Effects of geomagnetic storm on middle latitude ionospheric F2 during storm of 2-6 April 2004

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1 Indian Journal of Radio & Space Physics Vol 41, December 2012, pp Effects of geomagnetic storm on middle latitude ionospheric F2 during storm of 2-6 April 2004 B J Adekoya $,*, V U Chukwuma, N O Bakare & T W David Atmospheric/Ionospheric Physics Group, Department of Physics, Olabisi Onabanjo University, Ago Iwoye, Nigeria $ adekoyabolarinwa@yahoo.com Received 27 February 2012; revised 11 September 2012; accepted 27 September 2012 A study of the intense geomagnetic storm on 2-6 April 2004 has been carried out using ionospheric data obtained from ionosonde station in the mid-latitude stations with emphasis on the F2 region. Ionospheric storms represent an extreme form of space weather with important effects on ground and space-based technological systems. These phenomena are driven by highly variable solar and magnetospheric energy inputs to the Earth's upper atmosphere. The mid-latitude ionospheric stations are classified into two, depending on their geographic coordinates and ionospheric storm variation (i.e. positive and negative phases of ionospheric storm). The low-mid latitude are characterized with geographic latitude between 20 N< lowmid 36 N, while high-mid latitude are >36 N. The ionospheric storms are classified as positive storms (P-storm) or negative storms (N-storm) depending on whether maximum electron density variation is positive or negative following the onset of geomagnetic storms (small fluctuations of less than ±20% are neglected). The ionospheric storm produced at lowmid latitude was observed to be negative (N) ionospheric storm at the main phase, and also the declining phase of Dst was simultaneously depleted. The recovery phase is more pronounced with positive (P) ionospheric storm with noticeable effect than the initial and main phase. The D(foF2) response of high-mid latitude was observed with positive storm in the main phase. The initial phase responded with severe negative ionospheric storm compared to low-mid latitude with positive ionospheric storm. During the main phase of the geomagnetic storm, positive and negative phases of ionospheric storm prevailed in the daytime and at night. Keywords: Ionospheric F2 layer critical frequency, Ionospheric disturbance, Geomagnetic storm, Ionospheric storm, Magnetospheric storm PACS Nos: dj; Vv; Lr 1 Introduction Ionospheric storms represent an extreme form of space weather, which can have significant, adverse effects on increasingly sophisticated ground and space-based technological systems which are becoming more and more important to governments, corporations and the lives of ordinary citizens of our planet. Even in the early days of ionospheric research, it was noticed that geomagnetic activity is accompanied or quickly followed by marked changes in the F2 layer. These changes sometimes take the form of increases of the fof2 critical frequency, but more often there is severe decrease in the fof2 constituting phenomena. From numerous experimental and theoretical studies 1-3, a basic picture of global ionospheric storms has emerged. It is clear that the behaviour of the ionosphere during geomagnetic storms is greatly influenced by the coincident thermospheric storm, i.e. by the changes in neutral air winds and composition, which result in changes to rates of production and loss of ionization 4. Because of the close coupling between the ions and the neutrals, it is not possible to understand the ionospheric storm without considering the corresponding thermospheric storm and thermospheric storm without ionospheric storm 4. It is now generally believed that ionospheric storm has a close relationship with the thermospheric storm. The propagation of negative and positive ionospheric storms is strongly determined by the thermospheric disturbance spreading speed 5. The travelling atmospheric disturbances move the fof2 ionization upward to higher altitudes along the geomagnetic field lines, resulting in slower loss rates and higher electron densities. The increases in the midlatitude ionospheric F-region electron density and total electron content (TEC) are often observed in local dusk sector during magnetic storms and termed dusk effect 6. This is because the uplifting of F-layer by an eastward electric field and convergence in the east-west direction might be responsible for the dusk effect (Ref. 7 and

2 ADEKOYA et al.: EFFECTS OF GEOMAGNETIC STORM ON MID-LATITUDE F2 DURING 2-6 APRIL 2004 STORM 607 references therein). Sojka et al. 8 showed that a persistent electric field can also cause strong fof2 enhancement or positive storm phases near midnight at middle latitude. Furthermore, as a result of the local time variation of winds and neutral composition changes at middle latitudes, negative ionospheric storm effects are most often seen in the morning and positive storm effects in the afternoon and evening 9. Also, at middle latitudes, positive storm effects are more often seen or last longer in winter, and negative storm effects prevail in summer due to the greater latitude penetration of the equatorward winds and the composition disturbance zone in summer than in winter 4,5. The source of the high density plasma, seen during the positive storm phase, show that a magnetospheric electric field with an eastward component penetrates to mid-latitudes and increases local production on the dayside to a degree that is sufficient to account for the storm time density increases that have been observed. There are two mechanisms responsible for the positive ionospheric storm: downwelling of neutral atomic oxygen and uplifting of the F-layer due to winds 4. According to Mikhailov et al. 10, this mechanism works best during the daytime, while increases in O density causes positive storm effects at night. Mikhailov et al. 10 also found that an increase in the O density is more important than an increase in the O/N 2 ratio in causing positive storm effects. The negative phase is due to decreases in the O/N 2 and O/O 2 neutral density ratios 4. Seaton 11 first suggested that neutral composition changes, specifically increased O 2 density, could cause decreases in NmF2 observed during storms. The durations of both negative phase and positive phase have clear latitudinal, seasonal and magnetic local time (MLT) dependence 5. Sahai et al. (Ref. 3 and reference therein) also pointed out that the mechanisms associated with the development of the positive and negative phases are related to different magnetosphere-ionosphere interaction channels. In the present paper, an attempt was made to determine the mid-latitude differences between positive and negative ionospheric storm effects during a geomagnetic storm of 2-6 April 2004 using a study of the response of ionospheric parameter, fof2. According to physical mechanism of the positive ionospheric storms at low and mid-latitudes 12-14, the positive storms are expected to occur frequently at ±20 to ±30 magnetic latitudes and during the morning-noon onset of geomagnetic storms. The positive storms at high-mid latitudes involve the intensification and equatorward expansion of the subauroral electric fields 15,16. At equatorial latitudes and low-mid latitude, the eastward prompt penetration electric fields (PPEFs) during the main phase (MP) have effects which are opposite to those at higher latitudes 17,18. The different type of ionospheric disturbances recorded at a given station was furthermore associated with a triggering mechanism, using the time sequence of thermospheric-ionospheric storm effects. 2 Solar terrestrial data and ionospheric stations The geomagnetic index and solar wind data used in the study consisted of hourly values of the low latitude magnetic index Dst, interplanetary magnetic field component Bz, interplanetary electric field, proton number density, solar wind flow speed, plasma flow pressure, plasma temperature and plasma beta. These data were obtained from National Space Science Centre s NSSDC OMNIWeb Service ( The ionospheric data used in this study consisted of hourly values of fof2 obtained from Space Physics Interactive Data Resource (SPIDR s) network ( of ionosonde stations located in the equatorial and mid-latitudes regions. These stations are located in the European-African sector (Rostov, Athens, and Rome), East Asian sector (Guangzhou and Chongqing) and American sectors (Goosebay, Point arguello and Millstone Hill). Table 1 lists the stations showing geographic coordinates. Table 1 Ionosonde stations with their geographic coordinates Station (Code) Euro-African sector Geographic coordinates φ λ Difference between LST and UT, h Rostov (RV149) N E +3 Athen (AT138) N E +2 Rome (RO041) N E +1 East-Asian sector Guangzhou (GU421) Chongqing(094 29) American sector Millstone Hill (MHJ45) Goosebay (GSJ53) Point Arguello (PA836) N E N E N E N E N E -8

3 608 INDIAN J RADIO & SPACE PHYS, DECEMBER 2012 The present study of global ionospheric response to the geomagnetic and interplanetary phenomena forcing is concerned with variation in fof2 during 2-6 April However, the F2 region response to geomagnetic storms is conveniently described using a modified form of the analysis of Chukwuma 14, in terms of D(foF2), that is the normalized deviations of the critical frequency fof2 from the reference: fof 2 ( fof 2) ave D( fof 2) = 100% ( fof2) ave The D(foF2) variation are described in terms of percentage change in amplitude of critical frequency fof2 from the reference and following Ref. 19 and reference therein, positive and negative storms occur when the absolute maximum value of D(foF2) exceeds 20%. Furthermore, this limit is sufficiently large to prevent inclusion of random perturbation and disturbances of neutral atmospheric origin (gravity waves, etc.), thereby, making the indicated positive and negative storms represent real change in electron density not simply redistribution of the existing plasma. Hence, the data, that were analyzed, consisted of D(foF2) of respective hourly values of fof2 for the afore mentioned periods, while the reference for each hour is the average value of fof2 for the hour calculated from the three quite days April The use of D(foF2) rather than fof2 provides a firstorder correction for temporal, seasonal and solar cycle variation so that geomagnetic storm effects are better identified 14. An important criterion, used in choosing the reference period, is that these days must be devoid of not only of any significant geomagnetic activity but also there must be an absence of any considerable solar activity. This followed the fact that Chukwuma 19 have shown the high solar flares activity results in ionospheric disturbances due to their effects on thermospheric neutral density Results 3.1 Interplanetary and geomagnetic response The first panel in Fig. 1 shows magnetic index (Dst) plot against time (hrs UT) for the period 2-6 April 2004 representing the plot covering two days before and two days after the storm event. The storm is summarized using the low latitude magnetic index (Dst) and is interpreted using available interplanetary data. However, storms are classified as weak (Dst > -50 nt), moderate (Dst between -50 nt and -100 nt) and intense (Dst < -100 nt) 21. From the plot beginning 00:00 hrs UT, the storm was weak till 14:00 hrs UT, after which an intense storm was observed with peak value of -112 nt at 00:00 hrs UT before it returned to its weak storm appearance around 07:00 hrs UT on 5 April. It was observed that in the process of recovery, the storm emerged to second phase maximum value of -81 nt at about 19:00 hrs UT on the same day. On 6 April, it gradually decreased for the rest of the day. It is noteworthy that the storm main phase occurred in near coincidence with the sharp southward turning of interplanetary magnetic field (IMF) at the magnetic cloud boundary. The Bz plot shows a northward rotation during 00:00-06:00 hrs UT with peak value of 2.6 nt at 03:00 hrs UT. The orientation of interplanetary magnetic field of the southward component Bz was very low during the quiet day with maximum northward orientation of 6.5 nt at 11:00 hrs UT. The period of pre-storm was observed with a southward turning, for about 12 h, before it emerged into northward direction. The Dst minimum value coincided with the northward Bz peak of 12.9 nt at 16:00 hrs UT. The storm originally recovered completely before a second depletion with southward orientation of Bz at 17:00 hrs UT with nt peak. This is confirmed by Gonzalez & Tsurutani 22, referring to intense storms with peak Dst -100 nt They have suggested threshold values of Bz 10 nt and T 3 h. Also, preliminary studies of moderates storms with -100 nt < peak Dst -50 nt confirm earlier suggestion made by Rusell et al. 23 for associated threshold values of Bz 5 nt and T 2 h. The period of second Dst depression was noted to coincide with northward turning of IMF. This was preceded with a southward turning of nt peak at 17:00 hrs UT. Two interplanetary structures are important for the development of such class of storms: the sheath region just behind the forward shock and coronal mass ejection (CME) itself. According to Gonzalez et al. 24, these structures lead to the development of intense storms with two-step growth in their main phase. These structures also lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream 24,25. The plasma temperature plot indicates a rather steady temperature value between 00:00 and 12:00 hrs UT on 2 April. The temperature, however, began to fluctuate having different peak values till about 08:00 hrs UT on 4 April. A low temperature value of the plasma was then observed from 12:00 hrs UT noon to 12:00 hrs UT mid-night on 5 April. Thereafter, the temperature of the plasma increased abruptly to a

4 ADEKOYA et al.: EFFECTS OF GEOMAGNETIC STORM ON MID-LATITUDE F2 DURING 2-6 APRIL 2004 STORM 609 Fig. 1 Composition of interplanetary and geomagnetic observations for 2-6 April 2004

5 610 INDIAN J RADIO & SPACE PHYS, DECEMBER 2012 peak value of K at the onset period, thus, sudden increase in temperature denotes the arrival of the storm. The flow speed plot emerged with a high speed stream at the early hours of 2 April, and it maintained a rather steady speed till around 09:00 hrs UT on 3 April, thereafter, the flow speed increased to 503 kms -1 at 17:00 hrs UT on 3 April. This increase extended to the main phase with peak value of 610 kms -1 at 14:00 hrs UT on 6 April. The coincidence time of minimum Dst and IMF northward turning was recorded with flow speed increase of 504 kms -1. Gonzalez et al. 26 attributed these to the higher relative velocity, stronger shock and field compression. If shock runs into a trailing portion of a high-speed stream, preceding it, there may be exceptionally high magnetic fields 25. The plot of flow pressure was recorded with a low pressure from early hours of 2 April till around 10:00 hrs UT on 3 April. Thereafter, the flow pressure increased and attained a peak pressure value of npa at 15:00 hrs UT, the storm onset period. This increase extended to the main phase period with a maximum peak of 6.7 npa at 19:00 hrs UT; the time of minimum depression was recorded with flow pressure of 2.40 npa. After the maximum flow pressure, the flow sharply decreased as Dst was recovering. The higher plasma density and the higher velocity combined to form a much larger solar wind ram pressure. This pressure compressed the Earth s magnetosphere and increased the field magnitude near the equator 27. The electric field emerged from the southward direction in the early hours of 2 April to the northward with peak field record of 3.38 mv m -1 at 21:00 hrs UT on 3 April. The low field penetration to the Earth s magnetosphere was continued till around 12:00 hrs UT on 3 April. The electric field rather gradually increased in the northward direction to 3.96 mv m -1 on the storm main phase at 22:00 hrs UT. It, later, orientated southward and attained a minimum peak field of at 16:00 hrs UT during the first recovery phase. It is evidently seen from the plot that solar wind dawn to-dusk electric fields directly drive magnetospheric storm. These fields are caused by a combination of solar wind velocity and northward interplanetary magnetic field. The plasma beta responded with a high value at the initial phase, the pre-storm period recorded a high plasma beta of 3.54 and the main phase recorded a low beta. The field reversals typical within magnetic clouds feature magnetic field reconnection during the period of southward field and general lack of reconnection and solar wind injection into the magnetosphere during the part of northward field Ionospheric response The D(foF2) plots in Figs 2 and 3 represent the low-mid and high-mid ionospheric station in different sector of the earth s ionosphere as presented in Table 1. The low-mid latitudes are characterised with geographic latitude between 20 N and 36 N, while high-mid latitude are > 36 N. The low-mid latitude stations are: Guangzhou, Point Arguello, and Chongqing; and high-mid latitude stations are: Athens, Rome, Rostov, Millstone Hill, and Goosebay. The ionospheric storm effect to the geomagnetic storm can be classified into five different categories of maximum electron density 29. The storms are classified as positive storms (P-storm) or negative storms (N-storm) depending on whether maximum electron density is positive or negative following the onset of geomagnetic storms (small fluctuations of less than ±20% are neglected). If ionospheric storms show initial positive electron density (>20% for more than three hours) followed by negative electron density, it is classified as PN-storms. If initially, electron density is negative followed by positive density, they are classified as NP-storms. There are also non-significant ionospheric storms (NS-storms) for which D(foF2) is weak (less than ±20%). Examples of ionospheric storm are shown in Figs 1 and 2 for the low-mid and high-mid latitude of the earth s atmosphere. The dark region represents the storm main phase and time at which the severity of the storm was recorded Low-mid latitude The D(foF2) of Guangzhou emerged to a positive intense ionospheric storm of 28% at 02:00 hrs UT on 3 April, the initial phase of the storm. Starting from 14:00 hrs UT, the geomagnetic storm main phase began to decline, the ionosphere recorded a depletion of electron density with peak depletion of 54% at 21:00 hrs UT on 3 April. Thereafter, the ionosphere electron density enhanced to the positive phase and recorded a non-significant (NS) ionospheric storm (i.e. the electron density concentration below the reference level) which lasted for about 15 h. At the recovery phase of the storm, the ionosphere recorded a negative and positive ionospheric storm with peak

6 ADEKOYA et al.: EFFECTS OF GEOMAGNETIC STORM ON MID-LATITUDE F2 DURING 2-6 APRIL 2004 STORM 611 Fig. 2 Variation in D(foF2) for low-mid latitude station during 3-5 April 2004 depletion of 39% and enhancement of 58% at 19:00 and 02:00 hrs UT on 4 and 5 April, respectively. The ionospheric F2 response of Point Arguello showed a non-significant ionospheric storm starting around 00:00-03:00 hrs UT on 3 April. Thereafter, electron density in the plasma depleted to 34% at 04:00 hrs UT sharply; it enhanced to the positive phase of the storm with 27% and 24% enhancements at 09:00 and 13:00 hrs UT. The main phase at this station was preceded with NS till about 03:00 hrs UT on 4 April, when it sharply depleted to 31% at 04:00 hrs UT and thereafter, it increased to positive phase with 21% enhancement at 10:00 hrs UT on the same day. The recovery period was recorded with more

7 612 INDIAN J RADIO & SPACE PHYS, DECEMBER 2012 Fig. 3 Variation in D(foF2) for high-mid latitude station during 3-5 April 2004

8 ADEKOYA et al.: EFFECTS OF GEOMAGNETIC STORM ON MID-LATITUDE F2 DURING 2-6 APRIL 2004 STORM 613 pronounce storm than the initial and main phase with maximum electron density of 34%, 34%, and 39% at 16:00, 11:00 and 19:00 hrs UT on 4 and 5 April, respectively. The negative phase storm recorded at this period was NS ionospheric storm. The D(foF2) of Chongqing did not record any pronounce storm during 00:00-13:00 hrs UT (i.e. the electron concentration was not above the reference level). At about 14:00 hrs UT, the atmosphere recorded a negative storm with 44% depletion, this period was observed as the declining period of the main phase, the period at which Bz orient southward. This negative storm was emerged to an NS ionospheric storm which did not last for more than 5 h before a second depletion at the main phase with 35% at 22:00 hrs UT on 3 April. The recovery period had a negative storm response of 28% at 19:00 hrs UT on 4 April, positive with maximum peak of 72% at 02:00 hrs UT and 36% depletion at 13:00 hrs UT on 5 April High-mid latitude The atmosphere at Athens began its response with a negative ionospheric storm at about 04:00 hrs UT with electron density concentration of 22%; thereafter, D(foF2) sharply enhanced to the positive phase with NS ionospheric storm. The main phase depleted from 27% electron density concentration at 20:00 hrs UT to 20% at about 04:00 hrs UT on 4 April. Thereafter, it increased to the positive phase and recorded an NS storm for more than 17 h before it started decreasing and attained depletion value of 24%, 27% and 29% at 04:00, 07:00 and 20:00 hrs UT in the recovery phase. The D(foF2) of Rome emerged with NS ionospheric storm for the first 6 h on 3 April around 07:00 hrs UT, fof2 started increasing till 10:00 hrs UT when it recorded a maximum depletion value of 34%. Thereafter, it increased sharply and recorded a positive storm of 21% electron density at 13:00 hrs UT. The main phase was preceded with NS storm until about 08:00 hrs UT on 4 April when it recorded an enhancement value of 24%. The recovery phase recorded a positive storm of 23% at 22:00 hrs UT on 4 April and negative storm of 44% at 21:00 hrs UT on 5 April. Rostov atmosphere did not show any significant ionospheric storm till around 01:00 hrs UT on 4 April when the occurrence of geomagnetic storm started increasing the electron density concentration in the ionospheric plasma to 21% enhancement and later depleted to 21% at 04:00 hrs UT. The recovery phase of the ionosphere at this station was recorded with a severe negative ionospheric storm with maximum peak electron density of 37% at 04:00 hrs UT on 5 April. The atmosphere at Millstone Hill responded to the geomagnetic storm with more intense positive storm than negative. The intense negative storm was recorded only at the initial phase, when the ionosphere responded with 35% depletion at 09:00 hrs UT. The main phase was emerging with positive intense storm. As Dst started declining to the minimum value, the ionosphere at this station responded with a depletion in electron density and enhancement in Dst phase greatly increased the electron density to 76% at 08:00 hrs UT on 4 April. Thereafter, it decreased sharply but remained in positive phase with NS ionospheric storm and later increased back to an enhancement value of 41% at 18:00 hrs UT on the same day. The recovery phase recorded a negative storm of 21% depletion at 09:00 hrs UT on 5 April. This period also registered a positive storm of 31% enhancement in electron density at 12:00 and 15:00 hrs UT. The storm intensity at Goosebay ionosphere was more severe compared to remaining high-mid latitude stations. At about 03:00 hrs UT, the electron density was 31% and at 09:00 hrs UT, it was 32%. An enhancement of 27% at 23:00 hrs UT was observed during the period of maximum Dst. Also, the negative phase of the storm recorded NS storm. The maximum electron density concentration was observed on 5 April with 32% enhancement at 04:00 hrs UT. The negative storm was recorded at 08:00 and 11:00 hrs UT with 21% and 22% depletion, which exceeded the maximum positive storm record. 4 Discussions According to Huang et al. 30, during geomagnetic activities, perturbed electric fields at middle and low latitudes of the ionosphere may result from the effect of prompt penetration from high latitudes and the disturbance dynamo mechanism. The dominant interplanetary phenomena causing intense magnetic storm are the interplanetary manifestation 21,31. Two interplanetary structures are important for the development of such class of storms: the sheath region just behind the forward shock and the coronal mass ejection (CME) itself. However, these structures lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary

9 614 INDIAN J RADIO & SPACE PHYS, DECEMBER 2012 structure accompanying another stream 24,25. However, Zhao et al. 32 found that internal interplanetary coronal mass ejection (ICME) field orientation may indeed exhibit a preference for the prevailing solar field pattern, suggesting that these fields also contribute to the seasonal pattern of geomagnetic storms. The great (or intense) storms are those with peak of Dst -100 nt, moderate storms fall between -50 and -100 nt, and weak storms are those between -30 and -50 nt (Ref. 26). In light of this characteristic, the storm was an intense storm and was driven by magnetic cloud. Geomagnetic activity is known to increase dramatically whenever the IMF stream is toward negative z-direction 33. According to Adekoya et al. 34, the magnitude of Bz turning into southward direction from northward highly depends upon the severity of the storm; and the variation in F2 layer parameter at the time of geomagnetic storm is strongly dependent upon the storm intensity. Also, the storm driver is characterized by low plasma beta, high magnitude of magnetic field component, large coherent rotations; often include large and steady north-south components and higher proton temperature. The dark regions in Figs (1-3) represent the main phase of the storm. This period at low-mid latitude was observed to record more of negative ionospheric storm and also, the declining phase of Dst was simultaneously depleted. The positive phase of the storm at this period was observed with NS ionospheric storm. The recovery phase is more pronounced with P storm and has a larger effect of ionospheric storm than initial and main phase. The D(foF2) response of high-mid latitude responded mainly with positive storm in the main phase; and the negative phase of the storm at this period was observed with no-significant ionospheric storm. The initial phase responded with severe negative ionospheric storm compared to low-mid latitude with positive ionospheric storm. From the above characteristics, it is noteworthy that ionospheric storm response at mid-latitude to the studied geomagnetic storm were different. They either responded with positive or negative, depending on the pre-storm response (i.e. PN-storm or NP-storm). Also, during the main phase of the geomagnetic storm, positive and negative phases of ionospheric storm prevail in the daytime and at night, respectively. These followed David et al. 35, that electron density enhancement in the dayside ionosphere are often seen during the geomagnetic storm. 5 Conclusions In the present work, an analysis on the global changes was conducted in fof2 using normalize deviation of critical frequency F2 [D(foF2)] on the ionosphere at mid-latitude stations in order to verify the geomagnetic storm effect of 2-6 April Ionospheric storms represent an extreme form of space weather with important effects on operational problems in space communication and navigation systems affecting everyday human activity. These phenomena are driven by highly variable solar and magnetospheric energy inputs to the Earth's upper atmosphere. The mid-latitude ionospheric stations are classified into two types depending on their geographic coordinates and ionospheric storm variation (i.e. positive and negative phases of ionospheric storm). The low-mid latitude are characterized with geographic latitude between 20 N < low-mid 36 N, while high-mid latitude are > 36 N. Also the storm are classified as positive storms (P-storm) or negative storms (N-storm) depending on whether maximum electron density is positive or negative following the onset of geomagnetic storms (small fluctuations of less than ±20% are neglected). If ionospheric storms show initial positive electron density (>20% for more than three hours) followed by negative electron density, it is classified as PN-storms. If the storm initially has negative electron concentration, followed by positive density; they are classified as NP-storms. There are also non-significant ionospheric storms (NSstorms) for which D(foF2) is weak (less than ±20%). The ionospheric storm produced at low-mid latitude was observed to be negative (N) ionospheric storm in the main phase; and also, the declining phase of Dst was simultaneously depleted. The recovery phase was more pronounced with P storm and had a larger effect of ionospheric storm than initial and main phase. The D(foF2) response of high-mid latitude responded mainly with positive storm in the main phase. The initial phase responded with severe negative ionospheric storm compared to low-mid latitude with positive ionospheric storm. Dayside of the main phase at both low and high-mid latitude are characterized by positive storm while the night time mainly characterized by negative storm. Lastly, it may be noted that magnitude of Bz turning into southward direction from northward highly depends upon the severity of the storm and large variation in F2 layer electron density in the main

10 ADEKOYA et al.: EFFECTS OF GEOMAGNETIC STORM ON MID-LATITUDE F2 DURING 2-6 APRIL 2004 STORM 615 phase of geomagnetic storm are strongly dependent upon the storm intensity. This storm was observed to be driven by magnetic cloud which is characterized by low plasma beta, high magnitude of magnetic field component, large coherent rotations; often include large steady north-south components and high proton temperature 36. Acknowledgements The authors are grateful to National Space Science data Centre s (NSSDC) OMNI database ( and also to the National Geophysical Data Center s SPIDR (Space Physics Interactive Data Resource) network s ( The authors also appreciate the reviewers for suggesting some useful references, which were of great importance to this paper. 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