Statistical study of the DP2 enhancement at the dayside dip-equator compared to low latitudes

Abstract. It has been largely investigated and established that the DP2 events are enhanced at dip-equator compared to low latitudes. Studies by several authors showed various enhancement ratio values of DP2 amplitude at the dayside dip-equator relative to those at low latitudes. In order to quantify this enhancement ratio, we carried out a statistical study on this phenomenon using the ground magnetometer data at locations ranging from polar cap to dip-equator over the African, Asian and American sector. Our result gave an enhancement ratio which shows a diurnal variation with a maximum value around 12:00 LT and is correlated with the regular variation of the geomagnetic H-component. The longitudinal variation of the enhancement ratio of the DP2 exhibits high values over the American sector compared to Asian sector and African sector which have lowest values. This longitudinal dependence is similar to that of the equatorial electrojet magnetic effect.


Introduction
The geomagnetic disturbances observed generally at low latitudes are attributed to the magnetospheric electric fields and currents that connect the high-latitude ionosphere with the low-latitude ionosphere penetrating through the middle latitude (Nishida et al., 1966;Reddy et al., 1979;Kikuchi et al., 1996;Kobea et al., 1998).Studies by Nishida et al. (1966) found that a type of disturbance called DP2, is associated with fluctuations in north-south component of interplanetary magnetic field.The DP2 events are characterized by magnetic fluctuations with periods of 30 to 60 min at Correspondence to: N. M. Mene (medardmene@yahoo.fr)low latitudes (Nishida, 1979).These events exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip-equator (Kikuchi et al., 1996).In fact the DP2 events are caused by the penetration of magnetospheric electric field at low-latitude and are affected by the redistribution of hot plasma that is the region-2 field-aligned currents into and out of the ionosphere (Vasyliunas, 1972;Senior and Blanc, 1984;Fejer, 1979).Under steady state conditions these region-2 currents tend to minimize the penetration of ionospheric electric fields and currents between the auroral region and low-latitude, producing a shielding electric field (Karlson, 1979;Block, 1979;Vasyliunas, 1972;Southwood, 1979).During time-varying disturbances, the distribution of magnetospheric plasma and the accompanying shielding currents do not have the time to develop the appropriate shielding electric field.The magnetospheric electric field therefore can penetrate at the mid-, lowand equatorial latitude ionosphere (Jaggi and Wolf, 1973;Kelley et al., 1979;Spiro et al., 1988;Fejer et al., 1990;Sobral et al., 2001;Abdu et al., 1998).This penetration causes the DP2 events.It has been largely investigated and established that the DP2 magnetic fluctuations were enhanced in amplitude at the dayside dip-equator compared to those at low latitude (Kikuchi et al., 2008;Nishida, 1971).Studies by Kobéa et al. (2000) and Kikuchi et al. (2008) reported DP2 amplitude enhancement factors of about 4 and 2.7 respectively.These results show variation in DP2 enhancement ratio and efforts must be made towards having a good understanding of the DP2 events at the equator in order to improve the modeling of this phenomenon.The DP2 enhancement at the dip-equator is caused by the high values of the Cowling conductivity at low latitudes, responsible for the equatorial electrojet (EEJ) circulation having the regular variation of geomagnetic H-component (S r (H )) as its signature (Fambitakoye and Mayaud, 1976a;Baker and Martyn, 1953;Hirono, 1952).So, the DP2 enhancement ratio should present a diurnal variation which can be estimated by the S r (H ) values.The purpose of the present paper is to study the diurnal and longitudinal variation of DP2 enhancement ratio at the dayside dip-equator relative to those at low latitudes and to show the correlation between the enhancement ratio and the S r (H ) values.Section 2 presents the data used and the method of analysis.Section 3 is devoted to the presentation of DP2 disturbances during the 11 March 1993 magnetic storm.In Sect.4, we carry out the study of the latitudinal profile of DP2 magnetic fluctuations which occurred during the 11 March 1993 magnetic storm.Section 5 deals with a statistical study that shows the diurnal variation of DP2 enhancement average ratio and its correlation with the S r (H ) variation.Section 6 presents the longitudinal dependence of the DP2 enhancement ratio.Section 7 is the summary and conclusion.

The data used and the method of analysis
We use ground-based magnetometers data recorded at high, mid and low latitudes over the American, African and Asian longitude sectors.The geographic and the geomagnetic coordinates of the magnetometer stations are listed in Table 1.Data from those three sectors are geomagnetic minute values.The selected storm days are given in Tables 2 and 3.The times mentioned on these tables are the times of the beginning of DP2 disturbances.In order to investigate the DP2 events, we use Dst, AU, AL indices and the horizontal com-ponent of geomagnetic field H .The Dst index gives an estimate of the sum of magnetospheric currents and depicts the different phases of magnetospheric disturbances (Akasofu, 1964).In fact, it illustrates the magnetic effects of the Chapman Ferraro currents (DCF), the ring current (DR) and the tail current (DT) during the magnetic storm.The AU and AL indices are used to evaluate the auroral electrojets currents amplitude and the arrival of magnetospheric particles in the high latitudes ionosphere.The H component gives an estimate of the equatorial electrojet current intensity; it should be noted that it integrates the effects of all currents flowing in the Earth's environment.These effects can be gathered in two folds: the daily local time variation of the Earth's magnetic field during quiet days (S r ) and the disturbed variation (D) due to the electric currents generated by the various disturbance mechanisms.The DP2 events are generally observed during the main phase of the magnetic storm (Huang et al., 1979).During this period, DCF, DT and DG are negligible and only the ring current (DR) in the magnetosphere is intense (Cole, 1966;Fukushima and Kamide, 1973) and is evaluated by the Dst index which gives a good approximation of the symmetric ring current.We may infer the magnetic disturbance DP (Disturbed Polar) that represents the magnetic effects of the ionospheric electric current systems; generated by the penetration of magnetospheric convection electric field at equator (DP2) and the ionospheric disturbance dynamo (Ddyn) as follows: Where L is the geomagnetic latitude of the station and S r is the average of the five quietest days of the month.The DP2 events are characterized by coherently variations of the DP from high latitude to dip-equator (Kobéa et al., 2000).

Definition
Geomagnetic storm-time variations often show magnetic fluctuations occurring coherently at high latitude on the dayside dip-equator where they affect the regular variations (Onwumechilli et al., 1973;Nishida et al., 1966).Those magnetic fluctuations called DP2 caused by the alternating domination of R1 and R2 FACs transmit the magnetospheric convection electric field to high latitudes ionosphere (Kikuchi et al., 1996).The time signature of DP2 fluctuations is a period of 30-60 min (Nishida, 1979) and these fluctuations continuous for several hours during the main phase of magnetic storms (Kikuchi et al., 2010).The main characteristics of the DP2 events are a falloff in amplitude with decreasing latitude and an enhancement of their amplitude on the dayside dip-equator compared to those of low latitudes (Kikuchi et al., 1996(Kikuchi et al., , 2000)).In the following section we present some DP2 disturbances and their enhancement at dip-equator during the magnetic storm of 11 March 1993.

DP2 disturbances during the magnetic storm of 11 March 1993
The equatorial Dst time-variations (Fig. 1a) shows that the storm started on 11 March 1993 at 01:00 UT with a low compression phase (+20 nT) that ended at 09:00 UT.This period is followed by the main phase of the storm characterized by the decrease of the Dst that peaked at a value of −120 nT at 18:00 UT followed by the recovery phase.Figure 1b shows the plot of the auroral AU, AL indices; we note an intense auroral activity on 11 March 1993 indicating the occurrence of an important magnetospheric convection process during this time period.Figure 1c shows the geomagnetic H component variations at Niellé (NIE) located on the dayside during the disturbed time, overlaid in a dotted line by those of the reference quiet day, that is, 5 March 1993.During the main phase of the storm, we note that the H component variation tracks the Dst because Niellé (NIE) is located at the dip-equator where the ring current (DR) effect is preponderant.However, appreciable increases in H component are clearly observed between 09:00 and 10:00 UT, at 12:00 and 13:00 UT while the Dst is decreasing.Figure 2 shows the DP latitudinal variations which exhibit nearly coherent increase at all stations (from polar cap to dip-equator) during the time interval 09:30-09:54 UT and 12:15-12:30 UT.These disturbances present a decrease in magnitude with decreasing latitude and an enhancement at dip-equator compared to low latitude.According Nishida (1971), these observations are the characteristics of the influence of a penetration of magnetospheric convection electric field effects at low latitude (DP2  the low latitudes (Tamanrasset).These enhancements detected successively at Mopti (MOP) and Niellé (NIE) confirm the circulation of currents in the equatorial ionosphere.

Latitudinal profiles of DP2 events
The latitudinal profile of the DP2 is characterized by a rapid falloff in amplitude with decreasing latitude toward the low latitude and the amplitude increase at the dip-equator due to the Cowling conductivity effect (Hirono, 1952;Kikuchi et al., 1996Kikuchi et al., , 2000)).Figure 3 shows the latitudinal profiles of the DP2 events on 11 March 1993 at 09:45 UT and 12:15 UT.The enhancement ratio (the DP2 amplitude at the dip-equator (Niellé) divided by that at low latitude (Tamanrasset)) obtained is 1.82 at 09:45 UT and 3 at 12:15 UT.We note that the DP2 enhancement depends on the local time.In order to show evidence of this dependence, we carried out a statistical study of several magnetic storms in 1993.Table 2 gives the analyzed magnetic storm events and the DP2 time of occurrences.and [20:00, 00:00 LT]), the DP2 amplitude is not enhanced at the dip-equator because the Cowling conductivity is weak during these time periods.During the daytime, the DP2 enhancement at the dip-equator starts around 07:00 LT with an average ratio of 1.2; it increases until a maximum average ratio of 3.3 around 12:00 LT and decreases, ending at 19:00 LT.The observed standard deviations can be related to the equatorial electrojet variability.

Correlation between the DP2 enhancement ratio and H component regular variation (S r (H ))
The DP2 enhancement owing to the Cowling conductivity effects can be noticeable throughout the variation of the geomagnetic H component (S r (H )) which is typical of the equatorial electrojet (Fambitakoye and Mayaud, 1976a,b).Therefore, the DP2 enhancement ratio can be estimated by the H component regular value.Figure 5 shows the reference days average S r (H ) values (dashed line) and the DP2 enhancement ratio at dip-equator (Niellé).The S r (H ) variation and     variations of H component disturbances (DP) from North polar cap latitudes to dip-equator during these magnetic storms.Over these three sectors, we observe increases of the DP simultaneously at the polar region, middle and low latitudes around 12:00 LT with considerably enhancement at the dipequator According to Nishida (1971), these disturbances are the signature of DP2 events.Table 4 gives the DP2 amplitudes respectively at the low latitude and the equator and the enhancement ratio between the dip-equator and the low latitude over the American, African and Asian longitude sectors for the days 7 October 1998, 10 March 1998 and 19 October 1998, respectively.We note that around the same local time, we have different values of DP2 enhancement ratio over these three longitude sectors.Figure 7 illustrates this result throughout the local time variations of the DP2 enhancement ratio in these three longitude sectors.The Huancayo (HUA) station (American sector) exhibits the highest average value of the DP2 enhancement ratio which peaked up 8 compared to the value of 5.7 of Yap (YAP) station (Asian sector) and 4.5 of Addis Ababa (AAE) station (Asian sector); it is important to note that the AAE station presents the relatively lowest average value.In the same vein, the DP2 enhancement starts early in the morning (05:00 LT) to end in the late afternoon (21:00 LT) over the American sector compared to the African and the Asian sectors where it starts at 06:00 LT and ends at 20:00 LT.The above observations show that the DP2 presents a longitudinal dependence evidenced by the longitudinal variation of the equatorial electroject when we use the average values of the S r (H )(quiet time reference) observed in 1998 (Fig. 8).The Huancayo station (American sector) exhibits the highest intensity of EEJ compared to those Yap station (Asian sector) and Addis Ababa station (African sector) where the EEJ presents lowest intensity.This longitudinal variation of the EEJ is in accordance to the results obtained by Doumouya et al. (2003).The DP2 enhancement ratio and the EEJ have similar longitudinal dependences.

Conclusion
The DP2 events are enhanced at the dayside dip-equator compared to low latitude due to the Cowling conductivity effect.The enhancement ratio is characterized by a diurnal variation which is correlated with the regular variation of geomagnetic H component with a good correlation coefficient of 0.95.This correlation can be used to estimate the DP2 enhancement ratio from the S r (H ) values.The DP2 enhancement ratio at dip-equator and the EEJ presents similar longitudinal variation with the highest values over the American sector compared to those of Asian and African sectors.The African sector exhibits relatively lowest values.Moreover, it is worth to note that over the American sector where the EEJ has a high intensity, the DP2 enhancement starts early in the morning and ends late in the afternoon.The DP2 enhancement ratio presents a diurnal and longitudinal variations correlated with the equatorial electrojet magnetic effect variations.
Similar studies on other longitude sectors should help in obtaining a good evaluation of these parameters in order to contribute to a better understanding of DP2 events at the dip-equator.Such results are useful for the improvement of simulation models of electric field at the dip-equator during the magnetic storms.

Fig. 1 .
Fig. 1.Variation of the Dst index (A), the auroral indices AL and AU (B) and the geomagnetic H-component (C) on 11 March 1993 (solid line).The dotted line (C) shows the quiet time variation (S r (H )) on 5 March 1993 (reference quiet days).

Fig. 5 .
Fig. 5. African sector, results of the year 1993: Variation of DP2 enhancement ratio average at NIE (solid line) and the S r (H ) average (dotted line).The correlation coefficient is 0.95.

Fig. 7 .
Fig. 7. Results for the year 1998: Diurnal variations of the DP2 enhancement ration average in 1998 at dip-equator compared to low latitude over the American (HUA), African (AAE) and Asian(YAP) sectors.

Table 1 .
Locations of magnetic stations.

Table 2 .
Magnetic storms days selected in 1993 and times of the beginning of DP2 disturbances at Niellé (NIE) over African sector.

Table 3a .
Magnetic storms days selected of 1998 and times of the beginning of DP2 disturbances at Addis Ababa (AAE) over African sector.

Table 3b .
Magnetic storms days selected of 1998 and times of the beginning of DP2 disturbances at Huancayo (HUA) over American sector.

Table 3c .
Magnetic storms days selected of 1998 and times of the beginning of DP2 disturbances at Yap (YAP) over Asian sector.

Table 4 .
DP2 amplitudes at low latitude and the equator and the enhancement ratio of DP2 between the dip-equator and the low latitude over the American, African and Asian longitude sectors.