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Calibration Constant

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B W Lites – 1st expert on this subject based on the ideXlab platform

  • weak field magnetogram Calibration using advanced stokes polarimeter flux density maps i solar optical universal polarimeter Calibration
    Solar Physics, 2002
    Co-Authors: T E Berger, B W Lites

    Abstract:

    Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms’ with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map’ with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field’ plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data.

  • Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration
    Solar Physics, 2002
    Co-Authors: T E Berger, B W Lites

    Abstract:

    Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes  V / I `filter magnetograms’ with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map’ with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field’ plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm^−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. ^*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. ^†The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Akihiro Yamazaki – 2nd expert on this subject based on the ideXlab platform

  • The instrument Constant of sky radiometers (POM-02) – Part 1: Calibration Constant
    Atmospheric Measurement Techniques, 2018
    Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro Yamazaki

    Abstract:

    Abstract. Ground-based networks have been developed to determine the spatiotemporal
    distribution of the optical properties of aerosols using radiometers. In this
    study, the precision of the Calibration Constant ( V0 ) for the sky
    radiometer (POM-02) that is used by SKYNET was investigated. The temperature
    dependence of the sensor output was also investigated, and the dependence in
    the 340, 380, and 2200 nm channels was found to be larger than for other
    channels and varied with the instrument. In the summer, the sensor output had
    to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm
    channels and by 4 % in the 2200 nm channel in the measurements at
    Tsukuba (36.05 ∘  N, 140.13 ∘  E), with a monthly mean
    temperature range of 2.7 to 25.5  ∘ C. In the other channels, the
    correction factors were less than 0.5 %. The coefficient of variation
    (CV, standard deviation/mean) of V0 from the normal Langley method,
    based on the data measured at the NOAA Mauna Loa Observatory, is between
    0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas
    absorption was less than 1 % in the 1225, 1627, and 2200 nm channels.
    The degradation of V0 for wavelengths shorter than 400 nm ( − 10 %
    to − 4 % per year) was larger than that for wavelengths longer than
    500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from
    the reference POM-02 was 0.1 % to 0.5 %. Here, the data were
    simultaneously taken at 1 min intervals on a fine day, and data when the air
    mass was less than 2.5 were compared. The V0 determined by the improved
    Langley (IML) method had a seasonal
    variation of 1 % to 3 %. The root mean square error (RMSE) from the
    IML method was
    about 0.6 % to 2.5 %, and in some cases the maximum difference
    reached 5 %. The trend in V0 after removing the seasonal variation
    was almost the same as for the normal Langley method. Furthermore, the
    Calibration Constants determined by the IML method had much higher noise than
    those transferred from the reference. The modified Langley method was used to
    calibrate the 940 nm channel with on-site measurement data. The V0
    obtained with the modified Langley method compared to the Langley method was
    1 % more accurate on stable and fine days. The general method was also
    used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm)
    with on-site measurement data; the V0 obtained with the general method
    differed from that obtained with the Langley method of V0 by 0.8 %,
    0.4 %, and 0.1 % in December 2015, respectively.

  • the instrument Constant of sky radiometers pom 02 part 1 Calibration Constant
    Atmospheric Measurement Techniques, 2018
    Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro Yamazaki

    Abstract:

    Abstract. Ground-based networks have been developed to determine the spatiotemporal
    distribution of the optical properties of aerosols using radiometers. In this
    study, the precision of the Calibration Constant ( V0 ) for the sky
    radiometer (POM-02) that is used by SKYNET was investigated. The temperature
    dependence of the sensor output was also investigated, and the dependence in
    the 340, 380, and 2200 nm channels was found to be larger than for other
    channels and varied with the instrument. In the summer, the sensor output had
    to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm
    channels and by 4 % in the 2200 nm channel in the measurements at
    Tsukuba (36.05 ∘  N, 140.13 ∘  E), with a monthly mean
    temperature range of 2.7 to 25.5  ∘ C. In the other channels, the
    correction factors were less than 0.5 %. The coefficient of variation
    (CV, standard deviation/mean) of V0 from the normal Langley method,
    based on the data measured at the NOAA Mauna Loa Observatory, is between
    0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas
    absorption was less than 1 % in the 1225, 1627, and 2200 nm channels.
    The degradation of V0 for wavelengths shorter than 400 nm ( − 10 %
    to − 4 % per year) was larger than that for wavelengths longer than
    500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from
    the reference POM-02 was 0.1 % to 0.5 %. Here, the data were
    simultaneously taken at 1 min intervals on a fine day, and data when the air
    mass was less than 2.5 were compared. The V0 determined by the improved
    Langley (IML) method had a seasonal
    variation of 1 % to 3 %. The root mean square error (RMSE) from the
    IML method was
    about 0.6 % to 2.5 %, and in some cases the maximum difference
    reached 5 %. The trend in V0 after removing the seasonal variation
    was almost the same as for the normal Langley method. Furthermore, the
    Calibration Constants determined by the IML method had much higher noise than
    those transferred from the reference. The modified Langley method was used to
    calibrate the 940 nm channel with on-site measurement data. The V0
    obtained with the modified Langley method compared to the Langley method was
    1 % more accurate on stable and fine days. The general method was also
    used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm)
    with on-site measurement data; the V0 obtained with the general method
    differed from that obtained with the Langley method of V0 by 0.8 %,
    0.4 %, and 0.1 % in December 2015, respectively.

  • The instrument Constant of sky radiometer (POM-02), Part I: Calibration Constant
    , 2018
    Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro Yamazaki

    Abstract:

    Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of aerosols using radiometers. In this study, the accuracy of the Calibration Constant (V0) for the sky radiometer (POM-02) which is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels, and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba. In the other channels, the correction factors were less than 0.5 %. The accuracy of V0 from the normal Langley method is between 0.2 and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for shorter wavelengths was larger than that for longer wavelengths. The accuracy of V0 estimated from the side-by-side measurements was 0.1 to 0.5 %. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 to 3 %. The RMS error from the IML method was about 0.6 to 2.5 %, and in some cases, the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. The Calibration method for water vapor in the 940 nm channel was developed using an empirical formula for transmittance. The accuracy of V0 was better than 1 % on relatively stable and fine days. A Calibration method for the near-infrared channels, 1225, 1627, and 2200 nm, was also developed. The logarithm of the ratio of the sensor output can be written as a linear function of the airmass, by assuming that the ratio of the optical thicknesses between the two channels is Constant. The accuracy of V0 was better than 1 % on days with good conditions.

T E Berger – 3rd expert on this subject based on the ideXlab platform

  • weak field magnetogram Calibration using advanced stokes polarimeter flux density maps i solar optical universal polarimeter Calibration
    Solar Physics, 2002
    Co-Authors: T E Berger, B W Lites

    Abstract:

    Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms’ with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map’ with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field’ plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data.

  • Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration
    Solar Physics, 2002
    Co-Authors: T E Berger, B W Lites

    Abstract:

    Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes  V / I `filter magnetograms’ with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map’ with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field’ plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm^−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. ^*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. ^†The National Center for Atmospheric Research is sponsored by the National Science Foundation.