Instruments For Observing The

The only way to study the Sun from the Earth is by examining the solar quantities that reach us, which includes particles, magnetic fields, and electromagnetic radiation. In addition, the action of the Sun's gravity on other bodies in our solar systems may be used to infer the solar mass. There is a great deal to learn from the electromagnetic radiation. The visible light and its directional variation is the foremost source of information on what happens on the Sun. It is through the visible light that we can see the Sun directly with our own eyes1 or by photographs.

1 It is harmful to stare directly at the Sun!

Telescopes were therefore the first instrument that were used for solar studies. During total solar eclipses, it is possible to observe the coronal structures directly. Past observations also have indicated that the solar diameter is not quite constant, but may vary as a result of solar activity. Heliography involves photographic studies of the Sun, and has been used to record sunspots. This observational technique reveals the spatial structures on the Sun.

2.2.1 Measuring the total solar irradiance

One instrument for measuring total radiative energy flux is the bolometer (Fleagle and Businger, 1980). A water-cooled black target has often been used to estimate the total solar irradiance (TSI) (also known as the "Solar constant"2), which is estimated from the energy budget of the black plate and the cooling system. Satellites have been recording the TSI since the late 1970s when a solar irradiance monitoring system finally was put into orbit.3 There have been a series of different satellites measuring TSI4 starting from 1978. Frohlich and Lean (1998a) attempted to merge the various readings from the different satellites. The differences between the different satellites suggest that the TSI estimates from different satellites are not exactly the same and that these may be in the range 1365-1373 W/m2 (see Figure 2.1). There is some debate as to whether the TSI level is the same during the two sunspot minima or whether the TSI at solar minimum has increased. Willson (1997) has suggested a trend of 0.036% (0.09 W/m2) increase per decade.

2.2.2 There is more than total irradiance

There is more information about the Sun carried away by the electromagnetic radiation than the total energy. The radiation has several properties which are dependent on the solar conditions during the emission of light. The electromagnetic waves have an amplitude (or photon density associated with the light intensity), a wavelength (which in terms of photons reflect the energy of each particle), and polarisation. The effect of the Sun's gravitation on the objects in our solar system can also be used to estimate the solar mass.

Temperature is a bulk measure of the vibrational energy of the atoms. The wavelength of light's so-called continuous spectrum, is associated with the black body radiation which is dependent on the Sun's temperature. The simplest way to picture thermal radiation is to consider electric fields extending from electric charges. Electric charge is one of the four elementary physical quantities that we know, the others being to mass, time, and distance. All physical concepts may be explained in

2 This label is a misnomer.

3 There was a lot of debate about whether this project really was necessary, as everyone "knew" that the "solar constant'' was constant. It is only in hindsight that we know the value of these observations.

4 HF/ERB on NIMBUS-7, ACRIM I & II on SMM and UARS, the solar monitor on ERBS, SOVA2 on EURECA, and VIRGO on SOHO.

TSI measurements from satellite

TSI measurements from satellite

1980 1985 1990 1995 2000

date

Data source: NASA (ftp://daac.gsfc.nasa.gov/)

Figure 2.1. Comparison between four different satellite-based TSI measurements. Data from NASA's Climatology Interdisciplinary Data Collection.

1980 1985 1990 1995 2000

date

Data source: NASA (ftp://daac.gsfc.nasa.gov/)

Figure 2.1. Comparison between four different satellite-based TSI measurements. Data from NASA's Climatology Interdisciplinary Data Collection.

terms of these entities. For instance, Einstein showed in his theory of special relativity that mass and energy are related by E = me2, and are in fact two sides of the same thing. When charge accelerates (oscillates or vibrates) then the electric field is disturbed, and moving charges produce magnetic fields (a relativistic effect). In a "classical physics interpretation'', the perturbation of the fields is not instant everywhere, but the disturbances propagate at the speed of light, c, and hence take the form of a wave. Materials with absolute temperature above zero are made up of atoms and molecules that vibrate, and the charged protons and electrons follow the motion of the atoms. Thus, the electromagnetic radiation associated with these oscillating charges reflects the temperature. In Section 3.2.5.1 the total solar irradiation is used to infer the temperature of the solar surface.

The particles do not all vibrate with the same energy and frequency, but some oscillate more vigorously than others. There is a spread in individual energies which follows a common statistical law describing random motion. By assuming a

Gaussian distribution5 in the vibrational energies of the atoms, it is possible to derive a spectral distribution law (Planck's law) of emitted thermal emission as a function of temperature.

On top of the continuous spectrum are spectral lines associated with narrow wavelength bands, which are a result of atomic transition from an unstable energetic state to a more stable state. The latter type of radiation is known as line emission (Fraunhofer lines), and is a central topic in atomic and quantum physics. Each element is associated with line emissions of different wavelength, and by examining the line spectra it is possible to learn about which elements are present in the Sun. It is possible to decompose light into its spectral components by passing it through a prism. Light with a short wavelength bends more than light with a longer wavelength. Other spectral techniques include diffraction gratings. The study of the different wavelengths is known as spectroscopy.

Because of the overlap between the two types of emission, one may get a wrong temperature estimate unless both types are taken into account. There are two ways of inferring the black body continuum: (i) by measuring the continuum intensities between the spectral lines ("Continuum windows'') or (ii) broad spectral band measurements with line spectrum corrections (Maltby, 1992)

2.2.3 Spectrography and polarisation

Instruments that observe the solar spectrum are based on spectrography, which measures the intensity of light as a function of its wavelength. When electrons change energy states (AE), they may emit or absorb light of a wavelength (A) according to AE = hv = hc/A (line emission). In addition to depending on the atomic energy levels, the line spectra are also affected by the presence of magnetic fields during the line emission. A quantum physical theory, known as the Zeeman effect, says that the electrons bound by an atom can only have a spin with the axis oriented in certain directions. The presence of the magnetic field thereby modifies the atomic energy levels, usually splitting these into three levels: a Zeeman triplet. There is, however, also more complicated Zeeman splitting, or in some cases, the magnetic field has no effect on the atomic energy levels. There is furthermore the inverse Zeeman effect, where light is absorbed by matter in the presence of a magnetic field, rather than being emitted. The Zeeman effect and the inverse Zeeman effect have different effects on the line spectra. Instruments for measuring the Zeeman split include the magnetograph.

By studying the Zeeman splitting of Fraunhofer lines6 it is in principle possible to learn about the solar magnetic field. The Zeeman splitting is illustrated schematically in Figure 2.2.

The Zeeman triplet consists of two or three components, depending on the direction of the magnetic field, H, compared to the observer. When looking straight into the field (Figure 2.2a) there are two components which have the wave-

5 Also called normal distribution.

6 Named after the discoverer.

Left

Right c

(a) Longitudinal field

(b) Transverse field

Figure 2.2. A schematic showing the three components in a Zeeman triplet when viewed into the beam (a) and perpendicularly to the beam (b). The horizontal axis is wavelength.

lengths A0 — SAH and A0 + SAH, often referred to as av and aR. By looking in a direction normal to the field (transverse field), three components can be seen, with the wavelengths A0 — SXH, A0, and A0 + SXH. The two symmetric components are av and aR, and the central line is called the k component.

The electromagnetic wavelengths may also be affected by the relative motion between the emitting material and the observer due to the Doppler effect. One example of the Doppler effect that probably most readers are familiar with is the change of pitch heard when an approaching train passes an observer. Likewise, the frequency of light is shifted when the object moves with respect to the observer. The apparent frequency, f', can be expressed in terms of the true frequency f0, the phase speed of the waves (often the speed of light or sound), and the velocity of the observer and object. If the observer is moving away from the wave source at a speed vo, then this apparent frequency f ' = f0(c — vo)/c (vo is negative if the observer is approaching the source), but if the source is moving away from the observer (vs) then the appropriate expression is f '= f0c/(c + vs). The Doppler effect will affect all the wavelengths equally and is not influenced by SAH. Through the study of the Doppler shifted light, it is possible to estimate the velocities of the solar material. One example is the discovery of the motion in the light sunspot regions, known as the Evershed effect.

The polarisation of the light emitted from the Sun may hold some information about the Sun. For instance, the components of the Zeeman triplets have different polarisation. In the longitudinal field, the av and aR are left- and right-handed circularly polarised respectively. The av and aR components in the transverse case are transversely polarised with respect to the magnetic field, whereas the k component is polarised parallel to the magnetic field.

2.2.3.1 The magnetic field associated with sunspots

The strong magnetic fields in the sunspots give rise to Zeeman splitting of the line spectra. It is usually assumed that the lines examined are Zeeman triplets, however, more complicated forms of Zeeman splitting may also take place. In the sunspots, it is the inverse Zeeman effect, caused by the absorption of light under the influence of a magnetic field, which is believed to dominate. The measurable quantities from light emitted from the sunspot regions are the wavelength, the line intensity, and the polarisation of light. The relation between the intensities of the three lines of a transverse field Zeeman triplet is IaR : : Iay = - (1 — cos2 7) : 4 sin2 7 : 4 (1 — cos2 7) (Bray and Loughhead, 1964). The magnetic field in the sunspot is assumed to be uniform. The shift in the wavelength from its real value, A0, due to the Zeeman effect is:

Plate analysers may be used to examine the polarisation of the light and Figure 2.2 shows how each line in the triplet is polarised. It is in principle possible to infer the strength and direction of the magnetic field from these quantities, given an idealised situation where there is only an inverse Zeeman effect (only absorption under the influence of a magnetic field, and no emission), and the triplet is dominating over all other forms. The magnetic field must also be uniform and not change with depth.

2.2.3.2 Potential problems associated with spectrography

In practice, spectrographs have in the past been limited by low spatial resolution and intensities, and long exposure time is often required. Because the observations are influenced by fluctuating atmospheric conditions, there may be need for "seeing correction". Contaminating stray light from scattering in the atmosphere may also introduce errors in the observations. Light may furthermore be subject to undesired partial polarisation by the mirror arrangement, and it is therefore important to carefully design the instruments to minimise these effects. It is usually assumed that there is no interference from light absorbed or emitted in the path of the light between the Sun and the Earth, but if this assumption is false then the observations may be distorted. Recent space-borne instrumentation and technological advancements have improved the situation.

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  • sophia
    Why is it necessary to have a special instrument for observing the sun?
    8 years ago

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