Global Helioseismology Products
Global helioseismology is the study of the resonant modes of
oscillation inside the Sun in order to infer the properties of the interior.
Algorithm for calculation:
The global helioseismology pipeline begins with a spherical harmonic
decomposition. At present only dopplergrams are used as input, but other
observables will be added in the future. Images are remapped to a uniform
grid in longitude and sin(latitude), apodized, and an inner product is
taken with a set of target spherical harmonic masks, yielding a complex
amplitude as a function of spherical harmonic degree l and azimuthal order
m for each image, up to l=1000. For each l and m a timeseries is
constructed, typically of length 72 days, which are then detrended and
gapfilled. Fourier transforms of these detrended and gapfilled timeseries
are fitted to yield the mode parameters.
Keywords
All of the above dataseries have T_START as their first primekey, which is
slotted with a width of one day and an epoch of 1993.01.01_TAI-the MDI
epoch (see AllAboutJsocNames). The
exception is hmi.V_sht_gf_72d, which is a linked dataseries created for
browsing. It has a slot width of 72 days so that one may search on any
date and find the timeseries containing that date. When applicable, the
next two primkeys will be LMIN and LMAX, although these will be equal for
all timeseries and power spectra beyond hmi.V_sht_1d (they are retained
for consistency among the dataseries). The last primekey will always be
NDT, the number of points in the corresponding timeseries. The gaps data
series have only T_START and NDT as primekeys, the rest have all four.
These and other keyword definitions are given in the following table:
- T_START (time)- the beginning of the time interval a record corresponds
to, specified as a date string or as an offset from the MDI epoch
1993.01.01_TAI
- LMIN (integer)- minimum spherical harmonic degree represented.
- LMAX (integer)- maximum spherical harmonic degree represented.
- NDT (integer)- number of time points represented.
- T_STEP (float)- length of a time step in seconds, always a constant. The
length of the timeseries is then NDT times T_STEP.
- T_STOP (time)- the beginning of the following timeseries, or T_START +
NDT * T_STEP.
- T_OBS (time)- the midpoint of a timeseries as given by (T_START + T_STOP)
/ 2.
- MAPMMAX (integer) - maximum azimuthal order m in the remapping, usually
constant.
- SINBDIVS (integer) - number of increments in sin(latitude) in the
remapping, usually constant.
- DATE (time) - the time at which the record was created
Available Products
- hmi.V_sht_1d (archived) - raw timeseries of length 1 day, chunked in l.
- hmi.V_sht - raw timeseries of any length with one l per record.
- hmi.V_sht_gaps - window functions before gapfilling.
- hmi.V_sht_gf (archived) - detrended and gapfilled timeseries.
- hmi.V_sht_gf_gaps - window functions after gapfilling.
- hmi.V_sht_gf_72d - a link to detrended and gapfilled timeseries of length 72 days (see below).
- hmi.V_sht_pow - power spectra of detrended and gapfilled timeseries.
- hmi.V_sht_modes (archived) - ascii tables containing the mode parameters.
Modules
The global pipeline consists of five modules and accompanying scripts:
jv2ts - takes dopplergrams as input and outputs timeseries chunked in l.
It provides the combined functionality of three historic modules: v2helio,
helio2mlat, and qdotprod.
jretile - input and output are timeseries, changes tiling in time and
chunking in l.
jtsfiddle - detrends and gapfills timeseries. output can be timeseries,
fourier transforms, power spectra, etc.
jtsslice - performs fourier transforms on a section of a timeseries to
save on I/O.
jpkbgn - extracts mode parameters from fourier transforms. currently input
are timeseries, output are ascii tables.
Joint Science Operations Center (JSOC)
