C     ****
C     ****     /FIELD/ CONTAINS MAGNETIC QUANTITIES READ FROM MASS STORE
C     ****
       real :: alatm,alonm,xb,yb,zzb,bmod,dmlat,rjac,av,p,rmag11,rmagc,
     |  rmag2,rmag22,rjacd
      COMMON/FIELD/ALATM(ZIMXP1,0:ZJMXP1),ALONM(ZIMXP1,0:ZJMXP1),
     1  XB(ZIMXP1,0:ZJMXP1),YB(ZIMXP1,0:ZJMXP1),ZZB(ZIMXP1,0:ZJMXP1),
     2  BMOD(ZIMXP1,0:ZJMXP1),DMLAT(ZIMXP1,0:ZJMXP1),
     3  RJAC(ZIMXP1,0:ZJMXP1,2,2),AV(ZIMXP1,0:ZJMXP1,3,2),
     4  P(ZIMXP1,0:ZJMXP1),RMAG11(ZIMXP1,0:ZJMXP1),
     5  RMAGC(ZIMXP1,0:ZJMXP1),RMAG2(ZIMXP1,0:ZJMXP1),
     6  RMAG22(ZIMXP1,0:ZJMXP1),RJACD(ZIMXP1,0:ZJMXP1)
      integer,parameter :: nwfield = zimxp1*(zjmxp1+1)*23
C     ****
C     ****     WHERE:
C     ****       ZIMX=72, ZJMX=36, ZIMXP1=ZIMX+1, ZJMXP1=ZJMX+1
C     ****       ALATM = GEOMAGNETIC LATITUDE AT EACH GEOGRAPHIC GRID
C     ****               POINT               (RADIANS)
C     ****       ALONM = GEOMAGNETIC LONGITUDE AT EACH GEOGRAPHIC
C     ****               GRID POINT           (RADIANS)
C     ****       XB    = NORTHWARD COMPONENT OF MAGNETIC FIELD
C     ****       YB    = EASTWARD COMPONENT OF MAGNETIC FIELD
C     ****       ZB    = DOWNWARD COMPONENT OF MAGNETIC FIELD
C     ****       BMOD  = MAGNITUDE OF MAGNETIC FIELD        (IN GAUSS?)
C     ****       DMLAT = DIPOLE LATITUDE CORRESPONDING TO APEX OF
C     ****               FIELD LINE
C     ****       RJAC(2,2) = SCALED DERIVATIVES OF GEOMAGNETIC
C     ****                   COORDINATES W.R.T. GEOGRAPHIC COORDINATES
C     ****         RJAC(1,1) = COS(THETAS)/COS(THETA)*D(LAMDAS)/D(LAMDA)
C     ****         RJAC(1,2) = COS(THETAS)*D(LAMDAS)/D(THETA)
C     ****         RJAC(2,1) = 1./COS(THETA)*D(THETAS)/D(LAMDA)
C     ****         RJAC(2,2) = D(THETAS)/D(THETA)
C     ****
C     ****         WHERE  (LAMDA,THETA) ARE GEOGRAPHIC COORDINATES
C     ****                (LAMDAS,THETAS) ARE GEOMAGNETIC COORDINATES
C     ****       RJACD = DETERMINANT OF RJAC
C     ****       AV    = THE TWO MAGNETIC VECTORS A1 AND A2
C     ****         AV1 = A1
C     ****         AV2 = A2/COS(THETAS)
C     ****       P     = SCALE FACTOR BETWEEN GEOGRAPHIC AND GEOMAGNETIC
C     ****               SYSTEMS DIVIDED BY  SIN(DIP)*COS(THETAS)
C     ****             = RS**2*COS(THETA)/(BMOD*COS(THETAS))*
C     ****                 D(LAMDA,THETA)/D(LAMDAS,THETAS))
C     ****       RMAG11 = (A1.A1)/P*SIN(I)*COS(THETAS)
C     ****       RMAGC  = (A1.A2)/P*SIN(I)
C     ****       RMAG2  = 1./BMOD
C     ****       RMAG22 = (A2.A2)/P*SIN(I)/COS(THETAS)
C     ****
C