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Exploring Solar Radio Spectral Irradiance: Historical Insights and Measurement Techniques

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This presentation by Christophe Marqué at the Royal Observatory of Belgium delves into the history and techniques of solar radio astronomy. Originating during WWII, the conversion of radar equipment into radiotelescopes enabled observations across a wide spectral range. Key topics include the emission mechanisms of the active and quiet Sun, critical calibration procedures for flux measurements, and the influence of atmospheric conditions on data accuracy. The insights gained contribute significantly to our understanding of solar activity and its impacts on space weather.

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Exploring Solar Radio Spectral Irradiance: Historical Insights and Measurement Techniques

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  1. Solar radio spectral irradiance Christophe Marqué Royal Observatory of Belgium SOLID Meeting WP2

  2. The beginning • Solar radio astronomy started during WWII • Conversion of radar equipments into radiotelescopes • No choice of frequencies SOLID Meeting WP2

  3. The beginning Active Sun • Sporadic activity linked to solar eruptive events • Accelerated electrons (non thermal populations) • Different spectral types SOLID Meeting WP2

  4. The beginning Non active Sun • Thermal emission from the corona (T~106K, Pawsey,1946) • Bremsstrahlung, Gyroresonance • 2 components: Quiet Sun & SVC SOLID Meeting WP2

  5. The beginning “The excitement of the eclipse observations [at 10.7 cm] was soon followed by the sobering thoughts that solar radio emission from sunspots would be variable…” A. Covington, Proc. NRAO Workshop, 1983 SOLID Meeting WP2

  6. Chromosphere Unknown Thermal bremsstrahlung Transition region Gyrosynchrotron Thermal bremsstrahlung Low corona Thermal bremsstrahlung Gyroresonance Plasma emission Gyrosynchroton High corona Thermal bremsstrahlung Plasma emission Interplanetary medium Solar radio observations The Sun is observed in radio in a large spectral window: from far I.R to km wavelengths

  7. Solar radio spectral irradiance • Continuum emission: no lines • Flux density: W.m-2.Hz-1 • Solar Flux Unit: 10-22 W.m-2.Hz-1 = 104 Jansky • Intensity as Brightness Temperature SOLID Meeting WP2

  8. Solar Flux radio Observatories 1000 MHz 2000 MHz 3750 MHz 9400 MHz 17000 MHz 2800 MHz 245 MHz 410 MHz 610 MHz 1415 MHz 2695 MHz 4995 MHz 8800 MHz 15400 MHz SOLID Meeting WP2

  9. Flux Measurement • Full Sun • Parabolic dish • Horn antenna • Pentincton: strict calibration procedure 3 times per day SOLID Meeting WP2

  10. Flux measurement: calibration • Tanaka (1973) paper • Toyokawa: reference station • Flux correction (0.9 for F10.7) • Standard for absolute calibration • Precision 1-2 % SOLID Meeting WP2

  11. Flux measurement: natural sources of error • Atmospheric absorption • Rain • Humidity (dew/snow) on antenna • Temperature variation • Ionospheric disturbances SOLID Meeting WP2

  12. Flux measurements: examples Ground interference Ground interference Quiet Sun level Solar flare calibration Quiet sun level

  13. Cycle 23-24 SOLID Meeting WP2

  14. Emission mechanism After Schmahl & Kundu 1998 SOLID Meeting WP2

  15. Spectrum of QS &SVC • QS : S~f2 -> Thermal Bremstrahlung • SVC: ~Flat spectrum Gyroresonance/Free-Free 2001-2002 SOLID Meeting WP2

  16. Free-Free / Gyroresonance SOLID Meeting WP2

  17. Quiet Sun: essentially Free-Free Zirin, 1991 SOLID Meeting WP2

  18. SVC S. White SOLID Meeting WP2

  19. Last words • Long term spectral irradiance with good stability (with proper calibration procedure) • Care should be taken about the meaning of daily values • No Instrument degration (apart from rust & spare parts availability) • Possibility to use radio data for EUV/UV calibrations • Gold mine for long term studies of coronal evolution (n, T, & B) SOLID Meeting WP2

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