The daytime equatorial electrojet is a narrow band of enhanced eastward current flowing in the 100 to 120 km altitude region within ±2° latitude of the dip equator. A unique way of determining the daytime strength of the electrojet is to observe the difference in the magnitudes of the Horizontal (H) component between a magnetometer placed directly on the magnetic equator and one displaced 6 to 9 degrees away. The difference between these measured H values provides a direct measure of the daytime electrojet current, and in turn, the magnitude of the vertical ExB drift velocity in the F region ionosphere. This paper discusses a recent study that has established the quantitative relationship between the vertical ExB drift velocity in the ionospheric F region and the daytime strength of the equatorial electrojet in the South American (west coast) longitude sector.
 In the low latitude, F region ionosphere, the ambient ion and electron density distributions are determined through the combined physical processes of: (1) Production by solar EUV radiation; (2) Loss through charge exchange with N2 and O2 and; (3) Transport parallel to geomagnetic field lines by diffusion and neutral winds and perpendicular to B by ExB drift. In the daytime E region (90–120 km), dynamo processes generate eastward electric fields, which are transmitted to F region altitudes (150–800 km) by equipotential geomagnetic field lines, causing both ions and electrons to drift upward, perpendicular to B with an ExB/B2 drift velocity. At the same time, forces parallel to B due to gravity and plasma pressure gradients act to transport plasma along the magnetic field lines. The net effect is to create crests in electron density on either side of the magnetic equator at ±15 to 18 degrees dip latitude, known as the equatorial anomaly. Trans-equatorial neutral winds transport ionization from one hemisphere to the other causing asymmetries in both peak electron densities and peak altitudes in the equatorial anomaly (see Moffett  for an excellent review). The primary transport mechanism for creating the equatorial anomaly is vertical ExB drift in the F region. Fejer  and Scherliess and Fejer  discuss the large day-to-day variability in vertical drift velocities as measured by the Jicamarca Incoherent Scatter Radar (ISR) located at the magnetic equator in Peru. This day-to-day variability in ExB drift is responsible for the large day-to-day variability in the low latitude F region ion and electron density distributions with altitude, latitude and local time [Anderson, 1973]. Because the daytime upward ExB drift is so critical, it is very important to be able to specify the drift on a day-to-day basis, since the Jicamarca ISR makes drift observations only two or three times a month.
 The purpose of this paper is to demonstrate that there exist quantitative relationships whereby the vertical ExB drift velocity in the equatorial F-region can be estimated using ground-based magnetometer observations. Such quantitative relationships have been developed for the South American sector, during the Solar Maximum period, 1998–1999. This represents the first time such a unique relationship has been quantitatively established. The paper (1) Describes briefly the physics of electrodynamics of the equatorial electrojet; (2) Outlines the data sets that were used to develop the relationship; (3) Validates the relationship by demonstrating how well the inferred daytime ExB drift velocities compare with actual measured drifts throughout the day and (4) Summarizes a few of the scientific studies that can now be carried out knowing the day-to-day variability in the magnitude of ExB drift velocities.
2. Low Latitude Electrodynamics
 It is well known that the effect of neutral winds together with diurnal and semi-diurnal tidal components in the atmosphere cause currents to flow in the 100 to 130 km altitude region. This is the so-called Sq (Solar quiet) wind dynamo current system in the E region. Resulting from this current system is an electrostatic field directed eastward from dawn to dusk at low latitudes. The strength of this electric field is about 0.5 mV/m and is responsible for the upward ExB drift velocities of ∼20 m/sec measured by the Jicamarca ISR. As a result of this electric field, within ±2° of the magnetic equator, an enhanced eastward current flows (between 100 and 110 km altitude) known as the equatorial electrojet (see Richmond ; and Reddy  for in-depth reviews of the neutral wind dynamo and the equatorial electrojet, respectively).
Figure 1 depicts the eastward electric field (yellow arrow), the consequent vertical electric field (red arrow) and the current systems that are associated with the electrojet. The view is to the North at the magnetic equator viewing the dayside region. If an eastward electric field exists and is perpendicular to B, then a Hall current is generated in the downward direction. Because of the particular geometry at the magnetic equator where magnetic field lines are horizontal, the Hall current, carried by upward moving electrons, quickly polarizes the ionospheric E layer so that an upward directed polarization electric field is produced. This electric field (red arrow) is about 5 to 10 times stronger than the eastward electric field (yellow arrow) that produced it. It is this vertical electric field that is responsible for the eastward equatorial electrojet current (carried primarily by electrons drifting westward with an ExB/B2 velocity). This current produces the strong enhancement in the H component observed by magnetometers within ±2° of the magnetic equator.
Figure 2 is a schematic plot of typical noontime magnetometer H component observations as a function of latitude. Note the 100 nanoTesla (nT) increase near the dip equator superimposed on the “global” Sq current magnetometer observations. When the H component observations from a magnetometer 6 to 9 degrees away from the magnetic equator is subtracted from the H component value measured by a magnetometer on the magnetic equator, the difference is only related to the electrojet contribution which, in turn, is directly related to the eastward electrostatic field that created the electrojet current. Carrying out this subtraction to provide a ΔH value is necessary in order to eliminate the Dst ring current component in H as well as the global Sq Dynamo component of H. The resulting ΔH value is then only related to the ionospheric electrojet current and hence the east-west electric field. This eastward electric field might originate from the Sq Wind dynamo mechanism or could be associated with a penetration electric field from high latitudes, or both. It is emphasized that the electric field is ionospheric in origin and is not associated with the Dst “ring” currents, or the Tail currents.
3. Calculating the Relationship Between ΔH and ExB Drift
Rastogi and Klobuchar  suggested and demonstrated that the strength of the daytime equatorial electrojet could be measured using two magnetometers, one situated on the magnetic equator and the other displaced by 6 to 9 degrees away. Using this technique they were able to infer whether the daytime vertical ExB drift velocity in the F region was large or small. They compared the difference in the Horizontal (H) component values between magnetometers at Trivandrum (8.5°N, 77.0°E., 0.5°S dip lat.) and Alibag (18.5°N, 72.9°E, 13.0°N dip lat.) with the observations of Total Electron Content (TEC) measured by a chain of polarimeters as a function of latitude and local time in the Indian Sub-Continent. It is well known that large upward ExB drift velocities produce the equatorial anomaly with crests in the peak electron density, Nmax, and TEC at ±15° dip latitude while the absence of ExB drift does not create the anomaly. They verified that a weak equatorial electrojet was accompanied by an absence in TEC crests, while a strong electrojet (large ΔH values) was accompanied by observed daytime crests in TEC at ±15 ° dip latitude. They also found that measuring the day-to-day fluctuation in H at only one station — Trivandrum — was not a realistic measure of the strength of the equatorial electrojet. Anderson et al.  subsequently carried out theoretical calculations of TEC as a function of local time and latitude and compared these with the Indian TEC observations. They found excellent agreement for both “weak” and “strong” electrojet days. Neither Rastogi and Klobuchar  nor Anderson et al.  presented a quantitative relationship between ΔH and ExB drift — only a qualitative one. In the current investigation, however, we do determine this relationship, quantitatively. In order to develop the relationship between the strength of the equatorial electrojet and the daytime, vertical ExB drift in the F region, data sets from two magnetometer sites in Peru were obtained. In addition, the observed vertical drift velocities were obtained from the Jicamarca Incoherent Scatter Radar (ISR) facility between June 1998 and July 1999. The 1-minute averaged Horizontal (H) component observations from Canete, Peru (0.8°N. dip lat.) and Piura, Peru (6.8°N. dip lat.) were obtained from the Circum-Pan Pacific Magnetometer Network [Yumoto, 2001] and the Jicamarca ISR observed ExB drifts, from Dr. Erhan Kudeki, University of Illinois.
 For each of the magnetometer data sets at Canete and Piura, the nighttime baseline in H was first obtained for each day and then subtracted to give the daytime values. This produced daytime H component values at each of the two stations for 10 days in 1998 and 1999. In order to investigate the ΔH vs ExB drift relationship, all of the 10 days when the Jicamarca ISR observed ExB drift velocities anytime between 10 and 13 LT were plotted against the ΔH values observed at those same times. The smallest time interval is 5 minutes and this is dictated by the Jicamarca drift observations. We separate the data points into two sets, those contained in the region where ΔH and ExB drift are both positive and those where they are both negative. Figure 3 displays the relationship between ΔH and ExB drift when both are positive. The least-squares, straight line fit to the points in Figure 3 is given by ΔH = 2.3 * ExB drift + 14.0nT. The error associated with the assumption of a linear, least-squares fit to the data points is 26 nT which is equivalent to an ExB drift velocity of 5 m/sec. The linear-correlation coefficient, r, is 0.9. Figure 4 displays the relationship when ΔH and ExB drift are both negative. The least squares, straight line fit to the points is given by ΔH = 6.1*ExBdrift − 11.0nT. To avoid discontinuities for small ExB drifts, the relationship that is used is ΔH = 28.1 * ExBdrift − 11.4nT. It should be emphasized that these results apply for the South American (west coast) sector under solar cycle maximum conditions. It is entirely likely that the relationship will be different at different longitudes and for different solar cycle conditions.
 In order to verify that the relationships derived above are realistic and can be applied at other local times, Figures 5 and 6 plot the ΔH-inferred ExB drifts for two days when the Jicamarca ISR measured ExB drifts throughout the day. Figure 5 displays the comparison on July 16, 1998 (day 197) during the daytime hours. The agreement is excellent. Not only does the ΔH-inferred drifts agree with the measured ExB drift values over the longer, hourly time scales but also over the tens-of-minutes time scales. To emphasize this point, Figure 6 compares the drifts on October 21, 1998 (day 294) when rapid changes in ExB drifts occur, due primarily to the penetration of high latitude electric fields to the equatorial region. Again, both the ISR measured ExB drifts and the ΔH-inferred drifts are in good agreement. The daytime RMS error for Figure 5 is 4.8 m/sec while the RMS error for Figure 6 is 3.7m/sec. The RMS error is defined by sqrt (∑(ExB drift (observed) — ExB drift (inferred)) 2)/sqrt (N), where N is the number of 5 minute local time intervals for each day.
 In a paper by Stening , the electrojet current, eastward electrostatic field and the Horizontal (H) component in the Peruvian sector are calculated and compared with observations. Moderate solar cycle activity conditions (F10.7 ∼ 140) are assumed. Stening incorporates his “equivalent circuit method” [Stening, 1968] to self-consistently calculate both the electrojet current and the electric field perpendicular to B. The resulting magnetic field variations at the Earth's surface are compared with magnetometer H and Z component observations. He assumes that 40% of the total H component value is due to induced currents within the Earth. His Figure 6 displays the calculated and observed H values as a function of latitude near noontime in the Peruvian sector. In this case, his H values represent the daytime H value after subtracting the nighttime, baseline H values. The calculated, eastward electrostatic field that generates this latitudinal variation in H is 0.94 mV/m. In the South American longitude sector where the magnitude of B is ∼0.3 gauss, 1mV/m corresponds to 40 m/sec (ExB/B2 drift = E/B). Therefore, a value of 0.94 mV/m is equivalent to an upward ExB drift velocity of 38 m/sec in the F region. Although Stening does not try to establish the relationship between this ExB drift value and the difference between the H value at the magnetic equator and at ±6 to 8 degrees dip latitude, from his Figure 6, this relationship turns out to be [210 nT − 110 nT]/38 m/sec = 2.6 nT/m/sec. This is surprisingly close to the slope of 2.3 nT/m/sec that our current investigation has established.
 This paper discusses a recent study that has established the relationship between the vertical ExB drift velocity in the ionospheric F region and the daytime strength of the equatorial electrojet in the South American (west coast) longitude sector. Magnetometer H component observations from Canete (0.8 N. dip lat.) and Piura (6.8 N. dip lat.) in Peru and daytime, vertical ExB drift velocities measured by the Jicamarca Incoherent Scatter Radar (ISR) Facility have been used to establish this relationship. The magnetometer observations and the ISR drift measurements were obtained for the period between July, 1998 and June, 1999. It is found that when ΔH and ExB drift are both positive the relationship is given by ΔH = 2.3 * ExBdrift + 14.0nT and when ΔH and ExB drift area both negative then the relationship is ΔH = 6.1 * ExBdrift − 11.0nT. Excellent agreement is achieved when the magnetometer-inferred vertical, daytime ExB drift values are compared with the Jicamarca ISR observations of ExB drifts. As a result, a number of important Space Weather science investigations can now be carried out relating to the day-to-day variability of the low latitude ionosphere.
 It should be emphasized, that these results are based on only 10 days of observations and only in the Peruvian longitude sector. It is to be expected that the relationship would be different at different longitudes, different solar activity levels and different seasons. A much larger database of observed ExB drift velocities and magnetometer measurements are needed to ascertain the dependence of these relationships on various conditions. In addition, these observed relationships need to be compared with future theoretically calculated values of the eastward electrostatic field and self-consistent calculations of the resulting daytime equatorial electrojet strength and resulting H component magnitudes. These comparisons will then put the observed relationships on a firm understanding of the physical processes that are taking place.