Using extreme-ultraviolet (EUV) spectroheliograms from the first intentional postflare observations with the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we determine relative line-of-sight velocities and their temporal evolution during the gradual flare phase of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams in lines of O v, Fe xvi, and Fe xix, with formation temperatures of, respectively, 0.25, 2.0, T max and 8.0 MK show strong velocity gradients coincident with the Ha ribbons, visible in Big Bear Solar Observatory (BBSO) images. These gradients are perpendicular to and moving with the Ha ribbons. Bright downflowing plasma seems to be prevalent in the regions, between the ribbons and the magnetic neutral line, that coincide with the ends of postflare loops seen with the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma on the outer side of the ribbons is less bright in the EUV but shows strong relative blueshifts. This pattern of upflows and downflows demonstrates, for the first time in transition region and coronal lines, the existence of chromospheric evaporation during the late gradual phase of a flare and provides evidence for ongoing reconnection.

Abstract. We present a statistical analysis of 132 dayside (LT 0700-1700) bow shock crossings of the AMPTE/IRM spacecraft. We perform a superposed epoch analysis of low frequency, magnetic power spectra some minutes upstream and downstream of the bow shock. The events are devicded into categories depending on the angle θ Bn between bow shock normal and interplanetary magnetic field, and on plasma-β. In the foreshock upstream of the quasiparallel bow shock, the power of the magnetic fluctuations is roughly 1 order of magnitude larger (δB ∼ 4 nT for frequencies 0.01-0.04 Hz) than upstream of the quasi-perpendicular shock. There is no significant difference in the magnetic power spectra upstream and downstream of the quasi-parallel bow shock; only at the shock itself, is the magnetic power enhanced by a factor of 4. This enhancement may be due to either an amplification of convecting upstream waves or to wave generation at the shock interface. On the contrary, downstream of the quasi-perpendicular shock, the magnetic wave activity is considerably higher than upstream. Downstream of the quasi-perpendicular low-β bow shock, we find a dominance of the left-hand polarized component at frequencies just below the ion-cyclotron frequency, with amplitudes of about 3 nT. These waves are identified as ioncyclotron waves, which grow in a low-β regime due to the proton temperature anisotropy. We find a strong correlation of this anisotropy with the intensity of the left-hand polarized component. Downstream of some nearly perpendicular (θ Bn ≈ 90 • ) high-β crossings, mirror waves are identified. However, there are also cases where the conditions for mirror modes are met downstream of the nearly perpendicular shock, but no mirror waves are observed.

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Abstract. We analyze data of the AMPTE/IRM spacecraft downstream of a supercritical, quasi-perpendicular bow shock with an upstream/• (ratio of thermal to magnetic pressure) greater than unity. The observed ion temperature anisotropy (Tpñ > Tpl]) satisfies the mirror instability criterion, and the mirror growth rate is positive. We investigate the low-frequency magnetic fluctuations and find that several minutes downstream of the shock ramp the compressive mode dominates in two frequency intervals below the local proton gyrofrequency. Band-pass filtering of the two frequency ranges shows that the particle pressure and magnetic pressure vary in antiphase, as is typical for the mirror mode. In addition, we estimate the proton compressibility, that is, the ratio of the relative fluctuations of proton density and magnetic field, and the Alfv6n ratio, that is, the ratio of fluctuations of the bulk velocity and the Alfv6n velocity. Combining these results, we find strong indications for the existence of mirror waves in the region downstream of the bow shock. The observed spectrum of the mirror waves has a two-banded structure.

Upflows of several tens of km/s have been observed by SOHO-CDS in the late gradual phase of the M6.8 two-ribbon flare on April 29, 1998. These upflows observed in EUV lines formed at coronal temperatures are interpreted as chromospheric evaporation which fills the postflare loops with hot plasma. In order to achieve chromospheric evaporation, the chromospheric plasma has to be heated to coronal temperatures. The energy for this heating process is assumed to be provided by magnetic reconnection. The mechanism which transports the energy from the reconnection site to the chromosphere must be either thermal or non-thermal. ·V.le compare the observed upflow velocities with the velocities derived by different chromospheric heating models in order to decide which mechanism might account for the chromospheric heating. From non-thermal models we take the electron energy flux necessary to achieve the observed velocities and calculate the expected hard X-ray counts in Yohkoh/HXT for non-thermal thick-target Bremsstrahlung generated by this electron flux. We conclude that energetic (> 15keV) non-thermal electrons are unlikely to cause the chromospheric heating since a significant number of HXT counts are expected from the resulting electron energy flux but not observed. Recent thermal conduction models seem to be more appropriate for explaining the observations.