ORCID Profile
0000-0003-4924-878X
Current Organisation
University of Adelaide
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Publisher: American Geophysical Union (AGU)
Date: 08-2022
DOI: 10.1029/2022JA030481
Abstract: 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.
Publisher: American Geophysical Union (AGU)
Date: 04-2019
DOI: 10.1029/2018JA026440
Publisher: American Geophysical Union (AGU)
Date: 08-2018
DOI: 10.1029/2018JA025288
Publisher: American Geophysical Union (AGU)
Date: 2019
DOI: 10.1029/2018JA025997
Publisher: American Geophysical Union (AGU)
Date: 09-2018
DOI: 10.1029/2018JA025457
Publisher: American Geophysical Union (AGU)
Date: 12-2019
DOI: 10.1029/2019JA027188
Abstract: We present an empirical model of thermospheric winds (High‐latitude Thermospheric Wind Model [HL‐TWiM]) that specifies F region high‐latitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HL‐TWiM represents the large‐scale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical high‐latitude wind measurements it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground‐based stations (Fabry‐Perot Interferometers and Scanning Doppler Imaging Fabry‐Perot Interferometers) located at various northern and southern high latitudes and two space‐based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL‐TWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HL‐TWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HL‐TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Model‐data intra‐annual comparisons in this study show semiannual oscillation‐like behavior of GOCE winds, rarely observed before in wind data.
Publisher: American Geophysical Union (AGU)
Date: 03-2021
DOI: 10.1029/2020JA028505
Abstract: Strong Thermal Emission Velocity Enhancement (STEVE) is a nightsky optical phenomenon of great research interest in recent years. Recent findings indicated that STEVE likely represents certain extremely intensified chemiluminescence airglow instead of traditional aurora. In this study, we investigate the patterns and variations of the neutral wind and temperature before the STEVE emergence using joint scanning Doppler imager (SDI 630 nm) and optical all‐sky imager (ASI) observations, and make an initial effort to explore the potential preconditioning role of neutral winds in the STEVE production. Neutral winds enhance in westward and southward directions following substorm auroral intensification, and show an equatorward propagating trend from auroral latitudes. However, in STEVE events the enhanced equatorward winds feature a steep stop/reversal at certain subauroral latitude, and strong wind convergence is developed there. This pattern sustains for ∼15–20 min, and then STEVE arises at about this stop latitude. The strength of the southward wind intensification and wind convergence is in general weaker or absent in nonSTEVE substorm events. We propose that enhanced equatorward winds may transport relevant neutrals species that are key to the STEVE airglow production to subauroral latitudes, and pile up at the stop latitude of the equatorward winds due to the strong convergence there. Such a transport ileup effect led by the neutral winds may prepare a reservoir of neutral constituent which, when further aided by subauroral ion drift, leads to a dramatic increase of the airglow production and the STEVE occurrence.
Publisher: American Geophysical Union (AGU)
Date: 28-09-2021
DOI: 10.1029/2020JA028916
Abstract: We have measured auroral zone thermospheric neutral winds in the midnight local time sector, using ground‐based optical Doppler spectroscopy of the 630.0 nm emission from atomic oxygen, originating at around 240 km altitude over Alaska. One of the most prominent features seen in winds at these latitudes is the cross‐polar jet emerging from the polar cap at local times around magnetic midnight. The standard view is that wind flows anti‐sunward in the midnight sector and spills equatorward over magnetic latitudes extending well below those of the auroral zone. The purpose of this paper is to show that this view is too simplistic. From our observatory at Poker Flat, Alaska (∼ N), the anti‐sunward flow is frequently seen to stall over surprisingly short horizontal distances (100–200 km), without spilling further equatorward. This behavior is most prevalent during a low solar activity at mid‐winter when the combination of pressure gradient established by solar heating and the ion drag is not enough to allow the jet to push through the background atmosphere on the nightside. At higher latitudes, by contrast, the flow is relatively uniformly anti‐sunward around magnetic midnight even during quiet conditions. During periods of high solar and magnetic activity, the expected spilling of the midnight sector cross‐polar jet to lower latitudes often is indeed observed over Alaska. Our observation of abrupt stalling during quiet solar and geomagnetic conditions is a very significant difference from the model predictions, with potentially important ramifications‐ which is the motivation for the present study.
Publisher: American Geophysical Union (AGU)
Date: 04-2020
DOI: 10.1029/2019JA027654
Publisher: American Geophysical Union (AGU)
Date: 27-04-2022
DOI: 10.1029/2021JA029988
Abstract: Intense sunward (westward) plasma flows, named Subauroral Polarization Stream (SAPS), have been known to occur equatorward of the electron auroras for decades, yet their effect on the upper thermosphere has not been well understood. On the one hand, the large velocity of SAPS results in large momentum exchange upon each ion‐neutral collision. On the other hand, the low plasma density associated with SAPS implies a low ion‐neutral collision frequency. We investigate the SAPS effect during non‐storm time by utilizing a Scanning Doppler Imager (SDI) for monitoring the upper thermosphere, SuperDARN radars for SAPS, all‐sky imagers and DMSP Spectrographic Imager for the auroral oval, and GPS receivers for the total electron content. Our observations suggest that SAPS at times drives substantial ( m/s) westward winds at subauroral latitudes in the dusk‐midnight sector, but not always. The occurrence of the westward winds varies with AE index, plasma content in the trough, and local time. The latitudinally averaged wind speed varies from 60 to 160 m/s, and is statistically 21% of the plasma. These westward winds also shift to lower latitude with increasing AE and increasing MLT. We do not observe SAPS driving poleward wind surges, neutral temperature enhancements, or acoustic‐gravity waves, likely due to the somewhat weak forcing of SAPS during the non‐storm time.
Publisher: Wiley
Date: 26-10-2022
Publisher: American Geophysical Union (AGU)
Date: 11-2018
DOI: 10.1029/2018JA025806
Publisher: American Geophysical Union (AGU)
Date: 05-09-2020
DOI: 10.1029/2020GL089339
Abstract: This study explores intra‐annual oscillations (IAOs) in upper thermospheric winds using GOCE cross‐track wind measurements between 70°S and 70°N. Due to the Sun‐synchronous dawn‐dusk orbit of GOCE, the cross‐contamination between seasonality and local time variations in winds is minimal, which makes it a unique space‐based data set to extract IAOs and study their latitudinal variation. Our analysis reveals that the annual (AO), semiannual (SAO), and terannual (TAO) oscillations are robust features in thermospheric winds. The AO is strongest at middle latitudes SAO and TAO litudes increase with increasing latitude. The latitudinally averaged litudes of the AO, SAO, and TAO for dusk/dawn are 30.0/35.0, 8.5/11.3, and 6.0/6.6 m/s, respectively. The phase of AO reverses around the equator. SAO and TAO phases vary with latitude but do not reverse like the AO. For both the SAO and TAO, the average phase at dusk and dawn differs by ∼ 30 days.
Publisher: American Geophysical Union (AGU)
Date: 05-2022
DOI: 10.1029/2021JA029805
Abstract: 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.
Publisher: American Astronomical Society
Date: 31-07-2023
Publisher: American Geophysical Union (AGU)
Date: 2021
DOI: 10.1029/2020JA028193
Abstract: During magnetospheric substorms, high‐latitude ionospheric plasma convection is known to change dramatically. How upper thermospheric winds change, however, has not been well understood, and conflicting conclusions have been reported. Here, we study the effect of substorms on high‐latitude upper thermospheric winds by taking advantage of a chain of scanning Doppler imagers (SDIs), THEMIS all‐sky imagers (ASIs), and the Poker Flat incoherent scatter radar (PFISR). SDIs provide mosaics of wind dynamics in response to substorms in two dimensions in space and as a function of time, while ASIs and PFISR concurrently monitor auroral emissions and ionospheric parameters. During the substorm growth phase, the classical two‐cell global circulation of neutral winds intensifies. After substorm onset, the zonal component of these winds is strongly suppressed in the midnight sector, whereas away from the midnight sector two‐cell circulation of winds is enhanced. Both pre and postonset enhancements are ≥100 m/s above the quiet‐time value, and postonset enhancement occurs over a broader latitude and local‐time area than preonset enhancement. The meridional wind component in the midnight and postmidnight sectors is accelerated southward to subauroral latitudes. Our findings suggest that substorms significantly modify the upper‐thermospheric wind circulation by changing the wind direction and speed and therefore are important for the entire magnetosphere‐ionosphere‐thermosphere system.
Publisher: American Astronomical Society
Date: 31-07-2023
Publisher: American Geophysical Union (AGU)
Date: 09-2023
DOI: 10.1029/2022JA031069
Publisher: American Geophysical Union (AGU)
Date: 11-2020
DOI: 10.1029/2020JA028165
Abstract: Two‐dimensional thermospheric wind fields, at both E and F region altitudes within a common vertical volume, were made using a Scanning Doppler Imager (SDI) at Poker Flat, Alaska, during a substorm event. Coinciding with these observations were F region plasma velocity measurements from the Super Dual Auroral Radar Network (SuperDARN) and estimations of the total downward and upward field‐aligned current density from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This combination of instruments gives an excellent opportunity to examine the spatial characteristics of high‐latitude ionosphere‐thermosphere coupling and how a process which is triggered in the magnetosphere (the substorm) affects that coupling at different altitudes. We find that during the substorm growth phase, the F region thermospheric winds respond readily to an expanding ionospheric plasma convection pattern, while the E region winds appear to take a much longer period of time. The differing response timescales of the E and F region winds are likely due to differences in neutral density at those altitudes, resulting in E region neutrals being much more “sluggish” with regard to ion drag. We also observe increases in the F region neutral temperature, associated with neutral winds accelerating during both substorm growth and recovery phases.
Publisher: American Geophysical Union (AGU)
Date: 14-09-2023
DOI: 10.1029/2022JD037967
Abstract: This paper investigates the lower‐to‐upper atmosphere coupling at high latitudes ( N) during the northern winter months of 2012‐2013 years, which includes a period of major Sudden “Stratospheric” Warming (SSW). We perform statistical analysis of thermosphere wind disturbances with periods of 30 minutes to 70 minutes, known as the medium scale travelling atmospheric disturbances (MSTADs) in atomic oxygen green line (557.7 nm) near ∼120 km and red line (630.0 nm) emissions near ∼250 km observed from Scanning Dopplers Imagers (SDIs) over Alaska. The SDI MSTADs observations (60N‐75N) are interpreted in conjunction with the previous daytime MSTID observations by SuperDARN midlatitudes (35N‐65N) radars in the F‐region ionosphere and western hemisphere, which confirm findings from the SDI instruments. Increases in MSTAD activity from SDIs show correlations with the increasing meridional planetary wave litudes in the stratosphere derived from MERRA2 winds. Furthermore, a detailed study of the lower atmospheric conditions from MERRA2 winds indicates that the lower atmospheric sources of MSTADs are likely due to the stratospheric generated GWs and not orographic GWs. Favorable stratospheric propagation conditions and polar vortex disturbances resulting from the increased planetary wave activity in the stratospheric region both appear to contribute to increased MSTAD activity in the thermosphere. Additionally, the results show that the MSTID activity from SuperDARN HF radars at mid latitudes during the January 2013 SSW is lower than the MSTAD activity in SDI winds at high latitudes.
Publisher: Wiley
Date: 13-05-2020
No related grants have been discovered for Mark Conde.