Periodic fluctuations in the geomagnetic field during magnetic storms

Abstract

Abstract Some of the characteristics of the periodic fluctuations occurring in the horizontal intensity of the Earth's magnetic field during magnetic storms are investigated, using the five minutes values of H at Huancayo, Trivandrum, Alibag, Honolulu and San Juan during the storm of 8 July 1958. The study has been extended to sixteen sudden commencement and twenty gradual commencement storms at Huancayo during 1958 using 5-min values of H at Huancayo. Adopting the method of auto-correlation analysis, we have studied the period of oscillations of the significant fluctuations, and find that most commonly the period is about 40 min. The oscillations occur generally during the initial and main phases of the storm. The fluctuations are in phase at the two equatorial stations of Huancayo and Trivandrum or Huancayo and Alibag. They are therefore simultaneous at stations round the Earth close to the geomagnetic equator. At Honolulu or San Juan, the fluctuations are irregular and are poorly correlated with those at the equatorial stations. The situation at a high latitude station such as Sitka has not been studied. On the assumption that the periodic fluctuations are due to large scale inhomogeneities in the solar plasma impinging on the magnetosphere, we have calculated the average scale length of the inhomogeneities in the plasma to be 0.02 a.u. This scale length is comparable to the scale length of inhomogeneities derived by McCracken (1) from solar flare cosmic ray events and the scale length indicated in the experimental results by Bryant et al (2) relating to energetic particles measured by Explorer XII.