RESPONSE OF POST-SUNSET VERTICAL PLASMA DRIFT TO MAGNETIC DISTURBANCES

Transcription

1 CHAPTER 6 RESPONSE OF POST-SUNSET VERTICAL PLASMA DRIFT TO MAGNETIC DISTURBANCES 6.1. Introduction 6.2. Data Analysis 6.3. Results 6.4. Discussion and Conclusion

2 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances Introduction The study of electric fields and plasma drifts at the ionospheric F region is a major topic in the understanding of solar terrestrial energy coupling processes. The process of energy transfer is complicated due to the complexity of the parameters involved. Extensive studies have been conducted on the behaviour of ionospheric and magnetospheric current and electric field during magnetically disturbed periods, namely geomagnetic storms (Gonzales et at., 1979; Reddy et at., 1979; 191; Somayajulu et at., 1991). The ionospheric plasma motion is decided by the motion of the neutral atmosphere and the disturbance in the magnetosphere due to the solar wind. During daytime, since the ionospheric conductivity is very high at the equatorial region, it is very difficult to identify the magnetospheric contribution. However, during post- sunset period, when ionospheric conductivity is low, response of the ionospheric electric field to magnetospheric disturbance is easier to identify. Radar measurements of F region plasma drift and related electric field during magnetically disturbed period can be used to investigate the physical processes involved in magnetosphere-ionosphere coupling. Vertical plasma drift at the equatorial F region is found to deviate from the regular diurnal pattern during magnetically disturbed periods. The low latitude electric field perturbations are usually westward during daytime and eastward during nighttime They are also associated with large and sudden decreases in high latitude convection driven by rapid northward changes of IMF (Rastogi and Patel, 1975; Fejer et al., 1979; Kelley et al., 1979; Fejer. 196). The daytime eastward and nighttime westward electric field perturbations associated with sudden increases in convection are less common and have smaller lifetime and amplitude compared to the disturbance electric fields associated with northward B z changes. These deviations are common near the dusk and in the early morning-noon periods (Fejer, 196). Radar measurements of F region vertical plasma drift at the equatorial region is used to determine the dependence of equatorial zonal electric fields on auroral electrojet indices during storm time. These disturbance drifts resulted from the prompt penetration of high latitude electric fields and the dynamo action of storm

3 6.1. Introduction 123 time winds which produce largest perturbations of few hours after the onset of magnetic activity. This equatorial signature of the electric field disturbance changes depending on the contribution of the two components (Fejer and Scherliess, 1995). Studies have shown that the plasma drift deviated from their normal pattern during both quiet and disturbed periods. The reversals during disturbed periods are of shorter duration and are due to magnetospheric and high latitude phenomena (Fejer et al., 1976). Balsley and Woodman (1971) using Jicamarca radar (December 20-21, 1967) have shown that the electrojet drift velocity has anomalous night time reversal. These anomalous reversals and upward drift during the disturbed periods may be associated with the appearance of spread F especially in the post midnight period during solar minimum (Chandra ancl Rastogi, 1972; Bowman, 1975). Work by Onwumechilli et ai., (1973), Carter et ai., (1976) have shown a good correlation between auroral and equatorial zone magnetic field and drift velocity fluctuations. During geomagnetically disturbed conditions, ionospheric electric fields and currents at middle and low latitudes were found to differ from their quiet day pattern (Rastogi et al., 1972; 197; Blanc, 197; Fejer et al., 1979; Kelley et ai., 1979; Reddy et al., 1979). As explained in Chapter V, there are two source mechanisms for the observed disturbances. They are the solar wind magnetosphere dynamo and ionospheric disturbance dynamo. The disturbance dynamo electric field perturbations are observed at the magnetic equator about hours after a geomagnetic storm (Fejer et al., 193). They decrease the upward drift velocity during the day and downward drift during the nighttime (Fejer et ai., 199). The observation of post-sunset vertical plasma drift at equatorial station Trivandrum shows a typical characteristic featur~ with a sharp increase in the plasma drift during post- sunset period with a peak value around 1900 hours 1ST and a sharp reversal in the direction of drift around 2000 hours 1ST with the drift remaining negative for a few hours afterwards. During certain days, the vertical plasma drift showed anomalous deviation from the typical pattern. In this Chapter, these anomalous features in the vertical drift have been studied and we have tried to link these features with the dynamo electric field by comparing the vertical drift velocity at Trivandrum with the corresponding horizontal component of geomagnetic field.

4 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances Data Analysis The typical feature of the evening F region is a sharp enhancement in the plasma drift with a peak around 1900 LT and a reversal to the negative direction with a negative drift during night. For the present study, we have selected ten days (Table 6.1.) having marked deviation from the typical pattern. To compare the F region plasma drift (Vd) of these days with the magnetic field variations, the horizontal component of geomagnetic field (H) at Trivandrum is used. In the present analysis fifteen minute data of Vd and H have been used. The H data is obtained by digitising the magnetograms supplied by the Indian Institute of Geomagnetism, Bombay. The variation in H, (6.H) values are obtained by subtracting the mean midnight value of the day from the digital value for every fifteen minutes. The mean midnight value is obtained by averaging the midnight value of the previous and following days of analysis. Date A p Table 6.1 List of days selected for the analysis and the corresponding A p values The hourly D st values are obtained from the world data centre (WDC). These D st values are subtracted from the corresponding (6.H) values (b.h - D st) which provides a measure of the ionospheric part. These are hourly values. To eliminate the shorter period fluctuations of less than an hour present in the vertical drift, the vertical drift velocity is subjected to a suitable smoothing window of 75 minutes. The smoothed vertical drift velocity is compared with b.h.

5 6.3. Results Results Figures 6.1.a. to 6.1.j. depict the pattern of the variation of vertical drift velocity, horizontal component and hourly values of (f:lh - D st). The smoothed vertical drift velocity is compared with H in Figures 6.2.a to 6.2.e. On all the days selected, the vertical drift velocity show an anomalous reversal towards the positive direction around 2200 hours 1ST with slight day- to-day variation in the reversal time. On 7th September (Fig. 6.1.a.) and 24th September (Fig. 6.1.b.), 1993, the velocity reversed towards positive direction soon after 2000 hours 1ST. Comparing the vertical drift velocity with the horizontal component of geomagnetic field it is seen that they show an almost similar gross pattern on most of the days with slight variation during certain period on few other days. Among the ten days selected (Table. 6.1.), the pattern during 23rd October, 1994 (Fig. 6.1.i.) is peculiar in the sense that on this day, the vertical drift and horizontal component show variations in opposite direction. On this day, when vertical drift decreases and when it reverses to the negative direction at 1900 hours 1ST, the horizontal component increases towards the positive direction. Corresponding to the reversal at 1900 hours, the b.h component shows a steep increase. The negative peak in vertical drift at 1930 hours coincides with the peak in the positive direction in f:lh. The gross pattern of (f:lh - D st) is also similar to that of V,l and H. The small oscillations seen in the vertical drift are, however, not observed in (tlh - D st) due to the fact that (f:lh - Dsd are hourly values whereas vertical drift is taken at fifteen minutes interval. Although there is a general similarity in the pattern of the two parameters (f:lh and Vd), differences are observed in the pattern during certain periods on some days. For example, on 30th September, 1993 (Fig.6.1.e.), during the period 2000 to 2200 hours 1ST, the vertical drift is negative and also show small oscillations of less than an hour period. But the f:lh component is positive eventhough its amplitude decreases slightly at the time corresponding to the negative reversal in V,1. An increase in the b.h component is seen corresponding to the positive Vd reversal. Similarly on 6th December, 1993 (Fig.6.1.f.), till 2100 hours 1ST, both the pattern are not exactly the same. After 2100 hours the pattern is the same

6 6. Response of Post-sunset Vertical PI., asma Drift to M... agnetlc Disturbances 126 (a),,-... l2 ~ c I 4 <) Ap ( b),,-... ~ 0 c...- I <J - jv Ap 'cj)~,- E 1;) - _0-0' >I - f <] L4 V -24 i Fig.6. I. Vertical drift velocity Vd (-->, Horizontal component of geomagnetic field 6H (-H-) and AH- Dsf (-+-) on (a) and (b)

7 6.3. Results 127 ~ I- c~ 24 ' ~~ 0... II 0 I<l <J Cc) o 4 50 "0 I >I <J ' I Cd) ~ ~ 4 I <l 0 r--~l--l--,... 4,...,...1 e/) C 4,...,...,...te/) C, h'''-~-...ot,-'"--'\--..l.-...j0 "0 I >I <l -_._ Fig 6.1. Vd C-),6H(-*-) and 6H - Dst C-+-) on (c) and (d)

8 6_ Response of Post-sunset Vertical Plasrr,(j Drift to Magnetic Disturbances 12 ~ 32 L 24,,-.. ' ~ '-",... 'tnt- OC... II ~ <J 0 ( e) Ap ti ~ _: -l -12, ( f ), Ap - 6 'c...,,-.. 2 'tntoc I I 0 I<) <) -2 t t-4~ -6 r ~ - t" _ Fig.6.1. Vd (--), D.H(---*-) and 6H - Dst(---';--) on (e) and (f),

9 6.3. Results r----;: ~'32 24 \6 r- c -... I <J -\6 t l2-93. Ap ,,-... I/)r- " c - E ~... I/) > I -24 i ~ -32 t -40,...4 ( h) Ap- 26 S,,-... r- 0 0,,-...r- I/)... c,,~ I_oS - E... 0 I/) <l fs I - J'I tt ,,-... C Fig.601. Vd (-), 6H(-*-) and 6H - Dst(~) on (g) and (h)

10 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances 130 although the small oscillation seen in vertical drift are not present in 6.H. On 7th December, 1993 (Fig.6.l.g.) the 6.H component reverses towards the negative direction almost two hours earlier around 100 hours 1ST. Here also from 2000 hours the pattern is similar although there is a time lag with V'd leading the!:lh component. On 22 October, 1994 (Fig.6.l.h.), the pattern is almost similar till 2000 hours and after 2215 hours. In between these two periods, both 6.H and V';l show almost opposite pattern. Corresponding to the peak in the positive direction at 2045 hours in V';l the 6.H component shows a peak in the negative direction. During this interval, the (6.H - D st) is similar to that of V'd. On 24th October (Fig.6.l.j.) also, the pattern is different till 2000 hours 1ST. After that they are similar although vertical drift show oscillations of smaller period which are not as such reproduced in the H curve. The post-sunset vertical drift velocity is found to have fluctuations of period less than 50 minute superposed on the gross pattern during both quiet and disturbed periods. These vertical drift velocity fluctuations with period less than 50 minutes is analysed in Chapter IV. To study the gross pattern, the vertical drift velocity is smoothed by using a smoothing window of 75 minutes. This can be done only for those days which do not have any data gap in between. This analysis is done for five days, for which data is available without any data gap. The vertical drift after smoothing is compared with the horizontal component of geomagnetic field as shown in Fig.6.2.a. to 6.2.e. The ten days data considered for the present analysis include both disturbed and quiet days. Comparing the similarity in the pattern of the three parameters, vertical drift, horizontal component of geomagnetic field and (6.H - D st) for the quiet and the disturbed days, it is seen that the pattern show better similarity during quiet days compared to the disturbed days. The duration of the reversed drift is comparatively larger in the case of quiet days than disturbed days. The observed vertical drift velocity pattern in the post- sunset period is decided by the ionospheric disturbance dynamo and the neutral wind. Since during the period of observation, the HF Doppler radar was working in the single receiver

11 6.3. Results 131 ( i ) 24 i4 - La ~ -c: - 11I_ -... E III k _0 <]-24 q -24 >k -0 1 t ~e ~,""64 ( j ) _c: ;: -r - rii...,. - c: 'E c5' I- - -I <] tt t Fig.6.1. Vd (-), ~H(-4 and 6H - 0st(-+-) on (i) and (1)

12 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances 132 (0) Ap- 12,... en E I <J t 4 01-l------;:!7~r_---lI-~--lO ;2.2, _ 4 ' J -4 4 ~ t ( b),... I- c ""-" Ap I 4 <3 t-:,... en... E ""-" 4 "0 > t Fig.6.2. Vertical drift velocity Vd (--e--), 5 point running mean value and Horizontal component of geomagnetic field ~H(~) on (0) and (b)

13 6.3. Results 133 mode, the meridional wind pattern could not be evaluated. Considering the fact that at the magnetic equator the F region vertical drift is decided by the EXB drift and at locations away from magnetic equator, the meridional component of plasma drift also contributes to the vertical drift (Rishbeth et al., 197) it is possible to evalu'ate the auroral pattern of neutral wind by considering such two stations. Hence making use of the nighttime hif data of Trivandrum and SHAR the nighttime thermospheric meridional wind pattern (U) can be calculated. During the ten days of HF Doppler observation the Trivandrum and SHAR ionosonde data is available for only one day ie., Fig 6.3. shows the vertical drift velocity measured using HFD radar at Trivandrum compared with hif values measured using ionosonde at Trivl1ndrum and SHAR. Fig 6.4. shows the dh' F / dt for Trivandrum and SHAR which gives the ionization vertical drift at the two stations. The meridional velocity for is calculated using the method adopted by Krishnamurthy et al., (1990). The method can be explained as follows. Let V represent the velocity at a station away from the equatorial station and VD represent the vertical drift at the equatorial station. Then v = VDcos/ - Ueos/sin/ - WJ)sin 2 / (1) where U is the meridional neutral wind, VVD is the plasma drift due to diffusion and / is the magnetic dip at the station a\vay from the equator. Hence from (1), U can be obtained as; U = [2(VD eos / - V)]/ sin 2I - VV'D tan / (2) In this equation the drift velocity calculated is the sum of true drift velocity and apparent drift velocity due to chemical loss. The terms for calculating f3 is obtained from Anderson and Rusch (190) and the number density of N 2 and 02 and neutral temperature are obtained from MSIS-6 model (Hedin, 197). The pattern of the meridional wind thus obtained is shown in Fig The wind pattern is as follows. The wind is northward from 100 hours to almost 2000 hours 1ST. At about 2000 hours 1ST, the wind becomes southward and remains like that for an hour. Again at 2100 hours, it becomes northward till 0100 hours except for a short southward wind at about 2200 hours. From 0 I00 hours the wind is almost southward except for one or two very small interval northward wind.

14 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances 134 (c) Ap ,...,... en c - E....- '-'" I "0 <J > t (d) Ap ,... en c:, E -I "0 > <J -~2-32 t t (-) and bh (...-) on (c) Fig.6.2. agd V (d)

15 6.3. Results 135 (e) Ap ""' ""' Il) t-... c E I <J t-4-4 t > "' Fig.6.2. V d (-&-) and 6H(-if-J on (e)

16 CJ. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances u ~ , --,, \, \ ~ 1:7 29 Time (1ST) Fig.6.3. hi F data of Trivandrum (contin~ line) and SHAR (dotted lin.) of r ~..--, I/) , E 0 >. :t: () 0 -\0 G) :> , I \ 'J ':' ~ Time (1ST) 29 Fig Velocity of Trivandrum (continued line) and SHAR (dotted line) of ~

17 6.4. Discussion and Conclusion Discussion and Conclusion The vertical drift velocity at the equatorial F region is a direct measure of the electric field. The general pattern of the electric field in the evening F region is that the electric field pattern is eastward in the post-sunset period. Around 1900 to 2000 hours 1ST, the electric field becomes westward and remains westward afterwards. But the anomalous positive reversal seen in the vertical drift around 2200 hours 1ST for the selected days shows the existence of a superposed eastward electric field over the already existing westward electric field. During disturbed period, Fejer (196) has shown a close coupling between the horizontal component of geomagnetic field of Adisababa and east-west electric fields measured at Jicamarca. Studies show that there is a close relation between the auroral and equatorial ionosphere especially during magnetically disturbed period. A strong relation is found to exist between interplanetary, auroral and equatorial magnetic field changes (Nishida, 196a:b). Gonzales et ai., (1979) have shown that horizontal component of geomagnetic field and eastward component of the electric field show similar features. Similar results are obtained by Ichinose et ai., (1994) also. According to Blanc and Richmond (190) there are mainly two source mechanisms responsible for the temporal and spatial behaviour of electric fields at low and middle latitudes during geomagnetically active periods. They are the magnetospheric dynamo and ionospheric dynamo. In the magnetospheric dynamo, interaction between solar wind and magnetosphere causes the flow of electric current connecting the magnetosphere and high latitude ionosphere. Part of this current penetrate directly into the low latitude through the conducting ionosphere. The thermospheric wind produced by auroral heating alter the global heating and generate electric fields and currents at middle and low latitudes by ionospheric dynamo action. But the time scale involved is longer in the second mechanism compared to the first. Hence the ionospheric dynamo can be a possible source mechanism for the anomalous pattern in vertical plasma drift. Fejer et al., (199) have studied the seasonal variation in the drift velocity. They have explained the increase of upward drift velocity in the late afternoon

18 6. Response of Post-sunset Vertical Plasma Drift to tvlagnetic Disturbances en E 0 r----t--+--h---\---/l\---a---i :::> Time ( 1ST) hrs Fig Meridional wind pattern (U) of ~ Winter AVQ F10.7 =200 units 40 -III... E 0 -::l ) Looot ",eon time (Hrs) Fig Nocturnal vori,ation of U for winter of (continued line) The dcshed curve show HWM90 mod.1 wind (H.din 1991) for the some season ( from Hori and Krilhnomurthy I 1995 )

19 6.4. Discussion and Conclusion 139 and early morning period for winter months as due to the penetration of magnetospheric electric field associated with increase in polar cap potential drop. The large efficiency in the penetration during evening sector in winter may be due to asymmetry in the conductivity distribution between the two hemispheres during solstice. The decrease in the pre-reversal enhancement peak during equinox is consistent with perturbations resulting from disturbance dynamo electric fields. The direct penetration of high latitude effects into the low latitude ionosphere can generate large amplitude short lived zonal electric field perturbations occurring simultaneously at different latitudes and longitudes (Gonzales et al., 1979; Fejer, 196; Fejer et al., 1990a). The perturbation electric fields are large enough to cause a reversal in the direction of the upward/northward drift and of current flow in the equatorial electrojet. But there is no evidence for the penetration of large amplitude meridional electric field (zonal plasma drifts) into the equatorial ionosphere even during strongly disturbed conditions (Fejer et al., 195; 1990a). Fejer et al., (191) made use of VHF backscatter data on equatorial electrojet in an empirical method to delineate the ionospheric and magnetospheric contributions. They have observed that during the initial phase the magnetospheric and ionospheric current show fluctuations that are in antiphase. There is direct and indirect evidence for the prevalence of disturbance electric fields at equatorial latitudes during disturbed geomagnetic conditions (Gonzales et al., 1979; Blanc and Richmond, 190; Gonzales et ai., 193; Takahashi et al., 197). The electric field enhancement may vary from night to night (Haerendal and Eccles, 1992). The equatorial electrojet current during evening hours is significant in determining the post-sunset enhancement of horizontal electric field. The conductivity reduction in the E region due to the recombination of ionization and plasma drift enhances the horizontal (east-west) electric field and thereby increases the speed of uplift. Thus the dynamic adjustment of ionization has an unstable feed back which explains the night to night variability of horizontal electric field. Ground based magnetometer and ionosonde observation indicate that delayed equatorial electric field disturbance do not always follow enhanced magnetic activity even in the presence of ionospheric storm effects at mid latitudes (Sastri, 19a). Hence in addition to the geomagnetic disturbances, other effects such

20 6. Response of Post-sunset Vertical Plasma Drift to Magnetic Disturbances 140 as changes in the chemical composition of mid latitude ionosphere and long period gravity waves playa role in the occurrence of equatorial disturbance dynamo electric fields. But it is difficult to compare the measured disturbance dynamo electric fields with theory since the amount of high latitude heating cannot be easily estimated (Mazaudier and Venkateswaran, 1990). At the magnetic equator, there are several observations which shows relevance to the disturbance dynamo (Blanc and Richmond, 190). Rastogi et al., (1972) found that the average drifts on disturbed days are reduced in magnitude compared to that of quiet days. Carter et ai., (1976) found that eastward electric field reversed during magnetically disturbed period. Results of Rastogi and Patel (1975) and Patel (197) suggest a close correlation between electric field reversals at the equator and south to north reversals of the vertical component of IMF. They have explained this as strong IMF reversals from southward to northward impose an electric field on the ionosphere opposite to the normal 5'/ field thereby decreasing or reversing the normal ionospheric drift direction. But during certain events, IMF reversals do not produce any change in the equatorial ionospheric electric field and also anomalous reversals do occur at times when IMF is steady and southward. This suggests that equatorial effects are not directly related to IMF but are the results of changes in the convection and high latitude substorm phenomena which may be triggered by changes in IMF (Fejer et ai., 1979). The meridional velocity calculated for one day ( ) shows deviations from the winter meridional pattern shown by Hari and Krishnamurthy (1995) using the same method for (Fig.6.6). The ~iouthward wind seen at 2000 hours 1ST and also from 0100 hours 1ST is not present in the winter pattern obtained by Hari and Krishnamurthy (1995). This discrepancy may be due to the disturbance dynamo effect. When there is a disturbance effect in the meridional wind pattern, it will affect the zonal wind pattern also which in turn is shown ip. the vertical drift pattern obtained at Trivandrum. This gives a supportmg evidence that disturbance dynamo may be the possible explanation for the observed anomalous reversals in the vertical drift at Trivandrum obtained using the HF Doppler radar.