The observations of TEC night-time enhancement in equatorial anomaly region Chen Yanhong Ma Guanyi Center for Space science and Applied Research,Chinese Academy of sciences, Beijing, China

Outline  Introduction  Observation  Summary and discussion

Introduction  Equatorial spread F (ESF) refers to the density irregularities, with a wide spectrum of scale sizes occurring in the nightside equatorial F region.  Among various ESF phenomena, the one with largest scale sizes is localized plasma depletions, sometimes referred to as “plasma bubbles”.  Their characteristics and morphology has been the subject of extensive investigations over the past several decades.  Theoretical and observational investigations indicate that they are generated on the bottomside of the nightside equatorial F region and rise to higher altitudes as a result of nonlinear evolution of the generalized Rayleigh-Taylor and E×B instabilities.

 Oya et al.(1986) and Watanabe and Oya(1986) first reported observations of localized regions of plasma density enhancements in addition to plasma depletions in the nightside equatorial F region. They called the phenomenon plasma “blobs”.  Le(2004) analyzed the plasma density enhancements in detail and their association with ESF plasma depletions using data from ROCSAT-1 and DMSP spacecraft. Observations of density enhancement

Figure from G.Le (JGR,2004)

 Dashora (2005) reported the TEC localized enhancement using GPS data in Udaipur （ 24.6N,73.7E ， geomagnetic latitude:15.6 ） in India. They found this enhancement is not connected with the scintillation index S4. Figure from Dashora (Ann. Geophy.,2005)

 In 2005, we have established 4 GPS stations in the low- latitude area of China, that is Fuzhou (26.1N, 119.3E), Xiamen (24.5N, 118.1E),Guangzhou (23.1N, 113.2E)and Nanning (22.8N, 108.3E). Our GPS stations

 We surveyed the GPS data gathered on the four stations and also found this kind of TEC enhancement.  Because we only want to analyze the relative TEC variations. Therefore, we only use slant TEC from carrier phase observation.  The time resolution is 1 second. Data we used

The first case is on Oct 5,2005. This day,we can find the geomagnetic activity is relatively quiet. Figure from u.ac.jp/dstdir/ Case 1

obviously enhancement in Fuzhou and Xiamen,but in Guangzhou and Nanning,it’s not obviously. The TEC increased about 2TECU, and continued about one hour’s long. And the latitude is about 25 degree.

The second case is on Sep. 12,2005. This day is in the recovery time of a geomagnetic storm. Figure from Case 2

The distribution of IPP (el>20)

PRN 2 LT=(UT+8)

PRN 7

PRN 10

The IPP’s position

Case on Feb. 20,2006 Case 3

Summary  The TEC enhancements are observed in the four stations. And it is occurred more often in Fuzhou and Xiamen than in Guangzhou and Nanning.  This TEC increased about 2TECU, and the duration is from 30 minutes to one hour’s long.  This TEC enhancement can happen pre-midnight and also after midnight, and also in geomagnetic quiet time and disturbed time.  The observation indicate that the enhancement region is discrete. The latitude is limited to geographical latitude (about geomagnetic latitude).

Thanks for your attention!