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Difficulty: HardIdentifying Underlying Assumptions and Premises

### Passage
Heating of the Solar Corona

The solar corona—the outermost layer of the Sun’s atmosphere—is hot, with temperatures exceeding 1,000,000 K1,000,000\text{ K}, while the underlying photosphere is only about 5,800 K5,800\text{ K}. Two astrophysicists discuss competing models for how energy is transported and dissipated to heat the corona.

Astrophysicist 1
The corona is heated primarily by magnetohydrodynamic (MHD) waves, specifically Alfvén waves. These waves are generated by the mechanical motion of plasma in the convective zone and travel upward along magnetic field lines into the corona. Because the corona has an extremely low density, these waves become non-linear and dissipate, transferring kinetic energy directly to the coronal particles as heat. This wave propagation and dissipation process occurs continuously across the entire solar surface.

Astrophysicist 2
The corona is heated by thousands of small, discrete magnetic reconnection events called nanoflares. The turbulent motion of the photosphere twists and braids the magnetic field lines that extend into the corona. When the tension in these braided lines becomes too high, the magnetic fields rapidly snap and reconnect, releasing stored magnetic energy. This energy is converted into thermal energy, heating the local plasma. These reconnection events are localized and intermittent, but their high frequency across all magnetic loops accounts for the high coronal temperature.

Question
In order for Astrophysicist 1’s model of coronal heating to be valid, which of the following implicit assumptions must be true regarding the solar atmosphere between the convective zone and the corona?

  1. A
    The density of the solar atmosphere increases continuously from the convective zone to the corona.
  2. B
    Alfvén waves are converted into magnetic reconnection events before they reach the corona.
  3. The intermediate layers of the solar atmosphere do not absorb or reflect the majority of the upward-propagating Alfvén waves.Answer
  4. D
    The mechanical motion of the plasma in the convective zone is driven entirely by nanoflares occurring in the photosphere.

Answer

The intermediate layers of the solar atmosphere do not absorb or reflect the majority of the upward-propagating Alfvén waves.
For the wave-heating model proposed by Astrophysicist 1 to be valid, the energy generated as Alfvén waves in the convective zone must actually reach the corona. Because the waves must travel through the intervening layers of the solar atmosphere to do so, the model relies on the implicit assumption that these intermediate layers do not absorb or reflect the majority of the wave energy before it can reach the corona.

Step-by-Step Solution

1
Analyze Astrophysicist 1's model to determine the source and destination of the heating energy.
The energy is generated as Alfvén waves in the convective zone and must travel along magnetic field lines to reach the corona, where they dissipate.
Identifying the start and end points of the energy transfer helps isolate the intermediate path where assumptions may lie.
2
Identify the physical barrier or path between the convective zone and the corona.
The waves must travel through the intermediate layers of the solar atmosphere (the chromosphere).
Since the waves must pass through this region to reach the corona, the properties of this region are critical to the model's validity.
3
Evaluate the implicit requirement for wave propagation through these intermediate layers.
If these intermediate layers absorbed or reflected the waves, the waves would never reach the corona to heat it. Therefore, the model implicitly assumes these layers allow the waves to pass through.
This is a necessary underlying premise for the proposed energy transport mechanism to function.

Key Concept

Identifying underlying assumptions in scientific models
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Topics

Identifying Underlying Assumptions and Premises

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