Relation between magnetospheric state parameters and the ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A04323, doi:10.1029/2010JA015521, 2011

Relation between magnetospheric state parameters and the occurrence of plasma depletion events in the nighttime midlatitude F region Ilgin Seker,1 Shing F. Fung,1 and John D. Mathews2 Received 31 March 2010; revised 23 December 2010; accepted 9 February 2011; published 30 April 2011.

[1] Studies using all‐sky imagers have revealed the presence of various ionospheric irregularities in the nighttime midlatitude F region. The most prevalent and well known of these are the medium‐scale traveling ionospheric disturbances (MSTIDs) that usually occur when the geomagnetic activity is low and midlatitude spread F plumes that are often observed when the geomagnetic activity is high. The inverse and direct relations between geomagnetic activity and the occurrence rate of MSTIDs and midlatitude plumes, respectively, have been observed by several studies using different instruments; however, most of them focus on MSTIDs only and use only Kp to characterize geomagnetic activity. In order to understand the underlying causes of these two relations and to distinguish between MSTIDs and plumes, it is illuminating to better characterize the occurrence of MSTIDs and plumes using multiple magnetospheric state parameters. Here we statistically compare multiple geomagnetic driver and response parameters (such as Kp, AE, Dst, and solar wind parameters) with the occurrence rates of nighttime MSTIDs and plumes observed using an all‐sky imager at Arecibo Observatory (AO) between 2003 and 2008. We also present seasonal and annual variations of MSTIDs and plumes at AO. The results not only allow us to better distinguish MSTIDs and plumes, but also to shed further light on the generation mechanism and electrodynamics of these two different phenomena occurring at nighttime in the midlatitude F region. Citation: Seker, I., S. F. Fung, and J. D. Mathews (2011), Relation between magnetospheric state parameters and the occurrence of plasma depletion events in the nighttime midlatitude F region, J. Geophys. Res., 116, A04323, doi:10.1029/2010JA015521.

1. Introduction [2] MSTIDs and spread F plumes (or plasma bubbles) are the two most common irregularities observed with all‐sky imagers at midlatitudes. When observed with various single‐ point instruments such as ionosonde, radar, GPS‐TEC, and plasma probes on satellites these two midlatitudes phenomena are usually confused because they both appear as plasma depletions. Although it is also not easy to distinguish one type of depletion from the other in the 2D airglow images, MSTIDs and plumes each have several distinct characteristics (which are based on observations made with an all‐sky imager at AO) as listed in Table 1 and demonstrated in Figure 1. For example, in all‐sky images, the MSTIDs typically appear as periodic parallel bands whereas the plumes usually appear as a single structure with fractal‐like fingers. Furthermore, during nighttime at midlatitudes in the northern hemisphere, MSTIDs are usually aligned northwest to 1 Geospace Physics Laboratory, Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 2 Radar Space Sciences Laboratory, Department of Electrical Engineering, Pennsylvania State University, University Park, Pennsylvania, USA.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JA015521

southeast and propagate southwestward [Tsugawa et al., 2007b] as opposed to the plumes which are usually aligned more parallel to the geomagnetic field lines as compared to MSTIDs and propagate west (or initially east then west) as they vertically rise at equator and map to northward/higher altitudes along geomagnetic field lines. At low latitudes the MSTIDs usually appear (in all‐sky images) from the north to northeast and plumes usually appear from the south. It should be noted that although these are the most common features of MSTIDs and plumes (as observed with an all‐sky imager), exceptional events do exist. [3] Although a number of theoretical models have been proposed for these ionospheric phenomena, most notably the Perkins instability [Perkins, 1973; Zhou et al., 2006] for MSTIDs and the Rayleigh‐Taylor instability [Makela, 2006] for spread F plumes, how the actual physical mechanisms may operate under different geomagnetic activity levels have not yet been firmly established. Finally, and perhaps most importantly, it has been tentatively found that, at midlatitudes, the MSTIDs usually occur during geomagnetically quiet times [Saito et al., 2002; Shiokawa et al., 2003; Kotake et al., 2006; Tsugawa et al., 2007a; Candido et al., 2008; He and Ping, 2008; Seker et al., 2008] whereas the plumes usually occur at midlatitudes when the geomagnetic

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SEKER ET AL.: F REGION DEPLETIONS AND MAGNETOSPHERE

Table 1. Different Properties of MSTIDs and Plumes as Observed With the Penn State All‐Sky Imager Property

MSTIDs

Plumes

Structure Appearance Alignment Propagation Wavelength and period Theory Occurs when Kp is

parallel bands extending from north northwest‐southeast toward southwest 100–300 km, ∼1 h Perkins instability low

single plume from south north‐south east‐west not periodic Rayleigh‐Taylor high

activity is high [Garcia et al., 2000; Sahai et al., 2000; Seker et al., 2007]. [4] Saito et al. [2002] found using the MU radar and GPS network in Japan that the occurrence of the traveling iono-

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spheric disturbances (TIDs) is anticorrelated with solar activity. Shiokawa et al. [2003] reported the inverse dependence of the MSTID occurrence with solar activity at midlatitudes. After the analysis of 3 years of GPS‐TEC data, Kotake et al. [2006] revealed a similar inverse relation between solar and MSTID activities only at nighttime over Japan and Australia but not during the day and not over North/South America or Europe. Tsugawa et al. [2007a] reported the seasonal dependence of the inverse relation of the MSTID occurrence with solar activity. After analyzing 7 years of data but only 28 MSTID events from an all‐sky imager in Brazil, Candido et al. [2008] reported that that there were no occurrences of dark bands during high solar activity (HSA), 3% during medium solar activity (MSA), and 11% during low solar activity (LSA). He and Ping [2008] found a similar relation using Kp and radar data

Figure 1. (a) An intense MSTID event consisting of parallel depletion bands. (b) An intense spread F plume event illustrating its fractal plume‐like shape. (c) A moderately intense MSTID event. (d) A moderately intense plume event. The bright (dark) regions indicate enhanced (depleted) density regions. Plumes (MSTIDs) are observed at midlatitudes usually when the geomagnetic activity is high (low). 2 of 11

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from 2000. A recent paper by Martinis et al. [2010] used all‐ sky imager data from Arecibo and showed an inverse relation between MSTID occurrence and solar flux. [5] However, there are only a few studies on the relation between the low‐latitude plumes and solar activity and we have found only a few case studies and no statistical studies on the relation of “midlatitude” plumes and geomagnetic/ solar activity. The statistical study of transequatorial plasma bubbles by Sahai et al. [2000] using an all‐sky imager in Brazil (∼16°S dip latitude) revealed that the plumes occur more frequently at HSA (55% versus 33%) and reach very high apex (above equator) altitudes more often (66% versus 34% of plumes) as compared with LSA. They found no difference in the seasonal dependence of plume occurrence between LSA and HSA. However, using sounders, Whalen [2002] found an inverse relation of spread F and bubble occurrence with Kp, the strength of which depends on the season. Since their observations were made at low latitudes, the spread F bubbles they observed are probably plumes, and not MSTIDs. The plumes, which reach higher altitudes over equator during high geomagnetic activity, are thus expected to reach higher latitudes following the field lines, meaning that the plumes would be expected to be observed at midlatitudes more often during high Kp than low Kp which agrees with our observations. The plumes at very high altitudes might be missed by ionosondes which might explain the decrease in spread F occurrence with Kp near equator found by Whalen [2002]. Similarly, Hysell and Burcham [2002] statistically analyzed the data from the JULIA radar in Jicamarca and found that the equatorial plumes occur at and penetrate to higher altitudes with higher solar flux and geomagnetic activity. They attributed this to the electric fields controlled by the meridional circulation which is affected by the disturbance dynamo in the auroral zones during geomagnetic storms. Garcia et al. [2000] studied a midlatitude plume event which occurred during a geomagnetic storm using all‐sky imager, GPS, and Digisonde observations. They called it “midlatitude spread F” which has also been studied using the Arecibo radar by Mathews et al. [2001]. Here, we prefer to call it “midlatitude plume” to emphasize the distinct shape of these plasma depletions in all‐sky images. Sobral et al. [2002] studied the statistical relation between the plasma bubbles and seasons, solar cycle, and geomagnetic activity using airglow data from Brazil (33°S, 28°S dip latitude). However, they have not distinguished between MSTIDs and plumes and considered both as bubbles even though MSTIDs and plumes behave very differently with respect to seasons, solar cycle, and geomagnetic activity. They also studied the effect of time delay and found that the postsunset bubble development is inhibited/enhanced by presunset/postsunset magnetic activity increases. The fact that the nighttime bubbles are found to occur more often with higher Kp (when minimum time delay is applied) suggests that most bubbles they observed are actually plumes. [6] In summary, most studies focused on the inverse relation between geomagnetic activity and MSTIDs and used Kp as the only indicator of solar or geomagnetic activity. There are a few studies on midlatitude plumes but they are based on a few case studies and do not provide any statistics. Here, we attempt to statistically characterize both MSTIDs and midlatitude plumes using 5 years of all‐sky

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imager data and multiple magnetospheric state parameters instead of Kp only. These parameters are introduced in section 2.

2. Method [7] In order to better understand the relation between the geomagnetic activity and occurrence of MSTIDs and plumes at midlatitudes, the data from the Penn State All‐Sky Imager (PSASI) at Arecibo Observatory (AO) from 2003 to 2008 were categorized and analyzed according to solar wind and geomagnetic conditions. The type, intensity, and start/ end times of the F region events observed during 2003– 2008 using the 630 nm airglow filter were recorded. The event type is either MSTID or plume classified on the basis of the characteristics listed in Table 1. The most easily distinguishable feature of midlatitude MSTIDs and plumes (as observed with an all‐sky imager) is their distinct shapes (parallel bands for MSTIDs versus fractal shaped plumes). Another distinct characteristic is that the plumes at AO always appear from south whereas MSTIDs never appear from south and almost always propagate southwestward. [8] The event intensity is based on the intensity of depletion in the airglow images and consists of three levels: weak, moderate, intense. This classification was initially done visually from the all‐sky movies created for each night mostly to eliminate the very weak events that could have been miscategorized. To validate this categorization, we calculated the percentage of depletion intensities numerically from the original data for a full year of sample data and it was found that the calculated intensities mostly agreed with the visually assumed intensities. Yet, it should be kept in mind that even the calculated intensities are not very reliable because the airglow intensity also depends on height of F layer in addition to plasma density and the background intensity is highly variable owing to city lights, tower lights, clouds, etc. Once again, our goal for this classification was to discard the unreliable weak events since it is more difficult to distinguish between MSTIDs and plumes if the depletion is weak. [9] Although the event start and end times often depend on the duration of the event itself, they can also be influenced by other factors which prevent airglow observations such as cloud coverage and moonrise/moonset or sunrise/ sunset times. Nevertheless, the start and end times always represent a time interval during which an event is occurring. [10] To determine the relationships between geomagnetic activity and the occurrences of midlatitude MSTIDs and plumes, we obtained the magnetospheric state parameters from the NASA Magnetospheric State Query System (MSQS) [see Fung and Shao, 2008; S. F. Fung, Announcement of a “Magnetospheric‐State Query System,” AGU SPA Section Newsletter, XI(9), 2004]). The parameters chosen in this study are Kp, Dst, AE, solar wind (SW) magnetic field (B), SW magnetic field north‐south component (Bz), SW electric field east‐west component (E), SW velocity (V), SW flow pressure (P), SW density (N), and SW temperature (T). SW B is more commonly known as interplanetary magnetic field (IMF), but here we use SW B to emphasize its origin. The magnetospheric state parameters can be classified as the driver (input) parameters (solar wind electromagnetic fields, velocity, flow pressure, density, and temperature) which are measured by satellites (such as ACE and WIND) and the

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Figure 2. A sample magnetospheric state parameter versus time plot showing the time variation of Dst for individual intense MSTID and plume events. The duration of each airglow event is highlighted with thicker lines. response (output) parameters (Kp, Dst, and AE indices) which are measured by ground magnetometers. Kp index is a global average of geomagnetic activity at midlatitudes sampled at 3 h intervals; the Dst index, which has 1 h time resolution, is the average of geomagnetic field perturbations measured at various low‐latitude observatories and measures the ring current; and the AE index measures the magnetic perturbations at high latitudes with 1 min resolution providing a measure of currents in the auroral ionosphere induced by substorms. In this study, hourly averages are used for each parameter. As each of these parameters measures a different region of the magnetosphere, they can be used to characterize the onset conditions and electrodynamics of ionospheric plasma irregularities such as MSTIDs and plumes. [11] For each MSTID and plume event, each of the magnetospheric state parameters is recorded from ∼10 h before sunset (1000 LT) to ∼10 h after sunrise (1600 LT). A sample time variation plot is given in Figure 2 illustrating the difference in the Dst values for intense MSTIDs and plumes. The durations of the observed events are highlighted with thick lines which always fall between 1900 and 0600 LT since the imager can operate only at night. Instead of showing the time variation plots for other parameters, which

show a similar trend with Dst, and which is impractical for displaying all the events, we present histogram plots in Figure 3 showing the occurrence rate of both moderate and intense MSTIDs and plumes for different value intervals of each parameter. In these plots, the occurrence rate distribution is calculated from the ratio of number of events that occurred when a specific parameter had a value in a specific bin to the number of total available data (defined as clear nights with at least 1 h of observation) in that bin. This relative occurrence rate is used to remove the effects of data distribution over different parameter bins. In particular, since intense geomagnetic storms are much less likely to occur than weak storms or quiet times, very few events are observed for those extreme bins, so the division by the number of available data is intended to remove that background variation. However, the occurrence ratios for the extreme bins (e.g., intense geomagnetic storms) are not very reliable as there are very few available data for such extreme conditions. To obtain more reliable results, we have reduced the value of the maximum bin until there is sufficient data in those bins while keeping bin intervals equal for all other bins. [12] The monthly and annual variations of MSTID and plume occurrence rate are given in Figures 4 and 5,

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Figure 3. Histograms showing the distribution of the occurrence ratio of plumes and MSTIDs over magnetospheric state parameters Kp, Dst, AE, SW B, SW Bz, SW E, SW V, SW P, SW N, and SW T. The ratios are calculated by dividing the number of events that occurred when a specific parameter had a value in a specific bin to the number of total available data for that bin. Weak events are unreliable and thus are ignored. All parameters are averaged over 1 h, except Kp, which has a 3 h time resolution. SW, solar wind; B, magnetic field; Bz, magnetic field north‐south component; E, electric field east‐west component; V, velocity; P, flow pressure; N, density; T, temperature. 5 of 11

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Figure 4. Average monthly occurrence rates of intense, moderate, all MSTIDs, and all plumes at Arecibo Observatory. 6 of 11

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Figure 5. Annual occurrence rates of MSTIDs and plumes at Arecibo Observatory. respectively. The occurrence rates for these plots have been calculated differently from the histogram plots. Instead of using the ratio of actual number of events in a specific month or year to the total number of events, we used the ratio of number of events for a specific month or year to the number of available days (e.g., clear skies, imager working) in that month or year in order to avoid misleading results that would occur for months or years with only a few available days.

3. Results [13] The relation between various magnetospheric state parameters and the occurrence of MSTIDs and plumes observed with PSASI between 2003 and 2008 can be visually seen from the plots given in Figures 2 and 3. Figure 2 shows the time variation of the Dst index for intense MSTIDs and plumes. The durations of each MSTID or plume event observed is highlighted with a thick line. Figure 2 clearly demonstrates the distinction of Dst values for MSTIDs and plumes. Since the average behavior is more important than individual time variations, histogram plots are used for each parameter. The histogram plots in Figure 3 illustrate the difference in magnetospheric state parameters between MSTIDs and plumes. The average values for each parameter for both moderate and intense plumes and MSTIDs are listed in Table 2 quantitatively revealing the relationships between the magnetospheric state parameters and these ionospheric events. These values are found by averaging each parameter first over the time duration of each event observed with the

all‐sky imager and then over all MSTID or plume events in the same intensity category. [14] Results in Figure 3 suggest that although individual midlatitude plume events tend to occur during geomagnetic storms, they might also occur during quiet times but much less often. For example, the average plume occurrence rate is 5% (14 nights out of 288) for Kp < 4 and 38% (9 out of 24) for Kp > 4. However, the results clearly show that MSTIDs usually occur at geomagnetically quiet times and hardly occur during geomagnetic storms (e.g., 3 out of 24 or 13% for Kp > 4 versus 122 out of 288 or 42% for Kp < 4) further confirming the previous aforementioned studies. Similar trend is observed for all other parameters (Dst, AE, SW B, SW Bz, SW E, SW V, SW P) except for solar wind density (SW N) and temperature (SW T). In other words, it seems that the MSTIDs usually occur when the magnitude of the parameter is low (e.g., geomagnetically quiet times) and although midlatitude plumes can occur during both quiet times and storms, they occur much more often during high geomagnetic activity. Inversely, it can be said that MSTIDs almost never occur during intense storms. [15] Interestingly, among the 11 intense midlatitude plume events, by far the lowest average Kp (1.82) and highest average Dst (3 nT) occurred on 25–26 December 2003 when a depletion plume turned into a brightness plume and MSTID bands appeared afterward. This exceptional inversion event has been recently investigated in detail by Martinis et al. [2009]; however, no information on geomagnetic activity was provided. This example suggests that, for some reason, the inversion plumes might occur only

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Table 2. Average Values of Each Geomagnetic State Parameter for Plumes and MSTIDsa Plume

MSTID

Parameter

High Intensity: 11 Events

Total: 23 Events

Moderate Intensity: 12 Events

Moderate Intensity: 88 Events

Total: 125 Events

High Intensity: 37 Events

Kp AE (nT) Dst (nT) SW V (km/s) SW E (mV/m) SW B (nT) SW Bz (nT) SW P (nPa) SW N (cm−3) SW T (103 K)

4.79 654 −86 542 3.9 15.1 −5.9 4.10 6.26 128

3.84 415 −58 520 1.9 10.6 −2.8 3.15 5.46 127

2.97 203 −30 506 0.5 5.9 −0.8 2.28 4.72 126

1.84 182 −15 451 −0.1 5.2 0.2 1.97 5.36 120

1.81 171 −13 452 −0.2 5.3 0.4 1.94 5.11 123

1.75 144 −8 458 −0.5 5.6 0.9 1.87 4.54 131

a These values correspond to the total average of the thick lines in Figure 2. Abbreviations are as follows: SW, solar wind; B, magnetic field; Bz, magnetic field north‐south component; E, electric field east‐west component; V, velocity; P, flow pressure; N, density; T, temperature.

during low Kp; however, these exceptional events are not the focus of the current study. [16] In particular, it can be seen from the Bz histogram that while midlatitude plumes usually occur when the interplanetary magnetic field (IMF) is pointing south (negative Bz), the MSTIDs usually occur when it is pointing north (positive Bz). This further strengthens the argument that midlatitude plumes tend to occur during geomagnetic storms and MSTIDs tend to occur during quiet times since it is much more likely for geomagnetic storms to occur during southward IMF. Similarly, it can be seen that the midlatitude plumes and MSTIDs usually occur when interplanetary electric field (IEF) is east and west, respectively. [17] These relations become more significant when the parameters are averaged over all MSTID or plume events as illustrated in Table 2. For example, the average Kp, AE, and Dst for the 11 intense plume events are 4.8, 654 nT, and −86 nT, respectively, as compared to 1.8, 144 nT, and −8 nT for the 37 intense MSTID events, a significant difference. Although the intensity classification was done to eliminate the weak events, we also noticed an empirical relation between the depletion intensity and magnetospheric parameters which is not unexpected. The average Kp, AE, and Dst values for moderate MSTIDs and moderate plumes fall between the average values for intense MSTIDs and intense plumes. For example, for moderate plumes and moderate MSTIDs, the average Dst values are −30 nT and −15 nT, respectively. The average Kp, AE, and Dst values for moderate plumes are higher (in magnitude for Dst) than for moderate MSTIDs, as also expected. Similar trends are observed for the other (solar wind) parameters shown in Table 2 although the strength of the relation varies for each parameter. [18] Figures 4 and 5 present the monthly and annual variations of MSTID and plume occurrence rates. The year 2008 was discarded in Figure 5 since there is data only from the first two months of 2008 owing to a hardware problem. The seasonal variation is shown separately for moderate and intense MSTIDs whereas for plumes it is shown only for all events since there are not enough intense or moderate plume events for each month. The largest peak for MSTID in Figure 4 occurs in February and second peak occurs in July. The largest minimum occurs in September (fall equinox) and the other minimum occurs in March (spring equinox).

However, Martinis et al. [2010] did a similar study and found peaks in January and June and minima in March and October. For intense MSTIDs only, large peaks occur closer to solstices in July and December whereas the minima occur in April–May and September–October. For moderate MSTIDs only, large peaks occur in February and May whereas the minima occur closer to equinoxes in March and September. [19] However, it can be observed that the plumes occur a lot more often around October and far less between March and May. Interestingly, in our data set no plume event has occurred in August during the 5 years of observation period. In general, the plumes occur much less often than MSTIDs (as can be seen by comparing the percentages in Figures 4 and 5) but it is unlikely that this is just a sampling issue considering that there are 92 August nights with dark (moon‐down), clear skies in the data set. [20] The annual variations in Figure 5 suggest that the occurrence of midlatitude plumes is directly correlated with solar cycle. There is especially a sharp decrease in 2007 when the solar activity has become very low. At equatorial latitudes, Hysell and Burcham [2002] and Sahai et al. [2000] found a similar correlation of plume occurrence with solar activity using radar and imagers, respectively. As expected, the MSTIDs start to occur slightly more often as the solar activity diminishes except in 2007 when an unexpected decrease in MSTID occurrence is also observed. The ionosphere during the recent unusually low solar minimum has been very weak so this unexpected decrease might simply be due to lack of plasma in the nighttime ionosphere.

4. Discussion [21] In this study, we have investigated, for the first time, the relation between multiple magnetospheric state parameters and the occurrence of midlatitude plumes and MSTIDs in the nighttime F region. The results given in Table 2 and Figure 3 suggest that there is a direct and inverse relation between the occurrences of midlatitude plumes and MSTIDs, respectively, and the geomagnetic activity. An empirical relation between the depletion intensity and magnetospheric parameters was also found. However, it was found that these relations apply to a set of events rather than individual events. On the basis of these results, it can be concluded that, in general, plumes tend to occur at high Kp,

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low (highly negative) Dst, and at high solar wind velocity, pressure, magnetic/electric field (in magnitude) and during IMF south and IEF east whereas MSTIDs tend to occur when the magnitude of these parameters are low and during IMF north and IEF west. It seems that this empirical relation is especially valid for Kp, AE, and Dst and not valid for solar wind density and temperature. It should be noted that when the solar activity is high (e.g., due to a CME or flare targeting Earth), it is expected that the magnitude of most of the magnetospheric parameters increase so it is not surprising to see similar effects for most of the parameters used in this study. However, solar wind density is generally anticorrelated with solar wind speed which might explain why plumes seem to occur more often with high solar wind speed but less often with high solar wind density. [22] It is concluded that although it is impossible to define a single depletion event just by looking at the magnetospheric state parameters, it might be possible to forecast the occurrence probability of a midlatitude plume or MSTID event under certain conditions, and inversely, to tell whether an ambiguous depletion event in the all‐sky data is likely a plume or MSTID. Most importantly, these results suggest a relation (although somewhat stochastic) between the state of the magnetosphere and ionospheric phenomena such as plumes and MSTIDs. So, it can be concluded that even if these F region irregularities are seeded at lower altitudes by gravity waves as suggested by various studies [Nicolls and Kelley, 2005; Shiokawa et al., 2006; Tsugawa et al., 2007a; Vadas, 2007; He and Ping, 2008], their occurrence in the F region and intensity can be affected by the condition of the magnetosphere and solar wind. [23] Several possible physical causes of the inverse relation between geomagnetic/solar activity and the occurrence of MSTIDs have been proposed. Saito et al. [2002] attributed this relation to Perkins Instability, the growth rate of which anticorrelates with the solar activity as Kelley and Fukao [1991] first showed. Whalen [2002] suggested that the decrease of the maximum prereversal eastward electric field during high geomagnetic activity suppresses the bubble occurrence; however, what they called bubbles are probably equatorial plumes and not MSTIDs. Kotake et al. [2006] also attributed this inverse relation to enhanced Perkins Instability owing to lower neutral density during low solar activity. After analyzing ionosonde data from 1957 to 1990, Rishbeth and Mendillo [2001] concluded that the F2 layer variability is mostly caused by the geomagnetic activity. They found that the variability is much higher at night and attributed this to enhanced auroral energy input and lack of the strong photochemical control of the F2 layer at night. [24] The only statistical study on the relation of (low‐ latitude) plumes with geomagnetic activity by Sahai et al. [2000] found that the equatorial spread F bubbles reach higher altitudes at high solar activity owing to stronger F region vertical plasma drift. More specifically, around sunset at the magnetic equator the F layer rises owing to prereversal enhancement and this process is greatly enhanced during geomagnetic storms owing to prompt penetration of high‐latitude electric fields. The rise of equatorial plumes enables them to reach midlatitudes following the geomagnetic field lines. We think this is currently the most plausible explanation for the high rate of occurrence of midlatitude

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plumes during geomagnetically active periods. However, the average eastward electric field is in general weaker during low solar activity than high solar activity which makes it more difficult for equatorial plumes to reach midlatitudes. So during low solar activity, the plume occurrence at midlatitudes is expected to depend on geomagnetic activity even more. On the contrary, Garcia et al. [2000] refuted this mechanism and suggested that the midlatitude spread F might be caused by enhanced Perkins instability owing to the expansion of the equatorial anomaly region upward and northward in response to the polarization by a penetrating eastward electric field. However, midlatitude spread F might refer both to plumes and MSTIDs and Perkins instability applies only to midlatitudes whereas the plumes are usually generated at equatorial latitudes and propagate or grow to midlatitudes. [25] In addition, MSTIDs have been found to occur more often around solstices and less often around equinoxes. This agrees with other studies done at midlatitudes [Kotake et al., 2006, Martinis et al., 2010]. Kotake et al. [2006] found this relation at all longitude sectors using GPS‐TEC data. They attributed this to the decrease in neutral density at solstices which causes gravity waves with larger amplitudes which could possibly seed more intense MSTIDs. Martinis et al. [2010], who used the all‐sky imager at Arecibo, embraced an alternative mechanism. They attributed the high occurrence rate around solstices to the interhemispheric flux tube coupling between conjugate ionospheric regions. Although both mechanisms are possible, considering that there is an inverse relation between MSTID occurrence and geomagnetic activity, we hypothesized that the low occurrence rate of MSTIDs around equinoxes could possibly be a result of the high occurrence rate of geomagnetic storms around equinoxes via the Russell‐McPherron effect (which is based on the orientation of the geomagnetic dipole with respect to the IMF which depend on seasons) [Russell and McPherron, 1973]. To test this hypothesis, we analyzed the seasonal variation of MSTID occurrence only for the geomagnetically quiet condition (Kp < 2) and the results (shown in Figure 4) still showed similar seasonal variation (except for January) suggesting that the seasonal variation of MSTID occurrence is probably not related to the seasonal variation of geomagnetic activity. In addition, it is observed that peaks occur closer to solstices for intense MSTIDs and minima occur closer to equinoxes for moderate MSTIDs. [26] However, it is observed that the midlatitude plumes occur often in October with a second (minor) peak in February, almost never in August, and very little between March and May. Again, the possible Russell‐McPherron effect is discarded as plumes that occurred only at high Kp still showed similar seasonal variation. Using an all‐sky imager at 23°S in Brazil, Sahai et al. [2000] found a minimum of plume occurrence between May and August and a maximum between October and March. This is somewhat similar to our result which is expected as the plumes are shown to be conjugate events. This seasonal dependence has in general been attributed to the prereversal enhancement in vertical drifts owing to seasonal changes in zonal electric field which controls the plasma instability occurrence and duration [Hysell and Burcham, 2002]. However, Tsunoda [1985] found a seasonal (high at equinoxes and low at solstices)

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and longitudinal dependence of equatorial spread F occurrence and attributed it to the alignment of solar terminator with the geomagnetic field lines during the equinoxes, thereby causing the E region Pedersen conductivity to change most rapidly. However, they admit that there are some discrepancies in this relation, and that there must also be other electrodynamic mechanisms involved which we agree as our results show peaks in October and February, not in September and March, and the minima are also not exactly during solstice months. Similarly, Maruyama and Matuura [1984] found a longitudinal and seasonal variation but suggested that it is caused by the suppression of Rayleigh‐Taylor instability through the effect of transequatorial thermospheric wind pattern that varies with season and longitude. [27] Using the solar wind parameters and the Kp, Dst, and AE indices it might be possible to infer the occurrence of ionospheric plumes relative to the phase of the geomagnetic storm or substorm. Also, since different magnetospheric indices usually represent different parts of the magnetosphere, by using time‐variation plots such as the one given in Figure 2 for Dst and appropriate time delays as suggested by Fung and Shao [2008], the sequence of events that eventually manifest as MSTIDs and plumes could be understood. For example, if AE index usually gets perturbed right before plume events that would indicate that the plumes are controlled by auroral processes; whereas if AE gets perturbed shortly after plume events, then it could be inferred that these ionospheric disturbances are somehow affecting the auroral electrojet and thus the magnetosphere via ionospheric outflows [e.g., Winglee et al., 2002]. [28] Furthermore, recent studies using satellite data also suggest that there might be a connection between the quasiperiodic MSTIDs and the periodicities observed in the solar wind [Huang et al., 2000; Livneh et al., 2009]. So, it might be very interesting to study in more detail the relation between the magnetospheric state parameters and various properties of these events (such as wavelength, orientation, scale, speed, and especially period). These studies, which require detailed analysis using high‐resolution satellite and imager data, are beyond the scope of this study and are going to be the topic of future research. [29] Acknowledgments. This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. The Penn State All‐Sky Imager System is supported under NSF grant ATM 07‐21613 to Pennsylvania State University. The monthly and annual variation plots have been provided by NASA summer intern Seok Youl Yoon. We thank the reviewers for their constructive comments. [30] Robert Lysak thanks Yuichi Otsuka and the other reviewers for their assistance in evaluating this paper.

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