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The noise mask of the magnetic field strength is a full-disk map that represents the noise of magnetic field strength over the Sun's disk. The noise mask of the magnetic field strength is a full-disk map that represents the noise of magnetic field strength over the Sun's disk. The noise values at a given location are used in some versions of the disambiguation code to optimize performance.
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The noise mask is derived from a group of FD10 full-disk magnetic field strength data. It has been demonstrated that the noise of field strength varies over the Sun's disk, and the pattern of this noise variation changes with the orbital velocity (Vr) of HMI relative to the Sun. Thus it is necessary to generate Vr-dependent noise masks. The method is described in below. The noise mask is derived from a group of FD10 full-disk magnetic field strength data. It has been demonstrated that the noise of field strength varies over the Sun's disk, and the pattern of this noise variation changes with the orbital velocity (Vr) of HMI relative to the Sun. Thus it is necessary to generate Vr-dependent noise masks. The method is described below.
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 . Second, the median of the field strength for each on-disk pixel is calculated from the selected field strength data. Strong field pixels where field > 300.0 Gauss, roughly 3 sigma of field noise, are excluded.  . Second, the median of the field strength for each on-disk pixel is calculated from the selected field strength data. Strong field pixels where field > 300.0 Gauss, roughly 3 sigma of field noise, are excluded. About 40 maps are considered for each velocity range.
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Changes of instrument settings took place at several instants (see the list below). Consequently the noise appears to change slightly. To reflect such change, a noise mask need to be generated specifically for each time periods. The masks currently available in JSOC are generated using FD10 data from April-June 2012. Thus they can be used for data after 19 January 2012. Changes of instrument settings took place at several instants (see the list below). When the settings change, the noise appears to change slightly. To reflect such change, a noise mask need to be generated specifically for each time periods. The masks currently available in JSOC are generated using FD10 data from April-June 2012. Thus they can be used for data after 19 January 2012.  Based on comprison of samples at Vr=0 over a longer interval, no significant changes in the noise mask were observed to occur between instrument reconfigurations.
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 . For each time interval we save polynomial coefficients that can be used to reconstruct the noise mask for any velocity.  . For each time interval we save sets of polynomial coefficients that can be used to reconstruct the noise mask for any velocity.
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 . Run when the instrument characteristic change.  . A new mask is needed when the instrument characteristics change.
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===== Issues needing resolution ===== ===== Issues =====
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  * Not all time periods are ready   * Not all time periods are currently available.

Noise mask of magnetic field strength for the inverted full-disk vector magnetic field data

The noise mask of the magnetic field strength is a full-disk map that represents the noise of magnetic field strength over the Sun's disk. The noise values at a given location are used in some versions of the disambiguation code to optimize performance.

The noise mask is derived from a group of FD10 full-disk magnetic field strength data. It has been demonstrated that the noise of field strength varies over the Sun's disk, and the pattern of this noise variation changes with the orbital velocity (Vr) of HMI relative to the Sun. Thus it is necessary to generate Vr-dependent noise masks. The method is described below.

  • First, the field strength data are sorted by the orbital velocity Vr. For example, for velocity Vr_center, the field strength data selected have a velocity in [Vr_center - delta_Vr, Vr_center + delta_Vr]. Currently delta_Vr = 50.

  • Second, the median of the field strength for each on-disk pixel is calculated from the selected field strength data. Strong field pixels where field > 300.0 Gauss, roughly 3 sigma of field noise, are excluded. About 40 maps are considered for each velocity range.

  • Finally, a 15-order 2D polynomial is performed to fit the median. The polynomial is the Chebyshev polynomials of the first kind. The fitted polynomial is the noise mask for Vr_center. The coefficients of the polynomials, which are used to reconstruct the noise mask, are archived in JSOC with a series name of su_yang.cheby_coef. The prime key is Vr.

Changes of instrument settings took place at several instants (see the list below). When the settings change, the noise appears to change slightly. To reflect such change, a noise mask need to be generated specifically for each time periods. The masks currently available in JSOC are generated using FD10 data from April-June 2012. Thus they can be used for data after 19 January 2012. Based on comprison of samples at Vr=0 over a longer interval, no significant changes in the noise mask were observed to occur between instrument reconfigurations.

Inputs
  • During each time interval approximately 40 ME computations made for each interval in velocity.

Outputs
  • For each time interval we save sets of polynomial coefficients that can be used to reconstruct the noise mask for any velocity.

Cadence
  • A new mask is needed when the instrument characteristics change.

  • Time Intervals

    • Time

      Instrument Status

      2010.05.01

      Initial HMI Configuration

      2010.12.13_19:47

      Change tuning from 500 to 600

      2011.07.13_18:35

      Change tuning from 600 to 620

      2011.07.13_18:40

      Change exposure time from 120ms to 125ms

      2012.01.18_18:15

      Change tuning from 620 to 640

Archive
  • The coefficients of the polynomials used to reconstruct the noise mask are archived in JSOC2 in the series su_yang.cheby_coef. The prime key is Vr.

Options
  • N/A

Noise Mask Reconstruction

  • Figure showing NOISE MASK vs Time

  • Description of module to reconstruct noise mask vs velocity

Issues
  • Not all time periods are currently available.
  • It takes some time after changing the instrument to reconstruct a new mask

Responsible scientist
  • Yang Liu

JsocWiki: MagneticNoiseMask (last edited 2013-05-01 04:35:27 by localhost)