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P.P. Kronberg (U. Toronto)
In the pioneering 1950's and '60's days of radio astronomy, observing frequencies were typically between 50 and 600 MHz, with an early ``stretch'' up to 1420 MHz to observe the F=1,0 line of neutral atomic hydrogen. These sub-GHz frequencies were determined by the antenna and receiver technology at the time, and the typical resolutions of a few arcminutes to degree scales were a great, but unavoidable disadvantage. Not surprising therefore that, with advancing antenna and electronic technology, radio astronomers moved to progressively higher frequencies. Apart from achieving resolution that improves linearly with increasing frequency, the problematic variable ionospheric refraction disappears above ~300 MHz, and thermally emitting sources also become observable.
The recent outfitting of the 35 km. VLA at the ``old fashioned'' frequencies of 330 MHz (in the late 1980's) and 75MHz (in 1998) provides impressive imaging resolutions of 7'' and 30'' respectively. These will further improve as interferometer baselines grow in future. Furthermore, phase closure-based calibration techniques allow the dynamic removal of ionospheric ``seeing'' to produce low frequency images of unprecedented resolution and clarity, over areas of sky up to 15o x 15o at 75 MHz. I show and discuss some spectacular recent examples. The WSRT(NL), the GMRT(India) have similarly been advancing the radio observational frontier in these bands.
A key virtue of lower frequencies is that the spectral density (W Hz-1) of optically thin synchrotron radiation increases. Also, importantly, the radiation lifetimes of the ``relevant'' lower energy CR electrons are higher, ~109 yr (at z ~0) at frequencies around 200 MHz. This means that low energy, hence low frequency emitting, extragalactic CR electrons have the longest time to diffuse from their acceleration sites and still be visible. They also trace the presence of magnetic fields over whatever regions that faint synchrotron ``glow'' can be seen. I use illustrative examples to show how diffuse, low frequency extragalactic synchrotron radiation can measure intergalactic ``weather'', and how it can measure a previously invisible large component of energy in the IGM of galaxy clusters and beyond.