E Region neutral winds in the postmidnight diffuse aurora during the atmospheric response in Aurora 1 Rocket Campaign

Abstract

Measured E region neutral winds from the Atmospheric Response in Aurora (ARIA 1) rocket campaign are compared with winds predicted by a high-resolution nonhydrostatic dynamical thermosphere model. The ARIA 1 rockets were launched into the postmidnight diffuse aurora during the recovery phase of a substorm. Simulations have shown that electrodynamical coupling between the auroral ionosphere and the thermosphere was expected to be strong during active diffuse auroral conditions (Walterscheid and Lyons, 1989). This is the first time that simulations using the time history of detailed specifications of the magnitude and latitudinal variation of the auroral forcing based on measurements have been compared to simultaneous wind measurements. Model inputs included electron densities derived from ground-based airglow measurements, precipitating electron fluxes measured by the rocket, electron densities measured on the rocket, electric fields derived from magnetometer and satellite ion drift measurements, and large-scale background winds from a thermospheric general circulation model. Our model predicted a strong jet of eastward winds at E region heights. A comparison between model predicted and observed winds showed modest agreement. Above 135 km the model predicted zonal winds with the correct sense, the correct profile shape, and the correct altitude of the peak wind. However, it overpredicted the magnitude of the eastward winds by more than a factor or 2. For the meridional winds the model predicted the general sense of the winds but was unable to predict the structure or strength of the winds seen in the observations. Uncertainties in the magnitude and latitudinal structure of the electric field and in the magnitude of the background winds are the most likely sources of error contributing to the differences between model and observed winds. Between 110 and 135 km the agreement between the model and observations was poor because of a large unmodeled jetlike feature in the observed winds (140 m s-1). Agreement between the present simulation and the earlier simulations of Walterscheid andLyons (1989) is favorable, although the winds in the present simulation are considerably weaker for particle precipitation of similar characteristic energy and flux. The reasons for the difference were the smaller latitudinal extent of the model diffuse aurora and the weaker electric fields in our simulation. We have shown that the enhanced electron densities and electric fields associated with the postmidnight diffuse aurora provide the potential for a rapid acceleration of the zonal winds as shown by Walterscheid and Lyons (1989). However, the modeled response to the large-scale electric field is too great. This suggests that the assimilated mapping of ionospheric electrodynamics (AMIE) electric field is also too large. The actual electric field is most likely reduced locally in regions of enhanced ionization and conductivity within the diffuse aurora. In addition, we have shown that the "exotic" jetlike wind feature between 110 and 13...

Sensitivity studies of the E region neutral response to the postmidnight diffuse aurora

“…Parish et al (2003) used ion density and electric field structures measured during the ARIA I campaign to provide auroral forcing inputs for a 3-D high resolution, limited area model, and compared the results of simulations with winds measured during the same campaign. The results showed much closer agreement with the ARIA I wind observations than previous attempts to simulate the winds from the same campaign (Brinkman et al, 1995). The results of Parish et al (2003) suggest that higher order semidiurnal tidal modes such as the (2,5) and (2,6) modes (with tidal nomenclature as in Chapman and Lindzen, 1970) may be the most important in generating the observed vertical structures, and showed that the observed winds are not well reproduced if only auroral forcing processes are present.…”

“…The sensitivity of the neutral response to changes in the amplitude and phase of propagating tides in the CTIP background winds has been determined using model simulations, for constant auroral forcing parameters. The electric field and ion density structures are those inferred from measurements made during the ARIA I campaign, as described in Parish et al (2003) and Brinkman et al (1995), and simulations run from 10:00 to 14.10 UT for the day of the ARIA I campaign, 3 March 1992, corresponding to the period over which coordinated observations were made. Our previous results (Parish et al, 2003) suggest that higher order (2,5) and (2,6) semidiurnal propagating tidal modes are most important in generating vertical structure with features similar to those observed during the ARIA I campaign.…”

“…Four time intervals were defined, i.e. 10: 00-11:30, 11:30-12:10, 12:10-13:20, and 13:20-14:10 UT, with different ion density and electric field structures for each interval (see Brinkman et al, 1995;Parish et al, 2003), and these values were used to provide auroral forcing inputs for the high resolution model. Zonal and meridional neutral winds measured during the ARIA I campaign are shown in Fig.…”

“…A rocket was launched during the campaign at 14:06 UT, making a trimethyl aluminate (TMA) release, which allowed the winds between 90 and 190-km altitude to be observed. Ion densities and electric fields were inferred from the measurements made throughout the campaign (see Brinkman et al, 1995). Four time intervals were defined, i.e.…”

“…More recent simulations with 2-D models suggest that large neutral winds of around 200 m/s, and even supersonic winds, may be generated in the auroral zone Fuller-Rowell, 1985;Lyons and Walterscheid, 1985;Walterscheid and Lyons, 1989) although the magnitude of these winds could be reduced by the effects of cross arc winds. Brinkman et al (1995) aimed to simulate the ARIA I observations using a high resolution 2-D model, making use of values measured during the campaign to give aurora forcing parameters, and they compared the simulated neutral winds with those observed during the campaign. The results of these simulations produced some general agreement with the observations, but the magnitudes, strong vertical structure and wind shears were not well reproduced.…”