DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A5, PAGES , MAY 1, 2001 DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum C. Y. Huang, W. J. Burke, a J. S. Machuzak, a L. C. Gentile, and P. J. Sultan a Abstract. The Defense Meteorological Satellite Program (DMSP)flights F9 and F10 crossed postsunset local time sectors approxima, tely 14 times per day in Sun-synchronous orbits at an altitude of -840 km. We have examined a large database of postsunset plasma density measurements a. cquired during,,15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We ha. re analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground-based measurements. That is, the seasonal varia. tions detected at 840 km in longitude bins hosting radar/scintillation observatoa'ies appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that rela, tively large equatorial magnetic fields at F layer altitudes in the Pacific ( G) sector more strongly inhibit the growth of the Rayleigh-Taylor instability than a,t Atlantic ( G) longitudes. Contrary to general belief, we found tha. t EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to da. ys during storm recovery phases. 1. Introduction The growth of this instability depends on the develop- Despite long and intense study, developing a realisment of steep, upward plasma density gradients after tic capability for predicting occurrences of equatorial sunset in the bottomside F layer, local electric fields plasma bubbles (EPBs) has proved to be elusive. The with an eastward component, and low values of the matter is not without practical urgency. EPBs create flux tube integrated Pedersen conductivity [Anderson environments that promote the growth of ionospheric and Haevendel, 1979]. The fact that ionospheric plasma plasma turbulence capable of causing the scattering or generally rises in altitude just prior to and shortly after sunset before falling attests to the presence of enhanced scintillation of radio waves. On ionosondes the signature of bubbles gives rise to diffuse echoes known as eastward electric fields in the evening sector [Farley et al., 1986]. However, substantive questions remain respread F. It is generally conceded that EPBs grow in the garding the causes of these eastward electric fields. A postsunset, low-latitude ionosphere through a generalnumber of empirical and theoretical approaches have ized Rayleigh-Taylor (R-T) type instability [Oft, 1978]. been taken to answer these questions. Large databases have been developed using measure- Boston College Institute for Scientific Research, Chest- ments from satellites [Mavuyamand Matuuva, 1984; nut Hill, Massachusetts. 'Air Force Research Laboratory, Space Vehicles Direc- Kil and Heelis, 1998; McClure et al., 1998] or groundtorate, Hanscom Air Force Base, Massachusetts. based radars [e.g., Woodman and LaHoz, 1976] and beacon-scintillation monitors [Basu and Basu, 1985]. Copyright 2001 by the American Geophysical Union. Figure 18 of the review by Aavons [1993] (hereinafter referred to as A-93) sketches the season versus longitude Paper number 2000JA (S/L) distribution of range spread F occurrence at eight /01 / 2000J A stations distributed around the globe. Range spread F 8131

2 8132 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES Table 1. Ground Detection Sites Station GLat, deg GLong, deg MLat, deg Declination, deg IB(G)l Accra ? Ascension Guam Huancayo I Kodaikanal Kwajalein Manila Natal primarily occurs in the evening sector and is associated with the development of upward moving plumes of turbulence that can be observed with radars. Table 1 gives the geographic and magnetic coordinates of the observation stations. The final two columns of Table 1 give the approximate local magnetic declination and the magnitude of the field IBI at an altitude of 400 km at the nearest point to the station on the magnetic equator. We note that the variance in IBI between the Pacific ( 0.34 G) and Atlantic ( 0.25 G)sectors is considerable. During solar maximum years, scintillation activity was experienced at all stations near the time of the equinoxes. Strong minima occur in the rates of scintillation activity during the December-January solstice at Asian-Pacific stations and during the June-July solstice at South American-Atlantic-African stations. Tsunoda [1985] suggested that the seasonal variations are largely controlled by the local declination. The generalized R-T instability is favored where the longitudinal components of flux tube integrated Pedersen conductivity gradients at the terminator are most severe. Near the December Other models have examined the possibility that the enhanced postsunset electric fields arise naturally as a result of an F layer dynamo [Farley et al., 1986]. Details of the coupling between the E and F layers in the vicinity of the terminator have been worked out by Haerendel and Eccles [1992] and Eccles [1998]. This model requires that a portion of the dayside equatorial electrojet diverts into the nightside F layer. Consequent polarization charges produce a standing vortex in the flow of low-latitude plasma, causing it to rise in the early postsunset hours and fall at later local times. Understanding of the role of geomagnetic activity in EPB formation has evolved in recent years. Early pa- pers stressed the fact that under conditions of prolonged activity such as magnetic storms, plasma bubble and irregularity formation diminishes in the evening sector and grows in the postmidnight sector [Burke, 1979; DasGupta et al., 1985; Aarons, 1991; Greenspan et al., 1991]. Kelley and Maruyama [1992] suggested that enhanced penetration of stormtime electric fields to low magnetic latitudes is responsible for the rising postmidnight ionosphere and consequent EPB activity in that local time sector. (June) solstice the gradients are strongest in regions of westward (eastward) declination. A-93 cites different scintillation morphologies observed at the Guam and Other research has focused on the effects of stormtime Huancayo stations, which have similar declinations, as electric fields on plasma drifts in the global equatorial indicating that this explanation is incomplete. Several ionosphere [Spiro et al., 1988; Fejer et al., 1990]. Scherinvestigators have proposed that transequatorial neu- liess and Fejer [1997] and Fejer and Scherliess [1997] tral winds act as potential suppressants for the pre- developed techniques to separate the effects of penetratreversal electric field [Maruyama and Matuura, 1984; ing electric fields [Nopper and Carovillano, 1978] from Mendillo et al., 1992; Kil and Heelis, 1998]. those of the stormtime dynamo [Blanc and Richmond, Periodic plumes of rapidly rising equatorial irregu- 1980]. Electric field penetration appears almost instanlarities detected by radars suggest that gravity waves taneously throughout the magnetosphere and continues propagating in the neutral atmosphere act as seeds for either as long as high potentials are imposed by the interplanetary medium or until ring current ions create shielding layers. On timescales of--.10 hours, Joule heating of the stormtime ionosphere has the effect of creating a second dynamo the polarity of which causes the ionosphere to rise in the postmidnight sector and fall in the evening sector. Thus the models of Scher- gue that the global morphology of bubble occurrence indicates that tropospheric disturbances are the main seeds for EPBs. Recently, Prakash [1999] developed a theoretical model that only requires gravity wave winds to couple to the E region altitudes. Electric fields associated with Hall, rather than Pedersen, currents couple liess and Fejer [1997] and Fejer and Scherliess [1997] to the F layer providing the seeds for bubble formation. predict that in the early stages of magnetic storms pen- This model predicts that the sources for gravity waves etrating electric fields cause the postsunset ionosphere are at low latitudes. Meridionally propagatin grav- to rise; in the recovery phase a reverse-polarity dynamo ity waves from auroral disturbances cannot be seeds for causes the postmidnight ionosphere to rise. Consistent spread F phenomena. with this prediction, Burke et al. [2000] found that

3 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 8133 during the June 1991 magnetic storm EPBs were detected in the evening sector during the main phase and in the dawn sector during the recovery phase. The deepest plasma bubble was detected in the early evening sector by the Defense Meteorological Satellite Program (DMSP) satellite F10 during a portion of the storm's main phase when the CRP ES satellite, which was flying in the inner magnetosphere, detected electric fields penetrating earthward of the ring current. The bubble moved upward with a speed of 02 km s -. Hanson et al. [1997] showed that fast-rising EPBs most commonly occur in the early postsunset ionosphere. The purpose of this paper is to report on the distributions of EPBs observed by DMSP satellites in the evening sector during the solar maximum years 1989 and Our emphasis is twofold. First, we demonstrate that D MSP satellites are effective platforms for monitoring the climatology of EPB occurrence. Second, we show that both individual cases and the data set as a whole consistently indicate that EPB activity in the evening sector is enhanced during the early stages of high geomagnetic activity but can be suppressed, sometimes for days, after prolonged elevated activity. Section 2 briefly describes the DMSP satellite measurement capabilities. The S/L statistical properties of EPB detections by D MSP satellites are then presented in sec- tion 3. We also examine the response of the evening sector, low-latitude ionosphere during 11 magnetic storms with minimum Dst < -150 nt. In section 4 we com- pare our results with previously reported satellite and ground-based climatologies and with predictions of the Scherliess and Fejer [1997] stormtime model. 2. Instrumentation This paper summarizes a large number of solar maximum observations taken at magnetic low latitudes in the evening local time (LT) sector by a suite of cold plasma sensors on the DMSP F9 and F10 satellites [Greenspan et al., 1986]. The ion drift meters measured the vertical and horizontal components of plasma drift transverse to the satellite trajectories [Rich and Hairston, 1994]. Nominally, DMSP satellites are in circular, Sun-synchronous polar orbits with inclinations of at km altitude. Geographically, the orbital plane of the D MSP F9 satellite was near the LT meridian. The F10 satellite was launched into a slightly elliptical orbit. Initially, the orbit's ascending node was at 1936 LT, but it precessed by 42 min toward later local times over the course of 1991 [Hanson et al., 1997]. The F9 (F10) spacecraft crossed the magnetic equator in the evening sector moving toward the southwest (northwest) about every 101 min. In the course of a day each satellite made evening sector equatorial crossings, with the subsatellite point on the Earth's surface shifting ø in longitude to the west. DMSP F9 was flying in 1989; both F9 and F10 were operational in As expected, the DMSP F8 satel- lite, flying near the dawn-dusk meridian in both years, never detected EPB density depletions in the evening sector. 3. Data Selection and Observations Data presented in this section were selected with two objectives in mind. The first is to establish the statistical distribution of encounters between DMSP satel- lites and EPBs according to season and longitude. The second objective is to characterize evening sector EPBs during large magnetic storms. The years 1989 and 1991 were chosen for analysis because experience showed that DMSP encountered many more EPBs during the maximum activity phase of the solar cycle. A total of 11 magnetic storms with Dst -150 nt occurred in these 2 years. The fact that both F9 and F10 were flying in 1991 allowed us to examine differences caused by the lo- E o 106 ' - ' 105 c2 104 o ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' Mar 03, 199.,1 "/... (,a! 18 2 U-!,,,/- w k 1917 LT 2,1øE Kp.: 5-,. i i i I i i i I i,,,,, I i, i, I ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' Mar 04, 199,1. j.,, 19.4 UT,//,I x,,,,, 19.7LT (,b) 3øE,Kp 5 I I I, I, - I I I I. I I I I I I,,,,, I I ß ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' Mar 04, 19 J,,,..., 17.8 U-I //%Jill X, 19.7LT "----!?!,,,,,,,,,,s,o,, ' ' I ' ' ' I ' ' ' i ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' Jun 05, 1991,, 6.0 UT r-,!,o!.,,...,..,iv" /\ 20.0LT.o,+,. South Mag Eq North Figure 1. Examples of the four categories of bubbles (M-0 to M-3) according to the depth of depletions AN with respect to the nearby undisturbed plasma density. Plasma density categories' (a) M-0 if AN < 2, (b) M-1 if 2 < AN _< 10, (c) M-2 if 10 < AN _< 1-0, and (d) M-3 if AN > 100.

4 8134 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES Table 2. Equatorial Plasma Bubble (EPB) Encounters Satellite/Year Sam pies EP Bs M-0 M- 1 M- 2 M-3 F9/ F9/ F10/ cal time separation of-- 2 hours. The remainder of this section is divided into three subsections that summa- rize our EPB categorization, the S/L statistics of EPB encounters, and equatorial ionosphere responses to elevated levels of geomagnetic activity Categorization of EPB Encounters To determine when either DMSP satellite encoun- tered an EPB, we examined the distributions of plasma densities measured in the evening sector and plotted as a function of magnetic latitude. Burke et al. [2000, Figures 5 and 6] show examples of plasma densities and drift components measured by D MSP F9 and F10 during encounters with upward drifting EPBs in the main phase of the June 1991 magnetic storm. For the present study we have somewhat arbitrarily divided bubble encounters into four categories (M-0 to M-3) according nient to present the longitudinal distribution of bubbles as percent of encounters (PE). Here PE- 100.(number of encounters/number of samples) in bins of 150 longitudinal width. Considering the distributions from both DMSP satellites, three empirical points can be made: (1) By and large the P E variesmoothly with longitude ( b) and is quite similar both between the two spacecraft and between the 2 years. The entire data set is well represented over the range 0 ø _< b <_ 3600 by the quadratic expression PE fi fi ', (1) with a regression coefficient of (2) During these solar maximum years the probability that D MSP satellites would encounter EPBs across the South American to east African (300øE to 30øE) sector was >30%. (3) The probability of a bubble encounter fell to a broad to the depth of depletions AN with respect to the mimimum (<20%) across the Asian-Pacific longitude nearby undisturbed plasma density No. Plasma densector (60øE to 240øE). sities plotted in Figure 1 show examples of these cate- Figure 2 shows the distribution in longitude of engories: (Figure la)m-0 if AN _ 2; (Figure lb)m-1 if counters with deep plasma depletions. Asterisks and 2 < AN <_ 10; (Figure lc) M-2 if 10 < AN <_ 100; (Fig- triangles represent M-2 and M-3 encounters, respecure ld) M-3 if AN > 100. If a satellite crossed multiple tively. The top and middle rows summarize DMSP F9 bubbles during a single orbit, the orbit was regarded as observations in 1989 and 1991, respectively. The bota single encounter and categorized by the depth of the tom row gives the distribution of F10 detections. Care deepest density depletion. A summary of plasma bub- must be taken in using data displayed in Figure 2 by ble encounters by DMSP F9 in 1989 and 1991 and F10 allowing for sampling biases caused by the varied angles in 1991 according to the M-0 to M-3 categories is listed at which DMSP satellites crossed magnetic field lines at in Table 2. Occasionally, DMSP satellites detected Ap- low latitudes. Moving to the northwest as it crossed the pleton anomaly-like distributions in which the plasma equator in the evening sector, DMSP F10 flew close to density at the equator was significantly lower than that the alignment of magnetic meridians in regions of westat 15 ø magnetic latitude [Hanson and Urquhart, 1994]. ward declination and across them in regions of eastward These equatorial troughs were not considered as EPBs declination. Conversely, as DMSP F9 moved toward the unless they also contained irregularities of a factor of 2 southwest it crossed a larger swath of magnetic longior greater. tude in regions of westward declination. Thus if EPBs The first column of Table 2 indicates the total numwere evenly distributed in longitude, F10 should have ber of equatorial samples acquired in each year. In 1991, detected more of them in regions of eastward declination DMSP F9 and F10 encountered EPB irregularities dur- (eastern Pacific) than in regions of westwardeclination ing 1025 (--.21%) and 669 ( 4%)of their equatorial (Atlantic). The opposite bias would appear in F9 obpasses, respectively. In 1989, DMSP F9 experienced servations. Data in Figure 2 show that high rates of 1112 (--.23%) bubble encounters. We note that the detection occurred in the South American-African secdeeper M-2 and M-3 depletions were predominantly ob- tor for both spacecraft. This indicates that in this lonserved by F10 at earlier local times. Essentially identigitude sector the probability of occurrence, and hence cal numbers of deep depletions were detected by DMSP detection, is very high. In the eastern Pacific sector, F9 in 1989 and F10 detected far more large EPBs than F9, indicating either this sampling bias or a local time effect. From the 3.2. S/L Distributions of EPB Encounters Indian Ocean to central Pacific sectors where the mag- We next consider the seasonal and longitudinal distri- netic field declination is low, neither satellite detected butions of bubbles. Throughout this paper it is conve- many intense EPBs. The intense bubble distribution

5 .,.. o.. HUANG ET AL' GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 8135 ' ß i Pacific Sector Atlantic.. _ ' Sector, 180 ø 210 ø ø 330 ø 0 o 30 ø 60 ø 90 ø 120 ø 150 ø 180 ø Figure 2. Longitudinal distributions of M-2 and M-3 EPB encounters by the DMSP satellites F9 in (top) 1989 and (middle) 1991 and (bottom) by F10 in Asterisks and triangles indicate crossings of M-2 and M-3 depletions, respectively. shown in Figure 2 is qualitatively representative of the entire data set. The percent distribution of EPB encounters according to month is given for the three data sets in Figure 3. These show a tendency for both spacecraft to intercept EPBs in the equinoctial months of February, March, April (FMA) and August, September, October (ASO) with greater frequency than at the solstices. However, the probability that DMSP would cross a bubble in the November, December, January (NDJ) season was greater than in May, June, July (MJJ); -- 15% versus < 10%. We direct the reader's attention to the fact that DMSP F10 experienced far fewer EPBs in January and February than it did in December. The converse is true of F9 in both 1989 and In the discussion section we argue that the disparity in F10 observations is an effect of the satellite's orbit precessing across a steep local time gradient in the occurrence of EPB activity. The seasonal and longitudinal distributions of EPBs are presented in Figure 4 as percents of encounters in longitudinal bins of 300 width. Both similarities and differences are apparent. (1) During the equinoxes the data show longitudinal spreads similar to the average distribution represented by (1). During the 1989 ASO interval, F9 detected a relative maximum in the western Pacific (-,150 ø) which did not appear in the 1991 data. Also, in 1991, F10 observed a local maximum in the eastern Pacific (-,240 ø) that was unseen by F9 in both years. As discussed previously, this probably reflects an orbital sampling bias. (2) Strong anticorrelational behaviors were found in the Atlantic and Pacific 401 [ i;!1111 i? i i?? i % F9/91 F9/89 t tions sectors were during relatively the solstice frequent seasons. near While 180 o during EPB detec- MJJ, 30 ß % 1'10/91 they were entirely absent in NDJ. The highest (lowest) rate of detections occurred in the Atlantic sector during ::i::i i::i::i NDJ (MJJ). Athe LT of F9, bubbles were crossed in o 70 to 80% of thequatorial passes. 20 :::, 3.3. Responses to Geomagnetic Activity To test for possible dependences of EPB occurrence at topside altitudes on the level of magnetic activity, 10 we compared the distributions of the three databases with those of the Kp index at the times of equato-,, index, rial crossings which primarily by the DMSP reflects satellites. auroral The activity 3-hour as Kp ob- 0 ' ' ' ' ' ' ' served at midlatitude stations, was chosen more for Feb Apr Jun Aug Oct Dec Month Figure 3. Monthly distribution of EPB encounters by DMSP satellites F9 in 1989 and 1991 and by F10 in Data are presented as percent of total equatorial crossings during a given month. its ready availability than its geophysical significance. Data presented in Figure 5a show two distributions of Kp. The solid lines represent the Kp distribution for all D MSP orbits. The dashed lines indicate Kp distributions for the orbits in which EPBs were detected. We

6 ß 8136 HUANG ET AL.' GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES loo 8o 6O (a) Feb-Mar-Apr May-Jun-Jul F F F O o o 0 1oo 80 f (c) Aug-Sep-Oct 60 (d) Nov-Dec-Jan Longitude Figure 4. Longitudinal distributions of EPBs observed by the DMSP satellites F9 in 1989 and 1991 and by F10 in Data are sorted according to season and presented as percent of total equatorial crossings in 300 bins. grouped Kp data by integral values. For example, Kp tions conducive to the growth of deep plasma bubbles values of and 3- were assigned numerical values are enhanced during high geomagnetic activity. of 2.0, 2.3, and 2.7, respectively, and grouped as integral value 2. The highest numbered group includes all For reasons to be explained later, we plotted the hourly average Dst index for the full 2 years and marked data acquired when Kp _ 8 ø. The distribution show all occurrences of EPB detections. During the solar that during 1991 magnetic activity was slightly higher maximum years 1989 and 1991 there were five and six than in In 1991, Kp was _ 40 (_ 60 ) 27.5% magnetic storms, respectively, with Dst <-150 nt. (5.9%) of the time. In 1989, the occurrence rate was Listed in Table 3 are the dates and the approximate 21.7% (3.5%) for the same range of Kp. All of the plots universal times and values of minimum Dst. Figure 6 have peak values at Kp - 2. Although the total sample and EPB encounter plots monotonically decrease for Kp 2, the rates of decrease differ. For example, while.6.0% of all F10 samples were acquired with Kp _ 6 ø, 8.8% of the EPBs were detected under these very high magnetic activity conditions. This suggests that highactivity EPBs may be overrepresented in the database. Figure 5b gives the percent of EPB encounters out of the total samples in a given Kp bin for the three data sets. The histograms in this figure show that the percent of EPB encounters decreased monotonically over the Kp range 0 to 4. At higher Kp values the percent shows plots of Dst centered on times of four large magnetic storms in March and November of 1989 and These storms were chosen for presentation because they occurred at about the same time of year. To help understand EPB prestorm conditions and responses to the storms, each plot shows 11 days of Dst measurements roughly centered on the maximum epoch of each storm. Superposed on each of the plots are vertical lines coincident in time with EPB encounters by F9 in 1989 and either F9 or F10 in Depletion categories M-0 and M-1 are represented by dotted lines, M-2 by dashed lines, and M-3 by heavy solid lines. Attention is diof encounters rose to values approaching or exceeding rected to two representative features of the stormtime those in the Kp- 0 bin. This bimodal distribution of data sets. First, during the early part of each storm, the percent of EPB encounters occurs in all three data sets and does not appear to be a random fluctuation caused by low-count statistics. EPB activity was discernible in plasma density measurements taken during a total of 171 DMSP satellite passes with Kp _> 6 ø. In the from the sudden storm commencement (SSC) to approximately the maximum epoch, intense EPB activity was almost always detected. Second, for several days after the beginning of a storm's recovery phase, EPB activity was suppressed in the evening sector. DMSP same context we note that 15 of the 21 M-3 depletions were detected with Kp >_ 5 ø. This suggests that condi- measurements in the dawn sector showed a marked increase in bubble activity during the recovery phase of

7 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 8137 summarize briefly, (1) during solar maximum, DMSP [{a) F9/89 F9 and F10 detected 2800 bubbles in the postsunset 1200 F9/91 sector during 15,000 equatorial crossings; (2)the fre- d//... FlO/91 / " ' ' quency of bubble occurrence peaked at both equinoxes andin hea lan icsec ør (300øE ø30øe (3) here is asymmetry the frequency of bubble occurrence 800 / ; occurrence between northern frequency summer/winter during NDJ solstices, compared with 15% 10% [ '. during MJJ. Our observations are in basic agreem wkh previously reported resuks as summarized in A-93 and indicate ha DMSP measurements provide a sound 4OOg Kp ß... 0 : ai'... :? ') a/j? " o {b) F9/89 FlO/91 I I I Figure 5. Distributions of EPB encounters as a function of the Kp index. (a) Solid lines plot the number of equatorial crossings while Kp was in a given range. The dashed lines indicate the number of equatorial crossings when EPBs were detected. (b) Histograms give the percent of equatorial crossings during which EPBs were observed while Kp was in a given range. storms [Burke 1979; Greenspan et al., 1991; Burke et al., 2000]. Finally, we note that in viewing the data set as a whole we were impressed with the fact that a large (undetermined) fraction of the EPB encounters occurred during intervals of rapidly changing Dst, independent of whether a large magnetic storm developed. An example of such a response appears near the end of March 9, 1989 (day 68 in Figure 6a). 4. Discussion In the previous section we presented DMSP observations of EPB encounters in the evening topside ionosphere. Our results show seasonal and longitudinal variations in agreement with those of past studies which employed ground and/or satellite-based measurements. To basis for an EPB climatology Local Time Effects Two recent reports based on measurements of plasma densities by the Atmosphere Explorer E (AE-E) satellite [Kil and Heelis, 1998] and of ion drifts by DMSP [Hanson et al., 1997] show local time variations of EPB activity. Both data sets were acquired under solar maximum conditions. In fact, the >800 m s -x bubbles reported by Hanson et al. [1997] were detected using the ion drift meters on the F9 and F10 satellites throughout We note that Kil and Heelis [1998, Figure 4] show that the probability that AE-E would encounter bubbles at altitudes >350 km increased rapidly from near zero at 1900 LT to a broad maximum extending past 2200 LT. Figure 3 shows that (1) during the first 6 months of 1991 DMSP F10 detected significantly fewer EPBs than F9 and (2) as F10 precessed.- 42 rain in local time the F9/F10 rates of bubble detection became comparable. This impression is reinforced by comparing the low rate of F10 bubble encounters during January 1991 with the high rate during the solstice months of November and December. Because different counting techniques were employed, simple comparisons between the longitudinal distributions of fast bubbles [Hanson et al., 1997] and the bubble encounters reported here from DMSP F9 and F10 in 1991 are difficult. In developing statistics for their Figure 4, Hanson et al. [1997] allowed for multiple encounters with upward moving bubbles in a single equatorial pass of DMSP. In this study we simply deter- Table 3. Major Magnetic Storms Date UT Dst, nt March 14, March 26, Sept. 19, Oct. 21, Nov. 17, March 25, June 05, July 07, Oct. 02, Oct. 29, Nov. 09,

8 8138 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES (a) Dst for March 8-18, i -:1:"1' I 1 : I:1: II I :: I '" II I: I ::/ 0 ß : :1: I : '-" :.--::::-11:::!:::::: I :::::::::::::::::::::::: ::.-.: :-'::::::::"--:'-'-': ', :: = ' :: i!i"!! ::! i!i!i!!! i:: Ux. '"t/!!!i! ii 1 '300:.' i i!illi!!!:: if! i i!! ::!! \,,lj!!!::! ii ,. I I---, i ',i,i-- [ -,Ig... dig- - -,... I,, ll! i'i :.,I :::l..'d_..i II.i. Ill :: :.:. i i.: i:: :11 i i: I..::i:.:l! ,.I...,..._..._ _. i.-,_._:..'k.i_i..,,i ;.....,.::..-..,,_. _ '" '7 :i... ':'ill ['"t""l ; : ; : -,,...,., H'f ;'"' :;-- t:' '-:: ":,F... ¾.- ',.,,,.. ','-": 11, ' ; '." r.i ,...,.-.v ;x.;, t 1!-I [ B-V'..'.':: :i. :::11:::: ;III :l,: ill"'-':-..,'. II. I' -"1 111::-" KI!..,'xl I: i::::l:::tl-m [, "i 1, ß,! i ß ß ß,i,,,,, ß ß i,,,,,.i,,.,,.,,,i,,,,.,,a,l,,-,14 ' -, ß t,,, ß i ß, ß t i t I = ,,-,,,, -.,.,,,,,,,.,,,,,, (b) Dst for March 19-29, i... i":_";';';':'_:'; ;-- [-.'.'1'.'.'. '11 ::, :111:::: ii II :1œ ::::1:::1-1- ['d". ll l ':...'.':.'ii[[_.'; ii_lli _ ill.:_ [I;i iil i_:;,'. _., _ i... i[ IIl!iil[lill [ : ß '...'" (c) Dst for November 12-22, 1989 :::::::::::::::::::::::: :::':::::: i. i iili _.. iili i iii iii.i i : " _ ii II Ill I1.1 I II [ : (d) Dst for November 3-13, Julian Day Figure 6. Dst measured near the times of four magnetic storms in (a) March 1989, (b) March 1991, (c) November 1989, and (d) November Each plot gives data from 11 days roughly centered on the storm's maximum epoch. Vertical lines mark times of DMSP satellite encounters with EPBs. Depletion categories M-0 and M-1 are represented by light dotted lines, M-2 by dashed lines, and M-3 by heavy solid lines. mined whether DMSP did or did not detect bubble ac- early stages of high magnetic activity periods and are tivity. In the case of multiple bubble detections within later suppressed. The largest depletions in electron dena given pass, the orbit was classified by the magnitude sity (M-3s in Table 2) occurreduring the main phase of its deepest depletion. We note, however, that (1) of geomagnetic storms, as illustrated in Figure 6. A de- F10 detected many more fast bubbles than F9, and (2) tailed description of a storm during which EPBs were all of the fast bubble examples given by Hanson et al. observed in the main phase was given by Burke et al. [1997] showe density depletions corresponding to our [2000], in which the role of the stormtime penetration M-2 and M-3 categories. It is interesting to note that electric field was discussed. During the main phase the the longitude distribution of M-2 and M-3 depletions penetration electric field enhanced the field associated shown in Figure 2 qualitatively resembles those of fast with the atmospheric dynamo in the postsunset sector bubbles. The two data sets suggest that in the early [Haerendel and Eccles, 1992], facilitating the triggering postsunset sector conditions develop that favor the for- of the R-T instability. mation of deep-depletion, fast-moving EPBs [Hanson et From theoretical [Spiro et al., 1988] and observational al., 1997]. [Burke et al., 1998, 2000] considerations it is clear that when large potentials are first imposed by the interplan Relation to Geomagnetic Activity etary medium their effects are quickly felt throughout Data presented in Figures 5 and 6 show that, contrary the inner magnetosphere. These fields in turn drive the to the emphasis of earlier studies [e.g., DasGupta et al., convective motion of plasma sheet/ring current parti- 1985], EPBs develop in the evening sector during the cles. In time the different drift paths of ions and elec-

9 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 8139 trons produce layers of space charge that shield convec- penetration from the magnetosphere may combine to tion electric fields from the inner magnetosphere. Dur- produce the intense, fast-moving bubbles detected by ing periods of penetration of the inner magnetosphere, DMSP F10 in the early evening sector in It is this electric field maps to the low-latitude ionosphere near the dusk meridian that ionospheric conductivity where it drives a global ionospheric current system gradients are most severe, and equipotential lines must called DP-2 that is responsible for ground level mag- turn sharply poleward to maintain current continuity. netic perturbations reported by Niskida [1968]. DP-2 Such a superposition of fields would render the bottomis the system of Hall currents driven by the large-scale side of the F layer most unstable. convection system [Heppner and Mailnard, 1987]. We chose the Dst index as a proxy for electric field S/L Climatology of EPB Detections pentration. The Dst index responds to changes in Because of the different techniques used to study bubboth the magnetopause current and the ring current bles and other plasma irregularities, their interpreta- [Ma laud, 1980]. During rapid changes in the dynamic tions will never be without some ambiguity. The radars pressure of the solar wind the magnetopause current reat Jicamarca and Kwajalein used to probe the equasponds, causing the magnitude of the magnetic field in torial ionosphere have frequencies of 50 and 415 MHz the inner magnetosphere increase or decrease. These and respond to irregularities of meter scale sizes. At changes give rise to inductivelectric fields which afthe stations used in A-93 and listed in Table 1, satellite fect the entire magnetosphere-ionosphere system. Large beacon signals used to monitor transmissions for scintilnegative slopes in traces of the Dst index indicate that lations have widely different frequencies from 137 MHz the ring current is either growing energy or moving to 1.5 GHz. The Fresnel length for such irregularities is closer to the Earth. Both actions require the presence of kilometer scale sizes [Basu et al., 1976]. The density of intense electric fields in the magnetosphere. The ring depletions detected by the AE-E and DMSP satellites current can move closer to the Earth only if the maghave longitudinal scale sizes of tens of kilometers. By netospheric electric field has penetrated closer to the concentrating on the phenomenology of range spread F, Earth than the initial ring current location. Such penetration electric fields can affect the stability of the equa- A-93 deliberately chose to consider the types of evening sector irregularities associated with radar plumes and torial ionosphere in the evening sector. The question of why an expanded electric field pat- topside plasma bubbles. In our discussion we recognize tern develops the eastward component needed to enthat some plumes stagnate below 800 km but assume hance the growth rate of the R-T instability loft, 1978] that there exists some level of correspondence between still remains. A simple expansion of Heppner-Maynard scintillations and bubbles sampled by DMSP. This alconvection patterns [Heppr er and Ma mard, 1987] to lows us to compare the S/L climatologies of our bublow magnetic latitudes would result in an electric field ble encounters with scintillation results by sorting our with its dominant component in the meridional direc- DMSP observations during 1989 and 1991 in a format tion. Wolf [1970] and Nopper and Carovillar o [1978] similar to that of Figure 18 in A-93. have solved the ionospheric current continuity equa- Figure 7 shows the percent of bubbles detected by tion with realistic conductivity gradients. They demon- DMSP F9 during 1989 (circles) and 1991 (squares) strated that to satisfy the V. j - 0 condition requires while passing near the eight ground stations listed in that equipotentials twist poleward near the dusk ter- Table 1 which were used in the A-93 study. Each data minator. Consequently, associated electric fields gain point represents the number of detections within a given azimuthal components. Electric field coupling between month in a longitude bin of 15 o width. In agreement the ionosphere and magnetosphere in the presence of with A-93, most bubble production in the vicinity of the dusk-terminator conductivity gradient provides the these stations occurred during the equinox seasons. Figconditions needed to drive the large EPBs detected by ure 7 illustrates in detail that there are large longitudinal differences in EPB detections. Possible biases DMSP during the early stages of magnetic storms. The reverse dynamo model [Fejer et al., 1990; Scherliess caused by the orbital angles of F9 and F10 were preand Fejer, 1997; Fejer and Scherliess, 1997] suggests viously discussed. The largest bias appears to be in that as penetration electric fields abate in the recovery the region of eastward declination in the eastern Paphase, the effects of Joule heating at auroral latitudes cific where F10 crossed the greatest swath of magnetic radically alter neutral wind patterns on the nightside. longitudes at low latitudes. Figure 2 shows that F10 These winds cause the equatorial ionosphere to descend detected many more M-2 and M-3 EPBs than F9 in in the evening hours and rise in the postmidnight sector. this longitude sector. Data show similar occurrence fre- Data presented in this paper suggest that the effects of quencies by both spacecraft in the Atlantic sector where the stormtime wind patterns can inhibit the ability of detection by F9 should be favored. These high rates of the dayside dynamo to produce EPBs in the evening detection suggesthat EPB activity is more likely here sector for several days after a storm's main epoch. It than at any other longitude sector. In longitude sectors is interesting to speculate that electric fields caused by of small magnetic declination the detection rates of the the F region dynamo [Haerendel and Eccles, 1992] and two satellites were similar.

10 8140 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 100 Accra i\ IKødaikanal I Manila I Guam Kwajalein Huancayo [ Natal /O t AScensiOn ' " Month as a function of the month of the year. Data come from the viciniw of the eight scintillation stations listed in Table 1. "Fit" lines connecting data points ac as guides for he eye when comparing DMSP results with scintillation morphologies of A ors [1993, Figure 18]. Figures 8b, 12, and 13b of A-93 show the maximum rates of scintillation occurrences ranged between 60 and 80% at Accra, Natal, and Huancayo and were near 20% at Guam and Kwajalein. In Figure 7 the data for Guam show distinct differences between 1989 and 1991, although neither displays a broad maximum in MJJ months cited by A-93. Instead there is a maximum during the September equinox in 1989 and a less pronounced maximum during the March equinox of 1991 with a second maximum of near-equal value during the September equinox of that year. When comparing the results for Guam and Huancayo for these 2 years individually, we arrive at a very different conclusion from A-93 if we consider the 1991 data, and a slightly different conclusion ith the 1989 data. We suggest that the differences are the result of low sampling statistics, rather than a true difference in climatology. Noting the longitude for Guam (143ø), we see that the total number of EPBs observed during the entire year is 35 in 1989 and 20 in When considered over the 12 months plotted in Figure 7, it is not unlikely that low counting statistics might lead to large and deceptive variations. Similar fluctuations can also be seen at the Kodaikanal (77 ø) and Manila (121 ø) stations which are also in the Pacific sector. Finally, we consider possible explanations for the large longitudinal variations in the rates of EPB occurrence that appear in the data presented in Figures 2 and 7. Tsunoda [1985] suggested that much of the S/L variation can be explained in terms of gradients in the flux tube integrated Pealersen conductivity near the dusk terminator. Flux tubes with declinations aligning them most closely with the terminator are favored for the onset of the generalized R-T instability and the onset of spread F turbulence. Features anticipated by this model clearly appear in the DMSP data presented in Figure 5. In the central to eastern Pacific where the declination is eastward, D MSP detected enhanced EPB occurrence during the MJJ solstice. A strong increase in EPB activity was detected in the Altantic sector, where the declination is westward, near the NDJ solstice. However, the rates of detection were quite different. Far more bubbles were detected in the Atlantic sector than in the eastern Pacific. It is not clear that the model of Tsunoda [1985] speaks to this difference. Oft [1978] showed that an east-west electric field changes the "effective" gravity g in calculating the linear growth rate for the R-T instability g'= g- vin' (13 x B)/B 2, (2) where Pin is the ion-neutral collision frequency. Both g and the linear growth rate 9' for the R-T instability increase (decrease) when E has an eastward (westward) component. If we assume that the currents are driven by either leakage of the equatorial electrojet into the postsunset ionosphere [Eccles, 1998] or a low-latitude extension of the DP-2 current system [Nishida, 1968] and that ionospheric conductivities are independent of longitude, then the ratio E/B assumes a critical role in assessing 9'. In the right column of Table 1 we have approximated the magnitude of the magnetic field in Gauss at the magnetic equator near the eight scintillation stations used by A-93. We note that IBI varies from 0.34 to 0.23 G. Thus for the same electric field and pin the effective gravity would be stronger in the Atlantic sector than in the Pacific. In the postsunset hours the topside plasma of the Atlantic sector rests on a lighter fluid (weaker magnetic field) than in the Pacific sector. That is, plasma in the Atlantic sector is more R-T unstable than that in the Pacific. Given a finite period of time after sunset before the equatorial electric field reversal, one would expect that more bubbles that reach

11 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES 8141 DMSP altitudes can be generated in the weak magnetic field (fast growth) of the Atlantic sector Conclusions We have analyzed 3 years of EPB observations made with DMSP F9 (1989 and 991) and DMSP F10 During. 15,000 overflights of the postsunset sector made during these years, 2806 bubbles were detected. The S/L climatology of our bubble encounters is in general agreement with past ground- and satellite-based studies. We find that the largest plasma depletions occurred during the main phases of geomagnetic storms, preferentially closer to the dusk than to the midnight meridian. The recovery phase of storms is marked by the near-total absence of bubbles. We have extracted our data in the same longitudes as the ground stations used in A-93. We find that when counting statistics are high the results of Tsunoda [1985] are obtained. However, in the Pacific sector where frequency of occurrence is low, we do not see sufficient consistency in the data from year to year to come to a definitive conclusion regarding seasonal variations. We do, however, suggest that the relatively large equatorial magnetic field in the Pacific sector reduces the R-T growth rate thus rendering the nonlinear manifestation of EPBs at 840 km less likely. Acknowledgments. This work was supported by the U.S. Air Force Office of Scientific Research task 2311PL014 and by Air Force contracts F and F C-0039 with Boston College. The authors thank Frederick J. Rich of AFRL for his generous assistance in making the DMSP data available for this study. Janet G. Luhmann thanks J. Vincent Eccles and Jules Aarons for their assistance in evaluating this paper. 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12 8142 HUANG ET AL.: GLOBAL DISTRIBUTIONS- EQUATORIAL PLASMA BUBBLES Oft, E., Theory of Rayleigh-Taylor bubbles in the equatorial ionosphere, J. Oeophys. Res., 83, 2066, Prakash, S., Production of electric field perturbations by gravity wave winds in the E region suitable for initiating equatorial spread F, J. Oeophys. Res., 10 4, 10,051, Rich, F. J., and M. Hairston, Large-scale convection patterns observed by DMSP, J. Geophys. Res., 99, 3827, Scherliess, L., and B. G. Fejer, Storm time dependence of equatorial disturbance dynamo zonal electric fields, J. Geophys. Res., 10,, 24,037, Spiro, R. W., R. A. Wolf, and B. G. Fejer, Penetration of high-latitude electric field effects to low latitudes during SUNDIAL 1984, Ann. Geophys., 6, 39, Tsunoda, R. T., Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in the integrated E-region Pedersen conductivity, J. Geophys. Res., 90, 447, Wolf, R. A., Effects of ionospheric conductivity on convec- five plasma flow in the magnetosphere, J. Geophys. Res., 75, 4677, Woodman, R. F., and C. LaHoz, Radar observations of F region equatorial irregularities, J. Geophys. Res., 81, 5447, W. J. Burke, J. S. Machuzak, and P. J. Sultan, Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA, (william.burke2@hanscom.af.- mil; john.machuzak@psfc.mit.edu; peter.sultan@hanscom.- af. mil) L. C. Gentile and C. Y. Huang, Boston College Institute for Scientific Research, 402 St. Clement's Hall, 140 Commonwealth Avenue, Chestnut Hill, MA (louise.- gentile@hanscom.af. mil; cheryl.huang@hanscom.af. mil) (Received August 18, 2000; revised December 13, 2000; accepted December 28, 2000.)