Variation in Coronal Activity from Solar Cycle 24 Minimum to Maximum Using Three-Dimensional Reconstructions of the Coronal Electron Density from STEREO/COR1

Abstract

Three-dimensional electron density distributions in the solar corona are reconstructed for 100 Carrington rotations (CR 2054 – 2153) during 2007/03 – 2014/08 using the spherically symmetric method from polarized white-light observations with the inner coronagraph (COR1) onboard the twin Solar Terrestrial Relations Observatory (STEREO). These three-dimensional electron density distributions are validated by comparison with similar density models derived using other methods such as tomography and a magnetohydrodynamics (MHD) model as well as using data from the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO)-C2. Uncertainties in the estimated total mass of the global corona are analyzed based on differences between the density distributions for COR1-A and -B. Long-term variations of coronal activity in terms of the global and hemispheric average electron densities (equivalent to the total coronal mass) reveal a hemispheric asymmetry during the rising phase of Solar Cycle 24, with the northern hemisphere leading the southern hemisphere by a phase shift of 7 – 9 months. Using 14 CR ($\approx13$-month) running averages, the amplitudes of the variation in average electron density between Cycle 24 maximum and Cycle 23/24 minimum (called the modulation factors) are found to be in the range of 1.6 – 4.3. These modulation factors are latitudinally dependent, being largest in polar regions and smallest in the equatorial region. These modulation factors also show a hemispheric asymmetry: they are somewhat larger in the southern hemisphere. The wavelet analysis shows that the short-term quasi-periodic oscillations during the rising and maximum phases of Cycle 24 have a dominant period of 7 – 8 months. In addition, it is found that the radial distribution of the mean electron density for streamers at Cycle 24 maximum is only slightly larger (by $\approx30\%$) than at cycle minimum.

Appendix: The STEREO/COR1 Background Subtraction and Its Effect on the Coronal Mass Estimates

Removing instrumental stray light (referred to as background subtraction) is an essential step in the pB data reduction because raw COR1 signals are comprised of three components: the K coronal light, the scattered light (weakly polarized), and the F coronal light (unpolarized). The procedures for deriving the time-dependent COR1 instrumental background were described in detail by Thompson et al. (2010). Using their methods, two types of background images (namely the monthly minimum backgrounds and the combined monthly minimum and calibration roll backgrounds) are generated every ten days for each of the polarizer settings at 0^∘, 120^∘, and 240^∘. In SSW the routine secchi_prep is used to calibrate the COR1 data, including the process of background subtraction with a choice of using the regular monthly minimum backgrounds by default or using the combined background with the keyword /calroll. Table 3 lists the dates for the COR1 calibration roll maneuver during the period of our interest. The comparison of Figure 11 with Figure 19 shows that the differences of the total coronal mass calculated for COR1-A and -B are relatively larger as a result of using the calibration roll backgrounds compared to the case using the regular monthly minimum backgrounds. The reason may be that the calibration rolls of COR1-A and -B were performed typically four times a year and were also out of phase, and the calibration roll background images at other times than those listed in Table 3 have to be derived by interpolations (or extrapolations if a background change event occurred between the two closest calibration roll maneuvers). Long-term monitoring reveals that the COR1 background occasionally encounters a sudden increase, which is most likely due to a dust particle landing on the objective lens. For example, the dust landing event on 19 April 2009 for COR1-B is the largest event, and other events that significantly affected the COR1 background are listed in Table 4, where events due to the changes in image binning format and exposure time are also included. However, we caution that the background subtraction close to these events is generally poorer than normal, which may lead to the relatively larger uncertainties in the total coronal mass estimated around these events (see Figure 11b and Figure 19b).

Figure 19

Estimate of uncertainties in the measured total coronal mass from STEREO/COR1 when the pB images are processed with the regular monthly minimum background images. The annotation for all curves is same as Figure 11. The red bars at the top of panel (b) indicate the times of the events that caused a significant change in scattered light background for COR1-A (upper row) and COR1-B (lower row).

Figure 20

Ratios of the total coronal mass ($M_{\mathrm{CALROLL}}$) measured from the COR1 pB images that are processed with the combined monthly minimum and calibration roll backgrounds to that ($M_{\mathrm{REGULAR}}$) measured from the COR1 pB images that are processed with the regular monthly minimum backgrounds. The error bars are calculated using error propagation rules.

Table 3 Dates for COR1 calibration rolls performed during the period in which the data we analyzed were observed.

Table 4 Events that significantly changed the COR1 background during the period from 2007/03/04 to 2014/08/07.