Radio Science

Comparison of COSMIC occultation-based electron density profiles and TIP observations with Arecibo incoherent scatter radar data

Authors


Abstract

[1] In June 2006 an early opportunity arose to compare occultation-based electron density profiles with incoherent scatter radar data. The former were made available by the constellation of satellites called COSMIC. We find that the value of the peak plasma density in the F region is reasonably well characterized but that the altitude of the peak is overestimated above about 300 km and underestimated below that height. A simple Abel transform is not suitable for determining the E region profiles, even in daytime. We also compared the emission strength recorded by the Tiny Ionospheric Photometer (TIP) on board COSMIC to the Arecibo measurement of the TEC as part of an ongoing effort to calibrate the former. The TIP and Arecibo data both show the development of an ionospheric storm of the type referred to as high-intensity, long-duration continuous AE activity. It is surprising that such a modest magnetic storm (Dst ∼ −2.5 nT) resulted in the anomaly moving to nearly 30° magnetic latitude.

1. Introduction

[2] In June 2006 the Arecibo Observatory (AO) incoherent scatter radar (ISR) was operated in a mode suitable for comparison with occultation measurements of the Global Positioning System signals received by the COSMIC/Formosat receivers. The satellites were relatively close to each other in this portion of the mission so the locations of occultation paths are relatively close to each other. In this period, daytime passes occurred near 1430 LT and nighttime passes near 0230. Details concerning the Arecibo Radar can be found in the work of Isham et al. [2000] and for COSMIC in the work of Rocken et al. [2000].

[3] The Arecibo data used here come from both the vertically oriented line feed and the Gregorian feed, which was continuously in motion at a zenith angle of 15 degrees. The latter detector is advantageous because gradients in the medium, if present on a scale of 300 km, can be detected. Gradients create problems in turning occultation data into vertical profiles since, as we shall see, the occultation path covers a large horizontal distance.

[4] To the authors' knowledge, this is the first test of the Abel transform method using full measured ionospheric profiles up to the satellite altitudes.

2. Data Presentation

[5] The radar data were obtained during 160 h of continuous operation during a World Day period. To put these data in perspective, various interplanetary and geomagnetic data are presented in Figure 1. The z component of the interplanetary magnetic field (IMF) primarily fluctuated about zero but had a period of steady southward IMF during the first half of 28 June and was weakly southward most of the time on 30 June. Small negative values of DST and Sym-H developed at these times. In addition, shorter-period southward IMF periods created local depressions in the magnetic field the order of 5 nT.

Figure 1.

The z component of the interplanetary magnetic field along with the activity indices DST, Sym-H, and Kp for a 5-day period in June 2006.

[6] Figure 2 (top) shows the electron density measured at Arecibo for the period. Below are the three components of plasma drift in geomagnetic coordinates. The data gap occurred when the rotation of the feed was interrupted and vector drifts could not be measured.

Figure 2.

The logarithm of the plasma density measured using the Arecibo incoherent scatter radar along with the three components of the plasma drift in geomagnetic coordinates. VPN is the perpendicular northward (and upward) velocity, VPE is the perpendicular eastward drift, and VAP is antiparallel to the magnetic field.

[7] The AO line feed data are presented in Figure 2 for the entire period along with the three components of plasma drift. These data are typical for the midlatitudes region and illustrate the major differences between daytime and nighttime density profiles.

[8] The ISR has been normalized using the local ionosonde, and a comparison of NmF2 deduced from the ionosonde and from the ISR is presented in Figure 3. In Figure 3 and below, AST stands for Atlantic Standard Time for which UT = AST + 4.

Figure 3.

A comparison of the peak plasma density measured with the ISR and a local ionosonde.

[9] The daytime values of NmF2 on 28 June were about half as large as on the other 4 days, indicating a negative magnetic storm associated with the 30 nT decrease in the DST index. Curiously, after sunset on 29 June, a positive storm developed, leading to elevated densities all night when compared to the more normal evenings on the three previous nights. As we shall see, this elevation was due to a poleward thrust of the equatorial anomaly into Arecibo's field of view at almost 30° magnetic latitude. This burst of high nighttime density fortuitously occurred in the morning local time sector when the nighttime occultations occurred. There is evidence just before and just after the data gap that VPN and VAP were quite elevated in this period and, as is often the case, anticorrelated.

[10] In Figure 4 we expand the E region plasma content. In this format, the thin sporadic E layers become visible both day and night. These metallic ion-rich regions are tracers of shears in the tidal winds, which are most obvious as they track downward in the evening and stagnate below 100 km. The layer seen at low altitude near midday on 30 June is the remnant of a layer that descended the previous evening from higher altitudes.

Figure 4.

An expanded plot of the ISR-determined plasma density in the E region.

[11] We now turn to the main topic of this paper, comparison of these data with occultation-based profiles based on the Abel transform method. Figure 5 shows the location of the penetration point at 300 km for a given occultation, along with the outline of Puerto Rico and the location of the ISR. In each case the cross denotes the location, projected to the ground, of the peak in the F layer as deduced from the Abel transform, and the arrow shows the direction of the pass. The length of the line through each cross shows the total occultation distance between the interception points of the satellite-to-satellite line of sight and the 100 and 500 km altitude surfaces.

Figure 5.

The ground tracks for nine occultation paths whose Abel transform profiles are presented in Figures 7 and 8. The cross marks the location of the peak in the plasma density. The end points are plotted below the intercept points of the line of sight between the two satellites and surfaces at 500 and 100 km altitude. The symbols (s) and (r) stand for rising and setting occultations of the GPS satellite.

[12] The green arrow in Figure 5 shows the direction of the gradient in NmF2 deduced from data such as that shown in Figure 6, with the magnitude of the gradient given in the insert. In Figure 6, NmF2 is plotted from consecutive profiles determined during one azimuthal swing of the Gregorian feed, along with a sinusoidal fit to the data. The variation of NmF2 is due to a gradient in that quantity over the roughly 300 km-diameter circle traced by the radar beam in the F region. The sinusoid can be analyzed to yield a linear fit to the gradient in NmF2 using a spacing of 300 km. Although the data look noisy in this representation, only two of the data points differ by more than 2% from the fit.

Figure 6.

The plasma density as a function of position for one rotation of the Gregorian feed. The sine wave is a fit to the data, which are used to estimate the gradient in NmF2.

[13] Figures 7 and 8 show comparisons of the ISR profiles with the vertical profiles deduced using the Abel transform method developed independently at UCAR and JPL. The letters refer to the color code in Figure 5.

Figure 7.

A comparison between two Abel inversions of the occultation data and the Arecibo-determined density profiles. (Blue represents ground truth at Arecibo, red is UCAR, and black represents JPL.)

Figure 8.

An expanded E region plot of the same data presented in Figure 7. (Blue represents ground truth at Arecibo, red is UCAR, and black represents JPL.)

[14] Figure 7 is optimized for the F region, and Figure 8 is optimized for the E region. The time periods correspond to two consecutive passes of the COSMIC constellation on 30 June. Figure 9 presents data analogous to Figure 7 for the early morning period on the same day.

Figure 9.

Three nighttime occultations compared with the Arecibo profiles.

[15] Scatterplots of NmF2 and HmF2 measured during 32 occultations using the two methods are presented in Figures 10 and 11. The straight line at a 45° angle is not a fit but would, of course, correspond to identical parameter estimations.

Figure 10.

A scatterplot of NmF2 determined by the radar versus the occultation method.

Figure 11.

A scatterplot of HmF2 determined by the radar versus the occultation method.

[16] During the nighttime, another instrument on COSMIC provides ionospheric data. The instrument is called TIP, the Tiny Ionospheric Photometer, which looks downward and records an emission from recombination of O+ with electrons and which is proportional to the square of the electron density. Figure 12 shows TIP data for a series of consecutive nighttime passes on 30 June (day 181). Time runs from right to left in this presentation. The bright region over the Atlantic Ocean tracks the dashed line, which is nearly equidistant from the magnetic equator (the solid line) and clearly traces the Equatorial Arc due to the Appleton Anomaly. Inspection shows that this arc is the brightest and the most poleward for the pass nearest Arecibo. These data are from the pinhole detector, which is only sensitive to relatively high TEC. Thus, the most important scientific parameter we can deduce is the location of the equatorial anomaly which, for the modest activity period plotted, nicely follows the TIP intensity peaks. Figure 13 compares the TIP data for passes near Arecibo on three consecutive days in June. It is clear from these passes that the anomaly was located at an unusually high latitude on 29 June. The comparison we make here is part of the ongoing goal to calibrate the TIP data in vertical TEC units.

Figure 12.

The path of a single COSMIC satellite bearing a TIP detector using the pinhole aperture on 30 June 2006. The color scale shows the count rate, which becomes significant only when viewing the Appleton Anomaly. The detector has a false high count rate near the South Atlantic Anomaly (SAA). The highest meaningful count rate occurred near Puerto Rico. The emission used (135.6 nm) accompanies recombination of O+ with an electron and hence is proportional to the square of the total O+ content.

Figure 13.

A comparison between the TIP detector output (full aperture) for passes near Puerto Rico for 3 consecutive days. Day 181 corresponds to 30 June.

3. Discussion

3.1. Validation

[17] Study of Figures 7 and 9 shows that F region profiles deduced from the occultations, although not perfect, are reasonably accurate, given the distances from Arecibo and the long horizontal tracks involved. There are some surprising differences between the JPL and UCAR products, which show that data processing methods are not totally straightforward. The parameter estimates in Figures 10 and 11 show that the Abel transforms seem to determine NmF2 better than HmF2. The standard deviation of the former is 1 × 105 cm−3 and seems to be better at low densities in this data set. HmF2 values seem to be best near 300 km but are lower than Arecibo values below this height and higher than Arecibo values above it.

[18] Figures 7–10, on the other hand, show that the occultation/Abel transform approach in the E region is simply not suitable by any measure. This is not a huge surprise but, hopefully, will encourage developments in this area such as those begun by R. L. Bishop et al. (Comparison of nighttime E region density profiles obtained at the Arecibo Observatory and from GPS occultation measurements, submitted to Radio Science, 2008) and Hysell [2007]. The importance of E region density, in part, is due to the importance of E region conductivity in the development of Convective Equatorial Ionospheric Storms, aka Equatorial Spread F, and the prereversal enhancement of the vertical drift at the magnetic equator [Kelley et al., 2004].

3.2. Scientific Results

[19] It is a bit surprising that the midlatitude ionosphere reacted so strongly to the very modest magnetic storm. The daytime density was down by a factor of two for a storm with a maximum Kp value of only four. The fluctuations in the IMF were quite strong during the entire period of elevated Kp and were associated with a period of positive DST preceding the 30 nT decrease. It may be that a highly variable IMF leads to fluctuating electric fields at high latitudes that create significant Joule heating. This period may be of the type referred to as high-density, long-duration continuous AE activity [Tsurutani and Gonzalez, 1987] or more recently, high-speed solar wind stream [Denton et al., 2008]. The latter are characterized by disturbed A indices, but a weak ring current is observed here.

[20] The extension of the equatorial arcs to near Arecibo is also very surprising for a modest storm. The density was 8 × 105 cm−3 after midnight over Arecibo and, looking at the color plot, it is difficult to distinguish the nighttime data on 30 June from the daytime data above 200 km.

[21] The data dropout in the velocity data is quite unfortunate. However, as we suspected, the anticorrelation between VPN and VAP suggested by the data on both sides of the gap indeed continued throughout the period, indicating that a sizable eastward electric field was present. The VAP, then, is a response to the uplift as the plasma flows downward along the magnetic field. This hypothesis is consistent with both the positive storm and the poleward advance of the equatorial arc [Vlasov et al., 2003].

Acknowledgments

[22] The Arecibo radar is operated by Cornell University under a cooperative agreement with the National Science Foundation. Work at Cornell was supported by the Office of Naval Research under grant N00014-03-1-0243.

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