Abstract:
The formation of equatorial plasma bubbles (EPBs) associated with spread F irregularities are
fairly common phenomenon in the postsunset equatorial ionosphere. These bubbles grow as a result of
eastward polarization electric field resulting in upward E × B drift over the dip equator. As they grow they are
also mapped to low latitudes along magnetic field lines. The EPBs are often observed as airglow depletions in
the images of OI 630nm emission. On occasions the growth of the features over the dip equator is observed as
poleward extensions of the depletions in all-sky images obtained from low latitudes. Herein, we present
interesting observations of decrease in the latitudinal extent of the EPBs corresponding to a reduction in their
apex altitudes over the dip equator. Such observations indicate that these bubbles not only grow but also
shrink on occasions. These are the first observations of shrinking EPBs. The observations discussed in this work
are based on all-sky airglow imaging observations of OI 630.0nm emission made from Panhala (11.1°N dip
latitude). In addition, ionosonde observations made from dip equatorial site Tirunelveli (1.1°N dip latitude) are
used to understand the phenomenon better. The analysis indicates that the speed of shrinking occurring in
the topside is different from the bottomside vertical drifts. When the EPBs shrink, they might decay before
sunrise hours.
