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Ann. Geophys., 33, 333–344, 2015
www.ann-geophys.net/33/333/2015/
doi:10.5194/angeo-33-333-2015
© Author(s) 2015. CC Attribution 3.0 License.
Outflow of low-energy O+ion beams observed during
periods without substorms
G. K. Parks1, E. Lee2, S. Y. Fu3, M. Fillingim1, I. Dandouras4, Y. B. Cui3, J. Hong2, and H. Rème4
1Space Sciences Laboratory, University of California, Berkeley, CA, USA
2School of Space Research, Kyung Hee University, Yongin, Gyeonggi, Korea
3School of Earth and Space Sciences, Peking University, Beijing, China
4CNRS, IRAP, 9 Ave. Colonel Roche, Toulouse, France
Correspondence to: G. K. Parks (parks@ssl.berkeley.edu)
Received: 26 November 2014 – Revised: 6 February 2015 – Accepted: 25 February 2015 – Published: 17 March 2015
Abstract. Numerous observations have shown that ions flow
out of the ionosphere during substorms with more fluxes
leaving as the substorm intensity increases (Wilson et al.,
2004). In this article we show observations of low-energy
(few tens of electron volts) ionospheric ions flowing out
periods without substorms, determined using the Wideband
Imaging Camera (WIC) and Auroral Electrojet (AE) indices.
We use Cluster ion composition data and show the outflow-
ing ions are field-aligned H+, He+and O+beams acceler-
ated to energies of ∼40–80 eV, after correcting for spacecraft
potential. The estimated fluxes of the low-energy O+ions
measured at ∼20 000 km altitude are >103–105cm−2s. As-
suming the auroral oval is the source of the escaping ions,
the measured fluxes correspond to a flow rate of ∼1019–
1021 ions s−1leaving the ionosphere. However, periods with-
out substorms can persist for hours suggesting the low-
energy ions flowing out during these times could be a major
source of the heavy ion population in the plasma sheet and
lobe.
Keywords. Magnetospheric physics (Magnetosphere–
ionosphere interactions)
1 Introduction
The ionospheric ions that flow out into the magnetosphere
include the polar wind, upwelling ions from the cusp, polar
cap, and ion beams accelerated in the aurora by the electric
field parallel to the magnetic field direction. These escaping
ions have been observed by experiments from radars on the
ground (Wahlund et al., 1992) and on numerous satellites
including DE1, Polar, Geotail and Cluster (Yan and André,
1997; André and Yau, 1997; Moore et al., 1999; Maggiolo et
al., 2006, 2011; Nilsson et al., 2006; Seki et al., 2001). The
polar cap ions include the polar wind (Engwall et al., 2009;
Li et al., 2012) and cusp origin O+(Nilsson et al., 2012; Liao
et al., 2010, 2012). Cold polar cap ions sometimes consist of
mainly H+ions and can dominate the lobe outflow fluxes
(Engwall et al., 2009; Nilsson et al., 2010). Ions flowing out
with transpolar arcs (TPAs, also known as theta auroras) are
field-aligned and observed when the interplanetary magnetic
field (IMF) is northward (Kullen, 2012). Case events and sta-
tistical studies have shown the ion beams flowing out of the
polar cap are similar to auroral beams accelerated by field-
aligned potentials (Maggiolo et al., 2006, 2011; Nilsson et
al., 2004, 2006; Kronberg et al., 2014). However, cold O+
beams may form from velocity dispersion, and the beams can
look similar to those accelerated by an electric field (Horwitz,
1984; Liao et al., 2010).
The energetic ions flowing out of the auroral oval into
the magnetosphere occur mainly during auroral substorms.
Kistler et al. (2006) studied statistically the ion composi-
tion in the plasma sheet as a function of substorm activity.
In particular, they studied whether substorms occurring dur-
ing magnetic storms are different from substorms in non-
storm times. The role of O+ions in substorm dynamics is
still unresolved. Observations on the one hand have shown
that O+ions play a significant role in substorm dynamics
(Daglis et al., 1990; Korth et al., 2003; Fu et al., 2002; Nosé
et al., 2000), while others indicate the O+ions have no ef-
fects (Lennartsson et al., 1993; Grande et al., 2003; Kistler et
al., 2006).
Published by Copernicus Publications on behalf of the European Geosciences Union.
334 G. K. Parks et al.: Outflow of low-energy O+ion beams
Table 1. Observation intervals.
Date (2001) UT XGSE YGSE ZGSE Substorm
13 February 22:00–02:00a3.05 – (−3.73) −1.99 – (−1.19) −6.64 – (−0.5) 16:10 UT
23 February 10:00–13:30 3.44 – (−2.92) −2.52 – (−1.02) −6.45 – (−1.7) 03:16 UT
16 March 22:00–01:45a−1.1–(−2.81) −1.97 – (2.18) −6.45 – (3.19) 20:55 UT
19 March 04:00–09:00 2.34 – (−2.44) −3.79 – (−0.99) −7.04 – (−4.23) 23:28 UTb
28 March 20:00–23:30 −2.36 – (−1.76) −0.83 – (2.75) −4.6 – (3.96) nd
23 April 21:30–01:00a0.38 – (−2.87) −5.38 – (1.08) −7.41 – (−3.83) 20:54 UT
24 April 04:00–08:00 −0.79 – (4.36) 3.32 – (−0.59) 3.4 – (8.07) 20:54 UTb
26 April 09:00–16:00 −2.1 – (0.15) −0.73 – (0.48) −6.49 – (7.37) 06:21 UT
1 May 08:00–12:00 1.83 – (4.87) 1.73 – (−1.94) 6.25 – (8.46) nd
5 June 07:00–12:00 −3.21 – (−3.81) −15.1 – (10.5) −5.49 – (−7.28) 03:12 UT
8 June 03:00–09:00 −2.94 – (1.56) −2.09 – (2.52) −6.92 – (3.97) 11:07UTb
12 June 22:00–04:00a−2.65 – (2.34) −0.55 – (1.17) −6.5 – (5.26) 01:05 UT
14 July 01:00–06:00 3.28 – (0.29) 2.39 – (−4.7) 2.31 – (6.38) 23:43 UTb
5 October 04:00–12:00 1.41 – (−2.53) 2.13 – (−1.9) −6.44 – (7.42) 03:22 UT
7 October 17:00–21:30 3.4 – (−2.07) −2.71 – (−2.01) 0.98 – (7.14) 13:44 UT
12 October 00:00–08:00 −6.27 – (2.37) 8.84 – (1.01) −8.01 – (−5.69) nd
17 October 07:00–11:00 1.02 – (−467) −3.21 – (−0.05) 4.44 – (8.2) 01:25UT
19 October 11:15–13:00 2.56 – (3.38) 0.51 – (−1.76) −6.43 – (−2.95) 08:02 UT
29 October 04:00–06:00 1.14 – (−1.57) −3.68 – (−2.03) 3.05 – (6.28) 00:30 UT
31 October 04:00–09:00 −1.37 – (1.13) 8.81 – (3.75) −8.53 – (−7.71) 16:57 UTb
2 November 17:00–19:00 1.99 – (2.82) 1.85 – (−1.47) −6.87 – (−4.33) 13:24 UT
4 November 00:00–08:00 −9.75 – (−7.75) 16.6 – (16.77) 1.08 – (−3.4) 17:52 UTb
6 November 22:00–03:00a5.14 – (−2.73) 15.52 – (12.43) −5.87 – (−7.81) 19:07 UT
9 November 00:00–12:00 −7.01 – (−2.73) 17.7 – (12.6) −2.2–(−7.78) 04:26 UTb
14 November 00:00–07:00 −3.66 – (−1.31) 16.7 – (13.0) −5.26 – (−7.73) 20:18 UTb
18 November 00:00–08:00 −6.25 – (−5.06) 16.2 – (18.7) 4.66 – (0.28) 16:48UTb
19 November 06:00–12:00 1.69 – (2.42) 6.65 – (−0.49) −8.22 – (−5.83) 04:57 UT
17 January (2002) 21:00–02:00a−1.03 – (−2.42) −3.15 – (−3.31) −4.02 – (−1.88) 15:52 UT
22 February (2002) 21:00–02:00a1.97 – (8.87) 3.29 – (4.40) 7.64 – (8.31) 18:59UT
aday after, bday before, nd stands for no WIC data
POLAR Tide experiment together with Ultraviolet Imager
auroral images have established that the outflowing O+ions
are dependent on the substorm intensity, with outflowing
fluxes increasing as the substorm intensity increases (Wilson
et al., 2004). We have examined if the substorms are the only
source of auroral ions flowing out of the auroral oval. In this
article, we show evidence that auroral ions do not only escape
during substorms but also during periods without substorms.
Periods without substorms include “quiet” arcs, TPAs and
pseudo-breakup auroras. This article will focus only on ob-
servations during quiet arcs and pseudo-breakup auroras.
A pseudo-breakup aurora is different from a regular sub-
storm breakup. Pseudo-breakup auroras involve activation of
a small area of an arc that does not expand globally (Elvey,
1957). In pseudo-breakup auroras, a section of an arc bright-
ens “momentarily” and then fades. This process can occur
many times, but this behavior is different from a regular sub-
storm breakup, which includes the growth, expansion and re-
covery phases (Akasofu, 1964; McPherron, 1970). While a
distinction is made between substorms and pseudo-breakups,
numerous studies from ground and space have shown that
pseudo-breakup auroras include all of the same features as-
sociated with auroral substorms, except the intensities are
weaker. For example, Pi2 magnetic oscillations, which sig-
nify the onset of a substorm, are observed with the brighten-
ing of pseudo-breakup auroras but the amplitudes are smaller
(Koskinen et al., 1993). In the geomagnetic tail, the magnetic
field undergoes “limited” dipolarization, accelerating elec-
trons to several hundred kiloelectron volts and ions to a few
megaelectron volts but the fluxes are lower (Fillingim et al.,
2001; Parks et al., 2001). This article will add another feature
to the list of pseudo-breakup auroras: acceleration of H+, O+
and He+ions upward out of the auroral oval ionosphere, in
a nearly identical way to ions accelerated along the magnetic
field in regular substorms.
The outflowing ions during periods without substorms
have been measured by the ion composition experiment
known as Composition Distribution Function (CODIF) on
Cluster (Rème et al., 2001). Periods of substorms and those
without substorms are determined using auroral images ob-
tained by the Wideband Imaging Camera (WIC) on the IM-
AGE spacecraft (Mende et al., 2000) aided by the list of
Ann. Geophys., 33, 333–344, 2015 www.ann-geophys.net/33/333/2015/
G. K. Parks et al.: Outflow of low-energy O+ion beams 335
substorm onsets identified by Frey et al. (2004), Auroral
Electrojet (AE) indices (from the World Data Center for Ge-
omagnetism, Kyoto AE index service) and ground-based all-
sky camera records of the aurora obtained by MIRACLE
(Magnetometers – Ionospheric Radars – All-sky Cameras
Large Experiment) in northern Scandinavia.
A random survey of ion composition data from Cluster
(Rème et al., 2001) for 30 different days (Table 1) has re-
vealed that most of the outflowing ions occurring during pe-
riods without substorms are associated with the auroral oval
whose activities included the quiet arcs and pseudo-breakup
auroras. The measured energies of the escaping O+ions are
typically ∼20–40 eV, but after correcting for the spacecraft
potential, the ion beams have energies of ∼40 to 80eV.
These potentials are lower than the typical potentials associ-
ated with substorms which are >100eV to several kiloelec-
tron volts (Wilson et al., 2004; Marklund et al., 2010; Cui et
al., 2010).
Cluster measured the ions flowing out at heights of ∼2RE
to >10RE. The fluxes of outflowing ions are estimated to be
>103–105cm−2s. Assuming the source of these ions is the
auroral oval area sampled by the WIC, the estimated flow rate
of ions during periods without substorms is ∼1020 s−1. Not-
ing however that quiet arcs and pseudo-breakup auroras can
persist for hours, our results show that the non-substorm con-
tribution of the ions we measure can be very significant for
the plasma sheet. We have mapped the footprint of Cluster 3
using Tsyganenko 89 and 96 models to show that the space-
craft are located in the auroral oval. By comparing our results
with those in Nilsson et al. (2010) and with the discussion in
Nilsson et al. (2013), one may find that not much heating and
centrifugal acceleration is expected along these outflow paths
in the near-Earth lobes to the plasma sheet. The auroral ions
have similar and different features from the ions in the polar
wind, upwelling ions (cleft ion fountain) and polar cap (Yan
and André, 1997; Maggiolo et al., 2006, 2011; Nilsson et al.,
2006). In this first report, we will show three examples from
periods with different geomagnetic disturbance levels under
which the ion beams were observed flowing out of the iono-
sphere. Ions from both day and night sides flow out but we do
not distinguish between them here, nor will the observations
be compared to acceleration models (Yan and André, 1997;
Nilsson et al., 2008).
2 Observations
The list of events shown in Table 1 includes the time intervals
of the events, the position of Cluster covered during the inter-
val (first number in X,Y,Zrepresents start position and the
number that follows in parenthesis is the end position), and
the time of the last substorm that occurred just before the ob-
servations started, based on available WIC images (Frey et
al., 2004) and AE data. As can be seen, the observation inter-
vals did not include any substorm activity and they generally
Figure 1a.
started at least several hours after a substorm had occurred.
Three examples from this list are presented for further dis-
cussion: 31 October 2001, 19 March 2001 and 19 Novem-
ber 2001 (Fig. 1).
In Fig. 1a, b and c, panels 1, 3 and 5 show the differ-
ential number flux spectrograms of H+, He+and O+ions
with energies of ∼25eV–40 keV per charge, panels 2, 4 and
6, the density of these ions (statistically significant number
counts measured by the instrument corresponds to densities
≥0.01 cm−3). The overlapping IMF components and AE in-
dices for these times are shown in Fig. 2a, b and c. The
IMF Bzcomponent on 31 October was predominantly nega-
tive but small; on 19 March 2001 it was also negative from
04:00 to 06:00 UT after which it became positive (data miss-
ing from 07:30 to 09:00UT); and on 19 November 2001 it
fluctuated between positive and negative values. The IMF be-
havior here indicates that the events we discuss are different
from periods when the polar cap beams flow out (Maggiolo
et al., 2006; Nilsson et al., 2006).
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336 G. K. Parks et al.: Outflow of low-energy O+ion beams
Figure 1b.
The keograms covering the same period constructed from
the individual WIC Far Ultraviolet Imager images are shown
in Fig. 3a, b and c. In each keogram, the top panel shows the
auroral intensity as a function of MLat, the second panel as
a function of magnetic local time (MLT), and the third panel
the integrated photon flux averaged over 18:00–06:00 MLT
and 50–80◦MLat. The solid black lines are footprints of
Cluster 3 in the Southern Hemisphere using T89 and T96
models (the two models essentially gave the same results)
for 31 October, 19 March and 19 November 2001 (caveat: the
keograms come from auroras in the Northern Hemisphere).
The footprints of Cluster on 31 October were at MLat >80◦
so are not shown. Figure 4a, b and c shows examples of indi-
vidual auroral images to illustrate the activities in the auroral
oval and pseudo-breakup auroras. Examples of the velocity
space distribution of O+ions for the three days are shown in
Fig. 5. Figure 6 shows the velocity distributions of the three
ion species (H+, He+and O+), all from 19 November 2001.
31 October 2001: On this day, no significant H+
and He+were measured. Only O+ions were measured
Figure 1c. Three examples of low-energy ions flowing out of the
ionosphere during periods without substorms for different levels
of geomagnetic disturbances that included quiet arcs and pseudo-
breakup auroras. Panels 1, 3 and 5 show the differential number
flux spectrograms of H+, He+and O+and panels 2, 4 and 6 show
densities of these ions. The data shown come from SC3 but the ions
were also observed by SC1 and 4.
with very low density ∼0.01 cm−3after ∼05:40 UT. The
keogram (Fig. 3a) shows no significant auroral fluxes un-
til about 06:30 UT. The average fluxes corresponded to ∼
350 rayleighs (R) at 04:00 UT which increased to 500 R at
06:30 UT. The AE indices between 04:00 and 06:30 UT were
nearly zero (Fig. 2a), corroborating the keogram plot. The
WIC has a spatial resolution of ∼50–70 km, hence auro-
ral arcs are not spatially resolved. Thus, the WIC obser-
vations have been augmented using all-sky camera records
from MIRACLE. For this day, we find quiet auroral arcs,
measured at 557.7 nm, present until ∼03:00 UT when the
observations ended. There were no substorms recorded (not
shown).
Ann. Geophys., 33, 333–344, 2015 www.ann-geophys.net/33/333/2015/
G. K. Parks et al.: Outflow of low-energy O+ion beams 337
(a) (b)
(c)
-4
-2
0
2
IMF (nT)
Bx By Bz
31 Oct. 2001
0
50
100
150
200
250
AE index (nT)
AE
4 5 6 7 8
9
time (UT)
-250
-200
-150
-100
-50
0
50
AU/AL indices (nT)
AU AL
-5
0
5
IMF (nT)
Bx By Bz
19 Mar. 2001
0
100
200
300
400
500
AE index (nT)
AE
4 5 6 7 8 9 10
11
time (UT)
-300
-200
-100
0
100
200
300
AU/AL indices (nT)
AU AL
-10
-5
0
5
10
IMF (nT)
Bx By Bz
19 Nov. 2001
0
200
400
600
800
AE index (nT)
AE
6 7 8 9 10
11
time (UT)
-600
-400
-200
0
200
AU/AL indices (nT)
AU AL
Figure 2. Interplanetary magnetic field (IMF) Bx,Byand Bz, and AL, AU and AE indices covering the time period of low-energy ion
observations shown in Fig. 1.
Individual auroral images (Fig. 4a) show that the small au-
roral intensity increases observed at 06:30UT and 07:00 UT
occurred near midnight meridian at ∼70◦MLat and were
due to brightening of the aurora and pseudo-breakups, which
increased the AL indices to a peak value of ∼50 nT. The
increased auroral fluxes at ∼07:40UT are due to a more
intense pseudo-breakup aurora which increased the AL in-
dex to ∼100nT (Fig. 3a). However, no expansion occurred
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338 G. K. Parks et al.: Outflow of low-energy O+ion beams
(a) (b)
(c)
Figure 3. Keogram for 31 October 2001, 19 March 2001 and 19 November 2001 constructed from individual WIC images obtained by
IMAGE. The top panel shows the precipitated fluxes as a function of the magnetic latitude (MLat), the middle panel as a function of the
magnetic local time (MLT) and the bottom shows integrated photon fluxes from 18 to 06MLT. During periods without substorms, only quiet
arcs and pseudo-breakup auroras are observed.
and hence this intensification was considered due to pseudo-
breakup activity.
19 March 2001: Unlike 31 October 2001, significant
fluxes were measured in all of the ion species, H+, He+and
O+, between 05:00 and 07:00 UT (Fig. 1b). The density of
H+started out at ∼0.1 cm−3at 05:00 UT which decreased to
0.01 cm−3at 06:30 UT. The He+fluxes were weak with den-
sities of <0.01 cm−3, which however occasionally reached
∼0.01 cm−3. The O+fluxes were the highest with the den-
sity around 0.1 cm−3from 05:00 to 07:15 UT.
The geomagnetic activity was also slightly higher than
the previous example. The keogram for this day (Fig. 3b)
indicates that auroral activity between 04:00 and 08:00 oc-
curred from 62.5 to 70◦MLat (top panel) and the activ-
ity was in the evening and morning sectors (middle panel).
The integrated photon fluxes (bottom panel) corresponded to
∼550–1000 R between 04:00 and 09:00 UT. Note that Clus-
ter footprints initially traversed the region with small auroral
fluxes (∼07:00–09:30UT). Cluster then traversed regions
with slightly enhanced intensity, observed around 09:40 UT,
resulting in a 200nT AU increase (Fig. 2b). These auroral
fluxes are associated with pseudo-breakup aurora, which in-
cluded isolated activities at the northern and southern bound-
aries of the auroral oval, but no substorm expansion occurred
(Fig. 4b). According to Frey et al. (2004), a substorm on
19 March 2001 occurred at 09:25UT and the prior substorm
was on 18 March 2001 at 21:27UT (not shown). A substorm
growth phase was observed beginning around 09:15UT (see
the keogram) that led to substorm onset at 09:25 UT (Fig. 2b,
3b, 4b).
19 November 2001: Figure 1c shows the differential num-
ber flux spectrograms of the species H+, He+and O+
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G. K. Parks et al.: Outflow of low-energy O+ion beams 339
(b)
(c)
(a)
Figure 4. Individual auroral images for illustrating the type of activity observed for the three days, 31 October (a), 19 March (b) and
19 November 2001 (c) that cover the period of low-energy ion observations. The intensity of the aurora is shown in rayleighs.
www.ann-geophys.net/33/333/2015/ Ann. Geophys., 33, 333–344, 2015
340 G. K. Parks et al.: Outflow of low-energy O+ion beams
Figure 5. An example of velocity space distributions of O+ions
measured on 31 October 2001, 19 March 2001 and 19 Novem-
ber 2001. These distributions are shown according to the space-
craft’s coordinate system and the velocity space is defined in terms
of velocities parallel (Vpar) and perpendicular (Vperp) to the mag-
netic field direction. The scales are ±50 km s−1. The beams were
also observed by SC3 and 4.
(06:00–10:00 UT). The H+density was ∼0.08 cm−3, O+
density 0.32 cm−3and He+density <0.01 cm−3. This and
the last example show that the O+ions dominated H+ions,
though not all cases show this tendency. We calculated and
averaged the ratios O+/H+for each of the 24 events we ex-
amined that had simultaneous WIC images and Cluster ion
data. This resulted in half of them having an average ratio
O+/H+of ∼2.46±1.57 and the other half having a ratio of
∼0.08±0.04 (not shown). The ratios of O+/He+were all
>1 with the average equaling ∼25, except for two cases.
Our observations are different from previous observations
(for example FAST) where H+ions generally dominated
(Möbius et al., 1998; Maggiolo et al., 2006). However, FAST
observations came from disturbed substorm times, and the
Maggiolo et al. results are from statistical studies. Note that
in rare cases, He+ions dominate H+and O+ions (Lund et
al., 1998).
Of the three examples, this day (19 November 2001)
was the most disturbed with the average integrated photon
fluxes starting out around 1.25kR until ∼10:30 UT, when
a substorm onset occurred (also identified by Frey et al.
(2004)). The keogram shows (Fig. 3c) moderate auroral ac-
tivity during 06:00–10:00UT with pseudo-breakup activities
indicated by the individual images (Fig. 4c; comparison of
the first and fourth images shows a spot at 21MLT disap-
pearing and reappearing again). The AE for ∼10:30 UT sub-
storm peaked around 500nT (Fig. 2c), and the individual au-
roral images show an expansion of the aurora (not shown).
For this day, the footprints of Cluster were mapped to the
latter part of the auroral activity, hence we have no direct in-
formation on the regions of the pseudo-breakup activity.
Distribution function: Figure 5 shows examples of the
velocity space distributions of O+observed for the three days
discussed above. Ion beams are observed by all three Cluster
spacecraft (SC1, 3, and 4) but we only show data from Clus-
ter 1. These are 2-D cuts of the 3-D distributions presented in
the spacecraft frame where the coordinates are relative to di-
rections parallel (Vpar) and perpendicular (Vperp) to the mag-
netic field. The scales of the xand yaxes are ±50 km s−1. On
these three days, Cluster was traversing the Southern Hemi-
sphere, and the positive Vpar corresponds to ions flowing par-
allel to the magnetic field direction out of the ionosphere. All
of the ions are field-aligned beams occupying a very small re-
gion of the velocity space (one or two pitch-angle bins clos-
est to the direction of B), and the measured O+beams have a
velocity of ∼20 km s−1(for spacecraft potential correction,
see below).
While the O+beams can form from velocity filter effects
(Liao et al., 2010, 2012; Nilsson et al., 2004), we will inter-
pret our observations in terms of particle acceleration by par-
allel potential drops because the distributions we measured
were very much confined to the parallel direction (Marklund
et al., 2010). In this case, the energy per charge of an ion that
has gone through a potential drop 1φ measured by CODIF is
mv2/2q=(Wth/q)+1φ, where Wth is the thermal energy. If
the initial thermal energy of the ambient ionospheric plasma
(<1 eV) is assumed to be much less than the potential drop
(Wth q1φ ), then the shift of the peak of the ion beam will
correspond to the energy gained by going through the poten-
tial drop. The observed beam velocities of 20kms−1indicate
the O+ions have undergone a potential drop of ∼20–40 V
1φ =mv2/2q=16 ×1.6×10−27 ×(20 ×103)2/2×1.6×
10−19 =32 V along the magnetic field. However, the space-
craft was charged positively to ∼10–40V, indicating the ac-
tual beam energy is ∼50–80 V.
Figure 6 shows the velocity space distributions of the ions
measured on 19 November 2001. The measured velocity of
the H+beam is ∼100 km s−1, of He+is ∼50 kms−1and of
O+∼20–30 km s−1. The measured ratios of beam velocities
Vbof O+relative to H+is ∼4 and of He+∼2. These values
Ann. Geophys., 33, 333–344, 2015 www.ann-geophys.net/33/333/2015/
G. K. Parks et al.: Outflow of low-energy O+ion beams 341
Figure 6. An example of velocity space distributions of O+ions measured on 31 October 2001, 19 March 2001 and 19 November 2001.
These distributions are shown according to the spacecraft’s coordinate system and the velocity space is defined in terms of velocities parallel
(Vpar) and perpendicular (Vperp) to the magnetic field direction. The scales are ±50 kms−1. The beams were also observed by SC3 and 4.
are exactly the same as the theoretical ratios of the beam ve-
locities of O+and He+relative to the H+if all of the ions
had gone through the same potential drop: VH+=2VHe+and
VH+=4VO+. Thus, our observations indicate that the three
ion species originated from nearly the same height and went
through ∼50–80 V of potential drop. However, note that O+
ions showed velocities extending to higher values, indicating
that O+ions went through a larger potential range.
For a Maxwellian distribution, the width of the distribu-
tion corresponds to the temperature of the beams, hence our
observations show that the temperature of O+is larger than
H+and He+. The temperature is mass dependent. The esti-
mated temperatures (T) of H+, He+and O+ions (in units of
kT where kis the Boltzmann constant) using the distribution
function are ∼50, 75 and 200 eV, respectively. The results
indicate that ions are not only accelerated by the potential,
but they are also heated. Mass-dependent heating has been
observed previously (Collin, 1987; Reiff et al., 1988; Möbius
et al., 1998; Cui et al., 2010), but the details of how such a
heating mechanism works still remains unknown. Note that
for the O+we observed, there are counts in channels in the
adjacent bins relative to the magnetic field direction and the
temperature calculation includes their contribution. These
particles are probably pitch-angle scattered particles of the
original beam along the field. If only the field-aligned por-
tion is included, the temperature will be reduced to <100 eV.
Note also that because of mirror force, the temperature per-
pendicular to the magnetic field may indicate some heating
(see also Nilsson et al., 2012, for Cluster studies of ion heat-
ing throughout the polar cap magnetosphere).
3 Discussion
This paper has shown that low-energy ionospheric O+ions
flow out during periods without substorms. The O+ions are
originating from the auroral oval populated by quiet arcs and
pseudo-breakup auroras. The ion beams are observed by all
Cluster spacecraft although the details are different, indi-
cating the structures are smaller than the SC separations (a
few hundred kilometers). Preliminary results of test particle
simulation using Tsyganenko model (Tsyganenko and Sit-
nov, 2007) with a Weimer electric field (Weimer, 2001) show
these ions end up in the lobe and plasma sheet (not shown).
However, the results depend on the convective field, which is
not measured, and require further studies. Our observations
are consistent with previous results that indicate that O+ions
are expected to end up in the plasma sheet (Haaland et al.,
2012).
A qualitative picture that is emerging from these prelimi-
nary observations is that the field-aligned O+ions are accel-
erated along the magnetic field direction by a potential drop
very similar to ions accelerated during substorms (Marklund
et al., 2010). The observed values of the streaming veloc-
ity ratios of H+/ O+and H+/ He+support the field-aligned
acceleration interpretation. However, as noted earlier, a com-
peting mechanism for producing field-aligned beams is ve-
locity filter effects, which is not excluded (Liao et al., 2010;
Nilsson et al., 2004). The observed potential for periods with-
out substorms is a few tens of electron volts. The escaping en-
ergies and fluxes are a few orders of magnitude smaller than
in substorms (Wilson et al., 2004; Yan and André, 1997).
The auroral arcs are not resolved by the WIC, whose spa-
tial resolution is ∼50–70 km. Thus, it is not known if the
source of the low-energy ions includes only the quiet auroral
arcs and pseudo-breakup arcs or if it also includes the larger
auroral oval. There is freedom about what size we choose for
the source area. If we include the area of 18–06 MLT and 50–
80 MLat as in the keogram, the size is about 2.8×1017 cm2.
If we look at a smaller region near midnight, say 21–03MLT
and 65–75 MLat, the area is only 3.8×1016 cm2, about an
order of magnitude smaller. Based on these numbers the es-
caping fluxes correspond to a flow rate of 1019–1021 ions s−1.
This number is less than the cold ions escaping the polar cap,
1026 ions s−1(Engwall et al., 2009). However, considering
that the quiet auroral oval can persist for hours, the number
of ions escaping here (>1024 ions) can be higher than that
predicted by Seki et al. (2001) and comparable to the number
of energetic ions escaping during substorms (Yan and André,
1997).
www.ann-geophys.net/33/333/2015/ Ann. Geophys., 33, 333–344, 2015
342 G. K. Parks et al.: Outflow of low-energy O+ion beams
This article has focused only on the auroral oval that in-
cluded quiet arcs and pseudo-breakup auroras. However, au-
roras during periods without substorms also include TPAs,
observed during quiet solar wind conditions when IMF Bz
is northward. Superficial comparison of TPAs reported by
Kullen (2012) with overlapping FAST ion composition data
shows O+ions were flowing out in some of the TPA events.
The electrons of the TPAs have energy spectra similar to
the electrons in the plasma sheet (Meng, 1981), suggesting
that TPAs are connected to the plasma sheet. However, what
causes plasma sheet electrons to appear in TPAs in the polar
cap region is not precisely understood. Observations of TPAs
have recently been reviewed in Kullen (2012) and references
therein).
A fundamental question that still remains unanswered is
why pseudo-breakup auroras do not expand. Substorm on-
sets require dissipation of energy stored in the geomagnetic
tail into the ionosphere, and the ionosphere plays an impor-
tant role (Lysak, 1990). This has suggested that the reason for
the non-expansion is that the coupling between the magneto-
sphere and ionosphere is not adequate and inhibits current
flow. But at this juncture, the details remain unclear. Future
investigations of non-substorm auroras together with obser-
vations of O+ions escaping the ionosphere and radar mea-
surements of ionospheric conductivity (Wahlund et al., 1992;
Koskinen et al., 1993) could lead to a better understanding of
the coupling of the magnetosphere and ionosphere and the
role the ionosphere plays in substorm onset mechanisms.
Acknowledgements. This research work is in part supported
by a NASA grant to the University of California Berkeley,
NNX11AD49G-2/15. The work by E. Lee was in part supported
by the BK21 Plus Program and Basic Science Research Program
(NRF-2013R1A1A2010711) through the National Research Foun-
dation funded by the Ministry of Education of Korea.
Topical Editor E. Roussos thanks two anonymous referees for
their help in evaluating this paper.
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