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The flux density or monochromatic flux, , of a source is the integral of the spectral radiance, , over the source solid angle:
The unit is named after pioneering US radio astronomer Karl Guthe Jansky, and is defined as:
Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources; for example, the Third Cambridge Catalogue of Radio Sources (3C) reports results in Jy.
Jansky units are not a standard SI Unit, so it may be imperative to convert the unit to Watts. However, other unit conversions are possible with respect to measuring this unit.
The flux density in Jy can be converted to a magnitude basis, for suitable assumptions about the spectrum. For instance, converting an AB magnitude to a flux-density in microjanskys is straightforward:
The linear flux density in Jy can be converted to a decibel basis, suitable for use in fields of telecommunication and radio engineering.
The flux to which the jansky refers can be in any form of energy.
It was created for and is still most frequently used in reference to electromagnetic energy, especially in the context of radio astronomy.
The brightest astronomical radio sources have flux densities of the order of one to one hundred janskys. For example, the Third Cambridge Catalogue of Radio Sources lists some 300 to 400 radio sources in the Northern Hemisphere brighter than 9 Jy at 159 MHz. This range makes the jansky a suitable unit for radio astronomy.
Gravitational waves also carry energy, so their flux density can also be expressed in terms of janskys. Though gravitational waves have never been directly observed, typical signals on Earth are expected to be 1020 Jy or more. However, because of the poor coupling of gravitational waves to matter, such signals are difficult to detect.
It is important to understand the meaning of the per hertz component of the jansky unit. When measuring broadband continuum emissions, where the energy is roughly evenly distributed across the detector bandwidth, the detected signal will increase in proportion to the bandwidth of the detector (as opposed to signals with bandwidth narrower than the detector bandpass). To calculate the flux density in janskys, the total power detected (in watts) is divided by the receiver collecting area (in square meters), and then divided by the detector bandwidth (in hertz). The flux density of astronomical sources is many orders of magnitude below 1 W/(m2·Hz), so the result is multiplied by 1026 to get a more appropriate unit for natural astrophysical phenomena.
The millijansky, mJy, was sometimes referred to as a milli flux unit (m.f.u.) in the astronomical literature.
|110,000,000||Radio-frequency interference from a GSM phone transmitting 0.5 W at 1800 MHz at a distance of 1 km (RSSI of -70 dBm)|
|20,000,000||Disturbed Sun at 20 MHz (Karl Guthe Jansky's initial discovery, published in 1933)|
|4,000,000||Sun at 10 GHz|
|1,000,000||Milky Way at 20 MHz|
|2,000||Milky Way at 10 GHz|
|1,000||Quiet Sun at 20 MHz|
Note: Unless noted, all values are as seen from the earth's surface.