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Keywords:

  • storm enhanced density;
  • mid-latitude ionospheric disturbances;
  • sub-auroral polarization stream

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[1] Examples of storm enhanced density (SED) formation over Russia and Northern Europe are presented. These events, which persisted 15–20 hours, were fixed in local time near noon over Europe and then later observed over the American sector. The amount of total electron content (TEC) at the base of the SED erosion plume is found to be greatest in the American sector. A persistent, repeatable pattern is apparent in the time evolution of the latitude location of the SED plume base, although the latitudinal rate of change differs between the two sectors. In the European sector the invariant latitude (Λ) of the SED plume base is observed to be between 61°–63° Λ and at a time close to local noon. In the American sector, the position of the base of the plume shifts from local noon towards dusk, and moves to a lower latitude at a nearly fixed longitude.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[2] Storm enhanced density (SED) has been described as a spatially narrow, distinct, region of enhanced plasma density observed in the post-noon and pre-midnight sectors extending from the equatorward edge of the main ionospheric trough to the noon time cusp [Foster, 1993]. Under magnetically disturbed conditions, the plasma within the SED streams sunward carried by the equatorward edge of the sub-auroral polarization electric field known as the sub-auroral polarization stream (SAPS) [Yeh et al., 1991; Foster and Burke, 2002; Foster and Vo, 2002]. SED appears in two-dimensional global maps of GPS total electron content (TEC) as latitudinally narrow plumes of enhanced TEC [Coster et al., 2001]. Foster et al. [2002] found that these SED/TEC plumes are associated with the erosion of the mid-latitude ionosphere and outer plasmasphere by the SAPS electric field.

[3] SED plumes have been observed and investigated over the United States for more than a decade. Observations of SED have also been reported over South America [Coster et al., 2003], over Europe [Yizengaw et al., 2006], and over Japan [Maruyama, 2006]. Foster [1993] described the two dimensional extent of SED features over the American sector and examined their statistical occurrence using Millstone Hill incoherent scatter radar (ISR) measurements. The author found that although SED was most clearly observed during magnetically disturbed conditions (Kp ≥ 4), it could be identified at values of Kp as low as 2. Foster and Vo [2002] used Millstone Hill ISR measurements to report on the occurrence rate of SAPS responsible for the formation of SED. Those authors found that SAPS occurred at all Kp ≥ 4 and rarely at Kp as low as 2, in agreement with the earlier SED occurrence statistics.

[4] SED is associated with significant gradients in TEC which at times can exceed 100 TECU/degree. Such large gradients in TEC have instigated small scale irregularities which can induce phase scintillation [Skone et al., 2003; Doherty et al., 2004]. In the Northwest and Northeast regions of the continental United States, TEC gradients due to SED are known to have limited the availability of the Federal Aviation Administration's (FAA) Wide Area Augmentation System (WAAS) used for commercial air travel [Doherty et al., 2004]. This paper presents examples of SED development in the Russian and European sectors on October 01–03, 2001 and on April 18, 2002 between 4–7 UT. The point at which the poleward propagating SED plume separates from the region of mid-latitude TEC, which we define as the base of the plume, remains fixed near local noon as the Earth rotates underneath. As such, these events exhibit apparent westward motion at near-corotation speeds. Three of these four SED events are later observed over the American sector between 18–23 UT. During these events, the base of the plume also exhibits apparent westward motion, but at speeds slightly slower than corotation speeds. We find that SED/SAPS events can persist in the local noon to post-noon sector for 15 to 20 hours. We also present a European and American longitude sector comparison of SED. Our findings point toward consistently larger amounts of TEC at the base of the SED plume in the American sector and reveal a persistent, repeatable pattern in the latitude location of the base of the SED plume as the events progress. Additional comparisons with the Foster [1993] American sector SED characterization, which was based on ISR data from 1982 and 1990, affirm this highly repeatable pattern and point to the potentially predictive nature of the stormtime coupling of the ionosphere-magnetosphere system.

2. Data Sets and Instrumentation

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[5] The data sets employed in this study include October 01–03, 2001 and April 18, 2002 global maps of GPS TEC as well as October 02, 2001 Special Sensor for Ions, Electrons and Scintillation (SSIES) horizontal Ion Drift Meter (IDM) measurements taken onboard Defense Meteorological Satellite Program (DMSP) satellites F12, F13, F14 and F15. The acquisition and processing algorithms used to obtain vertical GPS TEC measurements have been described in detail by Rideout and Coster [2006]. DMSP satellites are polar-orbiting, sun-synchronous satellites which range in altitude between 835 and 850 km. The IDM data is sampled at a rate of 6/sec. Similar conditions of geomagnetic activity existed on each of the four days with Kp ranging between 6 and 7 and DST ranging between −100 and −200.

3. Observations

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[6] Many studies have focused on various aspects of SED development and behavior and its impact on the larger ionospheric-magnetospheric storm time environment in the American sector [Foster, 1993; Vo and Foster, 2001; Foster and Vo, 2002; Foster et al., 2002, 2004, 2005; Goldstein et al., 2003a, 2003b; Yin et al., 2004]. This investigation presents a comparison of SED formation as seen in different longitude sectors. On October 1–2, 2001 and April 18, 2002 SED events formed in the Russian longitude sector between 4–7 UT. For most of their duration, these SED events remained fixed in inertial (local time) coordinates near local noon while the Earth rotated under them. Each SED was subsequently observed over the American sector before dissipating. On October 3, 2001, SED formed at a later time near 9 UT in the European sector. However, this event dissipated before reaching the United States.

[7] Figures 1a and 1b present two averaged global TEC maps which highlight the occurrence of an SED event on October 2, 2001 whose base is initially in the European sector over Finland between 7–8 UT (Figure 1a) and later in the American sector between 20–21 UT (Figure 1b). The x-axis of the TEC maps is plotted in degrees from solar noon such that 12 LT is positioned at the center of Figure 1 regardless of the UT time range. In each TEC map, the base of the SED plume (indicated by an arrow) is identified as the point where the plume separates from a larger, broader region of enhanced TEC. In the American sector, this larger region of enhanced TEC has been noted by Foster et al. [2005] and has been referred to as a plasmaspheric bulge. For each of the SED events observed over the four days used in this study, the base of the plume remains near local noon in the European sector, and gradually shifts towards dusk over the American sector.

image

Figure 1. (a) Averaged GPS TEC map for October 2, 2001 07:00–08:00 UT and the corresponding DMSP F12 and F15 horizontal drift measurements (positive sunward). SED appears as a plume of enhanced TEC occurring near local noon over northern Europe. The location of the SED plume base is indicated with a black arrow. The corresponding geomagnetic latitude is drawn as a black line. DMSP F12 and F12 satellite tracks and horizontal drifts are plotted in red and green, respectively. The presence of SAPS electric fields is indicated by the red and green arrows. (b) Averaged GPS TEC map for October 2, 2001 20:00–21:00 UT and the corresponding DMSP F13 and F14 horizontal drifts measurements. SED appears as a plume of enhanced TEC occurring near local noon over the American sector. The location of the SED plume base is indicated with a black arrow. The corresponding geomagnetic latitude is drawn as a black line. DMSP F13 and F14 satellite tracks and horizontal drifts are plotted in red and green, respectively. The presence of SAPS electric fields is indicated with the red and green arrows.

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[8] In Figure 1a, the tracks of the DMSP satellites F12 and F15 between 7–8 UT are plotted on the TEC map in red and green, respectively. The corresponding ion drifts along the satellite paths are plotted to the right of the map with positive values indicating sunward drifts. The large sunwards drifts measured near 61° geodetic latitude by F12 and at 58° by F15 indicate the location of the SAPS electric field in the dusk sector. The tracks of DMSP satellites F13 (red) and F14 (green) are plotted in Figure 1b. DMSP F13 and F14 encountered SAPS at approximately 51° and 58° geographic latitude, respectively. Although not shown here, the presence of SAPS was confirmed for each of the SED events observed on the four days. The causative relationship of SAPS to SED is described by Foster et al. [2007].

4. Analysis and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[9] In this section we present a longitude sector comparison of the SED events observed on October 1–3, 2001 and April 18, 2002. The analysis focuses on the base of the SED erosion plume. The initial analysis is the same for each comparison. For each of the four days, the GPS TEC data were averaged into hourly 3° × 3° latitude/longitude bins. For each hourly averaged map, if the base of the SED plume could be distinguished, the averaged UT time, geographic latitude and longitude, invariant latitude, and maximum TEC value of the plume base were determined. The base of the SED plume was found by examination of adjacent TEC bins; it is defined as the location where the SED plume separates from the larger enhanced region of TEC. Determination of the SED plume base is not always possible primarily due to areas of sparse GPS coverage.

[10] Examination of the longitude dependence of the TEC magnitude at the base of the plume provides insight into the amount of TEC available in the source region of the SED. Figure 2 plots the magnitude of TEC at the base of the SED plume versus geographic longitude for October 1–2, 2001 and April 18, 2002. The data gap observed between −20° and −50° longitude is due to the lack of GPS receiver coverage over the Atlantic Ocean. As shown in Figure 2, for each day taken separately, the largest TEC values are seen in the American sector between −60° and −100° geographic longitude.

image

Figure 2. TEC magnitude at the base of the SED plume for three days under magnetically disturbed conditions, Kp = 6.

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[11] The higher TEC values in the American sector can be attributed to several possible mechanisms. For example, the dip of the geomagnetic field in this sector associates higher geomagnetic latitudes with the daytime electron densities of lower geographic latitudes. This effect could be further enhanced by mid-latitude penetration electric fields. Eastward electric fields raise plasma to higher altitudes where the recombination rates are lower. Lifting of the F-region plasma in this region has been reported by Yin et al. [2004] and modeled in simulation studies by Swisdak et al. [2006].

[12] Another mechanism for the higher TEC values in the American sector is suggested by Foster et al. [2005]. They report that at ionospheric heights, the SAPS electric field overlays the TEC reservoir at the base of the plume. The plasma is then drawn along the SAPS channel producing the SED plume. The authors suggest that the region of enhanced TEC is formed by the redistribution of low latitude plasma associated with an enhanced fountain effect due to penetrating electric fields during stormtime conditions. This effect was found to be most pronounced over the Americas in the vicinity of the South Atlantic Anomaly (SAA), potentially providing additional source plasma to the SED plume in the American sector.

[13] Figure 3 plots the geographic latitude location of the SED plume base versus UT for each of the four days included in this investigation under Kp = 6 conditions. The invariant latitude location of the SED plume base versus UT for April 18, 2002 is also shown, in order to illustrate the magnetic latitude variation of the base of the plume. In general, these SED events form over the Russian sector between 4–6 UT(∼ 120° to 90°E), subsequently pass over the European sector between 6–15 UT (∼ 90° to −30°E), and then move to the American sector between 18–24 UT (∼ 70° to 120°W). Three of the four events, October 1–2, 2001 and April 18, 2002 persisted for 15–20 hours. Foster et al. [2002] demonstrated that ionospheric SED plumes map into plasmaspheric drainage plumes observed by the IMAGE satellite extreme ultraviolet (EUV) imager. These plasmaspheric drainage plumes have been observed to persist on the order of 12 hours [Goldstein and Sandel, 2005, Spasojević et al., 2003]. We present here initial observations of similar lifetimes for their ionospheric SED counterpart.

image

Figure 3. Comparison of the latitude location of the base of the SED plume for four SED events under magnetically disturbed conditions, Kp = 6. The dashed and solid black lines indicate linear fits to the GPS TEC data. Foster [1993] SED characterization results using ISR measurements under Kp = 6 conditions are plotted with a grey dashed line.

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[14] From the GPS TEC data plotted in Figure 3 it is apparent that the latitude location of the base of the SED plume follows a highly consistent pattern. The dashed and solid black lines indicate linear fits to the data in the European and American sectors, respectively. There is a significant difference in the latitudinal rate of change of the SED plume base between these two longitude sectors, 1°/hour in the European sector and 2.4°/hour in the American sector. The change in slope occurs as the SED events pass over the Atlantic Ocean where there is a gap in GPS receiver coverage. We would expect that the two linear fits, data permitting, would join smoothly. Conversion of the data in Figure 3 to invariant latitude (Λ), shown for the April 18, 2002 event by the blue line, indicates that the base of the plume remains approximately constant between 61°–63° Λ in the European sector but moves to a lower Λ at a rate of 2.4°/hour in the American sector. In the data analyzed here, a minimum value of 48° Λ is observed over the American sector before the SED dissipates. The base of the SED plume remains near local noon for the events in the European sector and gradually moves towards dusk in the American sector.

[15] Detailed electric field models show that the position of the cusp varies with IMF By and Bz parameters [Weimer, 1996, 2001]. Observations in Figures 2 and 3 present characteristics of the base of the plume which is at an invariant latitude equatorward, and likely far equatorward, of the cusp. Further studies are needed to address the IMF dependence characteristics of the SED plume in the inner magnetosphere.

[16] Foster [1993] employed Millstone Hill ISR measurements taken between 1982 and 1990 to examine the magnetic activity dependence of the latitude location of the SED plume as a function of UT in the American sector. The results of that investigation for Kp = 6 are plotted in Figure 3 with a grey dashed line and are in excellent agreement with the GPS TEC data. This level of agreement, between data taken using different measurement techniques under varying solar cycle conditions, points to the potentially predictive nature of this feature. Foster [1993] also indicated how this latitudinal rate of change varies with magnetic activity. As additional examples of SED events spanning different longitude sectors and various levels of magnetic activity are identified in GPS TEC data, it will become possible to study the impact of magnetic activity on the observed repeatable behavior in the latitude location of the SED plume base.

5. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[17] We have presented observations of SED plumes in the Russian and European longitude sectors. These results provide evidence that the basic mechanism of producing SED by the overlapping of the SAPS electric field with the plasmaspheric boundary layer occurs in all longitude sectors. Our observations reveal that the SED and the physical mechanism responsible for its development can persist on the order of 15–20 hours. The point of separation of the plume from lower latitudes occurs near noon over the European sector. Over the American sector, the erosion point remains more closely fixed in geographic longitude and the base of the plume shifts from noon towards dusk Our longitude sector comparison for the four geomagnetically disturbed (Kp = 6) days revealed repeatable longitude sector differences in SED behavior. We find that the amount of TEC at the base of the plume is greatest in the American sector, suggesting additional plasma sources resulting from repeatable physical processes in this region. We observed that the latitudinal rate of change of the plume base differs between the European and American sectors having values of 1°/hour and 2.4°/hour, respectively. The invariant latitude of the plume base remains nearly constant, between 61°–63° Λ in the European sector but moves quickly to lower Λ at a rate of 2.4°Λ/hour in the American sector.

[18] The latitudinal location of the base of the plume with respect to UT exhibits a highly repeatable and consistent pattern. Comparison with earlier Foster [1993] results employing Millstone Hill ISR measurements indicates excellent agreement with the GPS TEC results. The repeatability of this pattern points to the potentially predictive nature of the stormtime coupling of the ionosphere-magnetosphere system.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Sets and Instrumentation
  5. 3. Observations
  6. 4. Analysis and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References