Quantifying the daytime, equatorial ExB drift velocities associated with the 
4-cell, non-migrating tidal stucture 

David Anderson, Eduardo Araujo-Pradere, Adela Anghel, Kiyo Yumoto, Archana 
Bhattacharyya, Maura Hagan, Astrid Maute 
 
In the daytime, equatorial ionosphere, the vertical ExB drift velocity is the 
major transport mechanism for creating crests in ionization on either side of 
the magnetic equator at +/- 15o to 18o dip latitude known as the equatorial 
anomaly. There is a large, day-to-day variability in these drift velocities due 
to tidal forcing from below, penetration of high latitude electric fields and 
disturbance dynamo generated electric fields. In this paper, we investigate the 
seasonal and longitude dependence of the daytime, vertical ExB drift velocities, 
on a day-to-day basis, using a recently-developed technique for inferring 
realistic ExB drifts from ground-based magnetometer observations. We have chosen 
only quiet days, Ap<10, from January, 2001 through December, 2002, so that the 
main contribution to the variability is due to the variability in the tidal 
forcing from below. In order to study the longitude dependence in daytime ExB 
drift velocities, we use appropriately-placed magnetometers in the Peruvian, 
Indian and Philippine sectors. To study the seasonal variability we have 
binned all of the quiet day observations in 2 month bins, e.g. January-February, 
March-April, November-December for both 2001 and 2002. 
Since we are particularly interested in quantifying the ExB drift velocities 
associated with the 4-cell, non-migrating tidal structure, we compare the 
seasonal and longitude ExB drift structure with recent satellite observations of 
neutral temperature and neutral winds that signify tidal forcing as well as 
satellite observations of daytime ExB drift velocities. In addition, we compare 
the ExB drifts with recent theoretically-calculated, daytime values in the 
Peruvian, Indian and Philippine longitude sectors. As an example, we find that 
the maximum, average ExB drift velocity for the March-April, 2002 time period, 
which corresponds to the IMAGE FUV observations of the 4-cell pattern, is 26 
m/sec in the Peruvian sector; 22 m/sec in the Indian sector and 23 m/sec 
in the Philippine sector. From the IMAGE observations, one of the four 
maxima in the 135.6 nm radiances occurred in the Peruvian sector and two
of the four minima occurred in the Indian and Philippine sectors, respectively. 
Implications of the season and longitudinal dependences in ExB drift velocities 
to tidal forcing will be discussed.