Lasco observations of the coronal rotation

The near-rigid rotation of the corona above the differential rotation of th e photosphere has important implications for the form of the global coronal magnetic field. The magnetic reconfiguring associated with the shear regio n where the rigidly-rotating coronal field lines interface with the differe ntially-rotating photospheric field lines could provide an important energy source for coronal heating. We present data on coronal rotation as a funct ion of altitude provided by the Large Angle Spectrometric Coronagraph (LASC O) instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecra ft. LASCO comprises of three coronagraphs (C1, C2, and C3) with nested fiel ds of-view spanning 1.1 R. to 30 R.. An asymmetry in brightness, both of th e Fe XIV emission line corona and of the broad-band electron scattered coro na, has been observed to be stable over at least a one-year period spanning May 1996 to May 1997. This feature has presented a tracer for the coronal rotation and allowed period estimates to be made to beyond 15 R., up to 5 t imes further than previously recorded for the white-light corona. The diffi culty in determining the extent of differential motion in the outer corona is demonstrated and latitudinally averaged rates formed and determined as a function of distance from the Sun. The altitude extent of the low latitude closed coronal field region is inferred from the determined rotation perio ds which is important to the ability of the solar atmosphere to retain ener getic particles. For the inner green line corona (< 2 R.) we determine a sy nodic rotation period of (27.4 +/- 0.1) days, whereas, for the outer white- light corona, (> 2.5 R.) we determine a rotation period of (27.7 +/- 0.1) d ays.