This is a blend between the outer circle and the inner ellipse.
- Draw both ellipse and circle, both with a stroke colour and no fill.
- Align their centers so they overlap.
- Select the ‘blend’ tool.
- Click once on the top anchor of the circle.
- Then click the right-hand anchor of the ellipse.
- Double-click the blend tool in the toolbox to have a dialog box pop up.
- In the box, enter the option ‘specified steps’ in the drop-down, and choose a number of steps around 20, then click OK.