Research ArticleASTROPHYSICS

Global conditions in the solar corona from 2010 to 2017

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Science Advances  14 Jul 2017:
Vol. 3, no. 7, e1602056
DOI: 10.1126/sciadv.1602056


  • Fig. 1 The changing appearance of the corona from solar minimum to maximum.

    These images are taken by AIA/SDO in EUV toward the end of the latest solar minimum activity period in May 2010 (left half) and during the current solar maximum period in December 2014 (right half). The three-color red-green-blue image channels are composed of observations made in three AIA channels: 171, 193, and 211 Å, respectively, corresponding to their most dominant emission lines of Fe IX, Fe XII, and Fe XIV with formation temperatures of ~0.7, 1.2, and 2.0 MK. The image has been processed using multiscale Gaussian normalization (64).

  • Fig. 2 Synoptic maps showing the varying properties of the corona and photosphere over a whole solar rotation.

    Shown here are (A) mean temperature, (B) EM, and (C) radial magnetic field component for a whole solar rotation during 04 March 2011. The y axis shows latitude on the solar disk, on a sine latitude scaling. The two horizontal dotted lines indicate the latitudinal range within ±40° of the equator (areas outside are not included for further study). Areas defined as active regions are shown within unbroken contours. Areas outside the dashed contours are considered as quiet corona. See Methods for more details on the identification of regions.

  • Fig. 3 Global means of various coronal and photospheric properties over the current solar cycle.

    Shown here are mean absolute magnetic field (blue), temperature (red), and EM (light green) for (A) quiet corona and (B) active regions. (C) Comparison of long-term changes in sunspot area (dark green), active region area (red), and quiet Sun magnetic field magnitude (blue). To reduce the dominant effect of the solar rotation and other short-term variation, all values have been smoothed in time with a Gaussian kernel of half-width 14 days. Daily sunspot area is sourced from

  • Fig. 4 The changing distribution of emission as a function of temperature over the current solar cycle.

    (A) Quiet corona and (B) active region DEM as a function of time over the current solar cycle. The y axis shows log temperature, with the DEM profiles strongly peaked at ~1.6 MK for the quiet corona and a broader peak at ~1.8 MK for active regions. The black curve shows the DEM-weighted mean temperature against time.

  • Fig. 5 The increasing global mean temperature and emission measure over the current solar cycle.

    Synoptic maps of mean temperature (left) and EM (right) for five solar rotations from 2010 to 2017. The Carrington rotation numbers are labeled in the plots, and each rotation is centered on dates 03 February 2011, 11 May 2012, 14 September 2013, 14 February 2015, and 11 December 2015.

  • Fig. 6 Comparing various global properties of the corona and photosphere throughout the current solar cycle.

    EM against temperature for (A) quiet corona and (B) active regions and EM against magnetic field magnitude for (C) quiet and (D) active regions. For the quiet corona, each point represents the mean value over a 30° rotation, whereas for active regions, each point represents the mean value of each region. The dashed line in (A) and (B) shows the expected power law for hydrostatic loops (β = 0.25; see Methods), and the dotted line gives the best-fit power law. The dashed and dotted-dashed lines in (C) and (D) show theoretical curves for ac (α = 0.625) and dc (α = 0.438) heating mechanisms (see Methods), whereas the dotted line shows the best fit. The normalized values for quiet corona and active regions are achieved by dividing by the all-time means for quiet corona and for active regions, respectively. The color of the points represents time from 2010 to 2017, progressing through black, blue, green, yellow, and red, as shown in the color bar. (E) Variation of normalized quiet coronal EM raised to a power of 7/8 against time. The shaded area gives the SD of values in each time bin of a Carrington rotation (~1 month).

  • Fig. 7 Estimates of the EUV irradiance of the corona over the current solar cycle.

    (A) Estimated short-wavelength (3- to 32-nm) EUV irradiance at Earth orbit (thin black line) calculated from the DEM maps covering the whole visible disk (see Methods). Also shown are the active regions (red) and quiet coronal (blue) components calculated for the limited latitudinal range used in the rest of this study. These are compared to measurements by EVE (black) integrated over the short-wavelength (3- to 32-nm) band. The short-band EVE measurements unfortunately end in May 2014. (B) Measured EVE irradiance in the short spectral band as a function of active region area. The color of the points represent time from 2010 to the end of EVE short-band measurements in 2014, passing through black, blue, green, yellow, and red, as shown in the color bar.

  • Fig. 8 Data from a whole Carrington rotation near solar minimum are used here to illustrate the DEM results.

    (A) DEM profiles averaged over the whole rotation for active regions (red) and quiet corona (green). The error bars show the SDs of DEM for each temperature bin. The quiet coronal DEM is strongly peaked near log T = 6.2. The active region DEM peaks near log T = 6.25 and decreases far more slowly with increasing temperature. (B to E) Maps of normalized DEM at four different temperatures, as indicated in the plot titles. The normalized DEM is the emission at that temperature as a percentage of the total emission at that pixel. Active regions are bounded by solid contours, whereas coronal holes are bounded by dashed contours.