AAS Meeting #194 - Chicago, Illinois, May/June 1999
PAPER 93.03

Supergranules and Photospheric Motions Near the Solar Poles

R. S. Bogart, J. G. Beck, R. I. Bush,& J. Schou

Stanford University



Since the SOHO launch in 1995, there have been seven opportunities for the Michelson Doppler Imager (MDI) to provide continuous Dopplergrams of the solar poles over extended intervals of time. Because of the nearly constant and uniform focus of the MDI images over small regions it is possible to resolve and track supergranules across the pole at these times. Seismic analysis of global modes from the MDI data has previously hinted at the possibility of a polar vortex in the upper convection zone. We report here on the first analysis of polar surface motions inferred from MDI data covering a total of about 2000 hours during five polar observing windows, including two full months around the south polar apparition of 1998.

Because of the lack of seeing and scattered light and excellent pointing stability, the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) makes excellent Doppler observations near the solar limb, despite its moderate spatial resolution (2"). Its ability to observe continuously for times comparable to and longer than the typical lifetime of supergranules makes it an ideal instrument for studying the behaviour of these features of predomiminantly horizontal flow whose maximum signal is at the limb. Although the SOHO orbit is inclined to the Sun's rotation axis at about the same small angle as the Earth (7.25 deg), the stability and continuity of MDI full-disc Dopplergrams make them a potential rich source of data on supergranules and large-scale flows near the solar poles.

Available Data

During the 3.5 years since SOHO launch on 2 Dec. 1995, there have been seven opportunities to observe the poles at their maximum tilt toward the observer. For a period of about two months around each such solstice, the inclination of SOHO to the solar equator |B| exceeds 6.25 deg, and the pole is more than 6" (three pixels) from the limb. (see Table I). The first of these occurred during the spacecraft and instrument checkout and calibration period before the commencement of nominal observations. Although there were two periods of more than a day when continuous full-disc Dopplergrams were made with MDI, the data were test data only, not taken in normal sequences, and they have not been calibrated. There may be useful information in them, paricularly when combined with the unique set of observations made on 5 Mar 1996, when the spacecraft was temporarily pointed to place the south pole within MDI's restricted field of high spatial resolution (0".65). These data are not considered here. The sixth opportunity for polar observing occurred during the extended period (25 Jun - 4 Nov 1998) when normal operations and communications were interrupted by a serious spacecraft emergency. Although there have been long periods of continuous "full-disc" coverage during the seventh polar observing opportunity (5 Feb - 7 Apr 1999), virtually all of these were made in a new observing mode that crops pixels near the limb out of the telemetry stream and so the polar region data were lost.

Table I.  Optimum opportunities for MDI observations of the solar poles

|B| > 6.25 (3 pixels from pole to limb)

1. 1996.02.05 - 1996.04.05
    1a	1996.02.17_20:00 - 1996.02.19_15:05 (hours 27428 - 27471) (43)
		(level 0 only)
    1b	1996.03.28_01:00 - 1996.03.31_07:30 (hours 28368 - 28447) (75)
		(level 0 only)

2. 1996.08.07 - 1996.10.10
    2a	1996.08.18_17:10 - 1996.08.23_03:00 (hours 31817 - 31922) (106)
    2b	1996.09.17_17:40 - 1996.09.22_01:35 (hours 32537 - 32641) (105)

3. 1997.02.04 - 1997.04.06
    3a	1997.02.13_10:30 - 1997.02.18_15:20 (hours 36106 - 36231) (126)
    3b	1997.03.12_14:20 - 1997.03.17_00:45 (hours 36758 - 36864) (107)

4. 1997.08.08 - 1997.10.09
    4	1997.08.10_14:30 - 1997.08.14_01:05 (hours 40382 - 40465) (84)

5. 1998.02.05 - 1998.04.06
    5	1998.02.05_00:00 - 1998.03.03_09:00 (hours 44664 - 45296) (633)
		part of Dynamics Campaign IIIa (beginning 1998.01.09_10:50)
	1998.03.05_02:20 - 1998.04.06_23:59 (hours 45338 - 46127) (790)
		part of Dynamics Campaign IIIb (ending 1998.04.10_08:00)

6. 1998.08 - 1998.10
		no data - spacecraft emergency

7. 1999.02.05 - 1999.04.07
	1999.02.11_03:30 - 1999.02.14_07:30	(63)
	1999.02.19_00:40 - 1999.02.23_02:40	(74)
	1999.03.13_01:20 - 1999.03.15_23:45 	(71)

The Movie

MDI Dopplergrams taken at a cadence of one per minute have been mapped and averaged together in one-hour frames for each of the five observing intervals to make the movies being shown. (The two parts of Dynamics Campaign III, separated by a 31-hour gap due to a spacecraft emergency but otherwise identical, have been combined into a single interval as the length of the gap is short compared to the overall duration.) The calibrated velocity data are first corrected for the line-of-sight component of spacecraft orbital motion and detrended with a uniform model of differential rotation and convective limb-shift. They are then projected onto the heliographic coordinate system using a stereographic projection centered at the appropriate pole and with a fixed direction for Carrington longitudes. The map angles are chosen so that the 0 longitude runs down from the center of the maps centered at the north pole and up from the center of those centered at the south pole. All mapped Dopplergrams within a clock hour, comprising a SOI dataset, are then averaged together. Mapping to the Carrington system before averaging has the effect of removing the spatial smearing that would result from rotation of features past the observer, at least to first order.

Although the overall data filling factor is high, typically about 97%, there are numerous missing or corrupted Dopplergrams, and occasional gaps of up to several hours. Gaps of up to five hours' duration are simply filled in by linear interpolation between the surrounding images - there are ten such gaps during the two-month south-pole series of 1998. Interpolated frames in the movie do not have date-time labels. Longer gaps are simply blanked out. Apart from the gap already mentioned, the only other is a 6-hour gap in the first north-pole sequence (1997 Feb).

Each of the four short (5-day) sequences 2a, 2b, 3a, 3b, and 4, is shown four times in the movie, twice showing a heliographic 15 deg grid for orientation and twice without the grid. The long 2-month sequence is then also shown twice with the grid and twice without, the second of each pair being sped up a factor of two. After that the two-month sequence at the south pole withthe grid overlay loops repeatedly.

The animation below shows the polar view for only sequence 2a, 1998.08.18_17:00 - 1996.08.23_03:00.


Supergranulation and Surface Rotation

Supergranules are plainly visible without evident distortion at very high latitudes in the movies. Naturally they exhibit substantial eastward proper motion relative to the Carrington coordinate system. We have made initial estimates of the differential rotation at high latitudes inferred from supergranule tracking simply by reprojecting and averaging the detrended Doppler data again, this time with a sinusoidal equal-area projection centered on a fixed Carrington longitude, so that physical displacements (in meters) along a parallel over a given time are proportional to image displacement at all latitudes. In temporal stack plots of individual latitudes the proper motion of a supergranule is proportional to the slope of the feature. Figure 1 shows examples of such stack plots for six selected latitudes from the 107-hour sequence around the south pole in March 1997.

Figure 1

Line-of-sight velocity as function of distance from the central Carrington longitude along the parallel (horizontal) vs. time (vertical, running from bottom to top), for several latitude strips

Latitude 60 S Figure 1: 60S

Latitude 65 S Figure 1: 65S

Latitude 70 S Figure 1: 70S

Latitude 75 S Figure 1: 75S

Latitude 80 S Figure 1: 80S

Latitude 85 S Figure 1: 85S

This initial set of maps of supergranulation shows striking evidence for a polar vortex, a spin-up in the Carrington frame near the pole (see Table 2). While the mean proper motion at -60° is quite consistent with standard measures of differential rotation at that latitude, and the differential rotation curve persists to -65°, there apears to be a turnover before -70°, with a velocity shear several times the magnitude of that of the mid-latitude differential rotation. The extrapolated polar rotation rate is about 2 microrad/sec, a period of 36 days.

Table II.  Mean zonal proper motions of supergranules at high latitudes

Latitude     u ± RMS  equivalent delta-rot (microrad/sec)

  -60°	   -165  (42)		0.47
  -65°	   -183  (48)		0.62
  -70°	   -175	 (44)		0.74
  -75°	   -128  (31)		0.71
  -80°	   -132  (65)		1.09
  -85°	   - 65  (13)		1.07

Polar Flows

Any steady cross-polar flow should be seen in the line of sight as a periodic signal with period equal to the rotation period at the pole, longer than the Carrington period but perhaps not as much longer as the standard models of differential rotation imply. In Figure 2 we show the average residual Doppler signal in a small (1 square degree) area centered on the south pole as a function of time during the two-month observing window of Feb. - Apr. 1998. Substantial quasi-periodic excursions of amplitude ~100 m/s are present, but the typical periods are only 5-10 days (slight peak in the power spectrum at 1 microHertz). Since this is much longer than the typical lifetimes of supergranules, it appears from this very preliminary result that there may possibly be a two- or three-sector structure of quasi-steady flows with a vertex away from the pole.

Research supported by SOI-MDI NASA grant NAG5-3077 at Stanford University.

Wednesday, 16-Jun-1999 16:30:01 PDT