European Helio- and Asteroseismology Network (HELAS)
Local Helioseismology Network Activity
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TrackingThe small regions of the Sun useful for helioseismology are generally observed for some period of time and it is necessary to track the same region as it crosses the Sun.The Sun rotates with an average synodic period of 27.8 days, but this varies from the equator to the poles. Each tracking method is determined by the latitude of the region of analysis. Two are listed here: Differential rotation rate: specified.Tracks at the rotation rate of the specified latitude (lambda), given by the formula omega (lambda ) = a0 + a2 sin2(lambda ) + a4 sin4(lambda ). The coefficients of the rotation minus the sidereal Carrington rotation (360 degrees/25.38 days = 2.865 micro rad/sec) used for most of the tracking done on the SOHO-MDI data here are, a0 = -0.02893, a2 = -0.3441, a4 = -0.5037, corresponding to the Mt Wilson 1982/84 differential rotation rate. The values in Table 1, C=-0.02988, B=-0.16784, A=3.1459, are used to compute A, B, C (or a0, a2 and a4) as given in Equation 11. Note that the coefficent for A (or a0) which is the rotation rate at the equator is actually slower than the Carrington rotation rate (and therefore all latitudes will be slower than Carrington). The coefficients are arbitrary offsets to the Carrington. Lagrangian rotation rate: This tracks at the differential rotation appropriate to the instantaneous latitude for each point in the region, rather than at a common rate for the whole region. This can take considerable computation time depending on the area of observation. More information on solar rotation.
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