How do you get particles to follow along the terrain?

Got it working eventually so posting here in case anyone else has the same issue. I have a function which updates particle system position with terrain height and a function to rotate individual particles with the surface normal of the terrain. The 2nd part was much harder than the first.

         private void $$anonymous$$oveParticlesOnTerrain(ParticleSystem ps)
         {
             int numParticlesAlive = ps.GetParticles(m_Particles);
             if (m_Particles == null || m_Particles.Length < ps.maxParticles)
                 m_Particles = new ParticleSystem.Particle[ps.maxParticles];
 
 
             // Change only the particles that are alive
             for (int i = 0; i < numParticlesAlive; i++)
             {
                 RaycastHit raycastHit;
                 Ray r = new Ray(m_Particles[i].position + Vector3.up * 100f, Vector3.up * -1f);
                 if (Physics.Raycast(r, out raycastHit, terrainLayer))
                 {
                     Quaternion rot = Quaternion.FromToRotation(Vector3.forward, raycastHit.normal);
 
                     m_Particles[i].axisOfRotation = Vector3.forward;
                     m_Particles[i].rotation3D = rot.eulerAngles;
 
                     m_Particles[i].position = new Vector3(m_Particles[i].position.x,
                         gravity.currentTerrain.SampleHeight(m_Particles[i].position) + .1f,
                         m_Particles[i].position.z);
                 }
             }
 
             ps.SetParticles(m_Particles, numParticlesAlive);
         }
 
         private void $$anonymous$$oveParticleSystemOnTerrain(ParticleSystem ps)
         {
             ps.transform.position = new Vector3(ps.transform.position.x,
                 gravity.currentTerrain.SampleHeight(ps.transform.position) + .1f,
             ps.transform.position.z);
         }
     

Hi, thank you! I was looking for a solution to create sand particles, and stumbled upon your post. You use raycasting here - how about performance with, let's say, 10000 particles? Also, why is there "gravity.currentTerrain.SampleHeight" -> you sample heighmap, offset by 0.1, but why "gravity" ? Thank you?