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Advances in Space Research 40 (2007) 1935–1940

IMF effect on ionospheric trough occurrence at equinoxes

Mirela Voiculescu a,b,*, Tuomo Nygren b

a Department of Physics, University ‘‘Dunarea de Jos’’ of Galati, St. Domneasca, No. 47, 800008 Galati, Romaniab Department of Physical Sciences, University of Oulu, P.O. Box 3000, Oulu FIN-90014, Finland

Received 11 November 2006; received in revised form 14 February 2007; accepted 14 April 2007

Abstract

Previous observations have shown that there is a relationship between the F region trough and both Bz and By components ofthe interplanetary magnetic field (IMF). Since IMF governs the polar cap convection, we investigate here if this relationship can beexplained by means of polar cap convection. The study is limited to equinox seasons. The poleward and equatorward edges of thetrough are determined from satellite tomographic observations and their locations are plotted in magnetic coordinates together withthe convection pattern given by Papitashvili and Rich [Papitashvili, V.O., Rich, F.J. High-latitude ionospheric convection modelsderived from DMSP ion drift observations and parameterized by the IMF strength and direction. J. Geophys. Res. 107, 2002,doi:10.1029/2001JA000264] using IMF measurements coincident with trough observations. The results indicate a close relationshipbetween the troughs and convection. Most of the troughs are seen within the dusk cell and the pattern of trough observationsrotates with the convection pattern, when By changes its sign. More dayside troughs are observed when Bz is negative than inthe opposite case, i.e. fast convective flow favours the dayside trough occurrence. Nightside troughs are observed more frequentlywhen By is negative. In both evening and morning sectors the trough is situated close to the edges of convection cells, which partlycontradicts previous results showing that the troughs are associated with the convection reversal. It is concluded that plasma con-vection has an important role in trough generation, although the effect of a strong electric field and other mechanisms like precip-itation certainly have a role of their own.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Ionospheric trough; IMF effect; Convection pattern

1. Introduction

The ionospheric trough is a plasma density depletionobserved at F region heights at geographic latitudes around55–75�. It is elongated in longitudinal direction and itswidth in the latitudinal direction is of the order of 5–10�.Usually, the trough is accompanied by the term mid-lati-

tude when it occurs lower than the equatorward edge ofthe auroral oval and high-latitude when it is located withinor poleward of the auroral oval (Rodger et al., 1992). We

0273-1177/$30 � 2007 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2007.04.108

* Corresponding author. Address: Department of Physics, University‘‘Dunarea de Jos’’ of Galati, St. Domneasca, No. 47, 800008 Galati,Romania.

E-mail addresses: mirela.voiculescu@ugal.ro (M. Voiculescu), tuomo.nygren@oulu.fi (T. Nygren).

will refer here simply to ‘troughs’, in accord with Whalen(1989), since it is difficult to differentiate between themid-latitude trough and high-latitude trough.

The observation of the trough started a long time agoand many of its properties are presently well established.One of these properties is the diurnal variation of the posi-tion of the trough. At a fixed geomagnetic level, the troughmoves towards lower latitudes with local time in the post-noon sector (Moffet and Quegan, 1983; Whalen, 1989;Rodger et al., 1992; Kersley et al., 1997; Voiculescuet al., 2006). Around local noon the trough reaches itsnorthernmost location. Another well-known feature is thelink to the geomagnetic activity. When the geomagneticactivity level is high, troughs occur at lower latitudes fora given local time or at earlier local times for a given lati-tude, as shown by Moffet and Quegan (1983), Rodger

ed.

1936 M. Voiculescu, T. Nygren / Advances in Space Research 40 (2007) 1935–1940

et al. (1992), Horvath and Essex (2003), Pryse et al. (1998),Voiculescu et al. (2006) and references therein. This rela-tionship is more obvious for the equatorward edge of thetroughs and, generally, does not depend on the season(Rodger et al., 1992; Voiculescu et al., 2006). Troughsobserved in summertime seem to be an exception in thesense that they are observed at high latitudes during med-ium or high Kp (Voiculescu et al., 2006).

The formation of the trough may involve more than asingle physical process. Some mechanisms relate to globalplasma convection. A local density minimum may appearwhen plasma of high and low density from different regionsof the ionosphere move next to each other. In the evening-premidnight sector, which is usually inside the dusk (even-ing) cell, the plasma flow stagnates in regions where thewestward convection is cancelled by the eastward corota-tion. In such stagnation regions, a trough forms becausethe plasma density decays due to the long stay in darkness(Whalen, 1989; Rodger et al., 1992; Pryse et al., 1998). Thisscenario was confirmed by Collis and Haggstrom (1988),using EISCAT data, and by Nilsson et al. (2005) whofound a good similarity between numerical estimationsand EISCAT results. In the postmidnight-morning sector,generally coinciding with the dawn (morning) cell, thereis no stagnation, but the plasma flow may be so slow atlow latitudes that there is a sufficient time for recombina-tion to produce a depletion (Whalen, 1989; Rodger et al.,1992; Pryse et al., 1998). The dayside troughs form at highlatitudes and might be due to low-density plasma con-vected from the nightside and replacing the high-densitydayside plasma (Whalen, 1989; Rodger et al., 1992; Pryseet al., 1998).

There are also local mechanisms which might contributeto the formation of a trough. One is the faster recombina-tion due to increased ion temperature, which leads to ero-sion in plasma density. The ion heating may be caused byhigh electric fields (Rodger et al., 1992; Nilsson et al.,2005) or by friction due to fast neutral winds (Crickmoreet al., 1997; Vlasov and Kelley, 2003; Rodger et al.,1992). Another local cause could be field-aligned plasmaupflow, produced by large horizontal winds, electron heat-ing or sub-auroral ion drifts. Joule heating at E region alti-tudes may also increase the molecular ion density in the Fregion, which induces faster recombination and could beone of the causes of the trough formation. Electron precip-itation plays a role in forming the poleward trough walls(Jones et al., 1997; Rodger et al., 1992) while proton precip-itation may be connected with the depletion itself (Nilssonet al., 2005). Each of these mechanisms should be consid-ered when analysing the formation of a specific trough.However, horizontal ion convection, bringing plasma withdifferent densities close to each other, plays the main rolefor most troughs (Moffet and Quegan, 1983; Whalen,1989; Rodger et al., 1992; Nilsson et al., 2005).

The convection pattern depends strongly on the projec-tion of the interplanetary magnetic field (IMF) in the yz-plane, and therefore the occurrence of the trough could

also be related to IMF. This possibility was investigatedin the statistical study of Voiculescu et al. (2006), hence-forth referred to as Paper I. They found that troughs occur-ring during low geomagnetic activity coincide with positiveBz and negative By while, at higher Kp values, troughsoccur more often when Bz is negative. In the present paperwe try to analyse to what extent the observed relationshipbetween the trough occurrence and the orientation of theIMF can be explained by the role of the convection patternin the trough formation.

2. Data analysis

The results of Paper I, on which the present paper isbased, were obtained from a database of satellite tomo-graphic observations collected during one year. Troughswere searched from this database in the following manner.The latitudinal profile of average electron density betweenthe heights of 200 and 400 km was calculated and troughswere identified from minima in these curves. The edges ofthe trough were defined as points where the height-inte-grated density drops to 50% from the outside value. Thisdefinition puts a criterion to the definition of the trough;if the average electron density at a minimum is too great,no edge can be defined and the observation is not countedas a trough. In many cases only one of the edges, either thepoleward or the equatorward one, can be determined. Anobservation is counted as a trough only, if a minimum isvisible within the field of view and the minimum is deepenough to allow the determination of at least one edge withthe 50% criterion.

The magnetic coordinates (latitude and local time) ofthe two edges (poleward and equatorward) were deter-mined and the values of Kp, By and Bz were collected foreach trough observation. The IMF data were providedby the WIND satellite for the first half of the year andby the ACE satellite for the second half. Since the IMFcomponents vary with time, 15-min averages were com-puted. The delay time from the satellite to the subsolarmagnetosphere was estimated using the solar wind datagiven by the same satellites. The trough edges are locatedbetween 56� and 72� CGM Lat, which is the satellite fieldof view. This latitude range practically excludes polarholes. For further details of the analysis, see Paper I.Because there are cases when only one of the two edgescan be determined, the poleward and equatorward edgesare investigated separately.

We limit the study to equinox seasons (i.e. troughs fromMarch–April and September–October; these constitute atleast one half of all observations). The reason is that thereis a seasonal variation (between summer, winter and equi-nox seasons) both in the convection pattern for a givenIMF direction (Papitashvili and Rich, 2002) and in thediurnal variation of the trough (Paper I). Fig. 1 showsthe seasonal dependence of trough observations for differ-ent IMF directions both at low and at high magnetic activ-ity. It is obvious that summer is different from the other

Fig. 1. Number of trough observations during spring (continuous line),summer (dash), fall (dot) and winter (dash-dot) for low and highgeomagnetic activity and for each of the four possible orientations ofIMF in the yz-plane.

M. Voiculescu, T. Nygren / Advances in Space Research 40 (2007) 1935–1940 1937

seasons. The distributions in spring and fall are rather sim-ilar, except for the fact that the total number of observedtroughs is much smaller in spring that at fall when Kp islow. A tentative explanation for the dependence of troughof both IMF and Kp was given in Paper I; here we willfocus mainly on the relationship between the location ofthe edges relative to the convection pattern.

Fig. 2. The equatorward (left) and poleward (right) edges of the observedtroughs in geomagnetic coordinates (CGMLat and MLT) for the fourpossible orientations of the IMF in the yz-plane, together with contours ofelectric potential given by the convection model byPapitashvili and Rich(2002). Observations with low (Kp 6 3) and high (Kp P 4) geomagneticactivity are marked with crosses and circles, respectively. The number oftrough observations is indicated at each subplot. The contour interval is4 kV; dashed lines stand for negative potentials, continuous lines forpositive potentials and the dash-dotted lines indicate the zero potential.The heavy circles show the highest and lowest latitudes of the satellite viewand the gray semicircle the extent of the dark side for equinox conditions.

3. General features of the trough

Fig. 2 shows the magnetic latitude and magnetic localtime of the observed trough edges, when the IMF pointsin each of the four quadrants in the yz-plane. The positionsof the poleward and equatorward edges are in the right andleft hand columns, respectively. Observations during low(Kp 6 3) and high (Kp P 4) geomagnetic activity are shownby different markers. The idea is to see whether the behav-iour of trough is different for high and low Kp for differentsectors of the IMF clock angle. This is of some interestbecause, regardless of the fact that there is a relationshipbetween Kp and IMF direction, high or low magnetic activ-ity may take place at any clock angle.

Superimposed in Fig. 2 are the convection patternsobtained by running the DMSP-based model of iono-spheric electrostatic potentials (http://www.sprl.umi-ch.edu/) described by Papitashvili and Rich (2002). In themodel, Bx was set to 1 nT, while the values of the othertwo components were averages of By and Bz at times ofthe observed troughs for each of the four cases. The greysemicircles portray the mean extent of night in equinoxconditions, i.e. the region where the ionospheric plasma isin darkness.

We will first note some general features of Fig. 2. Bothedges move toward lower latitudes in the evening andnight, reaching a minimum around 04–05 MLT and startretreating polewards around dawn, in accord with previousobservations (Pryse et al., 1998; Kersley et al., 1997; Kar-pachev et al., 1996). There is an indication that, at highgeomagnetic activity, troughs occur at lower latitudes,

but the latitudinal scatter at a given time is large. This sup-ports Rodger et al. (1992), Kersley et al. (1997) and Nilssonet al. (2005) who conclude that it is difficult to a establish aclear relation between the trough and Kp.

1938 M. Voiculescu, T. Nygren / Advances in Space Research 40 (2007) 1935–1940

It is known that the poleward wall of evening troughssometimes coincides with the equatorward precipitationboundary occurring in an already depleted ionosphere(Rodger et al., 1992; Jones et al., 1997; Horvath and Essex,2003). Thus the equatorward movement of the polewardedge with increasing geomagnetic activity may be explainedby the expansion of the auroral oval. However, this expla-nation does not hold for morning troughs, whose polewardedge is significantly further away from the equatorwardedge of the precipitation, except at high levels of geomag-netic activity (Rodger et al., 1992). Moreover, both Rodgeret al. (1992) and Paper I show that it is the equatorwardrather than the poleward edge of the trough that has astronger relationship with Kp.

Fig. 2 shows that, at least in the dusk cell, the equator-ward edge of the trough is closely associated with theboundary of the convection cell. When Bz is negative, theequatorward edges are more scattered in latitude than inthe opposite case, which seems to be connected to the sizeof the cell. Due to the relation between Bz and Kp, it is pos-sible that the observed southward movement of the equa-torward edge with increasing Kp is connected to theincreasing size of the dusk cell. In order to verify this, astudy on the actual size of the cell during individual troughobservations should be made.

Fig. 2 confirms some of the findings in Paper I.Troughs occur during high geomagnetic activity mostlywhen Bz is negative (there are more circles in panels(a) and (b) than in panels (c) and (d)). Of course, thisis not surprising since the probability of high Kp isincreased during southward IMF. A more interestingobservation is the By dependence, already visible inFig. 1 but also in Fig. 2. By comparing panels (a) withpanels (b) as well as panels (c) with panels (d) we seethat about 70% of the troughs at low geomagnetic leveloccur during negative By when Kp 6 3, while about 60%of them are found during positive By, when Kp P 4.Hence negative By favours troughs occurring at low geo-magnetic level while troughs at higher magnetic activityare slightly more numerous when By is positive .

4. Relation of the trough to IMF

4.1. Daytime troughs

There are more trough observations in the daytime (i.e.between 08 and 20 MLT) than at night and, out of these,the majority are associated with the dusk cell. The equator-ward edge is visible more often than the poleward edge.Close to the magnetic noon the poleward edge is hardlyever seen, which indicates that it must lie at high latitudes,outside the field of view of the satellite. For all IMF orien-tations, the trough observations in the postnoon sector areclearly linked to the orientation and size of the dusk cell.This is best seen in the locations of the equatorward edge,which clearly follows the margin of the dusk cell, for anyorientation of the IMF. It is obvious, especially in panels

(a) and (b), that the pattern of observed troughs rotatesclockwise together with the convection pattern, when thesign of By changes from negative to positive. This is a clearindication of the role of the convection pattern in throughgeneration. This also indicates that the equatorward wall ofthe dayside trough is composed of the normal daytime Fregion plasma and the trough is the result of sunward con-vection of low density plasma from the nightside, as pro-posed by Rodger et al. (1992) or Pryse et al. (1998).

Following Rodger et al. (1992) or Pryse et al. (1998),we suggest that the generation of the poleward wall ofthe daytime trough is contributed by the following trans-port mechanism. The plasma entering within the convec-tion cells from the nightside has a reduced density andthe amount of decay depends on the length of the timethe plasma has stayed in darkness. This, on the otherhand, depends both on the drift velocity and on the pathlength. Considering the dusk convection cells in panels(a) and (b), for instance, it is obvious that plasma flow-ing along the outer equipotentials has stayed a longertime in darkness than the plasma flowing along the innerequipotentials. Therefore its density will be lower thanthe density of the plasma at higher latitudes and it willalso travel a longer distance to the illuminated side,before the effect disappears due to the solar ionisation.Thus the poleward wall will be produced by a plasmawhich is either continuously illuminated or has travelledonly a short distance in darkness.

When Bz is negative, the plasma flow is fast especiallyin the dusk cell. Poleward edges of the daytime troughare seen more often when By is positive than in theopposite case. This is probably associated to the fact thatthe length of the equipotentials in darkness is longer inthis case. The electric field, which is stronger for positiveBy (Weimer, 2001; Papitashvili and Rich, 2002; Ruohon-iemi and Greenwald, 2005) and leads locally to a fasterdecay (Rodger et al., 1992), may also partly explainthe observation. The transport mechanism is less likelyto work in the morningside since, especially when By ispositive, the dawn cell is almost completely in darkness.The number of trough observations after sunrise is quitesmall, indeed.

When Bz is positive (panels (c) and (d)), plasma convec-tion from the nightside within the dusk cell is slower andmore limited in latitude than in the case of negative Bz.Then troughs are observed in the afternoon sector in a rel-atively narrow latitudinal band. Still, the pattern of troughobservations in the afternoon sector follows the rotation ofthe convection pattern when By changes its sign. In the pre-noon sector the number of trough observations is verysmall. When By is positive, plasma transport from nightsideis not possible and therefore the transport mechanism isnot even expected to be effective in this case. Another pos-sible factor affecting the low occurrence of daytime troughsfor positive Bz is the weakness of the electric field; thenrecombination is not so fast as for negative Bz, when theelectric field is strong.

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4.2. Night-time troughs

On the nightside the number of trough observations isgenerally smaller than on the dayside. The equatorwardedge of nightside troughs is visible almost exclusively inthe premidnight sector. Even there the number oftroughs is relatively small and, regardless of the sign ofBz, it seems to be favoured by negative By. In the post-midnight sector, only the poleward edge is seen, whichsuggests that the troughs must be located at low lati-tudes. The postmidnight troughs are also more oftenseen when By is negative than in the opposite case, justlike in the premidnight sector. Another observation isthat, regardless of the sign of Bz, Kp is mostly low whennight-time troughs are observed. This might be due tothe fact that during high magnetic activity these occurat lower latitudes, thus being often outside the field ofview of the satellite. Troughs occurring after midnightare co-located with the outermost equipotential lines ofthe convection cells. This clearly disagree with some pre-vious results (Rodger et al., 1992), which connect themorningside troughs to the connection reversal. Practi-cally none of our morningside observations can be con-nected to this part of the dawn cell.

Just like in the daytime, the pattern of trough observa-tions in the night-time seems to follow the rotation of theconvection pattern when By changes its sign. Thereforewe suggest that the horizontal transport of plasma playsan important role in the formation of nightside troughs.In brief, the mechanism is as follows: When the convectionpatterns carry the daytime plasma over the polar cap,recombination will reduce the plasma density in the dark-ness. The amount of decay depends on the drift velocityand the path length. In the case of evening cells, forinstance, the path length in darkness along the innerequipotentials is shorter than along the outer equipoten-tials, and therefore higher plasma densities are expectedat high latitudes than at low latitudes. The higher plasmadensities may contribute to the generation of the polewardwall of the trough. As a matter of fact, the flow along theouter equipotentials will lead the plasma into the stagna-tion region, and then quite low plasma densities will beexpected there.

When both Bz and By are negative, the flow path alongthe inner equipotentials of the dusk cell is essentiallyshorter in darkness than along the outer equipotentials(panel (a)) in Fig. 2). This favours the generation of thepoleward wall via the transport mechanism. When By

changes its sign (panel (b)), the convection pattern rotatesclockwise and the dusk cell expands towards dawn. Thenthe flow path in darkness along the inner equipotentialsgets essentially longer, which may prevent the generationof the poleward wall. A similar mechanism might alsowork on the morningside, at least close to the midnight.When By is negative (panel (a)), the dawn cell is partly illu-minated, but for positive By (panel (b)) it is completely inthe darkness. In the former case there is some chance that

the transport mechanism might work, but no such possibil-ity exist in the latter case.

When Bz is positive (panels (c) and (d)), the convectionpatterns behave essentially in the same way as in the case ofnegative Bz, with the exception that the plasma flows moreslowly and the dawn cell practically disappears when By

changes its sign from negative to positive. Therefore thetransport mechanism is expected to work qualitatively inthe same way as in the case of negative Bz. This is indeedobserved; the number of night-time troughs is clearly smal-ler when By is positive than in the opposite case. However,for positive Bz the convection pattern changes significantlywhen By turns from positive to negative, which might bethe reason for the strong By effect on the occurrence oftroughs during positive Bz that was found in Paper I.

5. Conclusions

In this paper we investigate the link between the Fregion trough in equinox seasons and the IMF orientationin the yz-plane. The location of the edges of the trough areobtained from a satellite tomography chain and they coverthe region between 56� and 72� CGM Lat. The IMF datawere obtained from the WIND and ACE satellites. Dueto the latitudinal range of the observations, we see moretroughs in the daytime than at night. Night-time troughslie further in the south than the daytime troughs, and there-fore they may often be invisible in our data. The compari-son with convection patterns also shows that the majorityof trough observations are associated with the dusk cell.

We see a close relationship between the patterns of equi-nox trough observations and F region plasma convectiongiven by the convection model by Papitashvili and Rich(2002). The most obvious feature in the results is that thepattern of trough observations rotates together with theconvection pattern when By changes its sign. Hence plasmaconvection must have an important role in troughgeneration.

Of course, there are several mechanisms which affecttrough generation and they are associated with ion produc-tion, ion recombination and plasma transport. Forinstance, electron precipitation often produces the pole-ward wall of the night-time trough and enhanced recombi-nation due to temperature increase may lead to plasmadepletion. In this paper, however, we mainly ponder thetransport mechanism which carries plasma with differentdensities to adjacent regions in the F layer. When a convec-tion cell is only partly illuminated, the plasma flowingalong the inner equipotentials in the cell is either continu-ously illuminated or stays only a short time in darkness,whereas the plasma flowing along the outer equipotentialsstays there for a long time. In the latter case recombinationleads to a greater plasma depletion. When these plasmasenter the dayside from the nightside, high plasma densitywill be located poleward of the low plasma density and adayside trough will be visible. In the nightside, the outerequipotentials of the dusk cell may guide the plasma into

1940 M. Voiculescu, T. Nygren / Advances in Space Research 40 (2007) 1935–1940

the stagnation region, which enhances the difference of theplasma densities at different latitudes.

A considerable qualitative agreement exists between thetransport theory and trough observations. When Bz is neg-ative, the plasma entering the dayside from the nightsidealong equipotentials within the evening cell travels a longerpath in darkness when By is positive than in the oppositecase. Then plasma flowing along them is also more likelyto make a depletion matching our criterion of troughsearch. This is in accordance with the fact that the numberof observed daytime troughs within the dusk cell is higherfor positive By than for negative By, when Bz is negative. Inthe nightside of the dusk cell the situation is different. Sincethe path of the plasma flowing along the inner equipoten-tials is longer when By is positive than in the opposite case,it is less likely that this plasma could make a wall on thepoleward side of the stagnation region. This is in agreementwith the observation that there are only a few observationsof the nightside trough within the dusk cell, when Bz isnegative.

In the case of positive Bz the transport mechanismworks within the dusk cell in a rather similar way. Themain difference is that the poleward edge of the troughappears within a narrow latitude range, which can beattributed to a smaller size of the convection cell.

The effect of the transport mechanism within the dawncell is reduced by its small size, which means that it isnot illuminated at all, when By is positive. Then it cannotcarry depleted plasma to the dayside. When By is negative,there is a possibility that dayside plasma flowing over thepolar cap may contribute the to the poleward edge of thetrough. This is in agreement with the observations.

It is clear that precipitation and other factors likechanges in recombination rate due to stronger electric field,increased temperature or plasma upflow, as well as solarionisation acting on a depleted plasma, have their effectson the trough. Our results indicate, however, that the ori-entation of the polar cap convection pattern, i.e. theIMF, has an important role in trough generation. The con-clusions made in this paper are only qualitative, of course,and the true significance of the transport mechanism rela-tive to the others can only be determined by means ofmodel calculations and, preferably, by a more extensivedata set.

Acknowledgements

The Scandinavian tomography chain is run by Sod-ankyla Geophysical Observatory, Finland. We are gratefulto T. Raita and J. Manninen for their continuous effort inmaintaining the receiver chain and to M. Lehtinen, M.Markkanen and late J. Pirttila for their efforts in develop-

ing the tomographic inversion routine. Ilkka Virtanen isacknowledged for his work on automatic selection oftroughs. We thank V. Papitashvili for developing theweb-based interactive interface to the Linear Models ofIonospheric Electrodynamics and making it available athttp://www.sprl.umich.edu/.

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