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The Sun in Time: Activity and Environment 21
of such active stars show excessive amounts of plasma around 10 – 20 MK (Güdel et al., 1997b),
which incidentally is the typical range of solar flare temperatures. This again led to the suggestion
that the high-T emission measure is in fact due to the superposition of a multitude of temporally
unresolved flares (see Section 5.8).
4.2.2 Inferences from coronal density measurements
Comprehensive surveys of stellar coronal electron density (ne) measurements based on X-ray spectroscopic
line-flux ratios were presented by Ness et al. (2004) and Testa et al. (2004), including
a sample of active solar analogs. These studies concluded that the surface filling factor (derived
from the emission measure, the measured ne, and a realistic coronal scale height) of magnetic loops
containing cool X-ray emitting material increases from inactive to moderately active stars but then
“saturates” at levels of about ten percent. In the most active stars, hot coronal loops are added,
with a sharply increasing filling factor, thus probably filling the volume left between the cooler
coronal magnetic loops. Observations of rotational modulation in very active solar analogs suggests,
however, that the coronal volume filling remains significantly below 100% (see Section 4.2.3
below).
4.2.3 Inferences from rotational modulation
Inhomogeneous coronae may reveal signatures in light curves as the star rotates, although success
of this method has been moderate given that coronal features evolve on time scales shorter than one
rotation (e.g., owing to flares). Two young, near-ZAMS solar analogs have shown clear signatures
of X-ray rotational modulation (EK Dra, Güdel et al. 1995c, and 47 Cas B, Güdel et al. 1995a),
pointing to a filling factor below unity. This is unexpected because such stars are in the empirical
X-ray saturation regime that has often been suggested to be due to complete filling of the surface
with X-ray bright coronal magnetic loops (see Section 5.5 below). EK Dra also showed evidence
for radio rotational modulation. The depth and length of the modulation (Figure 4a) constrains
the X-ray coronal height, and also the electron densities to ne > 4 × 10 10 cm −3 , in agreement with
spectroscopic measurements (Ness et al., 2004). This leads to the conclusion that much of the
emitting material is concentrated in large “active regions”.
Figure 4: Two examples of X-ray rotational modulation in young, active solar analogs: Left (a):
EK Dra (Güdel et al., 1995c); right (b): the supersaturated young solar analog VXR45 (Marino
et al., 2003a). Both light curves are phase-folded (reprinted with permission).
Living Reviews in Solar Physics
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