The critical frequency of the ionosphere is not a fixed value. It roughly varies with time of day, day of the year, and the 11-year long solar cycle. Short term variation is influence by many factors, of which the solar radiation and the earth magnetic field are the main drivers.

Solar flares may disturb the more usual arrangement of the ionosphere, and can be seen as anomalies in ionospheric radio wave propagation, sometimes even totally disrupting all ionospheric propagation.

The critical frequency of the F-layer is measured near real-time by ionosonde stations. Please take a look at them, they are very informative. The following link is from an ionosonde in Belgium, but there are several in your area as well:

http://digisonde.oma.be/latestFrames.htm

Is your interest driven by radio wave propagation issues, or earth science interest?

The cutoff frequency is the same as the critical frequency of a layer, in this case the F2 layer. It is the maximum value of the plasma frequency of the layer (denoted as foF2) that can reflect back a radio wave incident vertically. That means all frequencies above foF2 will penetrate the layer and escape to space (if the wave is transmitted from a ground transmitter). The foF2 can vary from a few MHz (from less than to 1 MHz) to more that 20 MHz depending upon the local time, location and solar activity levels, etc.

It can increase slightly (by a few MHZ) during some solar flares.

The cutoff frequency is the same as the critical frequency of a layer, in this case the F2 layer. It is the maximum value of the plasma frequency of the layer (denoted as foF2) that can reflect back a radio wave incident vertically. That means all frequencies above foF2 will penetrate the layer and escape to space (if the wave is transmitted from a ground transmitter). The foF2 can vary from a few MHz (from less than 1 MHz) to more that 20 MHz depending upon the local time, geographic location,solar activity levels, etc.

It can increase slightly (by a few MHZ) during some solar flares.

Cut-off frequency for ionosphere layer as referenced in many text is 30 MHz. Is not it the cut-off frequency for F2 layer is same as cut-off frequency of ionosphere as a whole?

I need a more forceful response on this. :)

If the cut off frequency as referenced in many text is 30 MHz, it should be the cut off frequency for ionospheric propagation at oblique angles, probably that is used by radio amateurs. But this is a nominal value, and it should also vary with local time season, solar activity etc.

The critical frequency of the ionosphere is not a fixed value. It roughly varies with time of day, day of the year, and the 11-year long solar cycle. Short term variation is influence by many factors, of which the solar radiation and the earth magnetic field are the main drivers.

Solar flares may disturb the more usual arrangement of the ionosphere, and can be seen as anomalies in ionospheric radio wave propagation, sometimes even totally disrupting all ionospheric propagation.

The critical frequency of the F-layer is measured near real-time by ionosonde stations. Please take a look at them, they are very informative. The following link is from an ionosonde in Belgium, but there are several in your area as well:

http://digisonde.oma.be/latestFrames.htm

Is your interest driven by radio wave propagation issues, or earth science interest?

I agree to all the answers but want to add some remarks from my own experience. The F2 layer theoretically does not have any cutoff frequency..During the IGY foF2 Defiinetly exceeded 25 Mhz at some of the stations situated near the EIA midlatitude peak.I have manually observed f0F2 upto 3 Mhz on some occasionin 1957-58 but it could not be recordrd photographically.

The instruments recording the reflected echoes do have limit at both minimum as well as maximum ends. The British NPL model ionosonde I had installed at Ahmedabad in 1953 had five bands from 0.7 Mhz to 25 Mhz divided in five bands .The receiver was made to track the transmitted frequency by a frequency descrimatind circuit which had a limited band with.This equipment worked well upto 14.3 Mhz but never worked beyond 14.3.The tables published by Physical Research :aboratory denoted these cases as D14.3 according to URSI rules.Some scientists have discarded he symbol"D" and have published papers even in JGR suggesting that there was a upper limit of f0F2 at Ahmedabad equal to 14.3 Mhz.This is utterly wrong and hese papers should be avoided in fture.

If I am told the equipment which records the foF2 to 8 Mhz I may be able to resolve the probem.

Prof. R.G.Rastogi,Physical Research Laboratory,Ahmedabad,India.

You can increase the critical frequency of F2 layer by releasing specific agents with lower ionization threshold. Such active experiments have been carried out in USSR and USA as well in 60s. The duration of enhancement is limited to less than one hour. Other technique is based on the usage of plasma generators (google North Star Active Rocket Experiment) with triggering the magnetosphere as a whole. The latter results in enhanced energetic particles precipitation, If their energy provides maximum of ionization at the F2-layer altitude, then you can also increase the foF2 frequency up to couple of hours.

the line 5 of my previous letter should be (observed foF2 upto 30 Mhz on some ocassions in 1957-58 but it could noot be recorde )

Prof. R.G.Rastogi '

In addition to the extended discussion above on the maximum attainable critical frequency of the ionosphere: sporadic E-layer ionization (abbreviated as 'Es' and 'foEs') can easily exceed 50 MHz, resulting in a Maximum Usable Frequency for a distance of 2000 km of more than 200 MHz. This happens sporadic and local, but still many times every summer season in Europe, allowing 144 MHz radio contacts with very strong signals during intervals of 5 minutes, sometimes as long as an hour. I have experienced several such events myself and they are well documented. During Es the E-layer critical frequency is higher than that of the F-layer.

The rates of ionospheric chemistry play a significant role on values of F2 layer critical frequency. When the rate is high, the density and refractive indices are expected to be high leading to higher F0F2 values. These values are location, local time, solar activity, geomag activity dependent. Like the previous responses, the values can span up to 30MHz, depending on the above stated parameters, but my experience is in the neighbourhood of 14 MHz.