Plasma bubbles that occur in the equatorial F-region make up one of the most distinguishing phenomena in the ionosphere. Bubbles represent plasma depletions with respect to the background ionosphere, and are the major source of electron density irregularities in the equatorial F-region. Such bubbles are seen as plasma depletion holes (in situ satellite observations), vertical plumes (radar observations), and emission-depletion bands elongated in the north-south direction (optical observations). However, no technique can observe the whole three-dimensional structure of a bubble. Various aspects of bubbles identified using different techniques indicate that a bubble has a “shell” structure. This paper reviews the development of the concepts of “bubble” and “shell” in this context.

1. INTRODUCTION
 2. EMERGENCE OF THE RISING BUBBLE MODEL
 3. CREATION OF BUBBLES BY RAYLEIGH-TAYLORINSTABILITY
 4. THREE-DIMENSIONAL BUBBLE STRUCTURE–PLASMA DEPLETION SHELL
 5. CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES

• Hyosub Kil(The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel MD 20723, USA) Corresponding author