Icarus the Eclipse Spectrograph
Using Icarus Eclipse Spectrograph during totality
During a partial solar eclipse the moon gradually covers the solar disc. About ten seconds before the second contact (start of totality) the photospheric light decreases dramatically and via the spectrograph (using the proper solar filter!) the Fraunhofer absorption (dark) lines become visible. About five second before the second contact and in a short interval time, the emission lines from the Chromosphere are visible. These emission lines are called "Flash Spectrum'', because of the instant reversion of the absorption spectrum to the emission spectrum.
As the eclipse progresses towards to the maximum, the emission lines gradually disappear, because the moon covers the East limb of the solar chromosphere. Close to the eclipse maximum, only the green and the red emission lines of ionized iron from the corona are detected (see ISS and solar spectroscopy theory). The intensity of these coronal lines depends on the solar activity. During a low solar activity, the green coronal emission line has a low intensity, because of the lower coronal temperature. After the eclipse maximum and up to a few seconds after the 3rd contact, the chromospheric emission lines gradually appear on the West limb.
During totality, the spectrograph also records a ‘’continuum color background’’ of the Flash spectrum, because of the scattered ‘’photospheric continuum’’ light in the corona. After totality the solar photosphere become visible again and the spectrograph detects the strong photospheric light.
!!! ISS it is not designed for partial eclipse spectroscopic observation. During the partial eclipse DON’T USE the ISS!!! There is danger to damage your eye or your camera.
Solar Chromosphere and Corona Spectroscopy - Flash Spectrum
The Icarus Slitless Spectrograph analyzes the incoming light of the Chromosphere and of the Solar Corona during the total solar eclipses. These layers are visible only when the lunar disc covers at least 99.8% of the solar disc and between the 2nd contact and the 3rd contact. The strong light from the solar Photosphere gradually disappears as the lunar disc covers the Sun and the red Chromosphere and the white corona become visible. These layers emit light composed of different colors (wavelengths), which are analyzed by the ISS grating. Then the objective lens focuses the different colors into different places on the camera CCD or CMOS, as the Flash spectrum.
The solar Chromosphere has a characteristic red color because of the presence of hydrogen (H) atoms - gas. The hydrogen emits a characteristic red color (and also turquois and blue colors with lower intensities). There are also other neutral chemical elements in the chromosphere such as helium (He), magnesium (Mg), sodium (Na), calcium (Ca), Iron (Fe) etc. These elements also emit their ‘’personal’’ characteristic colors. These colors are also detected by the Icarus Slitless Spectrograph. Stronger color emission intensity means a higher concentration of this element.
The solar corona also emits different colors:
- The light of the solar photosphere is scattered in the solar corona just like the solar light is scattered in a cloud. The photospheric light includes all colors of the spectrum and it is called ‘’continuum light’’.
- There are also other color emissions: the solar Corona has an extremely high temperature between 1.000.000-3.000.000 Kelvin!!! (up to now we don’t know the mechanism that heats the corona in these high temperatures…). In these temperatures the elements of the corona, mainly iron, calcium, nickel etc., continuously lose some of their electrons, thus becoming ionized and reemit characteristic lights. In the visual part of the spectrum there are two strong emission colors (in spectroscopy we use the word ‘’line’’ instead of color): the red emission line of iron that has lost 9 electrons, in places where the Corona temperature is about 1.000.000 Kelvin, is called FeX and the green emission line of iron that has lost 13 electrons, in places where the temperature of the Corona is about 2.000.000 K, is called FeXIV. From the relative intensity of these coronal emission lines, we can measure the corona temperature and estimate the general activity of the Sun. In the corona, more rarely, we can detect emission lines from 14 times ionized calcium (CaXV - yellow coronal line), 12 times ionized nickel (Ni XIII) etc.
- There are also other lights emitted by the corona, but these can only be detected with very special instruments.