Dayside electrodynamics observed by POLAR with northward IMF

Abstract.

Introduction

If the IMF retains a northward orientation for longer than a half hour significant changes appear in global ionospheric convection patterns. Maezawa [1976] reported detecting magnetic perturbations during the Antarctic summer, indicating that regions of sunward convection develop in the polar cap when IMF is northward. Electric and magnetic fields measured by the S3-2 [Burke et al., 1979], Atmospheric Explorer [Reiff and Heelis, 1994] and MAGSAT [Iijima and Shibaji, 1984] satellites suggested that with IMF B_Z > 0 and B_Y ~ 0, a four-cell convection patterns evolves. The pattern consists of two cells in the central polar cap, which are of opposite olarity to the standard negative afternoon cell and positive morning cell and are driven by merging at the poleward boundary of the cusp [Dungey, 1961; Russell, 1972], and a weak but standard-polarity pair of cells in the auroral oval, possibly driven by the low latitude boundary layer [Eastman et al., 1976]. Using electric field measurements of the DE 2 satellite, Heppner and Maynard [1987] showed that during periods northward IMF in which B_Y has large values, two distorted convection cells operate at high latitudes. The sense of cell rotation is the same as that observed with IMF B_Z < 0, but the axis of symmetry for the two cells is rotated far from the the noon--midnight meridian. The orbits used by Heppner and Maynard [1987] to infer the convection pattern with B_Z > 0 and B_Y > 0 were reexamined by Burke et al. [1994] using simultaneous measurements from the retarding potential analyzer [Hanson et al., 1981] and electron spectrometer [Winningham et al.}, 1981] on DE 2. These provided supplementary information about the directions of convection flow lines (equipotentials) crossing the selected DE 2 trajectories and about the source regions of precipitating electrons. They concluded that the afternoon (negative potential) cell is rotated into the prenoon sector and consists of two parts: (1) equipotentials whose associated magnetic flux is always open, and (2) equipotentials whose associated flux is both open and closed. Equipotentials with circulating open flux, hereafter referred to as lobe cells [Reiff and Burch, 1985], are embedded within the afternoon cell and have the same sense of rotation.

Passages of satellites through the dayside high-latitude ionosphere are marked by encounters with distinctive particle and field characteristics. Newell et al. [1991a, b] have identified the spectral properties of electrons and ions in the dayside ionosphere whose magnetospheric sources are the central plasma sheet (CPS), the low-latitude boundary layer (LLBL), the cusp and the mantle. In energy-versus-time spectrograms the cusp is marked by intense fluxes of low energy (<100 eV) electrons and energy-dispersed ions. The latter signature is a time-of-flight, velocity-filter effect. During periods of northward (southward) IMF, the highest energy ions are detected near the poleward (equatorward) boundary of cusp precipitation [Reiff et al., 1980; Burch et al., 1980]. Maynard [1985] showed that cusp entry is also marked by a significant increase in the level of low-frequency noise measured by electric field sensors. The rapid traversal of the cusp by satellites in low-altitude orbits only allows for the detection of waves in the Pc 1 band [Maynard, 1985; Basinska et al., 1992]. Satellite crossings of the ionospheric projections of magnetopause merging sites are frequently marked by electromagnetic spikes at the equatorward [Maynard et al., 1991] and poleward [Basinska et al., 1992] boundaries of cusp precipitation during periods of southward and northward IMF, respectively. On larger scales, ionospheric plasma convection in the cusp has a sunward (poleward) component when the IMF has a northward (southward) component. The azimuthal component of convection is controlled by the polarity of IMF B_Y. Convection is westward in the northern hemispheric cusp when IMF B_Y > 0. The opposite polarity relationship maintains in the southern hemisphere cusp. Finally we note that besides the large scale, Region 1/Region 2 systems, the dayside ionosphere is marked by cusp and mantle field-aligned current (FAC) systems, often referred to as Region 0 [Iijima and Potemra, 1976; Earlandson et al., 1988].

Empirical models for patterns of high-latitude potentials (convection) have been constructed for various solar wind/IMF conditions. These have been based on pattern recognition normalization technique using the DE-2 data set [Heppner and Maynard, 1987] and average values from the large DMSP data bases [Rich and Hairston, 1994]. Recently Weimer [1995, 1996] has developed a technique that uses least-squares fits of spherical harmonics that also includes effects of the dipole tilt angle. Published representations of the modeled convection patterns appear to be quite complex, especially for periods with IMF B_Z > 0 and |B_Y| <= B_. Multiple small cells evolve within larger convection cells.

This paper presents electric/magnetic field and particle measurements acquired by instrumentation on the POLAR satellite during two passes at dayside, high latitudes in extended periods of northward IMF in which |B_Y| <= B_Z. The following section contains brief descriptions of POLAR sensors used in this study. The observations section describes particle and field measurements taken at middle altitudes by POLAR on April 3, and 8, 1996. Interpretations of the observations in terms of POLAR encounters with middle altitude projections of previously identified magnetospheric plasma regimes are presented in the final section. We also compare POLAR measurements of high-latitude convection with the predictions of the Weimer [1995, 1996] model under prevailing interplanetary conditions.

Instrumentation

EFI consists of three dipoles to measure vector electric fields from potential differences between three pairs of spherical sensors. Two of the pairs of sensors are held at separation distances of 100 m and 130 m by wire booms that move in the spacecraft's spin plane. The third pair is held at a separation of 14 m by a pair of rigid booms, aligned with the spin axis. The two spin-plane components of the electric field are represented by the symbols E_{X-Y} and E_Z. E_{X-Y} is the projection of the spin-plane component of the electric field onto the GSE (geocentric solar ecliptic) plane. It is positive whenever the unit vector has a component in the -X_{GSE} direction. E_Z is positive toward the GSE north pole. The third component E_{56} points along the spin axis, positive in the sense that completes an orthogonal, right-hand coordinate system. Thus with POLAR orbiting in the noon meridian, components of the vector (E_{X-Y}, E_Z, E_{56}) are positive in the (-X_{GSE}, +Z_{GSE}, +Y_{GSE}) directions. Data are sampled at a rate of 40 s^{-1} by all three sensors. Corrections must be made for dc offsets and fields induced by satellite's -{\bf V_S} \times {\bf B} motion and by transformation of the measurements into the corotating frame. In addition, to eliminate magnetic wake effects at lower densities, data taken when a sensor axis was within an avoidance angle to the magnetic field were eliminated. The remaining data have been fit to a sine wave. The optimum avoidance angle value varies with density and is given for each data set below. Spin-fit measurements are presented below every 6 s for E_{X-Y} and E_Z. Measurements of E_{56} by the short booms are contaminated by unknown levels of dc offsets. To estimate their values we normally make independent calculations of E_{56} using the E . B = 0 condition. However, in the two cases chosen for analysis, the magnetic field vector lay very close the spin plane. This extrapolation then causes small uncertainties in the spin plane to be multiplied by large factors which creates large uncertainties in the extrapolated E_{56} value. For this reason we present in this paper only the electric field's spin-plane components.

MFI consists of two orthogonal, triaxial fluxgate magnetometers that are mounted on a nonconducting boom at separation distances from the nearest satellite surface of 5.97 m and 4.75 m. the outer sensor operates in two ranges +/-5525 nT and +/-694 nT; the inner sensor in the ranges +/-46,700 nT and 5860 nT. Data are sampled at a rate of 500 s^{-1} in all three components and averaged with a recursive filter to provide data at 100 samples per second and 8 samples per second. Only 8 samples per second data are available for the times under study here. In this study we are mostly concerned with magnetic perturbations produced by currents that couple the high-latitude to the magnetosphere or magnetosheath along closed or open magnetic field lines, respectively. Since these large-scale FAC systems generally extend much further in longitude than latitude [Iijima and Potemra, 1976], associated magnetic perturbations are mostly in the azimuthal component B_{56}. Positive-slope deflections in B_{56} with time (latitude) are detected as the northward-moving POLAR crosses FAC sheets directed into the ionosphere.

The HYDRA instrument consists of two pairs of electron and ion/electron spectrometers, each mounted 180 degrees apart on the spacecraft body. In this paper we only use ion and electron measurements from the Duo Deca Electron Ion Spectrometer (DDEIS). As the name suggests, DDEIS is made up of six pairs of 127 degrees electrostatic analyzers looking in different directions outward on a unit sphere. Each measures the fluxes of ions and electrons in 55 energy steps between 10 eV/q to 10 keV/q with a resolution in energy of delta E/E = 6%, and angle of 10 degrees. Fully three-dimension distribution functions for electrons and ions are acquired in 0.5 s.

Observations

The remainder of this section is divided into two parts which present POLAR observations first from April 8, then April 3. Here our task is to present physical quantities measured by the three sensors and point out empirical relationships between them. Interpretations, especially of potentially controversial aspects, are relegated to the discussion section. Because of their greater familiarity for identifying source regions, we show HYDRA measurements before those from EFI/MFI.

April 8, 1996:

On a purely empirical basis, the particle measurements from April 8 divide into four time intervals.

• (1) Prior to 1500 UT the highest ion count rates were at energies in the 8 to 3 keV/q range. The energy of peak counts decreased with increasing ILT. Electron counts were high (>100) at energies below 60 eV and low (<10) at energies above 100 eV. Their sharp spectral cutoff suggests that low-energy population mostly consists of atmospheric photoelectrons [{\it Doering et al.}, 1976].
• (2) The period between 1500 and 1505 UT is marked by a burst of electrons with energies between 200 eV and 2 keV. A similar increase in the flux of ions with $\sim$ 5 keV/q was detected simultaneously.
• (3) The third period extends from 1505 to 1540 UT and is marked by nearly constant fluxes of electrons with energies $<$200 eV and dynamic variations in the ion measurements. From about 1505 to 1515 UT the energies of highest ion counts decreased from 4 to 0.2 keV/q. With notable small-scale variations, average ion energies remained within 0.2 - 0.3 keV/q until 1526 UT when they began to increase systematically to 2 keV/q at 1537 UT. Phenomenologically, we describe the changing ion spectral characteristics at 1501 -- 1508 UT and 1526 -- 1537 UT as ``standard'' and ``inverse'' ion dispersion events.
• (4) After 1540 UT, DDEIS detected electron fluxes whose intensities and energies were significantly reduced, and ion count rates near background levels. Based on experience with the spectral characteristics of ion and electron fluxes observed at low altitudes [{\it Newell et al.}, 1991a, b], we identify the source regions encountered by POLAR during the first interval as the central plasma sheet (before 1500 UT), and during the fourth interval as the polar cap (after 1540 UT), respectively.

Figure 1 shows the spin-plane components of the electric field, the electric potential distribution along POLAR's trajectory, and the spin-axis components of the plasma drift V_{56} and magnetic field B_{56} with the T96 model field [Tsyganenko, 1996] subtracted. A magnetic field avoidance angle of 0 degrees up through 1550 UT and 45 degrees thereafter was used for the electric field spin fits. Potential values are for the corotating reference frame for easy comparison to the ionospheric patterns. Since the satellite velocity V_S lies very close to the spin plane, estimates of the potential distribution along the trajectory

Phi = { - E . V_S dt should be accurate, despite our lack of knowledge about E_{56}. Its utility for understanding geomagnetic processes requires that the convection pattern be relatively steady over the interval in which the measurements were acquired. The steady conditions observed by WIND in the interplanetary medium provides justification for this an assumption. Likewise our calculation of the V_{56} component of

 V = E x B

	   -----

	    B^2

Attention is directed to the following seven points. (1) Prior to 1452 UT, the absolute values and variability of electric field was <1 mV/m. After this the amplitudes of variations grew. (2) From 1503 to 1542 UT the amplitude of electric field variations assumed values <5 mV/m. The periods of the variations ranged from a few tens of seconds to a few minutes (Pc 1 to Pc 4). (3) Average (quasi dc) values of the E_{X-Y} and E_Z show similar variations, with the polarity of both components reversing near 1525 UT. (4) The third panel shows POLAR crossed two regions of negative potential that bound a region of positive potential (1512 -- 1534 UT). Viewed from above the north pole, the sense of rotation for plasma convection is clockwise/counterclockwise in regions of negative/positive potential (for instance, see patterns of {\it Heppner and Maynard} [1987]). In this case the potential of the counterclockwise rotating cell is ~3 kV. (5) The east-west component V_{56} had low values prior to 1504 UT. From 1504 to 1526 UT POLAR detected eastward flow (positive V_{56}), with an average value of ~10 km/s, as it crossed the region of ``standard'' ion dispersion and of low-energy, ions/electron fluxes. An average westward flow (negative V_{56}) of ~10 km/s to the west marked the ``inverse'' dispersion event. This is consistent with counterclockwise plasma rotation in the cusp derived from Phi measurements. In the polar cap V_{56} assumed low, steadily eastward values. (6) Variations in the trace of B_{56} mimic those of quasi-dc values of E_{X-Y} and E_Z. This indicates that POLAR crossed several large-scale FAC sheets which close via Pedersen currents in the ionosphere [Smiddy et al., 1980]. (7) Positive (negative) slopes in the $B_{56}$ trace indicate FACs into (out of) the ionosphere. Consistent with a postnoon MLT trajectory, the positive (before 1505 UT) and negative (1505 -- 1512 UT) slopes in B_{56} appear to be encounters with the afternoon Region 2 and Region 1 systems [Iijima and Potemra, 1976], respectively. The remaining FACs belong to the Region 0 system [Earlandson, 1988]. As expected, the main Region 0 current has a polarity opposite to that of the adjacent Region 1. A small region of upward current is seen poleward of the main Region 0 current.

April 3, 1996:

Electric and magnetic field measurements from the same period are given in Figure 2. A magnetic field avoidance angle of 30 degrees was used throughout for the electric field spin fits. Variations with time-scales of a few minutes or more in the spin-plane components of the electric field, E_{X-Y} and E_Z track well with each other and with those of B_{56}, with the model field subtracted, over the entire interval. We see that large-amplitude fluctuations with periods near 10 s appear continuously between 1233 and 1300 UT, and in bursts near 1330 UT. The potential distribution detected by EFI, indicates that POLAR was in a region of negative potential, characteristic of the afternoon convection cell. The plasma flow component V_{56} = (E_{X-Y} B_Z - E_Z B_{X-Y})/B^2 was weaker than observed on April 8, and after 1235 UT was predominantly toward the east. On large-scale, the trace of B_{56} appears similar to that observed on April 8. Positive slopes, indicating FACs into the ionosphere were observed before 1236 UT (region 2) and from 1248 to 1308 UT; a negative trend in the slope of B_{56} (Region 1) appeared between these intervals. A significant difference between the B_{56} traces of Figures 1 and 2 is the abundance of low-frequency fluctuations with amplitudes of a few nT. Their correlation with electric field variations indicates that POLAR encountered Alfven waves in the Pc 4 and 5 bands, as well as the large-scale FAC systems and suggests a more dynamic environment than that of the April 8 pass. Finally we note that the sharp negative turns in the B_{56} trace at 1300 and 1304 UT coincide with small-scale bursts of electron found in Plate 1B.

Discussion

Figure 3a schematically represents the data constraints on the convection pattern encountered by POLAR on April 8, projected onto the ionosphere. From 1505 to 1512 UT the plasma had an eastward drift component and the ion spectrogram displayed a ``standard'' dispersion signature. During periods of southward IMF, such ion-dispersion structures are normally regarded as time-of-flight effects as ions move from magnetic-merging sites on the magnetopause toward the ionosphere. With a northward IMF, the inner (low-latitude) portion of the LLBL is dominated by high-energy ions from the plasma sheet and the outer (high-latitude) portion by low-energy ions from the magnetosheath [{\it Eastman et al.}, 1976]. The apparent ``standard'' dispersion results from the superposition of these two populations. The separation of the two sources is even more evident in the second event (Plate 1B). During the period of the apparent ``standard'' dispersion, POLAR moved about 1.1 degrees in invariant latitude. This is the approximately the same latitudinal width of the LLBL observed above the ionosphere [Smiddy et al., 1982]. An examination of the potential distribution measured between 1452 and 1512 UT, indicates that equipotentials in the CPS continue into the LLBL. Finally, our interpretation of the particles detected by HYDRA as coming from the LLBL is consistent with theoretical arguments developed by Siscoe et al. [1991] that the LLBL is the current generator for the dayside Region 1.

From 1512 to 1537 UT POLAR crossed a positive potential cell in which plasma convection had an eastward (westward) drift component in its equatorward (poleward) parts. The ``inverse'' ion dispersion structure appeared in the poleward part. The simplest interpretation of these observations is that POLAR went through the mid-altitude projection of the cusp. Magnetic merging with the northward IMF occurred at the cusp's poleward boundary. In this scenario, POLAR crossed the magnetic mapping of the merging line (the heavy line in Figure 3a) where it encountered the highest energy ions of the ``inverse'' dispersion structure. After entering the polar cap, POLAR again found itself in a region of negative potential. The question arises as to whether equipotentials with values >-2 kV in the polar cap connect with those in the LLBL. The answer appears to be no. From the POLAR trajectory in Figure 3a, it is clear that plasma drifting along such equipotentials would have to move with a west component. In fact, data in Figure 1 show that the plasma had an eastward drift component. For this reason we have represented the second negative potential cell as being made up of open flux circulating clockwise in the polar cap. HYDRA measurements of polar rain fluxes indicate that the satellite must have passed to the poleward side of this cell's merging line.

Figure 3b provides a sketch of the data constraints on the convection directions in the pattern crossed by POLAR on April 3, based on the potential distribution found in the third panel of Figure 2. There are two classes of equipotentials with westward drifting plasma in the region of CPS fluxes. Equipotentials crossed at the lowest latitudes are not intersected again by the orbit. At higher latitudes within the CPS, we find equipotentials that turn and are intersected for a second time between 1240 and 1330 UT as the potential increased from about -8 to -1.5 kV. It encompasses the Region 1 and Region 0 current systems and spans the invariant-latitude range 76.5 degrees to 82.6 degrees. Ion fluxes observed across the entire interval are generally weak with broad distributions in energy. In the equatorwardmost portion (1240 -- 1255 UT) they appear bimodal in energy indicating that they are of LLBL origin [Eastman et al., 1976].

Electric potential and particle measurements are most simply explained if POLAR remained within a projection of the boundary layer until it entered the polar cap. First, throughout the 1255 -- 1330 UT period EFI measurements indicate that the plasma drift had an eastward component. During periods of IMF B_Y > 0, independent of the sign of IMF B_Z, magnetic tension forces on newly opened flux causes plasma to drift toward noon (west of the POLAR orbit) then into the morning side of the polar cap. This is opposite to the observed eastward direction. Indeed the Tsyganenko [1996] model predicts that field lines near 80) and 1500 MLT map to the night rather than to the dayside of the magnetosphere. The observed fluxes must be coming from boundary layers somewhere back in the tail. Second, electron and ion fluxes observed between 1255 and 1330 UT show gradual, rather than abrupt, transitions to polar cap levels. This is seen most clearly in the electrons, whose average energies monotonically decreased with increasing invariant latitude. It may be viewed as due to an adiabatic cooling of electrons as flux tubes grow in volume and convect down stream along the flank of the magnetotail. The latitudinal width of the projection of this boundary layer is larger than that reported by Smiddy et al. [1982]. We note, however, that potential drop of ~8 kV measured across the boundary layer is similar to potentials reported by Smiddy et al. [1982] and in ISEE 1 observations of potentials across the low latitude boundary layer on the dusk side Mozer [1984]. Independent evidence of the boundary layer hypothesis is provided by the TIMAS ion mass spectrometer on POLAR [Shelley et al., 1995] which detected weak, variable low-energy fluxes of He++ during the interval which must have entered the boundary layer some distance down the tail.

Figures 4a and 4b present ionospheric potential distributions in ILT/MLT predicted by the model of Weimer, [1995, 1996] for interplanetary conditions prevailing during the periods of interest on April 8 and April 3, respectively. Below the circular grids are listed minimum and maximum potentials predicted for the afternoon and morning cells; -28 and +12 kV on April 8, -18 and +9 kV on April 3. POLAR trajectories, mapped to the ionosphere using the T-96 magnetic field model [Tsyganenko, 1996], are provided for reference. In both cases the model correctly predicts that POLAR should detect measurable convection poleward of 72 degrees ILT, and that over the studied periods, it should remain within the afternoon cell. The morning (positive potential) cell is restricted to the postmidnight -- predawn quadrant. In both cases the model predicts that small lobe cells should develop in the noon sector, poleward of 80 degrees ILT. The sense of rotation for convection in these cells is clockwise, similar to that reported by Burke et al. [1994] for northward IMF with B_Y > 0. Attention is directed to the fact that even though IMF clock angles for the two events are similar, their differences, along with dipole tilt angle distinctions, are sufficient to produce clear differences in predicted convection patterns for the early afternoon sector. The model correctly predicted that POLAR would pass through a more stagnant region near 80 degrees ILT on April 3 than on April 8.

A comparison of the potential distributions shown in Figures 3 and 4 also reveals some significant differences. Obviously the model did not predict POLAR's encounter with a positive potential cell during the April 8, pass. The data are more suggestive of a four-cell pattern [Burke et al., 1979; Reiff and Heelis 1994 than a distorted two-cell pattern with an embedded lobe cell [Burke et al., 1994]. As suggested by Rich and Hairston [1994], the four-cell pattern may occur so rarely that it is not reproduced by empirical models developed from large data bases. However, the model of Weimer [1995, 1996] does produce the positive potential lobe cell on the dusk side of noon for pure northward IMF in a structured four-cell configuration, and this may be the start of the development of that cell.

Acknowledgments.

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