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Question recommendation for atmosphere mag field setup  

Forum: Fields: Magnetic and Otherwise
Date: 01 Feb, 2009
From: Scott Nutter <Scott Nutter>

I am working on a simulation of the production of synchrotron radiation by very high energy electrons traversing the Earth's magnetic field.  I have to have times of synchrotron photons arriving at a certain height in the atmosphere good to better than a nanosecond. I have been having trouble getting this kind of accuracy.  I base this conclusion on an algorithm to reconstruct the zenith angle looking at the time difference of two spatially nearby photons at a certain height, which arrive on essentially parallel trajectories.  The resulting calculated zenith angles indicate the photons are all arriving from nearly vertically above. This algorithm works fine in a simple monte carlo using geant3 RK tracking and simple straight projection of photon tracks from production positions returned by the g3synge generator.

In reading about magnetic field setup in the application users manual, I noticed a small reference to "integrating over time" in the section on stepper choice. (The code fragment has "nvar=8" and subsequent comments in the version for G4.9.1, the section is 4.3.2.4.)  Since the atmosphere is a completely different scale than the typical HEP experiment, I think it best if I get some recommendations on setting up a field properly, so that times of synchrotron photon production are very carefully computed.  

The field characteristics are:
o The purely magnetic field B~0.5 gauss strength, typical Bperp ~0.1 gauss.  The field varies very little (<25% over 360 km). 
o Atmosphere geometry is G4Orbs setup like "Russian dolls" with radii the size of the Earth + different heights.  The resulting layer thicknesses range from several meters to 50 km.  The thickness of each atmosphere layer is determined by requiring it to have a column density of 20 mg/cm2. Thus the G4Orbs are filled with air of different densities. The lowest density of the air is 1e-22 g/cm3, the lowest limit I could get G4.9.2 to accept. I track from 400 km height above the Earth's surface to 38 km height.
o Electron energies are from 2-50 TeV. 
o My test case is a straightdown electron. With all physics off, the deflection for a constant perpendicular field is in agreement with calculations, and is ~19.5 m for Bperp=0.1 gauss over 362 km.
o My current code for defining the field:
        myField = new CrestMagField(latitude, longitude, time);
        G4FieldManager* fieldMgr
            = G4TransportationManager::GetTransportationManager()
              ->GetFieldManager();
        fieldMgr->SetDetectorField(myField);
        fieldMgr->CreateChordFinder(myField);

Any recommendations? There is much discussion of special cases and fine tuning of the field.  I believe the default tracker is tracking correctly now, but I do not believe the times yet.
Thanks.

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