Solar radio noise temperature: At microwave frequencies relevant to satellite links, the effective noise temperature of the Sun typically exceeds what threshold (in kelvin)?

Introduction / Context:
The Sun is a strong natural noise source that can raise the system temperature when the antenna main lobe or sidelobe intercepts the solar disk, notably during Sun outages (Sun–satellite conjunctions).

Given Data / Assumptions:

• Quiet Sun noise temperatures in the microwave bands can be on the order of 10^4 K (varies with frequency and solar activity).
• During active events, effective temperature can be much higher.

Concept / Approach:

Because the Sun’s radio brightness temperature is typically many thousands of kelvin, using “more than 5000 K” as a conservative threshold is appropriate for exam-style questions. This highlights that the Sun dramatically elevates system noise when in the antenna beam.

Step-by-Step Solution:

Verification / Alternative check:

Operational guidance for Sun outage windows warns of severe C/N degradation due to solar noise loading the antenna beam.

Why Other Options Are Wrong:

• 500 or 1000 K: too low for solar radio noise temperature in most microwave contexts.
• 100000 K: can occur during bursts, but the question asks a typical “more than” threshold; 5000 K is the best general choice.

Common Pitfalls:

• Treating optical surface temperature (~5800 K) as the same as radio brightness temperature; the latter depends on frequency and solar activity.

Final Answer:

5000 K