List of Figures

View Image Figure 1:
Two views of the Sun during a total solar eclipse. Left: the pink chromosphere is visible, its color stemming from Hα emission at 6563 Å (image courtesy Dr. William Cohen; used by permission). Right: the corona is visible in scattered photospheric light (image by Bill Livingston/NSO/AURA/NSF).
View Image Figure 2:
Top: Roberts (1945) published photographs of chromospheric spicules taken eight minutes apart (image A and B), illustrating their rapidly changing character; note especially the appearance of two well-defined spicules at left in image B (from The Astrophysical Journal). Bottom: An image of the Sun obtained by the Solar and Heliospheric Observatory (SOHO) in the light of He ii at 304 Å. The huge prominence at right (which Huggins, Lockyer, and their 19th century contemporaries would have called a “red flame”) dominates the scene, but elsewhere are numerous macrospicules, especially at top. The chromosphere is highly inhomogeneous, dynamic, and topologically complex (image courtesy SOHO/EIT; ESA and NASA).
View Image Figure 3:
Figure 1 from Vernazza et al. (1981). The temperature structure derived from a semi-empirical model of the solar chromosphere is presented, along with the formation heights of important lines and continua. Very roughly, the solar chromosphere lies between the temperature minimum at right and the rapid rise toward transition region and coronal temperatures at  2300 km. A few important points: (1) heating and cooling via hydrogen is sufficiently central to chromospheric energy balance to form part of a definition of a chromosphere, (2) other than a very limited set of features such as Ca ii H & K, Hα, and the Ca ii infrared triplet, chromospheric lines lie in the UV or beyond and are not accessible from the ground, and (3) a given point in a real chromosphere may look profoundly different from this “average” chromosphere.
View Image Figure 4:
The extended composite total solar irradiance (TSI) record since 1975, from the World Radiation Center (PMOD-WRC) in Davos, Switzerland (Fröhlich).
View Image Figure 5:
Left: a well-known white light image of the Sun, illustrating sunspots and faculae, the latter being apparent as bright regions near the limb (image courtesy NSO/AURA/NSF). Right: Ca ii K image showing chromospheric emission from active regions and network (image courtesy Big Bear Solar Observatory/New Jersey Institute of Technology). The chromospheric extensions of the faculae (the so-called “plage”) are readily visible across the disk.
View Image Figure 6:
Left: A closeup of a sunspot, clearly showing the penumbral spot structure and surrounding photospheric granulation (image courtesy Dutch Open Telescope). Right: a sunspot, granulation, and faculae observed from a more oblique angle (image courtesy Goran Scharmer and Mats G. Löfdahl, Institute for Solar Physics of the Royal Swedish Academy of Sciences).
View Image Figure 7:
Left: Observations of photospheric granulation in the CH G band (4300 Å) show the granulation and bright points in the dark lanes; these are where the line of sight intersects flux tubes whose rarefied interiors allow one to see to deeper, hotter gas. Right: the upper photosphere, viewed in the Ca ii H line, exhibits reversed granulation (images courtesy Dutch Open Telescope).
View Image Figure 8:
Three contemporaneous views of the Sun. Top left: The solar photosphere, observed in the light of the λ 4300 CH G band on 2003 November 2. Active region AR10486 is apparent at center. Top right: The solar chromosphere in the same location and time, observed in Ca ii H (images courtesy Dutch Open Telescope). Bottom: A λ 171 Å image of an arcade of magnetic loops following a large flare that erupted from AR 10486 two days after the other images were taken (image courtesy Transition Region and Coronal Explorer).
View Image Figure 9:
Left: Representative time series from the MWO HK Project, illustrating periodically variable (top), irregular (center), and flat (bottom) chromospheric activity, expressed in terms of the dimensionless S index, running from 1966 – 1991 (from Baliunas et al., 1995). Right: Analogous series from the Lowell Observatory SSS project, showing the flux-calibrated time series for the Sun (top), the cycling solar twin 18 Scorpii = HD 146233 (center), and the relatively inactive solar analog 16 Cygni B = HD 186427. Quantities in brackets at the top of each panel are the S values derived from the spectra. The time span of the SSS series is 1994 – 2006 (from Hall et al., 2007b).
View Image Figure 10:
The distribution of activity in 815 southern Sun-like stars. Left panel: color versus activity, displaying four qualitative activity levels and a relative lack of intermediate-activity stars of R’HK  –4.65. Right panel: the distribution of activity is bimodal, though with a small low-activity excess that may represent Maunder Minimum stars (from Henry et al., 1996).
View Image Figure 11:
The distribution of activity F, G, and K dwarfs as a function of metallicity. Metal-poor stars show a singly-peaked distribution (left panel), while for stars with [M/H] > –0.2, the distribution is bimodal (from Gray et al., 2006).
View Image Figure 12:
The so-called “Vaughan–Preston Gap,” indicated in the figure with red ellipses, is a relative absence of F and G stars of intermediate activity levels. In addition to appearing in the large surveys by Henry et al. (1996) and Gray et al. (2006), it is present in the activity means obtained by the synoptic programs at MWO (left panel; CRV is the Mount Wilson instrumental color index) and Lowell (right panel) (adapted from Vaughan and Preston, 1980 and Hall et al., 2007b).
View Image Figure 13:
Figure 8 from Hempelmann et al. (2006), the first and clearest long-term observation of coherent chromospheric and coronal activity. The relative variability of the star is shown for the ROSAT and XMM-Newton observations (filled circles) and the combined Mount Wilson and Lowell HK series (open dots).
View Image Figure 14:
Figure 18 from Livingston et al. (2006). Top: the Ca K record, showing the direct correlation with the magnetic activity cycle. The scale is not shown in this figure, but the amplitude of the cycle in Ca K is about 25%. Center: the record for five photospheric iron lines, showing the lack of response of the lines to the 1996 Cycle 22/23 minimum. Bottom: control series, an O2 line normalized to air mass 1.0.
View Image Figure 15:
Left panels: long (top) and short (bottom) term brightness variations as a function of activity level and color. Filled circles indicate inverse correlations of brightness and HK emission; open circles indicate direct correlations. Right panels: HK variability (top) and photometric variability (bottom) versus overall activity level. Relative to comparable stars, the Sun has a relatively strong chromospheric cycle but low photometric variability (from Radick et al., 1998).
View Image Figure 16:
The brightness variations and chromospheric activity of the solar twin 18 Scorpii over 10 years. At top are the Lowell H & K seasonal means (blue diamonds), along with the solar seasonal means for comparison (purple squares). The 18 Sco photometric means are shown at bottom, relative to individual comparison stars (yellow and orange) and the weighted mean (red) (from Hall et al., 2007a).
View Image Figure 17:
Left: HD 140538 appears to have made a transition from a flat activity state to an unusually short cycle in 2000 (from Hall et al., 2007b). Right: HD 3651 shows evidence of having entered a flat activity state around 1980 (from Baliunas et al., 1995).