DOWNFLOWS UNDER SUNSPOTS DETECTED BY HELIOSEISMIC TOMOGRAPHY

SUNSPOTS are areas of cooler gas and stronger magnetic fields in the S un's photosphere (its 'surface'), but just how they form and are maint ained has long been a puzzle. It has been proposed(1) that small verti cal magnetic flux tubes, generated deep within the Sun, develop downfl ows around them when they emerge at the surface. The downflows bring t ogether a large number of flux tubes in a cluster to form a sunspot, w hich behaves as a single flux bundle as long as the downflows bind the flux tubes together. Until now, however, it has not been possible to test this model with subsurface observations. Here we use the recently developed technique of travel time helioseismology(2) to detect the p resence of strong downflows beneath both sunspots and the bright featu res known as plages. The flows have a velocity of similar to 2 km s(-1 ), and they persist to a depth of about 2,000 km. The data suggest, ho wever, that the vertical magnetic field can be a coherent Bur bundle o nly to a depth of similar to 600 km; below this depth it is possible t hat the downflows hold together a loose collection of flux tubes to ma intain the sunspots that we see.