The last week in April has been quite stable following a minor disturbance on the 23rd apparently caused by a coronal hole. Details at: http://www.dxlc.com/solar/
From 27th to the 30th, the index has remained at 5 and the K at 2. The first week in May will be quite disturbed. On the 2nd May, the A index will peak at 20 and the K at 5, then by the 8th May, conditions will settle with the A index at 5 and the K index at 2 until May19th when conditions are likely to deteriorate with the A index rising to 10 and the K index to 3. The solar flux has a downward trend remaining at 75 for the last week in April, then dropping to 70 by 2nd May where it will remain till the 19th May when it again rises to 75.
The Ionosphere: A collection of ionized particles and electrons in the uppermost portion of the earth's atmosphere which is formed by the interaction of the solar wind with the very thin air particles that have escaped the earth's gravity. These ions are responsible for the reflection or bending of radio waves occurring between certain critical frequencies with these critical frequencies varying with the degree of ionization. As a result, radio waves having frequencies higher the lowest usable frequency (LUF) but lower than the maximum usable frequency (MUF) are propagated over large distances.
The lowest part of the ionosphere, the D-layer appears at an altitude of 50-95km. This layer has a negative effect on radio waves because it only absorbs radio-energy, particularly those frequencies below 7MHz. It develops shortly after sunrise and disappears shortly after sunset. This layer reaches maximum ionization when the sun is at its highest point in the sky and this layer is also responsible the complete absorption of sky waves from the 80m and 160m amateur bands as well as the AM broadcast band during the daytime hours.
This part of the ionosphere is located just above the D-layer at an altitude of 90-150km. This layer can only reflect radio waves having frequencies less than 5MHz. It has a negative effect on frequencies above 5MHz due to the partial absorption of these higher frequency radio waves. The E-layer develops shortly after sunrise and it disappears a few hours after sunset. The maximum ionization of this layer is reached around midday and the ions in this layer are mainly O2+.
Highest part of the ionosphere. The F-layer appears a few hours after sunset, when the F1- and F2-layers merge. The F-layer is located between 250km and 500km in altitude. Even well into the night, this layer may reflect radio waves up to 20 MHZ, and occasionally even up to 25 MHZ. Ions in the lower part of the F-layer are mainly NO+ and are predominantly O+ in the upper part.
Thanks to Mike Terry and Ken Fletcher for regular updates.