Help with particles and raycasting

I have a 3D side scroller style game in which the player flies along avoiding stuff, you know, side-scrollery things.

I'd like to add an effect (particle system) to the player when they get close (say 1.6 units (meters)) to the terrain (dangeZone), like a dusty dragging cloud under them sort of thing. I'm familiar with the particle system and raycasts but I don't know how to marry the concepts together to achieve what I'm after.

I'd also be hoping to make the particle system 'grow' the closer the player gets to the terrain if that makes sense. There is also speed to consider, so the closer to the ground and faster the player is should have an effect on the particle system.

My Thoughts:

I already have a score multiplier that uses a raycast to check the player's position from the ground/terrain and increases the closer they get.

void Update() {

    force = speed *2;

    RaycastHit hit;
    Ray downRay = new Ray(transform.position, -Vector3.up);

    if (Physics.Raycast(downRay, out hit, dangerZone)) {

        var distanceToGround = hit.distance;
        float hazardMultiplier = Mathf.Round( (transform.position.y - distanceToGround)*100 ) /100;

        if (hit.collider.tag == "Terrain") {
            playerData.scoreMulitplier = hazardMultiplier;
        }
    } else {
        playerData.scoreMulitplier = playerData.baseScoreMulitplier;
    }
}

I'm thinking I can use the raycast I already have to instantiate a particle system on the terrain/at the raycast hit point but I'm not sure how exactly to go about this.

Any help is appreciated and thanks in advance.