Stratification observed by the in situ plasma density measurements from the Swarm satellites

Stratification phenomenon is investigated using the simultaneous in situ plasma density measurements obtained by the Swarm satellites orbiting at different altitudes above F2 peak. For the first time, the continuous distribution morphology and the exact locations are obtained for the nighttime stratification, which show that the stratification events are centered at the EIA (equatorial ionization anomaly) trough and extend towards the two EIA crests with the most significant part being located at the EIA trough. Another new discovery is the stratification in southern mid-latitudes; stratification events in this region are located on a local plasma peak sandwiched by two lower density strips covering all the longitudes. The formation mechanism of the stratification for the two latitudinal regions is discussed, but the stratification mechanism in southern mid-latitudes remains an unsolved problem.


Introduction
Stratification is a kind of phenomenon appearing in the ionospheric F2 layer at low-latitudes 26 near geomagnetic equator, where additional layer is shown above the F2 layer peak due to the 27 combined effect of the upward E×B drift at the geomagnetic equator and the meridional neutral 28 wind (Balan et al., 1997(Balan et al., , 1998Jenkins et al, 1997). This additional layer was called G layer and 29 renamed to F3 layer by Balan et al. (1997) due to its same chemistry as the F region. 30 Since stratification was first reported in the mid-20th century (Sen, 1949;Skinner et al., 1954), 31 many studies have been conducted to study the formation mechanism, diurnal, seasonal and solar 32 activity dependence of this phenomenon using different measurements, such as ground-based The features and formation mechanism of the ionospheric F2 layer stratification have been 2 extensively investigated for several decades, but unsolved problems, such as the exact locations and 3 distribution morphology that are useful to understand this phenomenon, still exist due to the 4 scattered and limited spatial coverage of the observations used in previous studies. So far, most of 5 these studies are based on ground-based or satellite-based ionograms. For the former, stratification 6 can only be observed during the period when the peak density of the stratification layer exceeds that 7 of the F2 layer; and for the latter, only during the period when the peak density of the stratification 8 layer is lower than the F2 peak. Continuous global distribution of the stratification cannot be 9 obtained from these scattered observations though local season and solar activity dependence 10 features can be obtained from these long-term observations. Moreover, there are contradictory 11 results in these studies. Whereas, simultaneous satellite-based in situ observations at different 12 altitudes above the F2 layer peak can provide spatial coverage of more extensive region, which can 13 incorporate all local time and longitudes. And the most important is that the morphology of the 14 stratification along the latitudinal direction can be obtained using the continuous measurements. 15 In this paper, for the first time, the in situ plasma measurements from the Swarm satellites are 16 used to study the precise locations, distribution and morphology of the stratification phenomenon. 17 Nighttime stratification on the southern mid-latitudes is found, which is never mentioned in previous 18 studies. Our results can provide new perspective for the stratification phenomenon, which is helpful 19 to the insight of the ionospheric F2 layer. 20

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Separation of the three satellites follow different schemes as shown in Fig. 1(a). To simplify the 4 calculation, we use only the measurements from Swarm A and B as the two satellites are closer to 5 each other, and they have more co-located orbit tracks after altitude separation. Therefore, co-6 located in situ plasma density measurements from Swarm A and B are selected using the criteria 7 defined below to conduct this study. 8 We also give the solar and geomagnetic activity indices during the select period, as shown in 9 there are few geomagnetic events as shown in Fig.2(c), we won't distinguish the data into disturbed 11 and undisturbed cases here. Therefore, all the selected co-located orbit pairs are used in this study.

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To detect stratification events, measurements of the co-located orbit pairs from the two satellites 17 are compared directly. Spatial and temporal criteria to search co-located orbit tracks are defined as: 18 (1) longitude difference between two orbit tracks near the equator region is within 5°, which can 19 keep true for nearly all the mid-latitudes as the orbit tracks of the Swarm satellites are almost parallel 20 to longitude for mid-and low-latitudes. This 5°spatial difference in longitude direction is 21 reasonable because of the little and negligible variation of the ionosphere in a small spatial scale. 22 According to Shim et al. (2008), the longitudinal correlation can vary from 23°at mid-latitudes 23 and 15°at low-latitudes during the day to 11°at mid-latitudes and 10°at low latitudes during the 24 night; and (2) time difference of measurements at similar latitude between two orbits is less than 30 25 min as appearance of stratification is normally much longer than this criterion (Balan et al., 1997); 26 moreover, variations of electron densities within 30 minutes can be neglected comparing to the 27 diurnal variation under geomagnetic quiet conditions. 28 A search of the dataset from Swarm A and B using the criteria, 1313 matched orbit pairs are found 29 from January to June 2014. Here, matched orbits indicate ascending (from south to north) or 30 descending (from north to south) half orbit tracks as a satellite passes the same location twice a day, 31 corresponding to daytime and nighttime respectively, as shown in Fig.1(c), which gives the local 32 time (LT) of Swarm B for both ascending and descending orbit during the selected data period. 33 Using these co-located orbit pairs, stratification events are identified by the following process. (1) 34 In situ plasma density measurements along the orbit tracks are down-sampled by averaging the data 35 over 1°latitude range; (2) The down-sampled data at same latitude from the two satellites are 36 compared, then data points, where average plasma density from Swarm B is greater than that from 37 Swarm A, are picked out; (3) Stratification events are identified only when at least 5 continual data 38 difference (Swarm B minus Swarm A) points are positive, which means a continual latitude of at 1 least 5°. Adoption of continual 5° at latitudinal direction is because the auto-detecting process may 2 make wrong decisions if less data points are considered due to the small data fluctuations of the 3 observations. Some very small stratification may be discarded in this way, but it won't affect the 4 final results. After all the stratification events are determined, the morphology along the latitude, 5 the location, and the global distribution of the stratification events are then studied based on the 6 detected events. 7

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The global distribution of the detected stratification events from all the co-located orbit pairs from 9 January to June 2014 is given in Fig. 3, also given in this figure is the variations of the occurrence 10 number with local time and month. As more than one event may be detected from one orbit track, 11 when comparing to the total number of co-located orbit pairs. The lower event number in January 27 is because the altitudes separation began at the end of this month, its occurrence rate (detected events 28 divide by total) is comparable to that in February, May, and June. As to the very small event number 29 in March and April, it is necessary to point out that the local time of the two satellites coincides to 1 dawn and dusk during these two months, as shown in Fig.1(c), which may be the reason why 2 detected events are fewer during this period. Fewer stratification events during March and April is 3 consistent with the fewer events during the day, which we will discuss in Section 4. 4 It can be seen clearly from Fig.3(a) that stratification events are concentrated on two geomagnetic 5 latitudes, one is the geomagnetic equator region, where most previous studies are concentrated on; 6 and the other is the mid-latitude region on the Southern Hemisphere, where the distribution of the 7 stratification events also show the feature of being parallel to the geomagnetic equator. It should be 8 noted here that the geomagnetic equator and latitude shown by grey dash line in Fig.3(a) are from 9 dipole coordinates, and dip equator shown by black dash line is also given as a comparison. 10 Geomagnetic control of the stratification events in southern mid-latitudes is obviously shown 11 according to its distribution feature. In Fig.3(a), stratification events near the geomagnetic equator 12 can occur in each month from January to June, whereas stratification on the southern mid-latitudes 13 only occur in May and June, just the local winter. Swarm A, namely on places where blue color curves are above red color ones. Also given in Fig.  27 4(b) are the ground tracks of the two satellites, which can be used to locate the longitude of the 28 stratification events. 29 As shown in Fig.4(a) and Fig.5(a), morphology of the nighttime stratification events, located near 30 the geomagnetic equator, shows that the stratification is centered at the equator ionization anomaly 31 (EIA) trough, where the geomagnetic equator is located at or near for most event cases, and extend 32 towards the EIA crests on both hemisphere. Occurrence of the nighttime phenomenon can be 33 accompanied by or without plasma depletion as shown in Fig.4(a) and Fig.5(a)

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We also give some examples in Fig.5(b) to show the typical morphology for daytime stratification. 6 These latitudinal distribution morphologies demonstrate clearly the results, reported by many 7 ground-based studies, that daytime stratification can appear on one side, frame 1 and 2 in Fig.5(b), 8 or both sides, frame 4 in Fig.5(b), of the EIA crests. We also show an example of the daytime 9 stratification centered at the EIA trough, frame 3 in Fig.5(b), which is seldom observed from ground-10 based ionograms. An interesting point in daytime data is that there is a small spike centered at the 11 EIA trough occasionally as shown by frame 4 in Fig.5(b), which is never seen in nighttime 12 measurements and needs further confirmation. As there are only a few daytime stratification events, 13 no statistical results can be obtained from these data. We only focus on nighttime stratification in 14 this study.

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There is another southern mid-latitude regions where the detected stratification events are 6 concentrated on and can cover all the longitudes as shown in Fig.3(a). Stratification phenomenon in 7 this region is never mentioned in previous studies. All the detected stratification events in this region 8 occur at local nighttime in May and June as mentioned above. Typical stratification events in this 9 region are located on the local plasma peak along latitudinal direction, which is sandwiched by two 10 lower density strips as shown in Fig. 6. Stratification events in this region can occur simultaneously 11 with that located near the geomagnetic equator region as indicated by frame 11 and 12 in Fig.6. The 12 morphology and locations of the stratification events in southern mid-latitudes are quite different 13 from that in geomagnetic equator region, which may imply the different formation mechanism for 14 the two situations.

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To get the peak height range of the ionospheric F2 layer during the selected data period, a 4 statistical analysis is performed using the radio occultation (RO) measurements from the COSMIC 5 mission. Of all the 16753 RO events located between ±20°geographical latitude, only 994 events 6 have peak height greater than 450km, including those with false peak height caused by disturbed 7 data, which indicates that the normal F2 peak height is lower than 450km at the equatorial region. 8 A small number of RO events, located along geomagnetic equator with a higher peak height, show 9 the morphology of two peaks on the profiles, which may imply stratification phenomenon. 10 Therefore, we can determine that the orbiting altitudes of the Swarm satellites are above the F2 peak 11 height for most cases, and the stratification events, appearing at the orbiting altitudes, can be 12 detected by the in situ plasma density measurements from the Swarm satellites. 13 The continuous morphology of the stratification and its global distribution obtained in this paper 14 give us a more intuitive image of the stratification phenomenon and are very useful to understand 15 this phenomenon. But some problems requires further analysis. 16 Stratification events detected in this study are mainly concentrated during nighttime period, 17 which is different from previous studies that occurrence of stratification is mainly during daytime, Using the measurements of the Swarm satellites, the continuous latitudinal morphology and the 2 exact locations of the stratification events are shown clearly for the first time, which demonstrate 3 that the nighttime stratification can cover all the latitudes continuously between the two EIA crests 4 with the most significant part being located at the EIA trough. This arch-like distribution 5 morphology is in accord with those studies using satellite-based observations (Depuev and Pulinets, 6 2001;Lockwood and Nelms, 1964;Wang et al., 2019). Depuev and Pulinets (2001) suggest that the 7 intensity of the stratification has a maximum just above the equator and decreases poleward within 8 ±10°dip; and Lockwood and Nelms (1964) also report that the field-aligned ledge is concentrated 9 above the geomagnetic equator, producing a dome-like cross-section in the equatorial region. Wang that post-sunset stratification is different from daytime stratification due to the different solar 26 activity and season dependence features and they suggest that post-sunset stratification is formed 27 due to the PRE upward plasma lifts and the existence of ionization production at the high altitudes 28 of F2 layer after sunset. However, this formation mechanism cannot explain the midnight/post-29 midnight stratification shown in this study and mentioned in previous studies (Depuev and Pulinets,30 2001; Uemoto et al., 2011), we therefore deduce that nighttime stratification may be resulted from 31 a different formation mechanism. According to some studies (Balan et al., 2008;Paznukhov, 2007), 32 the mechanism responsible for the storm time stratification is similar to that in quiet periods but 33 with a much faster processing time due to the rapid uplift of the F layer by an upward E×B drift 34 resulting from an eastward penetration electric field. We therefore speculate that the upward plasma 35 drift caused by the PRE will produce the same effects as that of the magnetic disturbances. As a 36 result, plasma can be lifted up quickly by the PRE from lower altitudes to higher altitudes, which 37 can lead to the higher densities at higher altitudes and plasma depletion and plasma bubbles at lower 38 altitude. After the PRE, the downward vertical drift resulting from the reversal electric field will 39 replenish the depleted region by carrying the plasma from higher altitudes to lower altitude, as a 40 result stratification can be formed during the downward carrying process at the EIA trough. At the 41 same time, the field aligned diffusion of the uplifted plasma can maintain the EIA structure on both 42 sides of the geomagnetic equator and form stratification at low latitudes. By this way, the nighttime 43 stratification morphology, centering at EIA trough and extending towards the two EIA crests as 44 shown in Fig.4(a) and Fig.5(a), can be formed. The EIA structure, which accompanies all the cases 1 of stratification near geomagnetic equator, is supposed to be the necessary condition to form the 2 stratification in this region. The existence of nighttime EIAs is common during geo-magnetically 3 quiet conditions, and re-appearance of EIA is triggered by the occasionally reversed upward vertical 4 plasma drift as nighttime vertical velocities are normally directed downwards (Yizengaw et al., 5 2009). In addition, the nighttime downward vertical velocities are greater after midnight than before 6 midnight, both during magnetically quiet and perturbed times (Rajarm, 1977). Combining these 7 results, we can explain the formation process of the nighttime stratification at and near geomagnetic 8 equator and why most of the nighttime stratification events are concentrated between post-sunset to 9 midnight period as shown in Fig.3(c). In addition, according to Balan et al. (2000), The new discovery in this study is the stratification on the southern mid-latitudes, which has never 15 been mentioned in previous studies. Wang et al. (2019) propose that a small stratification may exist 16 on southern mid-latitudes when comparing the in situ electron densities observed at different 17 altitudes by the same payload onboard DEMETER satellite, but a definite conclusion cannot be 18 given as the data is not observed simultaneously. The results in this paper further confirm their 19 proposal. However, we also notice that the season and solar activity of the data used in their study 20 are different from that in this study. Whether stratification on southern mid-latitudes can occur in 21 all seasons or only in summer (Wang et al., 2019) or winter (in this study), both their studies and 22 ours cannot give a definite answer due to the limit data coverage, which requires further studies their cases are distributed randomly on both hemisphere. As no literatures can be referenced on the 28 stratification located in the southern mid-latitudes, a brief discussion on its possible formation 29 mechanism is given here. 30 As shown in Fig.6, stratification events in this region are located on the local plasma peak, and it 31 seems that the more obvious the local peak is, the more obvious the stratification is. Plasma 32 enhancement in southern mid-latitudes is noticed by Tsurutani et al. (2004). They call the local peak 33 "shoulder", and this "shoulder" can be found from TOPEX, SAC-C, and CHAMP data sets, as well 34 as ground GPS data. Yizengaw et al. (2009) also report TEC enhancement in southern mid-latitudes 35 and attribute it to the meridional thermospheric wind that drives the F layer plasma upward as this 36 is the region where the wind-induced uplifting is most efficient. The morphology of the daytime 37 "shoulder" is similar as the nighttime TEC enhancement and local peak in this study. As this local 38 peak can exist both during the day and at night as well as under geomagnetic disturbed and quiet 39 conditions, we suppose that it is a normal phenomenon on southern mid-latitudes. Another Hemisphere, similar as the feature that mid-latitudes stratification occur only on Southern 42 Hemisphere. We speculate that the stratification in southern mid-latitudes is closely related with the 43 local peak structure according to their common feature. Tsurutani et al. (2004) suppose the "shoulder" 44 is likely the signature of the plasmapause, which can be used as a downward plasma source to form 1 the stratification in the mid-latitudes, but this cannot explain why this phenomenon doesn't appear 2 in northern mid-latitudes. 3 Abdu et al. (2005) suggest that precipitation of low energy (<10 keV) electrons in the SAA 4 (South Atlantic Anomaly), namely source of ion production, together with the ionization loss 5 process, might be a mechanism for the F2 layer stratification at mid-latitudes, but the locations of 6 their stratification are on the southern EIA crest, quite different from the locations in this study. 7 Moreover, precipitation mechanism cannot explain why the stratification can cover all the 8 longitudes. 9 According to Lin et al. (2005), large (storm time) upward E×B drifts can lift the ionospheric layer 10 to higher altitudes, and therefore can expand the EIA peaks to higher latitudes. However, the 11 proposal, transporting of equatorial plasma to higher geomagnetic latitudes by the super fountain 12 effect, still cannot satisfactorily explain the stratification in southern mid-latitudes. For one reason, 13 field-aligned diffusion of the uplift plasma by super fountain may lead to the mid-latitude 14 stratification, but it cannot explain the trough between the local peak and the southern EIA crest as 15 shown in Fig.6; the second reason, when there is no EIA signature near the geomagnetic equator, 16 and as a result no super fountain effect, there are still many stratification cases in this region; and 17 the third reason, this mechanism cannot explain the absence of stratification in northern mid-18 latitudes either. As no existing research results can satisfactorily explain the formation mechanism 19 of the stratification in southern mid-latitudes, we put it as an open question here, and subsequent 20 studies are anticipated. 21

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Stratification above F2 peak is investigated in this paper using the continuous in situ plasma 23 densities observed simultaneously by the Swarm satellites orbiting at different altitudes, some 24 refined features and new discovery on the F2 layer stratification are summarized as follows: 25 (1) It is the first time that stratification phenomenon is investigated using direct in situ plasma 26 density measurements. 27 (2) Most of the detected stratification events occur after sunset, and cluster between about 18:00 28 to 23:00 LT. 29 (3) The continuous morphology of the nighttime stratification events, located near geomagnetic 30 equator, shows that it centers at the EIA trough and extends towards both sides, but 31 sandwiched by the two EIA crests. This distribution feature is quite different from the 32 daytime stratification, which is located near but not the equator. 33