Vertical rise velocity of equatorial plasma bubbles estimated from Equatorial Atmosphere Radar (EAR)observations and HIRB model simulations

Abstract

The vertical rise velocity (Vr) and maximum altitude (Hm) of equatorial plasma bubbles (EPBs)were estimated using the two-dimensional fan sector maps of 47 MHz Equatorial Atmosphere Radar (EAR),Kototabang, during May 2010 to April 2013. A total of 86 EPBs were observed out of which 68 werepostsunset EPBs and remaining 18 EPBs were observed around midnight hours. The vertical rise velocities ofthe EPBs observed around the midnight hours are significantly smaller (~26–128 m/s) compared to thoseobserved in postsunset hours (~45–265 m/s). Further, the vertical growth of the EPBs around midnight hoursceases at relatively lower altitudes, whereas the majority of EPBs at postsunset hours found to have grownbeyond the maximum detectable altitude of the EAR. The three-dimensional numerical high-resolutionbubble (HIRB) model with varying background conditions are employed to investigate the possible factorsthat control the vertical rise velocity and maximum attainable altitudes of EPBs. The estimated rise velocitiesfrom EAR observations at both postsunset and midnight hours are, in general, consistent with the nonlinearevolution of EPBs from the HIRB model. The smaller vertical rise velocities (Vr) and lower maximum altitudes(Hm) of EPBs during midnight hours are discussed in terms of weak polarization electric fields within thebubble due to weaker background electric fields and reduced background ion density levels.