JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. A2, PAGES , FEBRUARY 1, 1995

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. A2, PAGES , FEBRUARY 1, 1995 Interplanetary magnetic field control and associated field-aligned currents of mantle precipitation Dingan Xu and Margaret G. Kive!son Institute of Geophysics and Planetary Physics and Department of Earth and Space Science, University of California, Los Angeles Ray J. Walker Institute of Geophysics and Planetary Physics, University of California, Los Angeles Patrick T. Newell and C.-I. Meng Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland Abstract. Dayside reconnection, which is particularly effective for a southward interplanetary magnetic field (IMF), allows magnetosheath particles to enter the magnetosphere where they form the plasma mantle. The motions of the reconnected flux tubes produce convective flows in the ionosphere. It is known that the convection patterns in the polar cap are skewed to the dawnside for a positive IMF By (or duskside for a negative IMF By) in the northern polar cap. Correspondingly, one would expect to find asymmetric distributions of mantle particle precipitation, but previous results have been unclear. In this paper the correlation between By and the distribution of mantle particle precipitation is studied for steady IMF conditions with southward IMF. Ion and electron data from the DMSP F6 and F7 satellites are used to identify the mantle region and IMP 8 is used as a solar wind monitor to characterize the IMF. We study the local time extension of mantle precipitation in the prenoon and postnoon regions. We find that, in accordance with theoretical expectations for a positive (negative) IMF By, mantle particle precipitation mainly appears in the prenoon region of the northern (southern) hemisphere. The mantle particle precipitation can extend to as early as 0600 magnetic local time (MLT) in the prenoon region but extends over a smaller local time region in the postnoon sector (we did not find mantle plasma beyond 1600 MLT in our data set although coverage is scant in this area). Magnetometer data from F7 are used to determine whether part of the region 1 current flows on open field lines. We find that at times part of the region 1 sense current extends into the region of mantle particle precipitation, and is therefore on open field lines. In other cases, region 1 currents are absent on open field lines. Most of the observed features can be readily interpreted in terms of the open magnetosphere model. Introduction distribution of mantle plasma precipitation in the ionosphere and on associated field-aligned currents (FACs). After Heikkila and Winningham [ 1971 ], Winningham and The mantle was originally observed at mid altitude and Heikkila [1974] and Winningham et al. [1975] reported in the tail [Rosenbauer et al., 1975; Sckopke et al., 1976; on polar region particle precipitation, low-altitude polar Hardy et al., 1975, 1979]. According to Rosenbauer et al. region satellite observations of precipitation were accepted [1975], the plasma mantle is a persistent layer of tailward as evidence for an open magnetosphere. Because the various flowing deenergized magnetosheath plasma inside of and domains of the magnetosphere map to the ionosphere, the adjacent to the magnetopause. It is formed of plasma on precipitating plasma measured in low altitude polar orbits tailward drifting flux tubes from the cusp region. Since ions may serve as a probe of the global magnetosphere and its on these flux tubes move along the magnetic field at a speed dynamics. In particular, the spatial distribution of regions that increases with energy while moving across the field of precipitation in the ionosphere is expected to respond at a speed independent of energy, the lower-energy ions systematically to solar wind conditions. In this paper, we spend a longer time transmitting to and reflecting from the report on interplanetary magnetic field (IMF) control of the ionosphere than do the higher energy ions. Therefore, in the magnetosphere at a fixed distance from the polar cusp, the energy of the plasma decreases with distance inward from Copyright 1995 by the American Geophysical Union. the magnetopause boundary. The corresponding signature Paper number 94JA of the mantle layer in the polar ionosphere should be a /95/94JA decrease of the density and energy toward higher latitudes 1837

2 1838 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION along the antisunward direction. Recently, Sanchez et al. [1990b] used data from low-altitude polar orbital satellites DMSP F6 and F7 and Newell et al. [1991a] used data from DMSP F7 and F9 to study mantle particle precipitation. They found regions showing ion data with smooth, continuous and monotonic decreases in average energy and density with increasing latitudes. They identified these regions as the plasma mantle from the energy dispersion discussed above. Newell et al. [ 1991 a] also compared the densities, energies and temperatures of the identified mantle plasma at low altitudes with high altitude mantle observations and obtained consistent results. These results support the expectations based on the descriptions given by Rosenbauer et al. [1975] and establish the usefulness of the approach to identifying mantle plasma at low altitudes. The distribution of mantle plasma is expected to depend on IMF By. For example, Crooker [1979] proposed that flux tubes newly merged on the dayside magnetopause drift along skewed ionospheric convection patterns when there is a finite IMF By. For a southward IMF, this model shows the ionospheric convection contours skewed to the dawnside for a positive IMF By or the duskside for a negative IMF By in the northern hemisphere. Heelis [1984] using AE- C satellite data and Burch et al. [1985] using DE data found experimental evidence of such skewed convection patterns. Later studies based on larger data sets have shown that the whole convection pattern is rotated toward prenoon [Heppner and Maynard, 1987; Rich and Hairston, 1994], a characteristic that was also observed by Heelis [1984]. Since the mantle layer forms on reconnected flux tubes for rotation of the whole magnetotail as proposed by Cowley [1981]. The only study using polar orbital satellites to investigate the relation between the IMF By and the mantle precipitation was a brief paper by Sanchez et al. [1990b]. From F6 satellite data taken in a dawn-dusk meridian plane near local noon, they found that the monotonic decreasing energy trend of the mantle ions could be from dawn to dusk or from dusk to dawn depending on the sign of IMF By. However, the spatial distribution of the mantle precipitation appeared to be uncorrelated with IMF By, although they emphasized that an expanded database would allow more accurate determination of a possible asymmetry. In this paper, we report on a more direct study that expands on the preliminary work of Sanchez et al. [1990b]. In the dayside reconnection model for IMF Bz<0, the mantle starts just poleward of the polar cusp, and is expected to exist stably near the noon region poleward of the polar cusp independent of the IMF By. Therefore we selected passes displaced farther away from the cusp to study how IMF By controls the dawn-dusk asymmetry of the distribution of the mantle particle precipitation. We also used higher time resolution IMF data than previous studies and required steady IMF conditions with southward Bz. We believe that these restrictions provide reasonable assuranc& that the magnetosphere has adopted a quasi-static configuration, and we find a clear correlation between the IMF By and the distribution of the mantle particle precipitation. In this paper we present results not only on the distribution of the mantle precipitation but also on associated FACs. It a southward IMF, we anticipate that the mantle precipitation has long been accepted that FACs can flow on polar cap field in the ionosphere is also distributed asymmetrically between lines for northward IMF [e.g., Iifima et al., 1984; Zanetti the dawnside and the duskside for finite IMF By. Previous attempts to correlate the distribution of the mantle et al., 1984; Iijima and Shibafi, 1987]. These currents are located poleward of the region 1 currents and flow in the precipitation with IMF By have yielded inconsistent results. opposite direction to the region 1 currents. Recently,de la Sckopke et al. [1976] using midaltitude Heos 2 and IMP 6 satellite data reported that the appearance and thickness of the mantle layer is correlated with southward IMF but found no clear correlation between IMF By and the distribution of the mantle layer. However, Hardy et al. [1979] using lunar Beaujardiere et al. [1993], using ground observations and DMSP data, reported cases in which part of the region 1 currents flowed on mantle field lines. In their study, concurrent IMF measurements were available for only one of the eight passes studied, so the IMF orientation was observations verified that plasma in the lobes, whose sources inferred from data within two hours of the passes. In only are the low latitude boundary layer (LLBL) and the plasma one case was the IMF identified as southward, and in five mantle, is strongly correlated with the IMF By; the probability cases it was identified as northward. In later studies, Ohtani of observing lobe plasma is greater on the dawnside of the et al. [1994] showed that part of the region 1 currents northern lobe and the duskside of the southern lobe for a flow on mantle field lines in one of their four multicurrent positive IMF By. The converse situation is also found. Some layer crossing cases. The IMF was unknown for this case. studies based on ISEE 3 distant tail observations confirmed G. Lu et al. (Characteristics of ionospheri convection and the correlation between the lobe plasma and the IMF By [e.g., Gosling et al., 1985]. In addition to the observational studies, there have been field-aligned current in the dayside cusp region, submitted to J. Geophys. Res., 1994) also showed one case with part of the region 1 current on mantle field lines in the prenoon some theoretical treatments of the formation of the mantle region of southern hemisphere with IMF By<0 and Bz<0. layer. In the Coroniti-Kennel MHD model of an open Previously from very limited ISIS 2 satellite data, we showed magnetosphere, the magnetosheath plasma passes through that the region 1 current can flow partially on open field a slow mode expansion fan to form the plasma mantle, lines for southward IMF. We correlated the locations of which is threaded by lobe field lines [Coroniti and Kennel, these currents with IMF By and explained the result by using 1979]. Siscoe and Sanchez [1987] and Sanchez et al. dayside reconnection and ionospheri convection models as [ 1990a] provided explicit one- and three-dimensional MHD proposed in Crooker's [1979] dayside merging model [Xu solutions, and found that magnetosheath plasma properties and Kivelson, 1994]. In this paper we use magnetometer transform smoothly into mantle and lobe plasma properties data from F7 to extend the earlier study. through the rotational discontinuity-slow mode expansion The paper is organized as follows: after we discuss the fan. In these calculations, finite IMF By does not introduce data set that is used in this study, we describe the correlation any dawn-dusk asymmetry in the distribution of the mantle layer other than a rotation of the mantle layer due to the between the distribution of the mantle precipitation and the sign of the IMF By component. Then we use simultaneous

3 ,,, XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION 1839 measurements from F6 and F7 to show that the local time extension of the mantle precipitation differs in the prenoon and postnoon regions. Next we study the relationship of region 1 sense FAC signatures to the plasma mantle. Discussion and conclusions are given at the end. By>0. Initial selection of the data relied on "an automated on-line data base" previously documented by Newell et al. [1991b]. The information is extracted more efficiently if the requests include multiple consecutive passes instead of individual passes. Data Observations DMSP F7 was in a Sun-synchronous nearly circular orbit at about 835 km altitude in a prenoon-premidnight local time Mantle Distribution for Different IMF By meridian with orbital inclination of 98.7 ø. DMSP F6 was Figure 1 is a plot showing the coverage of all the selected similar to F7 but with an orbit in the dawn-dusk meridian F7 passes with By>0. The northern and southern hemisphere plane. Identical SSJ/4 instrumental packages were installed on the F7 and F6 satellites to measure electrons and ions from 32 ev to 32 kev in 20 logarithmically spaced steps [Hardy et al., 1984]. The satellites were three-axis stabilized; the detector apertures were always oriented toward the local zenith with a field of view < 8 ø. At a magnetic latitude of 65 ø and altitude of 835 km, the loss cone is about 50 ø, and the field inclination to the zenith is 13 ø, so the passes have been overlaid. The passes from the upper right corner to the lower left corner are northern hemisphere passes. Noon (MLT 1200) is on the top of the plot and dawn (0600) is on the right side of the plot. Southern hemisphere passes are directed from the upper left to the lower right and are viewed from below the south pole. For these passes dawn (0600) is to the left and dusk is to the right. It is generally supposed that the convection pattern in the DMSP satellites above the polar regions always measured prenoon region of the southern hemisphere is approximately precipitating particles. The data used in this study are principally from DMSP F7. The portions of the passes useful for our study were the same as the pattern in the postnoon region of the northern hemisphere. With the assumption of such reflection symmetry, the passes in Figure 1 can represent passes in the prenoon region. (Some southern hemisphere passes nominally in a single hemisphere from which we can crossed the postnoon region, but their trajectories were almost tangential to the azimuthal direction, so that these passes were not useful for our study.) In addition, we used some data from the dawn-dusk meridian plane satellite F6, especially when the times of passes were close to the times of the F7 measurements. These data were useful for investigating the two-dimensional structure of the mantle precipitation. investigate the dawn-dusk asymmetry of the distribution of mantle precipitation. Among the 487 passes in Figure 1, we selected a small subset away from the local noon for visual inspection of the color spectrograms to study the dawn-dusk asymmetry of the distribution of the mantle particle precipitation. In doing so, we used passes that occurred during selected UT IMP 8 is used as a solar wind monitor. There are data hours. For the Sun-synchronous orbital satellite DMSP F7 gaps in the IMP 8 measurements, caused by gaps in tracking the geomagneticoordinates of the orbits vary diurnally as or because the spacecraft was inside the magnetosphere and thus not monitoring the IMF. Passes that occurred during intervals of persistent southward IMF were selected. Various 1200 considerations led to our selection criteria for the duration of the IMF orientation. It is known that the magnetosphere shows dramatically different configurations for different IMF directions. In order to limit the observation to cases in which the magnetosphere has reached a relatively stable configuration, we first required that the IMF maintain a southward orientation with a single sign of By for at least 30 min prior to the measurement. (If the ionospheric convection speed is 1 km/s, it takes about 30 min for a flux tube to convect from the cusp region to the dawn-dusk meridian plane.) We used IMF data at a time resolution of 10 min instead of the hourly averages that have been used in many previous studies. In studying structures that are established on timescales of about 30 min, we think that the 10-min resolution is adequate. The data in this study were obtained between December 1800 (1800) 1983 and December Because of the large size Figure 1. The DMSP F7 passes available for this study for of the DMSP data sets, we were able to impose more stringent stability requirements than the estimated minimum requirements and still select more than 1000 passes for examination. For example, we required that the IMF Bz be stable and southward for at least 200 min with By By>0. The northern and southern hemisphere passes have been overlaid. The passes from the upper right corner to the lower left corner are northern hemisphere passes. Noon (MLT 1200) is on the top of the plot and dawn (0600) is on the right side of the plot. Southern hemisphere passes not changing sign for 110 min. We eliminated passes are directed from the upper left to the lower right and are that occurred in the initial 30 min of each interval. We still viewed from below the south pole. For these passes the obtained 567 passes with IMF By<0 and 484 passes with IMF dawn (0600) is to the left and dusk is to the right.

4 1840 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION a result of the Earth's spin. For a specific UT, the orbits are almost identical from day to day. Figure 2 shows the subsets Table l. UT Intervals Covered for Figures 2a and 2b of passes for different IMF By. The corresponding UT IMF By Hemisphere Year UT range intervals covered are tabulated in Table 1. For By>O, there are 35 passes in the prenoon region of northern hemisphere northern > and 24 passes in the prenoon region of southern hemisphere (Figure 2a). For IMF By(O, there are 33 passes in the southern prenoon region of southern hemisphere and 24 passes in the prenoon region of northern hemisphere (Figure 2b). In northern trying to choose symmetrically distributed passes around the < cusp center, slightly different UT hours were used for the southern year 1986 and other years, but we still could not choose exactly symmetric passes. Also because of the orbital bias, we could not choose precisely the same number of passes in electrons (upper panel) and ions (lower panel). Different the prenoon regions of northern and southern hemispheres. regimes (identified from both the neural-network method and For the passes in Figures 2a and 2b, mantle regions are identified first by the neural-network method [Newell et al., visual inspection) are labelled just below the spectrogram. 1991b] and all mantle identifications are then confirmed From left to fight, CP is the central plasma sheet (CPS), BP is the boundary of plasma sheet (BPS), LL is the low-latitude and corrected if necessary by visual inspection of color spectrograms. Occasionally, the automated identification boundary layer (LLBL), MA is the mantle, and PR is polar scheme misidentified the mantle regions. Plate l a is rain. Other labels that do not appear on this particular plate representative of passes that include a clear mantle region are VO (the void) and CU (the cusp). crossing in the prenoon region of the northem hemisphere. The enhanced ion flux with a monotonically decreasing The total time interval is 4 min with 4 sec between two tick energy trend from about 09:13:19 to about 09:14:25 UT in the ion spectrogram of Plate l a is identified as the manfie marks. The two color spectrograms show differential energy flux from 32 ev to 32 kev in units of ev/cm2-s-sr-ev for precipitation region. Usually, the maximum ion energy is less than about 1 kev. This example of the manfie precipitation is very similar to the examples of mantle precipitation The Distributions of Passes and Mantle Crossings published by Newell et al. [1991a]. for Southward IMF For this pass, our visual inspection basically confirms the identifications by the neural network method (except for a (a) Passes By>0 (N:35 S:24) (b) By<0 (N:24 S:33) Mantle Crossings (c) By>0 (N:31/35 S:3/24) (d) By<0 (N:8/24 S:29/33) ', 18(0l ) ( t 106(1 8s poleward shift of the equatorward edge of the mantle region). The IMF components were (1.1, 2.1, -7.8)nT in GSM coordinates for this case. We looked at all the 59 passes for IMF By>0 shown in Figure 2a. Among the 35 cases in the prenoon region of northern hemisphere, 31 passes include clear mantle crossings like the one shown in Plate la. Among the 24 passes in the prenoon region of southern hemisphere, only three passes include mantle crossings. Figure 2c shows the distribution of the mantle crossings for the passes of Figure 2a. We also analyzed all the passes for IMF By<0 shown in Figure 2b. There are 29 mantle crossings among 33 passes in the prenoon region of southern hemisphere, and 8 mantle crossings among the 24 passes in the prenoon region of northern hemisphere. The distribution of the mantle crossings is shown in Figure 2d. In summary, the mantle precipitations were observed mainly in the prenoon region of the northern hemisphere for positive IMF By and in the prenoon region of the southern hemisphere for negative IMF By. Measurements From the F6 Satellite The orbit of the DMSP F6 satellite was close to the dawndusk meridian plane. We can study dawn-dusk asymmetry by using F6 alone and estimate the local time extension of the mantle precipitation by using simultaneous measurements Figure 2. The distribution of selected passes (a and b) and of both F7 and F6 satellites. corresponding mantle crossings (c and d) for IMF By>0 (a In this study, we looked at F6 passes only when the times and c) and for IMF By(0 (b and d). The number of passes of F6 passes were close to the times of F7 passeshown in in each hemisphere (northern N; southern S) are indicated Figures 2a and 2b. Since the F6 and F7 satellites did not on the top panels while the fractions of the passes on which normally cross the same polar region at the same time, it was the mantle was observed are indicated on the bottom panels. difficult to find simultaneous measurements from F6 and F7.

5 ,... XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION 1841 a) F7 Dec/03/86 ß. Z 3- ' - --3, I '. -"! cp ' l Illi'l jl ' '"' " "'"'"' ' "' ;'"" '[ ["' i ' 'J...'i' "' '' ' UT 09:11:30 09:12:04 09:12:38 09:13:12 09:13:47 09:14:21 09:14:55 09:15:30 MLAT MLT 10:16 10:17 10:18 10:20 10:22 10:26 10:29 10:37 2 LOG E FLUX ELEC ION 10 8 ß :i '1 b) F6 Dec/03/86 UT MLAT 69.4 MLT 06:52.,.', i -- --!.'-' ' ',, ':-"ll, ß... '... 1!-' I-t,1 [ I '_.. *--"I t!!,!i,r " t I,.I il,,,!t!..!. '!!.,tli,.' ' tl'lltl.: ' j'" '.,-'""i ß 3 '.'"' i' 'i i: i: '' II Ill!Ill' J ' JiiJ'l '! Jtt I"!'"' [. 2 ti,! i{l i, ':i ' 'I:" 1 I1 11 ' I;I', ' 4 ': '. t. i'ii I-li I'i' I I I iiit,i I I I!I I,I '1,:1 11t II I [,l ''J l[ '':' ' -- ',I I... l I... '!i':":i h'!,':,, I vo!... I,l,,',', lip el: ß, ",,' ',':'1'," ',, CP I BP, MA 08:44:00 08:45:51 08:47:42 08:49:34 08:51:25 08:53:17 08:55:08 08:57: :40 06:12 03:16 20:41 19:55 19:39 19:30 Plate 1. (a) A spectrogram from DMSP F7 plasma measurements. The two color spectrograms show differential energy flux from 32 ev to 32 kev in units of ev/cm2-s-sr-ev for electrons (upper panel) and ions (lower panel). Note that the ion energy scale in the spectrogram is inverted. Different regimes are labelled below the spectrogram. From left to right, CP is the central plasma sheet (CPS), BP is the boundary layer of the plasma sheet (BPS), LL is the LLBL, MA is the mantle and PR is polar rain. For this pass, the IMF is (1.1, 2.1, -7.8)nT in GSM coordinates. (b) DMSP F6 satellite measurements for a pass concurrent with that of Plate l a. It is displayed in the same format as Plate l a. Both dawn and dusk side crossings are shown. The mantle precipitation exists only on the dawnside.

6 1842 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION From the 116 passes of F7 data shown in Figures 2a and 2b (59 IMF B. >0 cases + 57 IMF B.,,<0 cases), we found about 30 F6 measurements which were acquired within +30 min of F7 measurements. Plate lb displays F6 data taken within 30 minutes of the pass shown in Plate l a. It is displayed in the same format as Plate l a showing the differential energy fluxes of both electrons and ions. The time interval for this pass is 13 min with 13 sec between two tick marks. This pass In summary, from the measurements taken at roughly the was near the terminator (around 0600 MLT). The mantle ion same UT from the F6 and F7 satellites, we conclude that precipitation is normally weaker than near noon, as reported the mantle precipitation can extend to as early as 0600 MLT by Newell et al. [1991a]. Also the energy dispersion of in the prenoon region, but we find no mantle plasma at the ion precipitation along a dawn-dusk orbital satellite is MLTs later than 1600 in the postnoon region. The absence not expected to be as clear as in the prenoon-premidnight of mantle plasma nearer to dusk appears to be independent orbital satellite F7 observation, as shown in the Plate l a. The of IMF By. region with weak energy dispersion of < lkev ions, and sheathlike electrons from about 08:46:27 to about 08:48:46 Field-Aligned Currents on Open Field Lines UT is identified as the mantle region. From the 30 pairs of F6 and F7 measurements, we found that when F7 measurement showed a clear mantle crossing in the prenoon region of either hemisphere and F6 was crossing the same polar cap at an earlier MLT as for Plates l a and lb, we could always identify a region of mantle precipitation along the F6 orbit on the dawnside of that hemisphere. The F6 mantle precipitation was found as early as 0600 MLT. But for duskside crossings with IMF By negative irethe northern hemisphere (positive in the southern hemisphere), we rarely found mantle plasma precipitation after 1600 MLT. The only mantle crossing that we found on the duskside along the F6 orbit is shown in Plate 2, which is in the southern hemisphere. The time interval and tick marks are the same as in Plate lb. The IMF was (-3.2, 4.3,-1.6)nT. For this strongly positive IMF By the mantle crossing from 10:07:26 to 10:09:12 UT is on the duskside before 1600 MLT. The data from F7 (around 1000 UT and 0630 MLT), which is not shown here, does not show a mantle crossing on the dawnside of the southern hemisphere. The previous F6 crossing (0823 to 0836 UT), also not shown here, crossed the southern dusk region at MLT 1700, and there was no clear mantle precipitation through this whole pass. In an initial study [Xu and Kivelson, 1994] based on very limited ISIS 2 electron and magnetometer data, we proposed that part of the region 1 current can flow on open field lines (R10 current) for southward IMF. We suggested that the distribution of the R10 current is controlled by the IMF B:,. Among the DMSP satellites, F7 is the only one launched to date carrying a magnetometer. At the present time, we have analyzed magnetometer data from a single year (1986). Among the 60 mantle crossings displayed in Figui'e 2c and 2d (31 for IMF By> IMF By<O), 32 occurred in Magnetometer data are available for 23 of these cases, 16 of them for IMF B?0 and 7 of them for IMF B..<0. We looked at all the 23 cases with magnetometer measurements and found 10 cases for which part of the region 1 current (R10 current) was accompanied by a mantle precipitation region (9 for IMFBy>0 and 1 for IMFB,,<0). Plate 3 is an example F6 Aug/07/84 LOG E FLUX ELEC ION 10!.8 z r,,l,l o r 3 UT 10:04:00 MLAT MLT 17:03 "; i l," ' ', "' '",',, ' I I ' " "I,,"I 1'1 ill I. till I ' I tlii t i I I!... ' MA i -i "" CP ]0:0S:S] ]0:07:42 ]0:0 :34 ]0:]]: ]0:]3:]7 ]0:-]S:08 ]0:]7: ].] ]6:30 ]S:2 ]3: ]0:4 0 :03 08:0 07:3S 5.'23 Plate 2. This plate is displayed in the same format as Plate l a. Clear mantle precipitation exists only on the duskside from about 10:07:26 to 10:09:12 UT. The IMF was (-3.2, 4.3,-1.6)nT in GSM coordinates.

7 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION 1843 F7 4 Jul/09/86 ß i 4 LOG E FLUX ELEC ION f I t I " I 2 z3 4OO 300 2OO N m OO RlC R10 i,, IIIilllllll IIIIIIIIIIIIIIIII. ß I I I I I, I I I I I I I III IIIIIIIllll IIIIllllllllllllllllill I I I I I,llllllllllllllllll'lllllllllll: Illllill II IIIIIIIIlilllllllllll CP I BPI MA I I I I...,... I... I...,..., 'lllllllll'llllll'111'll'l'llllllll'lllllllllllllllllllllllllllllllllllllllllll 3OO DMSP F7 Magnetometer 07/09/ I I I I I I I I I I I I I I I i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I UT MLAT 71.0 MLT 09:50 11:52:00 11:52:34 11:53:08 11:53:42 11:54:17 11:54:51 11:55:25 11:56: :47 09:46 09:41 09:37 09:31 09:20 09:05 Plate 3. An example of simultaneous measurements from plasma and magnetometer measurements. The top part of this plate is displayed in the same format as Plate l a for a different pass. Clear mantle precipitation region exists on the dawnside from about 11:53:42 to 11:55:15 UT. The lower part shows the magnetic field perturbations. The X direction is along the direction of the background field. The Y direction is along the projection of the satellite path onto a plane perpendicular to the background field, and the Z direction, which is basically along the azimuthal direction, completes the right-hand coordinate system. Different regimes identified from the plasma spectrogram are separated by several vertical lines. Region 1 sense currents are separated into two parts: the equatorward part corresponds to the BP and is labelled as R1C currents (region 1 current on closed field lines); the poleward part corresponds to the MA and is labeled as R10 currents (region 1 current on open field lines). The IMF is (4.7, 3.0,-3.0)nT in GSM coordinates.

8 1844 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION to show magnetic field perturbations from the DMSP F7 satellite, with part of the region 1 sense current associated with mantle particle precipitation. The top part shows the plasma data as shown in previous plates, the lower part shows the magnetic field perturbations. The time interval is 4 min with 3s between two tick marks. The X direction is along the direction of the background field. The Y direction the chance of encountering mantle plasma is high for passes close to the cusp region. A dependence of the cusp location on IMF By has been reported by Newell et al. [1989], who is along the projection of the satellite path onto a plane perpendicular to the background field, and the Z direction, found that the cusp shifts about 1 hour toward postnoon for a positive IMF By, and about one hour toward prenoon for which is basically along the azimuthal direction, completes a negative IMF By in the northern hemisphere. The shifts the right-hand coordinate system. Different regimes identified are in the opposite sense in the southern hemisphere. If from the color spectrogram are separated by several vertical the average cusp position shifts from local noon as IMF lines on the By panel. The clear region 1 sense current By changes, some of the passes may actually come close from about 11:53:42 to 11:54:25.5 corresponds to the mantle precipitation and is therefore labelled as R10 (region 1 current on open field lines). to the cusp location. For positive IMF By as shown in Figure 2a, the passes in the prenoon region of the southern hemisphere (mapped to the postnoon region in the northern hemisphere) are closer to the shifted cusp than the passes Results and Discussion in the prenoon region of the northern hemisphere. For the negative IMF By as shown in Figure 2b, the passes in the Mantle Distribution for Different IMF By prenoon region of the northern hemisphere are closer to the shifted cusp location than the passes in the prenoon region In the open magnetospheric model [e.g., Coroniti, 1985], of the southern hemisphere (mapped to the postnoon region reconnected magnetic flux tubes from dayside reconnection of northern hemisphere). We think this may be one reason are dragged tailward to form the mantle layer for southward why we occasionally observed mantle precipitation on the IMF. The dragging itself drives the convection cells in the side opposite to that predicted by the convection model. ionosphere [Russell, 1972; Crooker, 1979; Burchetal., 1985; As noted in the introduction, some previous studies did Heelis, 1984; Reiff and Burch, 1985]. Therefore, if different not find a dawn-dusk asymmetry of mantle precipitation. IMF By can skew the convection to either the dawnside or Sckopk et al. [1976] used Heos 2 data to study the mantle the duskside as proposed by Crooker [ 1979], the mantle layer layer. They found that the thickness of the mantle layer and its associated precipitation should appear predominantly depends on the IMF Bz, but that the distribution of the on the dawnside or the duskside. mantle plasma was not correlated with IMF By. They used Figures 2a-2d show that for stable and positive (negative) averaged IMF data with an average time resolution of 2 IMF By and Bz<0, there is an 88% chance of observing hours. The low temporal resolution of the IMF data may mantle particle precipitation in the prenoon region of the have obscured the By dependence. A study very similar to northern (southern) hemisphere, and only a 13% (33.3%) ours was carried out by Sanchez et al [1990]. They also chance of observing mantle particle precipitation in the used DMSP F6 and F7 data to study the mantle layer, but prenoon region of the southern (northern) hemisphere. The they found no strong By dependence. The passes they chose results are summarized in Table 2. Although only prenoon were mainly close to the noon region and initial IMF data passes were examined, the data come from both the northern used to choose passes were hourly averaged. As discussed and southern hemispheres. To an excellent approximation, in the introduction, the mantle starts poleward of the polar the distribution of mantle plasma depends on IMF By in the cusp for IMF Bz<0. Thus, on near noon passes, one might way that is described by an ionospheri convection model not expect to discern an IMF By signature in the distribution developed from the assumption of merging at the dayside of the mantle precipitation. This is why we chose passes magnetopause. away from local noon in our analysis of the asymmetry of Although the correlation between the distribution of the the mantle particle precipitation. mantle plasma and the sign of IMF By is evident, we would hemisphere Souhem hemisphere IMF By and about 1200 MLT for the negative IMF By). The entry mechanism for magnetosheath plasma during southward IMF implies that the mantle layer is routinely present immediately poleward of the cusp region. Therefore like to understand why we occasionally observed the mantle The Local Time Extension of the Mantle Precipitation plasma on what we shall call the unexpected side on the basis From nearly simultaneous measurements from F6 and F7, of the convection model. If we assume reflection symmetry and map the passes in the prenoon region of the southern we find that the mantle precipitation can extend to as early hemisphere to the postnoon region of northern hemisphere, as 0600 MLT in the northern hemisphere for a positive IMF the passes plotted in Figure 2a and 2b are symmetric about By or in the southern hemisphere for a negative IMF By. a meridian near local noon (about 1230 MLT for positive However, in our data set, the mantle precipitation is present over a smaller local time region (e.g., before 1600 MLT) Table 2. Prenoon Observations of Mantle Precipitation when the mantle precipitation is preferred on the duskside, that is, in the northern hemisphere for a negative IMF By By>0 By<0 or in the southern hemisphere for a positive IMF B,. Such a dawn-dusk difference of the local time extension of the Y N % Y N % mantle precipitation has not been reported previously. The asymmetry about noon may possibly be related to the skew Northern of convection patterns reported by Heppner and Maynard [1987], Lu et al. [1989] and Rich and Hairston [1994]. They found that ionosphericonvection patterns are always rotated to the prenoon region of the northern or southern hemisphere independent of IMF By. We suggest that the

9 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION 1845 intrinsic ionospheric processes that rotate convection patterns northern hemisphere for a positive IMF By and in the [Crooker and Siscoe, 1981] may also relate to the observed differences of the mantle precipitation between the dawnside and the duskside. On the other hand, the local time difference prenoon region of the southern hemisphere for negative IMF By. This result supports the ionosphericonvection model inferred from the open magnetosphere model. of the mantle distribution may reflect limitation of coverage. Limited F6 data, and especially the simultaneous Possibly in a larger data set, we might find a larger MLT extent of mantle precipitation. This is particularly true of the local time extension on the duskside as only one mantle crossing on the duskside was found. A study based on a larger data set of F6 is planned in the future. We should note that there have been several other reports of the distribution of mantle plasma and other magnetospheric plasma in the ionosphere, but none of them separated the mapping patterns measurements of F6 and F7 satellites, confirm the correlation between the sign of IMF By and the distribution of the mantle precipitation. We also find a difference of the local time extension of the mantle precipitation between the prenoon and the postnoon sectors. We have confirmed our previous report of region 1 current partially flowing on open field lines for southward IMF. In this study, we show that part of the region 1 currents for different IMF By. Vasyliunas [1979], using magnetotail corresponds to the mantle region and therefore, the region 1 measurements, identified different regimes and mapped these regimes onto the ionosphere. Newell and Meng [1992] used DMSP satellite data to identify different regimes and showed a similar plots. In all cases, the mantle plasma was viewed as symmetric about 1200 MLT. Our results show that the mantle precipitation is present near the cusp region and also currents can have a mantle source. Although RIO currents exist on the side of the polar ionosphere predicted by the dayside merging model, the possibility of observing the RIO currents is low (44%). Whether or not this is due to the seasonal effects, the IMF orientation, or other reasons will be studied in the future based on a larger data set. in either the prenoon or postnoon sector depending on the Acknowledgments. This work is supported by National IMF By. The local time extension of the precipitation also Science Foundation under grant NSF ATM Work differs between the dawnside and the duskside. at APL was supported by NSF Grant ATM (GEM) Field-Aligned Currents on Open Field Lines and NASA Grant 91-SSI-SR and T-62. We thank D. Hardy for the instrument calibration factors. We thank F. Rich and T. Potemra for the magnetometer data. The Editor thanks C. J. Pollock and E. Nielsen for their assistance in evaluating this paper. Our results support the concept that part of the region 1 current flows on mantle field lines, and therefore on open field lines. This confirms our previous result that RIO currents exist. However for IMF By>O, for which RIO current would be expected to appear in the prenoon region of the northern hemisphere, the probability of observing the RIO current in the mantle region is less than 60%. For IMF By<0 for which the RIO current would be expected in the prenoon region of the southern hemisphere, only one case was found (i.e., 1 pass of 7). We think there may exist several reasons why we did not observe more cases with RIO current inside the mantle regions. The existence of the R10 currents may depend on the observational season and on IMF orientation [Walker and Ogino, 1988; Saunders, 1989]. Mechanisms for driving the currents such as thermal pressure gradients, inertial currents, potential drops from reconnection or pressure anisotropy [Hasegawa and $ato, 1979; Vasyliunas, 1984; Stern, 1984; Southwood and Kivelson, 1991] may contribute to different degrees for different magnetosphericonditions. It may be necessary to examine a very large data set in order to assess the control by different parameters. We plan to acquire more magnetometer data from the F7 satellite to study possible seasonal and IMF orientation effects on the local time extension of the RIO currents. We also plan to examine a global MHD simulation with finite IMF By to determine whether the RIO currents are present and how they relate to the IMF orientation. Summary When there is a finite IMF B and Bz southward, dayside reconnected flux tubes will be dragged to the dawnside for a positive IMF B in the northern hemisphere. These reconnected flux tubes should produce an asymmetry of the distribution of the mantle precipitation between the dawnside and the duskside. From DMSP F7 data analyses, we find that the mantle precipitation occurs mainly in the prenoon region of the References Burch, J. L., et al., IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere, 1, Dynamics Explorer observations, J. Geophys. Res., 90, , Coroniti, F. V., Explosive tail reconnection: the growth and expansion phase of the magnetospheric substorms, J. Geophys. Res., 90, , Coroniti, F. V., and C. F. Kennel, Magnetospheric reconnection, substorms, and energetic particle acceleration, in Particle Acceleration in Planetary Magnetospheres, edited by J. Arons, C. Max and C. McKee, pp , American Institute of Physics, New York, Cowley, S. W. H., Magnetospheric asymmetries associated with the y-component of the IMF, Planet. Space Sci., 29, 79, Cowley, S. W. H., J.P. Morelli, and M. Lockwood, Dependence of convective flows and particle precipitation in the highlatitude dayside ionosphere on the X and Y components of the interplanetary magnetic field, J. Geophys. Res., 96, , Crooker, N. U., Dayside merging and cusp geometry, J. Geophys. Res., 84, , Crooker, N. U., and G. L. Siscoe, Birkeland currents as the cause of the low-latitude asymmetric disturbance field. J. Geophys. Res., 86, , de la Beaujardiere, O., J. Watermann, P. Newell, and F. Rich, Relationship between Birkeland current regions, particle precipitation, and electric fields. J. Geophys. Res., 98, , Gosling, J. T., D. N. Baker, S. J. Bame, W. C. Feldman, R. D. Zwickl, and E. J. Smith, North-south and dawn-dusk plasma asymmetries in the distant tail lobes: ISEE 3, J. Geophys. Res., 90, , Hardy, D. A., H. K. Hills, and J. W. Freeman, A new plasma regime in the distant geomagnetotic tail, Geophys. Res. Lett., 2, , 1975.

10 1846 XU ET AL.: IMF CONTROL OF MANTLE PRECIPITATION Hardy, D. A., H. K. Hills, and J. W. Freeman, Occurrence Sanchez, E. R., C.-I. Meng, and P. T. Newell, Observations of of the lobe plasma at lunar distance, J. Geophys. Res., 84, solar wind penetration into the Earth's magnetosphere: The 72-78, plasma mantle, Johns Hopkins APL Tech. Dig., 11, , Hardy, D. A., L. K. Schmitt, M. S. Gussenhoven, F. J. Marshall, H. C. Yeh, T. L. Schumaker, A. Hube, and J. Pantazis, Precipitating electron and ion detectors (SSJ/4) for the block 5D/flights 6-10 DMSP satellites: Calibration and data presentation, Rep. AFGL-TR , Air Force Geophys. Lab., Hanscom Air Force Base, Mass., b. Saunders, M. A., Origin of the cusp Birkeland currents,geophys. Res. Lett. 16, , Sckopke, N., G. Paschmann, H. Rosenbauer, and D. H. Fairfield, Influence of the interplanetary magnetic field on the occurrence and thickness of the plasma mantle, J. Geophys. Hasegawa, A., and T. Sato, Generation of field-aligned currents Res., 81, , during substorm, in Dynamics of the Magnetosphere, edited by S. I. Akasofu, D. Reidel, Hingham, Mass., Heelis, R. A., The effects of interplanetary magnetic field Siscoe, G. L., and E. Sanchez, An MHD model for the complete open magnetotail boundary, J. Geophys. Res., 92, , orientation on dayside high-latitude ionosphericonvection, Southwood, D. J. and M. G. Kivelson, An approximate J. Geophys. Res., 89, , Heikkila, W. J., and J. D. Winningham, Penetration of description of field-aligned currents in a planetary magnetic field, J. Geophys. Res., 96, 67-75, magnetosheath plasma to low altitudes through the dayside Stem, D., Magnetospheric dynamo processes, in Magnetospheric magnetospheric cusps, J. Geophys. Res., 76, , Heppner, J.P., and N. C. Maynard, Empirical high-latitude Currents, Geophys. Monogr. Ser., vol. 28, edited by T. A. Potemra, pp , AGU, Washington, D.C., electric field models, J. Geophys. Res., 92, 4467, Iijima, T. and T. Shibaji, Global characteristics of northward IMF-associated (NBZ) field-aligned currents, J. Geophys. Vasyliunas, V. M., Interaction between the magnetospheric boundary layers and the ionosphere, Proceedings of Magnetospheric Boundary Layer-Conference, Eur. Space Res., 92, , Agency Spec. Publ., 148, Iijima, T., T. A. Potemra, L. J. Zanetti, and P. F. Bythrow, Vasyliunas, V. M., Fundamentals of current description, in Stable patterns of large-scale Birkeland currents in the polar region during strongly northward IMF, J. Geophys. Res., 89, Magnetospheric Currents, Geophys. Monogr. Ser., vol. 28, edited by T. A. Potemra, pp , AGU, Washington, D. 7441, C., Lu, G., P. H. Reiff, M. R. Hairston, R. A. Heelis, and J. L. Karty, Distribution of convection potential around the polar cap boundary as a function of the interplanetary magnetic field, J. Geophys. Res., 94, , Newell, P. T., and C.-I. Meng, Mapping the dayside ionosphere Walker, R. J., and T. Ogino, Field-aligned currents and magnetospheri convection: A comparison between MHD simulations and observations, in Modeling Magnetospheric Plasma, Geophys. Monogr. Ser., Vol. 44, edited by T. E. Moore and J. H. Waite Jr., pp , AGU, Washington, to the magnetosphere according to particle precipitation D.C., characteristics, Geophys. Res. Lett. 19, , Winningham, J. D. and W. J. Heikkila, Polar cap auroral electron Newell, P. T., C.-I. Meng, D. G. Sibeck, and R. Lepping, fluxes observed with Isis 1, J. Geophys. Res., 79, , Some low-altitude cusp dependencies on the interplanetary magnetic field, J. Geophys. Res., 94, , Winningham, J. D., F. Yasuhara, S. -I. Akasofu, and W. Newell, P. T., W. J. Burke, C.-I. Meng, E. R. Sanchez, and J. Heikkila, The latitude morphology of 10-eV electron M. E. Greenspan, Identification of the plasm at low altitude, J. Geophys. Res., 96, 35-45, 1991a. fluxes during magnetically quiet and disturbed times in the MLT sector, J. Geophys. Res., 80, , Newell, P. T., S. Wing, C.-I. Meng, and V. Sigillito, The auroral oval position, structure, and intensity of precipitation Zanetti, L. J., T. A. Potemra, T. Iijima, W. Baumjohann and P. from 1984 onward: An automated on-line data base, J. Geophys. Res., 96, , 1991b. Ohtani, S., et al., Simultaneous prenoon and postnoon observations of three field-aligned current systems from VIKING and DMSP-F7, J. Geophys. Res., in press, F. Bythrow, Ionospheric and Birkeland current distributions for northward interplanetary magnetic field: Inferred polar convection, J. Geophys. Res., 89, , Xu, D., and M. G. Kivelson, Polar cap field-aligned currents for southward interplanetary magnetic field, J. Geophys. Res., Reiff, P. H., and J. L. Burch, IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere, 2, A , global model for northward and southward IMF, J. Geophys. M. G. Kivelson and D. Xu, Institute of Geophysics and Res., 90, , Planetary Physics and Department of Earth and Space Science, Rich, F. J., and M. Hairston, Large-scale convection patterns University of California, Los Angeles, CA ( observed by DMSP, J. Geophys. Res., 99, , mkivelson@igpp.ucla.edu; Rosenbauer, H., H. Griinwaldt, M.D. Montgometry, G. P. T. Newell and C.-I. Meng, Applied Physics Laboratory, Paschmann, and N. Sckopke, Heos 2 plasma observations Johns Hopkins University, Laurel, MD in the distant polar magnetosphere: The plasma mantle, J. R. J. Walker, Institute of Geophysics and Planetary Physics, Geophys. Res., 80, , University of California, Los Angeles, CA (e- Russell, C. T., The configuration of the magnetosphere, in mail: Critical Problems of Magnetospheric Physics, edited by E. R. Dyer, pp. 1-16, National Academy of Press, Washington, D.C., Sanchez, E. R., D. Summers, and G. L. Siscoe, Downstream evolution of an Open MHD magnetotail boundary, J. Geophys. (Received April 20, 1994; revised August 1, 1994; Res., 95, , 1990a. accepted August 8, 1994.)