Added brief instructions for new particle field

src/tracers/tracers.cpp

@@ -22,6 +22,7 @@
 #include "../main.hpp"
 #include "basic_types.hpp"
 #include "utils/error_checking.hpp"
+#include <cmath>
 
 namespace Tracers {
 using namespace parthenon::package::prelude;
@@ -78,6 +79,9 @@ std::shared_ptr<StateDescriptor> Initialize(ParameterInput *pin) {
     tracer_pkg->AddSwarmValue("B_z", swarm_name, real_swarmvalue_metadata);
   }
 
+  // MARCUS AND EVAN LOOK
+  tracer_pkg->AddSwarmValue("lnrho_0", swarm_name, real_swarmvalue_metadata);
+
   return tracer_pkg;
 } // Initialize
 
@@ -155,6 +159,9 @@ TaskStatus FillTracers(MeshBlockData<Real> *mbd, parthenon::SimTime &tm) {
   }
   auto &rho = swarm->Get<Real>("rho").Get();
   auto &pressure = swarm->Get<Real>("pressure").Get();
+  // MARCUS AND EVAN LOOK
+  const auto current_cycle = tm.ncycle;
+  auto &lnrho_0 = swarm->Get<Real>("lnrho_0").Get();
 
   // Get hydro/mhd fluid vars
   const auto &prim_pack = mbd->PackVariables(std::vector<std::string>{"prim"});
@@ -181,6 +188,11 @@ TaskStatus FillTracers(MeshBlockData<Real> *mbd, parthenon::SimTime &tm) {
             B_z(n) = prim_pack(IB3, k, j, i);
           }
 
+          // MARCUS AND EVAN LOOK
+          if (current_cycle % 1 == 0) {
+            lnrho_0(n) = Kokkos::log(prim_pack(IDN, k, j, i));
+          }
+
           bool unsed_tmp = true;
           swarm_d.GetNeighborBlockIndex(n, x(n), y(n), z(n), unsed_tmp);
         }