Hello,

The maximum altitude that can be reached by sounding rockets depends upon the type of sounding rocket used and how heavy the scientific payload is. I think the Black-Brant XII is currently the sounding rocket configuration used by NASA that is capable of reaching the highest altitudes. For a payload of about 300 lb, it can reach a maximum altitude of 1500 km (about 930 miles). If the payload is heavier, then the maximum altitude will be lower. When scientists build instruments for a sounding rocket mission, they have to be very conscious of how heavy their instruments are in order to make sure that the rocket can reach the right altitude to accomplish the scientific goals of the mission. You can read more about the different types of sounding rockets that NASA uses here:

http://sites.wff.nasa.gov/code810/vehicles.html

The links on the above page will bring up PDF files that include graphs of the maximum altitude and range for different payload weights and launch angles for each of the different sounding rockets.

The Sun can affect the Earth's ionosphere is different ways. One of the most important ways the Sun and solar activity affect the ionosphere is by changing something scientists call the "scale height" of the ionosphere. The scale height is basically a measure of the rate at which density changes with altitude. Scientists can divide the ionosphere into different layers based upon the electron density at different altitudes. Each of the layers (C,D, E, F) of the ionosphere has different densities, compositions, and radio absorption properties. There are daily and seasonal variations in the Earth's ionosphere due to the amount of light from the Sun reaching different regions of the atmosphere, as well as variations over the solar cycle caused by changes in solar and geomagnetic activity levels. Changes in the ionospheric scale height can affect radio wave propagation and the drag on satellites in low-Earth orbits, so understanding the ionosphere is very important.

Kris