Ionospheric and geomagnetic responses to changes in IMF B_Z: a superposed epoch study

. Superposed epoch studies have been carried out in order to determine the ionospheric response at mid-latitudes to southward turnings of the interplane-tary magnetic ﬁeld (IMF). This is compared with the geomagnetic response, as seen in the indices K p , AE and Dst . The solar wind, IMF and geomagnetic data used were hourly averages from the years 1967–1989 and thus cover a full 22-year cycle in the solar magnetic ﬁeld. These data were divided into subsets, determined by the magnitudes of the southward turnings and the concomitant increase in solar wind pressure. The super-posed epoch studies were carried out using the time of the southward turning as time zero. The response of the mid-latitude ionosphere is studied by looking at the F-layer critical frequencies, f o F 2, from hourly soundings by the Slough ionosonde and their deviation from the monthly median values, (cid:100) f o F 2. For the southward turnings with a change in B z of (cid:100) B z > 11 : 5 nT accompanied by a solar wind dynamic pressure P exceeding 5 nPa, the F region critical frequency, f o F 2, shows a marked decrease, reaching a minimum value about 20 h after the southward turning. This recovers to pre-event values over the subsequent 24 h, on average. The Dst index shows the classic storm-time decrease to about ) 60 nT. Four days later, the index has still to fully recover and is at about ) 25 nT. Both the K p and AE indices show rises before the southward turnings, when the IMF is strongly northward but the solar wind dynamic pressure is enhanced.


Introduction
The daily variation of the F region electron density distribution greatly affects the propagation of radio waves through the ionosphere.Radio waves that are most affected lie in the frequency range 3KHz-30MHz, where the extremes of this range correspond to signals in the Very Low Frequency (VLF) and High Frequency (HF) bands, respectively.Of particular importance is the maximum plasma density of the F layer.A radio wave incident vertically on this layer will be reflected if its frequency is less than the critical frequency of the layer, f o F2.This critical frequency (in Hz) is numerically equal to about nine times the square root of the maximum electron density (in m -3 ).The maximum usable frequency, or MUF, of an oblique propagation path is roughly proportional to f o F2, the ratio being known as the M-factor (e.g.Lockwood, 1983).The day-to-day variability of f o F2 is the major problem in predicting MUFs and, despite many attempts to find the cause of this behaviour (e.g.Aravindan and Iver, 1990), its origins remain unknown.More major disturbances, or storms, are better documented but not much better understood, being the subject of a vast number of publications (e.g.Pro ¨lss et al., 1991 and references therein).This variability greatly limits the efficiency of systems using HF frequencies such as communications, radar and navigation (e.g.Bradley, 1991).An example of a storm-type variation of ionospheric densities is shown in Fig. 1.In this figure, the crosses show the hourly values of f o F2 from Slough on 6-12 November, 1991.The circles show the monthly median values of f o F2 at the same UT (i.e. the value that is exceeded on half the days in the month at the UT in question).The median values reveal the diurnal cycle of f o F2, with larger values in the day (peaking in the early afternoon at about 12 MHz) and low values at night (falling to near 3 MHz just before sunrise).A storm commences shortly before midnight on 8th November (day 2).Prior to this there are some quiet days, on which plasma densities are higher than the monthly median values.(The monthly medians tend to be depressed by the presence of storm-type depletions within the month, Wrenn et al., 1987).For several hours after the onset of the storm f o F2 is either too low to be measured (sometimes because it is lower than f o E) or the F-layer cannot be seen because of enhanced absorption of the sounding waves in the D region.However, f o F2 recovers at sunrise on day 3, but is much lower than the monthly median on this day.By day 5, the variation is almost exactly the same as the monthly median and by day 6, it is almost back to the quiet day variation before the storm.
The storm-time behaviour is often quantified by taking the deviation of f o F2 from its monthly median values in Fig. 1, i.e. the value of f o F2 minus its monthly median value at that UT, f o F2.This has largest negative values ( f o F2 )5 MHz) around noon on day 3, but only very small negative values during the subsequent night when f o F2 is small even on quiet days.
A second, smaller minimum in f o F2 ( )4 MHz) is seen around noon on day 4, whereas f o F2 is very close to zero throughout day 5. Figure 2 places these values in context of the overall ionospheric variability.This is a plot of the distribution of all f o F2 values in the period of this survey , showing the number of occurrences of f o F2 in 0.1 MHz bins.The dashed line is the probability distribution (i.e. the probability of a given f o F2 value) and the solid line is the cumulative probability distribution (here meaning the probability that f o F2 is larger than a given value).It can be seen that a depression of as large as f o F2 )5 MHz (as in Fig. 1) is very rare indeed ( < 0.1% of all soundings).The distribution is not symmetric, such that negative f o F2 values (depletions) are slightly more common than positive f o F2 (enhancements) of the same magnitude.As a result most f o F2 lie in the range )4 MHz to 3 MHz.A large number of cases show f o F2 0, most of which are for night conditions.Note that other papers have discussed f o F2 as a ratio of the median value or the actual f o F2 values as a ratio of the median (e.g.Wu and Wilkinson, 1995) as they will not show as large a diurnal variation as f o F2.However, there is no guarantee that any one ratio will completely remove the diurnal variation and we prefer to work with the absolute deviation from the median and bear in mind that it will show a strong diurnal variation.One exceeding the value on that axis.Note the vertical scale is for the cummulative probability distribution and also applies, but divided by a factor of 10, for the probability distribution advantage of this approach is that f o F2 is directly proportional to the loss in bandwidth available to the operator of an HF system employing reflections from the F2 layer.
The objective of this study is to look at the effect of the north-south component of the interplanetary magnetic field (IMF) on the critical frequency of the ionospheric F layer as observed at the mid-latitude station.Because the ionospheric effect is well correlated with geomagnetic disturbance, as seen in a variety of magnetic indices (e.g.Wilkinson, 1995), we also look at the associated geomagnetic response.It is well established that the B z component of the IMF is the most important influence on the magnetosphere and highlatitude ionosphere, as it controls the fraction of the energy in the solar wind flow which is extracted by the magnetosphere.When B z is strongly negative, magnetic reconnection between the IMF and the geomagnetic field produces open field lines which allow mass, energy and momentum to be transferred from the solar wind to the Earth's magnetosphere, as first suggested by Dungey (1961).Many features of the Earth's magnetosphere and high-latitude ionosphere are known to depend on B z in a reference frame which allows for the orientation of the Earth's magnetic axis (such as the GSM frame).These include the cross polar cap potential (the ionospheric signature of the voltage across the magnetosphere) (Reiff et al., 1981;Doyle and Burke, 1983;Wygant et al., 1983;Cowley, 1984;Reiff and Luhmann, 1986); the patterns of the associated high-latitude F-region ionospheric flows (Heelis, 1984;Heppner and Maynard, 1987;Etemadi et al., 1988) and E-region currents (Nishida, 1968;Friis-Christiansen et al., 1985); field aligned currents (Iijima et al., 1978); the average size of the polar cap (Holzworth and Meng, 1984), the occurrence of global geomagnetic activity (e.g.Schatten and Wilcox, 1967;Arnoldy, 1971;Berthelier, 1976;Baker et al., 1981Baker et al., , 1983;;Clauer et al., 1981;Bargatze et al., 1985;Scurry and Russell, 1991), including magnetospheric substorms (Rostoker and Falthammer, 1967;Caan et al., 1977;Meloni et al., 1982;Samson and Young, 1986); the occurrence of reconnection signatures such as flux transfer events (Berchem and Russell, 1984) and high speed flow streams at the magnetopause (Scurry et al., 1994); the energy deposition into the high-latitude ionosphere and thermosphere (Akasofu, 1981;Bargatze et al., 1986;Weiss et al., 1992); and the global pattern of winds in the neutral thermosphere (Killeen et al., 1985).
It is the last of these, namely the influence of the IMF on the high-latitude energy deposition and the consequent thermospheric winds, that is of particular significance in the mid-latitude ionosphere.Most of the other phenomena listed are confined to high latitudes.For example, the convective flows and associated currents are confined by the magnetospheric ring current, which shields the mid-latitude ionosphere from the convective electric fields.As a result, the mid-latitude F-region is only weakly affected by these high-latitude electric fields and largely co-rotates with the Earth.The boundary between these two regions of plasma, one convecting and the other co-rotating, is close to the steep magnetospheric plasma density gradient at the plasmapause.The neutral thermosphere however, is not confined by any such a boundary and neutral winds can blow across field lines from high to middle and low latitudes.The high-latitude thermosphere, accelerated by the convecting plasma through ion-neutral momentum transfer and thermal pressure imbalances caused by auroral particle precipitation, generates winds in the mid-latitude thermosphere (Hernandez andRoble, 1978, 1984).Such an affect can clearly be seen in the ''storm time circulation'' pattern (Duncan, 1969;Mayr and Volland, 1972;Hays et al., 1973).In addition to being accelerated, the high-latitude neutral thermosphere undergoes heating.The latitudinal gradients in the heating result in changes to the global circulation of neutral air and the spatial distribution of molecular-rich air.(Pro ¨lss 1980(Pro ¨lss , 1987;;Pro ¨lss and Ro ¨mer, 1985).Both the winds and the composition changes (at a fixed pressure level) are known to alter the F region peak density (Rishbeth, 1975).In the absence of electric fields, the mid-latitude ionospheric plasma is confined to move along the geomagnetic field.A wind blowing meridionally has a component parallel to the magnetic field and this accelerates the plasma by collision.In addition, changes in the neutral composition alter the balance between production and loss in the plasma and this affects the peak density of the ionosphere.An increase in the percentage of molecular neutrals, as would be the case if the thermosphere were heated, would lead to a depletion in the ionospheric density (Seaton, 1956).Such effects have been reproduced by coupled models of the ionosphere -thermosphere system (see reviews by Rees and Fuller-Rowell, 1988;Sojka, 1989) and explain much of the storm-time depletions in the mid-latitude F layer (e.g.Rishbeth et al., 1987Rishbeth et al., , 1989)).It is expected therefore, that since the IMF-controlled flows and precipitation at high latitudes influence the mid-latitude ionosphere (via the neutral thermosphere) that there should exist a relation between the orientation of the IMF and the peak density of the ionospheric F layer at mid latitudes.The proposed mechanism for this process explains the decrease in f o F2 in terms of composition changes that result as a consequence of the increased energy deposition at high latitudes.Indeed studies of isolated substorms show global changes in ionospheric densities (Park and Meng, 1976).Large ionospheric currents cause joule heating of the atmosphere and if these currents are sufficiently long lived, they can cause an upwelling of the neutral thermosphere, bringing more molecular species such as N 2 and O 2 to the height of f o F2.At F region altitudes, these molecules are converted into molecular ions by charge exchange or ion-atom interchange reactions with oxygen ions and then relatively rapidly undergo dissociative recombination to cause a depletion in the F region plasma density.If meridional winds move molecular-rich neutral air to lower latitudes, this effect will be seen in ionospheric measurements there.The effects are expected, and indeed observed, to be a function of season (Pro ¨lss, 1977(Pro ¨lss, , 1980;;Wrenn et al., 1987).
However, there are other mechanisms which can give rise to depletions in the sub-auroral F-region and, in addition, f o F2 may be increased in positive phases which may have a variety of causes (Pro ¨lss et al., 1991;Rishbeth, 1989) and which have a variety of magnitudes and durations.One major mechanism in this respect is the effect of enhanced equatorward thermospheric winds which can lift the plasma to greater altitudes where the recombination chain is slower because fewer neutral molecules are available for the transfer reactions with O + ions.
The possibility that there may be an observable link between the IMF and the behaviour of the mid-latitude ionosphere has led to several investigations (Bremer, 1988;Aravindan and Iver, 1990;Mendillo and Schatten, 1983;Zevakina and Lavrova, 1980;Potapova and Shapiro, 1974;Triskova, 1983).Most of these studies found that changes in the average mid-latitude f o F2 occurred following a change in the orientation of the IMF.These studies however have mainly been based on the polarity of the sunward component of the IMF, B x , i.e. on the toward and away sector boundary crossings.For example, Bremer (1988) employed a superposed epoch study to show that sector boundary crossings were associated with systematic changes in the ionosphere at mid latitudes, similar to the effects in total electron content discovered by Mendillo and Schatten (1983).Sector boundary crossings were determined using high-latitude magnetograms.This inference relies on two facts: that the dawn-dusk IMF component B y is highly anti-correlated with IMF B x (because of the Parker spiral in the IMF) and that the B y component induces east-west Hall currents in the dayside polar cap, because of the tension force of newly-opened field lines (the Svalgaard-Mansurov effect).Bremer (1988) carried out a second study with a small set of IMF data and showed that the sector crossings were, on average, associated with changes in polarity of the northward IMF component, B z as predicted by the work of Russell and McPherron (1973).He ascribed the changes in the mid-latitude ionosphere to these IMF B z changes.We here carry out a similar study, but using direct in-situ measurements of IMF B z , rather than sector crossings inferred from ground magnetograms.
We also carry out an initial investigation of the convolution of the effects of IMF orientation with the effects of solar wind dynamic pressure, P, on the interaction chain discussed already.This pressure (corresponding to the kinetic energy density of the solar wind flow) has long been understood to be a factor in the development of major geomagnetic storms, as seen in the Dst index (e.g.Burton et al., 1975).Dynamic pressure is also a factor in generalised energy coupling functions used to analyse the dependence of auroral electrojet indices (AE AL) on the interplanetary conditions (Vasyliunas et al., 1982).Increased dynamic pressure has two competing effects: it increases the energy flux of the solar wind incident on the magnetosphere but it also compresses the dayside magnetosphere and the cross sectional area of the magnetosphere presented to the solar wind flow, thereby reducing the total power incident on the magnetosphere.The balance between these two effects is effectively altered by a parameter introduced by Vasyliunas et al. (1982) which must be determined empirically (Bargatze et al., 1986;Kan and Akasofu, 1982).Kan and Akasofu (1982) found that 1 by assuming that the cross-sectional area of the magnetosphere remains constant.In this case, the generalised coupling function reduces to the often-used epsilon factor (Akasofu, 1981) and the energy coupled into the magnetosphere is predicted to be independent of P.However, allowing for the change in the area, Bargatze et al. (1986) found 0.5, for which the input power increases with dynamic pressure as P 1/6 .Murayama (1982) found that the Dst index was better predicted by a P 1/3 dependence, which corresponds roughly to 0.4 (for which the power input increases as P 4/15 ).Scurry and Russell (1991) found that efficiency of the magnetic reconnection, responsible for most of the energy extraction, was indeed increased by higher P. If the energy extracted from the solar wind (for a given IMF B z and/or clock angle) depends on P, we expect the effects of that power extracted on the midlatitude ionosphere to depend on P also.

Method
For this study, we mainly use hourly ionospheric data from the Slough station (geographic latitude 51 N, longitude 0.2 W) because that forms the longest continuous sequence of ionospheric data, reaching back to 1931.We also make use of 22 years' magnetometer and plasma data from the Interplanetary Monitoring Platform (IMP8) satellite.From hourly averages of the IMF data, all southward turnings of the B z component of the IMF were identified.Note that these events do not include short-period ( < 1 h) fluctuations in B z .For each event, the change in B z was determined by B z B z (t))B z (t1h) where B z (t) is positive and B z (t 1h) is negative.The distribution of B z for the events is shown in the cumulative occurrence frequency curve in Fig. 3. From this it can be seen that over 3000 southward turnings with B z 2 nT were identified in the period November 1963 to December 1987.This falls to about 1000 events for B z 6 nT and under 100 for B z 14 nT.In order to define the ionospheric response to these IMF changes as clearly as possible, the largest of these events were selected.We chose a threshold of B z 11.5 nT, this being a compromise between studying the largest, most clear-cut events, and maintaining a large enough sample of the data to be statistically useful.This threshold gave a set of 150 events.The effect of varying this threshold was also investigated.
The north-south component of the IMF B z controls the coupling of the solar wind to the magnetosphere, but also of relevance to the high-latitude energy deposition is the energy density of the solar wind flow.Because this is dominated by the bulk flow of the solar wind plasma this is roughly equal to mnv 2 /2 P/2, where n is the solar wind density, m the mean mass, v the velocity and P the dynamic pressure.The largest B z events are when both B z (t) and B z (t1h) are large in magnitude.Bowe et al. (1990) showed that large |B z | was highly correlated with large solar wind dynamic pressure, P.This means the southward turning events will tend to be accompanied by large P. A threshold for the peak P (usually seen at or near the time of the southward turning) of 5 nPa was set as it divided the set for B z 11.5 nT into two approximately equal size subsets.Superposed epoch studies were then carried out for those southward turnings with B z 11.5 nT which were accompanied by a solar wind pressure of P > 5 nPa and those where the concomitant pressure was 5 nPa.A more detailed study of the effects of solar wind dynamic pressure will be presented at a later date: the above division of the dataset is, however, sufficient to reveal that P plays a key role in determining the geomagnetic and ionospheric response to southward turnings of the IMF.Inspection of parts (a) of Fig. 4 and 5 shows the mean B z variation for these two subsets is very similar for the times within the three hours of the southward turning but there are slight differences, with a gradual build up and decay occurring before and after this time for the high P subset.
Taking the southward turning to be time t 0, the average behaviour of B z , P, K p , Dst and AE, and the change in ionospheric F region critical frequency at Slough, f o F2, were studied for four days either side of the event.This was achieved by averaging each quantity in 1-hour bins of t.The results are shown in Figs. 4 and  5 where each panel shows the variation of the mean for that parameter, over ÿ96 < t < 96 h.Each mean is plotted as a point, with a bar which is plus and minus the standard deviation in the mean.Figure 4 is for all cases for which peak P exceeded 5 nPa, Fig. 5 for those cases where P 5 nPa.The top left panel shows the mean variation of IMF B z .This shows a bipolar signature, with < B z > of about 15 nT in both cases.Note that B z tends to be strongly positive prior to these large B z cases and that mean P increases with mean |B z | .In Fig. 4, <P> peaks at over 8 nPa at t 0, whereas in Fig. 5 there is a much weaker peak of 3.5 nPa.In both cases the rise in <P> starts about 1 day before the southward turn and last until one day after.For the high pressure cases, <B z > is positive for about 1 day prior to the southward turning, and negative for about 1 day after it, although the largest mean values around the time of the turning last for only 1-2 h.Only this second, bipolar spike-like feature is seen for the low pressure cases.

The geomagnetic indices
In order to understand the response of the geomagnetic indices to the southward turning of IMF B z , it is necessary to make a few points about how they are compiled and what conditions would induce them to change.The three indices used in this study are based on measurements made by ground-based magnetometers.Each of the indexes is compiled from instruments which lie in a restricted range of geomagnetic latitudes.
The planetary K index, K p , is the mean of twelve midlatitude (between 48 and 62º geomagnetic latitude) observing stations.The K index for each station is the range of field values in each 3-h period.In this way, the background diurnal variations are separated from times of magnetic activity.The K index is approximately logarithmic, with a maximum value of 9.Each level of K p is further divided into a plus, zero and minus subsection, e.g. 5, 5o and 5ÿ which we here quantify as 53, 50 and 47.As a result, the K p index, as represented in this work, extends from 0 to 90.The combination of high conductivity and high electric field in the auroral oval causes the auroral electrojets which are sensed by magnetometer stations at high latitudes (60 to 71 geomagnetic latitude), largely from the effect of the Hall currents.The difference of peak positive and negative disturbances seen by a ring of 12 selected stations is called auroral electrojet (AE) index.The AE index is, to some extent, enhanced by increased electric fields, ie by the high-latitude convection, but the major features are large increases caused by strong precipitation (giving high conductivities) in the presence of electric fields during substorm expansion phases.A ring of stations near the equator remotely sense the ring current around the Earth in the inner magnetosphere, and are used to generate the Dst index.
To give some idea of the typical variability of B z and the geomagnetic indices, in Fig. 6 we plot the cumulative probability distributions for each.These are shown in the same format as Fig. 2.These will be used later in the study to put a given size disturbance into context by its overall occurrence frequency.

Results
The most striking feature of the behaviour of B z in Fig. 4a and 5a is that in order to achieve a large southward turning, the IMF must first increase northward by the order of 8 nT before reversing to a southward field of ÿ8 nT.The persistence of B z is such that an increase in average values above zero starts around 24 h before reversal and the decline in negative B z continues up to 24 h afterwards.If we compare with the probability distribution shown in Fig. 6a, we find that such a large magnitude of B z is rare, with B z > 8 nT and < ÿ8nT for only 2.5% of the time in each case.

The effect on K p
It can be seen in both Figs.4c and 5c that K p rises before the southward turning of the IMF, roughly with the rise in the solar wind dynamic pressure, P. It decays more slowly than it rises, after the southward turning and remains elevated even after 96 h.To some extent, this reflects the fact that K p is correlated with both southward IMF and P. The mean K p peaks in the same hour as the southward turning at values near 53 (5) and 43 (4) for P > 5 nPa and P 5 nPa, respectively.
From Fig. 6b we see that K p exceeds these values for 13% and 30% of the time.

The effect on Dst
In both Fig. 4d and 5d, Dst averages about )15 nT prior to the southward turning.Unlike K p , however, this mean Dst is almost completely unaffected until the time of the southward turning, at which point it decreases rapidly to values below )60 nT and )35 nT for P > 5 nPa and P 5 nPa, respectively.This represents a significant enhancement of the ring current.Figure 6c shows that such enhancements occur for only about 5% and 30% of the time.The decay of the ring current is a complex combination of several ion loss processes, some of which have long time constants.By t 96 h, Dst has still not recovered to the initial value and remains at about )25 nT.The average Dst values show the classic form of a negative or main phase of a magnetic storm.Individual magnetic storms often start with a short-lived positive phase as the field is compressed.No clear positive phase can be seen in either Figs.4d or 5d.This implies that the factors generating the positive phase do not bear a fixed relation to the southward turning of the IMF (Burton et al., 1975).The sudden decrease of Dst starts at t 0.5 h and Dst reaches minimum values at about t 8 h.

The effect on AE
The studies discussed in the introduction, and many others, provide clear evidence that AE is enhanced by a prior period of southward B z , in the form of isolated substorms (with AE enhancements in the expansion and recovery phases lasting about one hour) or sequences of multiple substorms if the IMF remains southward.There is a delay of about 0.5)1 h prior to the onset of the expansion phase, i.e. the growth phase.This can indeed be recognised in Fig. 4e and 5e, in which a pulse of enhanced AE is seen for 1-2 h after the southward turning, with peak values of 650 nT and 560 nT for P > 5 nPa and P 5 nPa, respectively.From Fig. 6d, hourly AE exceeding these values occurs only 7% and 12% of the time.
The behaviour of AE is more complex than K p or Dst however, since there is an initial rise in AE which starts about 24 h before the southward turning.There is a short-lived minimum immediately before the pulse discussed in both cases, and after the pulse, AE decays slowly from an elevated value, rather like K p .Also as with K p , values are still elevated at t 96 h.This latter effect is probably due to the ring current having an effect on the high-latitude stations.The high values immediately after the pulse probably result from the average southward IMF remaining negative for several hours and the fact that the energy stored in the magnetotail during a period of southward IMF may not all be lost in a single period of substorm activity.

The effect on f o F2
Prior to the southward turning, f o F2 shows slightly negative mean values.The reasons for this are not yet clear, but it may be a result of large B z southward turnings tending to occur during periods of generally enhanced solar wind dynamic pressure, P. The average of f o F2 prior to the southward turning is about )0.25 MHz which is a depletion which Fig. 2 shows to be exceeded roughly 35% of the time.This enhanced pressure may have other effects, for example the decay in Dst may be initially opposed by the compressive effect of enhanced P.
In Fig. 4f there is a clear decay in f o F2 following the southward turning and the enhanced energy input to high latitudes, as indicated by the AE index.The largest amplitude of the effect is about 1.4 MHz, and from Fig. 2, larger depletions (lower f o F2 values) are seen for only 3% of the time.The minimum densities are seen roughly 20 h after the southward turning, by when Dst is recovering and AE has dropped to the weak enhanced values that appear to be due to the ring current and not substorm activity.The recovery takes place gradually over the next 3 days, over which the mean f o F2 shows a marked diurnal cycle which will be discussed in the next section.The same study, carried out for P 5 nPa is shown in Fig. 5f.The results show a slight depression of f o F2 values after the southward turning, but without the clear negative storm-type effects seen in Fig. 4f.It is thus immediately clear that although negative IMF B z is important in enabling the extraction of solar wind energy and thus driving global ionosphere/thermosphere disturbances, the solar wind energy density (proportional to P ) must be high for coherent changes to be seen at mid latitudes.The full dependence of f o F2 prior to the southward turning appears to be largely a summer phenomenon.The summer data show a minimum of f o F2 at )1.7 MHz and at time t 13 h, i.e. deeper and earlier than for the averages for all times of year.The winter cases produce a shorter-lived depression of f o F2, with minimum f o F2 at )1.2 MHz and at time t 23 h, i.e. less deep and later than for both the summer and allyear plots.
In order to confirm these findings a similar study was carried out using Argentine Island, a Southern Hemisphere station at geographic latitude )65.3 and longitude 295.8.The results are shown in Fig. 7c, d.In this case, the summer depletion was again much clearer, with a minimum f o F2 of )1.8MHz at t 15 h.The winter effect is weaker, indeed it is very weak with f o F2 falling to only )0.6 MHz.Thus comparison with Argentine Island confirms some of the effects reported for Slough are common to other stations, but the behaviour appears to vary with location.A study is now underway to analyse further stations in the way presented here.
One possible factor in f o F2 values, in addition to the changes produced by thermospheric composition variations, is the latitude of the mid-latitude trough (in ionospheric density), with respect to the station.This trough lies equatorward of the auroral oval and poleward of the co-rotating mid-latitude ionosphere (e.g.Spiro et al., 1978).Following southward turning of the IMF, the expansion of the convection pattern causes the trough to move to lower latitudes and this may sometimes contribute to the drop in f o F2 values seen at Slough.However, the trough shows a strong seasonal variation being weak or absent in summer.This is not consistent with the seasonal variations shown in Fig. 7.In addition, the trough is predominantly a nightside phenomenon, whereas the f o F2 variations shown tend to peak when Slough is near noon.

4.5
The effect of the timing of the southward turning Park and Meng (1976) report depressions in f o F2 caused by isolated substorms which form in the noon local time sector and then co-rotate with the Earth.From a statistical survey, Hargreaves and Bagenal (1977) found co-rotating and evolving regions of positive and negative storm phases.Such results mean that there should be, for any one station, a variation in the size of the depletion with the UT of the southward turning.
Generally there is an expectation that substorms taking place when the station is on the nightside will have greatest effect, but that effect only becomes apparent when the station rotates through sunrise onto the dayside.Figure 8 shows the effect of grouping the data according to the UT of the southward turning, again for the case of B z > 11.5 nT, P > 5 nPa (Fig. 4f).The four cases Fig. 8a-8d are for southward turnings in the UT ranges 03-09, 09-15, 15-21 and 21-03 h.
The depression of f o F2 is seen in all cases but the behaviour is very different in each case.The deepest depression is seen in Fig. 8c, for which the Slough station will be in the dusk/pre-midnight sector at the time of the southward turning (15-21 h.UT) and the pulse of enhanced AE commencing roughly one hour later.A small ionospheric depletion is seen at the time of the AE pulse (t equal to 1-2 h), when the station is in the pre-midnight sector.In this case, the peak depletion is 2 MHz at time t 18 h.By this time, Slough will have rotated into the noon sector.Minima in f o F2 are seen on the next three days when Slough returns to the noon sector, while nighttime f o F2 returns to near the same values seen prior to the southward turn ( f o F2 )0.2 MHz). Figure 8d is for southward turnings at 21-03UT, when Slough is in the midnight sector.
In this case the deepest depletion is )1.6 MHz at t 13 h, when, as for the case in Fig. 8c, Slough has rotated into the noon sector.Although a depletion is seen 48 and 72 h later, when Slough again returns to the noon sector, only a very weak minimum is seen on the first day at this UT.This effect appears to be real as it is found in the data for each of the 2 sunspot cycles when analysed separately: however the reasons for this are not known.Figure 8a is for turnings at 03-09 UT, when Slough is in the dawn sector.In this case, the depletion appears to start even before the southward turning, but this is probably because the depletion in this case is small ()0.8 MHz) and comparable in magnitude to the other fluctuations seen during the 96 hours, prior to the turning.The depletion lasts throughout the next 24 hours maintaining large ( 0.8 MHz) values even through the night, and the largest depletion ()1.2 h) is seen when Slough returns to the noon sector at t 28 h.
Weak minima are then seen each time the station subsequently returns to the dayside sector. Figure 8b is for turnings at 09-15 UT when Slough is in the noon sector.This is the one time in all the superposed epoch studies presented here that a positive phase is seen, with enhanced values (positive f o F2) peaking at t 6 h, Fig. 8a-d when Slough is near dusk (Hargreaves and Bagenal, 1977).A clear minimum is seen at t 23 h, and each subsequent time that the station returns to the noon sector, each minimum being weaker than the previous one as the ionosphere recovers.

Response magnitudes as a function of the change in B z
These studies were carried out using the arbitrarilychosen threshold for B z of 11.5 nT. Figure 9 investigates the effect of this choice of threshold.Parts a, b and c show, respectively, the minimum value of Dst in the storm depletion following the southward turn, the maximum of AE in the burst immediately after the southward turn and the subsequent minimum f o F2.The number of cases as a function of the B z threshold is shown in Fig. 3.It can be seen that the effect on Dst, AE and f o F2 all increase systematically with increasing B z .Note that the plot is as a function of B z threshold: large B z events will be included along with lower B z events as the threshold is lowered.This means that the dependence on B z value (rather than threshold) will be steeper than shown in Fig. 9.

Conclusions
We have studied the ionospheric and geomagnetic response of large southward turnings of the IMF, selected by the magnitude of the turning in hourly data.This set was then subdivided into two sets relating to large and small associated changes of the solar wind dynamic pressure.It was found that the response of the three magnetic activity indices followed similar trends in the two sets, but that for those events accompanied by small changes in the solar wind dynamic pressure, the magnitude of the effects was much reduced.The response of f o F2 was especially strongly dependent on the solar wind dynamic pressure, with little effect being seen when the pressure change was small.The results are consistent with the mechanism for the depletion of F region plasma density caused by enhanced solar wind magnetosphere coupling: this causes enhanced energy deposition in the high-latitude thermosphere, upwelling in response to the heating and transport of molecularrich air from the auroral zone to mid latitudes, where it alters the ion chemistry and leads to dissociative recombination of molecular ions.The depletion is greater in summer than in winter and is also seen when the station rotates around into the noon sector.For Slough, southward turnings taking place in the 15-21 UT range have the most effect, for which the station is in Comparison with the occurrence distributions of the various parameters shows that the events studied are large and quite rare, for most parameters the effects are only exceeded less than 10% of time.However, analysis for smaller thresholds of the change in the IMF B z shows the same effects but with reduced magnitude.This implies that the storm time changes and much of the day-to-day variability of the ionosphere have similar, if not the same, causes and the difference would then be largely one of degree.

Fig. 1 .
Fig. 1.A typical storm-time behaviour of F2 layer critical frequency, f o F2, observed at Slough.The crosses are the hourly values observed on 6-12 November 1991, the open circles show the diurnal variation of the monthly median values.The former minus the latter is here termed f o F2

Fig. 3 .
Fig. 3.The number of southward turnings with the change in the hourly B z values, B z , exceeding a given value for the years 1967-1989

Fig
Fig. 4a-f.Superposed epoch study for southward turnings with B z > 11.5 nT and peak solar wind dynamic pressure P > 5 nPa for 1967-1989.Plots are for a B z , b P, c K p , d Dst, e AE f f o F2.Time zero is the time of the southward turning; points are the mean value for each time bin and error bars are plus and minus one standard deviation in the mean ᭣

Fig
Fig. 6a-d.As for Fig. 2 for a B z , b K p , c Dst and d AE

Fig
Fig. 7a-d.Same as Fig. 4(f) for 30 day periods around summer solstice (a and c) and winter solstice (b and d). a and b are for Slough and c and d are for Argentine Island

Fig
Fig. 9a-c.The geomagnetic and ionospheric responses as a function of the threshold size of the B z change in the southward turning B z : a the minimum in Dst; b the maximum in AE and c the minimum in f o F2 ᭤