# GSW coordinate system

SHORT EXPLANATION

The Geocentric Solar Wind (GSW) coordinate system takes into account the actual direction of the solar wind at the Earth's location. The GSW system differs from the standard GSM only in that its $X$-axis is directed anti-parallel to the observed solar wind flow, rather than to Sun’s center, which more accurately takes into account the aberration effect.

## Difference between the GSM and GSW coordinate systems​

The most frequently used coordinate system in solar-terrestrial physics is the Geocentric Solar Magnetospheric (GSM) system. The standard GSM has its $X$-axis pointing toward the Sun, the $Z$-axis is perpendicular to $X$ and belongs a plane formed by the $X$-axis and the Earth's dipole axis, and the $Y$ completes a right-handed orthogonal set.

To be more precise, one should also take into account that the solar wind does not arrive radially but approaches from $\alpha^{\circ}$ west of the Sun due to the Earth's orbital motion, where $\alpha=\arctan(V_E/V_{SW})$ and $V_E=29.77$ km/s. On average, $\alpha$ is equal to $\sim 4^{\circ}$ assuming the solar wind speed $V_{SW}\sim 425$ km/s. Here we should note that the OMNI data are provided with the already removed aberration from the solar wind velocity component (the correction $V_Y=-29.77$ km/s introduced).

If the actual direction of the solar wind is known, then even more precise Geocentric Solar Wind (GSW) coordinate system may be used. It differs from the GSM system only in that $X_{GSW}$ axis is directed anti-parallel to the observed solar wind flow, which more accurately takes into account the variable aberration effect. The GSW system was first introduced and described by Hones et al. (1986).

## GSM to GSW​

Below you can find an example of recalculating the GSM coordinates to the GSW coordinates using the Geopack 2008 subroutines.

ATTENTION: this example is for the single-precision version of Geopack, in case of the double-precision version, one should replace the corresponding lines with implicit real*8 (a-h,o-z) and CALL RECALC_08 (IYEAR,IDY,IHOUR,MIN,ISEC,-400.d0,0.d0,0.d0).

c     !Here we have the GSM Cartesian coordinates XGSM,YGSM,ZGSM (float);c     !we know the date and UT (IYEAR,IDY,IHOUR,MIN,ISEC - all integeres);c     !and we know the solar wind velocity components in the GSE system (VX,VY,VZ)      implicit real*4 (a-h,o-z)      COMMON /GEOPACK1/ AAAA(15),PS,BBBB(28)  ! Common block should be placedC                             ! in the beggining of the program to use the Geopack properly      ...      CALL RECALC_08 (IYEAR,IDY,IHOUR,MIN,ISEC,-400.,0.,0.)    !1st step: call RECALC_08 withc                                  ! the radial SW direction, i.e., VX0=-400., VY0=VZ0=0 km/s      CALL GSWGSE_08 (XGSM,YGSM,ZGSM,XGSE,YGSE,ZGSE,1)         !2nd step: convert GSM to GSE      CALL RECALC_08 (IYEAR,IDY,IHOUR,MIN,ISEC,VX,VY,VZ)       !3rd step: call RECALC_08 withc                                                          ! an actual SW direction VX, VY,VZ      CALL GSWGSE_08 (XGSW,YGSW,ZGSW,XGSE,YGSE,ZGSE,-1)        !4th step: convert GSE to GSW      ...