1. Measuring the Near-Nothingness of Interstellar Space with Radio Astronomy Steven R. Spangler University of Iowa

2. An intuitive argument for the emptiness of space: the clarity of distant stars Yardstick for interstellar distances: the light year 1 light year = 9.46E+17 cm = 63,235 au Vega: 26 ly Altair: 17 ly Antares: 390 ly

3. How empty is interstellar space? A comparison with the Earth’s atmosphere

4. Point of talk: how radioastronomical measurements can diagnose the properties of incredibly tenuous gas in interstellar space

5. The North Liberty (Iowa) Radio Telescope

6. The importance of the interstellar medium: where do stars come from? Mass of star = 330,000 mass of Earth, mean density = 1.5 gm/cc

7. The interstellar medium is the gas from which stars ultimately form

8. Basic physics of extracting information on plasma in the interstellar medium: the radio refractive index Radio refractive index primarily diagnoses plasma density. Is secondary diagnostic of B. Radio refractive index in ISM differs by incredibly small amount from 1!

9. Basic physics of extracting information on plasma in the interstellar medium: the radio refractive index Radio refractive index primarily diagnoses plasma density. Is secondary diagnostic of B. Radio refractive index in ISM differs by incredibly small amount from 1! 10 -11 10 -7

10. Types of Scintillation Phenomena (i)Frequency scintillation (ii) Intensity cross- correlation

11. We measure line-of-sight integral of refractive index

12. The geometry of an ISM plasma measurement You are here Line of sight out of galaxy

13. The “signal generators” we use for probing the interstellar medium Polarization map of a radio galaxy at 1465 MHz

14. The Instrument: The Very Large Array Radiotelescope Operated by the National Radio Astronomy Observatory (NRAO)

15. The Very Large Array (VLA)

16. Physics of Faraday Rotation: the cartoon

17. Plasma Contributions to the Faraday Rotation Integral We need enough observations to sort out various contributions to coronal density and magnetic field

18. Faraday rotation through the ISM

19. What Faraday Rotation Observations have told us about the plasma of the interstellar medium

20. Summary Radioastronomical propagation measurements with the Very Large Array (EVLA) and Very Long Baseline Array (VLBA) radiotelescopes are sensitive to the ionized gas in the interstellar medium. They provide information on the plasma density, interstellar magnetic field, and properties of turbulence in the interstellar medium. These measurements provide insight into the “ecology” of the interstellar medium. GPAP Thanks