Ground measurements of the geomagnetic field, during the total solar eclipse of July 11, 1991, were carried out at three sites in Costa Rica located within the eclipse's path of totality. Near totality, there was a significant change in the total intensity, F. Its normal decrease in the afternoon hours was interrupted during a period of about 45 min, in which F varied very little. This was due mainly to a small variation in horizontal intensity, H, during such a period. The departure of F from normal lasted about 100 min; its maximum departure was near 10 nT and occurred about 38 min after totality, The declination, D, also experienced a small change (about 2.3 min) during the total period of the eclipse. The vertical intensity, Z, and the inclination, I, did not show significant changes.
