The ionosphere doesn't have a clear-cut starting point, as it's more of a gradual transition zone. However, it's generally considered to begin around 50-60 kilometers (31-37 miles) above the Earth's surface and extends up to about 1,000 kilometers (621 miles).

Here's a breakdown of the ionosphere's layers, with their approximate starting altitudes:

* D Layer: 50-90 km (31-56 mi)

* E Layer: 90-150 km (56-93 mi)

* F Layer (split into F1 and F2): 150-1,000 km (93-621 mi)

It's important to note that the ionosphere's layers can vary in height and density depending on factors like solar activity, time of day, and geographical location.