Dip Latitude Research Articles

Features of 3–7-day planetary-wave-type oscillations in F-layer vertical drift and equatorial spread F observed over two low-latitude stations in China

Abstract. Recent studies on the equatorial atmosphere–ionosphere coupling system have shown that planetary-wave-type oscillations, as an important seeding mechanism for equatorial spread F (ESF), play an important role in ESF irregularity development and its day-to-day variability in the equatorial latitudes. In this study, ionosonde virtual height and ESF measurements over Sanya (18.4° N, 109.6° E; 12.8° N dip latitude) and meteor radar neutral-wind measurements over Fuke (19.5° N, 109.1° E; 14° N dip latitude) during 2013 are used to investigate the features of planetary-wave-type oscillations in both the lower atmosphere and the ionosphere and their possible influences on ESF occurrence under the weak solar maximum year. The ∼ 3-day and ∼ 7-day planetary-wave-type oscillations have been observed in the neutral zonal winds and the time rate of change in F-layer virtual heights. According to the propagation characteristics, the 3-day and 7-day planetary-wave-type oscillations are basically recognized as ultrafast and fast Kelvin waves, respectively. With increasing heights, the 3-day wave oscillations are gradually amplified, while the 7-day wave oscillations are generally constant. By performing a cross-wavelet transform on the onsets of ESF and the vertical drifts of the F layer, we found that there are simultaneously occurring 7-day and 3-day common wave oscillations between them. The 7-day waves are mainly in the inversion phase, while the 3-day waves are mostly in an in-phase state, indicating that the 7-day waves may play a main role in ESF initiation. Approximate delays of 6 days for the 7-day waves and 5 days for the 3-day waves in their propagation upward from the lower atmosphere to the ionosphere are evaluated with wavelet power spectrum analysis. The estimated upward velocities from these time delays provide consistent evidence that the 7-day and 3-day waves propagate vertically upward with typical Kelvin wave characteristics. The results highlight the role of planetary-wave-type oscillations in the initiation and development of ESF in the Chinese low-latitude region.

Plasma blobs associated with plasma bubbles observed in the Brazilian sector

In this paper we present a case of plasma blobs associated with plasma bubbles which were observed by emission of OI 630.0nm airglow, using ground-based (all-sky images) and DMSP-F15 satellite data on 23 February 2007, over a low latitude station São José dos Campos (SJC) (23.21°S, 45.86°W; dip latitude 18.3°S) in the Brazilian sector. We calculated the zonal drift velocities of the plasma bubble and plasma blobs, and the longitudinal drift of the blobs that occurred that night using the linearization method presented by Pimenta et al. (2001). The north/south and east/west extensions of plasma blobs have also been estimated. The mean velocity of the plasma bubble is found to be 74±8m/s and the plasma blob zonal drift is 61±6m/s. The average velocity of the longitudinal drift of the plasma blob was 85±13m/s and the analyzed blobs had the mean north/south extension of 591km and east/west extension of 328 and 263km.

First observation of interhemispheric asymmetry in the EPBs during the St. Patrick's Day geomagnetic storm of 2015

AbstractIn this work, we have studied the characteristics of equatorial plasma bubbles (EPBs), such as their zonal drift and tilt, from the low‐latitude and dip equatorial region in the Indian longitude sector during the main phase of the 17 March 2015 storm. All‐sky airglow imaging observations from Tirunelveli (8.7°N, 77.8°E geographic, 1.7°N dip latitude) and Kolhapur (16.7°N, 74.3°E geographic, 11.5°N dip latitude) are utilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30–16:30 UT between 3°S and 15°N dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electric field initially appeared at higher dip latitudes almost 10 h after the storm onset, and subsequently, the same was observed at lower dip latitudes. The EPBs attained a peak westward drift at ~17:00 UT followed by a gradual decrease in their speed till ~18:30 UT. After regaining their westward speed, the EPBs continued to drift westward till 22:00 UT. Moreover, a latitudinal gradient in the drift motion of the EPBs was also observed on this night. Another interesting observation made from the images obtained from Tirunelveli was the presence of a large westward tilt of the EPBs. The most intriguing finding of this study, however, was the asymmetry in the tilt of the EPBs at conjugate points during the premidnight hours on 17 March 2015. In this study, the possible mechanisms that can explain these observations are discussed in light of the current understanding of the equatorial electrodynamics and EPBs.

Comparison of GPS-TEC measurements with IRI2012-TEC predictions in the Brazilian sector during the unusual solar minimum 2009

The total electron content (TEC) derived from two Global Positioning System (GPS) receivers localized at Palmas (PAL, 10.2oS, 48.2oW, dip latitude 5.5oS), near equatorial region, and Sao Jose dos Campos (SJC, 23.2oS, 45.9oW, dip latitude 17.6oS), under the southern crest of the equatorial ionization anomaly (EIA), in the Brazilian sector are used in this paper. TEC from the latest available IRI-2012 model at the same locations is also used. A comparative study between GPS-TEC and the IRI-2012 model is analyzed during the deep low solar activity period of 2009 in the unusual solar cycle 23. The measurements and the modeled data show that the IRI-TEC model gives the better estima- tes of the GPS-TEC during the nighttime from January to December months at PAL and SJC. The IRI-TEC model shows larger differences from the GPS-TEC in the afternoon hours, mainly at SJC. These di- screpancies observed at SJC in the afternoon hours for all months are possibly due to the dynamics of EIA in consequence of the equatorial fountain effect. At PAL and SJC, oscillations of GPS-TEC with pe- riods around 16 days in April and periods around 27-32 days between September and December possibly are noticed which could be related to a modulation of the semidiurnal tide and Sun’s rotation, respecti- vely. Also, oscillations with periods around 2-4 and 5-10 days from January to December are observed at PAL and SJC, possibly associated with the propagation of planetary waves.

Comparison of ionospheric profile parameters with IRI-2012 model over Jicamarca

We used the hourly ionogram data obtained from Jicamarca station (12° S, 76.9° W, dip latitude: 1.0° N) an equatorial region to study the variation of the electron density profile parameters: maximum height of F2-layer (hmF2), bottomside thickness (B0) and shape (B1) parameter of F-layer. The period of study is for the year 2010 (solar minimum period).The diurnal monthly averages of these parameters are compared with the updated IRI-2012 model. The results show that hmF2 is highest during the daytime than nighttime. The variation in hmF2 was observed to modulate the thickness of the bottomside F2-layer. The observed hmF2 and B0 post-sunset peak is as result of the upward drift velocity of ionospheric plasma. We found a close agreement between IRI-CCIR hmF2 model and observed hmF2 during 0000-0700 LT while outside this period the model predictions deviate significantly with the observational values. Significant discrepancies are observed between the IRI model options for B0 and the observed B0 values. Specifically, the modeled values do not show B0 post-sunset peak. A fairly good agreement was observed between the observed B1 and IRI model options (ABT-2009 and Bill 2000) for B1.

The effect of zonal wind reversal around sunset on ionospheric interhemispheric asymmetry at March equinox of a solar maximum year 2000

AbstractInterhemispheric asymmetry of the topside ionosphere around March equinox of a solar maximum year 2000 was investigated using ROCSAT‐1 total ion density (Ni) measurements. The asymmetry during the local time sector of late afternoon to midnight was analyzed for two longitude sectors with larger geomagnetic declinations (positive at 120°W–180°W and negative at 0–60°W). The asymmetry of equatorial ionization anomaly (EIA) crests is consistent with (but much weaker than) that of Ni at middle dip latitudes in each longitude sector, and the asymmetry is contrary between two longitude sectors. The zonal wind was confirmed to be the primary reason for the asymmetry; it induces opposite vertical plasma transports between two hemispheres under the effect of geomagnetic declination. For the asymmetry of EIA crests, the effect of vertical plasma transport dominates over that of transequator plasma transport. Zonal wind reversal around sunset results in Ni at middle dip latitudes increases in one hemisphere while quickly decreases in the other one, which induces a quick reversal of asymmetry at about 18:00 LT. Nighttime ion composition at 600 km was found to have similar asymmetries with those of Ni, indicating that wind‐induced vertical plasma transport is important for topside ion composition. Asymmetry intensity is somewhat different between two longitude sectors, and the difference depends on dip latitudes. That, on the one hand, is related to the longitudinal variation of the fountain effect and, on the other hand, implies that neutral winds are possibly longitudinally dependent.

The variability of low‐latitude ionospheric amplitude and phase scintillation detected by a triple‐frequency GPS receiver

AbstractIonospheric scintillation is a manifestation of space weather effects that seriously affect the performance and availability of space‐based navigation and communication systems. This paper presents results from an investigation on the characteristics of the phase and amplitude scintillation of Global Positioning System signals at the L1, L2C, and L5 frequencies. Field data obtained by a scintillation monitor installed in São José dos Campos (23.1°S, 45.8°W; dip latitude 17.3°S, declination 21.4°W), Brazil, a station located near the southern crest of the equatorial ionization anomaly, were used for this purpose. The analyzed data were collected during 150 nights from November 2014 to March 2015, an epoch of moderate solar activity close to the recent solar maximum. Only measurements corresponding to an elevation mask of 30° and values above standard threshold levels were used in the analysis. Outstanding characteristics of amplitude and phase scintillation are analyzed and compared in this study. The different characteristics of the scintillation focused in this study include (1) the statistics of their occurrences at the three frequencies; (2) the local time distributions of the amplitude and phase scintillation at different intensity levels; (3) azimuth‐elevation (spatial) distributions at different levels of the standard deviation of phase fluctuations; (4) scintillation enhancement and loss of phase lock conditions due to field‐aligned (longitudinal) propagation; (5) the relationship between amplitude and phase scintillation parameters for the L1, L2C, and L5 frequencies; and (6) the frequency dependence of the amplitude and phase scintillations. Important results on these different characteristics are presented and discussed, and some outstanding problems for future investigations are suggested.

High-resolution coherent backscatter interferometric radar images of equatorial spread F using Capon's method

Abstract. We present results of Capon's method for estimation of in-beam images of ionospheric scattering structures observed by a small, low-power coherent backscatter interferometer. The radar interferometer operated in the equatorial site of São Luís, Brazil (2.59° S, 44.21° W, −2.35° dip latitude). We show numerical simulations that evaluate the performance of the Capon method for typical F region measurement conditions. Numerical simulations show that, despite the short baselines of the São Luís radar, the Capon technique is capable of distinguishing localized features with kilometric scale sizes (in the zonal direction) at F region heights. Following the simulations, we applied the Capon algorithm to actual measurements made by the São Luís interferometer during a typical equatorial spread F (ESF) event. As indicated by the simulations, the Capon method produced images that were better resolved than those produced by the Fourier method. The Capon images show narrow (a few kilometers wide) scattering channels associated with ESF plumes and scattering regions spaced by only a few tens of kilometers in the zonal direction. The images are also capable of resolving bifurcations and the C shape of scattering structures.

Some new insights of the characteristics of equatorial plasma bubbles obtained from Indian region

All-sky imaging observations of OI 630.0nm airglow were carried out in campaign mode from Panhala (16.8°N, 74.1°E geographic; 11.1°N dip latitude), India, during January to March 2008. On 14 of 37 nights, equatorial plasma bubbles were observed. The drift speeds were observed to decrease with time in concurrence with the previous results. The tilts were mostly westward while on rare occasions the plasma bubbles tilted eastwards. The drifts were found to be relatively lesser on disturbed nights while the tilts appear to be marginally larger. The interdepletion distances (or bubble spacings) also showed a decreasing trend with time till midnight indicating that the bubbles approach each other with the passage of time. Such a behavior is not reported earlier and it seems to have important implications for understanding the time evolution of plasma bubbles. On occasions, the bubbles occurred in groups. An ionosonde operating over Indian dip equatorial site Tirunelveli (1.1°N dip latitude) was used to study the variations in the base height of the ionosphere during the plasma bubble observations. The ionosonde measurements indicate lack of significant pre-reversal enhancement (PRE) during geomagnetic quiet days in which the bubbles were observed.

First observation of presunset ionospheric F region bottom‐type scattering layer

AbstractThe bottom‐type irregularity scattering layer (BSL) that can appear in the ionospheric F region bottomside has been observed generally after sunset, serving as a possible telltale of equatorial spread F (ESF). Using simultaneous multibeam radar measurements over two low‐latitude stations, Sanya (18.3°N, 109.6°E; dip latitude 13°N) and Fuke (19.3°N, 109.1°E; dip latitude 14°N) in China, we report, for the first time, a thin BSL that initially occurred at presunset (~1720 LT), much earlier than the occurrence of BSL generated from the equatorial plasma shear vortex‐driven instability. The presunset BSL was situated around 225 km altitude and continued to exist until the appearance of ESF plumes after sunset (~1930 LT). Interestingly, the Doppler velocities of the presunset BSL echoes measured by the radar and the F layer virtual heights obtained from the collocated Digisonde measurements over Sanya both show oscillations with a period of about 1 h, suggesting a close link between the occurrences of the BSL and of F region plasma density large‐scale wave structure before sunset. These observations could imply an important role of gravity waves in the generation of the presunset F region bottom‐type irregularities.

On the role of horizontal wind shears in the generation of F0.5 layers over the dip equatorial location of Thiruvananthapuram: A numerical simulation study

A numerical simulation is carried out to estimate the rate of convergence of ionization required to produce a F0.5 layer with peak plasma frequency (foF0.5) of 3.2MHz from three different background layer densities, over Thiruvananthapuram (8.5°N; 77°E; dip latitude ~ 0.5°N), a dip equatorial station in India. Further the simulation study is extended to understand the convergences required by considering the seasonal mean peak F0.5 layer frequencies also. One possible mechanism by which this convergence can be produced is by a horizontal shear in the meridional wind. The corresponding shears required to generate the layer with the above convergence conditions are estimated. It is found that gravity waves are capable of generating wind shears, leading to the pooling of ionization and the generation of the layer over the dip equator. A meridional wind with the gravity wave induced wind shear is numerically estimated. Finally, the short scale gravity waves of periods around 3–23min have been inferred to be more efficient in generating the wind shear when compared to large scale horizontal waves leading to the generation of F0.5 layer.

An observing system simulation experiment for FORMOSAT-5/AIP probing topside ionospheric plasma irregularities by using DEMETER/IAP

The ion density probed by IAP (Instrument d'Analyse du Plasma) on board the DEMETER (Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions) satellite is used to find whether the science payload of Advanced Ionospheric Probe (AIP) on board FORMOSAT-5 can be employed to observe space weather of ionospheric plasma irregularities. The low-latitude irregularities within ±15° dip latitudes of the DEMETER/IAP ion density are nighttime phenomena, and become prominent in the South America-Central Africa sector almost all year round, especially during May to August. The high-latitude irregularities of the DEMETER/ IAP ion density appear around ±65° dip latitude worldwide in both daytime and nighttime, and become very intense in the winter and the equinox month/hemisphere. DEMETER/IAP results show that FORMOSAT-5/AIP can be used to monitor space weather of ionospheric daytime/nighttime plasma irregularities in not only the low-but also high-latitude ionosphere.

Ionosonde observations of daytime spread F at low latitudes

AbstractSpread F on ionograms has been considered to be a phenomenon mainly occurred at nighttime. This study presented a case study of daytime spread F observed by the ionosonde installed at Puer (PUR; 22.7°N, 101.05°E; dip latitude 12.9°N), where daytime spread F that lasted for more than 2 h (about 08:30 LT~10:45 LT) was observed on 14 November 2015. To investigate the possible mechanism, ionograms recorded at PUR and Chiang Mai (18.76°N, 98.93°E; dip latitude 9.04°N) were used in this study. We found that traveling ionospheric disturbances were observed before the occurrence of daytime spread F. Meanwhile, the movement of the peak height of the ionosphere was downward. We suggested that downward vertical neutral winds excited by traveling atmospheric disturbances/atmospheric gravity waves might play a significant role in forming daytime spread F over PUR during geomagnetic storms.

Seasonal and solar activity variations ofF3layer and quadruple stratification (StF‐4) near the equatorial region

AbstractThe study of multiple stratification of theFlayer has the initial records in the midtwentieth century. Since then, many studies were focused onF3layer. The diurnal, seasonal, and solar activity variations of theF3layer characteristics have been investigated by several researchers. Recently, investigations on multiple stratifications ofFlayer received an important boost after the quadruple stratification (StF‐4) was observed at Palmas (10.3°S, 48.3°W; dip latitude 6.6°S—near‐equatorial region), Brazil. The present study reports the latest findings related with the seasonal and solar activity characteristics of theF3layer and StF‐4 near the equatorial region during the period from 2002 to 2006. A significant connection between StF‐4 andF3layer has been noticed, since the StF‐4 is always preceded and followed by aF3layer appearance. However, theF3layer and the StF‐4 present different seasonal and solar cycle variations. At a near‐equatorial station Palmas, theF3layer shows the maximum and minimum occurrences during summer and winter seasons, respectively. On the contrary, the StF‐4 presents the maximum and minimum occurrences during winter and summer seasons, respectively. While theF3layer occurrence is not affected by solar cycle, the StF‐4 appearance is instead more frequent during high solar activity.

Shrinking equatorial plasma bubbles

AbstractThe 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 630 nm 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 630.0 nm 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.

Influence of uncertainties of the empirical models for inferring the E-region electric fields at the dip equator

Daytime E-region electric fields play a crucial role in the ionospheric dynamics at the geomagnetic dip latitudes. Due to their importance, there is an interest in accurately measuring and modeling the electric fields for both climatological and near real-time studies. In this work, we present the daytime vertical (Ez) and eastward (Ey) electric fields for a reference quiet day (February 7, 2001) at the Sao Luis Space Observatory, Brazil (SLZ, 2.31°S, 44.16°W). The component Ez is inferred from Doppler shifts of type II echoes (gradient drift instability) and the anisotropic factor, which is computed from ion and electron gyro frequencies as well as ion and electron collision frequencies with neutral molecules. The component Ey depends on the ratio of Hall and Pedersen conductivities and Ez. A magnetic field-line-integrated conductivity model is used to obtain the anisotropic factor for calculating Ez and the ionospheric conductivities for calculating Ey. This model uses the NRLMSISE-00, IRI-2007, and IGRF-11 empirical models as input parameters for neutral atmosphere, ionosphere, and geomagnetic field, respectively. Consequently, it is worth determining the uncertainties (or errors) in Ey and Ez associated with these empirical model outputs in order to precisely define the confidence limit for the estimated electric field components. For this purpose, errors of ±10 % were artificially introduced in the magnitude of each empirical model output before estimating Ey and Ez. The corresponding uncertainties in the ionospheric conductivity and electric field are evaluated considering the individual and cumulative contribution of the artificial errors. The results show that the neutral densities and temperature may be responsible for the largest changes in Ey and Ez, followed by changes in the geomagnetic field intensity and electron and ions compositions.

Quiet-time foF2 characteristics at an equatorial station in the African sector

A study on the morphology of the critical frequency of the F2 layer (foF2) has been done using data obtained with Ionospheric Prediction Sounder (IPS-42) located at Ouagadougou, Burkina Faso (geo. Lat. 12.40N, Long. 358.50, magnetic dip latitude 5.90N) an equatorial station in the West African sector. Data covering a whole solar cycle period from 1985 – 1995 were used to characterize the quiet time morphology of the F2-layer for that region. Results show that foF2 exhibit diurnal, seasonal and solar activity dependence. Diurnal morphology of foF2 is characterized by two peaks; pre-noon and post-noon peaks with the presence of noon-time “bite-out”. While the pre-noon peak is more pronounced during high solar activity periods, the post-noon peak is dominant at low solar activity. Also, while the post noon peaks are more pronounced at equinoctial months during low solar activity, the pre-noon peak is more pronounced at the solstices during high solar activity periods.

Tucumán ionospheric model (TIM): Initial results for STEC predictions

Most ionospheric models can calculate vertical total electron content (VTEC) predictions, but only a few are suitable for calculating slant total electron content (STEC). This ionospheric magnitude is generally measured for electron content determinations, with VTEC particularly corresponding to an elevation of 90°. This is generally obtained by applying a mapping function to STEC measurements, which leads to important calculation errors. Moreover, the equatorial region has unique characteristics, such as the fountain effect and the equatorial electrojet, which lead to significant errors in the model’s calculations. In this paper, the Tucumán ionospheric model (TIM) is presented as a novel alternative for calculating the STEC in low-latitude regions (−24 to 24 dip latitude). The model is based on spatial geometry where the considered trajectory is segmented, and the corresponding electron density calculations for the resulting segment end points are determined using the semi-empirical low-latitude ionospheric model (SLIM) with reference to their corresponding magnetic coordinates and height. Finally, the electron density values are integrated along the path to obtain the STEC. This work describes the TIM and tests their STEC predictions for five ray paths around the world (totaling 16 cases under study), which are compared with experimental data from satellites and with those calculated by the NeQuick model. Moreover, the TIM performance for VTEC predictions is also checked and compared with VTEC data obtained from Global Positioning System (GPS) signals, IRI model, and NeQuick model predictions, for six GPS receiver stations during the equinox and solstice (totaling 12 cases studied). Comparisons of the TIM predictions with experimental data show that 53% of the calculation has, in general, deviations <30%. For the considered cases, TIM reproduces the experimental data better than the other models.

The study of equatorial plasma bubble during January to April 2012 over Kolhapur (India)

&lt;p&gt;Over 53 nights of all sky airglow imager data collected during January-April 2012 from the low latitude station Kolhapur (16.68°N, 74.26°E; 10.6°N dip latitude) have been analyzed to study the F-region dynamics through the imaging of OI 630 nm emission line. The observed night airglow data were supported by the ionosonde measurements from Tirunelveli (8.7°N, 77.8°E; 0.51°N dip latitude). Well defined magnetic field aligned depletions were observed during the observation period. Out of 53 nights, 40 nights exhibited the occurrence of north-south aligned equatorial plasma bubbles. These plasma bubbles were found moving towards east with drift speed in range between 70 to 200 m s&lt;span&gt;&lt;sup&gt;-1&lt;/sup&gt;&lt;/span&gt;. We have analyzed the zonal drift velocity variation and relation of bubble occurrence with the base height of the ionosphere together with the effects of the geomagnetic Ap and solar flux F&lt;span&gt;&lt;sub&gt;10.7&lt;/sub&gt;&lt;/span&gt; cm index in its first appearance.&lt;/p&gt;

Inter-hourly variability of Total Electron Content during the quiet condition over Nigeria, within the Equatorial Ionization Anomaly region

The inter-hourly variability (IHV) of the Total Electron Content (TEC) over Nigeria during the quiet days (Ap<4) of the year 2013 was examined using ground-based GPS receivers installed at seven (7) different locations across Nigeria by the Nigerian Global Navigation Satellite Systems (GNSS) Reference Network (NIGNET) operated by the office of the surveyor general of Nigeria. Nigeria is a country that lies within Equatorial Ionospheric Anomaly (EIA) region. The IHV was calculated by converting the observed hourly slant TEC (STEC) value into the hourly vertical TEC (VTEC) and the differencing (∆TEC) with its corresponding hourly value from the previous day. There is a clear variation which depicts the expected temporal variability. The IHV in TEC in all the stations ranges between 0 and 20 TECU (TEC Units). The seasonal variation of the IHV of TEC over Nigeria maximizes (5–20 TECU) during Equinoctial months and minimizes (1–10 TECU) during the Solstice months. The IHV of TEC in September equinox period is higher than that of March equinox. Minimum value of IHV (~7 TECU at equinoxes and ~5 TECU at Solstice) was recorded at the Office of Surveyor General of the Federation (OSGF) station and the maximum value (~12 TECU at equinoxes and ~16 TECU at Solstice) was recorded at the Birni Kebbi Federal Polytechnic (BKFP) station which may be due to the fact that BKFP at 0.72° dip latitude is closer to the dip equator.