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Morphology of evening sector aurorae in λ557.7-nm Doppler temperatures


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An all-sky scanning Fabry-Pérot spectrometer was used to observe temperatures of auroral OI (557.7-nm) emissions over Poker Flat, Alaska (65.12N, 147.43W). The sudden temporal and spatial changes in Doppler temperatures observed are likely owing to the emission height changing as a response to variations in the characteristic energy of the precipitating electron population. Three cases were analyzed: (1) A Doppler temperature drop (~200 K) over the entire sky occurred immediately after an auroral brightening; the temperature remained lower after the auroral intensity had resumed its quiescent levels. (2) A local increase of Doppler temperature, colocated with a weak auroral arc, occurred 25 minutes before a westward propagating substorm onset. When the auroral luminosity suddenly increased the Doppler temperature had a sharp decrease. (3) The region inside a loop-like auroral arc showed elevated Doppler temperature relative to that of the arc itself. Auroral fading prior to onset was accompanied by increased temperatures.
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Morphology of evening sector aurorae in λ557.7-nm Doppler
J. M. Holmes1, M. Conde2, C. Deehr, and D. Lummerzheim
Geophysical Institute, University of Alaska Fairbanks
An all-sky scanning Fabry-Pérot spectrometer was used to observe temperatures of auroral
OI(557.7-nm) emissions over Poker Flat. The sudden temporal and spatial changes in Doppler
temperatures that were observed are likely due to the emission height changing as a response to
variations in the characteristic energy of the precipitating electron population. Three cases were
analyzed: (1) A Doppler temperature drop (~200K) over the entire sky occurred immediately
after an auroral brightening; the temperature remained lower after the auroral intensity had
resumed its quiescent levels. (2) A local increase of Doppler temperature, colocated with a
weak auroral arc, occurred 25 minutes before a westward propagating substorm onset. When
the auroral luminosity suddenly increased the Doppler temperature had a sharp decrease. (3)
The region inside a loop like auroral arc showed elevated Doppler temperature relative to that
of the arc itself. Auroral fading just prior to onset was accompanied by increased temperatures.
1. Introduction
Beginning with the original interferometric
measurements of the auroral λ557.7-nm line by Babcock
[1923], there has been considerable interest in the
measurement of auroral and airglow emissions and their
applications to upper atmospheric physics. Determination
of temperatures from Doppler-broadened emissions,
including both atomic [Armstrong, 1958; Wark, 1959;
Nilson and Shepherd, 1960] and molecular species
[Vegard, 1932; Lytle and Hunten, 1960], contributed to the
early quantification of thermospheric and mesospheric
temperatures. A discussion of the applications of optical
Doppler measurements for this era is given by Hunten
More recently, interferometric techniques have been
used with an emphasis on determining neutral winds from
observed Doppler shifts [Hernandez and Killeen, 1988].
Investigations of thermospheric dynamics have been made
using these techniques ranging from small-scale studies of
specific phenomena [Rees, 1984b] to global-scale
observations [Killeen et al., 1988].
With the advent of the ground-based all-sky Fabry-Pérot
spectrometer (ASI-FPS) [Rees and Greenaway, 1983; Rees
et al., 1984a; Biondi et al., 1995; Nakajima et al., 1995;
Ishii et al., 1997; Conde and Smith, 1995, 1997], it became
possible to record Doppler spectra in many locations across
the sky, as opposed to previous narrow-field Fabry-Pérot
spectrometers which routinely observed only the zenith
plus the four cardinal azimuths. This novel technique that
provides modest spatial resolution is now used to explore
the relationship between auroral intensity and Doppler
The auroral λ557.7-nm [O I] emission is a result of the
metastable transition between the excited 1D and 1S states
of atomic oxygen, the bulk of which is produced in the
height range of 100-150 km. The state has a lifetime of
0.74s, which provides adequate time for thermalization in
this altitude range. Thus, it can be tacitly assumed that the
emitting population is in thermal equilibrium with the
neutral atmosphere. Since the thermosphere has a positive
temperature gradient, higher energy electron precipitation,
which penetrates farther into the lower thermosphere,
produces more intense aurora with lower temperatures as
measured by all-sky instruments such as the Scanning
Doppler Imager [Rees, 1989].
This inverse relationship between Doppler temperature
and auroral intensity has long been known [Størmer, 1955],
and is known to persist in spite of increased localized Joule
heating [Ishii et al., 2001]. Narrow-field interferometric
measurements of auroral Doppler temperatures, when
compared with intensities, show an identical relationship,
although deviations from simple inverse-proportionality are
observed [Hilliard and Shepherd, 1966]. Owing to the
known relations between observed temperatures and
intensities, optical temperature data, in the absence of
auxiliary height information, are of limited use in
estimating the rate of auroral energy deposition and Joule
heating [Turgeon and Shepherd, 1962].
2. Instrumentation
The Poker Flat SDI makes measurements of the λ557.7-
nm [O I] emission using an imaging Fabry-Pérot
capacitance-stabilized étalon of 20-mm air-spaced gap and
105-mm working aperture. The all-sky optics of the
instrument have a 140º field of view. The imager is unique
in that unlike other imaging FPS instruments, the SDI
acquires spectra by repeatedly scanning the Fabry-Pérot
étalon plate separation through one order of interference,
producing spectra as a function of etalon gap (as opposed
1Now at the University Centre in Svalbard, Longyearbyen,
2Now at Latrobe University, Melbourne, Australia
Figure 1. Example SDI λ557.7-nm zone map with
fitted spectra (white). Exposure time: 1 minute.
Background shading depicts zone boundaries.
to the individual fringe shape) [Conde et al., 2001a]. This
allows the acquisition of images and spectra which are not
distorted by temporal and spatial brightness fluctuations in
the aurora during one scan [Conde and Smith, 1997].
To produce high signal-to-noise spectra with integration
times on the order of minutes, the SDI analysis software
divides images into arbitrary “zones”, the overall signal in
each zone yielding an individual spectrum. Figure 1 shows
the zones and their recorded spectra for a one-minute
integration. By fitting each spectrum with a suitable model
profile (in this case, a Gaussian emission function
convolved with a measured instrument function), the
Doppler broadening, Doppler shift and emission and
background brightnesses are determined. Doppler shifts
provide line-of-sight winds, while Doppler broadening,
when calibrated, returns the temperature of the emitting
For the cases analyzed, the SDI is configured to group
signal into 67 zones, thereby providing modest spatial
resolution without resorting to longer integration times
unsuitable to investigate the desired temporal auroral
structure. In order to ensure consistent uncertainties and
signal levels, the integration time is automatically adjusted
during observations according to signal brightness, with
exposure times varying from about 1 minute for bright and
7-8 minutes for very weak aurora.
Uncertainties in calculated temperatures vary depending
on auroral intensities and also the location of the zone
relative to the zenith, but are typically between 5 and 10
degrees Kelvin under bright auroral conditions. Figure 2(a,
b, c) are histograms of temperature uncertainty and their
variation with zone number (zone numbers increase with
zenith angle). Also, Figure 2(d) shows the decrease of
temperature uncertainty with auroral intensity. A
description of the treatment and determination of
uncertainties can be found in Conde [2001b]; a detailed
discussion of instrumental particulars can be found in
Conde and Smith [1995].
3. Observations
Three cases were chosen to illustrate different types of
dynamics of the λ557.7-nm optical temperature relative to
the motions of the visible aurora. All cases display
evening sector discrete aurora and/or substorm activity.
Although measurement of temperature derived from the
spectral width of auroral emissions can be used in part for
identifying Joule and particle heating, care must be taken
since it is well known that the height of emission (and
therefore the temperature in the ideal thermosphere) varies
primarily by changes in the energy spectrum of the particle
precipitation [Sica et al., 1996].
3.1 Case 1: December 2, 2002
The first case was observed on December 2, 2002.
Figure 3 shows the SDI data in an image sequence of
temperature maps. For reference, the Geophysical Institute
white light all-sky camera is shown above the maps.
Since the previous brightening event at 0844 UT (not
shown), a gradual increase in measured temperature
occurred, starting near zenith then nearly filling the entire
field of view of the instrument at about 0930 UT. This is
depicted in the first two images in Figure 3, at 0944 and
0947 UT. It is interesting to note that there was little or no
change in the measured brightness during this period of
The westward-propagating brightening event at 0949
UT displays an immediate temperature drop of roughly
200K within the intensified region. Note that after the
intensity enhancement had passed (either dimming
overhead or advecting out of the instrument’s field of
view), the lower temperature present during the event
persisted for many tens of minutes thereafter.
Figure 2. Temperature uncertainty histograms calculated
for 22 March 2003: all zones (a), zenith zone (b), edge
zone (c). A scatter plot showing temperature uncertainty
versus intensity is shown in (d).
3.2 Case 2: March 22, 2003
The second case was observed on March 22, 2002. The
sequence in Figure 4 shows a band of discrete arcs near
zenith accompanied by a relatively high temperature
(shown in yellow and red). Preceding the beginning of the
sequence, a thin arc of both increasing temperature and
brightness was observed slightly equatorward of the
brightening arc shown in the first frame in the figure.
Immediately after the emission intensity began to increase
at approximately 0852 UT, a temperature drop occurred
only in the vicinity of the brightening features, i.e. from
zenith eastward toward the edge of the temperature map
(0856 – 0858 UT). This case illustrates large gradients and
discrete features in the temperature data evolving in only
several minutes.
3.3 Case 3: March 24, 2003
The final and most interesting case occurred on March
24, 2002. Figure 5 shows the time evolution of a
westward-traveling arc. The SDI measured low to
moderate temperatures in the vicinity of the discrete loop-
like band, which is expected based on previous
measurements However, inside the band, a much warmer
region developed (0852-0857 UT) shortly before the band
broke up at 0859UT, filling almost the entire field of view
with aurora.
Figure 6 depicts the elevation of neutral temperature in
the zenith zone during the above time period. The ratio of
the λ630.0-nm [O I] emission to the λ427.8-nm (N2+)
emission, as measured by the Meridian Scanning
Photometer (MSP) at Poker Flat, was used to make an
estimate of the characteristic energy of deposited auroral
electrons in the magnetic zenith using a technique
Figure 4. A time series depicting an auroral brightening event and associated changes in the λ557.7-nm
optical temperature for 22 March 2003 from 0850 to 0909UT.
Figure 3. An image sequence depicting an auroral brightening event and associated changes in the λ557.7-
nm optical temperature for 2 December 2002 from 0944 to 0958 UT. The upper row shows the Poker Flat
All-sky Camera and the lower row shows the temperature maps produced by the Scanning Doppler Imager
Figure 5. A time series showing the WTS event on 24 March 2003 from 0848 to 0900UT. Note that as the
loop-like feature moves westward, the region of elevated optical temperature inside it appears to follow the
described in Lummerzheim, et al. [1990]. The figure
indicates a decreased characteristic energy, which follows
if it is assumed that the increase in Doppler temperature
resulted, at least partly, from an increase in the emission
4. Discussion
There exist commonalities between all three cases
discussed in this study. First, when considering discrete
arcs, the combined SDI and ASC data clearly show the
commonly accepted inverse relationship between auroral
intensity and temperature.
Next, as mentioned earlier, the data frequently exhibit
large temperature variations over very short spatial and
temporal scales. Several median temperature time series,
taken over the entire field of view, have shown changes in
temperature of hundreds of degrees Kelvin in a matter of
only several minutes. For the first case of December 2,
2002, shortly after 0800 UT (not shown), a near all-sky
brightening event occurred and the measured temperature
from the SDI dropped nearly 200K. Since the temperature
decreased, it is evident that auroral heating or possibly
other in situ thermospheric dynamics are not primarily
responsible for the bulk of the temperature change; the
drop occurred from a sudden lowering of the altitude of
auroral energy deposition, which corresponds to an
increase in characteristic energy. However, neutral heating
is almost certainly taking place, since it draws more than
50% of the power associated with auroral particle
precipitation [Rees et. al., 1983]. The presence of heating
in the emitting region causes the estimated emission height
to be overestimated (when simply converting from
temperature using a model such as MSIS); the actual height
of emission could in fact be lower depending on the
amount of heating.
It is also noteworthy that for the case of 0848-0900UT
on 24 March 2003, the portion of the loop-like feature
closest to the zenith decreased in intensity from 15 kR at
0854 UT to 3 kR immediately before the onset westward-
traveling surge. The λ427.8-nm channel of the MSP
revealed a similar reduction while the λ630.0-nm channel
remained approximately constant. This sudden increase in
the red/blue ratio and corresponding decrease in
characteristic energy is indicative of “auroral fading” as
described by Pellinen and Heikkila [1978]. Although the
softening of the auroral precipitation was not accompanied
by a significant increase in number flux, and the
equatorward hydrogen arc detected by the MSP λ486.1-nm
(Hβ) channel did not move significantly during this
“fading” period, the region of enhanced λ557.7-nm
Doppler temperature, located just equatorward of this arc,
increased in temperature by at least 100K in only several
minutes prior to surge onset.
5. Conclusions
The case studies presented here are only interpreted
qualitatively; they are used to illustrate some of the
phenomenology of the neutral thermosphere Doppler
temperature in the auroral energy deposition region and its
relation to optical auroral emissions. In addition, the
Doppler temperatures recorded using this imaging
instrument show variations with respect to auroral activity
that are consistent with older non-imaging instruments.
Although temperature data from the Scanning Doppler
Imager alone cannot be used to deduce the sources of the
dynamics observed, it is nonetheless an indispensable
instrument for the observation of spatially resolved
phenomena in the vicinity of auroral arcs and the
characterization of neutral atmospheric properties in the
context of auroral effects on the thermosphere.
Acknowledgments. This work was supported by a joint NASA
and NSF TIMED-CEDAR grant, number NAG5-10069. We
would also like to thank Brian Lawson for the operation of the
Poker Flat All-sky Camera and Meridian Scanning Photometer.
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... A possible issue remains, due to a combination of altitude variations in the green-line emission profile as a result of changing characteristic energy of the aurora (Holmes et al., 2005;Kaeppler et al., 2015), coupled with strong vertical shear of the horizontal wind field, that is known to be a common feature of the thermosphere at E-region heights (Larsen, 2002). One possible effect of spatially and temporally varying emission altitudes within regions of vertical shear could be to introduce spurious structures into the horizontal wind fields recovered by this analysis. ...
... The possibility remains that some portion of the structure that manifested in wind fields derived from the green-line spectra is a consequence of a combination of altitude variations in the green-line emission profile as a result of changing characteristic energy of the aurora (Holmes et al., 2005;Kaeppler et al., 2015), coupled with strong vertical shear that is common at these heights. Again, future work will address this possibility, by accounting for height variations using the Doppler temperature derived from the 558 nm spectra as a proxy for estimating the emission altitude. ...
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Simple scaling analysis of terms in the Navier‐Stokes momentum equation for Earth's atmosphere suggests that winds at heights above 120 km should be smooth and laminar, with little spatial variation over horizontal scale lengths smaller than several hundred kilometers. However, there is increasing evidence that this traditional understanding may fail to account for several important processes, including both waves and small‐scale ion‐neutral momentum coupling. Here, we examine the thermospheric neutral wind field over Alaska in unprecedented detail using observations from an array of four ground‐based all‐sky imaging Fabry‐Perot interferometers, processed using a new geophysical inverse algorithm, to derive high‐resolution maps of all three wind components, with a temporal cadence of 30 seconds. The reconstructed high‐resolution neutral winds showed synoptic‐scale agreement with prior observations and previously validated techniques, with all results exhibiting behavior in agreement with basic physics. However, stacked time‐series plots of vector wind components reveal significantly more spatial and temporal structure than previously reported. In particular, the observed responses included complex wave‐like behavior and highly geographically variable vertical winds. Local flow features were observed at spatial scales as small as 100 km at times, with temporal scales as short as a few tens of minutes. Instances of close spatial and temporal correlations were observed between the wind fields reconstructed from green‐line spectra and ionospheric flows observed independently by SuperDARN.
... Among other emissions, the SDI measures the 557.7 nm emission in order to infer the E region neutral winds and temperatures. Precipitating electrons with higher characteristic energies penetrate deeper into the thermosphere, causing a decrease in the neutral temperature estimated from the line width of 557.7 nm emission [Rees, 1989;Holmes et al., 2005;Hecht et al., 2006]. This effect is observed in the SDI and can be used to determine the neutral temperature in 115 azimuth/elevation sectors in the sky, thus enabling estimates of the characteristic energy over the whole sky. ...
... Therefore, changes in the altitude of the auroral precipitation, corresponding to changes in the characteristic energy of the precipitating electron flux, result in nonlinear neutral temperature response. A lower observed thermospheric temperature can be caused by electrons with a high characteristic energy, while a higher observed thermospheric temperature can be caused by precipitating electrons with a lower characteristic energy [Rees, 1989;Holmes et al., 2005;Hecht et al., 2006]. A key feature in this technique is the use of a neutral atmosphere model, such as Mass Spectrometer Incoherent Scatter (MSIS) [Picone et al., 2002] to specify the variation of the neutral temperature with altitude. ...
We present three case studies that examine optical and radar methods for specifying precipitating auroral flux parameters and conductances. Three events were chosen corresponding to moderate nonsubstorm auroral activity with 557.7 nm intensities greater than 1kR. A technique that directly fits the electron number density from a forward electron transport model to alternating code incoherent scatter radar data is presented. A method for determining characteristic energy using neutral temperature observations is compared against estimates from the incoherent scatter radar. These techniques are focused on line-of-sight observations that are aligned with the local geomagnetic field. Good agreement is found between the optical and incoherent scatter radar methods for estimates of the average energy, energy flux, and conductances. The Pedersen conductance predicted by Robinson et al. (1987) is in very good agreement with estimates calculated from the incoherent scatter radar observations. However, we present an updated form of the relation by Robinson et al. (1987), ΣH/ΣP=0.57〈E〉0.53, which was found to be more consistent with the incoherent scatter radar observations. These results are limited to similar auroral configurations as in these case studies. Case studies are presented that quantify auroral electron flux parameters and conductance estimates which can be used to specify the magnitude of energy dissipated within the ionosphere resulting from magnetospheric driving.
... This depth depends, in turn, on the characteristic energy of electron precipitation. It is possible to study the variation of this characteristic energy and resulting height variations in the 558 nm emission layer using Scanning Doppler Imager (SDI) instruments (e.g., Holmes et al., 2005;Kaeppler et al., 2015). The SDIs are ground-based all-sky imaging Fabry-Perot spectrometers that were first introduced by Conde and Smith (1997). ...
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Few remote sensing or in‐situ techniques can measure winds in Earth's thermosphere between altitudes of 120 and 200 km. One possible approach within this region uses Doppler spectroscopy of the optical emission from atomic oxygen at 558 nm, although historical approaches have been hindered in the auroral zone because the emission altitude varies dramatically, both across the sky and over time, as a result of changing characteristic energy of auroral precipitation. Thus, a new approach is presented that instead uses this variation as an advantage, to resolve height profiles of the horizontal wind. Emission heights are estimated using the Doppler temperature derived from the 558 nm emission. During periods when the resulting estimates span a wide enough height interval, it is possible to use low order polynomial functions of altitude to model the Doppler shifts observed across the sky and over time, and thus reconstruct height profiles of the horizontal wind components. The technique introduced here is shown to work well provided there are no strong horizontal gradients in the wind field. Conditions satisfying these caveats do occur frequently and the resulting wind profiles validate well when compared to absolute in‐situ wind measurements from a rocket‐borne chemical release. While both the optical and chemical tracer techniques agreed with each other, they did not agree with the HWM‐14 horizontal wind model. Applying this technique to wind measurements near the geomagnetic cusp footprint indicated that cusp‐region forcing did not penetrate to atmospheric heights of 240 km or lower.
... The airglow of the 557.7 nm atomic oxygen line occurs near the mesopause region, a little higher than the hydroxyl airglow. There exists a potential for temperature and wind observations by analyzing the 557.7 nm line Doppler broadening and shifting using aeronomic Fabry-Pérot interferometers that operate in high latitudes [7,8]. However, no comparison has been conducted with satellite data for such devices. ...
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... Since vertical temperature gradients in the lower thermosphere are generally steep, there are difficulties in quantitative analysis of the neutral temperature derived from the auroral green line measurement by the FPI. However, the green line temperature measurement is potentially useful in estimating an auroral energy deposition (Holmes et al., 2005). ...
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The rotational temperature and number density of molecular nitrogen (N2) in the lower thermosphere were measured by the N2 temperature instrument onboard the S-310-35 sounding rocket, which was launched from Andøya at 0:33 UT on 13 December 2004, during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. The rotational temperature measured at altitudes between 95 and 140 km, which is expected to be equal to neutral temperature, is much higher than neutral temperature from the Mass Spectrometer Incoherent Scatter (MSIS) model. Neutral temperatures in the lower thermosphere were observed using the auroral green line at 557.7 nm by two Fabry-Perot Interferometers (FPIs) at Skibotn and the Kiruna Esrange Optical Platform System site. The neutral temperatures derived from the look directions closest to the rocket correspond to the rotational temperature measured at an altitude of 120 km. In addition, a combination of the all-sky camera images at 557.7 nm observed at two stations, Kilpisjärvi and Muonio, suggests that the effective altitude of the auroral arcs at the time of the launch is about 120 km. The FPI temperature observations are consistent with the in situ rocket observations rather than the MSIS model.
... Note that the 557.7 nm Doppler temperature dropped noticeably during the passage of the bright auroras. This we associated with a drop in the 557.7 nm emission altitude [e.g., Holmes et al., 2005], and it is consistent with the ionosonde data. Figures 1e and 1o show the optical intensity ratio I 5577 / I 6300 (dotted line) from Figures 1b and 1l and the CTIP modeled height of the 557.7 nm optical emission (green dashed line) inferred from the SDI temperature observations in Figures 1d and 1n. ...
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We report the first observations of E-region neutral wind fields and their interaction with auroral arcs at meso-scale spatial resolution during geomagnetically quiet conditions at Mawson, Antarctica. This was achieved by using a scanning Doppler imager, which can observe thermospheric neutral line-of-sight winds and temperatures simultaneously over a wide field of view. In two cases, the background E-region wind field was perpendicular to an auroral arc, which when it appeared caused the wind direction within ~50 km of the arc to rotate parallel along the arc, reverting to the background flow direction when the arc disappeared. This was observed under both westward and eastward ion convection. The wind rotations occurred within 7-16 min. In another case, as an auroral arc propagated from the horizon toward the local zenith, the background E-region wind field became significantly weaker but remained unaffected where the arc had not passed through. We demonstrate through modelling that these effects cannot be explained by height changes in the emission layer. The most likely explanation seems to be greatly enhanced ion drag associated with the increased plasma density and localised ionospheric electric field associated with auroral arcs. In all cases, the F-region neutral wind appeared only slightly affected by the auroral arc, although its presence is clear in the data.
... [19] The peak altitude of the auroral OI 557.7-nm emission changes in the range of $105-140 km, depending on the energy of precipitating electrons [e.g., Holmes et al., 2005]. Since FPDIS observes neutral winds and temperatures around the emission peak altitude, apparent variations would occur if wind shear and temperature gradients existed in the vertical direction and the altitude of the emission peak changed. ...
It is important to investigate the small-scale dynamics of thermospheric neutrals and ionospheric plasmas at high latitudes since there are local and temporal variations in energy input and transfer processes associated with auroral activities. To clarify these ionosphere-thermosphere interactions in the auroral E region, we examined neutral winds and plasma motions obtained by coordinated Fabry-Perot imager and VHF radar observations at Syowa Station, Antarctica. From case studies on 11 and 12 September 1996, we found a good correlation between temporal variation (period < 1 hr) of E region neutral winds measured by the Fabry-Perot imager and the Doppler velocities of plasmas obtained from the VHF radar echoes. On the other hand, the absolute mean values of the neutral winds were around zero whereas the plasma Doppler velocities were 350-400 m/s, showing a significant discrepancy. Considering that the neutral-ion and ion-neutral momentum transfer collision frequencies in the E region are ˜10-6 Hz and ˜102 Hz, respectively, these good correlations are probably due to ion motions driven by neutral drag force, suggesting a strong coupling between neutrals and ions in the E region. The observed correlation and discrepancy between the radar Doppler velocity and the neutral wind velocity are consistent with the linear theory of E region irregularities that shows the radar Doppler velocity is the sum of two components; one is the E × B drift speed, and another is proportional to the neutral wind velocity.
... With the large vertical winds, large temperature enhancements of about 200 K in the lower thermosphere were also observed. However, recent FPI observations show that a correlation between the vertical winds in the lower and upper atmosphere does not always exist [Ishii et al., 1999 [Ishii et al., , 2001 [Ishii et al., , 2004 Kosch et al., 2000] and that an apparently inverse relationship between the estimated temperature and auroral intensity is explained by the tendency of the 557.7 nm emission to come from lower heights during bright aurora [Ishii et al., 2001; Holmes et al., 2005]. Although FPIs are valuable tools for simultaneously measuring neu- JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ...
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A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andøya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromsø. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70-140 K at 110-140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s-1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.
Neutral winds are a key factor in the dynamics of the ionosphere-thermosphere system. Previous observations have shown that neutral and ion flows are strongly coupled during periods of auroral activity when ion drag forcing can become the dominant force driving neutral wind flow. This is primarily due to increases in ion density due to enhanced particle precipitation as well as associated increases the strength of the electric fields that drive ion motions. Due to this strong coupling, numerical simulations of neutral dynamics have difficulty reproducing neutral wind observations when they are driven by modeled precipitation and modeled convection. It is therefore desirable whenever possible to have concurrent coincident measurements of auroral precipitation and ion convection. Recent advancements in high-resolution fitting of Super Dual Auroral Radar Network ion convection data have enabled the generation of steady maps of ion drifts over Alaska, coinciding with several optics sites. The Super Dual Auroral Radar Network measurements are compared with scanning Doppler imager neutral wind measurements at similar altitude, providing direct comparisons of ion and neutral velocities over a wide field and for long periods throughout the night. Also present are a digital all-sky imager and a meridian spectrograph, both of which provide measurements of auroral intensity on several wavelengths. In this study, we combine these data sets to present three case studies that show significant correlation between increases in F region precipitation and enhancements in ion-neutral coupling in the evening sector. We investigate the time scales over which the coupling takes place and compare our findings to previous measurements.
During March–April 2011 a campaign of coordinated observations was undertaken between the Poker Flat Advanced Modular Incoherent Scatter Radar and the Poker Flat Scanning Doppler Imager. These instruments provide horizontally resolved maps of plasma and neutral parameters in Earth's thermosphere. We report on data collected during the campaign, and use these data to investigate two key aspects of ion–neutral coupling, namely Joule heating and the ion–neutral collision frequency. Volumetric Joule heating rates were often well correlated with measured ion temperature enhancements. The contribution of the neutral wind dynamo to the observed heating rates was positive when the absolute horizontal magnetic field perturbation (|ΔH||ΔH|) was less than approximately 40 nT, and negative above that level. The total momentum–transfer ion–neutral collision frequency was estimated to be 1.02−0.152+0.179s−1 at an altitude of 260 km, which, for a neutral composition of 75% atomic oxygen, yielded an estimate of the O+–O collision frequency of 0.766−0.114+0.134s−1.
A numerical procedure is presented for analysis of backscatter spectra recorded by an atmospheric Doppler lidar experiment. The recorded spectra are modelled by an analytic function of five parameters. The procedure solves for the set of model parameters that yields the best least-squares fit between the model and the data. The model is formulated to allow for Doppler shifted and broadened scattering from both aerosols and from multiple molecular species, for a continuum background, and for the effects of instrumental broadening. All five model parameters are fitted in the signal domain. Three parameters are fitted by a direct analytic solution and two by numerical searching. If required, one or more parameters may be held fixed at externally-supplied values, while the remainder are fitted accordingly. The method's performance was examined by applying it to numerically simulated test spectra, for several example lidar configurations. Some implications of these results for the design of actual lidar experiments are discussed.
Observations of aurora at Saskatoon using a high-resolution photoelectric Fabry-Perot spectrometer have been continued. The results of 179 temperature measurements obtained from the Doppler widths of the atomic oxygen line at 5577 Å are presented. The values vary from 150 to 1140°K, with probable errors of about 30°K for the low temperatures and 45°K for the high temperatures. Ten temperatures were obtained from the OI line at 6300 Å, giving values from 1000 to 1900°K, each having a probable error of about 135°K. For the OI 5577 Å line results, the mean temperature for 1960 was close to 400°K, about 100°K higher than the mean for 1961. This may indicate a variation of auroral type, height, or temperature associated with the 11-yr solar cycle. Two other types of temperature variations with time were observed on a few occasions; changes over intervals of a few minutes, and changes during the course of an evening. Three measurements of temperature gradient, made by stepping the field of view upward across auroral forms, gave values of 5.0, 4.2 and 4.0°K/km. It is concluded that continued measurements of Doppler temperature can be useful, but would be much more so if heights of the emitting regions were determined simultaneously.
Simultaneous measurements of global-scale auroral luminosity distributions and neutral winds over the northern (winter) polar cap have been obtained using instrumentation on the Dynamics Explorers 1 and 2 spacecraft. Several examples of these simultaneous measurements are presented to illustrate the relationship between the circulation of neutral air in the high-latitude F region and the spatial distribution of the aurora. The initial study of these data indicates (1) that a definite correlation exists between boundaries in the neutral wind field and the location of the auroral oval with large-scale features of the neutral circulation tracking the substorm-dependent expansion and contraction of the auroral oval; (2) that the influence of ion drag from sunward convecting ions can extend to latitudes much lower than those normally associated with the auroral oval; (3) that the sunward neutral flow associated with the auroral oval in the dusk sector is in general, more pronounced than that associated with the dawn sector and that this asymmetry is ascribed to the different effect of the Coriolis force in the two sectors; (4) that the flow patterns for neutrals and ions within the geomagnetic polar cap are generally asymmetric with respect to the noon-midnight meridian, an effect considered to be controlled by the orientation of the interplanetary magnetic field; (5) that the region of the polar cusp and the apparent midday gap, or reduction in luminosities observed in the VUV wavelength auroral images near local noon, is closely associated with a large, antisunward surge in the neutral wind; and (6) that the morphology of the ion drag force in the polar regions is considerably more complex, even for very quiet geomagnetic conditions, than that computed by the National Center for Atmospheric Research thermospheric general circulation model for the steady state.
The physical and chemical processes responsible for neutral gas heating resulting from energetic electron bombardment of the atmosphere are quantitatively described. The neutral heating efficiency, defined as the local heating per unit energy deposition, is evaluated for a range of typical auroral electron spectra, and it is shown that the altitude profile of the efficiency is independent of the electron spectrum. The efficiency is sensitive to the neutral atmosphere (composition, density, temperature) but differences can be minimized by adopting a pressure coordinate in place of the altitude. Over half the energy input rate associated with energetic particle precipitation goes into neutral heating, while less than 5 percent leads to ion heating. The coupling between particle precipitation and joule heating is discussed, and a parameterized Pedersen conductance curve is presented.
A new all-sky imaging Fabry-Perot (ASIFP) spectrometer has been developed for ground-based mapping of upper atmospheric wind and temperature fields in the auroral zone. Although several other ASIFP spectrometers exist for atmospheric studies [Rees et al., 1984; Sekar et al., 1993; Biondi et al., 1995] these instruments have all operated with etalons of fixed optical gap, a method potentially subject to errors in the presence of auroral intensity gradients. In this instrument the etalon plate spacing is scanned periodically over one order of interference and each photon detected is assigned to a wavelength interval which is determined from both its arrival location on the detector and the etalon plate spacing prevailing at the detection time. Spectra accumulated this way are not distorted by spatial intensity gradients. Preliminary λ630 nm observations were made during the winter of 1994/95 from Poker Flat Research Range, Alaska. To illustrate some of the features we have observed in this study we present line-of-sight wind estimates derived for the night of December 7, 1994. The background wind matches averages presented previously by Sica et al. [1986] and is consistent with winds driven principally by momentum deposition from ionospheric plasma convection through ion-drag. Smaller scale curvature and divergence features are also discernable and are discussed.
Characteristics of vertical winds in the polar thermospheric region were examined using data sets generated with two types of Fabry-Perot interferometers at Poker Flat, Alaska (65.11°N, 147.42°W). The Communications Research Laboratory Fabry-Perot Interferometer (CRLFPI) simultaneously observed the O I 557.7 nm and O I 630.0 nm emissions, whereas the Geophysical Institute Scanning Doppler-Imaging Interferometer (GI-SDI) observed the O I 630.0 nm emission. The height of the O I 557.7 nm and O I 630.0 nm emissions were 100-140 and 200-240 km, respectively. The data were obtained from October 1998 to February 1999, and our findings were as follows: (1) Observations of the O I 630.0 nm emission showed that upward (downward) vertical winds were often present when bright aurora existed equatorward (poleward) of the observatory. This is consistent with previous studies [Crickmore et al., 1991; Innis et al., 1996, 1997]. (2) Comparison of vertical winds estimated from two different wavelengths (557.7 and 630.0 nm) showed that vertical winds were often observed simultaneously at both wavelengths, as reported by Price et al. [1995]. However, the vertical winds observed at different heights sometimes had different features when thin but bright aurora passed over the observatory. A similar observation was reported by Ishii et al. [1999]. (3) Vertical winds were often observed along with divergence and rotation of the horizontal wind field. Some vertical winds not associated with active aurora may be driven by the divergence in the horizontal wind.