Trying to add shadow to entities using CesiumJS

I'm trying to add a "shadow" of the in my replay but I'm not exactly sure if I should name that as a "shadow".

Please, look at my screenshot, you can see a red shadow below every entities (where I set the dots to highlight what I'm looking for)

You can see it live on https://ayvri.com/scene/z15y96gzjx/ck43xpxd500013e5ra7dh6s8e

I did try to find something documented for this feature, but so far, I didn't found. Neither on the Sandcastle.

I thought I would need to add particles to add a kind of tail, but I'm not even sure if it's the right direction to investigate on.

Thanks for any help!

Edit 2 : To see clearly what I need to reproduce, you can also check this screenshot. Below the white trace, you can see a red gradient (from opacity 0 to 1) starting from the beginning of the trace.

there is working example with ground tracer as I promised:

 var viewer = new Cesium.Viewer('cesiumContainer', {
scene3DOnly : true,
shadows : true,
timeline: false,
terrainShadows : Cesium.ShadowMode.ENABLED
});
var terrainProvider = Cesium.createWorldTerrain();
viewer.terrainProvider = terrainProvider;
//---------------------- Start position of the plane ---------------------------
var startLongitude = -123.0744619, startLatitude = 44.0503706;
viewer.camera.flyTo({destination: Cesium.Cartesian3.fromDegrees(startLongitude, startLatitude, 900)});
//--------------------------- CREATE A PLANE ENTITY ----------------------------
var plane = viewer.entities.add({
//   position: new Cesium.Cartesian3.fromDegrees(startLongitude, startLatitude, 600),  
model : {
uri : '../SampleData/models/CesiumBalloon/CesiumBalloon.glb',
minimumPixelSize : 128,
maximumScale : 20000,
color : Cesium.Color.BLACK.withAlpha(0.5),
}
});
//------------ Array of (time, position) samples along the plane route ---------
var airPathArray = [];
//------------ Array of the points of the ground path under the plane ----------
var groundPathArray = [];
//------------ Initialize arrays ----------
var timeNow = new Cesium.JulianDate.now();
airPathArray.push({time: timeNow, degrees: {longitude: startLongitude, latitude: startLatitude},
position: new Cesium.Cartesian3.fromDegrees(startLongitude, startLatitude, 600)});
groundPathArray.push(startLongitude);
groundPathArray.push(startLatitude);
//----------- Every 1 second a new point on the plane route is created ---------
var pathHandler = setInterval(function(){
var timeNow = new Cesium.JulianDate.now();
//timeNow = Cesium.JulianDate.addSeconds(timeNow, 2, new Cesium.JulianDate());
// At start it takes initial coordinates
// New next point coordinates are randomly taken
var lon = airPathArray[airPathArray.length - 1].degrees.longitude + 0.0005 * (1 - Math.random());
var lat = airPathArray[airPathArray.length - 1].degrees.latitude + 0.0005 * (1 - Math.random());
airPathArray.push({time: timeNow, degrees: {longitude: lon, latitude: lat}, position: new Cesium.Cartesian3.fromDegrees(lon, lat, 600)});
groundPathArray.push(lon, lat);
// The maximum number of the route points is fixed
if(airPathArray.length > 3){
airPathArray.shift();
groundPathArray.shift();
groundPathArray.shift();
}
// The updated route is ready for visualization
viewRoute();
}, 1000);
// Route visualization
function viewRoute() {
// console.log(airPathArray[0].position);
var trace = new Cesium.SampledPositionProperty();
for(var i = 0; i < airPathArray.length; i++) {
trace.addSample(airPathArray[i].time, airPathArray[i].position);
}
plane.position = airPathArray[0].position;
//plane.position.setInterpolationOptions({
//  interpolationDegree : 1,
//  interpolationAlgorithm : Cesium.LinearApproximation
//});
plane.orientation = new Cesium.VelocityOrientationProperty(trace);
}
//------------------------ CREATE A GROUND PATH ENTITY -------------------------
function updatePositions() {
var positions = new Cesium.Cartesian3.fromDegreesArray(groundPathArray);
return positions;
}
var goroundPath = viewer.entities.add({
corridor : {
width : 50.0,
material : Cesium.Color.BLUE.withAlpha(0.5),
positions: new Cesium.CallbackProperty(updatePositions, false)
}
});