Anasuya Aruliah

The seasonal behavior of high-latitude thermospheric winds and ion velocities observed over one solar cycle

The seasonal variation of nighttime thermospheric winds observed at Kiruna has been found to be significantly dependent on solar activity. Of particular interest is the observation that there is a large difference between the March and September equinox winds, despite similar levels of solar insolation. The September equinox winds are more December solstice-like. The average March equinox meridional winds are up to 70% larger than for September. The existence of an equinoctial asymmetry has not been predicted by either thermospheric or ionospheric model simulations, which assume that the equinoxes are fundamentally the same, and use forcing functions which are symmetric about the solstices. The average ion velocities measured at EISCAT are larger during the March equinox than the September at solar maximum, while the converse is true at solar minimum. In contrast, the March equinox nighttime thermospheric winds are larger for both solar maximum and solar minimum. Furthermore, the asymmetry is greater at solar maximum.

The combined effect of solar and geomagnetic activity on high latitude thermospheric neutral winds. Part I. Observations

Long-term averages of Fabry-Perot Interferometer (FPI) observations of the night-time OI (1D) emission at 630 nm from the F-region thermosphere at a high latitude site are presented here. The data base contains measurements of thermospheric neutral winds for every winter period from November 1981 to April 1989 inclusive. This covers nearly one complete solar cycle in terms of the radio and UV/EUV flux variation, from the last solar maximum to the present solar maximum. The instrument is located in Kiruna, Sweden, which is situated at the equatorward edge of the auroral oval at quiet to moderate levels of geomagnetic activity and beneath the oval at higher levels of activity. From this location, the FPI has sampled the response of the upper thermosphere winds at night to a wide range of geomagnetic and solar activity conditions. The data presented here show a significant correlation between the solar radio or EUV fluxes and the response of thermospheric neutral winds to geomagnetic conditions in the auroral oval. For K(p) < 2 the auroral oval is polewards of Kiruna and neutral winds are similar at solar minimum and solar maximum. At higher levels of geomagnetic activity, for a given level of activity, the neutral winds are a factor of two greater at high levels of solar flux, than at low levels of solar flux. Alternately, the same winds are seen at K(p) = 3 at high solar flux levels, as at K(p) = 5 for low flux levels. Comparing the situation for high and low solar activity, for a given level of geomagnetic activity, in the dusk auroral oval, the sunward or westward winds are a factor of two larger at solar maximum. In the midnight period, the equatorward wind is a factor of two larger at high solar activity, compared with low solar activity. In this paper, the observations will be discussed, with the implication that K(p) is a rather poor indicator of momentum and energy coupling from the solar wind to the upper thermosphere. In a second paper, the solar-terrestrial processes which cause this phenomenon will be discussed and modelled.

The trouble with thermospheric vertical winds: geomagnetic, seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms−1 during periods of quiet geomagnetic activity (Kp < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms−1 may occur.

First tristatic studies of meso‐scale ion‐neutral dynamics and energetics in the high‐latitude upper atmosphere using collocated FPIs and EISCAT radar

A unique experiment was undertaken during the nights of 27 and 28 February 2003. Tristatic Fabry-Perot Interferometer (FPI) measurements of the upper thermosphere were co-located with tristatic EISCAT radar measurements of the ionosphere. Tristatic measurements should remove assumptions of uniform wind fields and ion drifts, and zero vertical winds. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Mesoscale variations are observed on the scales of tens of kilometers and minutes. The magnitude of the upper thermosphere neutral wind dynamo field is on average 50% of the magnetospheric electric field and contributes an average magnitude of 41% of in-situ Joule heating. The relative orientations of the 2 dynamo field vectors produce a standard deviation of ±65% in the contribution of the neutral wind dynamo.

Ion-neutral dynamics in the high latitude ionosphere: first results from the INDI experiment

Abstract The INDI experiment consisted of a series of joint observations between EISCAT and a Fabry-Perot interferometer (FPI) situated at Kiruna. The FPI measured the 630 nm neutral oxygen emission at eight positions on a 30° elevation circle, giving a spatial average of the neutral wind field with a time resolution of about 15 min. The radar performed a seven-position, near-meridional scan in a region common to the optical measurements. Such simultaneous observations of the ionised and neutral components of the Earths atmosphere allow a study of the ion energy balance and the coupling between species. The first stage of the analysis was to derive the neutral wind from the EISCAT data using the simplified ambipolar diffusion and ion energy equations. This was then compared with the direct measurements from the FPI. There was good agreement between derived and measured meridional winds, but the zonal wind values, although showing the same trends, differed in magnitude by a factor of 3–5. The reasons for this are discussed. The meridional wind data was used to derive the ion-neutral collision frequency. This was a factor of 2 or 3 less than recent model values. Preliminary comparisons of the measured electron densities with the 630 nm emission intensity gave clues to the chemistry of the emission process.

An equinoctial asymmetry in the high‐latitude thermosphere and ionosphere

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Throughout the solar cycle, average nighttime thermospheric meridional winds are larger in spring than autumn despite similar levels of solar insolation. The average ion velocities are also larger in spring than autumn at solar maximum, but at solar minimum this position is reversed. Numerical simulations of the thermosphere and ionosphere have not predicted such asymmetries because they generally assume forcing functions that are symmetric about the solstices. The proposed explanation lies in the annual and diurnal variation in solar wind-magnetosphere coupling caused by changes in the orientation of the geomagnetic pole, and hence the magnetosphere, with respect to the average orientation of the IMF (the Russell-McPherron effect). This causes a 12-hour phase difference between the times of maximum solar wind-magnetosphere coupling at the two equinoxes. In addition, the orientation of the geomagnetic axis with respect to the average IMF is such that > 0 for the March equinox and By*Bz> < 0 for September. This results in a further source of asymmetry of forcing of the high-latitude ionosphere as the result of electric fields associated with the four sign combinations of By and Bz. Several predictions arise from the explanation given: for example, a high-latitude station measuring thermospheric neutral winds in Alaska, 180° in longitude from Kiruna, might be expected to see nighttime thermospheric winds that are larger in the autumn than in the spring.

Seasonal and solar cycle variations in high- latitude thermospheric winds

Thermospheric wind measurements have been collected systematically every winter for over nine years from a high-latitude site at Kiruna, Sweden (67.8{degree}N, 20.4{degree}E). The database contains 1,242 nights of data collected with a Fabry-Perot Interferometer (FPI), perhaps the largest single-site database of thermospheric winds. This analysis shows a marked seasonal and solar cycle variation. Particularly at high solar activity, sunward winds of the evening period (16-20 UT) are more than 50% stronger at Spring than at Autumn equinox. This large asymmetry in the behavior of high-latitude thermospheric winds at spring and autumn equinox has not yet been predicted by model simulations.

Observations of the lower thermospheric neutral temperature and density in the DELTA campaign

The rotational temperature and number density of molecular nitrogen (N2) in the lower thermosphere were measured by the N2 temperature instrument onboard the S-310-35 sounding rocket, which was launched from Andøya at 0:33 UT on 13 December 2004, during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. The rotational temperature measured at altitudes between 95 and 140 km, which is expected to be equal to neutral temperature, is much higher than neutral temperature from the Mass Spectrometer Incoherent Scatter (MSIS) model. Neutral temperatures in the lower thermosphere were observed using the auroral green line at 557.7 nm by two Fabry-Perot Interferometers (FPIs) at Skibotn and the Kiruna Esrange Optical Platform System site. The neutral temperatures derived from the look directions closest to the rocket correspond to the rotational temperature measured at an altitude of 120 km. In addition, a combination of the all-sky camera images at 557.7 nm observed at two stations, Kilpisjärvi and Muonio, suggests that the effective altitude of the auroral arcs at the time of the launch is about 120 km. The FPI temperature observations are consistent with the in situ rocket observations rather than the MSIS model.

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

Consequences of geomagnetic history on the high-latitude thermosphere and ionosphere: Averages

The thermospheric effects of “geomagnetic history” and the resulting ion-neutral interactions are determined through the analysis of a long-term database of high latitude neutral winds from Kiruna, Sweden, and simulations with the coupled thermosphere ionosphere plasmasphere model (CTIP). Three types of geomagnetic history are examined in detail with the data and the model: steady state conditions in which the Kp index for the current three hours is the same as the Kp index for the previous 3 hours; previously quiet conditions in which the Kp index for the current 3 hours is greater than the Kp index for the previous 3 hours; and previously active conditions in which the Kp index for the current 3 hours is less than the Kp index for the previous 3 hours. It is shown that during the hours of darkness at Kiruna, while the ionosphere responds immediately to changes in activity, the neutral gas can take between 3 and 6 hours to recover from the effects of any previous activity. Model simulations show that the rate of energy dissipation is also significantly dependent on geomagnetic history. For the previously active case the Joule heating and mechanical energy transfer rate are up to 4 times larger at certain latitudes as the steady state case. For the previously quiet case the heating rates are much smaller than the steady state case. There is a frequently made assumption that at high latitudes the mechanical energy transfer rate may be ignored as insignificant compared with the Joule heating rate. The results presented here show that this assumption is unreliable, particularly in the dusk sector and polar cap.