10 Years of SOHO - Nasa

SOHOescapes from the Sun in funnels of openmagnetic fields.SOHO, the Space Weather WatchdogWhile the Sun’s total radiative output isreassuringly constant, it is at the same timea dynamic and violent star. Besidesemitting a continuous stream of plasmain the solar wind, the Sun periodicallyreleases huge amounts of matter in coronalmass ejections (CMEs). These are the mostpowerful eruptions in the Solar System,with billions of tonnes of electrified gaspropelled from the Sun’s atmosphere intospace at millions of km/h. If they hit Earth,these immense clouds can cause largemagnetic storms in our magnetosphereand upper atmosphere. Researchers believethey are launched when solar magneticfields become strained and suddenly snapinto a new arrangement, like a rubber bandtwisted to breaking point.Apart from causing beautiful aurorae,CME disturbances can damage satellites,disrupt telecommunications, endangerastronauts, lead to corrosion in oil pipelines and cause current surges in powerlines. As our society becomes increasinglydependent on space-based technologies,our vulnerability to ‘space weather’becomes more obvious, and the need tounderstand it and mitigate its effectsbecomes more urgent. In recent years,forecasting the conditions in the near-Earth environment and the ‘geoeffectiveness’of CMEs and solar flares hasbecome one of the key research areas insolar and solar-terrestrial physics; SOHOis playing a pioneering role in this newdiscipline. While satellites that makein situ measurements in near-Earth orbitscan give only about a 2-hour alert of solarstorms, SOHO’s coronagraphs and EUVThis fiery coronal mass ejection shows stunning details in the ejected material, revealed by SOHO’s LASCO coronagraph. The directsunlight is blocked (red disc), exposing the surrounding faint corona. The white circle represents the approximate size of the Sunimager observe the source of CMEs andflares and thus provide up to 3 days’warning, sufficient to save costly equipment.SOHO’s LASCO and EIT are theprimary source of operational informationon the location, speed and orientation ofCMEs from the Earth-facing hemisphereof the Sun. These remote-sensing observationsare complemented by the CELIAS,COSTEP and ERNE in situ measurementsof the arrival of the CME andenergetic particles at the L1 Lagrangianpoint.The LASCO team has compiled anextensive catalogue that summarises themass, speed, acceleration, angular width,position and so on of the more than 10 000CMEs observed since launch. Thiscatalogue has been used in numerousstudies, including how the number ofCMEs varies with the solar cycle: the rateincreases from 0.5 per day during solarminimum to over 6 per day during solarmaximum. Using LASCO data, scientistcan also for the first time reconstruct 3-Dimages of CME structures. CombinedA coronal mass ejection heading almost directly towards Earth,observed by LASCO C2. The size of the Sun is indicated by acircle, and the x-marked circle on the Sun shows the origin ofthe CME. Panel (a) shows the total intensity (darker meansmore intense) as imaged directly by LASCO. Panel (d) is atopographic map of the material shown in panel (a). Thedistance from the plane of the Sun to the material is colourcoded;the scale in units of solar radii is shown on the side.Panels (b) and (c) show the intensity as it would appear to anobserver positioned to the side of the Sun or directly above it,respectivelywww.esa.int esa bulletin 126 - may 2006 29