Mapping of the Simultaneous Movement of the Equatorial Ionization Anomaly ( EIA ) and Ionospheric Plasma Bubbles Through All-sky Imaging of OI 630 nm Emission

An all-sky imager with 180° field of view has been operating at Kolhapur (16.8°N, 74.2°E; dip lat 10.6°N) in India to study the ionosphere-thermo­ sphere dynamics through the imaging of 01 630.0 nm oxygen emission line. It is observed that large number of events are characterized by the devel­ opment of strong Equatorial Ionization Anomaly (EIA). Two examples of equatorward movement of the Appleton Anomaly crests (reverse ioniza­ tion anomaly) with speed of 36-40 mis on the night of January 26, 1998 and January 18, 1999 during the observation of ionospheric bubbles have been reported showing the observed correlation between EIA and bubbles. Our results agree well with those of Sridharan et al. (1993) regarding the move­ ment of the reverse fountain from Indian low latitude region. Generally, the background emission rates were low when no bubbles were observed. (


INTRODUCTION
Ionospheric plasma irregularities associated with nighttime equatorial spread-F (ESF) phe nomena have been extensively studied at different longitude zones in a wide range of altitudes, starting from lower F-region to regions beyond 1000 km in the last two-three decades by using both radio and in-situ techniques (incoherent back scatter radar, rockets, satellites and UHF/ VHF scintillation, ionosondes, topside sounder) as well as optical techniques (conventional tilting and scanning photometers and wide angle imaging system) (Weber et al. 1978;Fagundes et al. 1999;Kil and Heelis 1998;Mendillo et al. 1997;Singh et al. 1997;Sahai et al. 1994Sahai et al. , 1996;;Taylor et al. 1997).The scale size of the irregularities varies between a few centimeters to several hundred kilometers in geomagnetic field perpendicular direction (Abdu 1993).
Several research workers have used wide-angle optical imaging technique of F-region nightglow emissions to study large-scale equatorial plasma irregularities.These are regions where the plasma densities decrease abruptly few orders of magnitude compared to the ambi-1 Indian Institute of Geomagnetism, Colaba, Mumbai -400 005 *Corresponding author address: Dr. G. K. Mukherjee, Indian Institute of Geomagnetism, Colaba, Mumbai -400 005; E-mail: gkm@iig.iigm.res.inent plasma densities.The importance of studies of ionospheric plasma irregularities grew be cause of their strong influence on ionospheric and trans -ionospheric communications.These low intensity regions which are optical signature of bubbles or depletions which have been mapped in the all-sky images of OI 630.0 nm, 557.7 nm and 777.4 nm emissions at a low latitude station, Kolhapur (Mukherjee et al. 1999).
The plasma in the magnetic field tubes tend to diffuse along F-region magnetic field lines while moving upwards because of greater conductivity parallel to the magnetic field lines than perpendicular to them.This will make the depleted plasma regions field aligned.While the bubbles drift eastward with the background plasma, the depleted plasma regions appear to tilt in the westward direction.These depleted regions are not homogeneous and within their bound aries exist the smaller scale size irregularities (a few meters to kilometers in size) which gives rise to strong VHF I UHF scintillations in satellite beacon signals.
The use of all-sky optical imaging system has been especially important in studying the spatial extent of the bubble and their motion relative to both the ground and to the surrounding ambient ionosphere (Mendillo et al. 1997).A CCD based all-sky imaging system (180° of view) developed in collaboration with Boston University, U.S., was operated from Kolhapur on clear moonless nights to obtain the signature and dynamics of plasma bubbles by studying the images at OI 630.0 nm, 557 .7 nm and 777.4 nm.The data for the generation and evolution of these large-scale irregularities is limited at the Indian longitude sector (Mukherjee et al. 1998).The location of the observing station is important as a single image frame at a given instant provides information regarding the dynamics of these irregularities over a large low latitude region starting from geomagnetic equator to equatorial ionospheric anomaly region.
These observations also permitted us to acquire a large comprehensive database for studying the growth and evolution of equatorial ionospheric plasma bubbles.
It is generally believed that the bubbles or depletions are produced in the bottom side of the F-region after sunset triggered by gravity waves under certain conditions when the bottom side of the F-region becomes unstable for the growth of a fluid type gradient instability mecha nism such as Rayleigh-Taylor Instability (Dungey 1956).The linear growth rate is given by Where Vn is the gradient in electron density n, V;n is the ion-neutral collision frequency, E and B are the electric and magnetic fields and g is the gravitational acceleration.
Basically, the thermospheric zonal wind that blows across the sunset terminator is respon sible for the F-layer dynamo electric field (Rishbeth 1971).The longitudinal gradient in the E layer Pederson conductivity and the gradient in the Hall conductivity result in the generation of an eastward (pre reversal) electric field which contributes to the vertical upliftment of the layer and it adds to the instability growth rate factor ( Y ).The electric field contributes to ( y) indirectly through its effect on g / v in and V n I n.Several studies have shown that the height of the nighttime F-layer is the single most important parameter controlling the generation of spread-F irregularities (Patel 1974;Woodman 1970).Recently Whalen (2000) showed a clear relationship and understanding among a bubble, range type spread-F and the development of ionization anomaly.There was no coordinated scintillation observations available during these measurements.However, the intersection of the bubble with the crest maximum is known to be the focus of maximum scintillation from many other experiments.The enhanced equatorial Appleton anomaly comprises of two regions of increased electron density maximum at ± 15 ° dip latitude.It was observed by Mendillo et al. ( 1992) that if the anomaly is well developed and symmetrical about the equator then it helps in the generation of Spread-F irregularities.
The generation of ionospheric plasma bubbles and the anomaly enhancement generally occur continuously following sunset.Rastogi (1989) has pointed out, that Spread-Fat equatorial stations is preceded by marked rise of h'F during post sunset hours.Sahai et al. (1996) also reported that in addition to uplifting of the post sunset F-layer at equatorial station, Fortaleza (3.9°S, 38.4°W) during the period of observation of plasma bubbles, there was a considerable increase in afternoon foF2 (critical frequency of the Flayer) values at off equatorial station, Cachoeira Paulista (22.7°S, 45.0°W) compared to the values at the equatorial station at Brazil sector.
In this report, we study the simultaneous observation of the movement of reverse ioniza tion anomaly and the development of ionospheric plasma bubbles from a low latitude station, Kolhapur in India.

INSTRUMENTATION
The design features of a CCD based imaging system have been explained by number of research workers (Baumgardner and Karandanis (1984); Mendillo et al. (1989)

Geometry of Observations
Figure 2 shows the location of the observing station Kolhapur with respect to geographic co-ordinates.The two circles indicate the coverage of the camera system for zenith distances 75 ° and go0 for an emission height of 300 km.during the night.Generally, the equatorial region at the extreme south of the image shows very low 630 nm intensity as the F-layer is situated at higher altitude in the evening hours, recom bination is low due to smaller values of ion-neutral frequency at F-region altitudes.The se quence of images in Fig. 5 shows two interesting phenomena.The bubble (low intensity) drifted eastward along with its movement towards the north while rising at higher altitudes at the equator while the equator ward movement of the anomaly (high intensity region) continues as inferred from the subsequent images.The inward motion (northward direction) of the top of the bubble is in opposite direction to the inward drift (ExB) of the bulk plasma towards the equator.Using the Fig. 3 and the sequence of images, the drift speeds were computed which vary between 36-40 mis.The bubble at 2150 hrs reaches 1500 km apex height (Fig. 4) and the average bubble rise velocity at the equator turns out to be 110 mis.Figures 6 (a

Observation on January 18, 1999
The average decimetric solar flux indices (Fl0.7 cm) for the day were 165 units and the average of the Kp indices during the period of observation was 2. It was a typically magneti cally quiet night with moderate solar activity.AU-sky images were taken with 630 nm filter with integration time of about one minute.that during the period of observation of bubbles at the low latitudes, the anomaly was well developed and fairly strong.

DISCUSSION AND CONCLUSION
The present study has brought out an integrated view of the phenomena like the develop ment of ionospheric plasma bubbles embedded in ambient plasma and the anomaly enhance ment in the low latitude Indian region.The drift speed of reverse EIA from north to south direction at night has been inferred from successive OI 630.0 nm images (Figs. 5 and 7) to be 36-40 mis when there was strong development of EIA shown by an arrow in the diagram.
The daytime F-region is controlled by a delicate balance between production of ionization through photo ionization and loss due to chemical and plasma transport processes.In the vi cinity of the dip equator, vertical diffusion of plasma is not possible without the imposition of an applied electric field because of unique configuration of the geomagnetic field.Such an electric field originates due to the global E-region dynamo driven by tidal winds.The E-region east-west electric field can be mapped to the F-region through the highly conducting geomag netic field lines and controls the plasma movement in the vertical direction through E x B drift (Heelis et al. 1974).Besides, as the gyro-frequency is much larger than the ion-neutral fre quency at F-region heights, the plasma once lifted upwards can diffuse along the geomagnetic field lines under the influence of pressure and gravitational gradient forces.Martyn (1947)  proposed the anomaly was due to the combined action of vertical drifts of ionization near dip equator and its subsequent diffusion to higher latitudes, which is well known as fountain effect.
Usually during the nighttime, the zonal electric field changes direction and becomes westward.
As a result the plasma moves equatorward as seen at an off equator latitudes, which is known, as reverse ionization anomaly.Hanson and Moffett (1966) also modeled a reverse fountain, which produced equatorial ridge of electron density.Since the 24-hour average of the vertical drift velocity must be equal to zero for the ionosphere, the upward drift by daytime must be balanced by downward drift at night.
Using the electron (e-h profile) density (N A and Nw) data of Ahmedabad (dip latitude 18. 6° N) and Waltair (dip latitude 10.6° N) in the Indian longitude zone, it was observed by Raghavarao et al. (1988) that there was an enhancement in the electron density ratio (N /N w ) by a factor of 8 to 30 between the two stations around local evening hours compared to their day time values (N /Nw) on spread-F days.This was shown as an evidence of intensification of northern crest of the equatorial ionization anomaly situated near Ahmedabad during spread F nights.No such enhancement in electron density ratio (N/Nw) was seen on nights without spread-F.Using the ALTAIR incoherent scatter radar and all-sky imaging technique at Kwajalein Atoll (Marshal Islands), Mendillo et al. (1992) showed ESF onset occurred when the airglow patterns associated with the equatorial anomaly was essentially symmetrical about the geomagnetic equator which is likely the case when the meridional winds are weak.But the visual representation of the anomaly or fountain movement has not been brought out earlier so clearly as the present imaging observation shows.As the all-sky camera covers a distance of 1800 km in north-south and east-west directions at 150° field of view at 300 km altitude, the spatial and temporal movement of the anomaly is well documented.The determination of the drift velocity of the fountain was done earlier from Indian longitude sector by a simple scan ning photometer unit by Kulkarni (1975) and obtained an estimate of the velocity of reverse fountain as 42mls.They also found that in the years of low sunspot activity the intensity of the EIA is weak and its southward velocity is less on magnetically disturbed nights.Also, using an imaging interferometer at Mt. Abu (24 ° 36' N, 72° 43' E), Sridharan et al. ( 1993) estimated the drift speed of the movement of the reverse fountain as 40 mis.However, the estimate was made from the movement of brightness structure within a narrow field of view ( < 6°) as seen by the interferometer.Our results (36-40 mis) reconfirm the earlier results from this longitude zone.It is well known that EIA displays a highly localized effect and it is generally well developed at a particular latitude and longitude sector and the effect might diminish at near by region.But the spatial extent (about 8°) in both longitude and latitude of the Appleton Anomaly has been inferred from the present study.Fejer et al. (1999) showed when upward drift velocities are larger than 5-10 mis near solar minimum, narrow unstable layer of weak irregularities are generated in all seasons at F-region heights at Jicamarca.If there is a rapid downward motion of the layer in the evening hours, the weakening of the unstable layer takes place due to unusually large downward drift velocities driven by westward electric field.Generally, a large downward drift followed by a positive drift which remains steady for more than an hour gives rise to spread-F irregularities.The motion of these bottom type irregularities is dominated by E-region dynamo electric fields.It is now well accepted that a perturbation electric field in the base of the F region is required to trigger the growth of plumes in the equatorial Spread-F.It is also believed that these perturba tion electric fields can be produced by gravity waves.Prakash (1999) also showed that ob served spacing of the plumes is much smaller than the horizontal wavelength of the gravity waves.This gives an idea that these perturbation electric fields are generated by the gravity wave wind in the E region connected to the base of the F-region via conducting geomagnetic field lines.
Fig. I. Schematic diagram of the all-sky camera ..

Fig. 2 .
Fig. 2. Field of view at 7 5° and go0 zenith angles at 300 km for the all-sky camera at Kolhapur.
on January 26, 1998 The average decimetric solar flux indices (Fl0.7 cm) for the day was (units of 10-22w m•2) 97 and the average of the Kp indices during the period of observation was 1.It was a typically magnetically quiet night with low solar activity.All-sky images were taken with 630 nm filter at every minute.The time is expressed in IST (Indian Standard Time), which is ahead of UT by five and half hours.The north-south aligned depletions were first observed around 2100 hrs and the Fig. 5 shows the movement of the Appleton anomaly (reverse ionization anomaly) Fig. 4. Projected altitude over the equator for OI 630.0 nm emission at 300 km for Kolhapur situated at the center.Dotted line represents the magnetic equator, dashed line represents a 75° zenith distance.

#
Figure 7 shows the 01 630 nm images taken during the night of January 18, 1999 with six frames; it depicts another example of the movement of the Appleton Anomaly during the night from north to south with drift speed of 36-40 mis.The high intensity region is shown by dark color.At 22:30 hrs the 01630 nm intensities are stron ger at the northern edge and very weak at the southern part of the sky.The north-south aligned plasma depleted region (ionospheric plasma bubble) is shown at the center, the structure moves towards the east gradually shown in the subsequent images.At 23:04 hrs the bubble reaches an apex altitude of about 1200 km, simultaneously another bubble (depleted in OI 630 nm intensity) appears at the western part of the sky at 22:42 hrs.The upper portion of the bubble breaks into two shown clearly on the images observed at 2304, 2314 and 2323 hrs IST.Apart from the north-south and eastward movement of the bubble, the background plasma or the reverse ion ization anomaly shown by an arrow also drifts towards the south(equator).This confirmedw

IFig. 5 .Fig. 6 .
Fig. 5. Movement of the reverse ionization anomaly with drift speeds of 36-40m/s on the night of January 26, 1998.Note also the eastward movement of the bubble along with its vertical rise in altitude extending increasingly northward.

Fig. 7 .
Fig. 7. Movement of the reverse ionization anomaly with drift speeds of 36-40 mis on the night of January 18, 1999.Note also the eastward movement of the bubble along with its vertical rise in altitude extending increas ingly northward.
a circular image of the sky 23 mm in diameter, which is directed into the collimator.Finally the light after passing through the filter is reimaged at f I 1 onto the photo cathode of the intensified CCD of Fairchild Model 3000 and intensifier type 4 727 ofITT.The detector is housed in a chamber thermoelectrically cooled to 60° C below ambient temperature to reduce the dark signal.Very weak nightglow signals of few Rayleigh in intensity can be monitored with the system with large integration time.With indexed color images, the system maintains a color table, of up to 256 colors with value 0 (blue) to highest value 255 (magenta) called the data number.The product of the data number and the constant (Rayleigh I data number) of the calibrated source would give the absolute intensity in Rayleigh.There are six fi lter positions, which can be used to study various emission lines originating at different ionospheric-thermo spheric heights.Since our main concern is to study F-region dynamics at night, we concentrate mainly on spatial and temporal characteristics of OI 630 nm emission.The spectral line at 630 nm is indicative of dissociative recombination involving 02 molecules and electrons near 300 km altitude (Mukherjee and Dyson 1gg2).
Schematic diagram of the all-sky camera .. produces