public double SemiAxis2Length {get; set;}

GeodesicEllipseParameter param = new GeodesicEllipseParameter();
param.AxisDirection = 4 * Math.PI / 3;
param.Center = new Coordinate2D(-120, 60);
param.LinearUnit = LinearUnit.Meters;
param.OutGeometryType = GeometryType.Polyline;
param.SemiAxis1Length = 6500000;
param.SemiAxis2Length = 1500000;
param.VertexCount = 800;

Geometry geometry = GeometryEngine.Instance.GeodesicEllipse(param, SpatialReferences.WGS84);
// geometry.IsEmpty = false

Polyline polyline = geometry as Polyline;
// polyline.PointCount = 801
// polyline.PartCount = 1