Getting error when initializing Rigidbody, A field initializer cannot reference the nonstatic field, method or property.

I created a simple script for a ball. The principle is, when I press space, it activates gravity and it falls, then respawns to another position, with gravity turned off, waiting to press Space to fall again. I followed the code on the scripting API page for Rigidbody, but I still get this error:

" Assets/Custom/sphereBehaviour.cs(7,31): error CS0236: A field initializer cannot reference the nonstatic field, method, or property `sphereBehaviour.ball' "

Here's the code:

 public class sphereBehaviour : MonoBehaviour {
 
     public GameObject ball;
     public Rigidbody rb = ball.GetComponent<Rigidbody>();
 
     // Use this for initialization
     void Start () {
         ball = GameObject.Find ("Sphere");
     }
     
     // Update is called once per frame
     void Update () {
 
         rb.useGravity = false;
 
         if (Input.GetKeyDown (KeyCode.Space))
             rb.useGravity = true;
 
         if(ball.transform.position.y < 1f)
             ball.transform.position = new Vector3 (0, setRndHeight (), 0);
     }
 
     float setRndHeight() {
 
         float rndHeight = Random.Range (3f, 11f);
 
         print ("Ball height is " + rndHeight);
 
         return rndHeight;
 
     }
 }