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Tomographic imaging of the polar-cap ionosphere over svalbard
JournalofAtmospherrc
Pergamon
PII: s1364-@2q97)000
and Solar-Terresrrrol
Physics.
Vol. 59, No. IS, pp. 1953-1959, 1997 0 1997 Elswier Science Ltd All nghts reserved. Pnnted in Great Britam 136&6826/97 $17.043+0.00
Tomographic imaging of the polar-cap ionosphere over Svalbard S.E.
Pryse, L. Kersley,
M. J. Williams,
I. K. Walker and C. A. Wilbon
Department of Physics, The University of Wales, Aberystwyth SY23 3B2, U.K. 6 January 1997; accepted 10 January 1997)
(Received 24 May 1996; in revisedform
results are presented from an experiment to image the electron density in the polar-cap ionosphere above Svalbard using tomographic techniques. The results show localised enhanced densities in the cusp and throat region, the elevated densities of a polar patch, relatively low densities in corotating plasma flux tubes in darkness at lower latitudes and the depleted densities of the polar hole. The reconstruction of these features, known to occur at polar latitudes, confirms the potential of tomography for imaging the ionosphere at these very high latitudes. 0 1997 Elsevier Science Ltd Abstract-First
INTRODUCTION
The plasma distribution in the polar ionosphere is dominated by convection, with a general antisunward flow across the polar cap and return flow at aurora1 latitudes. Driven by the solar wind, the precise form of the convection pattern is dependent on the orientation of the interplanetary magnetic field. The plasma distribution depends on the configuration of the convection flow relative to the sources of ionisation production, precipitation in the aurora1 oval and the cusp and solar radiation in sunlit regions. Observations have shown that the polar plasma may be broken up into patches, with various models being proposed to account for the very different electron concentrations on flux tubes in close proximity. The inhospitable conditions, with few suitable station locations, have resulted in the experimental studies often being made on a campaign basis at single sites. Whether by ionosonde, incoherent scatter radar or transionospheric propagation, the measurements have been largely confined to line-of-sight studies as a function of time. Ionospheric tomography offers a new method to image electron densities over a wide spatial area from a limited number of ground stations and so is well suited to conditions at high latitudes. The current paper presents a sample of the first experimental results of the application of tomographic techniques to give the spatial distribution of electron density over an extended vertical section of the northern polar ionosphere. Structures observed during two satellite passes in different local time sectors are consistent with those previously imaged by other experimental techniques and predicted by modelling
studies, and confirm the potential of tomography an observing technique at very high latitudes.
as
EXPERIMENT
Tomography involves measurement of the line integral of a parameter along many intersecting ray paths with subsequent inversion in a reconstruction algorithm to yield the two-dimensional distribution of the parameter. In the ionospheric context measurements of total electron content (TEC) are used to image the electron density in a vertical section. The experiment makes use of radio transmissions from navigational satellites in polar orbits. Details of the experimental method have been given in earlier papers (Kersley et al., 1993; Pryse et al., 1993) and will not be repeated here, though it should be noted that recent developments have been made in the reconstruction algorithms. It can also be noted that studies on the use of only two stations in ionospheric tomography, using observations by the European Incoherent Scatter (EISCAT) radar facility for verification, have shown that reliable results are obtained even under conditions of strong horizontal gradients, for a range of latitudes encompassing the stations, but distortions are introduced when the features imaged are far from the stations (Mitchell et al., 1997). For this reason, attention has been concentrated here on a sector of only some 9 degrees latitude, taken however, from a reconstruction grid that spans the entire latitudinal extent of a satellite pass of about 60 degrees. During a seven-day period in December 1994 satellite receiving systems were deployed at Ny Alesund
1953
S. E. Pryse et al.
1954
(78.9”N,12.0°E) and Longyearbyen (78.1°N,15.3”E) on Svalbard. It can be noted that with the convergence of longitude near the pole, the longitudinal difference in the stations does not introduce significant error into the results. Differential carrier phase measurements were made at both sites during a total of some 40 preselected satellite passes, which enabled the relative total electron content to be determined along satelliteto-receiver ray-paths. The measured electron content was subsequently used as input to the tomographic reconstruction routines.
ANALYSIS
The reconstruction technique applied to polar-cap measurements must give due consideration to the two fundamental problems of ionospheric tomography. The first occurs because of the need to calibrate the total electron content records. In past studies the twostation method of Leitinger et al. (1975) has been used, but this technique relies on a smooth horizontal gradient in the total content in the region of overlap. However, it has been found to be impossible to apply the method reliably to polar-cap data owing to their extremely structured character. The second fundamental problem arises due to the observing geometry with two closely-spaced stations, where all ray-paths through the image grid have a strong vertical bias. This results in the information contained in the measurements being mainly of the horizontal ionospheric structure, with information of the vertical ionisation distribution being missing. This problem has been surmounted in previous studies by using an ionospheric model like IRI-90 (Bilitza, 1990) to initialise the reconstruction process of an iterative algorithm. However, IRI-90 is essentially a mid-latitude mean ionospheric model and does not provide adequate background images for the initialisation of polar-cap tomography. In view of these two difficulties a simple, albeit computationally time-consuming, approach was taken for the reconstruction process. A database of 16384 (16 x 16 x 16 x 4) Chapman-type profiles was produced. The peak densities of these profiles covered the density range 0.2 x 10” to 1.7 x 10” m-3 in steps of 0.1 x 10” rnm3; the altitude of the F-layer peak ranged over 170-545 km in steps of 25 km, the scale height varied from 30-105 km in steps of 5 km and a linear gradient in the topside scale height was used, ranging from 100-250 m per km in steps of 50 m per km. The assumption made was that the actual vertical profile of the ionosphere at the time of a monitored satellite pass could be reasonably represented by one of these
profiles. Each profile was then used to set up a horizontally-stratified ionospheric model within the image grid. The offsets of the receiver TEC records for each model ionosphere were taken as the mean difference between the measured TEC values and the corresponding values calculated through the model. The implementation of the models into the reconstruction algorithm followed the method of Fougere (1995) where the TEC along horizontal paths through the models was used as input to the algorithm along with the measurements. These additional data contained no information on the horizontal structure but served only to constrain the vertical ionisation distribution of the reconstruction to that of the model. For the reconstruction, a grid spacing of 0.25 degrees of latitude by 30 km altitude was chosen. The MART (multiplicative algebraic reconstruction technique) algorithm, which was initialised by a uniform density distribution, was used to obtain the tomographic image corresponding to each model in turn. The assessment of the reconstructed image was based on a balance between rapid convergence of the modified initial ionosphere to the data and a low r.m.s. difference between the TEC input and the corresponding values through the reconstruction, with the final choice of image being based on the best combination of these.
RESULTS
Tomographic images are presented from two satellite passes in different local time sectors on 14 December 1994. The first occurred in the vicinity of the cusp with the satellite crossing the geographic latitude of 70”N at 10:34UT. The geometry of the 300 km ionospheric ray-path intersection is shown on a geomagnetic latitude vs MLT grid in Fig. 1. Around the time of the pass the WIND satellite measured the interplanetary magnetic field (IMF) in the solar wind. From 10:00 to 11: 15UT both the By and Bz components were weakly negative, with typical averaged values of - 1.9 nT and - 1.1 nT respectively during the time of the satellite pass. The electrical potential pattern, derived from the IZMEM model (Papitashvili et al., 1994) for the prevalent IMF conditions is also drawn on Fig. 1. The contours are at intervals of 5 kV with the zero level being set at 57”N and extending down the centre of the two-cell pattern from noon to midnight. Such contours can be considered to show the polar convection pattern as the plasma moves along the equipotential lines in the antisunward flow over the pole and return flow at lower latitudes. To the south the plasma co-rotates with the Earth.
Tomographic imaging of the polar-cap ionosphere
Fig. 1. Geometry of the ray-path intersection at a height of 300km for the satellite nass of 10:34UT on 14 December 1994 on a geomagnetic latitude vs MLT grid. The diamond indicates the position of the intersection at the start of the pass. Superimposed is the IZMEM electric potential pattern for the prevalent IMF condition,The dashed circle is drawn at the magnetic latitude of Ny Alesund, while the location of the site at the time of the pass is marked by the dot.
The resulting tomographic image of the electron density for the pass is shown in Fig. 2, on a height vs geographic latitude grid. Densities at the southernmost latitudes are relatively low and probably relate to the plasma tubes in this region either co-rotating or returning in the afternoon cell, but having been in winter darkness for an extended period of time. A distinct enhancement is observed directly above the receiving sites centred at about 78.5”N geographic latitude or 75.5”N geomagnetic latitude, with
Electron Density (x10” m” ) 1900
900-1
700
700
Height (km)5oo
500 300
Geographic Latitude Fig. 2. Tomographic image of ionospheric electron density reconstructed for the satellite pass at 10:34UT on 14 December 1994. The letters L and N denote the latitudes of Longyearbyen
and Ny kesund
respectively.
1955
maximum density some two-fold above values immediately to the south of the structure. There is indication from the upper regions that this feature slopes towards the south, suggesting alignment with the geomagnetic field, which is inclined at about 82 degrees from the horizontal. This enhancement maps into the throat region of the convective flow and may have been produced by particle precipitation into the cusp or may represent a cross section across a tongue of dayside ionisation being drawn into the polar cap. Towards the north a second enhancement is observed that is extended in latitude, with densities more than two times the background level of 0.6 x 10” rne3. This may be a section along a polar patch, extended in the antisunward flow, starting its journey across the polar cap. However, it must be noted that caution should be exercised in the detail of this electron density structure at its northern edge due to the limitation of twostation tomography to yield accurate images far from the stations. The tilt of the feature, apparently in the anti-sunward direction, may in fact be an artefact of the reconstruction method. No observations by an independent observing technique were available for verification of the reconstructed image. However, a comparison can be made with profiles from the EISCAT Svalbard Radar (GIVEME) model (Alcayde et al., 1994). Figure 3 shows the vertical profiles through the image obtained by averaging over intervals of 1” latitude, and also the profile given by the GIVEME model at 10:40UT for low solar activity. Whilst the height of the peak density given by the model is some 50 km lower than those through the image, it is encouraging to note that there is reasonable agreement in the magnitude of the peak density of the model with that of the background densities in the image immediately south of the enhancements. Densities through the enhancements are substantially larger than those given by the model. It can be noted that profiles shown here from the cusp region represent a worst case in terms of the correspondence with the model for the entire campaign. The profiles in other time sectors give better agreement. The second example presented here occurred in the local post-midnight sector, with the satellite crossing the latitude of 70”N at 23:04UT. The By and Bz components in the solar wind during the hour preceding the pass were relatively constant with Bz being weakly positive at about 1.6nT and By very weakly negative at about -0.3nT. The IZMEM electric potential at 23:OOUT is shown in Fig. 4 and may be considered as being representative of the plots for the conditions preceding the time of the pass. The satellite sub-ionospheric track at 300 km is superimposed on the con-
S. E. Pryse et al.
1956
I
I
a - -
800
76-77 Degrees
-
b - - - - -
77-78 Degrees
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d
--
e -f
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79-80 Degrees 80-81 Degrees GIVEME
lo:40
I.l’T
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0
0.5
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Electron Density ( xld’m-3 ) Fig. 3. Vertical electron density profiles through the tomographic image for the satellite pass at 10:34UT on 14 December 1994 together with the corresponding profile from the EISCAT Svalbard Radar (GIVEME) model for 10:40UT. 12:00 h4LT
18:00
Fig. 4. Geometry of the ray-path intersection at a height of 300 km for the satellite pass of 23:04UT on 14 December 1994 on a geomagnetic latitude vs MLT grid. The diamond indicates the position of the intersection at the start of the pass. Superimposed is the IZMEM electric potential pattern for the prevalent IMF condition._ The dashed circle is drawn at the magnetic latitude of Ny Alesund, while the location of the site at the time of the pass is marked by the dot.
tours of electric potential, indicating intersection with the dawn cell which has been somewhat displaced towards earlier times due to the prevalent IMF conditions. The tomographic image for the pass is shown in Fig. 5. Densities directly above and to the north of the receiving sites are low with values at the peak generally less than 0.4 x 10” me3 over some 3 degrees latitude. This region maps into the centre of the dawn convection cell, the polar hole, where the plasma circulates along the trajectories in permanent winter darkness. Densities increase slightly towards the north, while an enhancement in the aurora1 ionosphere is observed in the southern region of the reconstruction. While details of the precise form of the contours at the latitudinal extremes of the plot may not be exact due to the limitations of two-station tomography, nevertheless the general distribution with the depleted densities of the polar hole in the vicinity of the receiving stations can be accepted with confidence. Comparison of the GIVEME vertical profile with those through the reconstruction shows better agree-
1957
Tomographic imaging of the polar-cap ionosphere Electron Density (x10 I1 m-’ )
-700
700Height
model, whilst the most southern profile through the image at aurora1 latitudes shows peak densities larger than those of the model.
-900
900-
_ DISCUSSION AND CONCLUSIONS
-500
(km)500-
-300
300-w aWo.z -0.2--, loo-
I, 74”
0‘I
0
I
IN’,
76” 7i+ Geographic
i:4_
ti -100
/, 80” Latitude
1 52
Fig. 5. Tomographic image of ionospheric electron density reconstructed for the satellite pass at 23:04UT on 14 December 1994. The letters L and N denote the latitudes of Longyearbyen and Ny Alesund respectively.
ment for the height of the layer peak for this pass (Fig. 6). The magnitudes of the electron densities at the peak for the profiles intersecting the depleted regions of the polar hole are less than those given by the
First results have been presented from a campaign to image electron density in the polar ionosphere using tomographic techniques. One example showed localised field-aligned enhanced densities in the cusp region, the elevated densities of a polar patch in the antisunward plasma flow, and relatively low densities in co-rotating or return flow flux tubes in darkness at lower latitudes. The second example showed the depleted densities of the polar hole in the dawn cell where the flux tubes were circulating in permanent darkness. All of these structures correspond to features known to occur in the polar ionosphere under the winter observing conditions and so demonstrate the potential of tomography as an imaging technique in these regions. The new reconstruction method developed for this study is also validated by the gen-
I
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900 800
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76-77 Degrees
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c ---
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Degrees
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80-81 Degrees
f -
GIVEME
23:00 UT
600
Height (km) 500
a
0.5
1.0
1.5
Electron Density ( xl O”m-3 ) Fig. 6. Vertical electron density profiles through the tomographic image for the satellite pass at 23:04UT on 14 December 1994 together with the corresponding profile from the EISCAT Svalbard Radar (GIVEME) model for 23:OOUT.
2.0
1958
S. E. Pryse et al.
erally good agreement between the resultant profiles and those of the GIVEME model. It can be noted that the discrepancy in layer peak height found in the cusp region may relate to the different source mechanisms for the plasma, whether produced by localised softprecipitation or by convection of solar-produced ionisation from lower latitude. This is a topic for further investigation. The two-receiver experimental set-up corresponds to the simplest geometry that can be used for tomographic imaging. Studies of the dependence of the reliability of the reconstructed image on the number of ground receivers have been carried out in earlier tomographic work and are discussed by Pryse et al. (1995) and Mitchell et al. (1997). These showed that ionospheric features can be observed at latitudes in between the receiving sites and for a small range displaced north and south from the receiving stations regardless of there being four, three or even two receiving systems. However, the details of the features, particularly outside the range of the stations, are degraded with a decrease in the number of receivers. In the current study authentic images of the latitudinal distribution of ionisation are obtained between the receiver sites and the general form of the reconstructed features in the limited regions to the north and south may be relied on, though the details in the precise shape of these structures must be treated with caution. An improved coverage for the detail of structure would obviously be obtained by extending the latitudinal range with further receiving sites. It must be noted that the dynamic high-latitude ionosphere may undergo temporal variation during the satellite pass, but for this initial study any temporal change was neglected. However, the main features of the images presented in the figures can be obtained by using a portion of the satellite pass of only some 6 min duration. The features imaged here are consistent with published results from both experimental and modelling studies of the polar ionosphere reviewed by Carlson (1994). For example, early studies using the Chatanika incoherent scatter radar indicated that subauroral plasma is able to enter the polar cap forming a tongue of ionisation (Foster and Doupnik, 1984). Valladares et al. (1994), using a range of techniques, including measurements of the spatial distribution of electron density by the Sondrestrom radar, have investigated the formation and entry of patches into the polar cap, as have Rodger et al. (1994) using the Polar AngloAmerican Conjugate Experiment (PACE). Modelling studies of time-varying convection by several groups have been used to demonstrate the break-up of the tongue of ionisation into discrete patches (Anderson
et al., 1988; Sojka et al., 1993; Decker et al., 1994), whilst recently Valladares et al. (1996) invoke travelling convection vortices as a mechanism to create patches from the tongue. The polar hole of depleted densities formed by flux tubes circulating in permanent darkness in the dawn cell was suggested to account for observations in the southern hemisphere by Brinton et al. (1978). The results from this current tomographic study are in agreement with previously published work based on other experimental observing techniques and modelling studies. They serve to confirm the potential of ionospheric tomography as a new technique, complementary to current methods, for investigation of the spatial distribution of electron density in the polar ionosphere. The images obtained from this simple two-station receiving set-up suggest that the method is well suited to applications for monitoring of the ionosphere at remote and inhospitable locations.
Acknowledgements-This work has been sponsored by the U.K. Particle Physics and Astronomy Research Council. The assistance of the staff of the Norsk Polarinstitutt at Ny Alesund is gratefully acknowledged. The IZMEM electric potential model used for the study is attributed to V. Papitashvili, Space Physics Research Laboratory, University of Michigan.
REFERENCES
Alcaydt, D., Blelly, P.-L. and Lilensten, J. (1994) GIVEME - General Ionosphere Visualisation and Extraction from a Model for the EISCAT Svalbard radar. EISCAT Technical Note 94/52. Anderson, D. N., Buchau, J. and Heelis, R. A. (1988) Origin of density enhancements in the winter polar cap ionosphere. Radio Science 23, 5 13-5 19. Bilitza, D. (1990) International Reference Ionosphere 1990, National Space Science Data Center. Ref. 90-20, Greenbelt, Maryland, U.S.A. Brinton, H. C., Grebowsky, J. M. and Brace, L. H. (1978) The high-latitude winter F region at 300 km: Thermal plasma observations from AE-C. Journal of Geophysical Research 83,47674716. Carlson, H. C. (1994) The dark polar ionosphere: Progress and future challenges. Radio Science 29, 157-165. Decker, D. T., Valladares, C. E., Sheehan, R., Basu, S., Anderson, D. N. and Heelis, R. A. (1994) Modelling daytime F layer patches over Sondrestrom. Radio Science 29, 249-268. Foster, J. C. and Doupnik, J. R. (1984) Plasma convection in the vicinity of the dayside cleft. Journal of Geophysical Research 89,9 107-9 113. Fougere, P. F. (1995) Ionospheric radio tomography using maximum entropy, 1. Theory and simulation studies. Radio Science 30,429444. Leitinger, R., Schmidt, G. and Tauriainen, A. (1975) An evaluation method combining the differential measurements from two stations that enables the calculation of the
Tomographic
imaging
of the polar-cap
electron content of the ionosphere. Journal of Geophysics 41,201-213. Kersley, L., Heaton, J. A. T., Pryse, S. E. and Raymund, T. D. (1993) Experimental ionospheric tomography with ionosonde input and EISCAT verification. Annales Geophysicae 11, 10641074. Mitchell, C. N., Kersley, L. and Pryse, S. E. (1997) The effect of receiver location in two-station experimental ionospheric tomography. Journal of Atmospheric and SolarTerrestrial Physics 59, 141 I-1415. Papitashvili, V. O., Belov, B. A., Faermark, D. S., Feldstein, Y. I., Golyshev, S. A., Gromova, L. I. and Levitin, A. E. (1994) Electric potential patterns in the northern and southern polar regions parameterized by the interplanetary magnetic field. Journal of Geophysical Research 99, 13251-13262. Pryse, S. E., Kersley, L., Rice, D. L., Russell, C. D. and Walker, I. K. (1993) Tomographic imaging of the ionospheric mid-latitude trough. Annales Geophysicae 11, 144 149.
ionosphere
1959
Pryse, S. E., Mitchell, C. N., Heaton, J. A. T. and Kersley, L. (1995) Travelling ionospheric disturbances imaged by tomographic techniques. Annales Geophysicae 13, 13255 1330. Rodger, A. S., Pinnock, M., Dudeney, J. R., Baker, K. B. and Greenwald, R. A. (1994)A new mechanism for polar patch formation. Journal of Geophysical Research 99, 6425-6436. Sojka, J. J., Bowline, M. D., Schunk, R. W., Decker, D. T., Valladares, C. E., Sheehan, R., Anderson, D. N. and Heelis, R. A. (1993) Modeling polar cap F region patches using time varying convection. Geophysical Research Letters 20, 1783-1786. Valladares, C. E., Basu, S., Buchau, J. and Friis-Christensen, E. (1994) Experimental evidence for the formation and entry of patches into the polar cap. Radio Science 29,167194. Valladares, C. E., Decker, D. T. and Sheehan, R. (1996) Modelling the formation of polar cap patches using large plasma flows. Radio Science 31, 573-593.
Pergamon
PII: s1364-@2q97)000
and Solar-Terresrrrol
Physics.
Vol. 59, No. IS, pp. 1953-1959, 1997 0 1997 Elswier Science Ltd All nghts reserved. Pnnted in Great Britam 136&6826/97 $17.043+0.00
Tomographic imaging of the polar-cap ionosphere over Svalbard S.E.
Pryse, L. Kersley,
M. J. Williams,
I. K. Walker and C. A. Wilbon
Department of Physics, The University of Wales, Aberystwyth SY23 3B2, U.K. 6 January 1997; accepted 10 January 1997)
(Received 24 May 1996; in revisedform
results are presented from an experiment to image the electron density in the polar-cap ionosphere above Svalbard using tomographic techniques. The results show localised enhanced densities in the cusp and throat region, the elevated densities of a polar patch, relatively low densities in corotating plasma flux tubes in darkness at lower latitudes and the depleted densities of the polar hole. The reconstruction of these features, known to occur at polar latitudes, confirms the potential of tomography for imaging the ionosphere at these very high latitudes. 0 1997 Elsevier Science Ltd Abstract-First
INTRODUCTION
The plasma distribution in the polar ionosphere is dominated by convection, with a general antisunward flow across the polar cap and return flow at aurora1 latitudes. Driven by the solar wind, the precise form of the convection pattern is dependent on the orientation of the interplanetary magnetic field. The plasma distribution depends on the configuration of the convection flow relative to the sources of ionisation production, precipitation in the aurora1 oval and the cusp and solar radiation in sunlit regions. Observations have shown that the polar plasma may be broken up into patches, with various models being proposed to account for the very different electron concentrations on flux tubes in close proximity. The inhospitable conditions, with few suitable station locations, have resulted in the experimental studies often being made on a campaign basis at single sites. Whether by ionosonde, incoherent scatter radar or transionospheric propagation, the measurements have been largely confined to line-of-sight studies as a function of time. Ionospheric tomography offers a new method to image electron densities over a wide spatial area from a limited number of ground stations and so is well suited to conditions at high latitudes. The current paper presents a sample of the first experimental results of the application of tomographic techniques to give the spatial distribution of electron density over an extended vertical section of the northern polar ionosphere. Structures observed during two satellite passes in different local time sectors are consistent with those previously imaged by other experimental techniques and predicted by modelling
studies, and confirm the potential of tomography an observing technique at very high latitudes.
as
EXPERIMENT
Tomography involves measurement of the line integral of a parameter along many intersecting ray paths with subsequent inversion in a reconstruction algorithm to yield the two-dimensional distribution of the parameter. In the ionospheric context measurements of total electron content (TEC) are used to image the electron density in a vertical section. The experiment makes use of radio transmissions from navigational satellites in polar orbits. Details of the experimental method have been given in earlier papers (Kersley et al., 1993; Pryse et al., 1993) and will not be repeated here, though it should be noted that recent developments have been made in the reconstruction algorithms. It can also be noted that studies on the use of only two stations in ionospheric tomography, using observations by the European Incoherent Scatter (EISCAT) radar facility for verification, have shown that reliable results are obtained even under conditions of strong horizontal gradients, for a range of latitudes encompassing the stations, but distortions are introduced when the features imaged are far from the stations (Mitchell et al., 1997). For this reason, attention has been concentrated here on a sector of only some 9 degrees latitude, taken however, from a reconstruction grid that spans the entire latitudinal extent of a satellite pass of about 60 degrees. During a seven-day period in December 1994 satellite receiving systems were deployed at Ny Alesund
1953
S. E. Pryse et al.
1954
(78.9”N,12.0°E) and Longyearbyen (78.1°N,15.3”E) on Svalbard. It can be noted that with the convergence of longitude near the pole, the longitudinal difference in the stations does not introduce significant error into the results. Differential carrier phase measurements were made at both sites during a total of some 40 preselected satellite passes, which enabled the relative total electron content to be determined along satelliteto-receiver ray-paths. The measured electron content was subsequently used as input to the tomographic reconstruction routines.
ANALYSIS
The reconstruction technique applied to polar-cap measurements must give due consideration to the two fundamental problems of ionospheric tomography. The first occurs because of the need to calibrate the total electron content records. In past studies the twostation method of Leitinger et al. (1975) has been used, but this technique relies on a smooth horizontal gradient in the total content in the region of overlap. However, it has been found to be impossible to apply the method reliably to polar-cap data owing to their extremely structured character. The second fundamental problem arises due to the observing geometry with two closely-spaced stations, where all ray-paths through the image grid have a strong vertical bias. This results in the information contained in the measurements being mainly of the horizontal ionospheric structure, with information of the vertical ionisation distribution being missing. This problem has been surmounted in previous studies by using an ionospheric model like IRI-90 (Bilitza, 1990) to initialise the reconstruction process of an iterative algorithm. However, IRI-90 is essentially a mid-latitude mean ionospheric model and does not provide adequate background images for the initialisation of polar-cap tomography. In view of these two difficulties a simple, albeit computationally time-consuming, approach was taken for the reconstruction process. A database of 16384 (16 x 16 x 16 x 4) Chapman-type profiles was produced. The peak densities of these profiles covered the density range 0.2 x 10” to 1.7 x 10” m-3 in steps of 0.1 x 10” rnm3; the altitude of the F-layer peak ranged over 170-545 km in steps of 25 km, the scale height varied from 30-105 km in steps of 5 km and a linear gradient in the topside scale height was used, ranging from 100-250 m per km in steps of 50 m per km. The assumption made was that the actual vertical profile of the ionosphere at the time of a monitored satellite pass could be reasonably represented by one of these
profiles. Each profile was then used to set up a horizontally-stratified ionospheric model within the image grid. The offsets of the receiver TEC records for each model ionosphere were taken as the mean difference between the measured TEC values and the corresponding values calculated through the model. The implementation of the models into the reconstruction algorithm followed the method of Fougere (1995) where the TEC along horizontal paths through the models was used as input to the algorithm along with the measurements. These additional data contained no information on the horizontal structure but served only to constrain the vertical ionisation distribution of the reconstruction to that of the model. For the reconstruction, a grid spacing of 0.25 degrees of latitude by 30 km altitude was chosen. The MART (multiplicative algebraic reconstruction technique) algorithm, which was initialised by a uniform density distribution, was used to obtain the tomographic image corresponding to each model in turn. The assessment of the reconstructed image was based on a balance between rapid convergence of the modified initial ionosphere to the data and a low r.m.s. difference between the TEC input and the corresponding values through the reconstruction, with the final choice of image being based on the best combination of these.
RESULTS
Tomographic images are presented from two satellite passes in different local time sectors on 14 December 1994. The first occurred in the vicinity of the cusp with the satellite crossing the geographic latitude of 70”N at 10:34UT. The geometry of the 300 km ionospheric ray-path intersection is shown on a geomagnetic latitude vs MLT grid in Fig. 1. Around the time of the pass the WIND satellite measured the interplanetary magnetic field (IMF) in the solar wind. From 10:00 to 11: 15UT both the By and Bz components were weakly negative, with typical averaged values of - 1.9 nT and - 1.1 nT respectively during the time of the satellite pass. The electrical potential pattern, derived from the IZMEM model (Papitashvili et al., 1994) for the prevalent IMF conditions is also drawn on Fig. 1. The contours are at intervals of 5 kV with the zero level being set at 57”N and extending down the centre of the two-cell pattern from noon to midnight. Such contours can be considered to show the polar convection pattern as the plasma moves along the equipotential lines in the antisunward flow over the pole and return flow at lower latitudes. To the south the plasma co-rotates with the Earth.
Tomographic imaging of the polar-cap ionosphere
Fig. 1. Geometry of the ray-path intersection at a height of 300km for the satellite nass of 10:34UT on 14 December 1994 on a geomagnetic latitude vs MLT grid. The diamond indicates the position of the intersection at the start of the pass. Superimposed is the IZMEM electric potential pattern for the prevalent IMF condition,The dashed circle is drawn at the magnetic latitude of Ny Alesund, while the location of the site at the time of the pass is marked by the dot.
The resulting tomographic image of the electron density for the pass is shown in Fig. 2, on a height vs geographic latitude grid. Densities at the southernmost latitudes are relatively low and probably relate to the plasma tubes in this region either co-rotating or returning in the afternoon cell, but having been in winter darkness for an extended period of time. A distinct enhancement is observed directly above the receiving sites centred at about 78.5”N geographic latitude or 75.5”N geomagnetic latitude, with
Electron Density (x10” m” ) 1900
900-1
700
700
Height (km)5oo
500 300
Geographic Latitude Fig. 2. Tomographic image of ionospheric electron density reconstructed for the satellite pass at 10:34UT on 14 December 1994. The letters L and N denote the latitudes of Longyearbyen
and Ny kesund
respectively.
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maximum density some two-fold above values immediately to the south of the structure. There is indication from the upper regions that this feature slopes towards the south, suggesting alignment with the geomagnetic field, which is inclined at about 82 degrees from the horizontal. This enhancement maps into the throat region of the convective flow and may have been produced by particle precipitation into the cusp or may represent a cross section across a tongue of dayside ionisation being drawn into the polar cap. Towards the north a second enhancement is observed that is extended in latitude, with densities more than two times the background level of 0.6 x 10” rne3. This may be a section along a polar patch, extended in the antisunward flow, starting its journey across the polar cap. However, it must be noted that caution should be exercised in the detail of this electron density structure at its northern edge due to the limitation of twostation tomography to yield accurate images far from the stations. The tilt of the feature, apparently in the anti-sunward direction, may in fact be an artefact of the reconstruction method. No observations by an independent observing technique were available for verification of the reconstructed image. However, a comparison can be made with profiles from the EISCAT Svalbard Radar (GIVEME) model (Alcayde et al., 1994). Figure 3 shows the vertical profiles through the image obtained by averaging over intervals of 1” latitude, and also the profile given by the GIVEME model at 10:40UT for low solar activity. Whilst the height of the peak density given by the model is some 50 km lower than those through the image, it is encouraging to note that there is reasonable agreement in the magnitude of the peak density of the model with that of the background densities in the image immediately south of the enhancements. Densities through the enhancements are substantially larger than those given by the model. It can be noted that profiles shown here from the cusp region represent a worst case in terms of the correspondence with the model for the entire campaign. The profiles in other time sectors give better agreement. The second example presented here occurred in the local post-midnight sector, with the satellite crossing the latitude of 70”N at 23:04UT. The By and Bz components in the solar wind during the hour preceding the pass were relatively constant with Bz being weakly positive at about 1.6nT and By very weakly negative at about -0.3nT. The IZMEM electric potential at 23:OOUT is shown in Fig. 4 and may be considered as being representative of the plots for the conditions preceding the time of the pass. The satellite sub-ionospheric track at 300 km is superimposed on the con-
S. E. Pryse et al.
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lo:40
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Height (km) 500
0
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Electron Density ( xld’m-3 ) Fig. 3. Vertical electron density profiles through the tomographic image for the satellite pass at 10:34UT on 14 December 1994 together with the corresponding profile from the EISCAT Svalbard Radar (GIVEME) model for 10:40UT. 12:00 h4LT
18:00
Fig. 4. Geometry of the ray-path intersection at a height of 300 km for the satellite pass of 23:04UT on 14 December 1994 on a geomagnetic latitude vs MLT grid. The diamond indicates the position of the intersection at the start of the pass. Superimposed is the IZMEM electric potential pattern for the prevalent IMF condition._ The dashed circle is drawn at the magnetic latitude of Ny Alesund, while the location of the site at the time of the pass is marked by the dot.
tours of electric potential, indicating intersection with the dawn cell which has been somewhat displaced towards earlier times due to the prevalent IMF conditions. The tomographic image for the pass is shown in Fig. 5. Densities directly above and to the north of the receiving sites are low with values at the peak generally less than 0.4 x 10” me3 over some 3 degrees latitude. This region maps into the centre of the dawn convection cell, the polar hole, where the plasma circulates along the trajectories in permanent winter darkness. Densities increase slightly towards the north, while an enhancement in the aurora1 ionosphere is observed in the southern region of the reconstruction. While details of the precise form of the contours at the latitudinal extremes of the plot may not be exact due to the limitations of two-station tomography, nevertheless the general distribution with the depleted densities of the polar hole in the vicinity of the receiving stations can be accepted with confidence. Comparison of the GIVEME vertical profile with those through the reconstruction shows better agree-
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Tomographic imaging of the polar-cap ionosphere Electron Density (x10 I1 m-’ )
-700
700Height
model, whilst the most southern profile through the image at aurora1 latitudes shows peak densities larger than those of the model.
-900
900-
_ DISCUSSION AND CONCLUSIONS
-500
(km)500-
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300-w aWo.z -0.2--, loo-
I, 74”
0‘I
0
I
IN’,
76” 7i+ Geographic
i:4_
ti -100
/, 80” Latitude
1 52
Fig. 5. Tomographic image of ionospheric electron density reconstructed for the satellite pass at 23:04UT on 14 December 1994. The letters L and N denote the latitudes of Longyearbyen and Ny Alesund respectively.
ment for the height of the layer peak for this pass (Fig. 6). The magnitudes of the electron densities at the peak for the profiles intersecting the depleted regions of the polar hole are less than those given by the
First results have been presented from a campaign to image electron density in the polar ionosphere using tomographic techniques. One example showed localised field-aligned enhanced densities in the cusp region, the elevated densities of a polar patch in the antisunward plasma flow, and relatively low densities in co-rotating or return flow flux tubes in darkness at lower latitudes. The second example showed the depleted densities of the polar hole in the dawn cell where the flux tubes were circulating in permanent darkness. All of these structures correspond to features known to occur in the polar ionosphere under the winter observing conditions and so demonstrate the potential of tomography as an imaging technique in these regions. The new reconstruction method developed for this study is also validated by the gen-
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GIVEME
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Height (km) 500
a
0.5
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Electron Density ( xl O”m-3 ) Fig. 6. Vertical electron density profiles through the tomographic image for the satellite pass at 23:04UT on 14 December 1994 together with the corresponding profile from the EISCAT Svalbard Radar (GIVEME) model for 23:OOUT.
2.0
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S. E. Pryse et al.
erally good agreement between the resultant profiles and those of the GIVEME model. It can be noted that the discrepancy in layer peak height found in the cusp region may relate to the different source mechanisms for the plasma, whether produced by localised softprecipitation or by convection of solar-produced ionisation from lower latitude. This is a topic for further investigation. The two-receiver experimental set-up corresponds to the simplest geometry that can be used for tomographic imaging. Studies of the dependence of the reliability of the reconstructed image on the number of ground receivers have been carried out in earlier tomographic work and are discussed by Pryse et al. (1995) and Mitchell et al. (1997). These showed that ionospheric features can be observed at latitudes in between the receiving sites and for a small range displaced north and south from the receiving stations regardless of there being four, three or even two receiving systems. However, the details of the features, particularly outside the range of the stations, are degraded with a decrease in the number of receivers. In the current study authentic images of the latitudinal distribution of ionisation are obtained between the receiver sites and the general form of the reconstructed features in the limited regions to the north and south may be relied on, though the details in the precise shape of these structures must be treated with caution. An improved coverage for the detail of structure would obviously be obtained by extending the latitudinal range with further receiving sites. It must be noted that the dynamic high-latitude ionosphere may undergo temporal variation during the satellite pass, but for this initial study any temporal change was neglected. However, the main features of the images presented in the figures can be obtained by using a portion of the satellite pass of only some 6 min duration. The features imaged here are consistent with published results from both experimental and modelling studies of the polar ionosphere reviewed by Carlson (1994). For example, early studies using the Chatanika incoherent scatter radar indicated that subauroral plasma is able to enter the polar cap forming a tongue of ionisation (Foster and Doupnik, 1984). Valladares et al. (1994), using a range of techniques, including measurements of the spatial distribution of electron density by the Sondrestrom radar, have investigated the formation and entry of patches into the polar cap, as have Rodger et al. (1994) using the Polar AngloAmerican Conjugate Experiment (PACE). Modelling studies of time-varying convection by several groups have been used to demonstrate the break-up of the tongue of ionisation into discrete patches (Anderson
et al., 1988; Sojka et al., 1993; Decker et al., 1994), whilst recently Valladares et al. (1996) invoke travelling convection vortices as a mechanism to create patches from the tongue. The polar hole of depleted densities formed by flux tubes circulating in permanent darkness in the dawn cell was suggested to account for observations in the southern hemisphere by Brinton et al. (1978). The results from this current tomographic study are in agreement with previously published work based on other experimental observing techniques and modelling studies. They serve to confirm the potential of ionospheric tomography as a new technique, complementary to current methods, for investigation of the spatial distribution of electron density in the polar ionosphere. The images obtained from this simple two-station receiving set-up suggest that the method is well suited to applications for monitoring of the ionosphere at remote and inhospitable locations.
Acknowledgements-This work has been sponsored by the U.K. Particle Physics and Astronomy Research Council. The assistance of the staff of the Norsk Polarinstitutt at Ny Alesund is gratefully acknowledged. The IZMEM electric potential model used for the study is attributed to V. Papitashvili, Space Physics Research Laboratory, University of Michigan.
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