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Abstract: A-priori, rapid variations of solar activity that directly impact on the terrestrial environment should be expected to influence airglow brightness in the mesopause region via the photodissociative production of atomic oxygen, as it does on the time scale of the solar cycle. To find out whether this is supported by our midlatitude data, we analyze the strongest geoeffective solar activity events, in times when data from the Argentine airglow spectrometer were obtained. An alternative interaction path involving geomagnetic perturbations mediated by the solar wind can also be expected to affect the mesopause region. Daily mean values of different solar and geomagnetic activity indices, and more than 1400 nights of airglow brightness and rotational temperature measurements (mostly from El Leoncito, 31.8°S) are available for this study. The diagnostic value of this investigation is augmented by using information corresponding to two different nominal altitudes (87km for the OH(6–2) band, and 95km for the O2 b(0–1) band). Our approach ranks the (solar and airglow) events by their respective strength, which automatically provides emphasis on the more important cases. We conclude that if an airglow response to strong solar events exists, it is only short-lived and should therefore most easily be detectable by daytime observations. On the other hand, we did not find signatures in our airglow data that could convincingly be related to geomagnetic storms. [Copyright &y& Elsevier]

Abstract: Most available ground-based (GB) techniques for measuring temperatures in the upper mesosphere to lower thermosphere (or mesopause region) have systematic errors that are comparable to those of orbiting instruments. Determining these unknown biasses would normally require colocated observations that are only seldom feasible. Satellite measurements can be used as a “transfer standard” between GB observations that are not colocated. In this context, even with a reproducible or known bias in the satellite data, the comparison is still meaningful. Since Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) temperatures cover the mesopause region with very good accuracy (statistical errors do not exceed 1.5K and systematic uncertainties range from about 3–7.5K), they are quite suitable for this purpose. Because of the nearly constant precision over the height range of interest, also rotational temperatures of airglow emissions from different altitudes like the OH and O2 bands (or the OI 558nm line) can be successfully compared with each other. In spite of the limited number of overpasses during the relatively short CRISTA missions, the feasibility of such an intercalibration is demonstrated for widely separated GB sites. Here, the results obtained for GB measurements at eight different sites, using CRISTA-1 and CRISTA-2 data, are presented. For OH temperatures, the standard deviation between the different instruments is only 5.4K, confirming previous estimates. [Copyright &y& Elsevier]

Abstract: Bursts of strong day-to-day variations in airglow brightness and temperature for the mesopause region that last one or a few nights have frequently been observed at El Leoncito (LEO; 31.8°S 69.2°W), since 1997. After the start of the operation of the meteor wind radar at Cachoeira Paulista (CAP; 22.7°S 45.0°W, about 2600km further NNE) in March 1999, a number of the strongest airglow events at LEO were found to be followed, 1–3 days later, by negative (westward) zonal wind excursions of about that seem to be related. Meridional wind disturbances are absent or only weak. The zonal wind perturbation at CAP closely matches the altitude range defined by the airglow emission at LEO, i.e. an OH emission event corresponds to a lower-altitude wind signature, and an O2 emission event, to one at a higher altitude. The strong differences in seasonal occurrence patterns of airglow bursts at LEO observed earlier (with O2 bursts peaking in April, and OH bursts, in June) are confirmed by the more recent data presented here. Most of the observed features of the two-site events could be explained by anticyclonic vortices (with 2000km diameter) propagating zonally with the eastward background wind. International collaborations like the Global Airglow Transition Detection and Tracking (GATDAT) campaign are expected to provide the information required to further the understanding of these phenomena. [Copyright &y& Elsevier]

Abstract: Rotational temperatures and airglow brightness variations of the OH(6-2) and (0-1) bands measured at El Leoncito () from 1998 to 2002 are analysed to detect solar activity signatures. The emissions correspond to nominal altitudes of 87 and 95km, respectively. The homogeneous and consistent data set covers the rise, the two distinct maxima, and the start of the decline of the present solar activity cycle. The modulation of solar activity around the Gnevyshev gap (particularly pronounced in this cycle) has been so strong as to motivate the expectation to detect its signature in such a relatively short data set. While at 87km, we practically observe no correlation with solar radio flux, or at most a weak correlation in temperature, we find a moderate positive temperature and airglow brightness effect at 95km. This is a remarkable difference, in view of the small separation between the two altitude levels. The quantitative results lie close to the lower limits of our previous estimates in a recent trend analysis. However, the behaviour at 95km changes abruptly between anticorrelation, correlation, and no correlation. From December 1998 to July 2001 the correlation was very strong. From the standpoint of atmospheric nonlinear dynamics, variations in the degree of correlation in different years and at different observing sites should not be surprising. [Copyright &y& Elsevier]

In our data set of presently more than 750 nights of OH and O2 airglow observations at lower mid-latitudes, the night of April 25, 1999 stands out for several reasons: the greatest mean O2 intensity we have ever observed and by far the strongest quasi-monochromatic gravity wave signature in OH intensity (with period of about 38 min), accompanied by a prominent day-to-day variation in O2 intensity, and a strong tidal signature simultaneously present in intensities and temperatures at both emission heights. Whereas gravity wave events like this are relatively rare at our site, the slower variations are obviously regular features, similar to what we have seen on other occasions. The quantitative details of the observed phenomena are analyzed in order to establish possible relationships. We conclude from comparison with similar cases that high nocturnal means are frequently accompanied by strong gravity wave signatures, but not generally by strong tidal activity. Finally, different potential mechanisms, including solitons, are briefly reviewed. [Copyright &y& Elsevier]

Using our large data base from the Argentine site at El Leoncito that covers the years from August 1997 to May 2001, and four individual campaigns in 1986, 1987, and 1992, we search for trends in temperatures corresponding to heights of 87 and 95 km. Also the variations in OH and O2 airglow band intensities for these two altitudes supply complimentary information interesting in terms of trends. The seasonal variation is well documented with good reproducibility, since the end of a perturbation possibly due to the 1997 ENSO event, so that the time series of nocturnal mean values can be reliably freed from the seasonal effect. Some difficulties arise from the geophysical variations (mostly from planetary waves) still present in the data averages over 28 nights of observation. In spite of this, temperature trends of a fraction of 1 K/year should be detectable, using only the data from 1998 to 2001. However, only at 95 km, there is a significant positive temperature trend over the last 4 year. When the earlier campaigns are included, this 15 year data span shows a negative temperature trend of about −1 K/year and a positive (OH) intensity trend of about +1%/year, at 87 km. At 95 km, there is a similar (O2) intensity trend, but a temperature trend compatible with zero. These results are similar to previous findings for Northern Hemisphere midlatitude sites, by other investigators. As a by-product of the trend analysis, we also estimate values for the 1997 ENSO temperature anomaly of about −3.5 K at 87 km, and −6 K at 95 km. [Copyright &y& Elsevier]