Mark Conde

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

Spatial structure in the thermospheric horizontal wind above Poker Flat, Alaska, during solar minimum

A new all-sky imaging, wavelength scanning Fabry-Perot spectrometer was used to record high-resolution (R ≃ 200,000) spectra of the λ630 nm thermospheric optical emission above Poker Flat, Alaska. These spectra were used to derive spatially resolved maps of the horizontal wind vector at approximately 250 km altitude. We describe the procedure used to infer vector winds from line-of-sight Doppler shifts, along with its limitations. We present the time evolution of the vector wind fields obtained from this method for 6 nights of observation. Five of the 6 nights contained periods when we inferred the existence of significant curvature, divergence or shear in the thermospheric wind across our instruments ∼1000 km diameter field of view. The sixth night exhibited little spatial structure and is shown for comparison. We compare these results with a “generic” solar minimum winter time run of the National Center for Atmospheric Researchs Thermosphere, Ionosphere, and Electrodynamics General Circulation Model. While agreement was good at the start and end of the night, considerable differences were found in the late evening and midnight sectors. Some possible origins for these discrepancies are proposed. In particular, we suggest that the F region horizontal wind may be deflected by upwelling vertical winds, which are in turn driven by E region heating in the auroral zone. We note that both the instrument used and our high-latitude implementation of the analysis procedure are new experimental techniques. Thus the data presented here should be regarded as preliminary and, if possible, be validated by comparison with results from other techniques.

Vertical wind observations with two Fabry‐Perot interferometers at Poker Flat, Alaska

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.

Phase compensation of a separation scanned, all-sky imaging Fabry-Perot spectrometer for auroral studies.

We describe a new all-sky imaging spectrometer using a separation scanned Fabry-Perot étalon. It is intended for ground-based mapping of upper atmospheric wind and temperature fields in the auroral zone. Its major advantage is that recorded spectra are not distorted by spatial and temporal brightness fluctuations in the aurora. We present a discussion of previous approaches to field widening a Fabry-Perot spectrometer, then describe the principles underlying our method. This enables comparisons with the throughput and the response to brightness fluctuations provided by previous instruments. We also describe our instruments optical layout, its calibration, and data analysis.

Mapping thermospheric winds in the auroral zone

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.

MICA sounding rocket observations of conductivity‐gradient‐generated auroral ionospheric responses: Small‐scale structure with large‐scale drivers

A detailed, in situ study of field-aligned current (FAC) structure in a transient, substorm expansion phase auroral arc is conducted using electric field, magnetometer, and electron density measurements from the Magnetosphere-Ionosphere Coupling in the Alfven Resonator (MICA) sounding rocket, launched from Poker Flat, AK. These data are supplemented with larger-scale, contextual measurements from a heterogeneous collection of ground-based instruments including the Poker Flat incoherent scatter radar and nearby scanning doppler imagers and filtered all-sky cameras. An electrostatic ionospheric modeling case study of this event is also constructed by using available data (neutral winds, electron precipitation, and electric fields) to constrain model initial and boundary conditions. MICA magnetometer data are converted into FAC measurements using a sheet current approximation and show an up-down current pair, with small-scale current density and Poynting flux structures in the downward current channel. Model results are able to roughly recreate only the large-scale features of the field-aligned currents, suggesting that observed small-scale structures may be due to ionospheric feedback processes not encapsulated by the electrostatic model. The model is also used to assess the contributions of various processes to total FAC and suggests that both conductance gradients and neutral dynamos may contribute significantly to FACs in a narrow region where the current transitions from upward to downward. Comparison of Poker Flat Incoherent Scatter Radar versus in situ electric field estimates illustrates the high sensitivity of FAC estimates to measurement resolution.

Thermospheric vertical wind activity maps derived from Dynamics Explorer-2 WATS observations

Large-scale distribution of thermospheric vertical wind activity was studied using observations from the Wind and Temperature Spectrometer on the Dynamics Explorer-2 satellite. We calculated the vertical velocity standard deviation, σ(Vz), within a sliding window of width 120 seconds, corresponding to an along-track distance of ∼900 km. Maps of σ(Vz) in local magnetic time and invariant latitude reveal a high-latitude region of enhanced activity that largely fills the polar cap. Activity appears to be a maximum in the midnight-dawn sector of the cap, possibly at the nominal auroral oval latitude. We provisionally interpret our results as evidence of polar cap gravity waves with sources in or near the midnight-dawn auroral oval. Waves propagating poleward effectively fill the cap with vertical wind oscillations. Equatorward propagating waves move approximately parallel to the local thermospheric wind, resulting in the possibility of critical-layer dissipation.

First E region observations of mesoscale neutral wind interaction with auroral arcs

We report the first observations of E region neutral wind fields and their interaction with auroral arcs at mesoscale 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 plasma convection. The wind rotations occurred within 7–16 min. In one 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 modeling that these effects cannot be explained by height changes in the emission layer. The most likely explanation seems to be the greatly enhanced ion drag associated with the increased plasma density and localized ionospheric electric field associated with auroral arcs. In all cases, the F region neutral wind appeared less affected by the auroral arc, although its presence is clear in the data.

The first daytime ground‐based optical image of the aurora

Aurorae, spectacular phenomena in the polar night sky, also provide a convenient projection of effects of complex and energetic plasma processes of the outer magnetosphere. Much has been learned about the ionosphere and magnetosphere from night-time auroral images. However, similar imaging is extraordinarily difficult by day, due to the overwhelming background of atmospherically-scattered sunlight. This is unfortunate, since many auroral plasma processes may be unique to the sunlit ionosphere. A visible-light image of the aurora at λ630-nm wavelength was obtained from Kiruna, Sweden, at sunset on May 2, 1999, by an imaging spectrometer featuring excellent spectral resolution and out-of-band rejection. We believe this to be the first such image obtained from the ground under near-daytime conditions. These observations were obtained as a test of principle during the development of a prototype instrument. We believe this technique holds great promise for future ground-based studies of the daylit ionosphere and magnetosphere.

Observation and simulations of winds and temperatures in the Antarctic thermosphere for August 2-10, 1992

Optically derived upper thermospheric wind and temperature data, collected at Antarctic stations at South Pole (L = 14), Mawson (L = 9.3), and Halley (L = 4.6), and averaged over the low-activity period August 2–10, 1992, have been interpreted with the help of simulation by the National Center for Atmospheric Research thermosphere ionosphere electrodynamic general circulation model (TIEGCM) with inputs matching the average conditions of observation. The simulation provides a global background context upon which the widely-separated optical observations can be placed. The simulation shows three large-scale structures in the polar wind field: the morning vortex, the evening vortex, and the cross-polar wind jet. Each of these came within view of the group of observing stations during the diurnal cycle, providing arrival time observations and signatures which were examined relative to the TIEGCM simulation. Reasonable correspondence was found, indicating the capability of the model to agree simultaneously with observations at three widely spaced stations representative of the subauroral and auroral zones, as well as the polar cap. Simulated wind directions were in excellent agreement with observation, although wind magnitudes frequently exceeded measured values by up to 30%. Apparent divergent flows in the data from Halley and Mawson were explained as signatures of vortices from their presence in the simulated wind fields. Observed diurnal mean temperatures compared well with the simulation, confirming that heat inputs and the distribution of thermal energy in the model are, on average, reasonable. A significant and persistent difference between experimental and modeled temperatures was that the diurnal temperature variation observed at South Pole peaked at the nightside crossing of the jet and was minimum a few hours before noon magnetic local time, whereas the simulation indicated minimum temperatures on the nightside, in antiphase to the measurements. A simple calculation indicates that the observed temperature difference between the air parcels entering the polar cap, encountered on the dayside, and those leaving the polar cap on the nightside is reasonably matched to the heating due to the ion-drag acceleration process. No explanation of the lack of this temperature rise in the TIEGCM simulation is presently available.