Raw data

The Solar Oscillations Investigation project (http://soi.stanford.edu) provided us with a cube of data recorded by their MDI instrument. They transformed the coordinate system to Cartesian coordinates by projecting high-resolution data from an area approximately 18$^\circ$ square onto a tangent plane.

Figure 1 shows a time slice through a cube of raw velocity data from the MDI. The object in the center of the panel is a sun-spot. Figure 2 shows a time-distance section through the same cube. The sampling spacing is 1 minute on the time-axis and approximately 68 km on the two spatial axes.

Events on Figure 2 fall into two distinct spectral windows. The low temporal frequency events (<1.25 mHz) are related to solar convection (super-granulation), while the higher frequency events are related to acoustic wave propagation. We were interested in studying acoustic wave phenomena; so as a preprocessing step, we removed the lower frequency spectral window by applying a $\frac{1-Z}{1-\rho Z}$ low-cut filter to the data.

 

timedata
Figure 1 Time-slice through the raw data. The object in the center is a sun-spot. Distances are in Megameters (thousands of kilometers), and time units are kiloseconds.

 

txdata
Figure 2 Time-space section through a subset of the raw data.