Mid-Latitude Density Gradients
at the Ionospheric Trough
Atmospheric Sciences Group
MIT Haystack Observatory
in press J. Geophys. Res., Dec. 2000
ABSTRACT
The sub-auroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F-region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on trans-ionospheric propagation must be able to compensate for the effects of the sharp changes (> 10x) in total electron content (TEC) associated with the ionospheric trough and storm-time disturbance effects at mid latitudes. The Millstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at mid and sub-auroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering >20 degrees of latitude and altitudes between 150 km and 750 km have been used to identify the characteristics of the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. The ionospheric trough can be identified in ~35% of the Millstone Hill scans and for these we present a statistical characterization of average magnitude and location of the steepest TEC gradients associated with the equatorward wall of the trough. Larger TEC gradients occur at solar maximum, when the background TEC is higher. The steepest gradients occur in an environment of high TEC (solar max and adjacent to regions of storm-enhanced density, SED), when the processes which generate the trough are strongest (high Kp). High gradient values occur in the daytime zone, with maximum values of TEC gradient (~10 TEC/deg latitude; 1 TEC unit = 10 16 electrons m − 2 ) found in the post-noon ionosphere. Mean solar-maximum TEC gradient at 16 MLT is 3-4 TEC/deg for Kp < 2, increases linearly with Kp for Kp 2-4, and is nearly constant at a value of 7-8 TEC/deg for Kp 4-6. Storm enhanced density (SED), the bulk redistribution of F-region plasma by disturbance electric fields, can result in TEC > 100 over New England and TEC gradients of ~50 TEC / deg.
Introduction
The sub-auroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F -region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields [ Foster , 1995]. The sub-auroral ionosphere is not subjected to the precipitation-induced localized ionization enhancements which characterize the auroral oval. Instead, ExB plasma motions advect ionospheric plasma with significantly different characteristics through this region [ Foster , 1993], and chemical recombination of ions in the presence of large electric fields [ Schunk et al. , 1976] reduces plasma density producing the deep nighttime ionization trough. These disturbance effects are superimposed on the regular diurnal cycle of solar production and nighttime decay of the F region which has both a latitude and a solar-cycle variation. This paper presents an overview of the ionospheric effects of such mid-latitude phenomena, as observed by MIT's Millstone Hill incoherent scatter radar, and characterizes the spatial gradients of F -region total electron content (TEC) which are associated with their occurrence.
Ionospheric Trough Associated with Enhanced Electric Field
During geomagnetic disturbances, the electric fields and particle populations which characterize the auroral region expand equatorward and their effects are felt at previously sub-auroral latitudes. Intense convection electric fields appear in the expanded auroral oval (e.g. Yeh et al. , [1991]) and are responsible for density depletion and ionospheric trough formation due to the effects of enhanced recombination [ Schunk et al. 1976]. Collis and Haggstrom [1988] have derived an empirical relationship between the trough location and magnetic Kp index based on observations with the EISCAT radars, and an overview of the role of ion drift in the formation of the mid and high latitude trough was presented by Rodger et al. [1992]. The polarization jet [ Galperin et al. , 1974] or SAID (sub-auroral ion drifts) event is defined as westward convection with magnitude in excess of 500 m s − 1 , equatorward of the auroral precipitation, and often associated with an ionospheric trough [ Spiro et al. , 1979]. SAID are associated with disturbed conditions, and specifically with substorm recovery [ Anderson et al. , 1993]. The high velocities due to intermittent or repetitive SAID events lead to mid-latitude trough formation, leaving a longitudinally extended, conjugate fossil trough [ Evans et al. , 1983] which persists through the night, corotating with the earth.
Storm Enhanced Density (SED)
Global electric field models indicate a dominant two-cell pattern of plasma convection whose equatorward portion transports ionospheric plasma sunward toward the noon meridian from both the dawn and dusk auroral and subauroral regions. The effects of plasma transport, and the linkage of the driving electric field to the overlying magnetosphere, dominate ionospheric characteristics at auroral and sub-auroral latitudes. With increasing activity level, and particularly during geomagnetic storms, the equatorward extent of such effects expands to lower latitudes [e.g., Foster et al. , 1986]. Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F -region plasma is transported sunward and poleward from a source region at middle and low latitudes in the afternoon sector, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector [ Foster , 1993]. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F -region plasma observed in the polar cap during disturbed conditions. Within the region of SED, TEC is greatly enhanced and steep latitude gradients of TEC occur.
Millstone Hill Incoherent Scatter Radar
The Massachusetts Institute of Technology maintains an extensive radar research facility at Millstone Hill, located 35 miles northwest of metropolitan Boston at 42.6° N latitude, 288.5° E longitude. Multi-megawatt UHF transmitters and a fully-steerable 46-m antenna provide wide-ranging spatial coverage, spanning >30° of latitude and 4+ hours of local time at F region heights with ~30-min temporal resolution. The facility is situated at 54° invariant geomagnetic latitude (Λ) such that its extensive field-of-view for ionospheric observations encompasses the full extent of mid-latitude, sub-auroral, and auroral features and processes.
The location of Millstone Hill is such that significant ionospheric structure falls well within the radar's field of view. The ionospheric total density trough is observed nearly overhead at Millstone Hill almost every night and the equatorward offset of the north geomagnetic pole nearly along the Millstone Hill meridian brings high-latitude and auroral phenomena into the observatory's near field of view during geomagnetic storms (e.g. Buonsanto et al. [1992]). During disturbed conditions, intense convection electric fields occur at previously sub-auroral latitudes resulting in the formation of a deep, narrow F -region ionization trough. Equatorward of this, sunward advection of plasma from later local times and lower latitudes leads to a region of Storm-Enhanced Density (SED) [ Foster , 1993] in which TEC can exceed average values by a significant amount. These processes contribute to the formation of steep, localized plasma density gradients whose position and characteristics depend strongly on the magnetosphere-ionosphere coupling in that longitude sector.
The Millstone Hill incoherent scatter radar has been in operation through three solar cycles (since 1979 using the fully steerable 150-foot diameter antenna) and its data characterize ionospheric features and response over the altitude range 100 km to 1000 km with a typical altitude/spatial resolution of 50 km. Long-term operations as a part of the international Incoherent Scatter World Day program have built up a database of radar elevation scans which sample a span of 15° - 25° latitude with better than 1° latitude resolution and ~35 km altitude resolution. We have developed a semi-automatic technique to review and analyze every N-S elevation scan in the Millstone Hill radar database. Data from each radar elevation scan is sorted to produce vertical TEC in the altitude range 150 km - 650 km with 1° latitude resolution. The principal characteristics of each scan (gradients, trough location, geomagnetic activity conditions) are determined and cataloged and this reduced data set is analyzed to determine the best characterization of ionospheric gradients (depth of trough, location, variability) as functions of solar cycle, local time, and level of disturbance.
Figure 1 presents iso-density contours of electron concentration observed during a Millstone Hill elevation scan along the N-S meridian during disturbed conditions in the pre-midnight sector (02 UT = 21 MLT). Latitude coverage at F -region heights spans 25° latitude and altitudes 200 km - 800 km. A pronounced ionospheric trough was observed centered at 52° geomagnetic latitude, nearly overhead the radar facility. The trough is typically 2° - 6° in latitude width and many previous investigations have shown its great extent in the E-W direction. Foster et al. [1994] describe an event similar to that shown in Figure 1 and show that this type of ionospheric trough is often associated with the region of intense sunward plasma convection driven by the polarization jet electric field [ Galperin , 1974] near the ionospheric projection of the disturbed ring current and the inner edge of the plasmasheet. F -region total electron content (TEC), determined by integrating the electron concentration between altitudes 200 km to 800 km at each degree of latitude, is plotted below the elevation scan data. TEC is ~ 35 TEC units (1 TEC unit = 1016 electrons m-2) immediately equatorward of the trough and drops to < 5 TEC within the trough. The maximum TEC gradient on the equatorward wall of the trough is defined here as the greatest difference in F -region TEC determined over a 1 degree distance and is expressed as TEC per degree latitude. In this case, the TEC gradient is −12 TEC/deg and occurs at 50° magnetic latitude (~39° geodetic latitude). For the Millstone Hill meridian (285° E longitude), invariant magnetic latitude is approximately geodetic latitude plus 11°.
Method
We have examined over 11,000 N-S Millstone Hill elevations scans of the type shown in Figure 1 and have determined F -region TEC for each of them. These scans were taken over a span on nearly 20 years (1982-2000) during World Day and special purpose experiments run to build up a synoptic database to address mid-latitude ionospheric phenomena as well as to address specific events and activity conditions. Scans were taken throughout the 24-hr day and approximately equally distributed in solar maximum (F10.7 > 120) and solar minimum (F10.7 < 120) conditions. Individual elevation scans cover a 10-25 deg span of latitude at F -region heights in 10 minutes, with successive scans separated by ~30 minute intervals.
Figure 2a presents the distribution of all 11,000 scans in local time and as a function of the 3-hour Kp index, taken here as a proxy for the level of geomagnetic disturbance. Our experiments provide a normal distribution with Kp and uniform local time coverage. All parameters presented in this study have been sorted into 24 1-hr bins of local time and 21 Kp-index bins ranging from 0 to 9. Averages are determined separately for each bin and are presented without smoothing. As seen in Figure 2a, each Kp bin between 4 and 5 at each local time has been sampled by 20-40 separate scans. For Kp between 2 and 3, there are ~100 scans in each sampling bin. Insufficient statistics are available in our data set to present significant results for Kp > 6.
As will be seen in the statistical analysis to follow, the occurrence of an ionospheric trough and its steep equatorward density gradient within the Millstone Hill field of view is a function both of local time and magnetic activity. We were able to identify the equatorward edge of the trough in ~4,000 of the 11,000 scans, and the statistical analyses which follow present average characteristics of the trough density gradients based on this subset - those scans in which the trough could be identified clearly. Figure 2b presents percent-occurrence for the scans with troughs in the Millstone Hill database. A pronounced trough is least likely (< 20%) to be seen in the post-midnight/early morning sector where the overall density is low. A trough is most likely (> 50%) to occur in the post-noon and dusk sectors for high magnetic activity (Kp > 3). For each of the 4,000 identifications of the equatorward edge of the trough, we recorded Kp and F10.7, the latitude of the steepest gradient, the value of TEC at that point, and the maximum gradient value.
Statistical Results
Mid-latitude F -region peak density, and TEC, can be a factor of 3-10 higher during solar maximum due to the increased production of ionization by the enhanced solar EUV flux. Accordingly, larger TEC gradients occur at solar maximum, when the background TEC is higher. The steepest gradients occur in an environment of high TEC (solar max and adjacent to regions of storm-enhanced density, SED), when the processes which generate the trough are strongest (high Kp).
We have separated our database of ~4000 observations of the trough by Kp index, solar max/min (F10.7 > or < 120), and into one-hour increments of local time.
In Figure 3a we present the average value of the TEC gradient on the equatorward edge of the trough as functions of Kp and local time for solar maximum conditions. It is apparent that the large gradients are a daytime/disturbed-Kp phenomenon. Figure 3b presents the average TEC gradients for solar minimum conditions. For both solar min and solar max, the largest gradients are observed in the post-noon sector and for disturbed conditions with Kp > 3. The trough gradients in the post-noon sector are appreciably stronger for solar maximum conditions with the value of density gradient at 15 LT ~ 8 TEC/deg for solar maximum and ~ 3 TEC/deg for the solar minimum. Only weak gradients are seen in the post-midnight / pre-noon sectors for both cases.
Figure 4 presents the local time variation of the maximum gradient on the equatorward wall of the ionospheric trough for solar maximum conditions for Kp = 2 and Kp = 4. Average values are shown by the continuous lines and the variability which could be expected for similar conditions is shown by the standard deviation of our set of observations. Gradients are approximately the same at midnight and morning hours, but are significantly higher in the early post-noon hours for the higher value of Kp. We argue that this is an effect of the increase in background TEC in the sub-auroral post-noon ionosphere caused by advection from lower latitudes (SED) [cf. Foster , 1993]. TEC is enhanced in the post-noon sector with increasing level of geomagnetic disturbance during magnetic storms. High variability in TEC gradient is found in this post-noon region for both solar max and min, while a much smaller statistical variability is associated with the nighttime region. TEC gradients are ~ 8 TEC/deg in the dusk sector for Kp = 4 and ~ 4 TEC/deg for Kp = 2. The post-midnight gradient is ~2 TEC/deg independent of Kp and solar cycle.
In Figure 4 we see that the maximum gradient is found in the post-noon ionosphere, and in Figure 5 we present the magnetic-activity dependence of the trough TEC gradient for 16 MLT, in the local time sector of the largest gradients. The variation of the average TEC gradient with Kp index for solar maximum conditions is shown by the solid line, with the dashed contours indicating one standard deviation centered on the mean value. Solar minimum values of the mean are shown by Xs. Mean solar maximum TEC gradient at 16 MLT is 3-4 TEC/deg for Kp < 2, increases linearly with Kp for Kp 2-4, and is nearly constant at a value of 7-8 TEC/deg for Kp 4-6.
The latitude of the region of steep density /TEC gradient is significant for assessing the space weather impact of the ionospheric trough. Density gradient and the geodetic latitude of the equatorward wall of the trough during solar maximum are plotted against local time for four levels of the Kp index in Figure 6. (The latitude variation for solar minimum conditions is shown by the dashed line.) Gradients are steepest in the dusk sector with the highest values appearing at high latitudes shortly after noon. For Kp 3, 4, or 5, the average trough gradient region moves equatorward steadily at a rate of ~1° hr - 1 between 16 LT - 24 LT. In the post-midnight region, the latitude of the equatorward edge of the trough is near 44° geodetic (55° magnetic) independent of Kp, indicative of the association of the trough with the stagnation/recombination boundary at those local times. In the post-noon sector, poleward and noonward advection of solar-enhanced density from later local times (SED) sharpens and accentuates the equatorward edge of the trough during solar-maximum conditions, resulting in a higher latitude for the trough boundary in this sector at solar maximum.
We quantify our statistical results in tabular form in Tables 1 and 2 which present average TEC gradients and the geodetic latitude of the steepest gradient on the equatorward edge of the trough for solar maximum and solar minimum as functions of magnetic activity and local time. These tables and the graphical results shown in the figures are specific to the eastern North American sector where high magnetic latitudes occurs at relatively low geodetic latitude due to the offset of the geomagnetic pole toward this longitude sector.
Discussion
Whereas the main ionospheric trough is a nighttime feature characterized by a widespread reduction in plasma density, our study indicates that severe density and TEC gradients are mainly a daytime feature whose percentage occurrence peaks in the dusk sector for high magnetic activity.
Figure 7 presents a typical example of steep dusk-sector trough gradients observed near solar maximum on October 19, 1982 at 18 MLT when Kp was 4 − . Mid-latitude (Λ < 56°) plasma density exceeded 10 12 m -3 with F -region TEC ~30 TEC units. The steep equatorward wall of the trough (maximum gradient −12 TEC/deg) was seen at 59°Λ (48° geodetic latitude) some 6° poleward of the Millstone Hill site. TEC dropped to ~5 TEC units within the trough.
Many previous studies of the ionospheric trough have been based on in situ measurements with satellite probes [e.g. Grebowsky et al. , 1983] which provide a detailed picture at a specific height, or on ground-based ionosonde profiles [e.g. Whalen , 1989] which examine only the bottom side of the F layer. The incoherent scatter radar technique has been used to study the characteristics of the F region and the trough from 200-800 km altitude (e.g. Holt et al. [1983], Evans et al . [1983]). A quantitative statistical study of the equatorward edge of the trough and its associated density gradient using the extensive incoherent scatter radar database has not been done until now.
Figure 8. A severe example of storm-time mid latitude ionospheric density perturbation.
The trough-gradient characteristics presented in this study are average values, characteristic of conditions which can be expected to occur over the northeastern US. Space Weather effects and systems design often depend on a knowledge of the maximal conditions which can be expected - for density gradients, these occur during the sudden-onset phase of major geomagnetic storms. An example of the severity of TEC perturbations and gradients which can be observed at normally mid latitudes is provided in the Millstone Hill radar observations of the great magnetic storm of February 8, 1986 (Kp = 9). Millstone Hill observations spanned the interval of mid-latitude disturbance with wide-ranging elevation scans and sweeping azimuth scans which detailed the evolution of intense mid-latitude electric fields [ Yeh et al. , 1991] and their ionospheric consequences [ Yeh et al. , 1990]. Storm enhanced density in the North American sector was tracked at latitudes from Canada (L > 6) to the Caribbean (L < 2) [ Foste r, 1993] and associated TEC exceeded 100 *10 16 m -2 in the region of post-noon SED. Figure 8 presents iso-density contours observed by the Millstone Hill radar scanning N-S across the region of SED near local noon on February 6, 1986. Our determination of F -region TEC and TEC gradient in this region derived from the radar elevation scan, as done in this study, is shown in the bottom panels. TEC near 48° N geodetic latitude, within the region of SED, is nearly 100 TEC units. The latitude gradient in TEC, observed associated with the region of SED near 45° latitude, was ~50 TEC/deg. The February 8, 1986 event represents a severe example of the ionospheric density perturbation that can occur over the continental US during large geomagnetic storms. The study by Foster [1993] presents the related conditions and occurrence frequency associated with storm enhanced density over the northeastern US. SED is regularly seen throughout the afternoon sector at Millstone's longitude and can be identified at all disturbance levels down to Kp = 2.
Conclusions
Steep ionospheric density and TEC gradients are observed at sub-auroral and mid latitudes associated with the ionospheric trough and with advecting density enhancements during geomagnetically disturbed conditions. Latitude gradients in TEC associated with the equatorward wall of the trough are ~10 TEC units/degree of latitude for solar max condition, while gradients associated with regions of greatly-enhanced TEC (storm enhanced density, SED) are observed as great as ~50 TEC units/deg. Communications and navigation systems relying on trans-ionospheric propagation must be able to compensate for the effects of such sharp gradients in total electron content in the Northeastern American sector.
Acknowledgements
We thank the members of the Haystack Observatory Atmospheric Sciences Group at Millstone Hill for assembling and maintaining the database of radar observations which are the basis of this study. This research was supported by NSF Space Weather Grant ATM-9819598 to the Massachusetts Institute of Technology. Millstone Hill observations and analysis are supported by the National Science Foundation through a Co-operative Agreement with the Massachusetts Institute of Technology.
References
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Figure Captions
Figure 1. Iso-density contours of electron concentration (logarithmic over the range [10,12] m -3 with 0.2 m -3 spacing) are shown for a Millstone Hill elevation scan along the N-S meridian during disturbed conditions (02 UT = 21 MLT). A pronounced ionospheric trough is centered at 52° geomagnetic latitude, nearly overhead the radar facility. TEC determined by integrating the electron concentration between altitudes 200 km to 800 km is plotted below the elevation scan data. Maximum TEC gradient on the equatorward wall of the trough is −12 TEC/deg.
Figure 2. (a) Distribution of 11,000 Millstone Hill radar elevation scans in local time and as a function of the 3-hour Kp index. Our experiments provide a normal distribution with Kp and uniform local time coverage.
(b) Percent-occurrence of scans in which the trough can be identified in the Millstone Hill database. A trough is most likely (> 50%) to occur in the post-noon and dusk sectors for high magnetic activity (Kp > 3).
Figure 3. (a) Average value of the TEC gradient (in units of TEC/deg) on the equatorward edge of the trough is shown as a function of Kp and local time for solar maximum conditions. Large gradients are a daytime/disturbed-Kp phenomenon. Contour spacing is 1 TEC/deg.
(b) Average TEC gradients for solar minimum conditions are significantly less than those observed during solar maximum.
Figure 4. Local time variation of the maximum TEC gradient on the equatorward wall of the ionospheric trough for solar maximum conditions for Kp = 2 and Kp = 4. Average values are shown by the continuous and dashed lines and the 1-σ variability is indicated. Gradients are approximately the same at midnight and morning hours, but are significantly higher in the early post-noon hours for the higher value of Kp.
Figure 5. Magnetic-activity dependence of the trough TEC gradient at 16 MLT, in the local time sector of the largest gradients. The dashed contours indicate one standard deviation centered on the mean value. Solar maximum values are presented as the solid curve, with solar minimum values shown as Xs. Gradients are significantly enhanced for Kp > 3.
Figure 6. Average maximum density gradients and the geodetic latitude of the equatorward wall of the trough during solar maximum are plotted against local time for four levels of the Kp index. (The latitude variation for solar minimum conditions is shown by the dashed line.) The magnitude of the gradient is indicated by shading and is in the range [0,8] TEC/deg. Gradients are steepest in the dusk sector with the highest values appearing at high latitudes shortly after noon.
Figure 7. Iso-density contours of electron concentration observed on October 19, 1982 with a N-S elevation scan during typical dusk-sector disturbed conditions at solar maximum reveal a steep density gradient on the equatorward wall of the trough at 59°Λ (L ~ 4), some 6° poleward of the radar facility. TEC determined by integrating the electron concentration between altitudes 200 km to 700 km is plotted below the elevation scan data.
Figure 8. Iso-density contours observed by the Millstone Hill radar scanning N-S across a region of strong SED (storm-enhanced density) near local noon are presented with 0.25 m -3 spacing as a function of geodetic latitude (invariant latitude = geodetic latitude + 11°). F -region TEC and TEC gradient across the region are derived from the radar elevation scan and are shown in the bottom panels. The February 8, 1986 event represents a severe example of the ionospheric density perturbation that can occur over the continental US during large geomagnetic storms. TEC near 48° N geodetic latitude is ~100 TEC units and the latitude gradient in TEC associated with the region of SED near 45° latitude was ~50 TEC/deg.