Geomagnetic Latitude Range Research Articles

Characteristics of the ionospheric variability as a function of season, latitude, local time, and geomagnetic activity

An ionospheric F2 critical frequency database has been assembled to determine the variability of the F region as a function of local time, latitude, season, and geomagnetic activity. The database comprises observations from 75 ionosonde stations covering a range of geomagnetic latitude and includes 43 storm intervals. The database was previously used to develop the Storm‐Time Empirical Ionospheric Correction Model (STORM). The mean and standard deviation have been evaluated by sorting the data by local time, season (five intervals centered on equinox, solstice, and intermediate intervals), latitude (four regions each 20° wide in geomagnetic latitude), and up to eight levels of geomagnetic activity. The geomagnetic activity index was based on a weighted integral of the previous 33 hours of ap and is the same as that used by the STORM model. The database covers a full solar cycle, but insufficient information was available to sort by solar activity without compromising the estimates of variability on the other sorting parameters. About half the data were contained in the first level of geomagnetic activity, between 0 and 500 units of filtered ap corresponding to Kp ≤ 2, and half above that level. When local time dependence was included in the binning, sufficient data were available to sort into two levels of geomagnetic activity, quiet (Kp ≤ 2+) and disturbed (Kp > 3−). For all latitudes and levels of geomagnetic activity, the lowest variability was typically found in summer (10–15%), and the largest variability occurred in winter (15–40%), with equinox (10–30%) lying between the solstice extremes. The exception was low latitudes at equinox, which had surprising low variability (10%), possibly because of the weak interhemispheric flow at this time of year. At middle and low latitudes, the variability tended to increase with geomagnetic activity in winter and equinox but remained fairly constant in summer. At high latitudes, the surprising result was that in all seasons, and in winter in particular, the variability tended to decrease, probably because of the increased upwelling of neutral molecular species and stronger chemical control of the ionosphere. The data have also been used to build a table of estimated variability suitable for inclusion in the International Reference Ionosphere or any other climatological model. For periods where data were scarce or nonexistent, an estimated variability was provided on the basis of expectations of the consequences of physical processes. This was necessary to fill in the table of values in order to develop a module suitable for inclusion in the International Reference Ionosphere.

Low latitude model of Te at 600 km based on Hinotori satellite data

A new model of electron temperature (Te) variations in the low latitude ionosphere (600 km) has been developed by using Temeasurements onboard Hinotori satellite (February 1981–June 1982). The model is based on spline-approximation of the measured Te in 5-dimensional space comprised of solar activity (F10.7), month of the year, local time, geographic longitude and geomagnetic latitude. Each axis is divided into subintervals delimited by a number of nodes. To find Te at a point of interest, spline interpolation from the nearest nodes to the point of interest is used. During the lifetime of Hinotori mission, F10.7 varied between 140 and 230, so the model is constrained to this solar activity range. The geomagnetic latitude range is limited to ±42° due to the low inclination orbit of the satellite. The model error has been evaluated using the whole database, containing 17 months of data (4.2 × 10 5 measured values). The standard (mean square root) deviation of the model from the data is found to be around 14%. A detailed comparison between the model and IRI-95 predictions shows a discrepancy of 100–200 K in most cases, although some larger deviations are also found to occur. Contrary to IRI, the present model captures satisfactorily the morning overshoot and the afternoon enhancement of Te.

Plasmasphere electron temperature model based on Akebono data

Average electron temperature ( T e) distribution in altitude range 1000–10,000 km and in geomagnetic latitude range ± 70° are used to construct a simple analytical model of T e. T e distribution is considered constant during daytime (9–16 h) and nighttime (22–04 h) local time sectors. Transition between the constant thermal states is described by cubic splines. The vertical T e profiles at fixed geomagnetic latitudes are approximated by second order polynomials and then the three constants of these polynomials are approximated by another set of analytical expressions, being explicit functions of altitude, geomagnetic latitude and local time. No seasonal variation or hemispheric asymmetry is considered in the model. The average T e distributions at L shells greater than three were reconstructed, in order to avoid the unrealistically high temperatures measured in the regions with low plasma density. For this purpose, T e distributions above 3000 km along the L shells were calculated, taking the gradients obtained at L=2 (0.3 K/km during the day and 0.1 K/km during the night), reduced with latitudes as 1/( L−1). The accuracy and limitations of the model are discussed.

Spatial and temporal variability of the gamma radiation from Earth's atmosphere during a solar cycle

The Solar Maximum Mission satellite's Gamma Ray Spectrometer spent much of its 1980–1989 mission pointed at Earth, accumulating spectra of atmospheric albedo γ‐rays. Its 28° orbit ensured that a range of geomagnetic latitudes was sampled. We measured the variation with time and cutoff rigidity of some key γ‐ray lines which are diagnostic of the intensity of the Galactic cosmic radiation penetrating the geomagnetic cutoff and of the secondary neutrons produced in the atmosphere. We found that the intensities of nuclear lines at 1.6 MeV, 2.3 MeV and 4.4 MeV varied inversely with solar activity in cycles 21–22 as expected from the theory of solar modulation of cosmic rays. They were found to be strongly anticorrelated with cutoff rigidity, as expected from the theory of the cutoff, falling by a factor ∼3.6 between the lowest (<7 GV) and highest (>13 GV) rigidities sampled. The solar cycle modulation was particularly marked at the lowest rigidities, reaching an amplitude of 16%. The ratios of the intensities of the lines produced by nuclear de‐excitation (1.6 MeV, 2.3 MeV) and those from nuclear spallation (4.4 MeV) did not vary with either solar activity or cutoff rigidity, indicating that the shape of the secondary neutron spectrum in the atmosphere above 5 MeV was approximately constant over the times and regions sampled. If it is approximated by a power law in energy, we derive constraints on the absolute value of the power law index ∼−1.15–−1.45 and better constraints on its variability, ≤5% over a solar cycle, and ≤6% over SMM's range of sampled cutoff rigidities. We also measured the intensity of the electron‐positron annihilation line at 0.511 MeV. This line also varies with the solar cycle, but its variation with cutoff rigidity is weaker than that of the nuclear lines, falling by a factor 2 (rather than 3.6) over SMM's range of sampled cutoff rigidities. This can be understood in terms of the energy dependences of the cross sections for positron production and for the hadronic interactions which produce secondary neutrons.

Design and test results of the AMS RICH detector

The AMS-02 detector will operate for at least 3 years on the International Space Station, measuring cosmic ray ray spectra at about 400 km above sea level over a wide range of geomagnetic latitude. The proximity focusing ring imaging Čerenkov counter of AMS-02 will measure the particle velocity β with ≈ 0.1% uncertainty, making possible to discriminate Beryllium isotopes up to about 15 GeV/nucl. In addition its charge measurement will allow to study the elemental composition of cosmic rays up to Iron. A prototype of the RICH detector was tested with cosmic rays and on a ion beam accelerated by SPS, at CERN (October 2002).

Mid-latitude effects of the May 15, 1997 magnetic storm

We investigate the May 15, 1997 magnetic storm effects on the mid- and low-latitude ionosphere. The study is based on using the data from three chains of ionospheric stations located approximately along the meridians 20°, 140° and 280°E in the geomagnetic latitude range 13–65°N. Variations in f 0F2 are considered. Estimates of the zonal electric fields are made. Results of our analysis show that the main ionospheric effects of the storm under consideration are: (1) long-lasting intense negative disturbances during the storm main and recovery phases at subauroral and mid-latitudes; (2) positive disturbances at stations of the European and American chains observed prior to the storm, regardless of the local time; (3) a positive peak of Δ f 0F2 at stations of the Asian chain during the storm main phase in the evening hours; (4) a similarity of the form of the Δ f 0F2-variations at different latitudes and (5) the largest effect on the F region is observed at the Asian chain. The resulting differences of the Δ f 0F2-variations can be driven both by the local time of the sudden storm commencement and by magnetic dip. It is not mere chance that the largest differences are observed along the meridian 140°E where the difference between the geographic and magnetic poles is the largest.

Statistical average estimates of high latitude field-aligned currents from the STARE and SABRE coherent VHF radar systems

Abstract Two bistatic VHF radar systems, STARE and SABRE, have been employed to estimate ionospheric electric fields in the geomagnetic latitude range 61.1 – 69.3° (geographic latitude range 63.8 – 72.6°) over northern Scandinavia. 173 days of good backscatter from all four radars have been analysed during the period 1982 to 1986, from which the average ionospheric divergence electric field versus latitude and time is calculated. The average magnetic field-aligned currents are computed using an AE-dependent empirical model of the ionospheric conductance. Statistical Birkeland current estimates are presented for high and low values of the Kp and AE indices as well as positive and negative orientations of the IMF B z component. The results compare very favourably to other ground-based and satellite measurements.

Neutral winds in the lower thermosphere observed by WINDII during the April 4–5th, 1993 storm

Data from a major geomagnetic storm in April 1993 were selected for the first WINDII study of storm‐induced neutral winds in the lower thermosphere between 90 and 200 km. The storm commenced at ∼1200 UT April 4th, when the Kp index increased from 1.3 to 7.7. The geomagnetic latitude range of WINDII measurements from the daytime O(¹S) emission reaches the southern geomagnetic pole. The wind exhibits a dramatic change during the storm. In the polar region the maximum velocity of ∼200 ms−1 at 140 km altitude on April 2nd increases to ∼650 ms−1 at 200 km on April 5th. Winds plotted in geomagnetic coordinates show the familiar two‐cell pattern in that winds are much stronger in the dusk sector than in the dawn sector. This pattern penetrates down to ∼130 km. During the recovery period on April 8th, winds at higher altitudes return to normal while the two‐cell pattern persists at lower altitudes. The thermospheric O(¹S) emission rate is severely depleted by the storm to half of its normal value.

Properties and sources of low and very low latitude Pi2 pulsations

Data from a low‐latitude ground array of 10 fluxgate magnetometer stations spanning a geomagnetic latitude range from 38°N through the equator to 47°S, with a geomagnetic longitudinal extent from 185°E to 227°E covering ∼3 hours in magnetic local time, and an additional high‐latitude station (65°N) are used to study the spatial and conjugate properties of Pi2 geomagnetic pulsation wave frequency, amplitude, phase, and polarization. Using fast fourier transform and pure state vector analysis techniques, multistation observations show that individual frequency elements of the Pi2 waves are constant with latitude and longitude, but the dominant frequency may vary. In fact, the higher frequencies often dominate at low latitudes, and the lower frequencies often dominate at higher latitudes. The D component amplitude shows a clearly decreasing trend to a near‐zero minimum at the equator, while the H component amplitude does not show a simple behavior. The H component phase is nearly constant at low latitudes, while the high‐latitude station may be out of phase or in phase with the low‐latitude stations for different events. The D component phase is constant in each hemisphere with a 180° phase difference between hemispheres. Azimuthal wave numbers m obtained from interstation phase differences are small, typically |m| < 3, and show a dominance of westward propagation at most longitudes. It is concluded that the main features of low‐latitude Pi2 properties are most easily explained by magnetospheric cavity or wave guide mode resonances, and field line resonances are most likely responsible for the high‐latitude Pi2s. This supports the idea that midlatitude and low‐latitude Pi2s are two‐source phenomena.

Occurrence patterns for transient magnetic field signatures at high latitudes

Impulsive events marked by durations ranging from 5 to 15 min and peak‐to‐peak variations in the H component exceeding 40 nT are common in observations by high‐latitude day side ground magnetometers. We survey observations by a global network of high‐latitude ground magnetometers to distinguish between their proposed causes. Peak event amplitudes and occurrence rates occur prior to local noon in the range of geomagnetic latitudes from 70° to 76°. Although most events can be associated with abrupt changes in the interplanetary magnetic field (IMF) orientation, few can be associated with abrupt changes in the solar wind dynamic pressure. Many events can be observed during intervals of steadily northward IMF orientation, but only a few can be observed during intervals of steadily southward IMF orientation. We suggest an interpretation of the events in terms of a model in which changes in the IMF orientation modulate the fraction of the solar wind pressure applied to the magnetosphere.

A 1957 survey of cosmic ray intensity, 0 to 25 km altitude and 86°N to 73°S geomagnetic latitude

During 1957, as part of the program of the International Geophysical Year, the University of Iowa conducted a survey of cosmic ray intensity with equipment on balloon‐launched rockets (“rockoons”) over the range of geomagnetic latitude λ from 86°N to 73°S. This paper reports the results obtained up to altitudes of about 25 km during the flights of 26 balloons. The epoch of the observations was near the time of maximum solar activity. A comparison is made with similar observations at λ = 52° in 1953 at a time near minimum solar activity. The mean solar modulation factor is 0.69 ± 0.04 at altitudes ranging from 10 to 20 km altitude.

Spaced direction finding of nighttime whistlers at low and equatorial latitudes and their propagation mechanism

The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is single‐valued at any particular local time. (3) The ionospheric exit region of whistlers is very much restricted to the geomagnetic latitude range of 10°–14°, and there are no observed whistlers whose path latitude is between 14° and 20°. (4) The extent of their ionospheric exit region is very stable on different days, and the radius of the distribution is less than 40–50 km. (5) Surprisingly high occurrence of echo train whistlers is observed. (6) The propagation of whistlers in the Earth‐ionosphere waveguide after ionospheric transmission is more likely toward higher latitudes than toward the equator and the subionospheric propagation seems to exhibit a horizontal beaming around the magnetic meridian plane. We attempt to interpret these characteristics of the observations in terms of either nonducted or field‐aligned propagation, but discussion indicates that especially findings 4 and 5 are strongly indicative of field‐aligned propagation for very low latitude whistlers localized in geomagnetic latitudes of 10°–14°.

Wave normal direction and spectral properties of whistler mode hiss observed on the DE 1 satellite

A new study of magnetospheric hiss as a spatially and temporally enduring phenomenon is undertaken using a recently developed formalism that allows the representation of hiss by a field distribution function (FDF). This formalism explicitly takes into account the whistler mode relationships and the linear and spin motion of the satellite, so that on a spin‐stabilized satellite, it becomes possible to measure the wave propagation direction(s) from the observed fading patterns in the received electromagnetic field data. We have analyzed hiss signals received by electric and magnetic field antennae aboard the DE 1 satellite during a ∼3‐hour period on September 23, 1983. A band of hiss at frequencies <2 kHz was observed continuously from 0236 UT to 0539 UT over a range of geomagnetic latitudes from λm = 45°N to λm = 20°S and L shells of L = 4.3 to L = 5.3. Electron density deduced from in situ and remote measurements indicates that during this time the DE 1 satellite was near the boundary of the plasmasphere. Observations can be summarized as follows: (1) The general character of the electric and magnetic field spectrum remained the same throughout the 3‐hour‐long period, exhibiting an intensity peak at 1550 Hz. (2) The intensities of both the electric and the magnetic field decreased during this interval by ∼30 dB. (3) Well‐defined fading patterns at half the spin period (3.04 s) were observed throughout this period. These patterns were stable over a time scale of ∼1 min. (4) In addition, a slow fading pattern with a time scale of 30 s was observed approximately 30% of the time. (5) The hiss intensity also showed fading over a time scale of ∼10‐15 min. The spin fading patterns led to the measurement of wave propagation direction(s). The results of our analysis can be summarized as follows: (1) Near the geomagnetic equator (within ∼ ±3° latitude) we observe a wave normal angle of 60° ± 5° with respect to the local geomagnetic field. (2) Away from the equator, wave normal directions range from 30° to 80° with respect to the local geomagnetic field. (3) All wave normal directions observed over the 3‐hour‐long period were within ∼ ±45° of the plane normal to the local magnetic meridional plane. Results indicate that the hiss source radiates with initial wave normal angles in the range 30° < θ0 < θg (Gendrin angle) and 90° < ϕ0 < 270°, contrary to the common assumption that the source emits with wave normal angles closely aligned with the geomagnetic field. The FDF formulation has also permitted elucidation of the statistical nature of hiss. The slow time fading (∼30 s) is interpreted in terms of a coherence bandwidth Δω of about 0.2 rad/s.

Morphology of phase and intensity scintillations in the auroral oval and polar cap

The first long‐term measurements of phase scintillations at high latitudes from a quasi‐stationary Air Force satellite at 250 MHz are reported. The measurements were made from Goose Bay, Labrador covering the corrected geomagnetic latitude range of 64°–72°N and Thule, Greenland covering the corrected geomagnetic latitude range of 85°–89°N. Data from December 1979, March–April 1980 and March–April 1982 are presented from both stations. The sunspot numbers during these specific time periods were approximately the same. The scintillation magnitudes were higher during the vernal equinox at both stations than during the winter solstice. The Goose Bay data showed a well ordered diurnal variation with a nighttime maximum and daytime minimum, whereas the Thule data showed no such ordering. The limited phase scintillation data set from Thule is augmented by intensity scintillation measurements made during 1979–1982 using the same quasi‐stationary satellites. The long‐term Thule data shows that the major variation in scintillation activity is annual with minimum scintillations observed in the local summer, as was determined earlier by Aarons et al. (1981). In contrast to the pronounced annual variation, the dependence of overall scintillation activity on sunspot cycle is not greatly evident during 1979–1982 when the sunspot number varied approximately between 100 and 200. It was found that both Goose Bay and Thule could show phase scintillations of the order of 10 rad (with 82‐s detrend period) even during magnetically quiet times. One such case study, conducted during March 1982, is presented and second‐order parameters such as phase aad intensity spectral strengths and slopes and intensity decorrelation times are discussed for this event at both stations. The use of geostationary satellite data shows the importance of enhanced magnetospheric convection velocities on the observation of large phase scintillation values at both sites. This underscores the need for incorporating irregularity dynamics in the currently developed static models of ionospheric scintillation.

Large‐scale, near‐field magnetic fields from external sources and the corresponding induced internal field

Magsat data has been analyzed as a function of the Dst index to determine the first degree/order spherical harmonic description of the near‐earth external field and its corresponding induced field. The analysis was done separately for data from dawn and dusk with the following results: dusk external: q10 = 20.3 ‐ 0.68 Dst (nT); dusk internal: g10 = 29987.7 + 0.240q10 (nT); dawn external: q10= 18.62 ‐ 0.63 Dst (nT); dawn internal: g10 = 29992.3 + 0.287q10 (nT), where g10 and q10 are the degree 1, zero‐order, internal and external coefficients, respectively, in a spherical harmonic potential function describing the near‐earth magnetic field. Comparison with POGO data indicates that the constant term relating q10 and Dst has changed about 20 nT between 1970 and 1980, presumably due to an increase in the average ring current intensity. A local time variation of the external field persists even during very quiet magnetic conditions. Both a diurnal and 8‐hour period are present. A crude estimate of sq current in the ±45° geomagnetic latitude range is obtained for 1966–1970. The current strength, located in the ionosphere and induced in the earth, is typical of earlier determinations from surface data, although its maximum is displaced in local time from previous results.

Helium resonance and dispersion effects on geostationary Alfven/ion cyclotron waves

To understand observed structure and growth patterns of geostationary Alfvén/ion cyclotron waves, the properties of the hot proton cyclotron instability within a helium rich plasma are explored here. This exploration proceeds with an examination of the net linear wave amplifications that result as a wave propagates through the magnetic gradients of a realistic magnetic field model (linearity is discussed and justified). By taking care in generalizing a single pass model to a multiple pass system, the following conclusions have been reached: (1) The basic structure of the frequency gap that is observed close to the helium cyclotron frequency can be explained by the ‘stop‐gap’ dispersion effect; however, the helium cyclotron resonance effect contributes to the gap formation leading to He+ ion energization. (2) The presence of the magnetic gradients virtually insures that helium ion energization (by means of the low frequency wave branch only) is the inevitable consequence of the wave generation process. (3) The energized helium ions will sparsely populate a broad range of geomagnetic latitudes (±15 degrees) but will be concentrated strongly within several (2–4) degrees of the geomagnetic equator. (4) The presence of low percentages of helium ions is most likely to suppress the wave generation process (ATS 6 satellite observations support this conclusion for a portion of the wave spectrum).

STARE observations of long discrete echoes

Early studies of auroral radar echoes by Unwin1,2 showed the existence of a particular class of ‘long discrete’ echoes, distinguished as being weak and relatively long-lasting (1–60 min). The backscatter usually consists of several narrow (∼20 km) bands which drift slowly in latitude, and the height of the backscattering region appears not to exceed 100 km. Here, we report on observations of this type of radar aurora made during the past few years with the Scandinavian Twin Auroral Radar Experiment (STARE). Whereas Unwin's observations covered the range of geomagnetic latitudes corresponding to l-values ∼3−∼5, the present study is concerned with higher latitudes, corresponding to the range l ∼ 5.7−∼8.

Latitudinal reversal of polarization of the geomagnetic sudden commencement

Latitudinal variation of polarization of geomagnetic sudden commencements (sc) observed at geomagnetic latitude range 63°N–79°N is studied by the use of North American IMS magnetometer data from August 1978 to May 1979. Fourteen sc's out of 18 examined show clear polarization reversal between latitudes 64°N and 72°N. Among these, nine change their polarization direction from counterclockwise to clockwise with increasing latitude and the remaining 5 change it in the opposite sense. Clear local time dependence as shown statistically by Wilson and Sugiura (1961) could not be confirmed from this small data set. Their statistical results (Wilson and Sugiura, 1963), however, can be more reasonably interpreted by taking into account the existence of latitudinal polarization reversal described in this paper.

A Study of Two Polar Magnetic Substorms with A Two-Dimensional Magnetometer Array

Summary. The paper reports studies of the three-dimensional magnetospheric—ionospheric current systems which produced polar magnetic substorms on 1974 September 7 and September 18. The data were magnetic perturbation fields observed with a two-dimensional array of 23 three-component magnetometers located in western Canada beneath the auroral oval. In an earlier study of a substorm of September 11 (Bannister & Gough) the fields fitted calculated field for a Bostrom Type 1 current loop with field-aligned currents at east and west ends of the ionospheric segment, and with uniform current density across the width. The substorms here reported could not be modelled with uniform current density. An inverse method due to Oldenburg was therefore used to estimate current density distributions, and satisfactory fits of calculated to observed field resulted. Each substorm was modelled at six representative epochs. In general the principal ionospheric current seem by the array was westward. At four epochs of the September 7 substorm and throughout the September 18 substorm, significant eastward ionospheric current (or its equivalent in terms of the fields produced) was observed north of the westward electrojet. Northwestward bends in the ionospheric current segments were found at four epochs on September 7 and at three epochs on September 18. As in the September 11 substorm (Paper 1), these bends were either west of or close to magnetic midnight. In some cases the bends may follow the auroral oval, but in others they are sharper and may be associated with the Harang discontinuity. East of geomagnetic the ionospheric currents tend to run in a constant geomagnetic midnight latitude range. The developments of the three substorms, of September 7, 11 (Paper 1) and 18, are compared. They showed a variety of shifts in longitude, though all moved eastward relative to magnetic midnight.

The baselevel ionospheric trough

Isis 2 electron density profiles of the topside F region obtained during December 1971 have led to a detailed examination of the ionosphere's ‘main trough’ region under an optimum set of geophysical conditions. A total of 30 individual satellite passes during a 21-day period were used to determine the trough's characteristic features under the conditions of midwinter in the northern hemisphere, midnight local time, and very low magnetic activity. The morphological description obtained from this rigidly consistent data base is termed the ‘baselevel trough.’ The latitudinal profile of the trough at hmax over the 40°–80° corrected geomagnetic latitude range shows that while the equatorward edge and the trough minimum are wide features (7° and 10°, respectively), the poleward wall is sharp (∼1½°). The equatorward edge and the trough minimum tend to lose their distinct identities toward higher altitudes, while the magnitude of the poleward wall enhancement remains nearly constant at 2.15 from 450 to 950 km. The trough's poleward wall is found 1°–2° lower in latitude than the statistical position of the equatorward edge of the diffuse aurora. Ratios of topside densities to the peak density reveal that the shape of the Ne(h) profiles is constant from mid-latitudes to the trough minimum. Within the minimum a relatively rapid change in topside shape occurs which marks the light ion depletion region characterized by high H+/O+ transition heights. The northern terminus of the transition height enhancement agrees with the statistical position of the magnetospheric plasmapause. Estimates of plasma temperature (Te + Ti) show only modest enhancements within the trough minimum.