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\lefthead{MAYNARD ET AL.}
\righthead {DAYSIDE CONVECTION WITH NORTHWARD IMF}
\received{Month xx, 1996}
\revised{Month xx, 1996}
\accepted{Month xx, 1996}
\journalid{JGRA}
\articleid{ }{ }
\paperid{96JA00XYZ}
\ccc{0148-0227/96/96JA-00XYZ\$05.00}
\cpright{PD}{1996}
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%Author address material in alphabetical order
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\authoraddr{W. J. Burke,  
Phillips Laboratory, 29 Randolph Road, Hanscom AFB, MA, 01731-3010. 
e-mail: burke@plh.af.mil}

\authoraddr{R. P. Lepping, Laboratory for Extraterrestrial Physics, 
Code 696, Goddard Space Flight Center, Greenbelt, MD  20771.}

\authoraddr{N. C. Maynard and D. R. Weimer, Mission Research Corporation,   
One Tara Blvd., Suite 302, Nashua, NH, 03062.}  

\authoraddr{F. S. Mozer, Space Science Laboratory,   
University of California, Berkeley, CA, 94720.}  

\authoraddr{W. K. Peterson, Lockheed.}

\authoraddr{C. T. Russell, Institute for Geophysics and Planetary Physics,
University of California, Los Angeles, CA, 90049.}

\authoraddr{J. D. Scudder, Department of Space Physics and Astronomy, 
University of Iowa, Iowa City, IA, 52240.}

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%Front Material
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\begin{document}
\title {POLAR observations of convection with northward IMF at dayside high 
latitudes}
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\author{N. C. Maynard,\altaffilmark{1} W. J. Burke,\altaffilmark{2} 
D. R. Weimer,\altaffilmark{1}  F. S. Mozer,\altaffilmark{3}} 
\author{J. D. Scudder,\altaffilmark{4} C. T. Russell,\altaffilmark{5} 
W. K. Peterson,\altaffilmark{6} and R. P. Lepping\altaffilmark{7}}
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\altaffiltext{1}{Mission Research Corporation, Nashua, New Hampshire}
\altaffiltext{2}{Phillips Laboratory, Hanscom Air Force Base, Massachusetts.}
\altaffiltext{3}{University of California, Berkeley, California}
\altaffiltext{4}{University of Iowa, Iowa City, Iowa}
\altaffiltext{5}{University of California, Los Angeles, California}
\altaffiltext{6}{Lockheed Martin Space Sciences Laboratory, Palo Alto, 
California}
\altaffiltext{7}{Goddard Space Flight Center, Greenbelt, Maryland}
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\begin{abstract}
This study of dayside electrodynamics provides a first view of quasi dc 
electric fields 
measured by POLAR at middle altitudes.  Three high latitude passes chosen for 
analysis, occurred while POLAR was in the midday sector and the IMF 
maintained 
a steady northward orientation at clock angles near 40$^{\circ}$ in the 
Y$_{GSM}$ 
-- Z$_{GSM}$ plane.  At different times during the passes POLAR encountered 
electron 
and ion fluxes from the central plasma sheet (CPS), the polar rain and various 
magnetospheric 
boundary regions.  Although electric fields measured within the boundary 
layers were highly 
variable, we derived average components of plasma convection.  The results 
compare 
favorably with convection characteristics of four-cell patterns observed at 
ionospheric 
altitudes with northward IMF.   We show that POLAR encountered lobe cells both 
positive and 
negative polarity in the early postnoon sector.  The negative potential 
extends well into the 
prenoon sector.  Inverse, energy-dispersed fluxes of  He$^{++}$ were detected 
in 
regions of sunward convection.  Sunward convection extended into the region of 
polar rain 
north of the merging line's projection.  The magnetic merging signatures 
indicate that 
the process is both time varying and patchy.  Particle and field measurements 
are 
consistent with the afternoon, auroral convection cell closing in the low 
latitude 
boundary layer, which extend many tens of R$_E$ tailward of the dusk 
terminator.     
Taken in a larger context, the POLAR measurements provide new insight 
concerning 
the evolution of high-latitude convection from distorted two-cell patterns at 
large 
IMF clock angles to four-cells at low clock angles

\end{abstract}
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%Main Body Text

\section{Introduction}

	One of the principal scientific goals of the POLAR mission is to expand 
understanding of the electrodynamic coupling between the interplanetary medium 
and the Earth's magnetosphere.  This paper focuses on the analysis of particle 
and field measurements acquired during three high-latitude passes of POLAR 
in the midday sector. In each of the selected periods the interplanetary 
magnetic field (IMF) maintained relatively steady northward orientations.  In 
all cases 
IMF $B_Y$ was positive (dawn-to-dusk), however its magnitude relative to 
IMF $B_Z$ varied. Although the penetration of particles and electric fields 
into the magnetosphere during periods of northward IMF has been studied using 
a broad spectrum of ground- and space-based sensors, the results sometimes 
appear to be inconsistent.  

	The dayside magnetosphere quickly responds to changes in the polarities 
of IMF $B_Y$ and/or $B_Z$.  Within a few minutes (approximately an Alfv{\'e}n 
travel time) of the changes reaching the magnetopause, characteristic optical 
[{\it Sandholt}, 1991; {\it Sandholt et al.}, 1996; {\it Murphree et al.}, 
1990] and plasma convection signatures [{\it Clauer and Friis-Christensen}, 
1988] 
appear in the ionospheric projection of the cusp.  Global ionospheric 
convection patterns at high geomagnetic latitudes are not established until 
the IMF has
remained northward for longer than a half hour.  During the Antarctic summer 
{\it Maezawa}
[1976] observed magnetic perturbations whose explanation required sunward 
convection in the
central polar cap.  Electric and magnetic fields measured by the S3-2 [{\it 
Burke et al.}, 1979],
Atmospheric Explorer [{\it Reiff and Heelis}, 1994] and MAGSAT [{\it Iijima 
and Shibaji}, 1984]
satellites suggested that with IMF $B_Z > 0$ and $B_Y \approx 0$, a four-cell 
convection
patterns evolves.  This convection pattern consists of two cells in the polar 
cap, whose polarity
is opposite to the adjacent, standard 
negative potential (clockwise) afternoon and positive (counter-clockwise) 
morning 
cells.   The polar cap convection cells are driven by magnetic merging at the 
poleward 
boundary of the cusp [{\it Dungey}, 1961; {\it Russell}, 1972].  The residual, 
standard-polarity pair of cells at auroral latitudes are weak.  {\it Burke et 
al.} [1979] 
suggested that they are driven by the low latitude boundary layer (LLBL) [{\it 
Eastman et 
al.}, 1976].  


{\it Heppner and Maynard} [1987] showed that during periods of northward IMF 
in which $B_Y$ had large values, electric fields measured by the DE 2 
satellite were 
consistent with the existence of two standard-polarity convection cells at 
high 
latitudes.  Although distorted in shape, flow (equipotential) lines had the 
same sense of 
rotation as those observed when IMF $B_Z < 0$, but with the axis of symmetry 
rotated far from the noon--midnight meridian.  The same orbits used 
by {\it Heppner and Maynard} [1987] to infer the convection pattern with $B_Z 
> 0$ and $B_Y > 0$ were reexamined by {\it Burke et al.} [1994] using 
simultaneous 
measurements from the retarding potential analyzer [{\it Hanson et al.}, 1981] 
and electron spectrometer [{\it Winningham et al.}, 1981] on DE 2.  These 
provided 
supplementary information about the directions of convection flow lines 
crossing DE 2 trajectories and about the source regions of precipitating 
electrons. They concluded that the afternoon (negative potential) cell rotates 
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 are commonly referred to as 
lobe 
cells [{\it Reiff and Burch}, 1985].  In the cases studied by {\it Burke et 
al.} 
[1994] the lobe cells were embedded within the afternoon cell and had the same 
sense of 
rotation. Recently {\it Greenwald et al.} [1995] reported SUPERDARN 
observations of  the
dayside convection pattern evolving from two distorted cells into four 
cells as IMF $B_Y$ decreased in magnitude relative to $B_Z$.  The observed 
evolution of dayside convection compared favorably with prediction of the 
empirical model of {\it Weimer} [1996].

	Passages of satellites through the dayside high-latitude ionosphere are 
marked by encounters with distinctive particle and field characteristics. {\it 
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 LLBL, the cusp and the mantle.  {\it Newell and Meng} [1992, 1994] 
have 
utilized  the large DMSP data base to determine the probabilities of 
encountering these 
regions as functions of magnetic latitude and magnetic local time (MLT). 

 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 [{\it Reiff et al.}, 1980; {\it Burch et al.}, 1980]. {\it 
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 [{\it Maynard}, 1985; {\it Basinska et al.}, 1992].  
Satellite crossings of the ionospheric projections of magnetopause merging 
sites are 
frequently marked by electromagnetic spikes at the equatorward [{\it Maynard 
et al.}, 
1991] and poleward [{\it 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 [{\it 
Iijima and Potemra}, 1976; {\it 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 [{\it Heppner and 
Maynard}, 
1987] and average values from the large DMSP data bases [{\it Rich and 
Hairston}, 1994].  
Recently {\it 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 $|B_Y| \le B_Z$. 
Multiple 
small cells evolve within larger convection cells.

	This paper extends a preliminary report, mostly based on POLAR electric 
field measurements taken during two POLAR orbits [{\it Maynard et al.}, 1997].  


Expanding the study in local time and IMF clock angle, we present electric/
magnetic 
field and particle measurements taken during three example POLAR passes at 
dayside, 
high latitudes throughout which the IMF $B_Z$ and $B_Y$ were positive.  
Magnetic field 
mapping using the T96 model [{\it Tsyganenko}, 1996] and observed energies/
pitch angles 
of different ion masses are used to improve identifications of the source 
regions 
for electrodynamic signatures detected by POLAR.  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, 8,  and May 11, 1996.  Interpretations of the POLAR observations 
in terms of encounters with middle altitude projections of previously 
identified 
magnetospheric plasma regimes are presented in the final section.  In one case 
we were able to compare POLAR measurements with simultaneous plasma drift 
observations of global high-latitude convection by a Defense Meteorological 
Satellite 
Program (DMSP) satellite.  The DMSP measurements confirm that POLAR made 
direct 
measurements of a negative potential lobe cell that was part of a four-cell 
convection pattern. 

\section {Instrumentation}

	POLAR was launched into a 90$^{\circ}$ inclination orbit from the 
Western Test Range 
at Vandenburg, California on February 24, 1996.  The initial orbital plane was 
in the early post-noon/midnight local time sectors.  Apogee (above the 
northern polar 
cap) and perigee are at geocentric distances of 9 and 1.8 R$_E$, respectively. 
The 
spacecraft is spin stabilized at 10 rpm, with its spin axis perpendicular to 
the orbital 
plane.  The four instruments whose measurements are used in this study are the 
electric field instrument (EFI), the magnetic field experiment (MFE), the 
HYDRA 
electron/ion spectrometer and the torroidal imaging mass angel spectrometer 
(TIMAS).  
Comprehensive descriptions of these sensors are provided by {\it Harvey et 
al.} [1994], 
{\it Russell et al.} [1994], {\it Scudder et al.} [1994] and {\it Shelley et 
al.} [1994], 
respectively. Only brief summaries of their relevant capabilities are provided 
below.

	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 rotate 
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, called $E_{56}$, 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 the satellite's  $-{\bf V_S} 
\times 
{\bf B}$  motion and by a transformation of the measurements into the 
corotating frame.  
In addition, to minimize magnetic wake effects due to low plasma densities at 
POLAR altitudes, we eliminated 
data taken when dipole axes were within specified avoidance angles to 
the magnetic field.  The remaining data were fit to sine functions.  Optimal 
avoidance 
angles varied with plasma density.  Spin-fit measurements are presented for 
$E_{X-Y}$ and $E_Z$ every 6 s. 

	Measurements by the short booms of $E_{56}$ are contaminated by unknown 
levels of
dc offsets.  These have two major sources, asymmetries of the potential 
distribution in the sheaths around the spacecraft and differences in the 
contact potentials 
of the two sensors.  The first offset source occurs because the sensor 
elements are often 
embedded within the spacecraft sheath, to degrees that depend on the plasma 
density. To estimate these offsets and correct for them, we first subtract 
values that are 
proportional to the spacecraft potential.  Residuals are then compared to 
estimates of the 
$E_{56}$ component made using the assumption that $E \cdot B = 0$.  Systematic 
deviations are then used to estimate  the offset contribution due to  
differences 
in the surface contact 
potentials.  Whenever magnetic field vectors lies very close the spin plane, 
the  $E \cdot B = 0$ calculation amplifies small uncertainties  about 
satellite 
attitude by 
large factors, creating large uncertainties in the calculated $E_{56}$ value.  

In some cases the dc offsets of measured $E_{56}$ can be determined from 
comparisons 
with results at other times.  For other cases, such as the two April passes, 
no 
reliable values of $E_{56}$ could be determined.  

	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 
$\pm$5525 nT and $\pm$694 nT; the inner sensor in the ranges  $\pm$46,700 nT 
and 
$\pm$5860 nT. Data 
are sampled at a rate of 500 s$^{-1}$ in all three components and averaged 
with a recursive filter to provide data at 100 and 8 samples s$^{-1}$.  Only 
data 
sampled at the slower rate are available for this study.  Here, 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 [{\it 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$^{\circ}$ 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$^{\circ}$ 
electrostatic 
analyzers looking in different directions outward on a unit sphere.  The 
electron 
spectrometer measures fluxes in the 1 eV to 10 keV range. The other 
spectrometer 
measures the fluxes of ions with energiy per charge between 10 eV/q to 10 keV/
q, 
in 55 steps with a resolution in energy of ${\delta}E/E = 6\%$, and angle of 
10$^{\circ}$.  Fully three-dimension distribution functions for electrons and 
ions are acquired in 0.5 s.  To better represent the energies of in situ 
particles, 
we have adjusted the measured electron/ion count rates to reflect the effects 
of 
shifts induced by the floatimng potential of the spacecraft. 

	The TIMAS instrument uses a first-order, double focusing system of ion 
optics that simultaneously measures the spectral characteristics of positively 
charged ions in the mass per charge range 1 - 32 AMU e$^{-1}$, and energy per 
charge from 15 eV e$^{-1}$ to 32 keV e$^{-1}$ in 28 steps.  Nearly the full 
$4\pi$ sr 
solid angle is swept out each half spin (3 s) of the satellite.  In what 
follows we only display fluxes of He$^{++}$ ions which primarily come from the 
solar 
wind. They provide tracers for identifying direct (cusp/mantle) and indirect 
(LLBL) 
paths from the magnetosheath to the locations of POLAR.

\section{Observations}

	This section presents particles and fields measured by POLAR during 
three dayside 
passes in April and May of 1996.  In all cases the satellite was moving toward 
higher altitudes and northern-hemisphere latitudes.   Interplanetary 
conditions 
monitored by the WIND satellite near the times of interest are summarized in 
Table 1.   
Here we list the $X_{GSE}$ and $Y_{GSE}$ locations of the satellite in R$_E$, 
the average solar wind speeds $V_{SW}$ in km/s, the three GSM components of 
the 
IMF in nanotesla,  and the ammproximate time in minutes that it takes for a 
signal in the solar wind to propagate to the subsolar magnetopause.  
The solar wind speed was in the low-to-moderate range.  Interplanetary 
conditions were relatively stable for the periods of interest on April 3 and 
8. 
Since we are primarily interested in steady-state responses of the 
magnetosphere, 
knowing exact propagation times between WIND and the Earth are not critical 
for 
interpreting these events.  On May 11, WIND was near the dawn meridian, just 
outside the bow shock. The IMF clock angle was gradually increasing 
with the nominal value near 35$^{\circ}$.  Approximate propagation times used 
for selecting the appropriate WIND data are given in Table 1.  In all three 
cases 
the IMF had positive $Y_{GSM}$ and $Z_{GSM}$ components, and the IMF clock 
angle in the $YZ_{GSM}$ plane was near 40$^{\circ}$.  We note that the level 
of geomagnetic activity was relatively low  during the times of interest, 
with  $K_P$ indices of 2-, 2, and 2, respectively. 

	The remainder of this section presents POLAR observations each of the 
events starting with April 8.  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 and TIMAS measurements before those from EFI/MFE.

\subsection{April 8, 1996:}

	Plate 1 contains directional differential fluxes (cm$^{2}$ s sr keV/
e)$^{-1}$
of He$^{++}$ (top) and omnidirectional count rates of electrons (middle) and 
ions (bottom) acquired between 1430 and 1730 on APRIL 8, 1996. The data are 
in standard energy-versus-time 
spectrogram formats.  Color bars to the right of the spectrograms provide flux 
and count-rate magnitudes. The DDEIS spectra give averages of 72 measurements 
at the given energy steps, accumulated over 13.8 s intervals.  The 
measurements 
are presented as functions of universal time (UT), invariant latitude (ILT), 
magnetic local time (MLT) and geocentric distance in R$_E$.  
Values of the ILT, determined using IGRF 95 are related to the magnetic L 
shell by the familiar relationship $ILT = Cos^{-1}(1/{\sqrt L)}$.   

	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/e 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/e was detected simultaneously.  
(3)  The third period extends from 1505 to 1540 UT  and is marked by nearly 
constant fluxes of electrons with energies $<$200 eV and dynamic variations 
in the ion measurements.  From about 1505 to 1515 UT the energies of highest 
ion counts decreased from 4 to 0.2  keV/e.  With notable small-scale 
variations, 
average ion energies remained within 0.2 - 0.3 keV/e until 1526UT 
when they began to increase systematically to 2  keV/e at 1537 UT.  
Phenomenologically, we describe the changing ion spectral characteristics 
at 1501 -- 1508 UT and 1526 -- 1537 UT 
as ``standard'' and ``inverse'' ion dispersion events.  He$^{++}$ ions are 
observed starting near 1508 UT and are intense between 1515 and 1530 UT.  The 
inverse dispersion between 1526 and 1537 is also seen.  Fluxes of He$^{++}$ 
ions are low everywhere outside of this region.
(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. Interpretation of the source regions for 
particles 
observed during the second and third segments is deferred. 

	Figure 1 shows the spin-plane components of the electric field, the 
electric 
potential distribution along the trajectory, and the spin-axis components of 
the plasma drift $V_{56}$ and magnetic field $B_{56}$, the latter with the T96 
model field subtracted.  A magnetic field avoidance angle of 0$^{\circ}$ up 
through 1550 
UT and 45$^{\circ}$ 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 ${\bf V_S}$ lies very close to the 
spin plane, 
estimates of the potential distribution along the trajectory
$$
\Phi = \int {-{\bf E} \cdot {\bf V_S}}\, dt
$$
should be accurate, despite our lack of knowledge about $E_{56}$ for this 
pass.  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 
$$
{\bf V} = {{({\bf E} \times {\bf B})}\over {B^2}}
$$
requires no knowledge of $E_{56}$.  For reference, we have marked the four 
different particle-flux segments at the bottom of the figure.   

  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 electric field reverse polarity 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 $\sim$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 
$\sim$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 $\sim$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 [{\it 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 [{\it Iijima 
and Potemra}, 1976], respectively. The remaining FACs belong to the Region 0  
system [{\it 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.

\subsection{April 3, 1996:}

	Plate 2 presents TIMAS and HYDRA measurements from 1200 to 1345 UT on 
April 3, 
1996. Again, we divide the measurements as coming from four phenomenologically 
distinct regions.  
(1) Prior to $\sim$1229 UT two populations of low- and high-energy electrons 
are sampled. From spectral similarities to fluxes detected at the same 
invariant 
latitudes during the previous pass, we identify them as photoelectrons and 
electrons from the CPS.  
Over the same period energetic ($>$1 keV) ion fluxes, whose mean 
energies decreased with increasing invariant latitude, were observed.  
(2)  Fluxes of electrons with energies between about 500 eV and 4 keV 
underwent  
rapid increases and decreases at 1229 and 1238 UT, respectively.  In this 
interval fluxes of ions with energies $<$200 eV rose above background.
(3) From about 1238 to 1330 UT POLAR crossed a region of high fluxes of 
electrons whose mean energies, except for a few small-scale structures, 
decreased 
smoothly with invariant latitude. This region is marked by HYDRA observations 
of 
two ion populations with energies centered near 500 eV and 7 keV.  TIMAS 
recorded 
three weak bursts of He$^{++}$ ions with significant fluxes below 100 eV, 
at 1245, 1300 and 1315 UT.  The peak energy of each burst is less than that of 
it predecessor.In all cases however the flux of He$^{++}$ ions was nearly 
isotropic 
in pitch angle.
(4) After 1330 UT no  He$^{++}$ ions were detected and HYDRA measured polar 
rain 
electron and low ion flux levels, more typical of the polar cap. 
Again we describe particles from the first and fourth intervals as marking the 
CPS and polar cap, respectively. Sources for particles of the second and third 
are discussed below.

	Electric and magnetic field measurements from the same period are given 
in Figure 2.  A magnetic field avoidance angle of 30$^{\circ}$ 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 track well 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, but with smaller magnitude currents.   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 Alfv{\'e}n 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. 

\subsection{May 11, 1996}

	On May 11, 1996 the IMF varied gradually and the POLAR observations 
were more
complex than on the two April days.  Plate 3 presents TIMAS and HYDRA 
measurements 
taken between 1300 to 1400 UT. The POLAR trajectory was very near magnetic 
noon 
(1133 to 1237 MLT) and at high invariant latitudes (78.6$^{\circ}$ to 85.0$^{\
circ}$).  
Examining first the HYDRA electron spectrogram, we see that there is no flux 
enhancement found at the transition between regions of high and low energy 
electron 
dominance as observed on the previously.  For this reason we divide the 
particles into 
three phenomenological segments. 
(1) Prior to 1311 UT, electron and ion fluxes observed by HYDRA were at high 
keV energies, 
characteristic of CPS fluxes.  
(2) The period between 1311 and 1344 UT is marked by complex variations whose
characteristics 
we describe in more detail below. 
(3) After 1345 UT, HYDRA detected electron fluxes at polar rain levels.  Ion 
fluxes  
during the first and third intervals were below the sensitivities of TIMAS and 
HYDRA.   

 The properties of particle measurements in the second interval change at 1330 
UT.  
>From 1311 to 1330 UT there were two forward ion dispersion signatures in the 
HYDRA 
spectrogram, 1312 through 1318 UT and 1319 through 1325 UT.  Low-energy 
electron fluxes whose intensity gradually increased mark the interval.  The 
flux of He$^{++}$ ions ramped up after 1315 with their average energies 
increasing 
slightly at 1319 UT.  The intensity underwent a step like increases at 1324 
UT.  

The interval from 1330 to 1344 is characterized by multiple ion injections 
with 
reverse dispersion signatures.  The electron flux was intense and contained 
two major enhancements in both intensity and energy at 1332 and 1340 UT.  
Intense He$^{++}$ ion fluxes indicate access to the magnetosheath. The 
four peaks in ion energy with inverse 
dispersions observed by HYDRA at 1333, 1336, 1340 and 1344 UT have analogous 
features in the in the He$^{++}$ spectrogram.  We defer identification of 
source 
regions for the second interval to the discussion section.

	From top to bottom, Figure 3 presents the spin-plane components of the 
electric 
field, the potential distribution along the orbit track, the velocity 
components 
$V_{X-Y}$ and $V_{56}$ and the magnetic field component $B_{56}$ with the T96 
model 
field component subtracted.  The inclusion of $V_{X-Y}$ is possible because of 
a 
reasonable determination of the $E_{56}$ offset could be made for this orbit. 
The 
magnetic field 
avoidance angles used were 30$^{\circ}$ through 1315 UT, 0$^{\circ}$ from 1315 
to 1342 UT, and 60$^{\circ}$ thereafter.  Because POLAR remained close to a 
zero 
equipotential line until 1342 UT, we have 
calculated the potential distributions in both inertial (dashed line) and 
corotating (solid line) coordinates. The positive slope in the $B_{56}$ trace 
ending at $\sim$1311 UT  corresponds to interval (1) in the particle data of 
Plate 3. The signified downward current
appears in a narrow latitude band marked by a weak negative potential well.  
Its source is in the central plasma sheet, and probably coincides with the 
afternoon region 2 FAC. 

The adjacent Region 1 upward currents extend through the beginning of the 
second particle 
interval to 1319:30 UT.  Positive values of $V_{X-Y}$ at this time indicate 
that convection 
was antisunward (poleward) .  $V_{56}$ was initially toward dawn but reversed 
toward dusk near 1315 UT. 
>From 1319:30 through 1330 UT the FAC reverses sign, maintaining a nearly 
constant value.  $V_{56}$ again turned back toward dawn at 1323 UT.  
At this time $V_{X-Y}$ turns sunward (equatorward) and remained so throughout 
the remainder of the pass.  From 1319:30 through 1330 UT the potential had a 
slight positive excursion in the inertial system.  

	From 1330 to 1344 UT, particle and field patterns underwent further 
changes. 
A second relative maximum is seen in the potential pattern at 1336 UT.  The 
potential then starts a negative trend which continues through the end of the 
pass.  
The inverse ion dispersions with energy peaks near 1333 and 1336 UT occurred 
in regions of eastward plasma drift ($V_{56} > 0$).  The third and fourth 
inverse 
dispersions, noted in the discussion of Plate 3 are in regions of westward 
drift. 
Large sunward-velocity bursts accompany the first and third dispersions.  
Finally, we note that while the upward trend in the $B_{56}$ trace continued 
through 1334 UT, the other magnetic field components had changes of similar 
magnitude,  
indicating that the satellite passed obliquely through FACs that may have been 
more filamentary in structure. After 1345 UT the principal plasma flow was 
sunward, 
and the potential became more negative as the satellite penetrated the polar 
cap.

\section{Discussion}

	Data from the these POLAR passes with northward IMF exhibit both 
similarities and differences.  The three events provide slices at different 
local times 
through dayside high latitudes under similar IMF clock angle conditions 
($\sim$40$^{\circ}$). While it is easy to identify the source regions for data 
acquired at the lowest and highest latitudes of the passes as the central 
plasma sheet 
and the polar cap, the three passes have very different properties in the 
intervening 
regions.  Thus, each pass provides different perspectives on dayside 
electrodynamics 
with northward IMF and the sources that drive the electrodynamics. 

 During the POLAR passes of April 3 and 8, magnetic field variations showed 
the characteristic of the afternoon Regions 2, 1 and 0 systems of FACs 
(downward/upward/downward) with an additional, small upward current 
at the poleward edge.  At the highest and lowest invariant latitudes for which 
HYDRA spectral measurements are shown in Plates 1 and 2, one finds familiar 
characteristics of the CPS and polar cap.  Localized bursts of kilovolt 
electrons are found in both instances near the poleward boundary of Region 2 
currents (interval 2 in each case).  Since Region 2 currents are 
driven by pressure gradients in the plasma sheet [{\it Harel et al.}, 1981], 
and particle spectral characteristics do not point elsewhere, we suggest that 
the bursts originate in the outer CPS.  Physical processes responsible for the 
observed flux increases must operate deeper in the magnetosphere than the 
POLAR orbit.  The same current systems were observed during the May 11 pass.  
There was no burst of keV at the high-latitude boundary of CPS fluxes, so that 
interval (1) encompassed the entire Region 2 FAC

The most intriguing aspects of the observations in the intermediate latitude 
segments of the three cases (1505 -- 1540 UT on April 8, 1238 -- 1330 UT on 
April 3 and 1311 -- 1344 UT on May 11).  These times roughly span the Regions 
1 
and 0 FACs and part of the upward FAC at the poleward edge.  
In all cases the segments of interest began as POLAR entered the Region 1 
current sheet and ended as it entered the polar cap. Between these boundaries 
significantly different observations were made.  Our analysis and 
interpretation of the observations from this portion of the POLAR orbits 
follows for each case.

	Figure 4a schematically represents the constraints that the data places 
on our 
interpretation of 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 2).  
During the period of the apparent ``standard'' dispersion in Plate 1, POLAR 
moved about 1.1$^{\circ}$ in invariant latitude.  This is the approximately 
the same latitudinal width of the LLBL observed above the ionosphere [{\it 
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 {\it Siscoe et al.} [1991] 
that the LLBL is the generator for the dayside Region 1 currents.

	From 1512 to 1537 UT on April 8, 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 middle 
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 4a) 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 4a, 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.  
The counter-clockwise rotation of the first lobe cell is opposite to that of 
the 
normal afternoon convection cell and the embedded lobe cell observed by 
{\it Burke et al.} [1994].  It is similar to that observed by {\it Greenwald 
et al.} 
[1995].  We return to this point below.  The FACs associated with this cell 
are 
the downward Region 0 currents and upward current straddling the poleward 
edge.
   
	Figure 4b sketches 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$^{\circ}$ to 82.6$^{\circ}$. 
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 [{\it Eastman et al.}, 
1976].  

	Electric potential and particle measurements for April 3 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.  Thus, the drift expected from merging is opposite to the observed 
eastward direction. 
The TIMAS data (Plate 2) shows three bursts of He$^{++}$ ions in this region.  

The observed 
fluxes must be coming from boundary layers of the magnetotail.  To illustrate 
this, 
Figure 5 displays the projection of the POLAR orbit at each of the times of 
the bursts of 
He$^{++}$ ions using the T96 model.  A nominal magnetopause is also 
shown.  The particles enter the boundary layer from well behind the dusk 
terminator.  
Because of finite propagation times required to travel along the magnetic 
field lines to 
POLAR, the mapped position in the equatorial plane may be tailward of actual 
entry points in the boundary layer.   However, the conclusion that POLAR 
traversed 
the mapped boundary layer at points well behind the dusk terminator is clear.
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.  
We have noted that
each of the He$^{++}$ bursts is lower energy than the one preceding it, 
characteristic of sources being at greater distances down the tail.  Although 
the latitudinal width of the projection of this  boundary layer is larger than 
expected, the  potential drop of $\sim$8 kV 
measured across the boundary layer is similar to those reported by {\it Smiddy 
et al.} [1982] and {\it Mozer} [1984]. 

	The most complex particle and field variations were measured on May 11 
as the POLAR orbit straddles the noon magnetic meridian.  Cusp/cleft 
signatures 
were encountered at unusually 
high magnetic latitudes [{\it Newell and Meng} 1992, 1994].  We anticipate 
that the satellite's location mapped to several different geophysical regions 
over a small latitudinal span. 
Figure 6 is a three dimensional (3 D) image of magnetic field lines crossed by 
the 
POLAR trajectory at 5 minute intervals.  The T96 model with 
the prevailing DST value of -3 nT and appropriate solar wind conditions was 
used.  
Until 1315 UT, the field lines mapped to the prenoon dayside boundary layer.  
The field line crossed at 1320 UT mapped to the dawn flank of the magnetotail, 
well behind the terminator.  The remaining field lines map into the high 
latitude boundary of the magnetosphere.  Since the T96 model is very sensitive 
to 
input conditions, the map can only be used as a qualitative guide; however, it 
provides useful guidance for interpreting the POLAR measurements from this 
period.

	The first part of the second interval (1311 to 1319:30 UT) includes all 
of the duskside Region 1 currents and the first forward-dispersion ion event.  

We interpret this as part of the duskside convection cell because of its 
negative 
potential and the upward polarity of it FACs.  However, the magnetic mapping 
suggests a prenoon source.  

The latter part of the second interval (1319:30 to 1330 UT) contains the 
second forward-dispersion ion event and exhibits downward FACs as 
expected from afternoon Region 0, which has the same polarity as dawnside 
Region 1 currents.  Plasma velocities are initially toward dusk and then 
reverse toward dawn.  The potentials are all small and indicate that POLAR 
was moving close to a zero potential line between two convection cells.  

	Figure 6 suggests that POLAR locations during the latter part of the 
second interval mapped to the cusp/mantle region.  Here POLAR crossed 
four injection structures with 
reverse dispersion characteristics which are regarded as signatures of 
magnetic merging poleward of the cusp.  The azimuthal plasma velocity 
components of the first and second dispersion events were opposite to 
those of the third and fourth.  This suggests that the merging sites 
on the magnetopause mapped to the east and west sides of the 
POLAR trajectory.  The sporadic nature of the particle and field 
measurements indicate that POLAR encountered effects of temporally 
varying coupling between the magnetosphere and the interplanetary medium.  
This would take the form of time-dependent, patchy merging 
poleward of the cusp.  The strong fluxes of He$^{++}$ confirm that energetic 
particles had direct access from the magnetosheath.  The potential turned 
more negative at the end of this interval and the trend continued into the 
region of polar rain fluxes. Significant 
parts of that cell must have extended into the prenoon sector.

	To provide a larger context for the POLAR measurements just discussed 
we have examined 
particle and plasma drift measurements (Plate 4) taken during a nearly 
simultaneous dusk 
to dawn pass made by the Defense Meteorological Satellite Program (DMSP F13). 
Data presented
in the plate show that convection was weak ($<$250 m/s) in the afternoon 
auroral oval, and unmeasurably small in the morning oval. This is consistent 
with results of {\it Heppner 
and Maynard} [1987] who showed that with IMF $B_Y$ and $B_Z > 0$, the morning 
cell is confined to the predawn quadrant.  We note the presence of stronger 
($\sim$500 
m/s) convection along the flanks of the polar cap and an extensive region of 
sunward convection 
in the central polar cap.  These are the standard signatures of four-cell 
convection patterns [{\it Burke et al.}, 1979].        
Ephemeris given beneath the plate indicates that DMSP F13 crossed the noon 
magnetic meridian at about 1321 UT and 85$^{\circ}$ magnetic latitude.  Across 
the 
region of polar rain precipitation plasma convection had a steady sunward 
component.  
This is consistent with POLAR measurements of sunward convection up to 85$^{\
circ}$ 
near the noon meridian. Taken as a whole, the measurements from the two 
satellites 
suggest that the POLAR trajectory carried it along the boundary between 
positive and negative 
potential lobe cells until 1345 UT when it entered the negative potential 
cell.  

 	Synthesizing these three passes with $0 < B_Y < B_Z$ and similar IMF 
clock 
angles, solar wind particles had access  
to the high latitude magnetosphere along several different paths.  In all of 
the POLAR passes HYDRA and TIMAS detected energy-dispersed ion signatures and 
He$^{++}$ 
ion fluxes in regions that couple to the LLBL.  Some of the dispersion 
signatures reflect 
superposition of plasma sheet and magnetosheath populations; others from 
adiabatic 
cooling in flux tubes 
with expanding volumes.  During the April 3 event He$^{++}$ ions entered the 
boundary layer on the flank of the magnetotail well behind the dusk 
terminator.  
The April 8 and May 11 passes provide evidence for the existence of negative 
potential lobe cells that extend into the prenoon MLT sector.  On April 8 a 
small, positive-potential lobe cell was observed  in the early postnoon 
sector.
DMSP F13 measurements show that such a cell was also present at the time of 
the May 11 pass.  Figure 7 presents our empirical synthesis of the convection 
patterns encountered on the three POLAR passes.  It is critical that these not 
be 
regarded as static. Magnetic merging that drives the cells can be both 
temporally and 
spatially varying.  The afternoon boundary layer driven cell can extend into 
the prenoon 
sector.  DMSP measurements indicate that little of the morning side convection 
cell extends past the dawn terminator.

\section{Summary}

This study of the high-latitude dayside electrodynamics based on POLAR 
measurements, 
represents a first view of quasi dc electric fields at middle altitude.  
Comparisons of the measurements with those made at lower altitudes, not only 
provides 
confidence that EFI operated as intended, they also  have provided new insight 
on how 
convection patterns evolve with changing IMF clock angles and about the source 
regions for particles reaching the satellite.  The following conclusions, that 
either 
confirm or add to previous understanding, are derived from our analysis of 
POLAR 
particle and field measurements with northward IMF and with $B_Y > 0$, 
augmented 
with DMSP and WIND data.

1.    The center of a negative potential lobe cell, previously observed 
postnoon for 
larger clock angles [{\it Burke et al.}, 1994], is located prenoon for these 
clock 
angles (${\sim}40^{\circ}$).

2.  A positive potential lobe cell evolves in the postnoon MLT sector as the 
IMF clock angle decreases.

3.  The reverse energy-versus-time ion dispersions on the poleward edge of the 
cusp, observed on April 8 and May 11,are signatures of merging that drives 
lobe-cell convection. 

4.  Significant, isotropic fluxes of He$^{++}$ ions demonstrates that 
particles 
on newly merged field lines have direct access from the magnetosheath.  

5.  The magnetic merging that drives lobe cells may be patchy and/or vary with 
time.

6.  The changing number and position of the lobe cells provides a basis for 
understanding the evolution from distorted two-cell convection patterns at 
large IMF clock angles to four-cell patterns.

7.  Forward-dispersion ion signatures detected near the equatorward edge of 
magnetosheath like fluxes have sources in the LLBL.  Some entry occurs on the 
dawn side of noon and/or tailward of the terminator.

8.  He$^{++}$ ion fluxes were detected on field lines that mapped to the 
equatorial distances tens of R$_E$ down the dusk flank of the magnetotail. 

%-----------------------------------------------------------------
\acknowledgments         

This work was performed under funding from the NASA GGS Program (part of the 
International Solar Terrestrial Physics Program).  Work at UC Berkeley and 
Mission Research Corporation was supported under contract number NAS5-30367 
and grant number 
NAG5-3182. Work at UCLA was supported under grant number NAG5-3171.  Work at 
Iowa was supported under grant NAG S-2231.  WKP was supported by contract 
NAS5-33032. WJB was supported in part by NASA and in part by the U. S. Air 
Force Office of Scientific Research task 2311PL014.  We thank K. W. Ogilvie 
for 
the use of the WIND plasma data.

\newpage
%-----------------------------------------------------------------
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\newpage
%-----------------------------------------------------------------
%Figure Captions
%-----------------------------------------------------------------
\clearpage

%1
\begin{figure}
\figurewidth{35pc}   
\caption{Electric and magnetic field measurements from 1430 -- 1630 UT 
on April 8, 1996.  From top to bottom the panels give the electric field spin-
plane
components $E_{X-Y}$ and $E_Z$, the electric potential $\Phi$ derived from an 
integration along the POLAR trajectory, the plasma-drift $V_{56}$ and magnetic 
field $B_{56}$ component transverse to the orbital plane. We have The display 
quantity is the difference the measured $B_{56}$  and the T96 model field 
component 
[{\it Tsyganenko}, 1996]  Four particle regions described in the text are 
marked at 
the bottom of the figure for reference} 
\end{figure}

%2
\begin{figure}
\figurewidth{35pc}  
\caption{Electric and magnetic field measurements from 1200 -- 1345 UT 
on April 3, 1996, in the same format as Figure 1.}
\end{figure}

%3
\begin{figure}
\figurewidth{35pc}
\caption{Electric and magnetic field measurements from 1300 -- 1400 UT on May 
11, 
1996 in a format similar to Figure 1. Because $E_{56}$ could be calculated 
during this orbit, we have added the plasma velocity component $V_{X-Y}$.  It 
is 
positive when the plasma has an antisunward component of drift.}
\end{figure}

%4
\begin{figure}
\figurewidth{35pc}  
\caption{Schematic representation of ionospheric projection of potential 
distributions 
detected by POLAR near (A) 1515 UT on April 8, 1996 and (B) 1300 UT on April 
3, 
1996.  Directions of plasma flow are indicated by arrows.}
\end{figure}

%5
\begin{figure}
\figurewidth{35pc}  
\caption{Schematic representation of the projection of the regions of He$^{++
}$ fluxes 
detected by POLAR to the equatorial plane using the T96 magnetic field model.}
\end{figure}

%6
\begin{figure}
\figurewidth{35pc}  
\caption{Three-dimensional projection of magnetic field lines crossed by 
POLAR at 5 minute intervals from 1300 to 1400 on May 11, 1996, using the 
T96 magnetic field model.}
\end{figure}


%7
\begin{figure}
\figurewidth{35pc}  
\caption{Schematic representation of high-latitude convection/potential 
patterns encountered by POLAR on April 3 and 8 and May 11, 1996.  Approximate 
orbital traces relative to convection patterns for POLAR and DMSP F13 are 
indicated for reference.}
\end{figure}

\clearpage

%----------------------------------------------------------------------
\newpage
\setcounter{plate} {0}

%1
\begin{plate}
\platewidth{35pc}  
\caption {Particle measurements by TIMAS and HYDRA from 1430 to 1730 UT on 
April 8, 1996 in energy-versus time spectrograms.  The top panel shows 
directional differential fluxes  of He$^{++}$ ions in number 
(cm$^2$ s sr keV/e)$^{-1}$. Count rates for electrons with energies between 
1 eV and 20 keV and ions with energies per charge between 10 eV/e and 20 keV/e 
are given in the middle and bottom panels, respectively.  Note that the 
color-bar scales located to the right of the spectrograms, representing the 
intensities of measured quantities, are not the same for all plates.  The 
dashed lines on HYDRA spectrograms give mean energies derived from 
distribution 
functions associated with observed count rates.  We assume that the positive 
ions are H$^+$. }  
\end{plate}  

%2
\begin{plate}
\platewidth{35pc}  
\caption{Spectrograms in the same format as in Plate 1 from 1100 to 1400 UT on 
April 3, 1996.}
\end{plate}

%3
\begin{plate}
\platewidth{35pc}  
\caption{Spectrograms in the same format as in Plate 1 from 1300 to 1400 UT on 
May 11, 1996.}
\end{plate}

%
\begin{plate}
\platewidth{35pc}  
\caption{Directional differential fluxes of electrons (top) and ions (middle) 
measured by DMSP F13  between 1310 and 1330 UT on May 11, 1996.  The trace in 
the 
bottom panel gives the cross-track component of plasma velocity measured by 
the 
drift meter on the spacecraft.  It is positive in the sunward direction.}
\end{plate}

\clearpage

%-------------------------------------------------------------------------

\newpage
\clearpage
\begin{planotable}{ccrrrrrrrr}  
\tablewidth{41pc}  
\tablecaption{Interplanetary conditions}  
\tablenum{1}  
\tablehead{
\colhead{Date}            & 
\colhead{$X_{GSE} $}      &
\colhead{$Y_{GSE} $}      &
\colhead{$V_{SW}$}        &  
\colhead{$B_X$}           &  
\colhead{$B_Y$}           &       
\colhead{$B_Z$}           &
\colhead{Lag Time}}


\startdata   

4/3/96  & 77 &   2  &  353   & -0.3 $\pm$0.5  &  3.1 $\pm$0.8  &  5.1 $\pm$1.0  

& 25 \nl  

4/8/96  & 82 &  28  &  312   & -1.0 $\pm$0.8  &  6.4 $\pm$0.5  &  6.8 $\pm$0.4  

& 29 \nl  

5/11/96 & 2  & -29  &  300*  &  4.8 $\pm$0.4  &  1.9 $\pm$0.7  &  3.0 $\pm$0.5  

& 0  \nl

\end{planotable}
%-------------------------------------------------------------------------
\end{document}
\end 




