The K-corona, F-corona and E-corona (and T-corona)
Written: May 25, 2005
The white light corona is composed of three (four) components, distinguished by the
mechanisms which produce the light.
They are labeled, respectively, the K(Kontinuerlich), the F- (Fraunhofer) and the E- (Emission) corona. These three
components are formed by very different mechanisms and have very different properties;
it has also become common practice to identify a fourth component, the T- (Thermal)
emission. The radial gradients of the K-and the F-components are such that the Kcorona dominates inside about 2.5 R☉, while the F-corona dominates beyond (i.e. in
C2 and C3, see fig below extracted from the book ‘The solar corona’)
The K-corona displays a continuous emission spectrum and is found to be strongly
polarized. It arises out of photospheric light that is scattered by the free electrons
(Thomson scattering, e.g. Billings, 1996, ‘A guide to the solar corona’) of the coronal
gas. The K-corona is thus dependent on the presence of coronal gas, but it is actually
photospheric light that is scattered toward us by the electrons in the corona. This light
would be 100 % polarized if the sun were a point source and the scattering occurred at an
angle of Pi/2 to the line of sight (i.e. in the plane of the sky). However, because of the
complicated geometry of the actual coronal structures the polarization of the K-corona is
considerably less than 100% and varies from place to place in the corona.
The F-corona, so-called because it shows the same dark absorption lines that Fraunhofer
found in the on-disk spectrum is actually unrelated to the corona per se. It arises out of
the scattering of the photospheric light by small dust particles in the ecliptic plane
(between the orbits of Mercury and Earth) and it may more accurately be thought of as
the inner zodiacal light. Thus, the F-corona is again not true coronal light, but a
reflection (literally) of the bright photospheric light.
The Fraunhofer spectrum is visible in the F-corona because the Doppler shifts due to the
velocities of the dust particles are small compared with the widths of the Fraunhofer lines.
They are therefore not significantly broadened in the scattering process and remain visible
in the light that reaches us. In contrast these lines are nearly invisible to detect in the Kcoronal spectrum, because the high temperature of the corona makes the thermal velocities
of the scattering electrons quite large (v/c > 0.01). The Fraunhofer lines in the scattered
light are therefore broadened enough to be “washed out”, although sensitive
instrumentation can be used to observe the remnant spectral lines.
The T-corona is caused by the thermal (largely infrared) emission of interplanetary dust,
usually the same dust that is causing the F-corona.
The E-corona represents the only component thus far mentioned that arises out of true
emission of light by the actual coronal gas (like EIT). It consists of emission in
isolated spectral lines formed by the high-temperature coronal ions. (By 1940, the
emission lines had been identified; this was one of the great triumphs of astrophysics.)
The separation of the F- and K-coronae:
Separating the different components means identifying the contributions from the
different parts of the corona. To analyze the coronal electrons, we must subtract the Fcorona and discard it. The intensity of the K-corona gives the coronal electron
density. The two following methods describe how special observations can be made in
order to obtain the desired corona (F versus K mostly):
1. The traditional polarization analysis, which measures the fraction of polarized
light (doing measurements at three different angles). The disadvantage of this
method includes the fact that the K-corona is not completely polarized, that the Fcorona has some polarization (a few % per several R☉) and that instrumental
polarization may distort the signal. The method is most accurate in the brightest
and mostly highly polarized parts of the corona, usually in the streamers. The
method becomes less useful at distances beyond one or two R☉, when the Fcorona matches the K-corona intensity.
2. A better method, though more difficult, is to compare the depth of the
Fraunhofer lines in the photospheric and coronal spectrum. It is called the
Grotrian method and is used with C1 observations. This method requires
spectroscopic observations.
Best literature consulted:
Book ‘The solar corona’ (from JF, Authors: Leon Golub and J.M. Pasachoff, Ch1, Ch5)
Extracting the K-corona from total brightness observations (like LASCO)
In the large regime of LASCO observations (to 30 Rsun), the advantages of polarized
Brightness (pB) over the total brightness (B) observations disappear because (1)
instrumental stray light levels are low and (more importantly) constant and (2) the
polarized contribution of the F-corona beyond 5 Rsun cannot be ignored. Thus any pBbased inversion technique needs to first remove the polarized F coronal contribution, just
as in inverting total B. To enhance the scientific return of white light coronagraphic
observations, it would be extremely useful if coronal density information could be
extracted from the total B observations as has been done with pB observations so far.
It is the lack of a dependable F-coronal model that precluded the use of B observations to
calculate electron density even though total B observations are much easier to obtain than
pB. Note that the computation of electron density for transient phenomena such as
CMEs does not depend upon the knowledge of an F-coronal model because the preevent corona, including the F-corona, is subtracted off before doing the density
calculation.
For C2, the background F-corona can be empirically modeled by a pixel-by-pixel
minimum over 56 days observations centered on the day of the image (Ref 1, 2001). This
background model represents well the time-invariant brightness arising from the F-corona
far from the disk while preserving the transitory structures of the K-corona. However,
close to the limb, especially at the poles and other areas of low brightness, this resulted in
a large disagreement with the pB observations. This indicated that some K-coronal signal
was being inappropriately subtracted out with the F-corona, containing long-lived coronal
features. It was empirically found that most of the stationary background signal close to
the occulter is due to the K-corona. To account for this, a slope correction was applied to
the background model. The slope was empirically determined by a linear fit that varies
between 90% of the minimum near 3 Rsun and 100% at the edge of the image (~6 Rsun).
1. Hayes, Vourlidas, Howard, 2001, ApJ 248
http://cdsaas.u-strasbg.fr:2001/ApJ/journal/issues/ApJ/v548n2/52619/52619.web.pdf