Supplementary Material to:

An Introduction to Radio Astronomy

4^th edition Cambridge University Press 2019

Last updated 10/05/2019

Chapter 12: The Sun and the Planets

Further overview of the radio sun

As a complement to Chapter 12 see the short introduction to the radio Sun at https://www.radio2space.com/the-radio-sun/ and the introductory material at https://solar-radio.gsfc.nasa.gov/ in particular the pages on the Solar Imaging Radio Array (SIRA) mission concept.

Solar radio astronomers are presented with great observational challenges since the structure of the radio sun is highly frequency-dependent, covers a wide range of angular scales and is very time dependent. While solar radio astronomy is currently a somewhat neglected branch of our discipline research work continues. High frequency observations probe the solar photosphere and chromosphere while low frequencies are well-suited for studying the yet-unsolved mystery of coronal heating and for the study of “space weather”. The latter involves variations in the solar wind, in particular associated with Coronal Mass Ejections (CMEs), which can have major impact on the Earth, on satellite systems and on human spaceflight.

Low frequency work offers perhaps the greatest opportunities and in the near future LOFAR http://www.lofar.org/astronomy/solar-ksp/solar-physics-and-space-weather and the upgraded MWA have great potential http://www.mwatelescope.org/science/solar-heliospheric-ionospheric-shi

Looking further ahead the U.S. Astro2020 Science White Paper “Radio, Millimeter , Submillimeter Observations of the Quiet Sun” Bastian et al (2019) https://arxiv.org/abs/1904.05826 neatly summarises one major issue:

“An outstanding problem in solar physics and by extension, stellar physics, is how the dynamic chromosphere and corona are heated. The chromosphere and corona are only visible to the naked eye during solar eclipses, the chromosphere as brilliant, ruby-red ring of beads just above the occulted photosphere; the corona as a pearly white crown. The fundamental question is by what non-radiative mechanism(s) the temperature of the chromosphere is heated to > 10⁴ K, and how the corona is heated to several ×10⁶ K.“

The White Paper presents a concise multiwavelength summary of instruments capable of providing new observations of the solar “atmosphere” (including ALMA for the chromosphere) and points specifically to the need for new (low frequency) radio instrumentation to make further progress on understanding the hot corona.

The potential of the SKA for Solar Physics is covered by Nindos et al (2019) Adv. In Space Research 63, 1404 (see also https://arxiv.org/pdf/1810.04951.pdf)

Observations at metre wavelengths

Over and above the upcoming MWA and LOFAR observations (see above) many amateur radio astronomers and radio astronomy groups make systematic long-wavelength observations of the radio sun e.g. the Society of Amateur Radio Astronomers

http://www.radio-astronomy.org/node/33

The international e-Callisto (Compound Astronomical Low cost Low frequency Instrument for Spectroscopy and Transportable Observatory) project see http://www.e-callisto.org/ is aimed at a combination of science, education and outreach.

Observations at centimetre wavelengths

The Nobeyama Radioheliograph and its daily image of the Sun

The Nobeyama radioheliograph in Japan is one of the few instruments dedicated to daily solar radio observations. It observes for 8 hours per day at both 17 GHz and 34 GHz. The home page at https://solar.nro.nao.ac.jp/norh/ provides access to a full description of the facility and to the latest images of the solar disk.

The Expanded Owens Valley Solar Array http://www.ovsa.njit.edu/ is another dedicated instrument aimed at broad band (1-18 GHz) rapid solar observations see e.g. https://phys.org/news/2018-05-owens-valley-solar-array-reveals.html

Observations at millimetre wavelengths – from ALMA

At millimetre wavelengths the corona is transparent, and only a disc is seen, with a diameter of 0^o.5, essentially the same as the visible Sun.

The Solar Disc

A single dish of the ALMA millimetre wavelength array has a beamwidth of order one arcminute, well suited for imaging the whole disc. This image was made at 1.25 mm wavelength, showing structure related to complex magnetic fields in the two sunspot zones. The scanning must be rapid, since the rotation of the Sun changes the aspect of the surface by a beamwidth in one minute.

(Image credit ALMA (ESO/NAOJ/NRAO)

Sunspots

The Nobeyama radioheliograph images (at wavelengths ~18mm and 9mm) show the location of sunspots but imaging a sunspot in detail requires higher resolution. The image above is at 1.25 mm wavelength was made with ALMA at mm wavelengths (Image credit ALMA (ESO/NAOJ/NRAO).

The Planets

The early radiometric results on by Mayer, McCollough & Sloanaker (1958) described in Supplementary Material Chapter 5 were the first to show that the surface temperature of Venus was much hotter than the Earth. The surface temperatures of the Moon and planets are generally close to that expected from radiative equilibrium of a black body illuminated by the Sun. For a synoptic overview of planetary radio observation including the non-thermal radiation from Jupiter see the two reviews by I. de Pater

- “The Significance of the Microwave Temperatures of the planets in: PHYSICS REPORTS (Review Section of Physics Letters) 200, No. 1 (1991) 1—50. North-Holland (see also http://www.physics.purdue.edu/~lyutikov/Liter/sdarticle(10).pdf)

- “Radio Images of the Planets”: in Annual Review of Astronomy and Astrophysics. Vol. 28 (A91-28201 10-90).(1990)

A comprehensive summary of radio observations of the planets using large arrays can be found at https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011RS004752

ALMA is starting to have an impact on the study of solar system bodies at mm/sub-mm wavelelengths see https://almascience.nrao.edu/alma-science/solar-system

Detailed imaging of Neptune by Tollefsun et al (2019) https://arxiv.org/abs/1905.03384v1 probes the planet’s atmosphere and its latitudinal bands.