The formation of long-lived, sporadic E layers (Es) has been an intriguing subject of aeronomy for decades. and developed the wind shear theory, in which ions are swept together by the combined action of neutral collisions and Lorentz force; for recent reviews see or . Observation of persistent, narrow layers implies the important role of long-lived metallic ions (e.g., Fe+, Mg+, Si+, Na+, Ca+), which are abundant in the upper mesosphere and lower thermosphere through meteor ablation. Improved sensitivity of experimental techniques, such as incoherent scatter radar (ISR), shows that sporadic layers with a variety of plasma densities, widths, and lifetimes are a common presence throughout the E region e.g.,.

It was soon recognized that, near the magnetic equator and at high latitudes, the wind shear mechanism would not be as efficient as at mid-latitudes due to the small or large inclination of the magnetic field. At polar latitudes, convection electric fields often provide the important driver for convergence or divergence in the ion motion .

Through solving the equation of motion and neglecting terms with gravity, the vertical neutral wind and ion diffusion e.g.,, the vertical ion drift viz can be expressed by viz=cos⁡I1+ρi2EEB0-UNsin⁡I+ρicos⁡I1+ρi2ENB0+UE, where UN, UE, EN, and EE are the magnetic northward and eastward components of the neutral wind and electric field, respectively; I is the magnetic inclination; and ρi=νi/ωi the ratio of ion–neutral collision and ion gyro frequencies. The collision frequency increases exponentially with neutral density, and therefore the second term containing the zonal wind dominates the vertical ion drift below about 110 km altitude. In the absence of an electric field, the ion layer is observed to form near the zero in the zonal wind with a westward shear. However, it can also be seen that cos⁡I is small for high inclination, and the wind alone is less efficient.

Many sporadic layers (again, disregarding electric fields) are formed by tidal winds, e.g., semidiurnal or diurnal modes, but gravity waves may provide additional wind nodes . If the layer follows the zero wind node of an upward propagating gravity wave, it will descend in time. Eventually, collision frequencies become too large and diffusion processes “dump” the layer into the background ionosphere . Sporadic E layers are therefore rarely observed below 90 km.

The composition of the layers of metallic ions connects Es to meteor ablation processes and the formation of the neutral metal layers in the upper mesosphere . The detection of neutral and charged meteoric smoke or dust particles in the mesosphere has become a very active field, and different rocket techniques have been deployed e.g.,.

discusses the possibility of electron attachment to dust particles from meteor ablation. While deep electron depletions in connection with large, icy dust particles near the polar summer mesopause are well known e.g.,, observed electron loss in connection with dust layers also for winter nighttime conditions.

In this paper we report observations of sporadic E layers at high latitudes under geomagnetically very quiet conditions. The layers were observed by two rockets carrying fixed-bias Langmuir probes delivering very fine detail of the vertical plasma structure in the D and lower E region. A sequence of four rockets with trimethyl aluminium (TMA) releases provided measurements of the neutral wind in the upper mesosphere and lower thermosphere. The Poker Flat Incoherent Scatter Radar (PFISR) used five beams simultaneously adding some information about the time development and horizontal extent of the layers; however, electron densities were just above the observation threshold during this portion of the night when the rocket observations were made.

The primary goal of the experiment was the observation of the development of neutral turbulence around the turbopause . Our observation of Es was incidental, but worthy of attention, since simultaneous measurements of Es and neutral winds are relatively rare, and accurate wind measurements are difficult to obtain in this region. Many studies have focused on mid-latitudes and events above 100 km, where strong layers are observed and the wind shear mechanism is most effective, sometimes modified by electric fields e.g.,.

We focus on two key observations which we find significant for our case study of an Es at auroral latitude: (1) the close correlation between layer height and zonal wind shear node, and (2) deep electron depletions just below the layer of enhancement.

The paper is organized as follows: in Sect. 2 we briefly introduce the experimental techniques, in Sect. 3 we present the time development of wind profiles and layer altitude, and in Sect. 4 we discuss the electron depletion and possible reasons. In Sect. 5, we summarize our results.

The NASA sounding rocket experiment Turbopause was conducted on 17–18 February 2009 at Poker Flat Research Range (PFRR), Alaska (64∘ N, 147∘ W). The magnetic inclination and declination angles are 77.41 and 24.89∘. Local midnight is at 09:00 UT and magnetic midnight at 11:20 UT. The main purpose of the experiment was to study the transition from strong mixing and turbulence to more laminar behavior as visible in chemical trail releases. Results of the wind, temperature, and turbulence measurements were reported by , and a mixing event in the mesosphere observed in the sodium layer was studied by .

Four two-stage Terrier-Improved Orion sounding rockets were launched within a 2 h period starting at 09:59 UT (00:59 LT) northward along similar trajectories. Neutral winds were measured by all four rockets with the TMA chemical release technique . The second and third rocket carried additional instrumented daughter payloads measuring neutral parameters with the Combined Neutral and Electrons (CONE) ionization gauge , and negatively charged particles (electrons and negative ions) with dual fixed-bias Langmuir probes: a nose-tip probe at the front and the CONE instrument in the aft. The NASA payload designations and launch times are given in Table .

Figure  shows the proportions of the relatively short instrumented payload with a long, straight nose cone and the CONE sensor in the aft. Both instrumented rockets exited the lower atmosphere at relatively low body elevation angles (angle with the vertical) as determined by post-flight analysis of horizon sensor and magnetometer data. Above 70 km the atmospheric drag is minimal and the payload attitude remains constant due to the roll stabilization at 5 Hz. Near 95 km the angle between roll axis and velocity vector (angle of attack) was 35∘ on upleg and 90∘ on downleg. The payload orientation and cross-sectional area have an effect on payload charging and the sampling of neutral and charged particles by the probes in the front and back as discussed below.

Major ground-based support was provided by the Poker Flat Incoherent Scatter Radar (PFISR) and the sodium and Rayleigh lidars at PFRR . In this paper, we concentrate on detailed comparisons between the rocket probe profiles and the electron densities obtained by the incoherent scatter radar. In Fig.  the horizontal sampling is projected on a map of central Alaska. PFISR was configured with five beams (numbers 1–5; beam 1 was vertical). The four rocket trajectories are indicated by the long lines originating near beam 1 and labeled with numbers 76 through 79. The horizontal dashes indicate 95 km altitude.

The rocket campaign took place in the middle of the last solar minimum. The geomagnetic conditions were very quiet during the launch day. The planetary Kp index was zero for ∼18 h from 00:00 UT until 21:00 UT. The magnetogram for PFRR is shown in Fig.  for 15–19 February 2009 (days 46–50). It can be seen that conditions were disturbed on day 46, but variations were of the order of only 10 nT during the launch period on day 48 indicated by the black bar.

The magnetic field was similarly quiet or even quieter than during the sporadic E observations by with the European Incoherent Scatter Radar. During the quiet periods, EISCAT meridional electric fields were fluctuating by 5 to 10 mVm-1. PFISR data provided density estimates, but the backscatter signal was too low for estimating electric fields. We assume that the electric field probably did not exceed the quiet time values reported by and had probably only a small influence on the formation of Es.

Figure  summarizes the most relevant rocket observations in our context of sporadic E. The nose-tip probe on the first instrumented flight, 41.078, launched at 10:29 UT (first panel, blue line), observed a narrow sporadic E layer at 95 km, at the same altitude at which a zonal wind node was seen with the chemical releases (third panel). Thirty minutes later, during flight 41.079, the layer had descended about 1 km (red line), as had the wind node. The temperature profiles show a similar downward trend of the local temperature minima (last panel), as does the E-region ledge around 89 km. Due to changing lower atmosphere winds and launch azimuth adjustments, the two instrumented rockets followed slightly different paths through the atmosphere. However, as indicated in Fig. , the horizontal deviation between the two flights was only ∼10 km on upleg and about ∼20 km on downleg.

The current collected by the nose-tip probe (+5 V DC bias) on upleg is a good relative measurement of negative charge density (electrons and small negative ions), as has been demonstrated by comparisons with radio wave propagation measurements and incoherent scatter radar measurements . For this experiment we are able to convert the probe currents to electron densities matching the PFISR measurements (see below). High-resolution nose-tip probe currents in the D and E regions, ranging from 10-10 to 4×10-7 A, were obtained from 40 km upward. Notice the close agreement of the background current for the two flights in the upper D region and lower E region. The small-scale fluctuations attributed to mesospheric neutral turbulence were discussed in .

Significant electron depletions appear just below the enhancements, around 94 km (Fig. , first panel). The currents were only 10 % or less of the background values. Since these are measurements made by the nose-tip probe on upleg, the depletions were measured before the enhancements.

A second sporadic E layer was crossed first near 100.5 km (left panel, blue line), which had almost disappeared during the next flight (red line), while a third enhancement was present near 102 km. However, the layer structure is less clear in this altitude region, as can also be seen in the radar observations (see below). Therefore, we concentrate on the lower layer in the 95 km region.

The second panel displays the currents for each payload observed by the CONE electron probes on downleg. These probes were also biased with constant +5 V but collected about 3 times more current than the nose-tip probes due to different surface area and geometry. The general features of the profiles are similar to the upleg measurements; however, the narrow layers, depletions, and steep gradients are substantially smeared out or not visible at all. We ascribe this response to the unusually shallow angle with which the payloads entered the atmosphere, which in turn led to significant wake effects, and possibly also higher payload charging. We include these measurements since they demonstrate how important the position and orientation of the probe are, even for the collection of mobile electrons. Nevertheless, we note that the altitudes of the Es near 95 km are similar for upleg and downleg, which gives us an indication of the extent of the layer.

Figure  shows PFISR electron densities obtained for the E region with 10 min integrations and 750 m range resolution for all five beams within the observation period between 04:00 and 12:00 UT. The lowest detectable values were 5×108m-3, which was also a typical error estimate for the densities. Several faint layers passed through the radar beam within the 8 h observation period. A strong layer was obvious between 04:00 and 06:00 UT, which coincided with an apparent enhancement in the sodium layer . This event occurred early in the night before the rockets could be launched. The launch of the rocket salvo was triggered by perturbations in the topside of the sodium layer near 100 km as seen by lidar . The observation of Es was fortuitous, and radar electron densities helped us in calibrating the fixed-bias electron probe data. In the color image of the radar electron densities, a weak, narrow layer can be traced by eye in several beams as a faint descending line after about 08:00 LT until the end of the observations, when the layer is near 93 km. Although not very prominent in the radar data, we suggest that it is the same layer as seen in the rocket observations.

We again point out the very quiet conditions and very low ionospheric densities during that night. After 10:00 UT, auroral ionization caused higher background electron densities, which can be seen as the brighter pixels in the vertical and northward beams (beams 1 and 3). At this time, which is around local magnetic midnight, the magnetometer registered stronger perturbations (Fig. ).

Figure  illustrates the development of the horizontal wind components (top panel) over the 2 h period of the rocket observations. Features below 100 km clearly exhibit downward phase propagation. The zonal wind nodes descend at a rate of -1.3(±0.3) kmh-1, characteristic for long-period gravity waves. Based on the hodographs, identified two major wave motions with periods of 12 and 8 h and vertical wavelengths of 30 and 10 km, respectively.

The bottom panel shows the PFISR electron densities for beam 1 during the time of the rocket observations. Even with the guidance from Fig. , the sporadic layer near 95 km is difficult to identify. We performed a spline fit to each radar profile and marked the strongest local maximum in the relevant region with a diamond as our estimated layer altitude. Compared with the zonal wind node (descending black line), it appears that after 11:00 UT the layer remains about 1 km above the wind line. The blue and red rocket profiles are shifted in time and scaled to match the electron densities (see also Fig. 2 in ). Background densities are 8×109 m-3 and layer peak densities are 3 to 4×1010 m-3, which is low compared to many other Es measurements.

Evidently, the radar data are noisier than the rocket data and the Es layer is difficult to make out in the individual radar profiles. Weak sporadic layers may be patchy, and so electron densities vary in time and with beam direction. However, the slow descent of the layer and the matching wind nodes distributed over a 2 h period are significant observations in the context of previous Es and wind measurements.

We have presented in situ and radar measurements of E-region sporadic layers during nighttime and very quiet conditions at auroral latitudes. Here we summarize our most significant findings.

Persistent wind shears were likely the leading cause of Es formation at this relatively low altitude of about 95 km. The wind shear region was also responsible for the downward motion of the layer. Our observations are unique and perhaps somewhat surprising, since there are few measurements like ours and the neutral wind is generally considered inefficient at high latitudes. Based on our analysis, we infer that the convection electric fields were very small or negligible. Our results, however, are consistent with earlier incoherent scatter radar observations under very quiet conditions and also with modeling results.