Plasma blobs are observed to be weak density enhancements as radially-stretched structures emerging from the cusps of quiescent coronal streamers. In this paper, it is suggested that the formation of blobs is a consequence of an intrinsic instability of coronal streamers occurring at a very localized region around the cusp. The evolutionary process of the instability, as revealed in our calculations, can be described as follows, (1) through the localized cusp region where the field is too weak to sustain the confinement, plasmas expand and stretch the closed field lines radially outwards as a result of the freezing-in effect of plasma-magnetic field coupling; the expansion brings a strong velocity gradient into the slow wind regime providing the free energy necessary for the onset of the subsequent magnetohydrodynamic instability; (2) the instability manifests itself mainly as mixed streaming sausage-kink modes, the former results in pinches of elongated magnetic loops to provoke reconnections at one or multi locations to form blobs. Then, the streamer system returns to the configuration with a lower cusp point, subject to another cycle of the streamer instability. Although the instability is intrinsic, it does not lead to the loss of the closed magnetic flux, neither does it affect the overall feature of a streamer. The main properties of the modelled blobs, including their size, velocity profiles, density contrasts, and even their daily occurrence rate are in line with available observations.

[1] A 2.5-dimensional MHD analysis of Alfvén-wave-driven solar wind in the heliospheric meridional plane is presented in order to extend the one-dimensional model proposed by Weber and Davis [1967] to the two-dimensional case. In our model, no energy input is applied to the helmet streamer, while the solar wind along open field lines is assumed to be driven by Alfvén waves. The differential rotation of the Sun is considered. The numerical results obtained essentially match relevant Ulysses observations and give the spatial distribution of the azimuthal components of the flow velocity and magnetic field. The effect of the existence of these azimuthal components on the global properties of the solar wind is found to be negligible, and the loss rate of the angular momentum of the Sun due to the solar wind is estimated.

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Abstract. Recent observations of astrophysical magnetic fields have shown the presence of fluctuations being wavelike (propagating in the plasma frame) and those described as being structure-like (advected by the plasma bulk velocity). Typically with single-spacecraft missions it is impossible to differentiate between these two fluctuations, due to the inherent spatio-temporal ambiguity associated with a single point measurement. However missions such as Cluster which contain multiple spacecraft have allowed for temporal and spatial changes to be resolved, using techniques such as k filtering. While this technique does not assume Taylor's hypothesis it requires both weak stationarity of the time series and that the fluctuations can be described by a superposition of plane waves with random phases. In this paper we test whether the method can cope with a synthetic signal which is composed of a combination of non-random-phase coherent structures with a mean radius d and a mean separation λ, as well as plane waves with random phase.