Vector magnetographs

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  • Vector Magnetographs
definition
  • The MSFC magnetograph was developed in the early 1970s and has been operating as a vector system since 1976 in support of NASA space missions (Solar Maximum Mission and Spacelab), rocket and balloon experiments, and joint ground-based observations. The magnetograph consists of a symmetric 30-cm Cassegrain system which has a field of view of 6 x 6 arc-min giving an effective pixel size of 2.8 x 2.8 arc- sec in the 2 x 2 pixel-binning mode of the CCD camera. The absorption line Fe I 5250.2 is selected by a Zeiss birefringent filter (FWHM = 0.125A). The polarimeter consists of quarter-wave plates, a KD*P electro-optical modulator, and an analyzer provided by the sheet polarizers in the Zeiss filter. The resulting magnetic field sensitivities are 5 and 125 G in the line-of-sight and transverse components, respectively, with a temporal cadence of 5 min. The Zeiss filter can be tuned +/- 8 Angstroms about the Fe I 5250.2 line. This allows the three Fe I lines at 5247.0, 5250.2, and 5250.6 to be selected for performing flux tube analyses. Information on the method of calibration used for the interpretation of MSFC vector magnetograph data can be found in NASA TM-4048, 'The SAMEX Vector Magnetograph - A Design Study for a Space- Based Solar Vector Magnetograph' (M. J.Hagyard, G. A. Gary, and E. A. West, 1988). The capabilities of the MSFC vector magnetograph were extended in September 1989 by the addition of a coaligned H-alpha telescope with a CCD camera. This telescope was the engineering-backup model of the Skylab ATM H-alpha 1 telescope. The system is a 17-cm Cassegrain which feeds a Fabry-Perot filter with a 0.7 A bandpass. The H-alpha system provides video images of chromosphe- ric activity that are cospatial and cotemporal with the vector magnetograms. Research programs carried out with the coaligned instruments include studies of magnetic morphology, evaluation of magnetic shear at flare sites, calculation of vertical electric currents, analysis of magnetic field submergence/emergence, studies of magnetic canopies, evaluation of magnetic energy, coronal field extrapolations, Stokes profile analysis, and the study of magneto-optical effects. Because the MSFC magnetograph is a dedicated system, the MSFC team can observe each day that weather permits. The observational data are reported in: 'The MSFC Solar Observatory Report,' (published twice a year and distributed to the solar community). For further information on the MSFC Solar Vector Magnetograph, link to the web at "http://science.msfc.nasa.gov/ssl/pad/solar/magmore.htm"
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Abstract from DBPedia
    A vector magnetograph is a type of imaging telescope that can estimate the 3-D vector of the magnetic field on a distant body with a resolved line spectrum. Magnetographs are useful for studying the Sun because the surface magnetic field is important to the creation and maintenance of the solar corona, and gives rise to the phenomena of solar flares and space weather. Vector magnetographs measure the longitudinal (line-of-sight) component of the magnetic field separately from the transverse (image-plane) components, using different aspects of the Zeeman splitting that affects the wavelength of emission and/or absorption spectral lines in the presence of a magnetic field. The Zeeman splitting is caused by the fact that individual atoms are magnetized due to the circulating motion of electrons bound to them. Emission or absorption of a photon changes the magnetic moment of the atom. In a magnetic field, photons emitted with different polarizations gain or lose energy depending on their orientation relative to the surrounding magnetic field, changing the characteristics of the spectral line—some polarization components are blue-shifted or red-shifted relative to the line's reference wavelength, by a factor proportional to the field intensity. Specifically, the circular-polarized component of the light is shifted in wavelength proportional to the field strength in the direction of the observer, and the wavelength shift of the vertical and horizontal linearly-polarized components measures the field strength in those directions. A vector magnetograph works in a very narrow waveband around a single spectral line, for example the 525.02 nm 'Fe I' line from neutral (non-ionized) iron. The measured shifts in wavelength are fractions of a picometre. Measuring the full spectral profile of the line with this precision requires a high-dispersion spectrograph and a long time to collect sufficient photons to make the measurement with precision. For example, SOLIS requires about an hour to gather polarized spectral profiles over the whole Sun, and Hinode, the recently launched spacecraft with a 0.5-meter solar telescope on board, takes about an hour to cover a 164-arcsecond-square field (1% of the Sun) at very high spatial resolution. Other types of magnetograph use narrowband filter imaging to produce a measurement of the first few moments of the spectral line, and operate much more quickly: the HMI instrument on board the Solar Dynamics Observatory will produce a vector magnetogram every few minutes. The splitting effect is antisymmetric along the line-of-sight, but symmetric transverse to the line of sight, so the transverse component of the field can only be measured up to a factor of -1: there is a 180° ambiguity in vector magnetograph measurements of portion of the magnetic field that is perpendicular to the line of sight of the instrument. Notable existing vector magnetographs include the at the Mees Observatory in Hawaii, at Udaipur Solar Observatory, India, the SOLIS instrument at the National Solar Observatory (strictly speaking, SOLIS is a scanned spectropolarimeter), and the narrowband filtergraph instrument on the Hinode spacecraft. Planned instruments include a vector polarimeter at the Advanced Technology Solar Telescope slated to be built in the 20-teens, and the HMI instrument aboard the Solar Dynamics Observatory, launched in February 2010.

    (Source: http://dbpedia.org/resource/Vector_magnetograph)