JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A10226, doi:10.1029/2010JA015467, 2010
Quasistatic electric field structures and field‐aligned currents
in the polar cusp region
K. S. Jacobsen,1 J. I. Moen,1 and A. Pedersen1
Received 17 March 2010; revised 17 June 2010; accepted 26 July 2010; published 14 October 2010.
[1] Cluster data have been examined for quasi‐stationary electric field structures and
field‐aligned currents (FACs) in the vicinity of the dayside cusp region. We have
related the measurements to the Region 1/Region 2 (R1/R2) current system and the cusp
current system. It has been theoretically proposed that the dayside R1 current may be
located on open field lines, and experimental evidence has been shown for R1 currents
partially on open field lines. We document that R1 currents may flow entirely on
open field lines. The electric field structures are found to occur at plasma density gradients
in the cusp. They are associated with strong FACs with current directions that are
consistent with the cusp currents. This indicates that the electric field structures are
closely coupled to the cusp current system. The electric equipotential structures linking
the perpendicular electric fields seen at Cluster altitudes to field‐aligned electric fields
at lower altitudes fall into one of two categories: S shape or U shape. Both types are
found at both the equatorward edge of the cusp ion dispersion and at the equatorward edge
of injection events within the cusp. Previous studies in the nightside auroral region
attributed the S‐shaped potential structures to the boundary transition between the
low‐density polar cap and the high‐density plasma sheet, concluding that the shape of
the electric potential structure depends on whether the plasma populations on each side
of the structure can support intense currents. This explanation is not applicable for
the S‐shaped structures observed in the dayside cusp region.
Citation: Jacobsen, K. S., J. I. Moen, and A. Pedersen (2010), Quasistatic electric field structures and field‐aligned currents in
the polar cusp region, J. Geophys. Res., 115, A10226, doi:10.1029/2010JA015467.
1. Introduction
[2] Quasistatic field‐aligned electric fields are the main
mechanism for acceleration of auroral particles. They are
found at geocentric distances below 3.5 Earth radii (RE)
[Mozer and Hull, 2001]. The acceleration region has been
studied, for example, by the S3‐3, DE‐1, Viking, Akebono,
Freja, FAST, Polar, DMSP and Cluster satellites [e.g., Lundin
and Eliasson, 1991; Marklund, 1993; McFadden et al., 1999;
Mozer and Hull, 2001; Olsson and Janhunen, 2003; Hamrin
et al., 2006; Maggiolo et al., 2006; Borg et al., 2007]. Above
3.5 RE, there are no significant field‐aligned electric fields,
but strong bipolar or monopolar electric fields are observed
perpendicular to the magnetic field. The low‐ and high‐
altitude electric fields are connected by a U‐ or S‐shaped
potential structure [Carlqvist and Bostrom, 1970; Mozer et al.,
1980], as illustrated in Figure 2. The effects and the nature
of the electric field structures in the auroral region have
been the subjects of several studies in the last decade
[Marklund et al., 2001, 2004, 2006, 2007; Marklund, 2009;
Johansson et al., 2004, 2006, 2007; Figueiredo et al., 2005].
Marklund et al. [2004] found that the monopolar potential
1
Department of Physics, University of Oslo, Oslo, Norway.
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2010JA015467
structures occurred at the polar cap boundary, while the
bipolar structures occurred at the central plasma sheet/
plasma sheet boundary layer. They suggested that this could
be explained by the different plasma conditions on each
side of the boundaries. The plasma density in the polar cap
is too low to allow significant field‐aligned currents (FAC)
and corresponding ionospheric closure currents, and this
would be reflected in the potential structure. A statistical
study by Johansson et al. [2006] supported this interpretation.
[3] The signature in a energy‐time spectrogram of precipitating electrons accelerated by a field‐aligned electric field
is referred to as an “inverted V.” These are frequently
observed both in the nightside and dayside parts of the
auroral oval [Lin and Hoffman, 1982]. The dayside inverted
Vs are smaller and less energetic than those at the nightside
[Lin and Hoffman, 1982]. Menietti and Smith [1993] investigated the occurrence of dayside inverted Vs and found
that they appeared to span the region of both open and
closed field lines. However, they did not have a reliable
indicator of the exact position of the open‐closed boundary
(OCB). It has later been established that electrons of magnetosheath origin, which arrive in the ionosphere almost
immediately after reconnection, represents the most accurate
observable proxy of the OCB [Lockwood, 1997; Topliss et al.,
2001; Sandholt et al., 2002; Moen et al., 2004; Bogdanova
et al., 2006]. This is often referred to as the “low energy
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Figure 1. Illustration of the dayside current systems.
Upward current is blue, and downward current is red. Note
that this simple illustration does not give a complete picture
of the details, as it is only intended as a helpful reminder of
the general location and direction of the currents. In particular, the R2/R1 currents do not end as abruptly as shown
but are gradually weakened as one gets closer to noon. In
some cases, both the R2/R1 and the cusp currents may be
observed at the same MLT. Also, the cusp current are generally not perfectly centered on noon. See text for a more
complete discussion of current systems.
electron edge” (EE) of the low‐latitude boundary layer
(LLBL). With this reliable method of OCB identification,
the occurrence of the electric field structures associated with
inverted Vs is here placed firmly on open field lines.
[4] Cowley [2000] reviewed the literature concerning current systems in the vicinity of the OCB. The charge separation caused by the partial ring current forces current into
the ionosphere in the dusk hemisphere and out of the ionosphere in the dawn hemisphere. These currents are referred
to as Region 2 (R2) currents, and they flow on closed field
lines. Poleward of the R2 currents lies another current system, the Region 1 (R1) currents, whose currents are of
opposite polarity to the R2 currents. R1 currents are traditionally thought to flow on closed field lines to the outer
part of the plasma sheet, through which they are indirectly
linked to the solar wind dynamo [Stern, 1983; Cowley,
2000]. Stern [1983] stated that at local times near noon it
is plausible that the R1 current map directly to the solar
wind dynamo on open field lines, but noted that the experimental evidence for this was far from clear‐cut. There have
been some observations of R1 currents partially on open
field lines [de la Beaujardiere et al., 1993; Xu and Kivelson,
1994; Lopez et al., 2008]. It has been proposed that when
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the polar cap potential is saturated, the R1 current replaces
the Chapman‐Ferraro current as the primary force balance
versus the solar wind pressure [Siscoe et al., 2002a; Siscoe
et al., 2002b]. This requires a part or all of the R1 current to
flow on open field lines [Siscoe, 2006].
[5] At higher latitudes on the dayside there is a current
system associated with the dayside cusp, which is modulated in location and strength by the interplanetary magnetic field (IMF). The nature of these currents is discussed
by Taguchi et al. [1993], who referred to them as the low‐
latitude cleft current (LCC) and the high‐latitude cleft current (HCC). In this paper they are referred to as cusp
currents. They appear as pairs of latitudinal bands of FAC
around noon, with FACs in opposite directions. In the
Northern Hemisphere, the most equatorward FAC band will
be downward/upward for IMF BY positive/negative, respectively, and lies on the equatorward edge of the cusp or on
the equatorward edge of an injection event [Taguchi et al.,
1993]. The direction of the currents is opposite for the
Southern Hemisphere. The R2/R1 current system is at its
weakest at noon, where the cusp current is most likely to be
observed. Thus, it is not uncommon that only one of the
current systems is observed during a pass of the auroral
region. A simple illustration of the R2/R1 and cusp current
systems is shown in Figure 1.
[6] In this study we investigate electric field structures in
and around the OCB and the cusp on the dayside, using
Cluster data from 2001 to 2005. We place the occurrence
of bipolar and monopolar electric field structures in relation to the OCB and the plasma boundaries in the cusp
region. The main subject of this study is the relation
between the electric field structures observed in the cusp
region and the cusp current system. The cusp currents are
associated with the cusp or with plasma injections within
the cusp, and the direction of the cusp currents is controlled
by IMF BY. The statistical results on this subject are presented in section 3.5, with some single cases presented in
sections 3.1 and 3.2. A secondary subject is observations of
the R1 current system on open magnetic field lines. The
direction of the R1 current is downward in the dawn
hemisphere and upward in the dusk hemisphere. For this
subject, the number of cases is insufficient for a statistical
study. One very clear case is presented in section 3.6, and
two more cases are briefly discussed.
2. Method
[7] This study is based on data from the Cluster mission
[Credland et al., 1997; Credland and Schmidt, 1997] for the
years 2001 to 2005. Midaltitude (4–6 RE) cusp passes were
examined for electric field structures, using data from the
EFW instrument [Gustafsson et al., 1997]. Due to the
apogee of the Cluster orbit being below the equatorial plane,
Northern Hemisphere cusp crossings have a better geometry for midaltitude cusp crossings. Because of this, there
is only a small number of Southern Hemisphere crossings
for this study. The perpendicular electric field structures
measured at the Cluster orbit altitude are classified into
four categories; Converging, diverging, monopolar(+) and
monopolar(−).
[8] Bipolar electric field structures, which are either
converging or diverging, are associated with an U‐shaped
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Figure 2. Illustration of the electric potential structures linking high‐altitude and low‐altitude electric
fields. Not to scale. (a) U‐shaped potential structures for the North Hemisphere and South Hemisphere
cusp. (b) S‐shaped potential structures. The blue arrows show the general path of the Cluster satellites
through the cusps. The North Hemisphere pass shown in Figure 2b would produce a monopolar(−) signature in the electric field, as the electric field points in the direction opposite to the Cluster velocity
vector.
potential. See Figure 2 for an illustration of potential
structures. They are identified by a pair of electric field
excursion of opposite polarity, and a “hill”/“valley” in the
electric potential for a diverging/converging electric field,
respectively. A converging electric field is a sign of an
upward electric field at lower altitudes (<3.5 RE) and is
associated with an upward FAC. A diverging electric field
is a sign of a downward electric field at lower altitudes and
is associated with a downward FAC [Lyons and Speiser,
1985; Johansson et al., 2006]. Figure 2a shows U‐shaped
potential structures with converging electric fields. Both a
Southern Hemisphere pass and a Northern Hemisphere pass
would see the same signature; first an electric field pointing in
the direction of the spacecraft propagation, then an electric
field pointing in the opposite direction. At low altitudes
there is an upward electric field.
[9] Monopolar electric fields result in either an upward or
a downward electric field at lower altitudes, depending on
the configuration of the equipotential lines. They are identified by a single electric field excursion and a step in the
electric potential. The (+) and (−) designation is given based
on the sign of the monopolar electric field as seen when
projected along the velocity vector of the spacecraft. In the
Northern Hemisphere the cusp crossings are poleward and
in the Southern Hemisphere the cusp crossings are equatorward. Figure 3 is a polar dial plot of the cusp passes
included in the results presented in section 3.5 and shows
the orientation of the cusp crossings and the MLT span
covered. Note that the cusp passes are roughly radial.
Figure 2b shows S‐shaped potential structures with monopolar electric fields. The Northern Hemisphere pass sees a
monopolar electric field vector pointing in the opposite
direction of the spacecraft velocity vector, while the
Southern Hemisphere pass sees a monopolar electric field
pointing in the direction of the spacecraft motion. Whether
the low‐altitude electric field points upward or downward
depends on which way the electric potential lines are bent.
In Figure 2 they are shown bent toward the OCB, which
results in low‐altitude electric fields as shown in Figure 2. If
they were bent away from the OCB, the low‐altitude electric
field would point in the opposite direction. As the associated
FAC depends on the direction of the low‐altitude electric
field, we can determine which way the electric potential
lines were bent by considering the sign of the monopolar
electric field signature, the direction of the FAC and in
which hemisphere the measurements were taken.
[10] Forty‐nine cases were identified and manually inspected.
To determine if the electric fields structures are on open or
closed magnetic field lines, the OCB is identified by the EE
as seen by the electron spectrometer (PEACE) [Johnstone
et al., 1997] and the drop in high‐energy electron flux as
seen by the high‐energy electron instrument (RAPID)
[Wilken et al., 1997]. Events where IMF data was not
available, and cases where the IMF BY and/or BZ fluctuated
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Figure 3. Polar plot of the location of the cusp passes used for the statistics shown in Tables 2 and 3.
The cross marks the OCB, and the line extends to the end of the cusp. There are four cusp passes in which
two electric field structures were observed during the pass. Because of this, the number of lines do not
match the number of electric field structures in the tables. Passes with a negative IMF BY are colored blue,
and passes with a positive IMF BY are colored red.
between positive and negative just prior to or during the
electric field structure encounter, were removed. Events with
severely distorted cusp signatures as seen in the particle
spectrograms were also removed. The remaining events
were inspected for association with plasma boundaries, as
seen in density variations and energy‐time spectrograms
measured by the ion and electron spectrometers (CIS [Reme
et al., 1997] and PEACE). The CIS instrument of Cluster 2
does not work, so for these cases the boundary identifications are based on electron spectra and spacecraft potential
variations only. The spacecraft potential can be used as a
proxy for the plasma density [Escoubet et al., 1997], unless
the active spacecraft potential control [Riedler et al., 1997]
is operating. There is some difficulty in applying the
spacecraft potential density proxy in the cusp due to the high
density and variable temperature, resulting in uncertainties
in the absolute value of the density. Thus, although the
spacecraft potential has been used to identify density gradients, no corresponding density values were calculated.
Twenty‐seven cases were associated with plasma boundaries. After this inspection the events were sorted into two
groups, one with BY > 1 nT and one with BY < 1 nT, as the
direction of the cusp current system depends on the sign on
BY [Taguchi et al., 1993]. Although the data were not filtered by IMF the final set of events used in the study are
dominated by southward IMF, as this tends to produce a
cleaner, more orderly cusp.
[11] To determine the FACs associated with the electric
field structures, the residual magnetic field was calculated
from FGM [Balogh et al., 1997] magnetic field measurements by subtracting a fourth degree polynomial fit of the
magnetic field in an extended period around the event. The
FACs were then calculated under the assumption that they
consist of field‐aligned current sheets. These sheets are
assumed to lie perpendicular to the spacecraft trajectory,
and if not the actual current will be greater than the one
calculated.
[12] Some events proved suitable to address the subject of
the R2/R1 current system, specifically the location of the R1
current. These are not included in the statistics presented in
section 3.5, as they are clearly not cusp currents.
3. Results
3.1. Examples of Monopolar Electric Field Events
[13] Figure 4e shows an example of a monopolar(+)
electric field event observed by Cluster 1 on 9 October
2002, marked by the vertical lines. A clear positive electric
field excursion (Figure 4e) coincided with a plasma injection signature within the cusp, recognized by the sudden
appearance of precipitating ions of greater energy than the
background at 04:01:25 UT, and the subsequent energy
dispersion of these (Figure 4a). Enhanced electron fluxes at
greater energies than the background were also observed
(Figure 4c). At this time there was a sudden increase of ion
density from 7 to 17 cm−3 (Figure 4g). Although the
spacecraft potential (Figure 4h) is not required here, as ion
density measurements were available, it has been included
to show the similarity between the spacecraft potential graph
and the ion density graph. The IMF was stable, with a
positive Y component (Figure 4i). The potential difference
across the structure was 1.7 kV (Figure 4f), and the electric
field strength peaked at 20 mV/m (Figure 4e). A downward
FAC was observed at the center of the structure (Figure 4j).
The combination of a monopolar(+) electric field and a
downward FAC for a Northern Hemisphere pass indicates
that the electric potential lines were bent equatorward
toward the OCB, as discussed in section 2.
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Figure 4. Example of a monopolar(+) electric field structure observed by Cluster 1 on 9 October 2002
during a North Hemisphere cusp pass. Vertical lines mark the start and end of the electric field structure.
(a) ION+, downward ion energy flux; (b) ION*, upward ion energy flux; (c) EL+, downward electron
energy flux; (d) EL*, upward electron energy flux; (e) EFI, electric field projected along the velocity vector; (f) POT, integrated potential along the spacecraft trajectory; (g) IDNS, ion density; (h) SCP, inverse
spacecraft potential; (i) IMF, time‐shifted IMF (green, GSM X; red, GSM Y;blue, GSM Z; IMF data
shifted to the bow shock was obtained from OMNIWeb, and 10 min were added to account for the time
delay to the cusp); (j) FAC, FAC with positive (blue) being upward and negative (red) being downward.
(In addition, the label ENEL is used in other figures and is defined as “Energetic (≥50 keV) electron
energy flux.”)
[14] Figure 5d shows an example of a monopolar(−)
electric field event observed by Cluster 2 on 30 August
2001. A negative electric field excursion (Figure 5d), which
for this case is easier to spot in the potential plot (Figure 5e),
was seen in the cusp. At this time, the electron spectra
showed a substantial increase in the electron flux as well as
the electron energy (Figures 5b and 5c). The rapid drop in
the energetic electron flux (Figure 5a) and the sudden
appearance of disturbed electron spectra (Figures 5b and 5c)
seen at 15:31:40 UT indicate that Cluster passed the OCB at
this time. Thus, the sudden increase seen in the electron
spectra is an indicator of the equatorward edge of the cusp
bulk injection. The variation of the spacecraft potential
indicates a moderate density increase within the structure
(Figure 5f), but due to reasons mentioned in section 2 cannot
be taken as absolute density values. Note that the enhanced
inverse spacecraft potential in the time interval 15:31:00–
15:31:40 UT was caused by the high energetic electron flux
at closed magnetic field lines. The IMF BZ changed from
negative to positive shortly after exiting the structure, but BY
was stable at −4 to −6 nT (Figure 5g). The potential dif-
ference across the structure was 5.5 kV (Figure 5e), and
the electric field strength peaked at 48 mV/m (Figure 5d).
An upward FAC was observed throughout the structure
(Figure 5h). This indicates that the electric potential lines
were bent toward the OCB in this case also.
3.2. Examples of Bipolar Electric Field Events
[15] Figure 6d shows an example of a diverging electric
field event observed by Cluster 2 on 25 August 2001,
marked by the vertical lines. The very low energetic electron
flux (Figure 6a) shows that the spacecraft was at open
magnetic field lines. A prominent bipolar electric field
structure (Figure 6d) coincided with the equatorward edge
of the cusp bulk injection (Figure 6b). The variation of the
spacecraft potential indicates a drastic density increase
within this electric field transition region (Figure 6f). The
IMF was stable and dominated by BY, which had a value of
11 nT (Figure 6g). The height of the potential hill was 4 kV
(Figure 6e), and the electric field strength peaked at 72 mV/m
(Figure 6d). A strong downward FAC was observed
throughout the structure (Figure 6h), and the upward elec-
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Figure 5. Example of a monopolar(−) electric field structure observed by Cluster 2 on 30 August 2001
during a North Hemisphere cusp pass. Vertical lines mark the start and end of the electric field structure.
The labels are defined in the caption of Figure 4.
tron energy flux spectrogram clearly shows accelerated
electrons (Figure 6c) with predominantly sub‐keV energies.
[16] Figure 7d shows an example of a converging electric
field event observed by Cluster 3 on 22 September 2002. A
pair of electric field excursions (Figure 7d) were observed at
the beginning of a cusp ion dispersion signature (Figure 7b).
Electron spectra from PEACE were not available, but
energetic electrons measurements from RAPID (Figure 7a)
may be used to show that the event takes place on open field
lines. The energetic electron flux is quite low throughout the
entire interval, but show a clear break at 12:30:25 UT. The
OCB is located at this point or, if there is leakage of energetic electrons onto open field lines, some small distance to
the left in the plot. The event clearly takes place on open
field lines. At the same time the ion density increased from
0.1 to 2 cm−3 (Figure 7f), as part of a larger density increase
which eventually reached a density of 12 cm−3. The
spacecraft potential graph (Figure 7g) matches the density
graph. The IMF was stable, with a Y component of −4 nT
(Figure 7h). The depth of the potential valley was 0.6 kV
(Figure 7e), and the electric field strength peaked at 20 mV/m
(Figure 7d). An upward FAC was observed throughout the
structure (Figure 7i).
3.3. Lifetime of Electric Field Structures
[17] For some events similar electric field structures were
seen by several Cluster spacecraft, giving a measure of the
minimum lifetime of these structures. Table 4 shows a list of
those cusp passes where a similar structure was observed by
3 or 4 spacecraft. Note that not all of the individual cusp
passes by the different Cluster spacecraft were of sufficient
quality to be used for all other parts of this study, as discussed in section 2. Figure 8 shows the day with the largest
time difference between the first and last observation. The
spacecraft have been sorted by the order in which they
encountered the cusp, and particle spectra are included to
indicate its position. The main feature of the electric field is
the negative peak observed near the equatorward edge of the
cusp. Although the shape of the electric field graph varies
from spacecraft to spacecraft, the negative peak is present
for all of them, which indicates a lifetime of at least 50 min
for that particular structure.
3.4. Association With FACs
[18] Table 1 shows the direction of the FAC associated
with each type of electric field structure. As expected,
converging/diverging electric fields are associated with an
upward/downward FAC, respectively. Each type of monopolar electric field structure (±) was associated with one
direction of FAC, negatively peaking electric field structures
being associated with an upward FAC and positively
peaking electric field structures being associated with a
downward FAC. This means that the electric potential lines
were bent the same way, toward the OCB, for each cusp
pass. This kind of electric potential configuration is illustrated in Figure 2b.
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Figure 6. Example of a diverging electric field structure observed by Cluster 2 on 25 August 2001 during a North Hemisphere cusp pass. Vertical lines mark the start and end of the electric field structure.
The labels are defined in the caption of Figure 4.
3.5. Association With Cusp Plasma Boundaries
and IMF BY
[19] In most cases, the electric field structures were associated with a plasma boundary, either at the equatorward
edge of the cusp or a spatial structure/injection event within
the cusp precipitation region. The results are summarized in
Tables 2 and 3 for a positive and negative IMF BY,
respectively. Most of the electric field events were found at
or very close to the equatorward cusp ion edge. Poleward of
this, some events were associated with the equatorward edge
of a distinct ion injection event and some with structures
within the cusp which could not be positively identified as
injection events. 2 of the 3 unidentified events lacked ion
data, and did not have any clear signatures in the remaining
parameters. The last unidentified event had ion data, but the
signature was not consistent with an injection event. No
preference for monopolar or bipolar electric field structures
were found at any of these boundaries, although for positive
IMF BY only bipolar electric field structures were observed
near the equatorward edge of the cusp. For BY positive,
electric field structures whose associated current (see Table 1)
was consistent with the expected polarity of the cusp current
were observed in 9 of 11 cases. For BY negative, electric
field structures whose associated current was consistent with
the expected polarity of the cusp current were observed in
14 of 16 cases. Figure 3 shows the location of the cusp
passes presented in Tables 2 and 3.
3.6. Location of the R1 Currents
[20] Among the cusp passes investigated during this
study, there were some which provided very clear measurements of the R2/R1 currents. Figure 9 shows the 23 August
2004 cusp pass by Cluster 3. Two areas of FAC are clearly
visible (Figure 9e). A downward current was observed on
closed magnetic field lines, increasing in intensity closer to
the OCB. Immediately after passing the OCB, an upward
current was observed. At this time Cluster 3 was at a
postnoon MLT, and the SuperDARN [Greenwald et al.,
1995; Chisham et al., 2007] image (Figure 9f) confirms
that the foot point of Cluster 3 entered the dusk convection
cell. Thus we expect a downward R2 current and an upward
R1 current. IMF BY was positive (Figure 9d), which means
that if cusp currents were present we expect the direction of
the equatorward cusp current sheet to be downward.
[21] The downward current on closed field lines is identified as an R2 current (The FACs are shown in Figure 9e).
The upward current on open field lines is consistent with the
direction of the R1 current but not with the direction of the
most equatorward cusp current sheet. Thus, it seems that this
was indeed the R1 current, which at local times near noon
has been theorized to lie on open field lines [Stern, 1983].
Two other cases like this were also found, but are not shown
here. The 2 September 2003 cusp pass by Cluster 2, at
∼13:00 MLT, presented a very similar current picture to the
one shown here, but with a negative BY. The 5 October 2003
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Figure 7. Example of a converging electric field structure observed by Cluster 3 on 22 September 2002
during a North Hemisphere cusp pass. Vertical lines mark the start and end of the electric field structure.
The labels are defined in the caption of Figure 4.
cusp pass by Cluster 1 took place at ∼09:15 MLT with
currents as expected for a prenoon pass, with upward R2
currents located on closed magnetic field lines and downward R1 currents on open magnetic field lines.
[22] In the electric field shown in Figure 9c, there was a
converging electric field structure in the first half of the
upward current region, where the upward current was most
intense. An electric field structure was also present at the
boundary between the R2 and R1 current systems for the
two other cases mentioned earlier.
4. Discussion
[23] Five years of Cluster data have been examined for
electric field structures in the vicinity of the OCB and the
cusp. The perpendicular electric fields at high altitudes
(Cluster orbit) are connected to low‐altitude field‐aligned
electric fields through electric potential structures. For
bipolar electric fields at high altitudes the electric potential
structure is U shaped, while for monopolar electric fields the
electric potential structure is S shaped.
[24] The discussion has been divided into three parts,
covering different topics. Section 4.1 covers the topic of
electric field structures in the cusp. The similarities and
differences between this dayside study and previous nightside
studies, and the connection to the cusp currents, are discussed. Section 4.2 concerns the observations of R1 current
on open field lines. Section 4.3 discusses our observations
in relation to previous studies regarding the location of
dayside inverted Vs and a related phenomenon referred to as
a reversed flow event [Rinne et al., 2007; Moen et al.,
2008]. This is followed by a summary of the main results
in section 5.
4.1. Electric Field Structures in the Cusp Region
[25] Observations of the minimum lifetime of the electric
field structures are summarized in Table 4. Minimum lifetimes range from some minutes to some tens of minutes,
with one cusp pass indicating a lifetime exceeding 50 min
(see Figure 8). Minimum lifetimes on the order of minutes to
tens of minutes have been observed in previous studies
[Johansson et al., 2004, 2006; Marklund et al., 2001, 2004;
Figueiredo et al., 2005]. Thus, it is reasonable to treat the
electric fields and electric potential distributions as static
structures during the time it takes for Cluster to travel
through them, which is typically on the order of a minute or
less.
[26] Marklund et al. [2004] found that the monopolar
electric field structures occurred at the polar cap boundary,
while the bipolar structures occurred at the central plasma
sheet/plasma sheet boundary layer, and suggested that this
was due to the different plasma conditions on each side of
the boundaries. A statistical study of nightside auroral
electric fields by Johansson et al. [2006] supported this
interpretation. In this study we have examined electric fields
in the dayside cusp to see if there is a similar connection
there. The relevant boundaries are the equatorward edge of
the cusp ion dispersion and the equatorward edge of injec-
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Figure 8. Plot of the quasi‐static electric field structure with the longest observed minimum lifetime.
The four Cluster spacecraft have been sorted in the order in which they encountered the cusp. The labels
are defined in the caption of Figure 4.
tion signatures within the cusp. In addition, some plasma
structures within the cusp were not clear enough to be
positively identified as an injection signature and are
marked as unidentified. While the electric field structures
are observed to coincide with the plasma boundaries, there
was no preference found for either type of structure at any of
the boundaries. For positive IMF BY only bipolar electric
field structures were observed at the equatorward edge of
the cusp, but as the results for negative IMF BY show
6 bipolar and 5 monopolar electric field structures at the
equatorward edge of the cusp we cannot say that the
equatorward edge of the cusp ion dispersion is uniquely
associated with either type of electric field structure (bipolar
or monopolar). In the study by Johansson et al. [2006],
monopolar electric field structures were uniquely associated
with the polar cap boundary, at which the density on average
increased more than 500% from a value below 0.3 cm−3.
The bipolar electric field structures were uniquely associated
with plasma boundaries within the plasma sheet, where the
density on average increased 153% from a value somewhere
in the range of 0.1–3.0 cm−3. To emphasize the difference
between the results of that study and this study, we have
included some examples with an opposite behavior. In
Figure 4a monopolar electric field coincided with an injection signature within the cusp, at which the density
increased from 7 to 17 cm−3. In Figure 7a bipolar electric
field coincided with the start of the ion dispersion signature,
at which the density increased from 0.1 to 2 cm−3. Although
we did also observe events that fit the pattern of Johansson
et al. [2006], the number of events that do not fit the pattern
is significant: 4 of 8 events that took place at a plasma
boundary within the cusp had a monopolar electric field
structure, while 13 of 19 events that took place at the
equatorward edge of the cusp ion dispersion had a bipolar
electric field structure. This difference suggests that the
processes and/or conditions creating the electric field
structures in the dayside cusp are not to be controlled by the
density gradient at the boundary layer transition alone.
[27] The cusp current system [Taguchi et al., 1993] drives
current associated with the cusp and injection events in the
cusp. The close relationship between an imposed current
and the appearance of field‐aligned electric fields has been
studied by Knight [1973], Chiu and Schulz [1978] and
Lyons [1980]. The direction of the current flowing on the
equatorward edge of the cusp or an injection depends on the
sign of the Y component of the IMF. If the electric field
structures are related to the cusp current system, their form
should also depend on IMF BY. The events were sorted by
Table 1. Direction of FAC Current Associated With the Different
Types of Electric Field Structure
Converging
Diverging
Monopolar(+)
Monopolar(−)
9 of 13
Upward FAC
Downward FAC
Unclear
19
0
0
10
0
10
7
0
1
1
1
0
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JACOBSEN ET AL.: ELECTRIC FIELDS AND FACS IN THE CUSP
Table 2. Electric Field Structures Associated With Different Dayside Plasma Boundaries in the Cusp Region for
Positive IMF BY Conditionsa
BY > 1 nT
Equatorward cusp ion edge
Equatorward edge of injection structure
Unidentified boundary
Converging
Diverging
Monopolar(+)
Monopolar(−)
1 + 2S
0
1
3
2
0
0
2
0
0
0
0
a
The “+2S” in the upper left part of the table means that two converging electric field structures were observed during South
Hemisphere cusp passes. As the direction of the cusp currents is opposite in the two hemispheres [Taguchi et al., 1993], this is
equivalent to a diverging electric field structure for a North Hemisphere pass.
the sign of BY, requiring the magnitude of BY to be at least
1 nT, with the results shown in Tables 2 and 3. There is a
clear abundance of Converging and Monopolar(−) for negative BY, and an abundance of Diverging and Monopolar(+)
for positive BY. As seen in Table 1, each type of electric
field is associated with either upward or downward FAC,
and the pairs of structures occurring commonly for each BY
sign are associated with the same FAC. The FACs associated with the electric field structures are consistent with the
expected cusp currents, indicating that the electric fields are
closely coupled to the large‐scale current system. While this
explains the observations of electric field structures matching the FACs, it does not explain why these are sometimes
monopolar and sometimes bipolar. There is one major difference between a dayside and nightside pass through the
OCB. During a nightside pass, the OCB marks a sharp
boundary between the dense plasma sheet boundary layer
and the dilute plasma of the polar cap. However, during a
dayside pass, the OCB marks the boundary between the
central plasma sheet and the cusp, followed by a gradual
transition to the polar cap. (See Newell et al. [2004] and
references therein for statistics and other information
regarding precipitation regions.). It is important to note that
the density values derived from CIS do not, in some cases,
provide enough information to assess the capability of the
plasma to carry a current. While in the study by Johansson
et al. [2006] the low‐density plasma is found on the open
field lines of the polar cap, in this study the low‐density
plasma is found on the closed field lines of the central
plasma sheet. The central plasma sheet contains a relatively
large amount of high‐energy electrons and ions [Baumjohann
et al., 1989], so even if the density measured by CIS is as
low as the nightside polar cap measurements, the high‐
energy electrons and ions can act as efficient current carriers. The explanation suggested to explain the occurrence of
bipolar and monopolar electric field structures in the
nightside auroral study by Johansson et al. [2006] was that
the bipolar electric field structures appeared when the
field‐aligned current could couple in both (equatorward
and poleward) directions, while the monopolar structures
appeared when the current could only couple in one direction. The reason for the monopolar fields observed at the
polar cap boundary, which is the boundary between high‐
and low‐density plasmas on the nightside, was thus that the
polar cap plasma could not support any significant currents.
In this study we have observed a mix of bipolar and
monopolar electric field structures, both at the equatorward
edge of the cusp, which is a boundary between high‐ and
low‐density plasmas, and at the equatorward edge of
injection events within the cusp. As the plasma should be
able to support currents throughout the plasma sheet, cusp
and injection regions, [de la Beaujardiere et al., 1993;
Cowley, 2000; Taguchi et al., 1993] the monopolar electric
field structures we have observed cannot be explained in the
same manner as those at the nightside.
4.2. R1 Current on Open Magnetic Field Lines
[28] According to Stern [1983], it is plausible that at local
times near noon the R1 current lies on open magnetic field
lines. He noted, however, that the experimental evidence for
this was far from clear‐cut. Also, more recent MHD simulations indicate that some part of the R1 current must
sometimes flow on open field lines [Siscoe et al., 2002a;
Siscoe et al., 2002b; Siscoe, 2006]. There have been some
observations of R1 currents partially on open field lines [de
la Beaujardiere et al., 1993; Xu and Kivelson, 1994; Lopez
et al., 2008], but few, if any, observations of R1 current
flowing entirely on open field lines. We note that those
studies were performed with data from the dusk and dawn
regions, covering MLT ranges of ∼7:30–10:30 and ∼16:00–
20:00, while this study covers a near‐noon MLT range of
∼9:00–14:00. Figure 9 shows data from a cusp pass by
Cluster 3 on 23 August 2004. The pass takes place at a
postnoon MLT, and SuperDARN (Figure 9f) confirms that
the foot point of the spacecraft entered the dusk convection
cell. The directions of the dusk R2/R1 currents are downward/upward, respectively. The BY component of the IMF
was positive throughout the pass (Figure 9d). For IMF BY
positive the direction of cusp current at the equatorward
edge of the cusp is downward. The OCB is clearly recog-
Table 3. Electric Field Structures Associated With Different Dayside Plasma Boundaries in the Cusp Region for
Negative IMF BY Conditions
BY < −1 nT
Equatorward cusp ion edge
Equatorward edge of injection structure
Unidentified boundary
Converging
Diverging
Monopolar(+)
Monopolar(−)
6
0
1
1
0
0
1
0
0
5
1
1
10 of 13
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JACOBSEN ET AL.: ELECTRIC FIELDS AND FACS IN THE CUSP
Figure 9. Observations by Cluster 3 on 23 August 2004 during a North Hemisphere cusp pass. Downward R2 current is observed up to the OCB, while R1 current is observed on open field lines. The OCB is
marked by a vertical line. (a–e) Panels are marked by labels as defined in the caption of Figure 4. (f) A
convection map from SuperDARN, with the FAC shown in Figure 9e plotted along the foot point track.
nized by the EE in the electron spectrogram (Figure 9b), and
is marked by a vertical line. There are two distinct regions of
FAC, one downward at closed field lines and the other
upward on open field lines (Figure 9e). The current on
closed field lines is consistent with the R2 current. The R1
current is normally located immediately poleward of the R2
current. The current on open field lines flows in the same
direction as expected for the R1 current, but in the opposite
direction of what we expect for the most equatorward cusp
current sheet. We conclude that the upward current observed
on open field lines is indeed the R1 current. There is a
converging electric field structure in the upward FAC
region, indicating the presence of an inverted V at lower
altitudes. The electric field structure was also observed by
two other Cluster spacecraft, with a time difference of nearly
12 min between the first and last observation. The current
seen in Figure 9e has been plotted along the foot point track
of Cluster in the SuperDARN plot of Figure 9f. The
Table 4. Number of Spacecraft N That Observed a Similar Electric
Field Structure and Time T between the First and Last Observation
Date
N
T (s)
Type
4 Oct 2002
23 Aug 2004
21 Aug 2003
4 Sep 2003
30 Aug 2004
21 Sep 2003
4
3
3
3
3
3
3000
800
75
113
1225
180
Monopolar(−)
Converging
Converging
Converging
Converging
Monopolar(+)
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boundary between the R2 and R1 currents appear to coincide with the equatorward edge of the cusp convection. Two
other events which show R1 current flowing fully on open
field lines have also been mentioned (see section 3.6).
4.3. Inverted V Related Phenomena
[29] The signature in an energy‐time spectrogram of
precipitating electrons accelerated by a field‐aligned electric
field is referred to as an “inverted V.” Menietti and Smith
[1993] presented several observations of inverted Vs in
the region generally called the low‐latitude boundary layer
or cusp boundary layer, noting that the ion convection and
energy dispersion appeared to span both closed and open
magnetic field lines. They suggested that if reconnection
was happening at the time, the dayside inverted V may
signify a potential difference on a reconnecting field line.
Since then, reliable techniques for OCB identification have
been developed. Cluster, passing the cusp at an altitude of
4–6 RE, do not observe the inverted V particle spectrogram
signature but may instead see electric field structures that
signify the presence of a field‐aligned electric field at lower
altitudes. Of the 49 electric field structures identified, 2 were
located at the OCB and the rest were observed on open field
lines. These results indicate that the inverted Vs lie on open
field lines.
[30] In a study of ionospheric flow channels using the
European incoherent scatter radar at Svalbard, Rinne et al.
[2007] introduced the term reversed flow event (RFE) to
describe small and transient channels of plasma flowing
opposite to the background flow in the cusp. Moen et al.
[2008] further investigated the RFE phenomenon, and
found that each RFE was related to a thin auroral form,
consistent with a converging electric field and an upward
Birkeland current. While some RFEs moved poleward along
with poleward moving aurorals forms (PMAF), others
stayed with the background arc even though PMAFs were
observed. Two possible explanations for the RFE phenomenon were suggested. One of those explanations was that the
RFE might be the ground signature of an inverted V
acceleration region. As has been noted before, the observation of a converging electric field by Cluster is a signature
of an inverted V at lower altitudes. In this study we have
observed such electric field structures in association with
two different current systems; The cusp currents at the
equatorward edge of the cusp or a plasma injection, and the
R2/R1 current system. The minimum lifetime of these
electric field structures range from some minutes to some
tens of minutes, which is comparable to the average RFE
lifetime of ∼19 min [Rinne et al., 2007]. As a plasma
injection moves poleward, the associated FAC and electric
field structure will move with it. This could explain the
RFEs that move poleward along with a PMAF. RFEs that
stay with the background arc could be caused by the electric
field structure associated with the transition from the R2
current region to the R1 current region, as suggested by
Moen et al. [2008].
5. Summary
[31] Electric fields in the cusp region have been investigated, using five years of Cluster data. The main results are
as follows:
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[32] 1. Electric field structures which can be associated
with inverted Vs are observed only on open magnetic field
lines.
[33] 2. The electric equipotential lines of S‐shaped potentials in the dayside cusp region are bent toward the OCB.
[34] 3. As in a previous study of electric field structures in
the nightside auroral region by Johansson et al. [2006], the
electric field structures are commonly associated with a
plasma boundary, which in this study was either the equatorward cusp ion edge or the equatorward edge of an injection
event. However, the type of structure (monopolar or bipolar)
was not uniquely determined by which of those boundaries it
occurred at, nor by the density increase across the structure.
[35] 4. Each type of electric field structure (converging,
diverging, monopolar(+) or monopolar(−)) was associated
with either an upward or a downward FAC. The observed
FACs were in most cases consistent with the expected
direction of the cusp current for the IMF conditions at the
time, which indicates that the electric field structures are
closely coupled to the cusp current system.
[36] 5. A rare observation of R1 current flowing entirely
on open magnetic field lines has been documented.
[37] 6. The electric field structures investigated in this
study may be the driving force behind the RFE phenomenon, as suggested by Moen et al. [2008].
[38] Acknowledgments. We acknowledge the CIS, PEACE, RAPID,
FGM, and EFW instrument teams and the ESA Cluster Active Archive for
the use of Cluster data. We acknowledge the ACE instrument team for the
use of ACE data. Time‐shifted ACE data were obtained from the GSFC/
SPDF OMNIWeb interface at http://omniweb.gsfc.nasa.gov. Operation of
the SuperDARN radars is supported by agencies of the United States,
Canada, the United Kingdom, and France. SuperDARN convection
maps were obtained from the JHU/APL SuperDARN Web site at http://
superdarn.jhuapl.edu. This work has been supported by the Norwegian
Research Council.
[39] Robert Lysak thanks the reviewers for their assistance in evaluating this paper.
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