The ionosphere doesn't have a clearly defined "beginning" like a physical boundary. Instead, it gradually merges with the thermosphere, the layer of the atmosphere above the mesosphere.

However, we can define a general altitude range where the ionosphere is most prominent:

* It starts around 60 kilometers (37 miles) above the Earth's surface.

* It extends up to about 1,000 kilometers (621 miles).

The D, E, and F layers of the ionosphere, which are defined by their different electron densities, are all found within this range. The lower boundary of the ionosphere is influenced by the degree of ionization, which varies depending on solar activity.

It's important to note that the height of the ionosphere can fluctuate depending on factors like:

* Solar activity: During periods of high solar activity, the ionosphere expands and its lower boundary descends.

* Time of day: The ionosphere is typically thicker and more densely ionized during the day due to solar radiation.

* Geographic location: The ionosphere is generally thicker near the equator and thinner near the poles.

So, while it's not a sharp line, the ionosphere is generally considered to begin around 60 kilometers above the Earth's surface.