Layers of the Solar Atmosphere

Corona Chromosphere Photosphere

Details of solar activity can be seen more easily in the hotter outer layers, which are quite variable. Fortunately, Kepler is only sensitive to the photosphere.

Quiet and Active Sun

Ca IIK-lineimages.

These show both chromos-pheric and photos-pheric features.

1996 2001

Distribution of Stellar Activity Levels

We will be able to generate a similar plot from Kepler data alone (with vastly more stars, of course).

The basic variable here is stellar age. Proper motions will be nice too.

Ca II activity

Photometry for the Quiet and

Active SunThe amplitude and character of photometric variations change during the solar activity cycle. This is less of a problem at transit timescales.

Correlations between the Colors

Behavior in all 3 colors is well-correlated (and with total irradiance).As the activity level increases, the correlations get even better.

Plusses=1996

Diamonds=2001

Three Color Photometry of the Sun

Characterizing All Variability By Effective Temperature

VariationsThe color and brightness variations can be characterized with a single integrated variation in effective temperature. This must be made up of smaller hot and cool regions distributed on the surface.

One might start with a single temperature difference for each type of region, with a given dependence on the limb angle at which they are seen (due to 3-D effects).

Binned to Kepler timescale (15 min)

Detailed Images of a

Spot I hope to boil down the very complex solar surface to a few parameters, remembering that we will find only gross details on other stars. Of interest to Kepler is how photometry will look at other effective temperatures, rotation periods, and viewing angles.

Imaging of Spots on

StarsFor decades, people have produced simple 2-component photometric models of large starspots (based on lightcurves).

At the next level, there are 3-component maps of stars (with dark and bright regions) based on Doppler imaging (which contains longitude and latitude information). In some cases, the magnetic field has been imaged as well. This can only be done for rapid rotators.

Parameters for Simple Photometric Modeling

In order to construct a simple model for the observed photometric variability, one needs several parameters:

1) Stellar rotation period (found by Kepler). This is of interest for characterizing the age of the star (and should match activity levels). Can we find photometric binaries?

2) Stellar inclination: found from rotation period plus rotation velocity [spectroscopic vsini] and stellar radius (we will have an estimate, which will get better if we get parallaxes).

3) Temperature contrast(s) for hot and cold regions4) Size and location of regions on star5) Time history of regions6) Viewing geometry (and dependence on this)

The question is how much of this can be reconstructed from a well-sampled, precise light curve, whose continuous nature can substitute at some level for Doppler information.

Modeling Solar Light Curves

Whitelight

Magneticfield

Solanki and co-workers

What Causes the Temperature Variations?

My immediate goal is to understand the detailed solar photometric variations in term of images of the Sun, to see what sort of features cause what sort of dips and bumps at various timescales.

Temporal Imaging of Stellar Activity

We will not have the same sort of information that Doppler imaging gets. Polar spots, for example, are always visible, so only their time variability can be seen. We will know whether and when a feature is carried across both limbs. If something appears but does not disappear when it should, it is at “high” latitude (knowledge of inclination indicates how high). If it disappears early, that is a dissolution of the feature. There will be ambiguity between longevity and location which cannot be fully resolved. How well can different features be separated? Solutions won’t be unique.We will have better luck characterizing the general level of stellar activity, activity cycles, and frequency of Maunder minima for different stellar masses and ages. The age calibration should get much better. We should also get good information on flare frequencies. Cooler stars will generally be more variable (contrast is better for chromospheric temperatures). We will also know how often “transit-like” events occur as a function of spectral type and rotation rate.