* Global helioseismology is the study of the resonant modes of
oscillation inside the Sun in order to infer the properties of the interior.
*

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.

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

**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.