ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-33-371-2015Observation of electron biteout regions below sporadic E layers at polar latitudesLehmacherG. A.glehmac@clemson.eduLarsenM. F.CroskeyC. L.Department of Physics & Astronomy, Clemson University, Clemson, SC, USADepartment of Electrical Engineering, The Pennsylvania State University, University Park, PA, USAG. A. Lehmacher (glehmac@clemson.edu)20March201533337138010July201429January201525February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/33/371/2015/angeo-33-371-2015.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/33/371/2015/angeo-33-371-2015.pdf
The descent of a narrow sporadic E layer near 95 km altitude over Poker
Flat Research Range in Alaska was observed with electron probes on two
consecutive sounding rockets and with incoherent scatter radar during a 2 h period near magnetic midnight. A series of four trimethyl aluminum
chemical releases demonstrated that the Es layer remained just slightly
above the zonal wind node, which was slowly descending due to propagating
long-period gravity waves. The location of the layer is consistent with the
equilibrium position due to combined action of the wind shear and electric
fields. Although the horizontal electric field could not be measured
directly, we estimate that it was ∼2 mV m-1 southward, consistent with
modeling the vertical ion drift, and compatible with extremely quiet
conditions. Both electron probes observed deep biteout regions just below the
Es enhancements, which also descended with the sporadic layers. We discuss
several possibilities for the cause of these depletions; one possibility is
the presence of negatively charged, nanometer-sized mesospheric smoke
particles. Such particles have recently been detected in the upper
mesosphere, but not yet in immediate connection with sporadic E. Our
observations of electron depletions suggest a new process associated with
sporadic E.
Ionosphere (Auroral ionosphere; Ionospheric irregularities) – Meteorology and atmospheric dynamics (Instruments and techniques)Introduction
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=cosI1+ρi2EEB0-UNsinI+ρicosI1+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 cosI is small for high inclination, and the wind alone is less
efficient.
Designations for turbopause payloads with launch times and azimuth (clockwise from north).
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.
Experiment
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 .
Orientation for payload 41.078 at 95 km upleg and downleg. The
angle of attack (angle between roll axis and velocity vector) is critical for
gas flow around instruments and plasma collection.
Four payload trajectories (76, 77, 78, 79) and five PFISR radar
beams (1, 2, 3, 4, 5) projected on a map of central Alaska. The points of
intersection at 95 km altitude on upleg and downleg are marked with a short
dash. The diamonds indicate the launch site, Poker Flat (P), and the camera
sites for TMA observations at Poker Flat, Ft. Yukon (F), and Coldfoot (C).
Magnetometer measurements at Poker Flat for 15–19 February 2009
(day 46–50). The period of radar and rocket observations discussed in this
paper occurred on day 48, indicated by the black bar. Only very small
disturbances can be seen around magnetic midnight at day 48.47.
From left to right: nose-tip probe electron currents (ram, upleg),
CONE electron probe currents (ram, downleg), zonal winds (upleg), and CONE
neutral temperatures (downleg) for missions 41.078 (blue) and 41.079 (red).
Typical wind and temperature errors are indicated by black bars.
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 ∼18h 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.
Observations
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 ∼10km on upleg and about
∼20km on downleg.
Electron densities observed with PFISR. The times of the four rocket
launches (41.076, 78, 79, 77) are marked with the colored ticks at the
bottom.
The current collected by the nose-tip probe (+5V 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 4×10-7A, 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 +5V
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. ).
Top: zonal and meridional wind profiles (solid and dashed lines,
respectively) from all four flights shifted according to the launch time
(41.076 purple, 41.078 blue, 41.079 red, 41.077 yellow). The black bar at the
first profile represents ±50ms-1. The slanted line is a linear
best fit (slope -1.3(±0.3)kmh-1) to the zonal wind node in the
95 km region. Bottom: vertical profiles of electron density from PFISR
(black) and nose-tip probes (blue and red) shifted in time. The scale is
linear and the black bar at the fourth profile represents 1×1010m-3.
Error estimates for PFISR are 1 to 6×109m-3, increasing with altitude. The nose-tip probe profiles are scaled to
match the radar-derived densities in the region below 100 km. The diamonds
mark the strongest local maximum near 95 km, descending in altitude (best fit
slope -0.3(±0.5)kmh-1). The straight line is again the fit to the
wind nodes from the upper panel and is shown for comparison with the electron
density structure.
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 8×109m-3 and layer peak densities are 3
to 4×1010m-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.
DiscussionSporadic E layers
It was first suggested by that, at auroral latitudes (>60∘), sporadic E layers can be formed by the action of the electric
field alone, since the wind shear necessary for the Whitehead mechanism is
less effective (Eq. ), in particular below 110 km altitude.
studied the role of auroral electric fields in
simulations and comparisons with EISCAT observations and found that metallic
sporadic layers can be formed between 90 and 105 km when the electric field
is westward or southward to southeastward. Electric field effects can be
much stronger; for example E/B∼ (5 mVm-1/0.05 mT) ∼100ms-1,
while disturbed conditions can have horizontal fields of 40 mVm-1 or more.
Metallic ion lifetimes are of the order of 10 min at 94 km, which requires
persistent wind shears to maintain a layer, when formed by the wind shear
mechanism.
Similar calculations were presented by and supplemented
with measurements through steering the Sondrestrom radar by which they could
determine the horizontal extent of several Es layers .
refined their analysis and found that a 60 ms-1 tidal
wind requires 2–3 h to form a 2 km thin layer, which is “dumped” near 93 km
altitude. The narrowest layers were generated for a combination of a
strong wind and weak 2 to 5 mVm-1 southward electric fields. They also found
that the shape of the layer is more symmetric under the action of both wind
and electric field. These results based on simulations closely resemble our
observations.
Vertical ion drift as calculated based upon the observed wind shear
from flight 41.078 and different meridional electric fields.
Figure shows the vertical ion drift as estimated from the
observed wind shear for flight 41.078 and Eq. (). The
wind components were rotated from geographic coordinates to geomagnetic
coordinates. We used the measured neutral densities from the CONE ionization
gauge to estimate the ion collision frequency. The dotted blue line shows the
vertical ion drift for the wind profile and zero electrical field. The
vertical ion drifts are very small (10 cms-1) and convergence times for a
1 km
layer are of the order of hours , which requires a
persistent wind shear to form and maintain the layer. Note that the
horizontal wind shear is westward and generates the correct convergence;
however, the condition viz=0 is met near 96 km and not near 95 km.
The additional lines add a northward electric field of -2 (solid), -5
(dashed), and +5mVm-1 (dash-dotted). Apparently, a small southward field is
sufficient to shift the convergence height to 95 km, in agreement with the
data and the simulations by . A northward electric field
would not create a layer.
We have neglected the influence of the vertical wind on the ion drift (see
). This may be justified since the wind pattern is
described well by a large-scale gravity wave, for which the zonal and
vertical wind components are in phase, i.e., in the zonal wind node the
vertical wind is also zero. Figure indicates that the
descent of the layer slows after about 11:30 UT. This could be due to the
increasing collision frequency and larger influence of ambipolar diffusion;
however, beam 2 in Fig. shows enhanced densities until
12:00 UT, the end of the observations.
While did not find such a clear case in their data of
wind shear playing the leading role in the formation of Es, our data
indicate that, under very quiet conditions, a sporadic E layer can be
produced mainly by the wind shear process, but even minimal electrical fields
may be relevant at auroral latitudes.
Electron biteouts
The deep depletions (“biteouts”) just below the sporadic layer are striking
features in the electron current profiles (Fig. ). Just
like the sporadic layers, the biteouts appear to follow the downward motion of
the zonal wind node.
What could be the cause of such biteouts? First we consider an overall
depletion in plasma density, both electrons and ions. The background
molecular ions (O2+, NO+) are assumed to be in equilibrium between
ionization and recombination. It is also commonly assumed that metallic ions
and additional electrons are responsible for the sharply enhanced sporadic
layer, in which fast recombination would reduce the molecular ion density.
This effect has been observed directly by ion mass spectrometers
and by comparing ion and electron density
profiles . However, this cannot explain the spatial
separation of enhanced layer and biteout, since the stronger recombination
occurs within the sporadic layer.
Next we consider the formation of a layer of negatively charged particles.
argued that in the electric field action that
carries positive ions downward, negative dust particles would be carried
upward. However, in our case, wind shear convergence maintaining the positive
ion layer would result in divergence for negative ions. The lack of
additional information, e.g., for ions, particles, or local electric fields,
does not allow us to speculate further as to why the biteout regions appear closely
coupled to the Es layer.
On the other hand, it is now well accepted that a broad distribution of
meteoric “dust” or “smoke” particles exists in the mesosphere, which
are
generated by meteor ablation and subsequent chemical and microphysical
processes .
discusses the importance of neutral dust at the
“dumping”
region for sporadic E around 90 km and estimates the electron attachment rate
to neutral dust particles. and
pointed out the interaction between dust layers, sporadic E, and neutral
metal layers, and that the presence of a charged dust layer would reduce the
electric conductivity parallel to the magnetic field and result in charge
separation and polarization electric fields. find that,
above the atomic oxygen ledge (typically 88–90 km), negatively charged dust
particles of radii <1nm (and which contain silicon) are abundant and
predominant over negative molecular ions.
The nose-tip probe will generally not detect the relatively heavy dust
particles (1000–10 000 amu), since they are being deflected in the
supersonic air flow around the payload rather than attracted by the probe
electric field. This is well known from flights during polar summer, when
dusty ice particles (between 80 and 90 km) give rise to visible noctilucent
clouds and polar mesospheric summer echoes observed by radar. A recent
measurement of associated electron biteouts under polar latitude summer
conditions with the same nose-tip probe experiment is discussed in
. Nosetip probe currents ranged from 10-9A in
the biteout region to 10-7A in the unperturbed background, which
are quite similar to the values presented here but about 10 km lower in
altitude. The payload carried additional instrumentation that found clear
evidence for negatively charged particles in the biteout regions.
present recent rocket observations from December 2010
over Andøya, Norway, which included measurements of negative dust
particles with the ECOMA instrument between 80 and 95 km,
as well as simultaneous electron and ion measurements. Based on observations and
supported by simplified ion chemistry, they conclude that dust layers are
associated with electron deficits.
A third possibility for the low electron current is negative payload
charging, which would lower the collection efficiency of the positively
biased nose-tip probe. DC probes often collect significantly less current than
predicted by Langmuir probe theory . This also applies to the
nose-tip probe as shown for daytime and nighttime conditions
, but the relative density profiles can be normalized
with the help of other techniques e.g.,.
show profiles of a wide sporadic E layer between 95 and
100 km observed by the radio propagation technique, which provides an
absolute measurement of electron density, together with data from a payload
potential monitor for the same polar night flight in December 2010 mentioned
above. The payload potential is generally negative, between -2 and -1V, and
about 0.5 V more negative within the sporadic layer. However,
show that this variation had no significant effect on
the in situ DC probe measurements.
report rocket measurements of a sporadic E layer near 92 km
and suggest that triboelectric payload charging had significantly
decreased the current collected by a DC electron probe when compared to
simultaneous RF impedance probe measurements. In our measurements we do not
observe depressed electron currents in the sporadic E-layer peak.
Unfortunately, our instrumentation did not include dust detectors, ion
probes, or RF electron probes, which would have provided very valuable data
in the biteout regions. However, based on similar observations with the same
probe in the polar summer mesosphere, our data suggest that the biteout
regions below the sporadic E layers were indeed depletions of free
electrons. While more recent experiments find evidence for layering effects
in charged dust particles, our observations show, for the first time, electron
biteouts in connection with sporadic E. The circumstance of very low
ionization may have played a role in our observations, but more comprehensive
measurements are needed to shed light on the connection between dust
particles and sporadic E, which has been a topic of discussion for over 30 years.
Summary and conclusions
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.
Deep electron biteouts below the sporadic E layers at nighttime are a new
observation. By analogy with similar biteouts observed in the polar summer
mesosphere and evidence from other experiments, we suggest that the biteouts
are electron depletions and possibly associated with a layer of negatively
charged dust particles that our probe cannot collect. We find support in
recent simultaneous measurements of negative dust particles and electron
densities, as well as in modeling results showing a significant fraction of negatively
charged dust particles above 90 km. The wind shear theory does not offer an
explanation why the depleted layer seems connected to the Es layer. We
recommend future experiments with the capability to observe electrons, light
ions, and charged dust, with launches done in quiet nighttime conditions to
help in resolving the questions surrounding sporadic E and dust layers.
Acknowledgements
This project was funded by NASA grants NNX07AJ99G (Clemson University),
NNX07AK02G (Penn State University), and NNX08AC57G (University of Alaska
Fairbanks). We thank Craig Heinselman and Mike Nicolls for providing PFISR
electron densities. Topical Editor K. Hosokawa thanks M. Friedrich and two anonymous
referees for their help in evaluating this paper.
References
Axford, W. I.:
The formation and vertical movement of dense ionized layers in the ionosphere,
J. Geophys. Res., 68, 769–779, 1963.Barjatya, A. and Swenson, C. M.:
Observations of Triboelectric Charging Effects on Langmuir-Type Probes in Dusty Plasma, J. Geophys. Res., 111, A10302, 10.1029/2006JA011806, 2006.
Beatty, T. J., Collins, R. L., Gardner, C. S., Hostetler, C. A., Sechrist Jr., C. F., and Tepley, C. A.:
Simultaneous radar and lidar observations of sporadic E and Na layers at Arecibo,
Geophys. Res. Lett., 16, 1019–1022, 1989.
Bristow, W. A. and Watkins, B. J.:
Numerical simulation of the formation of thin ionization layers at high latitudes,
Geophys. Res. Lett., 18, 404–407, 1991.
Bristow, W. A. and Watkins, B. J.:
Incoherent scatter observations of thin ionization layers at Sondrestrom,
J. Atmos. Terr. Phys., 55, 873–894, 1993.
Chimonas, G. and Axford, W. I.:
Vertical movement of temperate zone sporadic E layer,
J. Geophys. Res., 73, 111–117, 1968.Collins, R. L., Lehmacher, G. A., Larsen, M. F., and Mizutani, K.: Estimates of vertical
eddy diffusivity in the upper mesosphere in the presence of a mesospheric
inversion layer, Ann. Geophys., 29, 2019–2029, 10.5194/angeo-29-2019-2011, 2011.Croskey, C. L., Mitchell, J. D., Goldberg, R. A., Blix, T. A., Rapp, M.,
Latteck, R., Friedrich, M., and Smiley, B.: Coordinated investigation of plasma and
neutral density fluctuations and particles during the MaCWAVE/MIDAS summer
2002 program, Geophys. Res. Lett., 31, L24S08, 10.1029/2004GL020169,
2004.Croskey, C. L., Mitchell, J. D., Friedrich, M., Schmidlin, F. J., and Goldberg, R. A.:
In-situ electron and ion measurements and observed gravity wave effects in the polar
mesosphere during the MaCWAVE program, Ann. Geophys., 24, 1267–1278, 10.5194/angeo-24-1267-2006, 2006.
Friedrich, M., Torkar, K. M., Goldberg, R. A., Mitchell, J. D., Croskey, C. L.,
and Lehmacher, G.: Comparison of plasma probes in the lower ionosphere, Proc.
13th ESA Symp. on Rocket and Balloon Programmes and Related Research, land,
Sweden, 26–29 May 1997, ESA SP-397, 381–386, 1997.Friedrich, M., Torkar, K. M., Lehmacher, G. A., Croskey, C. L., Mitchell, J. D., Kudeki, E., and Milla, M.:
Rocket and Incoherent Scatter Radar Common-Volume Electron Measurements of the Equatorial Lower Ionosphere, Geophys. Res. Lett., 33, L08807, 10.1029/2005GL024622, 2006.Friedrich, M., Rapp, M., Blix, T., Hoppe, U.-P., Torkar, K., Robertson, S., Dickson, S.,
and Lynch, K.: Electron loss and meteoric dust in the
mesosphere, Ann. Geophys., 30, 1495–1501, 10.5194/angeo-30-1495-2012, 2012.Friedrich, M., Torkar, K. M., Hoppe, U.-P., Bekkeng, T.-A., Barjatya, A.,
and Rapp, M.: Multi-instrument comparisons of D-region plasma
measurements, Ann. Geophys., 31, 135–144, 10.5194/angeo-31-135-2013, 2013.
Giebeler, J., Lübken, F.-J., and Nägele, M.: CONE – a new sensor for
in situ observations of neutral and plasma density fluctuations, Proc. 11th
ESA Symp. Europ. Rocket Balloon Progr., Montreux, Switzerland, ESA SP-355,
311–318, 1993.
Goldberg, R. A., Lehmacher, G. A., Schmidlin, F. J., Fritts, D. C.,
Mitchell, J. D., Croskey, C. L., Friedrich, M., and Swartz, W. E.: Equatorial dynamics
observed by rocket, radar, and satellite during the CADRE/MALTED campaign. 1.
Programmatics and small-scale fluctuations, J. Geophys. Res., 102, 26179–26190,
1997.Haldoupis, C.: A tutorial review on sporadic E layers, in: Aeronomy of the Earth's
Atmosphere and Ionosphere, edited by: Abdu, M. A.,
Pancheva, D., and Bhattacharyya, A., Springer, Dordrecht, the Netherlands,
381–394, 10.1007/978-94-007-0326-1_29, 2011.
Havnes, O., Trøim, J., Blix, Mortensen, T. W., Nuheim, L. I., Thrane, E., and Tønnesen, T.:
First detection of charged dust particles in the Earth's mesosphere,
J. Geophys. Res., 101, 10839–10848, 1996.
Heinselman, C. J., Thayer, J. P., and Watkins, B. J.:
A high-latitude observation of sporadic sodium and sporadic E-layer formation,
Geophys. Res. Lett., 25, 3059–3062, 1998.
Hunten, D. M., Turco, R. P., and Toon, O. B.:
Smoke and dust particles of meteoric origin in the mesosphere and stratosphere,
J. Atmos. Sci., 37, 1342–1357, 1980.
Kato, S., Aso, T., Horiuchi, T., Nakamura, J., and Matsuoka, T.:
Sporadic-E formation by wind shear, comparison between observation and theory,
Radio Sci., 7, 359–362, 1972.
Kirkwood, S. and Nilsson, H.:
High-latitude sporadic-E and other thin layers – the role of magnetospheric electric fields,
Space Sci. Rev., 91, 579–613, 2000.
Kirkwood, S. and von Zahn, U.:
On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers,
J. Atmos. Terr. Phys, 53, 389–407, 1991.
Kirkwood, S. and von Zahn, U.:
Formation mechanisms for low-altitude Es and their relationship with neutral Fe layers:
Results from the METAL campaign,
J. Geophys. Res., 98, 21549–21561, 1993.Knappmiller, S., Robertson, S, Sternovsky, Z., and Friedrich, M.:
A rocket-borne mass analyzer for charged aerosol particles in the mesosphere,
Rev. Sci. Instr., 79, 104502, 10.1063/1.2999580, 2008.Larsen, M. F.:
Winds and shears in the mesosphere and lower thermosphere: Results from four decades of chemical release wind measurements,
J. Geophys. Res., 107, A8, 10.1029/2001JA000218, 2002.Lehmacher, G. A., Scott, T. D., Larsen, M. F., Bilén, S. G., Croskey, C. L., Mitchell, J. D.,
Rapp, M., Lübken, F.-J., and Collins, R. L.: The Turbopause experiment:
atmospheric stability and turbulent structure spanning the
turbopause altitude, Ann. Geophys., 29, 2327–2339, 10.5194/angeo-29-2327-2011, 2011.Lynch, K. A., Gelinas, L. J., Kelley, M. C., Collins, R. L., Widholm, M., Rau, D., MacDonald, E., Liu, Y., Ulwick, J., and Mace, P.:
Multiple sounding rocket observations of charged dust in the polar winter mesosphere,
J. Geophys. Res., 110, A03302, 10.1029/2004JA010502, 2005.
MacLeod, M. A.:
Sporadic E Theory. I. Collision-Geomagnetic Equilibrium,
J. Atmos. Sci., 23, 96–109, 1966.
Mathews, J. D.:
Some aspects of metallic ion chemistry and dynamics in the mesosphere and thermosphere,
Handbook for MAP, 25, 228–254, 1987.
Mathews, J. D.:
Sporadic E: current views and recent progress,
J. Atmos. Solar-Terr. Phys., 60, 413–435, 1998.Nicolls, M. J. and Heinselman, C. J.:
Three-dimensional measurements of traveling ionospheric disturbances with the Poker Flat Incoherent Scatter Radar, Geophys. Res. Lett., 34, L21104, 10.1029/2007GL031506, 2007.
Nygren, T., Jalonen, L., Oksman, J., and Turunen, T.:
The role of electric field and neutral wind direction in the formation of sporadic E-layers,
J. Atmos. Terr. Phys., 46, 373–381, 1984.
Piel, A., Hirt, M., and Steigies, C. T.:
Plasma diagnostics with Langmuir probes in the equatorial ionosphere: I. the influence of surface contamination, J. Phys. D Appl. Phys., 34, 2643–2649, 2001.Plane, J. M. C., Saunders, R. W., Hedin, J., Stegman, J., Khaplanov, M.,
Gumbel, J., Lynch, K. A., Bracikowski, P. J., Gelinas, L. J., Friedrich, M.,
Blindheim, S., Gausa, M., and Williams, B. P.: A combined rocket-borne and
ground-based study of the sodium layer and charged dust in the upper
mesosphere, J. Atmos. Solar-Terr. Phys., 118, 151–160,
10.1016/j.jastp.2013.11.008, 2014.Rapp, M., Hedin, J., Strelnikova, I., Friedrich, M., Gumbel, J., and Lübken, F.-J.:
Observations of positively charged nanoparticles in the nighttime polar mesosphere,
Geophys. Res. Lett., 32, L23821, 10.1029/2005GL024676, 2005.Roddy, P. A., Earle, G. D., Swenson, C. M., Carlson, G. G., and Bullett, T.
W.: Relative concentrations of molecular and metallic ions in midlatitude
intermediate and sporadic-E layers, Geophys. Res. Lett., 31, L19807, 10.1029/2004GL020604, 2004.Smith, L. G. and Mechtly, E. A.:
Rocket observations of sporadic-E layers,
Radio Sci., 7, 367–376, 1972.
Smith, L. G. and Miller, K. L.:
Sporadic-E layers and unstable wind shears,
J. Atmos. Terr. Phys., 42, 45–50, 1980.
Turunen, T., Nygren, T., and Huuskonen, A.:
Nocturnal high-latitude E-region in winter during extremely quiet conditions
J. Atmos Terr. Solar-Phys., 55, 783–795, 1993.
Ulwick, J. C., Baker, K. D., Kelley, M. C., Balsley, B. B., and Ecklund, W. L.:
Comparison of simultaneous MST radar and electron density probe measurements during STATE,
J. Geophys. Res., 93, 6989–7000, 1988.Wakabayashi, M. and Ono, T.: Multi-layer structure of mid-latitude sporadic-E
observed during the SEEK-2 campaign, Ann. Geophys., 23, 2347–2355, 10.5194/angeo-23-2347-2005, 2005.
Whitehead, J. D.:
The formation of the sporadic E layer in the temperate zones,
J. Atmos. Solar-Terr. Phys., 20, 49–58, 1961.Whitehead, J. D.:
Production and prediction of sporadic E,
Rev. Geophys. Space Sci., 8, 65–144, 1970.Williams, B. P., Croskey, C. L., She, C. Y., Mitchell, J. D., and Goldberg, R. A.:
Sporadic sodium and E layers observed during the summer
2002 MaCWAVE/MIDAS rocket campaign, Ann. Geophys., 24, 1257–1266, 10.5194/angeo-24-1257-2006, 2006.
Zbinden, P. A., Hidalgo, M. A., Eberhardt, P., and Geiss, J.: Mass spectrometer
measurements of the positive ion composition in the D- and E-regions of the
ionosphere, Planet. Space Sci., 23, 12, 1621–1642, 1975.