One measure of the strength of the ionosphere is called foF2. It is the maximum frequency that will be reflected straight back. That is imagine the radio transmitter and receiver are located near each other, the radio waves go straight up, and are reflected straight back down to the receiver. foF2 is continuously varying, based on solar activity, and what part of the Earth the Sun is over. You can find a real time map at this URL: http://www.spacew.com/www/fof2.gif
As the distance between the transmitter and receiver increases, the radio waves are not perpendicular to the ionosphere, but instead strike it at an angle. This allows frequencies higher than foF2 to be reflected. The angle that the radio waves strike the ionosphere depends on the distance between the transmitter and receiver, and the height of the ionosphere, which unfortunately also varies. This is called the hmF2, and there’s a real time map of it also: http://www.spacew.com/www/hmf2.gif
The Maximum Usable Frequency (MUF) can be found by:
MUF = foF2 * sqrt( 1+ [D/(2*hmF2)]^2) where D is the distance in km.
Lately, foF2 has been reaching very low values once the Suns sets. This is what causes the 43 meter band to “go long”, making it difficult to impossible to hear stations even many hundreds of miles away. As an example, here is a plot of the measured foF2 value taken over Wallops Island, VA. Consider these values typical for much of the eastern US during this time period:
The blue trace is today’s foF2 values, red is yesterday’s, and the green trace is an average of the last five days.
foF2 was about 5 MHz at 2300z, dropping to 4.5 MHz by 0000z. This was evident in the loggings for The Crystal Ship. Many listeners who normally get a strong signal from this station had poor or no reception (as was my case). This was also the start of a geomagnetic storm, the K index at 0000z was 3, and has since risen to 5 as I type this.
The flip side of a low foF2 value is that listeners at a greater distance from a station can get stronger signals. The geomagnetic storm last night could also have actually enhanced reception for some listeners. Medium wave DXers have referred to geomagnetic storms as “stirring the gumbo”, bringing in a different mix of station than are normally heard.
Update – here is the link to the real time Wallops Island foF2 chart: http://www.ngdc.noaa.gov/stp/IONO/rt-iono/realtime/WP937_foF2.png and the current graph itself: