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Description of the objective:


The Heliospheric Current Sheet (HCS) is the interplanetary extension of the neutral line between the two magnetic polarities in the corona and is a topological magnetic boundary. The HCS is also vital in the motion of cosmic rays throughout the heliosphere: depending on the polarity of the solar cycle, ions or electrons tend to migrate to low latitudes and along the HCS as they enter the solar system.


The HCS is remarkably thin: just a few thousand km across (Zhou et al., 2005), but is surrounded by the much thicker, denser Heliospheric Plasma Sheet (HPS) (Vourlidas and Riley, 2007). It is not clear how thin the HCS and HPS are close to the Sun, which could provide clues to their origin the HCS seems actually to thin with distance, for example. Both the HCS and HPS are also highly variable: what is the origin of this variability? Reconnection appears to occur here (Gosling et al., 2006): how frequent is this close to the Sun?

Data from a number of sources (cosmic rays: Simpson et al., 1996; the IMF: Luhmann et al., 1988; geomagnetic activity: Mursala and Zieger, 2001; source surface models: Zhao et al., 2005) suggest that the Sun's neutral line, and also the HCS, are persistently displaced Southward during solar minimum. This is consistent with a solar dipolar field offset from the Sun's center, but why (or even if) this should occur is unknown. 

The large scale structure of the low corona is determined by active regions, filament channels and coronal holes whose presence and location have a clear evolution over the solar cycle. Higher up in the corona, active regions and filaments are over-arched by pseudo streamers and streamers that fade into the heliospheric plasma sheet. This magnetic connection region between corona-heliosphere is of particular physical interest because it is here that the structuring of the magnetic field is taken over by the plasma outflow. This intermediate region, or transition corona (say 1 to 3 Rsun above the solar limb) is poorly studied as it corresponds to the FOV gap between most EUV imagers and coronagraphs. Coronal/heliospheric simulation codes such as ENLIL or EUHFORIA typically bypass the complexity in this region completely by ad-hoc empirical laws (eg WSA) to obtain 'coronal' boundary conditions at 0.1 Rsun.

When trying to determine the coronal foot-point of features observed by the in-situ instruments, the transition corona introduces significant uncertainty as the Parker-spiral type of mapping has to be connected to  magnetic field extrapolations from the photosphere.  Deciphering this connection is thus of primordial importance for Solar Orbiter connection science, but the transition corona might also harbour interesting features of itself:

- this might/should be the source region of stealth CMEs,

- the HERSCHEL/SCORE rocket (prototype for METIS) observed eg "He-horns" (http://metis.oato.inaf.it/Score.html) and 

- the SWAP EUV imager observed long-lived "coronal fans" (https://ntrs.nasa.gov/search.jsp?R=20170008533).

- UVCS Lya/OVI observations above 1.5 Rsun showed how important is to observe this transition corona


We, therefore, have to:


  • Determine the local tilt and latitudinal extent of the HCS as a function of solar distance and relation to the time-varying source surface neutral line. Asymmetry of the HCS. Determine whether the HCS is offset from the equator in near-Sun space. 
  • Study the relation of the Heliospheric Plasma Sheet (HPS) to the HCS. (Vourlidas and Riley, 2007).
  • Measure the variability of the HCS and HPS in time and space, with the goal of determining the solar or local origin of the variations. Reconnection (Gosling et al., 2006)?
  • Determine the link between heliospheric plasma sheet and coronal streamers (Bavassano et al., 1997).

Remarks:


  • As for objective 1.1.3.2.1 How does the Sun's magnetic field change over time?, in situ observations are key during EMC Quiet periods, but full disk synoptic remote sensing observations will also be important for a better understanding. 
  • This objective can be addressed by SOOP L_FULL_HRES_LCAD_MagnFieldConfig.
  • It would be interesting to study the HCS crossings at the perihelia during the Cruise Phase.
  • Observations during one solar rotation will not be enough since we will have gaps due to the RS windows durations. High-latitude observations are also required.


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  • SPICE
    • Target: Close loop structures, active regions.
    • Observing mode: Composition mapping and CME watch.
    • Slit: 4''
    • Exposure time/cadence and number of X positions: 180 s, X=96 for Composition mapping; 30 s, X=96 for CME watch
    • Field of View: 6'x11'
    • Number of repetitions of the study:1 for Composition mapping followed by up to 10 for CME watch
    • Observation time: 4.8 hours for Composition mapping and up to 8 hours for CME watch
    • Key SPICE lines to be included: Ne VIII 770 Å, Ne VIII 780 Å, Mg IX 706 Å, O II 718 Å, O IV 787 Å, O V 760.4 Å, O V 761 Å, O VI 1032 Å, O VI 1037 Å, Ne VI 999 Å, Ne VI 1010 Å, Mg VIII 772 Å, Mg VIII 782 Å, C III 977 Å, Fe III 1017 Å - 2 profiles and 13 intensities or 4 profiles and 11 intensities (maximum of 15) for Composition mapping;
      C III 977 Å, O VI 1032 Å, O VI 1037 Å, Ne VIII 770 Å, Mg IX 706 Å, Fe X 1028 Å – 15 lines (5 profiles+10 intensities) for CME watch 
    • Observing window preference: perihelion preferred
    • Other instruments: All instruments
    • Comments: The choice of lines, and also the number of intensities and profiles, is flexible, although the sum of the intensities and profiles is constrained to a maximum (e.g 15 for composition mapping). While varying the number of intensities and profiles, within the maximum, has no effect on the duration of the study, it will have an effect on the telemetry.