Abstract:
In this paper, we present response of equatorial and low-latitude ionosphere to 22/23 June 2015
geomagnetic storm using a chain of ground-based ionosondes located at Tirunelveli (8.73°N,77.70°E;
geomagnetic latitude: 0.32°N), Hyderabad (17.36°N, 78.47°E; geomagnetic latitude: 8.76°N), and Allahabad
(25.45°N, 81.85°E; geomagnetic latitude: 16.5°N) along with a chain of GPS receivers. Uniqueness of this storm
is that in contrast to the equatorial plasma bubbles that were detected in the European sector, we see
suppression of plasma bubbles in the Indian sector. The observations suggest that westward penetration
electric field during local midnight caused abrupt decrease of virtual height (h’F (km)) to ~200 km and
suppressed plasma bubbles due to undershielding. Later, the layer increased to 500 km simultaneously due
to overshielding effect. On 23 June, we observed negative storm in the Northern Hemisphere while
positive storm in the Southern Hemisphere. In addition, absence of equatorial Es layers at Tirunelveli and
presence of F3 layer at Tirunelveli/Hyderabad seem to be associated with equatorial electrojet (EEJ)/counter
electrojet (CEJ) variations. However, on 24 June, we observed strong negative storm effects at
Allahabad/Hyderabad, while positive storm effect at Tirunelveli. Simultaneous enhancement of h’F (km) at all
three ionosonde stations at 20:30 UT on 23 June during recovery phase suggest eastward disturbance
dynamo (DD) electric field that caused presunrise spread F at Hyderabad/Allahabad but void of spread F at
Tirunelveli suggesting its midlatitude origin. Periodogram analysis of foF2 and h’F (km) in the present analysis
suggest the presence of shorter periods (~< 2 h) associated with prompt penetration (PP) electric fields
while larger periods (>2 h) associated with DD electric field/winds.
