Ultraviolet Spectroscopy of Polar Coronal Jets

Abstract

We have observed a total of five UVCS/SOHO polar jets that correlate with the Extreme-Ultraviolet Imaging Telescope (EIT) and Large Angle Spectrometric Coronagraph (LASCO) jet events. We analyzed spectroscopic observations of these jets and found that they typically undergo two phases: at the first phase the O VI lines show a brief intensity enhancement (by a factor of 1.4) and narrowing (by a factor of 0.8), while the H I Lyα line is not enhanced, and the second phase, about 25 minutes later, when the H I Lyα line shows an intensity enhancement (by a factor of 1.3) and narrowing (by a factor of 0.8), while the O VI line is relatively unchanged. We modeled the observable properties of the jets from 1997 August 5, detected at 1.71 R☉. We interpret the first phase as the fast, dense centroid of the jet passing by the UVCS slit. The empirical jet model was able to reproduce the observed line properties with electron density enhancement by a factor of 3.2 (with a resulting density of 4.5 × 106 cm-3), an electron temperature decrease (change by a factor of 0.50 to 750,000 K), and the centroid outflow velocity larger than 280 km s-1. During the second phase, the model required a further decrease in the electron temperature (change by a factor of 0.10, with a jet temperature of only 150,000 K), along with a weaker electron density (1.7 × 106 cm-3) and an outflow velocity of 205 km s-1. Possible scenarios of the electron temperature variations needed to account for observed conditions on 1997 August 5 indicate that some heating is required. We computed models of the temperature and nonequilibrium ionization state of an expanding plasma using various forms for the heating rates. We found that the jet had to leave the Sun at an electron temperature below 2.5 × 106 K and that a heating rate of the same order as the average coronal hole heating is required. Such low initial temperatures are consistent with the idea that the jets observed by LASCO, EIT, and UVCS are different than previously observed coronal X-ray jets.