Intermittent outflows at the edge of an active region – a possible source of the solar wind?

Abstract

Context. It has already been established that the solar wind may originate at the edges of active regions (ARs), but the key questions of how frequently these outflows occur, and at which height the nascent solar wind originates have not yet been addressed. Aims. We study the occurrence rate of these intermittent outflows, the related plasma activities beneath in the low solar atmosphere, and the interplanetary counterparts of the nascent solar wind outflow. Methods. We use the observations from XRT/Hinode and TRACE to study the outflow patterns. The occurrence frequency of the intermittent outflow is estimated by counting the occurrences of propagating intensity enhancements in height-time diagrams. We adopt observations of SOT/Hinode and EIS/Hinode to investigate the phenomena in the chromosphere associated with the coronal outflows. The ACE plasma and field in-situ measurements near Earth are used to study the interplanetary manifestations. Results. We find that in one elongated coronal emission structure, referred to as strand, the plasma flows outward intermittently, about every 20 min. The flow speed sometimes exceeds 200 km s −1 , which is indicative of rapid acceleration, and thus exceeds the coronal sound speed at low altitudes. The inferred flow speed of the soft-X-ray-emitting plasma component seems a little higher than that of the Fe ix/x-emitting plasma component. Chromospheric jets are found to occur at the root of the strand. Upflows in the chromosphere are also confirmed by blue-shifts of the He ii line. The heliospheric plasma counterpart close to the Earth is found to be an intermediate-speed solar wind stream. The AR edge may also deliver some plasmas to a fraction of the fast solar wind stream, most of which emanate from the neighboring CH. Conclusions. The possible origin of the nascent solar wind in the chromosphere, the observed excessive outflow speed of over 200 km s −1 in the lower corona, and the corresponding intermediate-speed solar wind stream in interplanetary space are all linked in our case study. These phenomena from the low solar atmosphere to the heliosphere near Earth in combination shed new light on the solar wind formation process. These observational results will constrain future modeling of the solar winds originating close to an AR.

Asymmetries of solar coronal extreme ultraviolet emission lines

“…It should be noted that the findings presented here are quite different from the studies of Doschek et al (2008) or He et al (2010) who also find evidence of a wind outflow in the periphery of an active region. However, they only investigate single Gaussian fits.…”

“…Thus one can speculate that the Doschek et al (2008) and the He et al (2010) data actually show the same effect of a high-velocity component, which will have to be checked by re-examining their data.…”

Active region upflows

“…These chromospheric small-scale ejections could be masked by the low spatial resolution of EIS and spatially appear as unresolved large-scale flows. As a result of the frequency of occurrence of these kinds of jets and their ubiquitousness, the chromospheric origin of the AR upflows has been proposed (McIntosh & De Pontieu 2009;He et al 2010;Nishizuka & Hara 2011). The IBIS data provide a unique opportunity for exploring this idea.…”

“…McIntosh & De Pontieu (2009) suggested that blueward asymmetries observed in the Si vii (log T max = 5.8 K), Si x (log T max = 6.1 K) and Fe xiv (log T max = 6.3 K) lines near ARs may be a result of spicular activity in the chromosphere. He et al (2010) used Hinode Solar Optical Telescope (SOT) data in Ca ii H to study the plage area underlying coronal outflows. The authors reported three jets that occurred in a small area over a period of one hour and "inferred that the intermittent coronal outflow might be caused by the chromospheric jets".…”

Active region upflows

“…Although several indirect observations (McIntosh & De Pontieu 2009;He et al 2010;Nishizuka & Hara 2011) have indicated a correlation between jets in the chromosphere and coronal upflows, in Paper I it has been shown through chromospheric spectroscopic imaging that the upflows persists despite the absence of chromospheric jets, indicating that the driving should be located higher in the atmosphere.…”