The Modified Range Time lntensity Technique for studying the height variation of the (back-scatter power of vhf radar echoes at Jicamarca has been used to show that the growth lime of E-region irregularities is less than 1 min. Further, these irregularities disappear or reappear precisely at the time when AH at an equatorial minus AH at a non-equatorial station is close to zero irrespective of absolute H over the dip equator. This is consistent with the idea that E-region irregularities over the dip equator scattering the hf or vhf radio waves are due to gradient drift instability and supports the double-current hypo1hcsis for the equatorial electrojet.
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