Four years of SOHO Discoveries-Some Highlights - Nasa

ulletin 102 — may 2000 bullFigure 18. Massive ‘halo’CME as recorded byLASCO C2 on 17 February2000. Displayed here is aso-called ‘runningdifference’ image,showing the variation inbrightness from oneframe to the next(courtesy of SOHO/LASCOConsortium)Figure 19. Sequence ofEIT difference imagesshowing the intensity(density) enhancementand following rarefactionassociated with a shockwave expanding acrossthe solar disc from thesite of the origin of aCME, recorded on 12 May1997. A halo CME wasobserved by LASCO.These images wereformed from thedifferences of successiveimages in the emissionlines of Fe XII near 195 Å;this ion is formed attemperatures of about 1.5million degrees. The wavefront travels at speeds of~ 300 km/s, typical of afast mode Alfvén shock inthe lower solar corona(from Thompson et al.)been missed during LASCO/EIT datagaps, bringing the possible associationrate to 18 out of 21 (86%).EIT has discovered large-scale transientwaves in the corona, also called‘Coronal Moreton Waves’ or ‘EITwaves’, propagating outward fromactive regions below CMEs. Theseevents are usually recorded in theFe XII 195 Å bandpass, during highcadence(< 20 min) observations. Theirappearance is stunning in that theyusually affect most of the visible solardisc (Fig. 19). They generally propagateat speeds of 200–500 km/s, traversinga solar diameter in less than an hour.Active regions distort the waves locally,bending them towards the lower Alfvénspeed regions. On the basis of speedand propagation characteristics, the EIT waveswere associated with fast-mode shock waves.Another interesting aspect of these coronalMoreton waves is their association with theacceleration and injection of high-energyelectrons and protons, as measured, forexample, by COSTEP and ERNE.Solar windOrigin and speed profile of the fast windCoronal-hole outflow velocity maps obtainedwith the SUMER instrument in the Ne VIIIemission line at 770 Å show a clear relationshipbetween coronal-hole outflow velocity and thechromospheric network structure (Fig. 20), withthe largest outflow velocities occurring alongnetwork boundaries and at the intersection ofnetwork boundaries. This can be consideredthe first direct spectroscopic determination ofthe source regions of the fast solar wind incoronal holes.Proton and O VI outflow velocities in coronalholes have been measured by UVCS using theDoppler dimming method. The O VI outflowvelocity was found to be significantly higherthan the proton velocity, with a very steepincrease between 1.5 and 2.5 R , reachingoutflow velocities of 300 km/s at around 2 R (Fig. 21). While the hydrogen outflow velocitiesare still consistent with some conventionaltheoretical models for polar wind acceleration,the higher oxygen flow speeds cannot beexplained by these models. A possibleexplanation is offered by the dissipation of highfrequencyAlfvén waves via gyroresonance withion-cyclotron Larmor motions, which can heatand accelerate ions differently depending ontheir charge and mass.Speed profile of the slow solar windTime-lapse sequences of LASCO white-lightcoronagraph images give the impression of acontinuous outflow of material in the streamerbelt. Density enhancements, or ‘blobs’, formnear the cusps of helmet streamers and appearto be carried outward by the ambient solarwind. Sheeley et al., using data from theLASCO C2 and C3 coronagraphs, have traceda large number of such ‘blobs’ from 2 to over25 solar radii. Assuming that these ‘blobs’ arecarried away by the solar wind like leaves on82