SFI - Solar flux index refers to the intensity of solar radio emissions at 10.7 cm (2,800 MHz).
Higher flux correlates with increased ionization levels of the ionosphere, enhancing HF radio propagation conditions.
The solar flux unit (sfu) is a unit of measure for spectral flux density, particularly used in solar radio observations.
Specifically, it's often used to express the F10.7 solar activity index.
One sfu is equal to 10,000 jansky (Jy), which is equivalent to 1022 Watts per meter2 per Hz.
A good solar flux unit for the F10.7 index (a measure of solar activity) is the solar flux unit, often abbreviated as sfu.
Values between 150 and 300 sfu are considered good for long-distance radio propagation.
SN - A daily index of SUNSPOT activity (R), defined as
R = k (10 g + s) where S = number of individual spots, g =
number of sunspot groups, and k is an observatory factor.
A good solar sunspot number is relative to the phase of the solar cycle.
During solar maximum, which is expected to occur in the next few months,
a "good" number would be considered relatively high, potentially around or above 120,
according to American scientists. SpaceWeatherLive.com reports a monthly mean sunspot
number of 73.4 for May 2025, while the National Weather Service indicates that during
solar maximum, sunspot numbers increase, leading to more solar flares and increased geomagnetic activity.
A INDEX - A 3-hourly "equivalent amplitude" index of local geomagnetic
activity; "a" is related to the 3-hourly K INDEX according to
the following scale:
K
0
1
2
3
4
5
6
7
8
9
A
0
3
7
15
27
48
80
140
240
400
K INDEX - A 3-hourly quasi-logarithmic local index of geomagnetic activity
relative to an assumed quiet-day curve for the recording site.
Range is from 0 to 9. The K index measures the deviation of the
most disturbed horizontal component.
X-RAY - A daily average background X-ray flux in the 1 to 8
angstrom range. It is a midday minimum designed to reduce the
effects of FLAREs.