[1] It is believed that parameterizations of the photospheric magnetic field can be correlated with solar activity (Falconer et al., 2002 ; Leka and Barnes, 2003a and 2003b; Schrijver 2007; Mason and Hoeksema, 2010).
[2] Collecting, storing, tracking, and analyzing data in the number of full-disk vector magnetograms necessary to follow an active region as it crosses the disk is difficult.
SHARP stands for Spaceweather HMI Active Region Patch. A SHARP is a DRMS series that contains
(2) 31 data segments (described in detail below), including each component of the vector magnetic field, the line-of-sight magnetic field, continuum intensity, doppler velocity, error maps and bitmaps. The data segments are not full-disk; rather, they are partial-disk, automatically-identified active region patches.
T_REC
, and HMI Active Region Patch Number, HARPNUM
.
There are four SHARP DRMS series:
[1] hmi.sharp_720s
: definitive data with segments in CCD coordinates wherein the vector B has been decomposed into azimuth, inclination, and field,
[2] hmi.sharp_cea_720s
: definitive data wherein the vector B has been remapped to a Lambert Cylindrical Equal-Area projection and decomposed into B_radial, B_phi, and B_theta,
[3] hmi.sharp_720s_nrt
: near real-time data in CCD coordinates wherein the vector B has been decomposed into azimuth, inclination, and field,
[4] hmi.sharp_cea_720s_nrt
: near real-time data wherein the vector B has been remapped to a Lambert Cylindrical Equal-Area projection and decomposed into B_radial, B_phi, and B_theta.
This page describes the methodology for creating SHARPs and uses an example results for NOAA Active Region 11429, which produced an X5.4-class flare on 7 March 2012.
The HARPNUM of this region is 1449.The SHARP data series contain the HMI team's current best effort attempt to determine the vector magnetic field and uncertainties and the data provide a great deal of information. However, the instrument has known limitations due to its spatial and spectral resolution. There are a variety of known and unknown systematic errors and limitations induced by the orbital and other factors. The inversion and disambiguation methods are also known to be far less than perfect. So, please use these data carefully and examine them critically.
See Vector Magnetic Field for more documentation.
Please contact any member of the Vector Field Team with questions or concerns. We welcome your comments and suggestions.
The HMI pipeline analysis code automatically detects
active regions in photospheric line-of-sight magnetograms and
photograms. This code (i) identifies a patch on the full-disk imagery
that encompasses each active region and (ii) tracks every active
region as it rotates across the face of the solar disk. One active
region crossing the face of the solar disk is referred to as an HMI Active Region
Patch (HARP) and is assigned a HARP number, similar in concept to
a NOAA Active Region number. (We depart from the NOAA Active Region
number because HARPs are detected before NOAA Active Region numbers are
assigned and because we identify many smaller magnetic regions at HARPs.
However, we include the NOAA Active Region number post-facto as FITS
keyword NOAA_AR
).
There are two kinds of HARPS: (i) near-real time HARPS and (ii) definitive HARPs. Near-real time HARPs are calculated as soon as possible; in this case, the heliographic bounding box for the active region can change from one instant in time to another. Definitive HARPs are calculated only after an active region has crossed the face of the disk or 5 days after an active region disappears (whichever comes first); in this case, the heliographic coordinates enclosed by the bounding box for the active region are constant over time. HARPs provide all of the required geometric and heliographic information needed to track active patches in HMI and other solar data sets. Note that HARPNUMs in the nrt and defnitive data series will be different.
The movie below (courtesy Mike Turmon) shows near-real time HARPs crossing the face of the solar disk in March 2012 (NOAA AR 11429 corresponds to HARP Number 1449):
Definitive HARP data are available at hmi.Mharp_720s
; near real time (nrt) HARP data are available at hmi.Mharp_720s_nrt
.
The HARP analysis code defines a rectangular (in CCD coordinates)
bounding box, which contains a smooth bounding curve. The bounding box,
which is centered at the flux-weighted centroid, is defined as a HARP;
the area within the bounding curve is defined as an active region. This
information is available within the BITMAP
segment of the SHARP data series.
The HMI Stokes I, Q, U, and V data are inverted within the HARP bounding box by using the Very Fast Inversion of the Stokes Vector (VFISV) code, which assumes a Milne-Eddington model of the solar atmosphere, to yield the vector magnetogram and other plasma parameter data Borrero et al, 2011).
The azimuthal component of the vector magnetic field is disambiguated
using a minimum energy method to resolve the 180° ambiguity
(Metcalf,
1994; Leka
et al., 2009). Input parameters to the disambiguation module determine
the rate of minimization and number of reconfigurations attempted per
minimization step. These parameters are necessarily different for the
nrt and definitive data as the nrt data must be produced quickly. A
non-linear mask determines the noise thresholds as a function of solar
radius. (Pixels above this mask threshold can be identified in the
CONF_DISAMBIG
segment
of the SHARP series). Pixels below the threshold are smoothed during the annealing; pixels
above the threshold are not.
Spaceweather quantities (listed in Results[1], below) are calculated
every 12 minuets on the inverted and disambiguated data wherein
the vector B has been remapped to a Lambert Cylindrical Equal-Area
projection and decomposed into Bx, By, and Bz. Some of the spaceweather
quantities are calculated directly on the data (such as z-component
of the current density; for quantities that require a computational
derivative, we use a finite-difference method with a 9-point stencil)
and others require a potential field model (such as mean photospheric
excess magnetic energy density). The potential field is calculated
using Green’s functions and a monopole depth of 0.00001 pixels. Only
data that are the logical AND of (i) pixels within the smooth
bounding curve in BITMAP
and (ii) pixels above the
noise threshold (indicated by the coded values greater than 60 in
CONF_DISAMBIG
) contribute to the spaceweather indices.
Below is an example for nrt SHARP 606 at 2012.10.28_18:24:00_TAI; see
hmi.sharp_720s_nrt[606][2012.10.28_18:24:00_TAI]
.
The SHARP analysis code calculates the following spaceweather quantities for the region (keyword, quantity, units):
USFLUX
Total unsigned flux in Maxwells
MEANGAM
Mean inclination angle, gamma, in degrees
MEANGBT
Mean value of the total field gradient, in Gauss/Mm
MEANGBZ
Mean value of the vertical field gradient, in Gauss/Mm
MEANGBH
Mean value of the horizontal field gradient, in Gauss/Mm
MEANJZD
Mean vertical current density, in mA/m^{2}
TOTUSJZ
Total unsigned vertical current, in Amperes
MEANALP
Mean twist parameter, alpha, in 1/Mm
MEANJZH
Mean current helicity in G^{2}/m
TOTUSJH
Total unsigned current helicity in G^{2}/m
ABSNJZH
Absolute value of the net current helicity in G^{2}/m
SAVNCPP
Sum of the Absolute Value of the Net Currents Per Polarity in Amperes
MEANPOT
Mean photospheric excess magnetic energy density in ergs per cubic centimeter
TOTPOT
Total photospheric magnetic energy density in ergs per cubic centimeter
MEANSHR
Mean shear angle (measured using B_{total}) in degrees
Plots of several quantities computed for HARPNUM 1449 = NOAA AR 11429, appear at the bottom of this web page. One can plot any keyword value directly in JSOC lookdata
under the
graph tab.
Below are descriptions for each segment (segment name, units, description) in order of appearance in Lookdata.
The remapped CEA (cylindrical equal area) segments are slightly different (as noted) and only a subset of quantities are present.
MAGNETOGRAM
, [Mx/cm^{2}]
The magnetogram segment contains HARP-sized line-of-sight magnetogram data from the DRMS series hmi.M_720s
.
CEA version: the field is remapped, but not projected, i.e. it is still the line-of-sight component relative to HMI.
BITMAP
, [dimensionless]
The bitmap segment identifies the pixels located within the smooth bounding curve by labeling each pixel with the following:
1 : weak field outside smooth bounding curve
2 : strong field outside smooth bounding curve
33 : weak field inside smooth bounding curve
34 : strong field inside smooth bounding curve
CEA version: same.
DOPPLERGRAM
, [m/s]
The dopplergram segment contains HARP-sized dopplergram data from the DRMS series hmi.V_720s
.
CEA version: the Doppler velocity is remapped, but not projected, i.e. it is still the line-of-sight component relative to HMI.
CONTINUUM
, [DN/s]
The dopplergram segment contains HARP-sized dopplergram data from the DRMS series hmi.Ic_720s
.
CEA Version: same.
In order to solve the inverse problem of inferring a vector magnetic field from polarization profiles, the Very Fast Inversion of the Stokes Vector (VFISV) module solves a set of differential equations that fit the following parameters:
INCLINATION
, [degrees]
The inclination segment contains the magnetic field inclination with respect to the line-of-sight field.
CEA version: not included, See below
AZIMUTH
, [degrees]
The azimuth segment contains the magnetic field azimuth. Zero corresponds to the direction of a column of pixels on HMI’s CCD; values increase counter-clockwise.
CEA version: not included, See below
FIELD
, [Mx/cm^{2}]
The field segment contains the magnetic field strength. Currently, the filling factor is equal to unity, so this quantity is representative of the magnetic flux density. Values of 120 Mx/cm^{2} or less are generally considered to be noise.
CEA version: not included, See below
VLOS_MAG
, [cm/s]
The vlos_mag segment contains the velocity of the plasma along the line-of-sight. Positive means redshift. [Note: The dopplergram data are in m/s, whereas these data are in cm/s.]
CEA version: not included
DOP_WIDTH
, [mÅ]
The dop_width segment contains the doppler width of the spectral line, which is assumed to be a Gaussian.
CEA version: not included
ETA_0
, [dimensionless]
The eta_0 segment contains the center-to-continuum absorption coefficient. This is a measure of the difference between the absorption from the center and the continuum of the spectral line. (Differences arise because the center of the spectral line is formed at a different height in the atmosphere than the continuum of the spectral line. These quantities are not constant across the solar disk due to (i) thermodynamic variations and (ii) gradients in height.)
CEA version: not included
DAMPING
, [mÅ]
The damping segment contains the electron dipole oscillation approximated as a simple harmonic oscillator. This quantity is affected by the characteristics of the electromagnetic field. In the current version of the code, this parameter is constant and set to unity.
CEA version: not included
SRC_CONTINUUM
, [DN/s]
The src_continuum segment contains the source function at the base of the photosphere. In the Milne-Eddington approximation, the source function varies linearly with optical depth.
CEA version: not included
SRC_GRAD
, [DN/s]
The src_grad segment contains gradient of the source function with optical depth. By definition, src_continuum + src_grad = observed continuum intensity.
CEA version: not included
ALPHA_MAG
, [dimensionless]
The segment alpha_mag is defined as the portion of the resolution element that is filled with magnetized plasma. However, in the current version of the code, this parameter is constant and set to unity.
CEA version: not included
CHISQ
, [dimensionless]
The segment chisq is a measure of how well the profiles are fit in the least squares iteration. Chisq is not normalized.
CEA version: not included
CONV_FLAG
, [dimensionless]
This is currently not calcuated.
CEA version: not included
INFO_MAP
, [dimensionless]
The info_map segment identifies the quality index of the inversion output. The index at each pixel is defined as follows:
0x00000000 : Good pixel, with no severe issue.
0x0[0-6]00000 : Same as the convergence index in CONV_FLAG
0x08000000 : Bad pixel, defined as the same criteria as #5 of CONFID_MAP
.
0x10000000 : Low Q or U signal. sqrt((Q_0 + ··· + Q_5)^2 + (U_0 + ··· + U_5)^2) was smaller than 0.206 sqrt(I_0 + ··· + I_5 )
0x20000000 : Low V signal; |V_0| + |V_1| + ··· + |V_5| was smaller than 0.206 sqrt(I_0 + ··· + I_5 )
0x40000000 : Low B_los value. |Blos| from magnetogram algorithm was smaller than 6.2 Gauss.
0x80000000 : Missing data
CEA version: not included
CONFID_MAP
, [dimensionless]
The confid_map segment identifies the confidence index of the inversion output. The index value at each pixel will take the integer value from 0 (best) to 6(worst), defined as the highest item number satisfying the following conditions:
0: No issue found in the input Stokes
1: Signals for the transverse field component in the input Stokes parameters (Q & U) were weak.
2: Signal for the line-of-sight field component in the input Stokes parameters (V) was weak.
3: Magnetic field signals of both LoS and transverse component were weak.
4: The ME-VFISV iteration did not converge within the iteration maximum.
In the test data release, we set very small , thus, the confidence ε index at most
pixels may be 4.
5: If the difference between the absolute value of the line-of-sight field strength
derived from magnetogram algorithm and the absolute value of the LoS
component from VFISV inversion |B cos(inclination)| is greater than 500
Gauss, we regard the inversion did not well solve the problem.
6: One (or more) of the 24 input Stokes arrays had NaN value.
CEA version: same for nearest un-remapped pixel
The following segments contain standard deviations
and correlation coeffients that can be used to determine
the statistical errors of the vector magnetic field. As described in Section 4
of the AR 11158 Vector Magnetic Field, the calculated variances
and covariances are only reliable if the VFISV solution is close to an
absolute minimum.
CEA version: not included
INCLINATION_ERR
, [degrees]
AZIMUTH_ERR
, [degrees]
FIELD_ERR
, [Mx/cm^{2}]
VLOS_ERR
, [cm/s]
ALPHA_ERR
, [dimensionless]
FIELD_INCLINATION_ERR
, [dimensionless]
FIELD_AZ_ERR
, [dimensionless]
FIELD_ALPHA_ERR
, [dimensionless]
INCLIN_AZIMUTH_ERR
, [dimensionless]
FIELD_ALPHA_ERR
, [dimensionless]
INCLINATION_ALPHA_ERR
, [dimensionless]
AZIMUTH_ALPHA_ERR
, [dimensionless]
The following segments are outputs of the disambiguation module:
DISAMBIG
, [dimensionless]
The disambig segment identifies the type of disambiguation solution for each pixel by setting the following bits (if multiple bits are set, this
indicates that the various solutions agree). Currently the SHARP code uses the minimum energy solution to determine the disambiguation.
bit 1 : set if the minimum energy solution is used
bit 2 : set if the potential fiend solution is used
bit 3 : set if the radial acute solution is used
0 : set if the pixel was not disambiguated
CEA version: not included
CONF_DISAMBIG
, [dimensionless]
The conf_disambig segment identifies agreements between various types of disambiguation solutions by labeling each pixel with the following:
50 : not all solutions agree (weak field method applied)
60 : not all solutions agree (weak field + annealed)
90 : all solutions agree (strong field + annealed)
0 : not disambiguated
CEA version: same for nearest un-remapped pixel
The CEA magnetic field values are presented differently, as rectilinear components Br, Btheta, and Bphi at each grid point. Statistical errors are provided for each field component, but no cross-correlations are provided.
Bp
, [Mx/cm2]Bt
, [Mx/cm2]Br
, [Mx/cm2]Bp_err
, [Mx/cm2]Bt_err
, [Mx/cm2]Br_err
, [Mx/cm2]Below are plots of nrt and definitive spaceweather quantities calculated on the HARP-sized vector magnetogram data that encapsulated NOAA Active Region 11429. The plots are from March 4 to 8, 2012. The GOES X-Ray flux for this event peaked at 0:40 UT on March 7, 2012, corresponding to an X5.4-class flare. The flare peak is indicated in the plots by a yellow vertical line. Wide scatter in the spaceweather parameters at about ~8 UT on March 7, 2012 is due to poor data quality following an eclipse. Further below are plots representing the percent difference between the nrt and definitive quantities. The active region is rotating toward disk center; therefore, the scatter in the percent difference plots decrease with time as expected.