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21
Dawn‐Dusk Asymmetries of Ionospheric Outflow
Kun Li4, Elena A. Kronberg1,2, Mats André3, Patrick W. Daly1, Yong Wei4, and Stein Haaland1,5
ABSTRACT
Ion outflow from high‐latitude region of Earth’s ionosphere is an important source of the ions in the magnetosphere. As
a part of ionosphere‐magnetosphere coupling, it is also an important driver of dynamics in the magnetosphere. The
dawn‐dusk asymmetries in the ion outflow are sometimes non-negligible for the outflow processes and plasma
circulation in geospace. The causes of the asymmetries are diverse in terms of the morphologies, efficiencies, and
responses to the solar‐wind conditions. In this review, we focus on the mechanisms of dawn‐dusk asymmetries in
energization of ion outflow and in the ion transportation in the magnetotail. Asymmetric energization processes are:
(1) asymmetry in auroral precipitation associated with electron heating, ion beams and conics; (2) asymmetry in
Poynting flux associated with ion heating; (3) asymmetry in cusp spatial distributions that causes various heating on
the dayside; (4) high‐altitude ionosphere‐magnetosphere‐coupled convection with IMF controlled dawn‐dusk
asymmetry which is essential to centrifugal acceleration for transportation of polar cap ion outflow.
21.1. INTRODUCTION
The presence of oxygen ions in the magnetosphere
indicates that ionospheric particles flow into the magnetosphere and fill the near-Earth environment. Recent
studies suggest that the ionospheric outflow is the main
contributor to the plasma in the magnetosphere [Moore
and Horwitz, 2007]. One of the sources of ion outflow
from the ionosphere can be found in the cusp region,
associated with various energy inputs [Lockwood et al.,
1985]. Under average solar‐wind condition, with a spiral
configuration of interplanetary magnetic field (IMF)
and small Bz, ionospheric outflow can provide substantial
Max Planck Institute for Solar System Research, Göttingen,
Germany
2
Ludwig Maximilian University of Munich, Munich, Germany
3
Swedish Institute of Space Physics, Uppsala, Sweden
4
Institute of Geology and Geophysics, Chinese Academy of
Sciences, Beijing, China
5
Birkeland Center for Space Science, University of Bergen,
Bergen, Norway
1
amounts of ions into the magnetosphere through so‐
called auroral outflow. Low‐energy ions, with energies
up to a few tens of electron volts, are found to dominate
the ionospheric outflow through the polar wind from
the polar cap. The characteristics of high‐latitude ionospheric outflow and its energization are summarized by
Yau and Andre [1997]. Outflow can also be found at low
latitudes, where the plasma plumes detach from the ionosphere and escape sunward [André and Cully, 2012].
Ions outflowing from the ionosphere can be found
throughout the magnetosphere, since it is highly modulated by magnetospheric convection and as a function of
geomagnetic activities [Li et al., 2013]. This convection is
considered to be a main driver of ion transport and one
of the mechanisms energizing the ion outflow.
Various mechanisms of ion outflow have been revealed
and summarized in Moore and Horwitz [2007, and references therein]. The ionosphere is formed by solar ultraviolet irradiation. To enable ion outflow, the ions gain
energy through mechanisms, such as (1) Joule heating, (2)
ion acceleration, (3) auroral heating by various waves and
auroral precipitation, and (4) parallel electric fields in the
Dawn-Dusk Asymmetries in Planetary Plasma Environments, Geophysical Monograph 230, First Edition.
Edited by Stein Haaland, Andrei Runov, and Colin Forsyth.
© 2017 American Geophysical Union. Published 2017 by John Wiley & Sons, Inc.
273
274 DAWN-DUSK ASYMMETRIES IN PLANETARY PLASMA ENVIRONMENTS
auroral zone. Joule heating results from collisions between
neutral particles at low altitudes and ions picked up by
convecting magnetic fields [Korosmezey et al., 1992], and
also due to electric current (mainly the Pedersen current)
in the ionosphere passing through plasma with finite conductance. Ion acceleration parallel to the magnetic field
can occur due to charge separation giving rise to the polar
wind [Banks and Holzer, 1968; Moore et al., 1997] or in
the auroral regions due to auroral precipitation, in which
ion temperature is strongly correlated to ion outflow [Seo
et al., 1997]. Further heating in the auroral zone can
occur due to electromagnetic ion cyclotron (EMIC)
waves, broadband extra low frequency (BBELF) waves or
electrostatic ion cyclotron (EIC) waves (see, e.g., André
[1997]), which are in ion gyrofrequency range and have
enough power to enable the outflow. Some of the waves
in this category are Alfven waves traveling along the magnetic field from the nightside plasma sheet, with higher
efficiency to heat the ionosphere [Keiling et al., 2003]. The
presence of parallel electric fields in the auroral zone
results in ion beams [Mozer et al., 1977]. Furthermore, a
DC electric field is observed in the cusp, identified by
upward ion beams and ion conics and associated with ion
acceleration [Pfaff et al., 1998].
The outflow of ions essentially always exists in regions
poleward of the plasmasphere. It can be observed when the
spacecraft travels through the polar region. In terms of
energy and height characteristics, the outflow is identified
as (1) polar wind in the polar cap region, composed of H ,
He and O , with total energy less than a few tens of electron volts; (2) ion bulk upflow at auroral latitudes dominated by O. The ion bulk upflow turns into ion outflow by
ion acceleration at those altitudes it can reach, although
normally the bulk upflow does not have sufficient velocity to escape; (3) ion conics associated with the auroral
oval, energized by perpendicular electric field oscillations at a frequency near to ion gyrofrequency; (4) ion
beams stemming from ion conics in a diverging magnetic
field and transverse heating by waves in the upward direction; (5) upwelling ions in the dayside cusp region, accelerated from a few eV, in both parallel and transverse
directions. Upwelling ions are characterized by asymmetric upward and downward flux distributions.
Chappell et al. [1987] suggested the ionospheric outflow
alone could supply the plasma in the magnetosphere.
It is also generally accepted that the ionospheric outflow
significantly impacts the dynamics in the magnetosphere.
For example, (1) presence of low‐energy ions in a plasma
plume changes ion densities at the reconnection site on
the dayside magnetopause, and consequently decreases
the reconnection rates [Walsh et al., 2014]; (2) outflow of
oxygen ions, which causes pressure imbalance in the inner
magnetosphere, is able to induce the sawtooth oscillations at the nightside geosynchronous orbit, as simulated
by Brambles et al. [2011]; (3) oxygen outflow is the main
carrier of the currents in the ring current during storm
times [Hamilton et al., 1988].
The outflows from dawnside and duskside of the ionosphere are differ greatly. Yau et al. [1984] found the outflow
of both H and O , with pitch angles between 100° and
160° and energy from 0.01 to 1 keV, is larger on the duskside. Howarth and Yau [2008] studied the trajectories of
polar‐wind ions with a single‐particle approach, with input
from Akebono measurements. They found polar‐wind
supplies more ions to the duskside of the plasma sheet
when the IMF is duskward. When the IMF is dawnward,
the distribution of polar‐wind ions in the plasma sheet is
more even. They suggest there is a dependence on IMF By
in the polar‐wind deposition on the plasma sheet. In
contrast, a study by Li et al. [2013] showed a persistent
dawn‐dusk asymmetry exists for low‐energy ions from
the ionosphere transported to the plasma sheet, without
corresponding changes to the IMF direction. By checking their origin in the ionosphere, no significant dawn‐
dusk asymmetry is found. These low‐energy ions are
mainly H with both thermal and kinetic energy lower than
70 eV. Liao et al. [2010] reported O from the cusp in the
Northern Hemisphere tends to supply more to the dawnside of the tail lobes when IMF By is positive. The asymmetry is reversed when IMF By is strongly negative.
However, the dawn‐dusk asymmetry has attracted less
attention than day‐night and hemispheric asymmetries,
which can largely be explained by differences in solar
irradiation and magnetic field compression. The dawn‐
dusk asymmetry shows more complicated features, and
needs more investigations to explain. Even under a condition with symmetric solar‐wind parameters, the dawn‐
dusk asymmetry may exist. As an important aspect of
magnetosphere‐ionosphere coupling, dawn‐dusk asymmetry in the ionospheric outflow is created by asymmetries in ionospheric heatings, which are caused by
various energy inputs from the magnetosphere.
Dawn‐dusk asymmetries in the ionospheric outflow
and heating are studied by both observations and simulations. Rather than listing all the mechanisms involved
in the ion outflow, we only summarize the asymmetries
in energization mechanisms and outflow transports.
Their dependence on solar‐wind conditions is also analyzed. In summary, we try to determine the relative
importance of the sources of asymmetries, and outline
problems to be solved.
21.2. ASYMMETRIC ENERGIZATION
IN THE IONOSPHERE
The ionosphere is created by the dayside upper atmosphere absorbing the ultraviolet (UV) component of solar
irradiation. In the ionosphere, the electrons are removed
Dawn‐Dusk Asymmetries of Ionospheric Outflow 275
from ionized atoms, become photoelectrons, and gain
high enough energies to escape the Earth’s gravity.
A simple dawn‐dusk asymmetric energy input can be
considered as the asymmetric heat absorbed from the
Sun. The corotation of the ionosphere with the Earth can
turn the day‐night asymmetric heating into a dawn‐dusk
asymmetric one. As the ionospheric plasma with higher
temperature on the dayside turns to the dusk sector, the
ionospheric plasma cooled down on nightside turns to
the dawn sector. This gives rise to higher temperature in
the ionosphere, and larger scale height of the ionosphere
on the dusk side. One could thus expect that there is more
ion outflow on the dusk side of the ionosphere. However,
at high latitudes, where the main source of ion outflow is
located, the ionosphere does not necessarily co-rotate
with the Earth thus this effect may be reduced. It is necessary to do more studies on the importance of this effect.
Figure 21.1 demonstrates the causal relationships
between asymmetric ionospheric outflow and various
asymmetric energy inputs. Each causal relationship is
indicated by an arrow with a number labeled beside it.
Some of correlations between energizations and the outflow have been quantitatively studied by Strangeway et al.
[2005] and Moore and Khazanov [2010]. In their studies,
the ionospheric outflow is enhanced due to an increase in
Poynting flux or electron precipitation. Below, we list
the theoretical analysis, simulations, and observations
(if available) of these causal relationships.
Energization in the ionosphere can be classified as ion
and electron energizations. For the ion energization of
the thermal-type, increases in the ion temperature
increases the scale height of ions, making more ion
upwelling possible (illustrated as chart flow 5 and 6). In
the kinetic-type energization, ions with higher velocities
can easily overcome the gravitational drag. The electron
scale height can increase with the electron temperature,
which is correlated to the electron precipitation as
depicted by chart flow 9. This generates a larger ambipolar electric field (chart flow 10), and enhances the ion
upwelling as depicted by chart flow 11. Ion upwelling
turns into ion outflow when waves interact with ions and
enable them to escape in the form of ion beams and conics (chart flow 7). At high altitudes, ions accelerated by
the ambipolar electric field are continuously accelerated
by centrifugal acceleration (chart flow 1) [Cladis, 1986;
Cladis et al., 2000].
Asymmetric ion outflow
1
ELF/VLF waves
7
Asymmetric centrifugal
acceleration(polar wind)
6
Asymmetric ion
upwelling
Asymmetric increase of
ion scale height
Asymmetry in
ionospheric convection
11
Asymmetric increase of
electron scale height
10
5
2
Asymmetric electron
heating
Asymmetric Joule heating
Asymmetric neutral wind
of low altitude atmosphere
4
9
Asymmetric poynting flux
3
12
Asymmetric conductance
of ionosphere
13
Asymmetric spatial
distribution of cusp region
8
16
Asymmetric
field-aligned currents
Asymmetric electron
precipitation
14
15
Asymmetric spatial
distribution of aurora
Figure 21.1 Chart of relationships between the dawn‐dusk asymmetry in ionospheric outflow and various
asymmetries in energy inputs from the magnetosphere and the magnetosheath. Each relationship with a number
labeled beside it is explained in the text.
276 DAWN-DUSK ASYMMETRIES IN PLANETARY PLASMA ENVIRONMENTS
21.2.1. Joule Heating in the Ionosphere
Asymmetric ion heating can be found at lower altitudes.
In chart flow 2, the Joule heating is caused by collisions
between neutral particles and ions, which are driven by
ionospheric convection and field aligned current (FAC) at
lower altitudes in the ionosphere. In an investigation of
collisions between neutral particles and ions, Bjoland et al.
[2015] conducted a statistical study of Joule heating in the
ionospheric F-layer in the Northern Hemisphere, using
data obtained by SuperDARN radar and CHAMP satellite. They found a significant dawn‐dusk asymmetry in this
heating mechanism. The heating maximises at postnoon
and postmidnight, which are collocated with maximum
eastward and westward neutral wind velocity respectively.
Noted by Strangeway et al. [2000], casual relationship is
found between ionospheric Joule heating and the Alfvénic
Poynting flux (chart flow 4). The Alfvénic Poynting flux is
highly related to both cusp and aurora region. Alfven
waves propagating along open magnetic field lines transmit magnetic perturbations to the cusp. It is suggested
that outflow rates in the cusp are enhanced during a coronal mass ejection pass. The location of the cusp may
change with IMF orientation and show dawn‐dusk asymmetry; this is discussed in section 21.2.3.
Alfvénic Poynting flux in the auroral region is associated with broadband precipitating electrons and ions,
which originate from the plasma sheet. This is usually
observed as wave/broadband aurora at about the time of
substorm onset dipolarization. Two other types of aurora
in a substorm cycle are discussed in section 21.2.2. They
are (1) diffuse aurora associated with electron and ion
precipitation from the plasma sheet, and (2) monoenergetic aurora in which the electron precipitation from
the plasma sheet is accelerated by the global upward
field‐aligned current system [Newell et al., 2010; Wing
et al., 2013 and Chapter 20 in this volume]. In chart flow
15, asymmetric aurora distributions will result in asymmetric electron precipitation.
Keiling et al. [2003] compared 1-year’s worth of Alfvén wave
data obtained by the Polar spacecraft at altitudes from
28,000 to 35,000 km with images of the northern aurora in
both visible and UV spectrum. They found the average
energy Poynting flux toward the Earth (downward) concentrates at regions in the ionosphere where the average
aurora intensifies (Fig. 21.2a). They concluded that the
Poynting flux is adequate to accelerate the ions within the
auroral oval, in addition to FAC, which are also found
over the auroral oval at those altitudes. This observation
is reproduced by the LFM global simulation in Zhang
et al. [2012], as shown in Figure 21.2b. Here, clear asymmetry in the downward Alfvénic Poynting flux is shown
in both observation (Fig. 21.2a) and simulation
(Fig. 21.2b).
To investigate the cause of the asymmetry, Zhang et al.
[2012] repeated the simulations with controlled spatial
­variation of conductance in the ionosphere. In Figure 21.2c,
there are asymmetries in Alfvénic Poynting fluxes toward
the Earth in an MHD simulation with enhanced conductance due to electron precipitation. The average downward Poynting flux on the nightside has a higher flux in
the premidnight sector. This asymmetry disappears when
the conductance is spatially uniform as we can see from
the simulation in Figure 21.2d. Note that the fluxes in
Figure 21.2a and b are 1 year and 24 h averaged fluxes,
respectively. With occasionally uniform conductance in
these time periods, they show relatively milder asymmetries than that in Figure 21.2c, which shows the fluxes
in much shorter time periods (1 h). Also note that the
Poynting flux in the cusp region in Figure 21.2c and d is
minimized by controlled steady upstream driving parameters. Alfvénic wave activity is weak at the subsolar point,
as well as at the cusp because the subsolar point connects
the cusp with a magnetic field line.
21.2.2. Electron Heating in the Auroral Region
Clear dawn‐dusk asymmetry can also be identified in
electron energy precipitation of diffuse aurora and monoenergetic aurora [Wing et al., 2013 and Chapter 20 in this
volume]. In Figure 21.3a and b are energy fluxes of diffuse
aurora electron precipitation and monoenergetic electron
precipitation, respectively. The data are obtained from
DMSP at an altitude of 845 km. Diffuse aurora electron
energy precipitation is more intense in the dawn sector of
the ionosphere, with peak energy input almost twice the
peak energy flux of monoenergetic electrons, while monoenergetic electron energy precipitation is more concentrated
in the premidnight sector of the ionosphere. Monoenergetic
precipitation is found to be enhanced in the winter hemisphere. They consider that the enhancement of monoenergetic electron precipitating energy flux may result from
enhancement of the magnetotail stretching and the region
1 FAC increase during the substorm growth phase.
Upward FAC accelerates electrons downward, enabling
them to reach the upper atmosphere and creating discrete aurora. The FAC exhibits dawn‐dusk asymmetry,
as studied by Iijima and Potemra [1976]. In the dawn/
dusk sector of the ionosphere, upward FAC is at relatively lower/higher latitude. The amplitude of FAC
changes with solar wind and geomagnetic activities,
and is indirectly influenced by magnetosphere‐ionosphere convection. Poleward of the region 1 FAC in the
dayside, the region 0 FAC is believed to be associated
with the mantle. It is found that the polarity of the
region 0 current changes with direction of IMF By and
upward region 0 current occurs more frequently in the
prenoon sector [Wing et al., 2010].
Dawn‐Dusk Asymmetries of Ionospheric Outflow 277
Average Alfvénic Poynting flux
6–180 s bandpass, mapped to 100-km altitude
POLAR
4–6 RE
Kp = 2–
12
mW/m2
(a)
0.8
18
80
70
60
06
0.4
LFM
1.8 RE
Kp = 3
12
mW/m2
1.1
(b)
18
80
70
60
0.5
06
0.0
Keiling et al. [2003]
24
Jan–Dec 1997
24
Simulated with enhanced conductance
(c)
0.0
4–5 Feb 2004
Simulated with uniform conductance 5 mho
12
S‖
(d)
12
S‖
60
06
0
24
70
18
60
0.5
mW/m2
70
18
mW/m2
1
06
0.0
24
Figure 21.2 Morphology of downward Alfvénic Poynting flux. (a) One year average flux measured by the Polar
spacecraft [adopted from Keiling et al., 2003]; (b) 24 h averaged flux simulated by controlled global MHD model,
as a comparison of observation in (a); (c) the dawn‐dusk asymmetry in Poynting flux with the same model but with
asymmetric conductance in the ionosphere due to electron precipitation; (d) the same as (c), but the symmetric
conductance of 5 mho is applied. Note that the fluxes are higher in (b) to (d) than in panel (a), since the altitude of
simulation in (b) to (d) is lower than that of observation in (a). (b) to (d) adopted from Zhang et al. [2012].
Moreover, fast flows in the magnetotail may be associated with monoenergetic aurora, as they are observed
faster in the dusk sector [Hori et al., 2000]. More recently,
Zhang et al. [2015] simulated the asymmetries in these
two types of aurora with the Lyon‐Fedder‐Mobarry (LFM)
global model. As shown in Figure 21.3c, both energy
fluxes are simulated and mapped to 100 km reference altitude. These simulated energy fluxes are observed with
similar morphologies. The causal relationship between
electron heating and auroral electron precipitation or
aurora is indicated as chart flow 9 or 15 in Figure 21.1.
In principle, the conductivity and conductance can
be enhanced when plasma density is high due to plasma
precipitation (chart flow 8). Ionospheric conductance
associated with electron precipitation is calculated by
Robinson et al. [1987], in which both Perdersen conductance
and Hall conductance are positively related to the energy
flux. High ionospheric conductance is observed in regions
where the electron precipitation is enhanced [Ohtani
et al., 2009]. With enhanced conductance, simulation by
Zhang et al. [2012] indicates a dawn‐dusk asymmetry in
Alfvénic Poynting flux (Fig. 21.2c). This nightside asymmetric conductance induced asymmetry in the Poynting
energy flux is indicated as chart flow 3 in Figure 21.1.
21.2.3. Cusp Precipitation
Plasma precipitation to the dayside ionosphere comes
directly from the magnetosheath through the cusp on
open magnetic field lines [Meng, 1981; Wing et al., 1996,
2001], in addition to the uniform electron polar rain.
The magnetosheath hot plasma precipitation is thus
278 DAWN-DUSK ASYMMETRIES IN PLANETARY PLASMA ENVIRONMENTS
Diffuse aurora electron precipitation
(a)
Energy flux
(c)
12
12
2.1 GW
mW/m2
15
4.7 GW
09
60°
1.0
70°
0.5
18
06
06
18
80
75
70
65
0
60
21
55
03
50
00
00
(b)
Mono
Monoenergetic electron energy flux
12
mW/m2
4
15
Diffuse
0
4
mW/m2
09
0.5
0.25
06
18
80
75
70
65
0
60
21
55
03
50
00
Figure 21.3 Asymmetric energy flux of auroral electron precipitations. (a) Observed diffuse aurora electron precipitation with higher flux in the postmidnight and dawnward sectors; (b) observed monoenergetic electron precipitation
favor in the premidnight sector of the ionosphere [after Wing et al., 2013]; (c) energy flux for both precipitations
with 1 h averaging. The fluxes are simulated by Global MHD model, with fixed solar‐wind parameters. Numbers at
the top of the panel are the total power input to the ionosphere. After Zhang et al. [2015].
collocated with the cusp and an important path for
solar‐wind entry [Wing et al., 2014]. At high altitudes, the
cusp energetic particle (CEP) precipitation is controlled
by IMF directions [Woch and Lundin, 1992] and correlates with ULF waves [Chen and Fritz, 1998]. Escoubet
et al. [2008] found particle precipitation is immediately
enhanced when IMF turns from southward to northward. Therefore, the location of the cusp is considered
as a proxy for cusp precipitation and all accelerations
involved in the cusp.
In a study by Newell et al. [1989], there is dawn‐dusk
asymmetry in the location of the cusp, which is modulated
by IMF directions. When IMF By is negative (positive),
the peak probability of the observed cusp location projected onto the ionosphere is shifted to the prenoon sector
for the Northern (Southern) Hemisphere. This shifting
effect is more pronounced when IMF Bz is southward.
The statistical location of the cusp is studied by Wing
et al. [2001], with DMSP observations. They suggest that
the cusp location changes as a function of IMF By and Bz.
Dawn‐Dusk Asymmetries of Ionospheric Outflow 279
Both high and low altitudes of the cusp show dawn‐dusk
asymmetry with regulation by the IMF. In conjugate
hemispheres, the aurora can be observed at places on
both hemispheres connecting each other with a newly
reconnected magnetic field line. This magnetic field line
is reconnected at the reconnection site in the dayside
magnetopause, which is influenced by the IMF orientation
because it can change the configuration of the magnetosphere. This effect is studied by simultaneous imaging
with IMAGE and Polar in ultraviolet wavelengths
[Østgaard et al., 2007]. They show the evidence that IMF
By and the geomagnetic dipole tilt angle are controllers
of the substorm onset location.
A MHD simulation has also been conducted, suggesting a dawn‐dusk asymmetry. In a study by Siscoe et al.
[2005], the cusp region at high altitudes moves towards
lower latitudes when the IMF turns from northward to
southward.
The ion precipitation through the cusp to the ionosphere
with dawn‐dusk asymmetry of ion‐neutral particle collisions causes asymmetric Joule heating (shown as chart
flow 13). While the asymmetry of electron precipitation in
the dayside also may be attributed to asymmetric spatial
distribution of the cusp, as indicated as chart flow 14.
21.3. ASYMMETRIC TRANSPORTATION
OF OUTFLOW IONS
21.3.1. Cusp Outflow
Cusp outflow is found convecting with the opened
magnetic field lines over the polar cap to the magnetotail
lobes. Two‐dimensional kinetic simulation by Horwitz
and Lockwood [1985] shows that the higher the outflow
velocity of the ion, the higher the altitude the ion can
travel and thus the farther along the plasma sheet it can
reach. At the specific region, the cusp outflow ions are
monoenergetic, whereas the location of the cusp on the
ionosphere is modulated by the IMF, as mentioned in
section 21.2.3.
In the cusp, O ions are efficiently accelerated in the
ionosphere and transported to the lobes even during nonstorm times. Liao et al. [2010] used Cluster measurements
to study the transport path of cusp outflow in the magnetosphere. Their results show IMF By is a main controller.
When IMF By is positive, cusp O from the northern/
southern hemisphere tends to supply the dawn/dusk side
of the magnetotail. The asymmetry is obviously reversed
when IMF By is strongly negative. This can be explained
by magnetospheric convection, which is modulated by
magnetic tension force after a magnetic reconnection in
the dayside. Noda et al. [2003] used Cluster data to conclude that IMF By controls the y component of magnetospheric convection velocity in the lobes. Their results
show that a clockwise/counterclockwise convection (seen
from the Sun) presents when IMF By is negative/positive.
21.3.2. Auroral Outflow: Ion Conics and Beams
Ion conics with peak flux at an angle to the local
magnetic field, as summarized by Yau and Andre [1997],
are associated with ion transverse heating. Ions with large
pitch angles (at or close to 90°) are interpreted as transverse accelerated ions (TAI). On the nightside, they are
observed at low altitudes from 400 km by sounding
rockets during active aurora, to 1700 km frequently by
the Freja satellite. In the dayside, they are observed at
3000 km by the Akebono satellite.
On the other hand, ion beams with peak flux along the
magnetic field are directed outward. These ions are
observed at high altitudes (>5000 km), and may also be
transformed from ion conics at low altitudes because of
the magnetic mirror force. However, statistical altitude
distributions of beams and conics reveal that the magnetic mirror force is not necessarily the only acceleration
of ion beams. Both ion beams and conics have energies
ranging from 10 eV to a few keV.
Statistical studies of upflowing ion beams and conics
have been conducted by Yau et al. [1984] and Kondo et al.
[1990]. They utilized DE 1 measurements to study upflowing ion beams and conics of H and O , in energy range
from 0.01 to 17 keV (see Fig. 21.4). They found that the
source of upflowing ion beams and conics is mostly confined to the auroral regions. Within the auroral regions,
they also found clear dawn‐dusk asymmetry in the occurrence frequency of ion upward beams and conics. At
geomagnetic quiet times (Kp ≤ 3+), the peak occurrence
frequency of beams with energy higher than 1 keV is in
the premidnight sector (21–24 MLT), whereas the minimum occurrence frequency is in the morning sector. This
asymmetry is absent for lower‐energy ion beams ( 1 keV).
The occurrence frequency of conics for both H and O
peaks on the dayside around 11 MLT, indicating possible
wave interactions in the cusp. At disturbed times (Kp 4−),
two peaks of beam occurrence frequency are found in
the dusk (18–21 MLT) and dawn sectors (03–09 MLT)
for energy above 1 keV, although there is no obvious
asymmetry found for beams with energy below 1 keV.
Conics with energy above 1 keV are found frequently to
appear in the dawn sector (06–09 MLT).
21.3.3. Ionospheric Convection
The chart flow 1 indicates that the asymmetric convection
velocities on the topside ionosphere can cause asymmetric outflow velocities, and consequently asymmetric
outflow flux at high altitudes. In high‐latitude regions of
the topside ionosphere, the ionospheric magnetic field
280 DAWN-DUSK ASYMMETRIES IN PLANETARY PLASMA ENVIRONMENTS
Kp≤ 3+
1.0
0.01–
1 keV
Kp≥ 4–
H+
0+
H+
4–
17 keV
0.5
0.0
Occurrence frequency
Occurrence frequency
0.5
1–
4 keV
0.2
0.1
0.0
0.10
0.0
0.50
0.25
0.00
0.2
0.1
0.05
0.00
0+
1.0
0
6
12 18 24 0
6
12
18
Magnetic local time
24
0.0
0
6
12 18 24 0
6
12 18 24
Magnetic local time
Figure 21.4 Occurrence frequencies of dawn‐dusk asymmetries of upward ion beams (solid circles) and conics
(open circles) for H and O in three energy ranges. Left and right panels are for Kp  3+ and Kp  4−, respectively. The
occurrence frequencies are integrated over altitudes from 8,000 to 23,300 km. Adopted from Kondo et al. [1990].
lines connect to the magnetosphere, where the convection
is highly controlled by the IMF direction [Dungey, 1961].
This leads to IMF modulation and dawn‐dusk asymmetry of the polar ionospheric convection (see also
Chapters 9 and 10). For example, Haaland et al. [2007]
used Cluster EDI data to study the ionospheric convection and electric potentials by mapping magnetospheric
convection. Figure 21.5 shows the electric potentials
mapped to 400 km altitude for the northern hemisphere.
The potentials are derived with Cluster EDI data from
2001 to 2006. The convection velocities VE B can be calculated by: VE B
B /B 2. At places where the equipotential lines are closer to each other, the electric field is
stronger. When the IMF is purely southward, the highest
convection velocities can be found on the meridian line of
noon‐midnight in the high‐latitude polar cap region with
antisunward directions. The convection shows a pattern
of symmetric convection cells. With positive/negative
IMF By, the convection cells rotate clockwise/counterclockwise. The dawn‐dusk asymmetry of ionospheric
con­vection is clear even for northward IMF, and this
phenomena is opposite in the other hemisphere.
Given the same magnetic field configuration, the asymmetric convection results in asymmetric centrifugal acceleration at high altitudes, where the centrifugal acceleration
is the main energization process.
ac VE
VE
B
B
dbˆ
dt
bˆ
bˆ
V
VE
t
S
(21.1)
B
bˆ
In equation (21.1), ac is the centrifugal acceleration
of the ion’s guiding center, VE B is the E B convection
velocity, b̂ is unit vector of magnetic field, and S is unit
length along the magnetic field line. The convection
velocity is important since it highly effects the total centrifugal acceleration. Nilsson et al. [2010] conclude that the
average outward centrifugal acceleration is about 5 m/s2.
Although it is small, the cumulative effect could result in a
significant high velocity even for low‐energy ions.
21.3.4. Polar Cap Outflow
In the polar‐wind scenario [Axford, 1968], charge separation is formed because ions usually have a lower scale
height than electrons. Consequently, an ambipolar electric field is generated. The ambipolar electric field can
accelerate the ions and enable them to escape. Rather
than transverse heating by waves in the auroral region,
ions escaping from polar cap are produced by centrifugal
acceleration at high altitudes.
Polar wind is characterized by low‐energy ions (few
10s eV), which are hard to detect by regular measurements due to the spacecraft charging problem [Chappell
et al., 1987]. Below, we refer to this type of ion as cold
ions. Few observations have been conducted before
[e.g., Su et al., 1998; Seki et al., 2003], with artificially
modulated low‐energy part of Maxwellian distribution,
or measurements in the illumination shadow of the Earth
where there is no spacecraft charging.
With the “wake” technique to measure the electric field
in the plasma wake formed by the positively charged
Dawn‐Dusk Asymmetries of Ionospheric Outflow 281
Sector 7: Bz+/By–
12
Sector 0: Bz+
12
∆U = 18.3 kV
Sector 1: Bz+/By+
12
∆U = 15.5 kV
∆U = 27.6 kV
0
18
06
3
3
–6
80
70
06
06
80
18
70
9
3
0
00
7.1 kV
–8.7 kV
Sector 6: By–
12
∆U = 33.5 kV
80
18
70
06
–13.3 kV
0
10
20
30
3
–6
–10
0
Potential [kV]
0
Sector 5: Bz– /By–
12
∆U = 51.3 kV
60
06
9
COR wmap < 5.0 km/s bias_> = _0.96 MP_dist_ > 2RE
15.5 kV
70
2
–1
–6
–12
–20
14.2 kV
00
80
18
–30
3
Sector 2: By+
12
∆U = 40.6 kV
06
9
00
6.8 kV
North Polar Cap
2001/02–2006/03
EDI C1–C3
3
–17.9 kV
00
06
15
–11.2 kV
06
–6
0
06
–6
18
70
0
80
–19.2 kV
00
21.4 kV
Sector 3: Bz–/By+
Sector 4: Bz–
12
∆U = 61.9 kV
12
80
70
60
06
18
80
70
60
06
15
21
–6
9
9
15
21
–2
18
2
–1
–12
–
–1 24
8
4
06
0
60
27
70
–18
–6
80
21
15
18
–12
–18
–6
∆U = 54.3 kV
3
3
0
–27.3 kV
00
24.0 kV
–27.3 kV
00
3
9
0
34.6 kV
–26.4 kV
00
27.9 kV
Figure 21.5 Electric potential of the ionosphere as a function of magnetic coordination in AACGM system and
IMF orientation for the Northern Hemisphere. IMF orientation is indicated at the top of each sector. Potential in
sectors is calculated with EDI measurements and mapped to 400 km altitudes, and color coded as in the colorbar
at the center. Equipotential lines overlapping the colors are with a 3 kV spacing. Maximum and minimum potentials are indicated at the bottom of the sector. Total potential drop ∆U is shown at top right of each sector. Adopted
from Haaland et al. [2007].
spacecraft, the bulk velocity of cold ions can be derived
[Engwall et al., 2009], as well as the density by means of
spacecraft electric potentials. Li et al. [2012] studied the
source of cold ions on the ionosphere by calculating ion
trajectories. The trajectory calculation takes into account the
centrifugal acceleration and ionosphere‐magnetosphere
convection enters as indicated in equation (21.1). They
found the cold ions originate from the polar cap region
without significant dawn‐dusk asymmetry. However,
in the follow‐up study, Li et al. [2013] show a clear
282 DAWN-DUSK ASYMMETRIES IN PLANETARY PLASMA ENVIRONMENTS
21.4. SUMMARY
(a)
Y[RE ]
IMF southward
10
X[RE ]
–60
–50
–40
–30
–20
–10
0
–10
(b)
Y[RE ]
10
X[RE ]
–60
–50
–40
–30
–20
–10
0
–10
Flux [s–1/cm2]
<=104
105
>=106
Figure 21.6 Cold ion outflow flux supply on the plasma sheet
during a period of southward IMF. (a) or (b) the outflow fluxes,
which can be traced back to the Northern or the Southern
Hemisphere, with ion trajectory calculation by taking into
account the accelerations in equation (21.1) and the magnetospheric convection. Consistent dawn‐dusk asymmetry is
also found during periods with IMF in other directions. After
Li et al. [2013].
asymmetry in the cold ion supply to the plasma sheet
(see Fig. 21.6). The cold ions consistently tend to supply
more to the dusk side of the plasma sheet regardless of
the IMF direction. This phenomenon exhibits in the
other hemisphere with similar pattern. As the orbit of
Cluster is higher in the southern hemisphere, cold ion
outflow is generally measured at higher altitudes in that
hemisphere than in the northern hemisphere. Since ions
at higher altitudes are transported to the plasma sheet
further in the tail, ionospheric cold ion outflow from the
southern hemisphere can be found further in the tail. This
is attributed to the hemispheric asymmetry shown in two
panels in Figure 21.6.
This asymmetry, with more cold ion flow on the dusk
plasma sheet, coincides with the fact that the signatures
of reconnection are more commonly observed on the
dusk side, as summarized in Walsh et al. [2014].
Asymmetries are found to be general in both ionospheric particle heatings and ionospheric outflow. In
section 21.2, the asymmetric heatings are found to exist
for both ions and electrons. The heating of electrons
essentially increases the electron scale height, resulting
in a larger ambipolar electric field that accelerates the
ions. An important asymmetric heating of electrons
depicted in previous studies is the electron precipitation
associated with asymmetric spatial distribution of the
cusp. Moreover, diffuse and monoenergetic aurora with
dawn‐dusk asymmetry plays a role in asymmetric electron heating. In auroral and cusp regions, ion outflow
composed of ion beams and conics are often observed
with dawn‐dusk asymmetry. Besides, the electron precipitation increases the ionospheric conductance, which
causes an asymmetric Poynting flux for both cusp and
auroral regions. Asymmetric Poynting flux coupled
with asymmetric atom‐neutral particle friction bring
asymmetric Joule heating. With asymmetric scale
height of ions, the ion upwelling is asymmetric. In the
polar cap, although there is no evidence of asymmetry
in ion energization, the asymmetries in the ionospheric
convection driven by the IMF play a role for ion outflow at high altitudes.
The ionosphere is heated up by solar irradiation on
the dayside. When dayside ionosphere corotates with
the Earth duskward, the plasma in the ionosphere with
higher temperature fills into premidnight sector. Like­
wise, the cooler plasma from the postmidnight sector
of the ionosphere rotates to the morning side. A dawn‐
dusk asymmetry with high temperature on the dusk
side is consequently considered to exist. However, no
clear evidence from observation of polar‐cap outflow
could confirm this situation. This requires further
investigation.
It is interesting that the cold ion outflow from the
polar cap is consistently found more on the duskside
of the plasma sheet, even without asymmetry in its
source region. This may cause duskside favored reconnection, and fast flow on the duskside of the magnetotail. The ionospheric conductance in the dusk sector
may consequently be enhanced. This can be observed
as an asymmetric Poynting flux, which is also simulated.
The open question remains: is asymmetric auroral ion
outflow caused by asymmetry in the Poynting flux, a
trigger of asymmetric supply of cold ion on the plasma
sheet?
Another open question is, how long is the time taken by
each energization process for the ion outflow? And how
much asymmetric energy input to the ionosphere could
generate the observable asymmetry in ion outflow?
Dawn‐Dusk Asymmetries of Ionospheric Outflow 283
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