The highly structured magnetic field and plasma properties observed in the heliospheric extension of the coronal streamer belt have been interpreted as evidence for multiple current sheets originating at coronal helmet streamers. We explore the linear stability of a simple case: a triple current sheet, as would exist above two neighboring helmets of the same polarity. The behavior of the triple current sheet when perturbed by small disturbances can be described (in the incompressible limit) by MHD Orr-Sommerfeld and Squire equations, which we solve with a Chebyshev-, method. We show the velocity and magnetic fields which characterize the three unstable modes and describe the modal dependence on fieldwise wavenumber and current sheet separation. At long wavelengths an unexpected phenomenon occurs: two modes degenerate into unstable traveling modes. We also explore the three-dimensional behavior and the modal variation with both large and small values of the resistivity and viscosity. We conclude that the magnetic topology in closely packed streamers is susceptible to instabilities with growth times of the order of hours. Our predictions indicate that the resultant plasmoid structures should be observable with the large angle and spectrometric coronagraph (LASCO) and ultraviolet coronagraph spectrometer (UVCS) instruments on the upcoming Solar and Heliospheric Observatory (SOHO) mission.
IntroductionHelmet streamers have been observed essentially as long as solar eclipses, yet their formation and evolution remain poorly understood. Once the basic magnetic flux element has emerged through the solar photosphere, it is unclear how the characteristic cusp and current sheet structure of the helmet streamer is formed: either partial opening of the closed, stressed magnetic field or reconnection of previously open field is thought to occur with the aid of the solar wind [Kopp, 1992, and references therein]. At solar minimum the streamer belt around the solar equator appears closest to a laminar configuration, which can be idealized as a wavy, azimuthal torus; farther from the Sun the streamers form a thick plasma sheet, in which a much narrower, complex current structure is em- bedded [e.g., Borrini et al., 1981; Gosling et al., 1981; Winterhalter et al., 1994]. On closer inspection the streamer belt consists of discrete low-latitude structures distributed irregularly in longitude, with both two-and three-dimensional (3D) structure. The 3D aspects become prominent near solar maximum, when the Sun is most likely to have numerous complex, neighboring flux systems over a range of latitudes.Once thought to be entirely stable, long-lived phenomena, helmet streamers now are known to be capable of rapid changes. The best studied and most disruptive manifestations of such transient activity are coronal mass ejections. However, the complexity of the current configuration within the heliospheric plasma sheet suggests another mechanism for short-This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysic...