Origins of the Slow and the Ubiquitous Fast Solar Wind

Abstract

We present in this Letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5R s , obtained during the Galileo solar conjunction in 1997 January, to establish the origin of the slow solar wind. Limits on the flow speed are derived from the Doppler dimming of the resonantly scattered component of the oxygen 1032 and 1037.6 Å lines as measured with the ultraviolet coronagraph spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). White light images of the corona from the large-angle spectroscopic coronagraph (LASCO) on SOHO taken simultaneously are used to place the Doppler radio scintillation and ultraviolet measurements in the context of coronal structures. These combined observations provide the first direct confirmation of the view recently proposed by Woo & Martin that the slow solar wind is associated with the axes, also known as stalks, of streamers. Furthermore, the ultraviolet observations also show how the fast solar wind is ubiquitous in the inner corona and that a velocity shear between the fast and slow solar wind develops along the streamer stalks.

Physical parameters of a mid-latitude streamer during the declining phase of the solar cycle

“…Many recent works have focused on this topic (e.g., Marsch 1997;Noci et al 1997;Raymond et al 1997;Habbal et al 1997;Woo & Martin 1997), but -at present -a firm identification of the slow wind-source regions inside streamers is still an unresolved issue.…”

A review of recent studies on coronal dynamics: Streamers, coronal mass ejections, and their interactions

“…They are the product of the dynamic equilibrium between confining magnetic fields and expanding hot plasmas [4,5]. Previous studies indicate that streamers and nearby regions are important sources of the slow solar wind [8][9][10]. The streamer magnetic topologies, including the cusp structure, the co-existence of open and closed fields at the streamer boundary, and the expansion of relevant flux tubes, have deep impacts on the acceleration of the slow wind [13,38].…”

“…Therefore, blobs are regarded as velocity tracers of the backround solar wind. It is believed that the solar wind from the streamer region is a part of the slow solar wind [8][9][10]. Therefore, the speeds of blobs are representative of those of this type of the solar wind.…”

“…As mentioned, it is generally believed that the type II frequency is mainly determined by the electron density at the shock, the spectral shape is therefore expected to be affected by coronal structures along the shock path. Streamers are the most prominent coronal structure, several times denser than their surroundings [9,13,15,48,149], it is therefore expected that the transit of a type-II shock across a streamer should result in an apparent change of the spectra. Once this causal relationship is established, it is possible to pinpoint the type II source site.…”

“…Below the cusp, the streamer is bright due to the presence of denser-than-surrounding plasmas confined by its closed fields; while above the cusp the streamer stalk is bright because of the presence of the dense solar wind plasmas there. The streamer region is believed to be an important source of the slow solar wind [6][7][8][9][10]. It has been shown that the streamer magnetic configurations are important to the dynamics of the associated solar wind [11][12][13][14][15].…”