How can I turn on/off Rigidbody.useGravity?

I have put together a simple rigidbody motor and am trying to have the Rigidbody's "Use Gravity" turned off (false) while the character is grounded, and turned on (true) when they are not grounded. Here is my motor script, it is written in C#:

 using UnityEngine;
 using System.Collections;
     
 public class CharacterMovement : MonoBehaviour
 
 {
     public float forwardSpeed = 10.0f;
     public float backwardSpeed = 6.0f;
     public float sidewaySpeed = 8.0f;
     public float jumpForce = 10.0f;
     public float airModifier = 0.8f;
 
     public void FixedUpdate()
 
     {
 
         bool grounded;
 
         //When the user presses the W or UpArrow key, move them forward at the forward speed
         Vector3 forwardMovement = Input.GetAxis("Forward") * transform.forward * Time.deltaTime * forwardSpeed;
 
         //When the user presses the S or DownArrow key, move them backward at the backward speed
         Vector3 backwardMovement = Input.GetAxis("Backward") * transform.forward * Time.deltaTime * backwardSpeed;
 
         //When the user presses left or right (or "a & "d") on the keyboard, move them side ways at the sideways speed
         Vector3 sidewaysMovement = Input.GetAxis("Horizontal") * transform.right * Time.deltaTime * sidewaySpeed;
 
         //Is the character grounded?
         if (Physics.Raycast(transform.position, -transform.up, 2)) {
             grounded = true;
             Rigidbody.useGravity = false;
 
         } else {
 
             forwardMovement *= airModifier;
             backwardMovement *= airModifier;
             sidewaysMovement *= airModifier;
             grounded = false;
         }
 
        //Jump when the user presses the space key AND the character is grounded
         if (Input.GetKeyUp("space") && grounded)
         {
             rigidbody.AddRelativeForce(transform.up * jumpForce, ForceMode.Impulse);
             Rigidbody.useGravity = true;
         }
 
         //Move the character
         transform.Translate(forwardMovement + sidewaysMovement);
         transform.Translate(backwardMovement + sidewaysMovement);
     }
 }

I am getting 2 errors about each of the "Rigidbody.useGravity" lines since I'm not sure how exactly to access the compenent correctly: An object reference is required to access non-static member `UnityEngine.Rigidbody.useGravity'

Would appreciate help with how I can fix these two lines. Thank you.