1. A Comparison of Solar Polar Coronal Hole Areas
Between Solar Cycles 23 and 24
S. A. Hess Webber [NASA GSFC/GMU] N. Karna [NASA GSFC/GMU] W. D. Pesnell [NASA GSFC] M. S. Kirk [NMSU]
Abstract
Synoptic MDI Magnetogram Analysis
The perimeter tracing method produces the fractional areas shown in Figure 4. The annual periodicity in the area data increases the uncertainty in area value estimates. However, the nesting of the three wavelengths in both images and area calculations suggests that the peak amplitude values are best estimates of the true area. The peak values are also the upper limit of the uncertainty in the data set, so the actual true PCH areas can be no larger than the best estimate. With this reasoning in mind, we can estimate the areas obtained from the perimeter tracing method. We observe the northern hole to be about 6.2% of the solar surface area at the minimum preceding Solar Cycle 23 (May,1996), and the southern hole measures about 5.4%. We further find that at the solar minimum between Solar Cycles 23 and 24 (December, 2008), the northern and southern hole areas are about 4.0% and 5.2% of the total solar surface, respectively (see Figure 4).
We have now completed the EUV synoptic data set through present, so that we can now make a direct comparison between all three methods across the entire time series. In Figure 5, the areas from both synoptic map methods show good agreement in the two solar minima. From the EIT data, the estimated fractional area for the Northern PCH is 6.1% of the total surface area in 1996 and about 4.0% in 2008, while the area of the Southern hole is 4.3% in 1996 and 4.3% in From MDI, the estimated areas in 1996 are 5.0% for the Northern PCH and 5.6% for the Southern PCH. We further find that in 2008, the Northern and Southern polar hole areas are about 3.1% and 4.7% of the total solar surface, respectively. There is strong correlation between the three EIT wavelengths and the magnetic field. The correlation between EIT 195 and MDI is around 81% (r-squared), and the same for EIT 171. For EIT 304, the correlation with MDI is about 82%.
Again, we see annual variations in the synoptic EIT data, consistent with those found in the perimeter tracing method. The variations are stronger here due to a signature of synoptic maps: meridional strips are concatenated to form the maps. This technique adds to the projection effects inherent in the images. However, we use the peak-estimate for these results as well, under the same reasoning.
In all three methods, there is a notable decrease in the northern polar hole area between the 22/23 and 23/24 minima. The southern PCH remains about the same size but may also show a slight decrease in area. We also are seeing a significant drop off in the northern data after early 2010, which is indicative of the decline of the polar field into maximum. This is not yet visible in the southern data because there is a time lag in the evolution of the south pole with respect to the north.
We incorporate magnetogram data with synoptic maps from SOHO’s MDI instrument. This is done through similar image analysis techniques as the EUV synoptic maps (see Figure 3). Magnetograms display the detected line of sight magnetic field strength and location on the solar disk. White and black areas indicate positive and negative magnetic field along the line of sight (respectively), whereas grey areas indicate that the line of sight magnetic field is close to zero. The boundary of the PCH is obtained by averaging the longitudinal magnetic field strength at each latitude. The PCH boundary is at the zero point containing the dominant polarity at each pole. We then calculate the fractional coronal hole area from that latitude.
We have used the perimeter tracing algorithm and analysis of EUV and magnetic field synoptic maps to extend our time series of polar coronal hole areas through solar minimum between cycles 23 and 24 (through 2010). Both EUV algorithms use 171, 195, and 304 Å images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO. The perimeter tracing algorithm measures the polar coronal hole boundaries as they appear on the limbs over each polar rotation and calculates the enclosed area, while the synoptic method calculates the area of the polar coronal holes from the meridional boundary through each Carrington rotation. Line-of-sight magnetic field synoptic maps from SOHO’s Michelson Doppler Imager (MDI) instrument are used to estimate the polar coronal hole areas via polarity signatures. We remain convinced that the northern polar hole area is measurably smaller in the recent minimum than it was at the beginning of cycle 23, while the southern polar hole area is roughly the same.
Figure 3: An MDI synoptic map and the corresponding average magnetic field with respect to latitude. The average field plot demonstrates the unipolarity at the poles from which we determine the PCH boundaries.
Detection Methods
Perimeter Tracing
Perimeter tracing is a method for the automated detection of the polar coronal hole (PCH) boundaries at the solar limb (Kirk et al., 2009), summarized in Figure 1. The objective in tracing these boundaries is to accurately estimate the perimeter and fractional area of the polar holes. To determine the area, we map out the opening angle of the boundary points through one complete Harvey Rotation (HR, ~33 days). This data is then fitted with a curve from which we derive the area. The tracing algorithm uses SOHO EIT images to detect the PCHs. We gather the data from three of EUV wavelengths: 304 Å, 171 Å, and 195 Å, corresponding to the He II, Fe XI/X, and Fe XII lines, respectively. This method has allowed us to measure PCH areas over the lifespan of SOHO: starting in mid 1996 through 2010.
Results
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Conclusions
Projection effects obstructing the PCH boundaries due to the B0 angle can produce annual periodicities that cannot be removed directly. The perimeter tracing method minimizes these effects. The wavelength data are nested, both in the images and the area analysis, suggesting the peak amplitude of the periodicity is the a reasonable approximation to the true area values. The true areas can be no larger than than these upper limit estimates.
We have extended the EIT synoptic maps up to present, which allows us to directly compare the areas of the most recent minimum between methods. In all three methods, the northern polar hole area is measurably smaller in the recent minimum than it was at the beginning of cycle 23, while the southern polar hole area is roughly the same. There is also evidence of the deterioration of the northern PCH as we approach the maximum of cycle 24.
The average fractional PCH areas, between all methods and wavelengths, at the cycle 22/23 minimum are approximately 5.8% and 5.1% in the northern and southern hemispheres, respectively. During the minimum between cycles 23 and 24, the northern average is about 3.7%, while the southern is roughly 4.6%.
We plan to expand this research into SDO data, using AIA and HMI, so that the time series continues into subsequent solar cycles. We have already begun creating EUV synoptic maps of the AIA data. However, some adjustments will have to be made to the perimeter tracing method. The AIA images show that the coronal holes are evolving in too complicated a fashion for the simple polar coronal hole assumption.
Figure 4: Perimeter tracing method area plots. (a) Northern polar coronal hole area and (b) Southern polar coronal hole area as a function of date from June, 1996 to December, 2010 (HR 1069-HR 1229) for the 195 Å, 171 Å, and 304 Å wavelengths. The region represented by the thin, dashed line is in the range of solar maximum, where the estimated uncertainties are too large. The black vertical line signifies the 23/24 minimum.
Figure 1: Illustrating the perimeter tracing method using a 195 Å image (A) after using morphological transform functions to blur the original image; (B) a smoothed binary image of the image shown in (A); (C) an annulus of the limb; (D) heliographic coordinates of the north and south PCH boundaries marked with circles.
Synoptic EIT Analysis
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(a)
Properties of PCHs can also be derived by analyzing synoptic maps from the EIT instrument on SOHO. Up through early 2007, the synoptic maps are provided (Benevolenskaya, et al., 2001). After this time, we are creating the synoptic maps. The same three wavelengths used in the perimeter tracing method are used here. The fractional coronal hole area is found by counting the number of black pixels in longitude at each latitude and summing in latitude while accounting for the projection effects in the synoptic map (see Figure 2).
References
Benevolenskaya, E. E., Kosovichev, A. G., & Scherrer, P. H. (2001), Detection of high latitude waves of solar coronal activity in extreme ultraviolet data from the SOHO EUV Imaging Telescope, ApJ, 554, L107; Data obtained from:
Harvey, K. L., & Recely, F. (2002), Polar coronal holes Cycles 22 and 23, Sol. Phys., 211, 31; Data obtained from: ftp://nsokp.nso.edu/kpvt/synoptic/.
Kirk, M. S., Pesnell, W. D., Young, C. A., & Hess Webber, S. A. (2009), Automated detection of EUV polar coronal holes during Solar Cycle 23. Solar Physics, 257,
Hoeksema, J. T., et al. (2000). Synoptic Magnetic Field Measurements. AAS, SPD meeting #31, #01.39; B.A.A.S., 32, 808; Data obtained from
Figure 2: An EIT synoptic map with its corresponding plot of the fractional coronal hole area percentage, plotted against latitude.
Figure 5: (a) Northern and (b) Southern Polar Hole fraction versus calendar date from June to March 31, 2007 (CR 1911-CR 2055) for 195 Å, 171 Å and 304 Å and from June to July 13, 2010 for MDI Magnetogram. The dashed lines in the MDI data indicate periods of missing or incomplete data.
The EIT images are courtesy of the SOHO/EIT consortium. Parts of the SSW library were used for image analysis and display. We would like to thank Elena Benevolenskaya for providing the online EIT synoptic map archive. We would like to thank the SOI project for providing the MDI synoptic map archive.
ATST EAST 2011 — Washington, DC
Project supported by NASA’s Solar Dynamics Observatory (SDO)