{\rtf0\ansi{\fonttbl\f0\fmodern Ohlfs;\f1\ftech Symbol;} \paperw9840 \paperh13060 \margl120 \margr120 {\colortbl\red0\green0\blue0;} \pard\tx960\tx1920\tx2880\tx3840\tx4800\tx5760\tx6720\tx7680\tx8640\tx9600\f0\b0\i0\ul0\fs20 Temporal Coordination of Helioseismic Observations\ R. S. Bogart, R. I. Bush, J. W. Harvey\ 1991.V1.6\ \ SOI Technical Note 024\ \ It appears highly probable that GONG will be making continuous helioseismic observations concurrently with most or all of the SOI observations. To facilitate intercomparison of data from the two projects, an effort should be made to assure that the observations are of coincident solar phenomena. Both experiments will use a free-running dynamical time synchronized with International Atomic Time (TAI), but the time of synchronization, i.e. the offsets from the TAI epoch of 1958.0, are still to be determined. It is the purpose of this note to establish the synchronization times for GONG and SOI.\ \ GONG will use time signals from the Global Positioning System or from the Omega system to independently synchronize planned one-minute integrations at each site to TAI. SOI, along with the other SOHO helioseismology experiments GOLF and Virgo, will use the SOHO on-board clock, which is maintained accurate to within 0.1 sec, synchronized with a daily pulse generated once every 86400 sec following a TAI time to be determined. Following agreement of the SOHO helioseismology working group, SOI and Virgo will commence one-minute integrations 30 seconds prior to the daily pulse. Other ground-based helioseismology experiments are likely to commence or center integrations on one-minute ticks of either TAI or Coordinated Universal Time (UTC). It is known that IRIS sites will operate at random phases and the data will then be resampled to a uniform UT grid (Gelly 1991); presumably it should be possible to resample at a phase approximately commensurate with SOHO and GONG, or precisely commensurate if a uniform TAI grid is used.\ \ SOHO will be in a halo orbit about the L1 Lagrangian gravitational equilibrium point approximately 4.99 light-seconds closer to the Sun than the center of the Earth (4.97 seconds closer than the subsolar point on the Earth surface). The halo orbit for the spacecraft has a solar distance range of about 1.3 seconds with a period of 0.5 year. We do not know the phase of this oscillation; it may depend on the exact launch date. It is also true of course that the distances of both the Earth and the L1 Lagrangian point from the Sun vary with a range of about 17 seconds with a period of 1 year due to the eccentricity of the Earth orbit. We also note that the solar radius (the time delay between signals from the limb and from the center) is about 2.3 seconds.\ \ The current value of \f1\fc0 D \f0 AT (TAI - UTC) is (as of June, 1991) 26 seconds. \f1 D \f0 AT tracks in a one-second stepwise fashion \f1 D \f0 T = TDT - UT = TAI + 32.184 s - UT (TDT = Terrestrial Dynamical Time; UT = Universal Time). The trend of \f1 D \f0 T predicted for the next few years suggests that values of \f1 D \f0 AT will probably increase from the current value of 26 sec to 28 sec in 1994 to 32 sec in 1998.(see Table 1 & Figure 1). This means that during the time period of interest one-minute integrations commencing on a TAI minute tick will be approximately centered on the UTC minute ticks, and vice-versa.\ \ We propose that:\ \ 1. GONG will begin 1 minute integrations 25 seconds prior to the TAI minute tick (TAI \fs18\dn8 [60] \fs20\dn0 = 35), i.e. approximately 3 to 7 seconds after the UTC minute tick (1994-98).\ \ 2. SOI, Virgo, and GOLF will begin 1 minute integrations 30 seconds before the TAI minute tick (as viewed by an observer equidistant from Earth and L1), i.e. approximately at the UTC minute tick (1996) and 5 sec before the beginning of the GONG integration.\ \ 3. To accomodate the current plan of commencing SOHO helioseismology one-minute integrations 30 sec prior to the daily pulse, SOI will formally request that the SOHO daily pulse occur at the TAI minute tick.\ \ 4. Should GONG or SOI at some point adopt a different integration time, a reasonable effort will be made to center integrations concurrently (as viewed from inside L1) at 5 TAI minutes ticks or at ticks on the lowest practicable multiple of 5 minutes.\ \ TABLE 1\ Observed and Predicted Differences Between Atomic Time and Earth Rotation Time\ (Predictions by D McCarthy, 1991, U. S. Naval Observatory; predictions of TAI - UTC are only guesses as to when leap seconds will occur)\ \ DATE TDT-UT1 ERROR TAI-UT1 UT1-UTC TAI-UTC\ \ 1985.0 54.34 22.16 22\ .5 23\ 1986.0 54.87 22.69 23\ .5 23\ 1987.0 55.32 23.14 23\ .5 23\ 1988.0 55.82 23.64 24\ .5 24\ 1989.0 56.30 24.12 24\ .5 24\ 1990.0 56.86 24.68 25\ .5 25\ 1991.0 26\ .25 57.779 0.009 25.595 0.405 26\ .5 57.956 0.019 25.772 0.228\ .75 58.102 0.027 25.918 0.082\ 1992.0 58.310 0.040 26.126 -0.126\ .25 58.721 0.214 26.537 -0.537\ .5 58.954 0.284 26.770 0.230 27 ?\ .75 59.132 0.361 26.948\ 1993.0 59.383 0.445 27.199\ .25 59.629 0.536\ .5 59.858 0.633 27.674 0.326 28 ?\ .75 60.034 0.837\ 1994.0 60.280 0.942 28.096\ .25 60.521 1.051\ .5 60.745 1.165 28.561\ .75 60.917 1.283\ 1995.0 61.161 1.405 28.977 0.023 29 ?\ .25 61.401 1.531\ .5 61.626 1.661 29.442\ .75 61.798 1.795\ 1996.0 62.044 1.933 29.868 0.132 30 ?\ .25 62.286 2.074\ .5 62.512 2.219 30.328\ .75 62.687 2.367\ 1997.0 62.934 2.519 30.750 0.250 31 ?\ .25 63.176 2.674\ .5 63.404 2.832 31.220\ .75 63.579 2.994\ 1998.0 63.827 3.158 31.643 0.357 32 ?\ .25 64.071 3.325\ .5 64.300 3.495 32.116\ .75 64.477 3.668\ 1999.0 64.726 3.844 32.542 0.458 33 ?\ .25 64.973 4.021\ .5 65.204 4.202 33.020\ .75 65.385 4.385\ 2000.0 65.638 4.570 33.454\ .25 65.887 4.757\ .5 66.121 4.946 33.937 0.063 34 ?\ .75 66.305 5.137\ \ REFERENCES\ \ Gelly, B. 1991. "The IRIS Data Management" (Proc. 2nd IRIS Workshop, Tashkent, Uzbekistan SSR, USSR, Sept. 1989) Solar Physics 133:31-42.\ \ McCarthy, D. 1991. Private communication }