An olden but golden EISCAT observation of a quiet time ionospheric trough

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010ja015557, 2010 An olden but golden EISCAT observation of a quiet time ionospheric trough M. Voiculescu, 1 T. Nygrén, 2 A. Aikio, 2 and R. Kuula 2 Received 11 April 2010; revised 8 June 2010; accepted 28 June 2010; published 16 October [1] Incoherent scatter measurements were carried out on 9 November 1987, showing the presence of an ionospheric trough in the F region. The experiment was made using the EISCAT UHF radar, and it consisted of an azimuthal scan with constant beam elevation and a meridional scan. Since the radar rotates with the Earth, beams with different directions from subsequent scans meet in the same MLT CGMLat pixel in nonrotating frame. If the ionosphere is not too variable, these can be combined to give an average value of electron density and ion/electron temperature in each pixel. Furthermore, since different beams passing through the same pixel give different ion velocity components, it is also possible to obtain the velocity vector. The geomagnetic conditions during the observations were quiet enough for assuming a quasi stationary ionosphere. It was found that both ion and electron temperatures have minima within the trough region and increase at the poleward wall. Ion velocity measurements, together with a convection model, suggest that the density depletion within the trough is due to recombination of F region plasma convecting for a long time in the dusk convection cell beyond the terminator. The northern edge of the trough is associated with soft particle precipitation. The southern edge is steeper than the northern edge, and is built by sunlit plasma brought to the trough region by corotation. The trough is thus a result of a combination of transport and precipitation processes rather than stagnation. Citation: Voiculescu, M., T. Nygrén, A. Aikio, and R. Kuula (2010), An olden but golden EISCAT observation of a quiet time ionospheric trough, J. Geophys. Res., 115,, doi: /2010ja Introduction [2] The ionospheric trough is a large scale depletion in F region plasma, observed at geographic latitudes around It is elongated in longitudinal direction and its width in the latitudinal direction is of the order of The term midlatitude trough is used for a density depletion with the poleward edge collocated to the equatorward edge of particle precipitation and the term high latitude is associated with a trough observed at higher latitudes (for review, see Rodger et al., 1992). However, Whalen [1989] considers that the midlatitude and high latitude troughs are practically the same phenomenon extending from midlatitudes to polar cap, although the formation processes might be different at different latitudes or at different times. This is supported by our previous findings showing that no threshold or clear difference between midlatitude and high latitude troughs could be observed [Voiculescu et al., 2006; Voiculescu and Nygrén, 2007]. [3] Several properties of the trough, as well as physical mechanisms that could explain the formation of the trough, 1 Department of Physics, ECEE, Dunărea de Jos University of Galati, St. Domnească, Galati, Romania. 2 Department of Physics, University of Oulu, Oulu, Finland. Copyright 2010 by the American Geophysical Union /10/2010JA have been deduced from a large number of satellite and ground based (mainly incoherent scatter) observations [Moffet and Quegan,1983; Whalen,1989; Rodger et al., 1992; Mallis and Essex, 1993; Kersley et al., 1997; Horvath and Essex, 2003; Voiculescu et al., 2006]. [4] A well established characteristic of the trough is its relationship to geomagnetic activity. When the geomagnetic activity level is high, troughs occur at lower latitudes at a given local time [Moffet and Quegan, 1983; Rodger et al., 1992; Pryse et al., 1998]. This is best seen at the equatorward wall [Voiculescu et al., 2006]. [5] Initially, the trough has been considered a nighttime phenomenon, typically within the postafternoon to dawn sector at MLT [Moffet and Quegan, 1983]. However, satellite observations have shown that troughs do extend to the daytime and are observed during all seasons, displaying high variability [Whalen, 1989; Mallis and Essex, 1993; Horvath and Essex, 2003; Kersley et al., 1997; Voiculescu et al., 2006]. In winter, troughs occur mostly in darkness [Voiculescu et al., 2006]. The position of the trough has a diurnal variation with its northernmost location around local geomagnetic noon. For a steady geomagnetic activity, the trough in the premidnight sector moves toward lower latitudes with local time, reaching a minimum latitude around midnight and moving back to higher latitudes in the morning [Moffet and Quegan, 1983; Whalen, 1989; Rodger et al., 1992; Voiculescu et al., 2006]. In winter, this latitude vari- 1of13

2 ation is large and the equatorward edge is related to the solar terminator. Summer troughs are seen mostly at high latitudes all day, except around magnetic local night, when they descend to slightly lower latitudes [Voiculescu et al., 2006]. Several relations connecting the latitude of the trough, the time of the day, and the magnetic activity have been found from different data sets [Rodger et al., 1992; Werner and Prölss, 1997; Horvath and Essex, 2003]. [6] Density gradients at the walls of the trough vary with geomagnetic activity. The poleward edge is reported to be steeper than the equatorward one during quiet magnetic periods, especially in the premidnight sector [Rodger et al., 1992; Jones et al., 1997; Kersley et al., 1997]. Voiculescu et al. [2006] found that the gradients, especially for the equatorward edge, increase with local time in the postnoon sector. There is a also latitudinal dependence of the gradient at both edges of the trough, with maximum gradients around 65 CGMLat. [7] Recently, IMF effects on trough formation and/or position have been investigated [Voiculescu et al., 2006; Voiculescu and Nygrén, 2007]. The IMF can affect the trough by controlling the ionospheric convection pattern via magnetosphere ionosphere coupling. The B z component has the most important effect on the convection pattern, and therefore it has the greatest effect on the trough as well. However, B y is also important, since the shapes of the convection shells are altered by its orientation. [8] Various mechanisms for the generation of the trough have been suggested. The decreased electron density could be the result of increased recombination, decreased production, or transport effects (horizontal or vertical). One should also notice that more than a single mechanism may contribute to the trough simultaneously. The importance of one or another depends on the geophysical conditions. [9] In the nightside, a trough can be formed if plasma stays in darkness for a time which is long enough to allow sufficient decay [Whalen, 1989; Rodger et al., 1992]. The idea of stagnation is that westward convection flow within the dusk cell is balanced by eastward corotation. Then plasma motion ceases or becomes very slow. In the dawn sector, the structure of the convection cell prevents stagnation. However, even there convection may drive plasma along paths where production due to sunlight is absent. [10] Production is not absent everywhere beyond the terminator, since particle precipitation takes place there. This has a major role in generating the poleward wall of the evening troughs, generally coinciding with the equatorward boundary of the precipitation region [Rodger et al., 1992; Jones et al., 1997; Aladjev et al., 2001]. Convection of plasma from the dayside may also contribute to the walls of the trough. During summer, the plasma path in darkness gets shorter with increasing latitude, thus the plasma density can remain high until it reaches a region where the density is depleted, forming the poleward wall of darkside region troughs. [11] The absence of production has a major role in the nightside, but it cannot explain the dayside troughs. In the daytime, troughs may be formed when high density dayside plasma is displaced by low density plasma carried sunward from the nightside [Whalen, 1989; Rodger et al., 1992; Pryse et al., 1998; Mallis and Essex, 1993]. [12] The mechanism of trough generation due to increased recombination is connected to enhanced electric field, which increases the ion temperature by means of friction with neutrals. Then the plasma density is reduced because of the temperature dependence of the rate coefficients [Rodger et al., 1992; Crickmore et al., 1997; Vlasov and Kelley, 2003]. Large westward ion drifts known as polarization jets (PJ) [Galperin et al., 1974] and usually referred to as subauroral ion drifts (SAID) [Spiro et al., 1979; Anderson et al., 1991], as well as the newly discovered eastward ion drifts in the premidnight sector, [Voiculescu and Roth, 2008] are connected with ionospheric troughs. In principle, heating of neutral atmosphere by strong electric fields also leads to upwelling of the neutral atmosphere. The upward neutral wind drives F region plasma to greater heights and also speeds up recombination, since the abundance of biatomic neutral molecules in F region is increased. This mechanism is expected to be slow, however, since the large mass of the neutral atmosphere cannot be easily accelerated and, consequently, it also implies high electric fields (see e.g., Rodger et al. [1992]). [13] In this paper, we investigate the results of an EISCAT experiment that took place on 9 November 1987, when a well defined ionospheric trough was observed. The particular design of the experiment allows the determination of ionospheric parameters over a broad latitudinal and longitudinal range, and this is used in the attempt to identify the mechanisms that contribute to the formation of the trough. Also, the capabilities of this particular experiment are highlighted. 2. Observations 2.1. EISCAT Experiment and Data Analysis [14] The measurements were made with the EISCAT UHF radar (69.59 N lat, E lon and mlat, mlon) on 9 November 1987, UT. The experiment consists of an azimuth scan with a fixed elevation of 30. The azimuth step is 12, which gives 30 beam directions in the full rotation. The measurement time for each beam direction is 18 s, and the beam swing takes 12 s. Hence, 30 s is needed, effectively, for each beam direction, and the whole azimuth scan takes 15 min. However, only data that come from the times of fixed beam direction are used. In addition, the experiment consists of a latitudinal scan in geomagnetic meridional plane with five positions at elevation angles 20, 28, 90, 130, and 160 from the north. The latitudinal scan also takes 15 min, so that the length of the total measurement cycle is 30 min. The number of available range gates is 12. The first gate is at a range of 275 km, and the gate separation is 75 km. [15] An example of the parameters obtained by a single azimuth scan is shown in Figure 1. The horizontal and vertical axes in each image indicate beam aligned ranges in kilometers in longitudinal and latitudinal directions, respectively (geographic north to the top). The direction of beam rotation is shown by the arrow. Plots of electron density (n e ), beam aligned ion velocity (v i, positive toward the radar), and electron and ion temperatures (T e and T i ) are shown. The electron density plot shows a clear longitudinally oriented minimum to the south of the radar, which is an indication of the trough. A prominent westward flow, 2of13

3 Figure 1. Example of observations on 9 November, 1987, during the azimuthal scan starting at 1800 UT. Geographic north is up and magnetic (CGM) north is 20 toward west. northward of the density minimum, is visible in the ion velocity plot. [16] During a single measurement cycle, the radar moves 290 km eastward in the nonrotating frame. The ground projection of the first gate is at 238 km from the radar. Therefore, subsequent azimuth scans overlap in nonrotating coordinates. This is shown in Figure 2, where electron densities from four azimuth scans are plotted in local time geographic latitude coordinates. This figure is only a sketch, since the radar rotates with the Earth so that a full rotation does not make a circle. The start times of these scans are at intervals of 2 hr; hence three more azimuth scans are available between each pair of subsequent scans. This demonstrates that several measurements are available from the same region in the nonrotating frame. [17] Figure 2 illustrates that the observations can be used for mapping the properties of the trough, if the trough does not change too much within a time interval of a few scans. Figure 3 shows the behavior of IMF together with AU, AL, and Kp indices during the experiment. The IMF data were supplied by the OMNI2 data set, based on IMP8 data with 1 hr resolution. The primary data of OMNI are near Earth solar wind magnetic field and plasma parameters. There is an unfortunate gap in the IMF data, but Bz is quite small both in the beginning and at the end of the observations. There is some magnetic activity in the auroral region both in the beginning and at the end of the experiment, but the middle part is quiet. The Kp index is also small, between 2 and 2+. A reasonable conclusion is that the ionosphere has been rather quiet at least during most parts of the observations. [18] To better map the properties of the trough, the data are processed in the following manner. The region covered by the observations is divided into pixels in MLT CGMLat coordinates. If the pixel size is not too small, the same range gates of beams from different scans may hit the same pixel. Then more than a single value of a given plasma parameter is obtained for this pixel at the height of this range gate. Using these values and their statistical errors, the best value of the parameter for this pixel can be calculated. Since the directions of beams passing through a given pixel and coming from different scans are also different, they give 3of13

4 Figure 2. A sketch of electron density in nonrotating frame (local time versus geographic latitude) as seen by four separate azimuth scans with start times separated by 2 hr. The radar latitude is indicated by the dashed circle. different components of ion velocity. Then the ion velocity vector can also be determined, provided that at least three beam directions are available. Since the analysis can be repeated for each range gate, a three dimensional picture of the trough region is obtained. The lowest gates cover a more limited latitude range than the highest gates Results [19] Figures 4 and 5 show the the electron density, the electron and ion temperatures and the zonal velocity at four F region gates at 264 km, 306 km, 345 km, and 392 km. The solar terminator is also shown. Since the measurement covers several hours, these plots are not snapshots of the ionosphere and some temporal changes may have taken place. However, the relatively quiet magnetic conditions give a reason to believe that the changes are small enough to allow the mapping of the main features of the ionosphere. Field aligned velocities have been also calculated, but they are rather noisy and show no regular behavior that could be connected in time or space with the ionospheric trough. [20] The electron density plots reveal a well defined trough close to 70 CGMLat after 1500 MLT. The trough moves toward lower latitudes with time and leaves the field of view close to 60 CGMLat at about 22 MLT. At about 23 MLT, the trough is completely out of the radar sight. The same behavior is observed at all four gates. This supports the view that the high latitude and midlatitude trough are in fact manifestations of the same phenomenon, observed at different times of the day at different latitudes [Whalen, 1989; Voiculescu et al., 2006]. One should notice that the trough is entirely located beyond the solar terminator. [21] Although the temperatures are rather noisy, they show enhancements at the northern edge of the trough. An indication of temperature minima somewhere in the trough region is also seen. The electron temperatures are higher than the ion temperatures. [22] The zonal ion velocity portrays a distinct westward flow within the trough region. The same flow direction is also seen at the poleward wall of the trough before 21 MLT, but after that the flow direction at the wall is eastward rather than westward. At the equatorward wall, the direction of zonal flow is eastward or close to zero. Field aligned and meridional velocities were also calculated, but they are too noisy to show any regular behavior. [23] For a better understanding of the behavior of ionospheric parameters within the trough, latitudinal profiles of density, temperatures, and zonal velocity at the four selected altitudes and at four different times have been plotted in Figure 6. The first observation is the well known southward progression of the trough with MLT. The position of the trough minimum slightly varies with altitude, but the equatorward displacement is clear at all heights. At 306 km Figure 3. B y B z components of (top) IMF and (bottom) AU and AL indices together with Kp index during the experiment. 4of13

5 Figure 4. Electron density (first row), electron temperature (second row), ion temperature (third row), and eastward ion velocity (fourth row) at (left) two F region gates at 264 km and (right) 306 km. The coordinates are MLT and CGMlat and the velocity is given in nonrotating system. The solar terminator is indicated by the heavy red or black lines. 5 of 13

6 Figure 5. Same as Figure 4 for gates at 345 km and 392 km. 6 of 13

7 Figure 6 7of13

8 altitude, for instance, the density minimum is located at 68 CGMlat at 17 MLT. It moves equatorward at a speed of about 1 CGMlat per hour, reaching 65 CGMlat at 20 MLT. Within this time interval, the minimum density also decreases from about m 3 to less than m 3. [24] At all altitudes, the equatorward wall of the trough is clearly steeper than the poleward wall in the afternoonevening sector (at least up to 19 MLT), in accordance with Voiculescu et al. [2006]. This contradicts the results by Jones et al. [1997], Kersley et al. [1997], and references in Rodger et al. [1992], who reported that, during quiet magnetic periods, the poleward edge is steeper than the equatorward one, especially in the premidnight sector. The equatorward density gradient in Figure 6 decreases relatively fast with altitude, while the poleward density remains roughly constant around m 3 at all altitudes. Our results also show that the density gradient is smaller at high altitudes, which is in agreement with general observations of Anderson et al. [1991] or Prölss [2007], who found that the density troughs observed in the upper F region heights are less visible. [25] Although the temperature results are more noisy than the electron density results, they show a systematic behavior which gives some faith in their reliability. The electron and ion temperatures generally behave in an identical manner, but the ion temperature is lower, typically by K, which is a normal situation in the auroral nighttime F region (see, e.g., Holt and van Eyken [2000]). The basic feature in the temperatures is a minimum either at or slightly equatorward of the trough minimum. This is not necessarily clearly visible in each subplot (e.g., in electron temperature at 306 km, 19 MLT); this may be attributed to inaccuracies of the results. The drop of the ion temperature inside the trough is noticeable already at 17 MLT. The temperature minimum moves toward lower latitudes with MLT together with the trough minimum, which is also visible in Figures 4 and 5. The temperature values at the minimum also tend to decrease with MLT, suggesting that plasma is slowly cooling in the absence of photoionization source. Another feature of the minimum is that it seems to get narrower in latitude with MLT. The validity of this conclusion is limited by the coarse latitudinal resolution. [26] The electron temperature at the equatorward wall of the trough is typically K, but K at the poleward wall. Both temperatures show irregular variations on the poleward side of the trough at 19 and 20 MLT, with amplitudes of a few hundred kelvin. They may be caused by irregular heating in the premidnight sector, but the possibility of measurement error cannot be ruled out. [27] The bottom images in the subplots of Figure 6 show the zonal ion velocity in nonrotating coordinates. A very clear pattern in ion velocity is observed. This pattern is quite similar at all altitudes, which is due to the fact that the drift in F region is due to the electric field only. At the southernmost latitudes the drift is eastward. Somewhere to the south of the trough minimum, the drift direction changes from east to west. At later times (19 and 20 MLT) another change of direction back to eastward is met at high latitudes. There is no well defined stagnation region, since velocities close to zero are scarcely seen in the afternoon evening sector. Velocities are westward inside the trough, with average values of m/s. This is better seen in Figure 6, where the trough minimum is collocated with westward (sunward) velocities of about m/s. [28] In addition to the F region observations, it is also of some interest to see the behavior of different parameters at lower altitudes. This is because they may give a clue to the generation mechanisms of the trough. Figure 7 is a plot, similar to Figures 4 and 5, from the upper E region (142 km) and the bottomside F region (182 km). At this time of year, the E region is poorly illuminated even in the daytime, as shown by the low electron density at 142 km. At 182 km, the daytime electron density is high. A more important observation, however, is that an increase in electron density appears at the latitudes of the poleward wall after about 18 MLT. The increase is seen at both altitudes, and this region extends to lower latitudes with increasing MLT. This must be caused by particle precipitation, which suggests that the elevated electron density at the poleward wall in the F region is also associated with precipitation. Although the temperatures are noisy, they still indicate increased values, supporting the role of precipitation in production of the electron density. The velocity patterns are roughly similar to those in the upper F region. This suggests that the velocity must be mainly due to the electric field, although the direction of the plasma flow is affected by ion collisions with neutrals, especially in the E region. [29] Since the F region convection pattern may have a major role in the generation of the trough, it makes sense to compare the results with a model convection pattern. For this purpose, the LiMIE convection model [Papitashvili et al., 1994, 1999; Papitashvili and Rich, 2002; Papitashvili et al., 2002] was used. The input parameters of the model are the date and the values of the IMF components. As seen in Figure 3, there is an unfortunate gap in IMF data during the radar observation. The values of the two components at 1400 UT are By = 7.4 nt and Bz = 1.2 nt. The geomagnetic activity during the gap in the IMF data is low, and therefore no drastic changes in the convection pattern are assumed during the radar observations, although there is a change in the sign of Bz somewhere within the gap (By = 2.6 nt and Bz = 3.4 nt at 1800 UT). Hence it seems reasonable to use the values Bx =1nTBy = 7 nt and Bz = 1 nt as model parameters. The equipotential contours are plotted at intervals of 1.5 kv in Figure 8 on the top of electron density and zonal ion velocity plots at the altitude of 345 km. Zero potential is indicated by heavy dash dotted lines, and positive potentials are plotted with continuous and negative potentials with dashed lines. The pattern is divided into dusk and dawn cells, but obviously only the dusk cell seems to be important for the observed trough. Since the convection Figure 6. Latitudinal profiles of density (black continuous line), electron (blue dashed line) and ion (red dotted line) (top) temperatures and (bottom) transversal velocity (small image, continuous black line) at each of the four selected altitudes at four MLT times: 1700, 1800, 1900, and 2000 MLT. 8of13

9 Figure 7. Horizontal distribution in CGM MLT coordinates of electron density (first row), electron temperature (second row), ion temperature (third row), transversal velocity (fourth row) in the E region and low F region altitudes: 142 km and 182 km. Shown is also the position of the solar terminator as a line of different colours, chosen for the sake of a good contrast with the background. 9 of 13

10 Figure 8. (left) Zonal ion velocity and (right) electron density at 345 km together with the convection pattern given by the LiMIE model with IMF values B x = 1 nt, B y = 7 nt, and B z = 1 nt. The contour interval is 1.5 kv, positive potentials are indicated by continuous and negative potential by dashed lines. patterns are given in rotating frame, the velocities are also calculated in the same frame of reference. [30] In the left image, the model is in good agreement with the velocity observations. The model indicates mainly westward flow within the region where the observed zonal velocities are westward (blue area). Between 22 and 24 MLT, at the highest latitudes, the observed zonal velocity is eastward and also there the model gives eastward flow. Some discrepancy is met at the lowest latitudes before 21 MLT. There the observed velocities are close to zero or slightly eastward, whereas the model shows predominantly westward flow. The misfit between the model and the observations at low latitudes is most likely due to the fact that, according to Papitashvili and Rich [2002], corotation potential is removed in convection model. [31] Altogether, Figure 8 (left) indicates that the model convection pattern is in reasonable agreement with the observations. Therefore, it can be used in interpreting the role of horizontal plasma transport in trough generation. It would be better to do this with observed velocities, but, unfortunately, the observed meridional velocities were too noisy to allow this. Figure 8 (right) shows that the plasma transported to the trough region comes from nightside low latitudes. Hence it has stayed for a long time in darkness and has had a long time to recombine. Plasma from the trough region travels farther within the dusk convection pattern inside the polar cap staying all the time in darkness. However, when it enters the northern edge of the trough, the situation changes because of local production due to particle precipitation. This increases the plasma density and the newly generated plasma will start its path with the convection pattern, which drives it to lower latitudes and toward magnetic midnight. This increased electron density is indeed observed between 22 and 24 MLT at latitudes Hence the northern edge of the trough at later magnetic local times is not necessarily caused by local production due to precipitation, but it may be caused at least partly by horizontal transport from higher latitudes and earlier magnetic local times. [32] The model convection pattern would indicate that even plasma at the equatorward edge would come from the nightside ionosphere, from lower latitudes. However, the measured velocities show eastward components indicating that the plasma at the equatorward edge may actually come from the dayside, having no sufficient time to recombine. 3. Discussion [33] Extensive reviews of trough studies are given by Moffet and Quegan [1983] and Rodger et al. [1992]. These papers discuss observations, modeling and possible theories of trough generation, which must be explained in terms of plasma production, loss, and transport, which are connected by the continuity equation. When considering the trough mechanisms, one should notice that the theory should explain not only the electron density minimum but also the edges. Precipitation as well as plasma transport may have their effects at the walls of the trough. [34] Horizontal transport in F region is due to E B/B 2 drift. Together with corotation, it may lead to stagnation, low velocities or long paths of plasma elements in the nightside ionosphere. This may lead to a low electron density in the trough region, since production is absent and the plasma has had a long time to decay, even if it came from the sunlit part of the ionosphere. The theory of stagnation is generally accepted for the explanation of the quiet time eveningside trough (see e.g., Rodger et al. [1992]; Nilsson et al. [2005]). Whalen [1989] suggested that horizontal transport could explain a trough in the dayside ionosphere. Low density plasma convected from darkness to dayside would lead to a dayside trough minimum. The observations by Pryse et al. [1998] support this theory. [35] A different cause for trough generation would be heating, which could affect the trough generation in various ways. First, elevated temperatures would decrease the plasma density via temperature dependence of the rate coefficients. Then the charge exchange of O + producing rapidly recombining molecular ions would be enhanced. Second, heating would lead to upwelling neutral air, which in turn would speed up the charge exchange of O + because it would increase the molecular neutral concentration at F region altitudes. However, upwelling of the neutral 10 of 13

11 atmosphere is necessarily a slow process, which requires a lot of energy. Finally, heating could also result in an upwelling of the F region plasma, which would decrease the electron density by means of field aligned transport from the trough region. The field aligned ion velocity results are unfortunately so noisy that nothing can be said about the vertical displacement of plasma. However, the temperature results seem to indicate that heating and the associated upwelling and increased recombination do not have a major role in generating the trough minimum. [36] A reason of heating could be a strong electric field. Anderson et al. [1991] reported subauroral ion drifts (SAIDs) with very high ion speeds together with simultaneous high ion temperatures and accompanying ionospheric troughs [Anderson et al., 1991]. Model calculations by Pintér et al. [2006] show that in subauroral polarization stream events (SAPS), the F region electron density rapidly decreases and the altitude of the F layer peak increases in the presence of high electric fields. Anderson et al. [1991] connect the troughs to upward ion velocities, whereas the calculations by Pintér et al. [2006] indicate the main reason to be temperature increase and upwelling of the thermosphere. One should, however, notice that SAIDs are relatively short lived, not more than a couple of hours, and therefore they can hardly be the cause of a persistent trough structure, at least in the dayside ionosphere. In the nightside ionosphere, the situation may be different, since, once produced, a trough may prevail in suitable circumstances. [37] The main finding in temperatures is that both electron and ion temperatures display a clear minimum within the trough region. This result is quite different from that by Prölss [2007], who presented an elevated mean electron temperature within the trough region. This shows that this particular trough is not formed as a result of heating caused by high electric fields. Throughout the present observations, electron temperatures are generally higher than ion temperatures. Temperatures increase at the poleward wall, having occasional peaks which may be due to heating effects but may also be due to unknown errors in the incoherent scatter data. Anyway, these peaks lie northward of the trough minimum. Elevated ion temperatures on the poleward side of the trough are not exceptional and are most likely due to frictional heating. Temperatures at the northern side are higher than at the southern side of the trough. [38] The observations in the present paper come from a relatively quiet period. The trough minimum in electron density is very clearly visible at all F region altitudes. A prominent feature in electron density is that the equatorward wall of the trough is almost invariably steeper than the poleward wall. The latitudinal electron density profile greatly resembles the mean profile presented by Prölss [2007] and Voiculescu et al. [2006], but clearly contradicts the results shown, e.g., by Jones et al. [1997] and Kersley et al. [1997]. The drops in all ionospheric parameters, density, ion temperature, and electron temperatures in regions of sunward flow, indicate that the trough is formed via transport of plasma from nightside that has traveled a long time in the nonilluminated ionosphere and decayed in the absence of any ionisation source. [39] The idea of stagnation is connected with this mechanism, but, of course, strict stagnation can only be valid at some specific points in the F region, not within large areas within the trough. Since the meridional velocity was too noisy for the determination of the convection pattern, the observed zonal velocity was compared with the convection pattern given by the LIMIE model (Figure 8). The resulting agreement gives a reason to interpret the trough generation in terms of the convection pattern. The convection pattern clearly suggests that the low density plasma in the trough region comes from the nightside. It has obviously been traveling for a long while in darkness so that the decay processes have had time to operate. This is much in accordance with Sims et al. [2005]. One should notice that the low density plasma in the trough does not come directly from the polar cap but rather from lower latitudes. As a matter of fact, the flow lines are nearly parallel to the orientation of the trough. Further evidence for the importance of convection is given by the temperature minimum within the trough region, since upwelling and enhanced recombination would imply high temperatures [Anderson et al., 1991; Prölss, 2007]. Low plasma temperatures are expected to result from loss of energy in the absence of solar illumination or particle precipitation. [40] It is not only the trough minimum but also the walls of the trough which require some consideration. Electron density in the E and lower F region indicates the presence of particle precipitation at the northern wall of the trough. Elevated temperatures are also observed there. The electron density is indeed higher at MLT at lower latitudes. The convection is expected to drive this plasma further to even lower latitudes, and from there to the trough region after decay. It should be noticed that direct plasma transport from the polar cap to the trough would not be favorable for trough generation, since it would drive the high density plasma directly to the trough region. The conclusion is that the poleward wall is connected with both particle precipitation and convection. Pryse et al. [2009] and Middleton et al. [2008] reached a similar conclusion for the dawnside trough. They attribute the formation of the poleward wall to polar cap ionization transported into the region with modulation by particle precipitation. [41] At the equatorward wall of the trough, the LiMIE model convection pattern shows clear westward zonal velocities (Figure 8). This contradicts with the observations, which indicate small zonal velocities, mostly eastward. It is possible that at subauroral latitudes the convection model might not reproduce correctly the ionospheric velocities, because of the role of corotation on the velocity field at low latitudes. Moreover, the IMF structure during this particular experiment implies that the effect of magnetospheric convective electric fields is limited to higher latitudes. Hence, on the basis of the observed eastward zonal velocities, it is plausible that corotation dominates convection so that plasma comes from the sunlit part of the ionosphere. If the path from the sunlit part is not too long, the decay processes have not had time to cause too much loss. This seems to be the explanation of both the enhanced electron density and the temperatures at the equatorward wall of the trough. Our measurements show that the equatorial wall is composed of plasma from the illuminated F region, decaying with time in the absence of photoionization. This is supported by the gradually decreasing electron densities, decreasing equatorward density gradients, normal values of both electron 11 of 13

12 and ion temperatures, and anti sunward ion flow mainly due to corotation. [42] Nilsson et al. [2005] have also investigated the evening time trough using the EISCAT UHF radar and DMSP satellite data. They argue that stagnation (convection within nighttime ionosphere) can usually explain the observations, but sometimes height profiles of electron density may differ from what is expected. They suggest that downward Region 2 currents may decrease the ionospheric electron density within the trough. [43] It should be pointed out that the results in this paper come from a relatively quiet ionosphere. Different mechanisms may be important during disturbed periods or high electric fields. For instance, Vlasov and Kelley [2003] show in their modeling study of a storm generated trough that the effect of horizontal plasma transport on the electron density is insignificant in comparison with the effects of neutral wind and increased recombination. 4. Conclusions [44] In this paper, we have used an EISCAT incoherent scatter experiment for investigating some properties and formation mechanisms of a quiet time ionospheric trough in the evening sector. The experiment consists of alternating azimuthal and meridional scans and therefore beams from successive scans can observe the same ionospheric volume in nonrotating frame. This gives the possibility of obtaining several measurements from the same volume and, especially, the ion velocity vector. [45] A trough was identified in electron density latitude profiles, which could be monitored for about 12 hr, over a 10 latitudinal extent. Although the other velocity components are noisy, the zonal velocity, together with the other plasma parameters, was used to draw conclusions on the generation mechanism of the trough. [46] The results confirm some previously observed characteristics of the trough; the postnoon trough moves equatorward with magnetic local time and gets wider and shallower at higher altitudes. The northern wall of the trough is due to precipitation, similar to what Jones et al. [1997] found. The electron temperature is higher than the ion temperature throughout the observations. [47] An important finding is that a pronounced ion and electron temperature minimum is collocated with the density minimum or the equatorward wall. Contrary to some previous results [Jones et al., 1997; Kersley et al., 1997] but in accordance with Prölss [2007], the equatorward wall is steeper than the poleward wall. Higher temperatures are observed on the poleward side of the trough than on the equatorward side. Finally, the observed zonal ion velocity has a reasonable, although not perfect, agreement with a model convection pattern, which indicates plasma flow from nightside roughly along the trough structure. [48] The low temperatures in the trough region indicate that the trough mechanisms connected to heating can be ruled out. The convection pattern indicates that the plasma within the trough has convected from the magnetic midnight to low latitudes before entering the trough. Hence, the plasma has spent a long time in darkness and the decay processes have had time to operate without any ionization. The poleward wall of the trough is caused by particle precipitation and partly by convection of plasma produced by this precipitation. Our explanation for a steep equatorward wall of the trough, observed especially at early MLT times, is that plasma of high density is carried by corotation from the dayside to the trough edge. [49] This particular EISCAT experiment proved to be very useful in investigating the quiet time ionospheric trough and in studying the physical mechanisms that contribute to its formation as well as to its evolution. However, the analysis method used in this paper might not be appropriate for disturbed conditions. The method uses several observations at different times from the same volume in nonrotating frame and assumes a stationary ionosphere. If the ionosphere is too variable, the assumption becomes obviously a poor one and the results cannot be reliable. [51] Acknowledgments. This paper was presented at the 14th International EISCAT workshop in Tromsø, EISCAT is an international association supported by China (CRIRP), Finland (SA), Japan (STEL and NIPR), Germany (DFG), Norway (NFR), Sweden (VR), and the United Kingdom (STFC). The work of MV was partially supported by a research fellowship of the University of Oulu, Finland. 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