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 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.
- 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.
- 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). 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.
Inputs
- During each time interval a number of ME results taken at each velocity.
How Many?
Outputs
- For each time interval we save polynomial coefficients that can be used to reconstruct the noise mask for any velocity.
Cadence
- Run when the instrument characteristic change.
- Changes of the instrument:
- ||
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 JSOC with a series name of 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 needing resolution
- Not all time periods are ready
- It takes some time after changing the instrument to reconstruct a new mask
Responsible scientist
- Yang Liu
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