect to their bright lateral branches (Fig. 2). Several different explanations for this feature have been proposed (e.g. Noci et al. 1997; Raymond et al. 1997; Ofman 2000; Frazin et al. 2003; Akinari, 2007). • Streamer elemental composition: Elemental abundances vary among the different coronal structures with respect to photospheric values. The determining parameter appears to be the First Ionization Potential (FIP) of different elements (i.e., the FIP-effect). This effect is much more pronounced in the SSW than in the fast solar wind (von Steiger et al. 2000) and therefore it can be very useful as a tracer of their sources. A quantitative analysis that includes the reduction of the drag force (due to differential proton-ion speeds) on the different ion species (Geiss 1982; Geiss et al. 1995) would be necessary to assess the relationship between wind speed and the relative enhancement in the low FIP elements. Ko et al. (2008) provided maps of coronal plasma physical parameters of streamers derived from observations that were compared with large-scale coronal properties of a 3D MHD model. • SSW and flux tube expansion factors: Strachan et al. (2012) used synoptic maps of electron density and coronal outflow velocity at ~2.5 Rsun to estimate flux tube expansion factors, fexp , as a function of latitude at solar minimum.