The most important assets of Solar Orbiter/PHI for helioseismology are:

  • Combine Earth-based, front-side observations with PHI observations from back-side (and if possible higher latitudes) to
    • be able to observe front and back side sun simultaneously for few days, which would strongly improve local helioseismology 
    • improve and calibrate the farside modeling based on helioseismology
    • Use PHI’s out-of-ecliptic observations to measure meridional flow at high latitudes

For local helioseismology, PHI would need to observe at 1 min cadence and good resolution. Only Dopplergrams are needed, i.e. 1 data product out of the 5 in PHI’s standard dataset.

Helioseismology observing strategies & remarks

  • The requirements for local helioseismology and feature tracking are a moderate spatial resolution (f-mode wavelength; a few Mm) with a temporal cadence of 60s.
  • For granulation feature tracking, one needs to resolve granules. The cadence should be 60s between a pair of observations, then a waiting time of 30 min can be introduced before recording the second pair of two 60s separated images. Onboard processing is possible to obtain the flow map.
  • For supergranulation tracking, images can be taken separated by 1h. Again, onboard processing is possible. Granulation needs to be suppressed, which requires averaging on board. Furthermore, the solar rotation needs to be taken into account. High-resolution images are best taken close to the disk center to minimize limb effects. 
  • Noise reduction requires long time series. Given the noise level that increases with latitude and the smaller number of features at the poles, the error on the horizontal flow speed goes up to >6m/s at the poles with a time series of 30 days length, i.e. the meridional flow might not be measurable at the poles. 
  • Compression can help reduce the data rates with little effect on time-distance helioseismology (JPEG) and local correlation tracking (quantization).
  • Studying the center-to-limb effect in local helioseismology could help understanding the physics of solar oscillation modes.
  • Different observing strategies are suggested in order to address the following science goals:

    • Near-surface differential rotation: for measuring differential rotation at high latitudes one set of observations would be helpful. This requires 30 days of data. Resolving the torsional oscillations requires repetition of measurements. These 30 days of data do not need to be recorded at once. Averaging of single power spectra obtained from observing several sequences of 10 days is possible. Both HRT and FDT could be used here: HRT is only necessary for granulation tracking but the two other techniques could be based on FDT data.

    • Near-surface meridional flow at high latitudes requires 120 days of data due to the low amplitude of the flow. Again averaging of data is possible, but this smears out cycle effects. Observations for this sub-objective make sense in the later phases of the mission as a minimum latitude of 15-20 degrees is required to start analyzing the poles (depending on B angle).

    • For studying convection at high latitudes, 7 days of data are needed for obtaining flow maps and useful power spectra. Weeks to months of data would be needed for statistical analyses.
    • Large-scale convective flows 

  • A possible orbit for all these measurements is MTP11 - 2024/01/01 - 2024/07/01, with Solar Orbiter at 45 degrees from Earth.
  • Furthermore, observations need to be done with FDT or HRT with 60 days of observation time: FDT at high latitude far from the Sun (0.7 AU) and HRT at high latitudes close to the Sun (0.4 AU).
  • In summary, observing times are the biggest constraint. As a high cadence of 60s is required for many science goals, the feasibility of the observing programs highly depend on the compression that can be applied. For each of the science objectives above, the best methods need to be defined as well as the most suitable compression.
  • Some of the helioseismology goals can be addressed with synoptic observations. PHI synoptic observations outside the RS windows could be helpful but this is not included in the current baseline of the mission operations.


Measurements and topology of the polar magnetic field

  • PHI is optimally used for measuring the solar polar magnetic field at maximum solar latitude and at minimum distance.
  • Co-observations should be done from Earth, i.e. at a large B0 angle in March and September when Solar Orbiter is observing the pole visible to Earth. September is preferable as observations are then possible from the solar telescopes on the Canary Islands. In the current planning (October 2018 Option E), a suitable orbit for this is MTP14 - 2025/07/01 - 2026/01/01.
  • These observations should be combined with other observables, especially EUI observing polar jets and SPICE.
  • Polar magnetic field useful publications include Tsuneta et al., 2008, ApJ 688, 1374 and Shiota et al., 2012, ApJ, 753, 157.


  • Magnetoconvection is present on the Quiet Sun and in active regions. 
  • The challenges are the various spatial scales that range from 100 km to 50 Mm, as well as the various temporal scales ranging from 1 min (small magnetic elements) to 1 week (active regions).
  • Observations from the ground are affected by solar rotation, i.e. the maximum observing time is 14 days. In addition projection effects influence the data.
  • Zeeman measurements exhibit 180° ambiguity in the azimuthal magnetic field component, which can be resolved by stereoscopic observations.
  • For this objective, it is important to use the good spatial resolution (pixel size of 110 km) and the large field of view 1000’’x1000’’ of PHI/HRT for combined observations between Earth-bases telescopes and Solar Orbiter near the almost co-rotating phases to allow long periods of AR tracking. This should enable studying the long-term behavior of active regions. The advantage of the almost co-rotation will allow observing many flares and follow the decay of active regions. 
  • Solar Orbiter's most important assets for magnetoconvection are:
    • longer observations of the same target due to near co-rotation phase close to the Sun,
    • Solar Orbiter is an observatory combining many remote sensing instruments,
    • combined with ground-based support or SDO, it will provide a new vantage point and allow stereoscopy.
  • It is suggested to use an observing program of 15 days during maximum and declining solar cycle 25 to follow up AR dynamics. 
  • An optimal orbit is MTP-07 (0.3 AU, 8-11.5 degree/day, latitude change of 15 degrees). MTP-10 would be fine too.
  • Another observing sequence should include the full remote-sensing package (PHI, EUI, SPICE) coordinated with DKIST and GREGOR. Observing targets should be quiet sun regions at the disc center, the limbs and polar regions. Possible orbits are the first orbit and later orbits of the mission with higher latitudes. The length of the time series should be 3h per pointing (telemetry intensive observations).


Other related goals

  • Measure solar oblateness and luminosity of the Sun. For this, rolls of the spacecraft are required, with a minimum of 8 positions, preferably close to the equatorial plane. This could be done during communication or instrument calibration spacecraft rolls.
  • Concerning the question whether a small-scale dynamo is acting on the Sun, the difficulty for observations is that the very convolved field is hard to resolve spatially. Solar Orbiter could look at the distribution of the magnetic field and the emergence rate per feature over latitudes. If emergence rate is independent of latitude, probably the feature is produced by local dynamo rather than local one.
  • Regarding the solar irradiance, Solar Orbiter has no measurements of the Sun's luminosity. However, it can help obtaining synoptic charts with extended viewing windows. Those could be used for modes of the total solar irradiance.
  • Furthermore, Solar Orbiter can help answer the question whether the observations of the Sun's chromospheric and photospheric activity is different from that observed from other stars. As one sees the Sun only from the ecliptic, comparisons with other stars might be biased, since they are observed from all viewing angles. By leaving the ecliptic, we might be able to test this hypothesis.


Related SOOPs 




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