Date of Award
Doctor of Philosophy (PhD)
Katharine B. Gebbie
Simultaneous satellite and ground-based observations of supergranular velocities in the Sun were made using the University of Colorado UV Spectrometer on OSO 8 and the Sacramento Peak Observatory diode array instrument. We compare our observations of the steady Doppler velocities seen toward the limb and at disk center in several spectral lines that span 1400 km in their heights of formation. Our observations indicate that supergranular flows are able to penetrate at least 11 density scale heights in the atmosphere. Seen at radius vector 0.8, the dynamic range of these motions increases from about 800 m/s in the photosphere to about 3000 m/s in the middle chromosphere. At disk center, regions of downflow tend to correlate with the bright Ca II network, those of upflow with the darker areas. The dynamic range of these vertical motions is only about 200 m/s in the photosphere but increases in the middle chromosphere to a value comparable with that seen at radius vector 0.8. Thus a distinct change appears to occur in the flow structure: whereas the horizontal component of the velocity predominates in the low photosphere, suggesting strong braking of the vertical momentum there, the motions higher in the atmosphere are more isotropic. These observations imply that supergranular velocities should be evident in the transition region. In addition to supergranular scale flows, our observations at disk center show a 7000 km scale of motion, which may be convective in origin. We call this mesogranulation. Mesogranules are most apparent in the MgI 5173A spectral line formed in the lower chromosphere. In the low photosphere, they are masked to some extent by the large signals from granulation and five-minute oscillations. Our SiII 1817A observations of the middle chromospheric motions indicate that a 7000 km scale of motion is also present there. If the scale of granulation is related to the depth of the HI ionization zone, and that of supergranulation to the Hell zone, then the mesogranules may represent the scale of motion driven in the Hel ionization zone. The decrease of the kinetic energy flux with height in the atmosphere suggests that major dissipation is occurring in these flows.
November, Laurence J., "Mesogranulation and Supergranulation in the Sun" (1979). JILA Graduate Theses & Dissertations. 1.