In solar observation and imaging, coordinate systems are used to identify and communicate locations on and around the Sun. Since the Sun is gaseous in nature, there are no permanent demarcated points that can be referenced.

Background

The Sun is a rotating ball of plasma at the center of the Solar System. It lacks a solid or liquid surface, so the interface separating its interior and its exterior is usually defined as the boundary where plasma becomes opaque to visible light, the photosphere. Since plasma is gaseous in nature, this surface has no permanent demarcated points that can be used for reference. Furthermore, its rate of rotation varies with latitude, rotating faster at the equator than at the poles.[1][2]

Heliographic

Heliographic coordinate systems are used to identify locations on the Sun's surface. The two most commonly used systems are the Stonyhurst and Carrington systems. They both define latitude as the angular distance from the solar equator, but differ in how they define longitude. In Stonyhurst coordinates, the longitude is fixed for an observer on Earth, and, in Carrington coordinates, the longitude is fixed for the Sun's rotation.[3][4][5][6]

Stonyhurst system

The Stonyhurst heliographic coordinate system, developed at Stonyhurst College in the 1800s, has its origin (where longitude and latitude are both 0°) at the point where the solar equator intersects the central solar meridian as seen from Earth. Longitude in this system is therefore fixed for observers on Earth.[6][3]

Carrington system

The Carrington heliographic coordinate system, established by Richard C. Carrington in 1863, rotates with the Sun at a fixed rate based on the observed rotation of low-latitude sunspots. It rotates with a sidereal period of exactly 25.38 days, which corresponds to a mean synodic period of 27.2753 days.[7]: 221 [1][2][3]

Whenever the Carrington prime meridian (the line of 0° Carrington latitude) passes the Sun's central meridian as seen from Earth, a new Carrington rotation begins. These rotations are numbered sequentially, with Carrington rotation number 1 starting on 9 November 1853.[8][9][10][5]: 278 

Heliocentric

Heliocentric coordinate systems measure spatial positions relative to an origin at the Sun's center. There are four systems in use: the heliocentric inertial (HCI) system, the heliocentric Aries ecliptic (HAE) system, the heliocentric Earth ecliptic (HEE) system, and the heliocentric Earth equatorial (HEEQ) system. They are summarized in the following table.[1][11][12][13]

Common heliocentric coordinate systems
NameAbbreviation+X-axis+Z-axis
Heliocentric inertialHCISolar ascending node on eclipticSolar rotational axis
Heliocentric Aries eclipticHAEFirst point of AriesEcliptic north pole
Heliocentric Earth eclipticHEESun–Earth lineEcliptic north pole
Heliocentric Earth equatorHEEQIntersection between solar equator and solar central meridian as seen from EarthSolar rotational axis

See also

References

  1. 1 2 3 Thompson, W. T. (April 2006). "Coordinate systems for solar image data". Astronomy & Astrophysics. 449 (2): 791–803. doi:10.1051/0004-6361:20054262.
  2. 1 2 Ulrich, Roger K.; Boyden, John E. (May 2006). "Carrington Coordinates and Solar Maps". Solar Physics. 235 (1–2): 17–29. doi:10.1007/s11207-006-0041-5.
  3. 1 2 3 Ridpath, Ian, ed. (2018). "heliographic coordinates". A Dictionary of Astronomy (3rd ed.). Oxford University Press. doi:10.1093/acref/9780191851193.001.0001.
  4. ↑ Sánchez-Bajo, F.; Vaquero, J. M. (1 May 2013). "Measuring solar rotation from digital camera images". European Journal of Physics. 34 (3): 527–536. doi:10.1088/0143-0807/34/3/527.
  5. 1 2 Stix, Michael (2002). The Sun: An Introduction (2nd ed.). Berlin, Heidelberg: Springer. ISBN 978-3-642-56042-2.
  6. 1 2 Çakmak, H. (November 2014). "Computer-aided measurement of the heliographic coordinates of sunspot groups". Experimental Astronomy. 38 (1–2): 77–89. arXiv:1407.1626. doi:10.1007/s10686-014-9410-5.
  7. ↑ Carrington, R. C. (1863). Observations of the spots on the sun. London: Williams and Norgate.
  8. ↑ "Synoptic Maps". National Solar Observatory. Retrieved 21 December 2023.
  9. ↑ "Solar-Terrestrial Coordinate Systems". Wilcox Solar Observatory. Retrieved 21 December 2023.
  10. ↑ Thompson, W. T.; Wei, K. (January 2010). "Use of the FITS World Coordinate System by STEREO/SECCHI". Solar Physics. 261 (1): 215–222. doi:10.1007/s11207-009-9476-9.
  11. ↑ Hapgood, Mike (July 1997). "Heliocentric coordinate systems". Mullard Space Science Laboratory. Retrieved 21 December 2023.
  12. ↑ Hapgood, M. A. (May 1992). "Space physics coordinate transformations: A user guide" (PDF). Planetary and Space Science. 40 (5): 711–717. doi:10.1016/0032-0633(92)90012-D.
  13. ↑ Fränz, M.; Harper, D. (February 2002). "Heliospheric coordinate systems" (PDF). Planetary and Space Science. 50 (2): 217–233. doi:10.1016/S0032-0633(01)00119-2.
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