Measures of solar Measures of solar oblateness using active-oblateness using active-

region maskingregion masking

H.J. Zahid, M.D. Fivian, H.S. Hudson

Space Sciences Lab, UC Berkeley

Abstract

RHESSI makes measurements of the solar radius using images from the linear CCDs of its aspect system (see poster SH53A-1076). Faculae confuse the measurement by causing excess brightness at the limb. Here we present a method for masking out the facular contamination using EUV images from SOHO/EIT. The 284A images have sharp facular contrast and therefore can serve as a proxy for masking facular regions in our measurement of the radius. We discuss how this masking tends to decrease both the magnitude and error in our measurement of oblateness.

Measurements of solar radius with RHESSI

Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is a rotating spectroscopic imager used in the study of high-energy solar events. The Solar Aspect System (SAS) on board the spacecraft provides knowledge of the spacecraft pointing. This system consists of three independent lens/sensor pairs measuring the location of the solar limb. Through a complex reduction scheme, this system of three independent rotating telescopes can be used to make six simultaneous, interdependent measurements of the solar radius as a function of position angle with high cadence (~100 points per second).

EIT and solar features at the extreme limb

The Extreme Ultraviolet Imaging Telescope (EIT) onboard SOHO is designed to image the solar transition region and corona in four different bandpasses in the extreme ultraviolet. We find that the Fe XV band at 284A shows the sharpest contrast between faculae and the quiet sun. By taking an annulus of ~4 arc sec at the extreme limb from full-disk EIT 284 images, we can compare measurements of the solar radius with solar features seen by EIT.

Synoptic Views of the limb

The top panel shows a synoptic view of the extreme limb using EIT 284A. The bottom panel shows a synoptic chart of our radius values, with the color scale covering ~150 mas. The X-axis gives position angle (N, W, S, E, N as shown). The EIT data have been interpolated to have the same time base as our radius data.

Comparing synoptic views

The synoptic charts represent two different views of the extreme limb. Solar features show up as a displacement of our measurement of radius whereas in the EUV they cause a brightness increase. The high contrast between the quiet sun and active regions seen in the EUV allows us to use these data as a sensitive proxy for masking solar activity in our measurement of the radius. We set a threshold value in the EIT data to serve as a definition for facular presence. We then use the related times and positions to mask our measurement of solar radius.

Application of the EUV mask

An application of the mask. Here the contours are overlaid for a specific threshold. We can change the threshold to vary the depth of masking. Note the underlying signal in the bottom panel due to oblateness.

Varying mask threshold

Top, the mask threshold value as a function of amount of data cut. Bottom, a histogram of amount of data in each 1-degree bin for an average over all orbits for two different threshold values corresponding to the ones shown above. The vertical dashed lines mark the equator. The data cuts efficiently remove the active regions.

Comparing the solar shape with varying masks

The three different measured shapes of the sun correspond to the three different data cuts shown in the previous figure. The data removed from the active zones help to suppress the apparent radius increases due to facular contamination in our measurement of the radius as a function of limb position. The smooth overlaid curves show fits for the oblateness.

Measure of oblateness

Top panel, the oblateness (axisymmetric quadrupole amplitude) derived from 943 orbits of RHESSI data. Bottom panel shows the corresponding error calculated from the RMS of the orbital measurements. If we take the minimum of the top curve as the measure of oblateness we get an oblateness of 5.16 0.087 mas for this data set.

Discussion of oblateness measurement

•The previous illustration shows the fitted oblateness as function of the amount of data cut. The oblateness decreases to minimum as we eliminate more and more facular contamination by our method of masking.

•Beyond the minimum, the data cuts are statistically favored in the non-active zones. Because on average we are removing the larger radius values, the radius in the non-active zones decreases more than in the active zones. This leads to an increase in the differential radius (equatorial minus polar), thus increasing the oblateness.

•The minimum of the oblateness fit very nearly coincides with the minimum of the error (bottom panel in previous plot). This indicates we are properly applying our mask. Our minimum measurement of oblateness corresponds to roughly a cut of 15% of our data.

Conclusion

Facular contamination revealed by EUV brightness systematically tends to increase the apparent oblateness. We use this relationship to mask out the faculae, allowing us to derive a cleaner measurement of oblateness. We are currently working on using masking in our initial data selection before averaging. These are preliminary results; the measure of 5.16 0.087 mas has systematic errors and represents an upper limit of the true oblateness (see Poster SH53A-1076).