Imaging of reconnection processes in hard X-rays

The Hard X-ray Spectrometer aboard the SMM detected several events of energy release late in the development of two-ribbon flares. One such event, at 21:12 UT on 21 May, 1980 (～ 20 min after the flare onset and 15 min after the peak of the impulsive phase) is studied in detail. The site of new brightening first became visible in hard X-rays (> 22 keV) and only afterwards showed up at lower energies. It was clearly located high in the corona so that one can identify it with energy release at the tops of newly formed post-flare loops. Thus, if the Kopp and Pneuman model of the loop formation is adopted, we may have imaged here a reconnection process in the solar corona. An attempt is made to estimate physical parameters at the reconnection site.     

Soho Observations of Density Fluctuations in Coronal Holes

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9 to 2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborate our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10–12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Coronal Heating: a Comparison of Ion-cyclotron and Gravity Damping Models

SOHO/UVCS data indicate that minor ions in the corona are heated more than hydrogen, and that coronal heating results in T_⊥ larger than T_‖. Analogous behavior has been known from in situ measurements in solar wind for many years. Here we compare and contrast two mechanisms which have been proposed to account for the above behavior: ion-cyclotron resonance and gravity damping. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Fall 2000 and Fall 2001 SOHO-Ulysses Quadratures

SOHO-Ulysses quadrature occurs when their included angle with the Sun is 90°. At these times the same plasma leaving the Sun in the direction of Ulysses can first be remotely analyzed with SOHO and then later be sampled in situ at Ulysses. Quadratures in Fall 2000/2001 are of special interest because Ulysses will be near the south and north heliographic poles, respectively, and it will be near sunspot maximum. But, the quadrature geometry is complex - Ulysses is not in a true polar orbit and the orbital speed of Ulysses and SOHO about the Sun will be comparable. In neither case is true quadrature achieved, but this works to the observer's advantage. Here we show plots of the relative positions of SOHO and Ulysses throughout the two quadrature intervals. This revised version was published online in August 2006 with corrections to the Cover Date.     

First results from UVCS/SOHO

We present here the first results obtained by the Ultraviolet Coronagraph Spectrometer (UVCS) operating on board the SOHO satellite. The UVCS started to observe the extended corona at the end of January 1996; it routinely obtains coronal spectra in the 1145 Å – 1287 Å, 984 Å – 1080 Å ranges, and intensity data in the visible continuum. Through the composition of slit images it also produces monocromatic images of the extended corona. The performance of the instrument is excellent and the data obtained up to now are of great interest. We briefly describe preliminary results concerning polar coronal holes, streamers and a coronal mass ejection, in particular: the very large r.m.s. velocities of ions in polar holes (hundreds km/sec for OVI and MgX); the puzzling difference between the HI Ly- image and that in the OVI resonance doublet, for most streamers; the different signatures of the core and external layers of the streamers in the width of the ion lines and in the OVI doublet ratio, indicating larger line-of-sight (l.o.s.) and outflow velocities in the latter.     

Bernard Fleck and Zdenek Švestka (eds.), The First Results from SOHO

Space Science Reviews -     

On the Lyman α and O VI Line Profiles in Streamers and Coronal Holes

The profiles of the Lyα line at 1215.6 Å and of the O VI doublet at 1031.9 Å and 1037.6 Å in the extended solar corona have been analyzed vs. latitude and radial direction, performing observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the ESA-NASA solar satellite SOHO (Solar and Heliospheric Observatory). The results show that these lines behave differently with latitude: the H I Ly α line has larger full width at half maximum (FWHM) values in the streamer region and narrower values towards the pole, while the FWHM of O VI lines has a minimum at the center of the streamer and slightly increases towards the polar regions. We briefly discuss the impact of the results on coronal heating theories.     

Ulysses-UVCS Coordinated Observations

We present results from SOHO/UVCS measurements of the density and flow speed of plasma at the Sun and again of the same plasma by Ulysses/SWOOPS in the solar wind. UVCS made measurements at 3.5 and 4.5 solar radii and Ulysses was at 5.1 AU. Data were taken for nearly 2 weeks in May–June 1997 at 9–10 degrees north of the equator in the streamer belt on the east limb. Density and flow speed were compared to see if near Sun characteristics are preserved in the interplanetary medium. By chance, Ulysses was at the very northern edge of the streamer belt. Nevertheless, no evidence was found of fast wind or mixing of slow wind with fast wind coming from the northern polar coronal hole. The morphology of the streamer belt was similar at the beginning and end of the observing period, but was markedly different during the middle of the period. A corresponding change in density (but not flow speed) was noted at Ulysses. This revised version was published online in June 2006 with corrections to the Cover Date.