J. L. Innis

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson, Antarctica

Abstract Zenith observations of the oxygen λ1630 nm auroral/airglow emission (produced at an altitude of ∼220 to ∼250 km) were obtained with the Mawson Fabry-Perot Spectrometer (FPS) during three ‘zenith direction only’ observing campaigns in 1993. The data show many instances of strong (50 to 100 m s −1 ) upwellings in the vertical wind, when the auroral oval is located equatorward of the zenith. Our data appear consistent with the existence of a region of upwelling up to ∼ 4° poleward of the poleward boundary of the visible auroral oval, rather than short duration, explosive heating events. The upwellings are probably the vertical component of wind shear produced by reversal of the zonal thermospheric winds, which occurs near the poleward boundary of the visible auroral oval. Zenith temperature was also seen to increase when the oval was equatorward of Mawson, showing rises of up to 300 K or more. However, this increase is at times unrelated to the upwellings, and seems to be caused by the expansion of the warm polar cap over the observing site. On a number of nights the boundary between the polar cap and the auroral oval was observed to pass over our site several times, occasionally showing a quasi-periodic expansion and contraction. We speculate that this quasi-periodic movement may be related to periodic auroral activity that is known to generate large-scale gravity waves.

Further observations of the thermospheric vertical wind at the auroral oval/polar cap boundary

Abstract We present further observations of thermospheric winds and temperatures, derived from observations of the λ630 nm oxygen auroral/airglow emission (from ∼240 km altitude), obtained with a Fabry-Perot spectrometer (FPS) at Mawson station, Antarctica (L = 9). We report further instances of large upward zenith winds, with velocities up to ∼80ms−1, often associated with increases in temperature of up to 200 K. These upward winds are mostly seen around 21 Z, which is when Mawson station passes under the poleward edge of the (discrete) auroral oval during intervals of low to moderate geomagnetic activity, and are yet further examples of the type of event presented in an earlier paper (Innis et al., 1996). We also find intervals, up to several hours in length, when the temperatures measured South and East of the station can be several hundred degrees higher than those measured in the North and West directions, which we ascribe to the expansion of the hot polar cap into the South and East viewing directions. We have observed examples of seemingly anomalous wind measurements in the South and East directions during these times, which appear to be related to the presence of an upward wind in the observing volume of the FPS. Our observations suggest, however, that the upward wind would cause only a small perturbation (relative to the horizontal component) of the high-latitude thermospheric neutral wind field. The size of the zenith wind events (up to ∼120 m s−1) seen in our observations at Mawson are comparable with the amplitude of oscillations seen in gravity waves propagating at ∼300 km altitude over the polar cap from the nightside to the dayside, detected by Johnson et al. (1995) from Dynamics Explorer 2 observations. This similarity and the fact that we see the upward winds at the night-time auroral oval/polar cap boundary suggest that the origin of the upward winds may be intrinsically linked to the processes that generate these gravity waves.

High‐resolution Fabry‐Perot Observations of mesospheric OH (6‐2) emissions

Over the 1992/93 summer the Fabry-Perot spectrometer at Mawson, Antarctica (67.6°S, 62.9°E), was modified to permit observations of two lines in the OH (6–2) band near λ 840 nm. Preliminary observations were obtained on 5 nights in early February. Analysis of these lines, permitting estimates of mesospheric winds and temperatures, will be presented. Lambda-doubling of OH (6–2) Q1(2) and P1(3) lines was measured to be 5.50±0.01 pm and 19.17±0.06 pm respectively. The results show that mesospheric temperatures can be obtained from Q1(2) profiles even though the doublet is not fully resolved. This emission is significantly more intense than the P1(3) emission so its use will allow better resolution of temporal variations.

Are polar cap gravity waves a heat source for the high‐latitude thermosphere?

Ground based measurements of thermospheric temperatures from Mawson, Antarctica, often show a marked spatial gradient, with data taken poleward of the discrete auroral oval up to 200 K or more warmer than measurements taken in or equatorward of the oval. This region of increased temperature is identified as the polar cap. We suggest a possible contribution to polar cap heating may be from thermospheric gravity waves.

Thermospheric gravity waves in the southern polar cap from 5 years of photometric observations at Davis, Antarctica

Five years of photometric data of the λ630 nm oxygen auroral/airglow emission (∼240-km altitude) over Davis, Antarctica (Λ=−74.6°), are searched for evidence of periodic oscillations due to atmospheric gravity waves. Data were selected for intervals when the auroral oval was equatorward of Davis and hence are from the thermospheric polar cap. A total of 106 cloud-free nights, each with a minimum of 2 hours in the cap, were subjected to a full spectral analysis. Wave-like features were seen on 45 nights (42%), with some nights showing evidence for two intervals of waves: A total of 53 waves were identified. Periods range from ∼13 to 40 min. We identify the oscillations as the signatures of gravity waves. Derived wave properties include horizontal wavelengths from 220 to 1600 km and phase speeds ranging from ∼200 to ∼850 ms−1. Around half of the waves are directed within ∼ ±40° of magnetic south, with most of the remainder being directed either approximately magnetic east or magnetic west. We interpret this as indicating the wave source regions are of significant linear extent and are aligned either parallel or perpendicular to the auroral oval. Source locations are estimated using the known dependence on frequency of the gravity wave angle of ascent, assuming the source is near the electrojet height (120 km) and ignoring the effects of background winds. The source locations are found to be near the magnetic latitude of the poleward edge of the auroral oval. Phase speeds and travel directions are similar to the speeds and direction of substorm surges, which is suggestive of the source of these waves.

A Theoretical Model Study of F-Region Response to High Latitude Neutral Wind Upwelling Events

Abstract Neutral wind upwelling events are well documented in both the northern and southern high latitude ionosphere. The vertical winds in the F-region frequently exceed 100 m / s , and winds in excess of 200 m / s have been observed. These upwelling events occur in a latitudinal band of 4°–6° width that extends in local time in the midnight-morning sector; this band always lies poleward of the auroral precipitation. Using the time dependent ionospheric model (TDIM), a series of sensitivity simulations are carried out, based on observational constraints provided by the spectacular upwelling event seen at both the Mawson and Davis stations in the Antarctic on 08 June 1997 (Innis et al., 1999). The model simulations indicate that the F-layer density at any given point may either increase or decrease during an upwelling event, depending upon the past history of the plasma flux tube. Because this past history of the F-layer convection is unknown for the specific upwelling events a detailed case study cannot be undertaken. Instead a series of sensitivity simulations based upon a range of possible convection histories will be studied to determine the relative effect of the upwelling. The absolute density is not dependent upon solar EUV production because of winter conditions, but is sensitive to the auroral electron precipitation. The best F-layer indicator of the upwelling is the height of the layer, hmF2. For upwelling events with vertical drifts of 100 m / s h m F 2 can be increased by 100 km in 10 min . Upon leaving an upwelling region, the hmF2 almost as rapidly decreases to its normal height. Resulting from this lifting of the O+ layer is the reduction in O+ recombination and 630-nm emission; this latter consequence is observed as a standard feature of the upwelling events. In the topside ionosphere the electron density is responsive to the upwelling. The total electron content (TEC) is not, in general, sensitive to the uplifting events, however, low elevation slant path GPS TEC measurements might well detect the rapid uplifting of the F-layer. The upwelling event observations are insufficient to constrain our understanding of their impact upon the ionosphere. This model study does imply that upwelling events can modify the F-layer height severely. Such layer height modification can have measurable effects on radio frequency ray paths through the ionosphere. To quantify such effects a fuller description of the upwelling events as well as the past history of ionospheric plasma is needed. Experiments with higher time resolution of both the neutral parameters and F-region at multiple locations are necessary to unravel these complex events.

Deceleration of the high‐latitude thermospheric wind by polar cap gravity waves

This letter explores the hypothesis that gravity wave drag may account for the observed deceleration of the thermospheric anti-sunward wind near the nightside auroral oval, which at times has been seen to reduce to near zero in a horizontal distance of a few hundred kilometres. Other high-latitude observations have revealed thermospheric gravity waves that appear to propagate across the polar cap from the nightside to the dayside, and hence are moving counter to the wind near magnetic midnight. Approximate constancy with altitude of the vertical velocity amplitudes of these waves suggests a moderate level of energy dissipation is present. Calculations indicate a wave-mean flow interaction could lead to a deceleration of the wind of magnitude comparable to that observed.

Thermal behaviour of the Antarctic thermosphere observed from Mawson

Abstract The behaviour of thermospheric temperature in the region of Mawson, Antarctica during 1992–1996 is presented. Temperatures have been derived from Fabry-Perot spectrometer observations of the 630-nm OI emission. A distinct nighttime temperature gradient is observed along the magnetic meridian with hotter temperatures towards the pole. Equatorward of Mawson the temperature data have been fitted by an equation based on diurnal and annual sinusoidal variations, sunspot number and a quadratic dependence on Kp. Temperature differences between observed temperatures and the mathematical description have a mean of 10K and a standard deviation of 102K.