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Solar spicules

Invited review paper

7. Conclusion

The author of a review article is undoubtedly the one who benefits most from it. Only by reviewing an entire subject does it become clear how much is known of it and in what areas more information is desired. In the past 10 years, spicules have probably been the best studied fine structures on the sun. A substantial amount of observational information on spicules is available as seen in Section 3. Some of the most important questions that remain to be answered in greater detail than available now are, in my opinion, the following:

  1. (1)

    Do spicules indeed diffuse after they reach their maximum growth and brightness? If so, is this an expansion of the spicule magnetic field, a change of the properties of the gases surrounding the spicule, or an actual diffusion of spicular matter across the spicular magnetic fields?

  2. (2)

    Is the group behaviour of spicules as that described by Lippincott (1957)? If true, it would imply that the spicule mechanism extends over a large area of the sun (100000 km), which conflicts with the spicule theories as given in Section 4.

  3. (3)

    Is the ‘tilt’ of spicule-emission lines real? If so, is it caused by spicule rotation and how does it differ between the various spicule-emission lines?

  4. (4)

    Is the spicule diameter different in the different spicule-emission lines?

  5. (5)

    Is it possible to measure the spicular magnetic field How large is it?

  6. (6)

    Do the physical conditions vary from spicule to spicule? How do they vary with height and time within one spicule? For this, one needs simultaneous spectra of spicules in many lines which have a different temperature behaviour.

  7. (7)

    Is it true that both the bright and dark elongated fine mottles seen in disk spectroheliograms are spicules? This requires renewed study of the solar disk.

  8. (8)

    After a definite identification of spicules with disk structure is made, what can one learn about spicule properties from the disk study? One can, for example, try to find a direct link with the solar granulation.

  9. (9)

    Are there spicules in active regions of the sun? How do they differ from spicules in quiescent regions?

  10. (10)

    Are chromospheric grains perhaps spicules which do not grow upwards because of a lack of magnetic fields? Are they perhaps related to granules?

  11. (11)

    What are the implications of a breakdown of the ‘statistically steady-state’ assumption in the spicule-intensity calculations?

The answers to many of these questions are of great importance in the precise understanding of a spicule, and in the derivation of a magnetohydrodynamic model for it.

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Beckers, J.M. Solar spicules. Sol Phys 3, 367–433 (1968). https://doi.org/10.1007/BF00171614

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Keywords

  • Magnetic Field
  • Temperature Behaviour
  • Solar Disk
  • Actual Diffusion
  • Group Behaviour