Recent investigations on the photospheric angular velocity pattern have shown that young and short-living tracers show rotation rates higher than those determined both by older tracers and by photospheric plasma. As a direct relationship between the age of the tracer and the angular velocity determination has been found (Zappala & Zuccarello 1991), it seemed very interesting to investigate whether also the ''lifetime'' parameter might have a role on angular velocity determinations. We have therefore analyzed the sunspot-group data reported in the Greenwich Photoheliographic Results during the 1874-1976 period. 9000 objects were selected as young sunspot-groups (YSG) and, using the lifetime parameter as selecting rule, we could catalogue 4463 objects having a lifetime between 2 and 10 d. The rotation rate of these objects as a function of their lifetime was calculated and the results obtained may be summarized in the following main points: 1. Independently of their lifetime, sunspots rotate during the first 2-3 days of life in the photosphere, at a higher rate than that of recurrent sunspots. 2. Sunspots with a lifetime ranging from 2 to 8 d are more efficiently decelerated than YSG, while 11-day living sunspots are less efficiently decelerated. 3. Sunspots in the equatorial belt (0-10-degrees) having a lifetime comparable to that of supergranule cells, rotate slower than the cells themselves. 4. The angular velocity measured during the last day of life is lower both than that of YSG and than that deduced by sunspots which disappear the day after. These results have been analyzed in the scenario of the sunspots cluster model (Schussler 1987). According to the conclusions drawn, the initial higher angular velocity of young and short-living sunspots is not a function of the cluster ''aggregation capability''; the rate of rise of the merging level through the convection zone is influenced by (or influences) the ability of the cluster to keep coalesced; finally, when the merging level rises too fast, the sunspot dissolves.
|Titolo:||INFLUENCE OF THE LIFETIME PARAMETER ON THE ROTATION RATE OF SUNSPOTS|
|Data di pubblicazione:||1993|
|Appare nelle tipologie:||1.1 Articolo in rivista|