In this section, I present a qualitative description of the chromosphere and its characteristics, prefatory to a similarly introductory definition of activity in Section 2.2.
The outer atmosphere of the Sun is invisible in white light, washed out by the brilliant radiation from the photosphere. Awareness of the Sun’s outer atmosphere developed during the 1700s and 1800s thanks to total eclipses, as observers began to take note of the extended corona, as well as the “red flames” we now call prominences and the pink ring of emission at the solar limb that we call the chromosphere (see Figure 1). These phenomena were confirmed to be of solar rather than terrestrial origin by the mid 1800s, and spectroscopic observations began with an eclipse visible in India and Malaysia in 1868. Observations were soon being made outside of eclipses as well, using prism spectrometers to image the Sun in narrow emission passbands. The early observations led promptly to the discovery in the solar spectrum of a new element, appropriately named helium, some two decades before it was discovered on Earth. Subsequent work led to the realization that spectra of both the chromosphere and the corona contain numerous emission features of high-temperature ionized species, indicating that temperature in the solar atmosphere, after dropping from ≈ 6500 K to 4400 K (and possibly down to 3800 K in places) through the photosphere, rises to an extended plateau at about 7000 K in the chromosphere, and then abruptly jumps to over 1,000,000 K in the corona. More fundamentally, the physical extent of the temperature rise is incompatible with thermal processes alone, so the escape of energy from the photosphere to empty space must involve mechanisms beyond simple equilibrium transfer of radiation.
The term chromo-sphere carries at least some implication that we are considering a “layer” of plasma overlying the photosphere, but it has long been known that the chromosphere is quite heterogeneous. Roberts (1945) published observations of “small spike” prominences, which he called spicules and which are both ubiquitous and evanescent features of the chromosphere, appearing and disappearing on timescales of minutes. They and their larger cousins, the macrospicules (Bohlin et al., 1975), are tightly collimated jets of plasma streaming upward through the chromosphere. Spicules are bright in Hα, giving the chromosphere its pink color, while the larger and hotter macrospicules are also prominent in extreme ultraviolet images (see Figure 2). Alongside these bright features, however, we observe widespread and extremely cool gas at chromospheric heights, revealed by the presence of CO bands and picturesquely described as a chromospheric “heart of darkness” (Solanki et al., 1994). These observations, along with others I will discuss later, put to rest any notion that the chromosphere constitutes a well-defined layer; rather, it is extremely inhomogeneous, variable on short and long timescales, and characterized by strongly confined and directed regions of hot plasma that suggest a complicated magnetic topology.
Small wonder, then, that researchers have struggled with the apparently simple question, what is a chromosphere? Numerous definitions have been advanced, variously based on height, temperature, physical processes, or some combination thereof. Reviewing this question herein will be a circular task, since activity itself defines the chromosphere to a significant extent, but for now let us consider Figure 3, from the definitive “early” model of the solar chromosphere (Vernazza et al., 1981). Height in the atmosphere increases to the left, and the lower x-axis shows the column mass density.
In this model, the temperature declines through the photosphere to a height of about 500 km, rises slowly to about 8000 K at 2000 km, and then rises sharply. For now, let us take a working definition of the chromosphere to be the regions of a stellar atmosphere (a) where we observe emission in excess of that expected in radiative equilibrium and (b) where cooling occurs mainly by radiation in strong resonance lines (rather than in the continuum as is mostly the case in the photosphere) of abundant species such as Mg ii and Ca ii. If we take the naive view that this defines a homogeneous, spheroidal shell in the solar atmosphere, then Figure 3 suggests that for the Sun, the chromosphere is roughly 1700 km thick and reaches perhaps 25,000 K in its uppermost part. As discussed above, the picture is considerably more complex, so our definition is intentionally worded to avoid speaking of parameters like heights and temperatures, but rather of two essential chromospheric physical processes, and giving no implication that these processes occur in a spatially uniform way.
Having developed at least an approximate definition of a chromosphere in Section 2.1, let us now define activity.
In a stellar atmosphere in radiative equilibrium (RE), energy transport through the plasma is purely by radiation, and any heat absorbed from the radiation field is balanced by the thermal emission of the plasma back to the photon flux, to maintain the outward flow of energy from the deep interior. It has been known for some time that the outward temperature rise one usually associates with a chromosphere can occur under special circumstances in radiative equilibrium (e.g., Cayrel, 1963; Auer and Mihalas, 1969; Skumanich, 1970), but also that emission reversals in prominent Fraunhofer lines such as Ca ii H & K are a sure sign of departures from RE; additional mechanisms of heating, generally termed activity, are required to explain the additional radiative losses in these and other lines. This activity takes two principal forms.
Babcock (1961) described a model by which a self-regenerating magnetic field could explain the principal features of visual and magnetic observations of the sunspot cycle, and since then the evolution and variability of solar and stellar magnetic fields has been found to account for much of what we observe as activity in the chromosphere and corona, via heating by Alfvén waves or transport of mechanical energy along the magnetic “conduits” into the outer atmosphere. Much of this review, therefore, will be concerned with our theoretical and observational understanding of the magnetic properties and behavior of stars.
An alternative form of activity was proposed by Biermann (1948) and Schwarzschild (1948), who discussed how the solar granulation, which occurs as myriad convective cells rise to the solar surface and release energy in the photosphere, could generate a continuous stream of acoustic waves that propagate into the outer atmosphere, heating it as they develop into shocks and dissipate. Dissipation of acoustic energy as a source of extra heating, and how it propagates into the outer stellar atmosphere, has since been widely explored.
This activity is not only central to the definition of a chromosphere, but drives its essential structure in the following qualitative way. As noted just above, in the Sun and stars like it, phenomena exist that dump mechanical energy into the atmosphere overlying the mainly neutral photosphere, causing heating beyond the expected RE values for the increasingly tenuous plasma. The plasma can balance the energetic books through a steadily increasing hydrogen ionization fraction as it warms from ≈ 5000 to ≈ 8000 K, which releases a large pool of electrons that allows the collisional radiative cooling that forms part (b) of our definition above. This happens over a relatively thick region, explaining the large extent of the chromospheres of Sun-like stars. Once hydrogen becomes fully ionized, however, the plasma loses this critical cooling mechanism; not surprisingly, this happens at the point near the left side of Figure 3 where the temperature rises rapidly from the chromospheric “plateau” to coronal temperatures.
“I’ll buy chromospheres for all types of stars,” said R.N. Thomas (in Jordan and Avrett, 1973, p. 48), and under some definitions one may indeed argue for a chromosphere in almost any star. However, the types of activity discussed above also strongly suggest in which stars we may expect to find chromospheres in the theoretical sense given in Section 2.1; i.e., an unexpectedly thick region of the stellar atmosphere characterized by non-radiative heating and cooling occurring predominantly in resonance lines rather than the continuum.
First, we expect to find thick chromospheres primarily in cool stars due to the structural considerations given in the previous section. Dissipation of excess mechanical heating can happen in cool stars via ionization of hydrogen as the plasma warms at increasingly large heights above the photosphere; hot stars with partially or highly ionized photospheres have already “used up” this electron pool at their visible surfaces, and thus cannot support the extended chromospheres we see in the cool half of the Hertzsprung–Russell (HR) diagram. Second, both the magnetic and non-magnetic sources of activity mentioned above imply the presence of surface convection, the former through its critical role in maintaining the magnetic dynamo via subsurface bulk mass transport, and the latter explicitly. In this review of chromospheric activity, therefore, I consider those stars for which a subsurface convection layer is present. This will occur in roughly in late A and cooler dwarfs, and in more massive stars as they leave the main-sequence and develop convective zones.
Observations amply support these theoretical arguments, and can be used to construct a “chromospheric HR diagram” showing rather precisely where Sun-like chromospheres are expected to be found, as well as illuminating important aspects of stellar structure and evolution.
Toward the thin convection zone limit, evidence has been found for chromospheric emission in dwarfs as hot as Altair, A7 IV-V (Freire Ferrero et al., 1995), and Simon et al. (2002) concluded from Far Ultraviolet Spectroscopic Explorer (FUSE) observations of a sample of A dwarfs that high temperature emission indicative of coronae, and by inference chromospheres, appears at about 8250 K. The chromospheres near this limit are of course quite weak; Simon et al. (2002) found emission for these stars to be at most a few percent of solar values. However, the onset of the emission appears to be abrupt and well determined, suggesting an equally abrupt transition from radiative to convective stellar envelopes at an effective temperature in good agreement with stellar structure models. These findings are also consistent with those of an earlier broad survey of C ii emission in solar-type stars (Simon and Landsman, 1991).
If the disappearance of a convective envelope implies the disappearance of a Sun-like chromosphere for hot stars, we might expect a similar change in behavior when the dynamo-generating interface between the convective zone and radiative interior disappears for fully convective, low mass stars. Initial investigations in this area focused on the so-called dMe stars, i.e., M dwarfs exhibiting Hα emission. An early, exhaustive survey by Joy and Abt (1974) suggested that dMe stars were ubiquitous beyond the point where full convection sets in (at about spectral type M5.5), but Giampapa and Liebert (1986), using deep echelle observations of 24 late M dwarfs, observed comparable numbers of dMe and non-dMe stars, and that the Hα emission in the dMe stars was correlated with kinematic class and, by inference, with age. The existence of an activity-age relationship for these stars implied that a rotation dependent dynamo was operating even in the fully convective limit. Fleming and Giampapa (1989) later followed up these observations with a Ca K survey of M stars, a much harder observational task due to the very low flux; the observations also suggested the presence of a chromosphere, albeit with increasingly inefficient non-radiative heating for redder spectral types. Recent semi-empirical models of M star atmospheres (Mauas et al., 1997) indicate the presence of a chromosphere even for “basal” (i.e., the lowest activity) M stars. Chandra observations have revealed quiescent coronal emission in the M8 dwarf VB 10 (Fleming et al., 2003) (and, very recently, in an L dwarf Audard et al., 2007). The similarity of the emission in VB 10 to that of the Sun’s quiet corona raises the interesting possibility that similar, non-tachocline dynamos operate in both the Sun and in extremely low-mass stars (and perhaps even brown dwarfs) where the principal cycle-generating dynamo cannot exist; it seems clear that significant magnetic flux is a pervasive component of M star atmospheres (Reiners and Basri, 2007).
The emphasis of this review is on “Sun-like” stars, but I also should make brief mention of the behavior of post main-sequence stars. Chromospheres are ubiquitous where subsurface convection zones are present, but Linsky and Haisch (1979), using some of the very earliest IUE data, discovered that emission we associate with transition regions and coronae was observed in giants of spectral type K1 and earlier, but not in later giants, possibly as a result of cool stellar winds. This division of giants into what Linsky and Haisch (1979) termed solar and non-solar giants initially seemed quite sharp, but more extensive samples revealed the existence of the so-called “hybrid” stars exhibiting evidence of both coronae and strong winds (Reimers, 1982; Judge et al., 1987). The division between coronal and non-coronal stars appears to apply only to giants, with all G and K giants with Mbol < –2 appearing to be X-ray sources (Reimers et al., 1996).
Rosner et al. (1995), in an exceptionally lucid Astrophysical Journal Letter, proposed that this overall behavior results from a change in the nature of the dynamo as a star evolves. Giants in the Linsky and Haisch (1979) solar-like category retain the large-scale dynamo we see in the modern Sun, leading to a generally Sun-like atmosphere and activity. (Along these lines, we have at least one textbook example from the long-term surveys, HD 81809, which comprises two G subgiants but exhibits a strong, well-defined 8.2 year activity cycle Baliunas et al., 1995; Hall et al., 2007b). As stars “cross” the dividing line, the activity becomes dominated by small scale fields with an open large-scale topology, permitting the development of massive winds but losing the large, closed magnetic structures associated with transition regions and coronae. Hybrid stars, which encompass a large range of coronal activity, appear to be in transition between the two stages.
Interestingly, one of the canonical “quiet” red giants, Arcturus, has been detected in X-rays by Chandra (Ayres et al., 2003), albeit with Lx / Lbol some 10–4 that of the Sun; the authors posit that the magnetic structures responsible for the emission may by the ancient giant analog of solar spicules, perhaps even responsible for driving the stellar wind itself. The details of giant star coronae are still poorly understood, but the recent observations make it clear that the magnetic nature that drives chromospheric activity on the Sun is retained by stars well past the end of their main-sequence lives.
Section 2.1: Students looking for a glimpse into the nature of scientific progress and debate can hardly do better than reading, cover to cover, the conference proceedings entitled Stellar Chromospheres (Jordan and Avrett, 1973). While the material is dated relative to our modern view of the chromosphere, the volume is exceptional both for its speakers’ outstanding presentations of the underlying physics, as well as the unusually lengthy and detailed discussion transcripts, which present singular insights from the leading workers of the day, as well as textbook examples of spirited but professional and humor-tinged debates about the fundamental issues. This may be contrasted with the acerbic salvos between Lockyer and Huggins, who launched chromospheres research by arguing bitterly about who was doing first and best at observing the Red Flames; compare it also with the general tenor of commentary, especially among the vox populi, when solar variability is examined for its influences on terrestrial climate.
A detailed examination of how the chromosphere, transition region, and corona may be defined (along with several other topics) can be found in the thorough review by Linsky (1980). Another useful review is given by Ulmschneider (1979); see also Ulmschneider et al. (1977) and subsequent papers in the series for treatment of the generation and propagation of acoustic waves in the solar atmosphere.
Section 2.3: Excellent reviews of the consensus that emerged following the watershed of IUE and Einstein observations in the late 1970s and early 1980s are given by Linsky (1985) and Simon (1986).
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