A peculiar young active region was observed in 1998 March with the Ultraviolet Coronagraph Spectrometer (UVCS) over the southwest limb. The spectra showed strong emission in the 974 line of fluorine-like iron, [Fe xviii], which is brightest at an electron temperature of 10 6.8 K, and lines of Ne ix, [Ca xiv], [Ca xv], Fe xvii, [Ni xiv], and [Ni xv]. It is the only active region so far observed to show such high temperatures 0.5 R above the solar limb. We derive the emission measure and estimate elemental abundances. The active region produced a number of coronal mass ejections (CMEs). After one CME on March 23, a bright post-CME arcade was seen in EIT and Yohkoh/SXT images. Between the arcade and the CME core, UVCS detected a very narrow, very hot feature, most prominently in the [Fe xviii] line. This feature seems to be the reconnection current sheet predicted by flux rope models of CMEs. Its thickness, luminosity, and duration seem to be consistent with the expectations of the flux rope models for CME. The elemental abundances in the bright feature are enhanced by a factor of 2 compared to those in the surrounding active region, i.e., a first ionization potential enhancement of 7-8 compared to the usual factor of 3-4.

Solar eruptions occur when magnetic energy is suddenly converted into heat and kinetic energy by magnetic reconnection in a current sheet (CS). It is often assumed that CSs are too thin to be observable because the electric resistivity in CSs is taken to be very small. In this work, we show the implications for the CS thickness h e d estimated from observations of three eruptions by the UVCS and the LASCO experiments on SOHO. We infer the effective causing the rapid reconnection, which predicts much faster reconnection in a thick CS than that h e caused by the classical and anomalous resistivities. We find that in these events CSs are observable and have extremely large values of d and , implying that large-scale turbulence is operating within CSs. We also discuss h e the properties of the so-called hyperresistivity caused by the tearing mode and the relation to our results.

No abstract

No abstract

We present spectra of a three-part coronal mass ejection (CME) observed by the Ultraviolet Coronagraph Spectrometer aboard SOHO on 2000 February 11. Images of the CME in different spectral lines show how the morphology depends on the temperature, density, and outflow speed of the ejected plasma. The H i Ly is the line that best resembles the white-light data, although it can be rather different where the outflow speed severely dims its radiative component. We estimate the ranges of temperature and density in the front, prominence core, and void. We also estimate the outflow speed that is the true speed of the ejecta as obtained from the Doppler dimming technique, its component projected on the plane of the sky, and the line-of-sight speed for the three components of the CME. The plasma in the front was denser, cooler, and more depleted in O and Si than the ambient coronal streamer. These characteristics indicate that it originated in the closed field core of the pre-CME streamer. The leading edge was not the projection of a simple spherical shell onto the plane of the sky. The line profiles suggest a wide looplike structure, although a more complete shell that was brighter in some areas could also match the data. The prominence has a structure in temperature and density with the hotter top end emitting in the Mg x and Si xii lines while the bottom end was much cooler and visible only in the H i Lyman lines. Emission in the void was rather faint. The outflow speed obtained from Doppler dimming of the radiative lines, the line-of-sight speed measured from the Doppler shift of the lines, and the plane-of-the-sky speed estimated from the comparison of the images taken at 2.3 and 2.6 R give speeds much lower than those estimated at greater heights (>4 R) from LASCO and indicate a stronger acceleration at lower heights.