UKPSC Prelims 2024 General Studies Paper-I · Question 3 of 147

Correct — A, Photosphere.

The Sun has no solid surface; the disc we see is the Photosphere, the layer from which most of the Sun's visible light escapes. It is a few hundred kilometres thick, at about 5,500 °C, and because ordinary sunlight comes from it, it is the bright layer the stem describes.

Above it lie the chromosphere and the corona. They are outer layers too, but far fainter, so they show themselves to the naked eye during a total solar eclipse, when the Moon covers the bright disc. Neither is offered among these options.

The idea to carry away: the photosphere is the Sun's visible, light-giving surface. The other three options are either a feature sitting on that surface or layers of Earth's own atmosphere.

• (b)
  Ionosphere — The ionosphere belongs to the Earth, not the Sun. It is the region of the upper atmosphere, from roughly 80 km upward, where solar radiation strips electrons from gas molecules and leaves electrically charged ions behind.

  Its examinable property is that it reflects radio waves back to the ground, which is what makes long-distance radio transmission possible. It is the right answer to a question on that, or on the layer of Earth's atmosphere lying above the mesopause.

• (c)
  Mesosphere — The mesosphere is also a layer of Earth's atmosphere, sitting above the stratosphere from about 50 km to 80 km. Temperature falls with height through it, and its top, the mesopause, is the coldest level of the atmosphere at roughly −90 °C to −100 °C.

  It is the right answer to the layer in which most meteors burn up, or to the coldest layer of the atmosphere. It shares the '-sphere' ending with the photosphere, but it is a layer of air above the Earth, not a layer of the Sun.

• (d)
  Sunspots — Sunspots are on the Sun, but they are a feature, not a layer. They are patches of the photosphere where strong magnetic fields poke through from the interior and suppress the upward flow of heat, so they run cooler than their surroundings and look dark by contrast.

  They are the right answer to the dark, cooler, magnetic patches on the solar surface whose numbers rise and fall over a cycle of roughly 11 years. The stem asks for a bright layer; a sunspot is neither bright nor a layer.

The Sun is a ball of hot gas with no solid surface, so its layers are defined by what they do rather than by hard boundaries. Energy is made in the core, carried outward through the radiative and convective zones, and released as light at the photosphere.

The photosphere is the visible surface: the level at which the gas becomes thin enough for light to escape into space. Outside it lie two layers of solar atmosphere, the chromosphere and the corona, which are hot but faint, and the whole is studded with features such as sunspots, flares and prominences.

• The photosphere is the Sun's visible surface, the layer that emits most of the sunlight we see.
• The photosphere's temperature is about 5,500 °C, and it is a few hundred kilometres thick.
• From the centre outward, the Sun's layers run core, radiative zone, convective zone, photosphere, chromosphere, corona.
• The chromosphere lies just above the photosphere and appears as a fine red rim around the Sun during a total solar eclipse.
• The corona is the outermost solar layer, seen as a white crown at totality, with temperatures reaching up to about two million degrees Celsius.
• Sunspots are areas of the photosphere slightly cooler than their surroundings, created where the Sun's magnetic field pokes out from the interior.
• Sunspot numbers rise and fall over a cycle of roughly 11 years.
• The mesosphere is Earth's atmospheric layer between about 50 km and 80 km, where temperature falls with height and meteors burn up.
• The ionosphere is Earth's belt of electrically charged particles above the mesopause, and it reflects radio waves back to the ground.
• Aditya-L1 was launched on PSLV-C57 on 2 September 2023 and reached its halo orbit around Sun–Earth L1 on 6 January 2024.
• On Aditya-L1, the SUIT telescope images the photosphere and chromosphere, while the VELC coronagraph studies the corona.

• Reading 'outer' and reaching for the outermost layer: the corona is farther out but faint, and it is not among the options; 'bright' is the operative word.
• Assuming that a word ending in '-sphere' must name a layer of the Sun: mesosphere and ionosphere are layers of the air above the Earth.
• Choosing sunspots because they are the best-known solar feature: they are dark, cooler patches on the photosphere, not a layer.
• Misplacing the chromosphere: it sits between the photosphere and the corona, not outside the corona.

No eligible match among this question's citable set.

• practice — not a real PYQ

  Arrange the following layers of the Sun from the innermost to the outermost: 1. Corona 2. Photosphere 3. Chromosphere Select the correct sequence:

  • (a)2, 3, 1
  • (b)3, 2, 1
  • (c)1, 3, 2
  • (d)2, 1, 3
  Answera — The photosphere is the visible surface, the chromosphere is the thin layer just above it, and the corona is the outermost halo.

  (b) puts the chromosphere beneath the photosphere; (c) inverts the whole order; (d) places the corona between the photosphere and the chromosphere.

• practice — not a real PYQ

  Sunspots appear dark against the solar disc because they:

  • (a)are openings through which the Sun's cooler interior is seen
  • (b)are regions of the photosphere cooler than their surroundings
  • (c)lie in the corona, which gives out no visible light
  • (d)are shadows cast on the Sun by the inner planets
  Answerb — Strong magnetic fields at a sunspot hold back heat rising from below, so the patch is cooler than the neighbouring photosphere and looks dark by contrast.

  (a) fails because the Sun's interior is hotter than its surface, not cooler; (c) fails because sunspots sit on the photosphere, not in the corona; (d) describes a planetary transit, a small round silhouette that crosses the disc in hours, not a sunspot.

• practice — not a real PYQ

  Which of the following are layers of the Earth's atmosphere and not of the Sun? 1. Mesosphere 2. Chromosphere 3. Ionosphere 4. Corona Select the correct answer using the code given below:

  • (a)1 and 3 only
  • (b)2 and 4 only
  • (c)1, 2 and 3 only
  • (d)1 and 4 only
  Answera — The mesosphere (about 50–80 km) and the ionosphere (above the mesopause) are layers of Earth's atmosphere.

  The chromosphere and the corona are the two layers of the Sun's atmosphere above the photosphere, so (b) names the solar pair, and (c) and (d) each mix a solar layer into the Earth set.

• practice — not a real PYQ

  The approximate temperature of the Sun's photosphere is:

  • (a)about 5,500 °C
  • (b)about 15 million °C
  • (c)about 2 million °C
  • (d)about 500 °C
  Answera — The photosphere, the visible surface, is at about 5,500 °C.

  (b) is the order of the core temperature, where fusion takes place; (c) is the order of the corona's temperature; (d) is far too cool for a layer that glows white-hot.