Generation Mechanisms of Ion Upflow
in the Polar Topside Ionosphere（極域上部電離圏におけるイオンアップフローの生成機構の研究）

Yasunobu Ogawa
小川 泰信

Division of Particle and Astrophysical Science
Graduate School of Science
Nagoya University, Japan
名古屋大学大学院理学研究科素粒子宇宙物理学専攻

February 2002

A Dissertation for the Degree of Doctor Scientiarum

Generation Mechanisms of Ion Upflow
in the Polar Topside Ionosphere（極域上部電離圏におけるイオンアップフローの生成機構の研究）

Yasunobu Ogawa
小川 泰信

Division of Particle and Astrophysical Science
Graduate School of Science
Nagoya University, Japan
名古屋大学大学院理学研究科素粒子宇宙物理学専攻

February 2002

Acknowledgments

I would like to express my deepest gratitude to my supervisor Prof. Ryoichi Fujii for his
continuous encouragement of my work on this thesis. This thesis could not have been
completed without his appropriate guidance. I would also like to express my sincere gratitude
to the other members of the advisory committee, Prof. Tadahiko Ogawa, Prof. Masayoshi
Kojima, and Dr. Kazuo Shiokawa, for their constructive comments on this thesis. I would also
like to express my respectful gratitude to Dr. Satonori Nozawa who took time to help withdata collection. He also read this thesis and gave me a number of helpful suggestions.
I would like to sincerely acknowledge Dr. Stephan C. Buchert at the Swedish Institute ofSpace Physics (IRF) in Sweden for the helpful discussions and encouragement on the entiretyof this thesis. I also deeply appreciate the discussions and suggestions of Dr. Francois Formeat Centre d’etude des Environnements Terrestre et Planetaires (CETP) in France, which
greatly helped me with research of Chapter 5 in this thesis.
I am deeply grateful to Dr. Shin-ichi Ohtani at the Applied Physics Laboratory, Johns
Hopkins University (APL/JHU) in the USA, for giving me a wonderful chance forcollaboration using the data of the DMSP satellites. He kindly helped me during my visit to
APL/JHU in May 2001, and gave me useful advice and comments on my study.
I would like to express my sincere appreciation to Prof. Shigeto Watanabe at Hokkaido
University, Prof. Sawako Maeda at Kyoto Women’s University, and Prof. Toshifumi Mukai
and Dr. Takumi Abe at the Institute of Space and Astronautical Science (ISAS), for their kindencouragement and useful advice on my study. I would also like to acknowledge thecontribution of Mr. Toshiaki Yamaguchi, Ms. Youko Yamaguchi, Ms. Judy Kishida, Dr. Shinichiro Oyama, Dr. Susumu Saito, Mr. Minoru Endo, Ms. Katsue Suzuki, Mr. Masahiko
Sugino, Mr. Hirotada Kataoka, Mr. Kazuhiro Morise, Mr. Syun-ichi Matsuo, Mr. Kazuhiro
Adachi, Ms. Seiko Imaida, Mr. Masaki Kono, Mr. Hiroyuki Iwahashi, Ms. Satoko Sumiyama,and Mr. Takafumi Tamagawa, who are the staff, graduate students, and graduates of the Solar-iTerrestrial Environment Laboratory (STEL) of Nagoya University, for their support andfriendship. I am also thankful to Mr. Naofumi Yoshida at Tohoku University, and Mr. Manabu
Yamada at Hokkaido University, for their studying and discussing with me the topics
associated with this thesis.
I am indebted to the director and staff of the EISCAT for operating the facility andsupplying data. EISCAT is an international association supported by Finland (SA), France(CNRS), the Federal Republic of Germany (MPG), Japan (NIPR), Norway (NFR), Sweden(NFR), and the United Kingdom (PPARC). The DMSP particle detectors were designed by Dr.
Dave Hardy of AFRL, and the data was obtained from JHU/APL. I also thank Dr. Dave Hardy,
Dr. Fred Rich, and Dr. Patrick Newell for the use of the detectors. I am also grateful to thestaff of the CDA web for providing public WIND and ACE satellite data.
Finally, I would like to express my deepest thanks to my family for their continuous
encouragement and especially to my parents for their permission for me to proceed with mystudies in the graduate school.

iiGeneral Notation
Eelectric field vector [V m-1]

B

geomagnetic field vector [T]Vvelocity vector [m s-1]

ggravitational acceleration vector [m s-2]

jcurrent density vector [A m-2]eelectric charge [C]ttime [sec]mmass [kg]nnumber density [m-3]T

temperature [K]P

pressure [Pa]I

geomagnetic dip angle [˚ ]kwave number [m-1]νcollision frequency [s-1]kB

Boltzmann constant (= 1.38 × 10-23 J K-1)

Subscript
eelectrons

i

ions

n

neutralsinion-neutralie

ion- electron

enelectron-neutraleielectron-ion//

parallel to the geomagnetic field line⊥

perpendicular to the geomagnetic field line

iii

iv

Abstract

We have investigated three aspects of ion upflow phenomena in order to understand thegeneration mechanisms of ion upflow. The first step was to investigate the relationshipbetween ion upflow and particle precipitation which is different between several
magnetospheric regions. The second step was to investigate the relationship between ion
upflow and heating, from the macroscopic point of view. The third step was to investigate thecharacteristics of naturally enhanced ion-acoustic lines (NEIALs), which may be caused byplasma instabilities and are strongly related to ion upflow from the microscopic point of view.
We have examined the regions where dayside field-aligned (FA) ion upflows occur, basedon a statistical analysis using approximately 170 simultaneous events between the EISCAT
Svalbard radar (ESR) and the DMSP satellites. This systematic analysis for ion upflow has
never been examined. We found that ion upflows occur not only in the cusp and cleft (thelow-altitude portion of the low-latitude boundary layer (LLBL)) which have been considered
as ion upflow regions, but also in the topside ionosphere connected to the mantle region. Ion
upflows seldom occur either in the Boundary Plasma Sheet (BPS) or in the Central Plasma
Sheet (CPS) in the dayside high latitudes. Almost all of the events in which the average FA
ion velocity is more than 100 m s-1 are associated with sufficient soft electron precipitation(differential energy flux of electrons at 100 eV > 107 eV cm-2 s-1 sr-1 eV-1). Although soft
electron precipitation also sufficiently exists in the BPS, the ion velocities are mostly less than
100 m s-1. The present results indicate that soft particle precipitation is the predominantenergy source driving ion upflow in the topside ionosphere, but it works on ion upflow
effectively in the higher latitude regions in the dayside and not in the BPS.
Plasma heating associated with FA ion upflow in the dayside topside ionosphere has beenpresented using data obtained simultaneously with the ESR and the EISCAT VHF radar. The
FA ion upflows observed at an altitude of 665 km in the dayside cusp are associated withsignificant anisotropy of ion temperature, isotropic increases of electron temperature andv

enhancements of electron density. The anisotropy factor reaches maximum values of 1.7.
There is no clear correspondence between the enhancements of the electric field strength andthe occurrence of the ion upflows. This is the first experiment to point out ion anisotropy
independent of frictional heating. The ion upflow associated with perpendicular heating seems
to closely correspond to NEIALs, because the FA ion velocity immediately changed aroundstrong NEIALs.
The wave number (k)-dependence of the received power in high signal-to-noise ratio(SNR) conditions, occurring for NEIALs and for real satellites, has been investigated. These
analyses help to specify generation mechanisms of NEIALs. In the case of NEIALs, we foundthat variations of the relative power between channels well above both the estimated andexpected 1-sigma level occur over a signal pre-integrated profile, and a few profiles where thepower radiated by NEIALs varies systematically with wavelength/wave number (k). Thisfeature of NEIALs does not seem to be caused by the ESR system. Furthermore, power
differences along one profile with NEIALs are obviously different from those of echoes fromsatellites at a similarly high SNR. The most plausible explanation we can suggest is that the kdependence of the power in NEIAL events has its origin in the scattering medium itself. The
frequency dependence of the power in NEIAL events and the characteristics of the enhanced
peak heights can be well explained by the parametric decay of Langmuir waves [Forme et al.,2001].
The results from these three investigations lead to the conclusion that the energy of soft
electron precipitation is considered as the main source of the ion upflow. The energy of soft
particle precipitation is supplied to the ions in the topside ionosphere via wave-particle
interaction, such as wave-induced transverse ion heating, and upward parallel electric field
due to anomalous resistively produced by plasma turbulence. Thus generation mechanisms ofion upflow must have the transversely ion heating and the upward acceleration by waveparticle interaction while the induced plasma waves decay. In addition to direct precipitation
effects, namely enhanced ambipolar diffusion and heat flux, wave-particle interaction may
hence play an important role in driving ion upflow.

viContents

AcknowledgmentsiAbstractv

Contents

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1

1

General Introduction
1.1 Ionosphere in the High Latitudes

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1.2 Particles in Each Magnetospheric Region
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1.2.2 Low-Latitude Boundary Layer
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1.6 Purpose of Thesis

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1.5 Naturally Enhanced Ion-Acoustic Lines

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Ion Outflow Processes From Ionosphere to Magnetosphere

1.4 Ion Upflow in the Polar Ionosphere

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1.2.4 Plasma Sheet and Plasma Sheet Boundary Layer
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Instruments

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2.1

EISCAT Radars

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DMSP Satellites

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Occurrence Regions of Ion Upflow in the Dayside Ionosphere
3.1 Instruments and Data
3.2

Results

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3.3

Discussion

3.4

Chapter Summary

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Ion Temperature Anisotropy Associated With Ion Upflow
4.1 Experiment

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4.2

Data

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Observations

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Chapter Summary

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k-Dependence of the Naturally Enhanced Ion-Acoustic Lines
5.1

Purpose

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Methods

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Results

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Comparison with Echoes From Satellites

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Discussion

5.6

Chapter Summary

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6

Conclusions and Suggestions for Future Studies

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A

Ion Motions in the Polar Topside Ionosphere89

A.1 Theoretical Formulation

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Observational Results

Bibliography

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viii

Chapter 1

General Introduction

The Earth’s magnetosphere is formed by magnetic fields originating from within the Earth,
the solar wind and the interplanetary magnetic field (IMF) originating from within the Sun,and plasma particles existing there. The plasma particles in the magnetosphere are notgenerated there, but are transported from other places. With regard to the locations supplying
plasma particles, it was considered in the past that the solar wind, composed by hydrogen ions(H+), helium ions (He+), and electrons, was the source of the magnetospheric plasma because
particles originating from within the Earth were gravitationally bound on the Earth’s surface.
However, observations over the last three decades clearly indicate that ionospheric ions, suchas molecular nitrogen ion (N2+), molecular oxygen ion (O2+), and oxygen ion (O+) are indeed
in the magnetosphere [Yau and André, 1997 and references therein]. Thus, it is now clear thatthe ionosphere is an important source of ions for the magnetospheric plasma [Chappel, 1988;
Shelly, 1988]. In other words, the outflows of ionospheric ions to the magnetosphere are also
important for the formation of the magnetosphere. Additional evidence exists demonstratingthat the geomagnetic activity seems to control the supply of magnetospheric ions from theionosphere. A higher geomagnetic activity enhances a greater abundance of ionosphericplasmas in the magnetosphere. On the other hand, plasmas in the plasma sheet seem to be atleast partly controlled by solar wind during geomagnetically quiet situations [e.g., Winglee,
1998]. This change of the magnetospheric ion composition is important, because increased
heavy ion such as O+ can have large effects on some important magnetospheric processes [e.g.,
Fu et al., 2001].
The ionosphere and the magnetosphere are closely linked together through the magnetic1

field lines. Ions, in particular, in the polar ionosphere obtain energy and momentum from themagnetosphere through particle precipitation, heat flux, propagation of electric fields andplasma waves. Consequently ions in the ionosphere occasionally can move up to higher
altitudes and subsequently flow into the magnetosphere and interplanetary region. Recently
characteristics of the magnetospheric ions originating in the ionosphere have become clear asa result of several studies using satellites [Yau and André, 1997 and references therein].
However, little is known about ion heating and acceleration mechanisms around the polar
topside ionosphere, where ions do start to flow up to the magnetosphere. In this thesis, wehave investigated the generation mechanisms of this ion upward flow occurring at the polar
topside ionosphere.
For a general introduction, characteristics of particles in the polar ionosphere and themagnetosphere are briefly reviewed in this chapter. We particularly describe distributions ofparticles in the polar ionosphere in Section 1.1, in several of the magnetospheric regions in
Section 1.2, and in several ion outflow phenomena observed mainly in the bottomsidemagnetosphere in Section 1.3. Characteristics of ion upflow in the topside ionosphere andnaturally enhanced ion-acoustic lines (NEIALs), which we mainly focus on this thesis, aredescribed in Section 1.4 and Section 1.5. Finally, the purpose of this thesis is described in
Section 1.6.

1.1

Ionosphere in the High Latitudes

The Earth is covered with the atmosphere, in which particles are gravitationally bound to low
altitudes and maintain their distribution under a diffusive equilibrium. Figure 1.1a shows
typical height profiles of the number density of different neutral species below 1000 km
altitude at the high latitude, calculated using the Mass Spectrometer and Incoherent Scatter(MSIS) neutral atmosphere model [Hedin, 1991]. Below approximately 150 km altitude, themajor neutral species are molecules of nitrogen (N2) and oxygen (O2), which are the same as
those found on the ground. Above approximately 300 km altitude, the number density ofatomic oxygen (O) exceeds those of N2 and O2. The atomic oxygen is produced byphotodissociation and energetic particle impact dissociation. A minor species, atomic nitrogen(N) and nitrogen monoxide (NO), is produced by molecular dissociation and various chemical
reactions. Above approximately 900 km altitude, the number densities of atomic helium (He)and hydrogen (H) exceed that of O. The transition height strongly depends on the solar2

activity and season. While O dominates the upper atmosphere even above 1000 km duringsolar maximum conditions, He and H dominate above 500 km during solar minimum andwinter conditions.
Around a few hundred km in altitude, various gases in the atmosphere are partly ionized
mainly by solar EUV. The region of these mixed ions, electrons, and neutral particles is called
the ionosphere. Solar irradiation of intensity decreases due to absorption downward in the
Earth’s atmosphere where the neutral density increases toward the Earth. The ionization at
higher altitudes (above approximately 400 km altitude) balances the diffusion of ions under adiffusive equilibrium, whereas the ionization at lower altitudes (below approximately 200 km
altitude) balances the recombination between ion and electrons under a photochemical
equilibrium. The ionosphere therefore has a peak at a certain altitude. Figure 1.1b shows
typical height profiles of the number density of electrons and different ion species below 1000km altitude, calculated using the International Reference Ionosphere (IRI) model [Bilitza,
1997]. A clear peak is seen at approximately 300 km altitude, as we have mentioned before.
The region around the peak is called the F region, and the peak is called the F peak.
Furthermore, a region above the F peak is called the topside ionosphere, and a region below
the F peak, around an altitude of 100 km, is called the E region. The number densities ofelectrons and total ions around the F peak are approximately 3× 1011 m-3, which is roughly
1000 times smaller than that of neutral particles. Hence the ionosphere consists of weakly
ionized plasma.
Regarding the composition of ionospheric ions, major neutral particles at each altitude, i.e.
N2, O2, and O, are ionized by the solar EUV and changed into a molecular nitrogen ion (N2+),
molecular oxygen ion (O2+), and oxygen ion (O+). These ions are changed into other ions or
neutrals through chemical reactions. First, N2+ recombines with an electron and dissociates
into two nitrogen atoms (N), or recombines with the abundant O2 and O and forms O2+ andnitrogen monoxide ion (NO+). Secondly, O2+ remains there, or recombines with an electron
and dissociates into two oxygen atoms (O). Finally, O+ also remains there, or recombines withthe abundant N2 and O2 and forms NO+ and O2+. Consequently NO+ and O2+ are dominant
below approximately 200 km, while O+ is dominant above 200 km. The composition ratios aswell as the absolute values change by solar zenith angles, particle precipitations, and theionospheric electric field [e.g., Schunk, 1975]. Above approximately 900 km altitude, thenumber density of the hydrogen ion (H+) exceeds that of O+. Although the transition heightfrom O+ to H+ as well as that from O to H strongly depends on the solar activity and season,3

dominate regions of the heavy ions such as N2+, O2+, and O+ are always maintained below theregion where H+ is dominant.
The temperature of the ionosphere is essentially controlled by the absorption of solar UV
radiation in the thermosphere. A typical model of temperature profiles for the neutrals, ions,and electrons is shown in Figure 1.2. The electrons have a larger mobility and heat
conductivity so that the temperature of the electrons usually becomes higher than thetemperature of the ions. The ions are heavier and interact by collision more strongly withneutrals than with electrons around the F peak. Because of the similar masses between thecolliding species, much of the excess energy of the ions is transferred to the neutrals. The
temperature of the ions is therefore smaller than that of the electrons, but higher than thetemperature of the neutrals around the F peak. Ion temperature increases with increasing
altitude and comes close to the electron temperature at the topside of the F peak because
collisions between ions and electrons become dominant at higher altitudes. These temperature
profiles are also significantly dependent on solar activity [e.g., Roble and Emery, 1983]. At
the high latitude, these temperatures are also strongly affected by electric fields andprecipitations from the magnetosphere.

1.2

Particles in Each Magnetospheric Region

As already mentioned, the ionosphere and magnetosphere are closely linked together throughthe magnetic field lines. Magnetospheric electric fields map down to the ionosphere, creating,for example, the plasma convection, frictional heating and plasma instabilities. At highlatitudes, particle precipitation from the magnetosphere ionizes the atmosphere. The heat flux
from the magnetosphere is also conducted down to the ionosphere. The characteristics ofparticle precipitation from the magnetosphere are quite different from one regime to anotherin the magnetosphere, such as the plasma sheet, low-latitude boundary layer (LLBL), lobe,
mantle, and cusp (see Figure 1.3). In particular, the dayside auroral precipitation consists ofmagnetosheath origin related boundary layer precipitation, and plasma sheet origin related
Boundary Plasma Sheet (BPS) and Central Plasma Sheet (CPS) precipitation considered as
extensions of the nightside precipitation regions. We first describe the characteristics of theseregions.

4

1.2.1Cusp

The magnetic field lines of the Earth can be divided into two parts according to their locationon the sunward or tailward side of the Earth (see Figure 1.3). Between these two parts on bothhemispheres are funnel-shaped areas with a near zero magnetic field magnitude called thepolar cusp. It allows the magnetosheath plasma to directly enter the ionosphere, which
generally occurs where the ion cutoff drops to at least 1 - 3 keV and below. The cusp position
and the precipitation fluxes strongly depend on the IMF and solar wind conditions [Newell etal., 1989; Zhou et al., 2000].
1.2.2

Low-Latitude Boundary Layer

Low-latitude boundary layer (LLBL), first discussed by Eastman et al. [1976], is the daysidemagnetospheric boundary layer at a low-latitude and continues to the magnetotail. It consistsof a mixture of plasmas of magnetospheric and magnetosheath origins; the magnetosheathplasma may leak diffusively through the magnetopause into this region. The density andtemperature are approximately 5× 105 - 107 m-3 and from less than 100 eV to 1000 - 2000 eV,
respectively. The LLBL is distinguished from the cusp by higher average particle energies andsmaller precipitating ion energy fluxes. The low-altitude portion of the LLBL is called thecleft.
1.2.3

MantleMantle, first defined by Rosenbauer et al. [1975], covers much of the high-latitude
magnetosphere, extending tailward from the cusp region (see Figure 1.3). Mantle ischaracterized by de-energized magnetosheath plasma. The density and temperature are a few
times 104 - 106 m-3 and approximately 100 eV, respectively. The particles flow tailward at avelocity of 100 - 200 km s-1. The density and energy often decrease when moving inward
from the magnetosheath towards the magnetosphere (velocity filter effect); at a low-altitude
this creates an energy-latitude dispersion. Ions originating in the ionosphere have also beenfound in the mantle/lobe regions [Mukai et al., 1994; Hirahara et al., 1996; Seki et al., 1998].

5

1.2.4

Plasma Sheet and Plasma Sheet Boundary Layer

Plasma sheet is located in the center of the tail near the equatorial plane (see Figure 1.3). The
Plasma sheet particles are hot, having energies in the keV range. The plasma density is a
slowly varying function of time, the average value being approximately 4 × 105 – 2× 106 m-3.
The ion temperature is approximately 7 times higher than the electron temperature[Baumjohann et al., 1989]. Plasma Sheet Boundary Layer (PSBL) is generally observed as atransition region between the lobes and the plasma sheet (see Figure 1.3). The number density
is typically 105 m-3, which is slightly smaller than that in the plasma sheet.
From ionospheric observations related to the plasma sheet and PSBL, two different typesof precipitations have been found. They were defined by Winningham et al. [1975] as Central
Plasma Sheet (CPS) and Boundary Plasma Sheet (BPS), respectively, because traditionally the
CPS is assumed to map to the plasma sheet in the magnetosphere and the BPS to the PSBL.
However, these interpretations have been modified by several observations in themagnetosphere [e.g., Feldstein and Galperin, 1985; Sandahl and Lindquist, 1990; Fukunishi
et al., 1993]. Through those observations, there is general agreement that the plasma sheet in
the tail covers the whole of the auroral oval in the nightside and the inverted-V region
associated with the (low-altitude) BPS must be mapped to the outer portion of the plasma
sheet, not to the PSBL. Thus, the CPS and BPS terms defined by low-altitude observations
have been used simply as the phenomenologically defined regions of the diffuse and discrete
aurora on the nightside of the Earth.
Dayside precipitation in the BPS closely resembles the poleward portion of the nightsideauroral oval. The electrons have a typical temperature of approximately 300 eV, which issomewhat higher than the LLBL. It remains an unsettled question whether BPS as well as
LLBL belongs to the closed field-line region or to the open one [Lockwood, 1997]. The CPS
is the hard zone of the electron precipitation whose typical energies are above 1 keV. The CPS
in the dayside consists of electrons that have been injected into the near-Earth region from thenightside and subsequently drift around the Earth.
Figure 1.4 shows the results of the mapping of several magnetospheric regions mentioned
above to the dayside ionosphere, according to particle precipitation characteristics.
Ionospheric ions have been found in almost all magnetospheric regions mentioned above. In
the next section, we describe several kinds of ion outflow processes from the ionosphere to6the magnetosphere.

1.3
Ion Outflow Processes From the Ionosphere to theMagnetosphereAn important phenomenon of the ionosphere-magnetosphere coupling is the formation of
upward ion flows from the ionosphere since they can be a significant source ofmagnetospheric plasma. Based on several observations around the bottomside magnetosphere,
ion outflow phenomena can be grouped into two categories. One is the bulk ions in theionosphere escaping to the magnetosphere. The other one is a fraction of the bulk ionsaccelerated and escaping to the magnetosphere. We review the characteristics of ion outflow
in order.
In the polar cap, bulk ions continuously escape without any special heating. The
phenomenon is called “polar wind.” Abe et al. [1993] shows the height distributions of thepolar wind in which ions start to flow up around 3000 km and the escape velocities become
faster with increasing altitude (see Figure 1.5). Ion species of the polar wind are considered tobe H+, He+, and O+, which are relatively lighter ions in comparison to NO+ and O2+.
The ion outflow associated with partially accelerated ions can be divided into several
types, such as ion beams, transversely accelerated ions (TAIs), ion conics, and upwelling ions
(UWIs). The characteristics are usually seen in the high-energy part of the ion velocity
distributions. Ion beams have a peak flux along the upward magnetic field direction and are
generally observed above 5000 km altitude. The field-aligned ion energization of several eV
results from parallel electric fields.
TAIs and ion conics were first observed by Sharp et al. [1977]. Ion conics have a peak
flux at an angle to the upward magnetic field direction, while TAIs have peak pitch angles ator close to 90˚. Ion conics are sometimes interpreted as TAIs from a lower altitude which
spiral up the field line as a result of the magnetic mirror force. On the dayside, TAIs are
regularly present down to approximately 3000 km on the Akebono satellite [Whalen et al.,1991]. On the nightside, TAIs are observed at 1400 km on the ISIS-2 satellite [Klumpar,
1979] and at 1700 km on the Freja satellite [André et al., 1994], and frequently down to 400
km on sounding rockets during an active aurora [Yau et al., 1983; Arnoldy et al., 1992]. Ion
conics also have been observed at altitudes down to approximately 1000 km on sounding
rockets and on the ISIS-2 satellite [Klumpar, 1979; Yau et al., 1983], and up to several Earth’s

7

radii on the PROGNOZ-7 satellite [Hultqvist et al., 1983].
TAIs and ion conics are related to transverse ion heating events with typical energies fromthermal to a few keV accompanied with upward flowing ions, electron precipitation anddensity depletions, and a variety of different resonant waves, such as lower hybrid (LH)
waves or low-frequency broadband electrostatic waves [e.g., Moore et al., 1996; André, 1997].
Recent satellite and rocket observations indicate that broadband low-frequency emissions are
the most important wave signatures associated with perpendicular ion energization, probably
followed by waves around a LH wave frequency. In a statistical study using data from theFreja satellite obtained near 1700 km [André et al., 1998], perpendicular O+ ion heating tomean energies above 5 - 10 eV were classified into the four types shown in Figure 1.6. Type 1
and 2 are both associated with broadband low-frequency waves (a few Hz). Type 3 isconnected with auroral electrons and waves near half of the hydrogen gyro frequency (about
200 Hz) or waves around the LH wave frequency (about 1 kHz), while type 4 is associatedwith precipitating H+ or O+ and waves around the LH wave frequency. Type 2 is also
associated with precipitating protons with keV energies and electrons with mean energies ofapproximately 100 eV. It is clear that broadband low-frequency waves are the most common
wave signatures in Figure 1.6. The large-amplitude electric field fluctuations can produce aperpendicular acceleration of ionospheric ions [e.g., Lundin and Hultqvist, 1989; Hultqvist,
1996].
Although observed ion conics and TAIs typically have energies from 10 eV to a few keV,lower-energy TAIs also exist and have been reported from sounding rocket flights [Garbe etal., 1992; Yau et al., 1983]. However, observationally they are often difficult to investigate indetail from satellite measurements due to limited detector energy resolution, spacecraft
velocity, and spacecraft charging.

1.4

Ion Upflow in the Polar Ionosphere

Around the auroral oval, bulk ions in the F region/topside ionosphere move upward along
the field lines with transient plasma heating. The phenomenon is called “ion upflow.” Ion
upflow is often observed with Incoherent Scatter (IS) radars located in the high latitudes [e.g.,
Keating et al., 1990; Foster et al., 1998; Endo et al., 2000]. The typical velocity of ion upflow
is a few 100 m s-1 to a few 1000 m s-1, which is smaller than the escape velocity from the
Earth. Hence it is still not understood how the ion upflow at high latitudes relates to the8magnetospheric ion outflow for which the ions have presumably overcome the Earth’s gravity
and succeeded to escape. However, ion upflow must certainly play an important role asplasma sources of the ion outflow, because heavy ions such as NO+ and O2+ found in the ion
outflow and in the magnetosphere should primarily exist only in the lower ionosphere so a
certain process must occur such as ion upflow to bring these heavy ions to the bottomsidemagnetosphere.
IS radar observations by Wahlund et al. [1992a] have shown ion upflow events associatedwith strong perpendicular electric fields, frictional ion heating, an elevated F region and low
electron densities below 300 km indicating a small amount of auroral precipitation. This could
be explained by strong pressure gradients produced by an increased ion temperature, and theconsequent pushing of ions upward. It is possible that these upflow events are bulk plasma
upflows with both ions and electrons moving upward. Observations with IS radars haveshown that rather strong DC electric fields heat ions anisotropically by ion-neutral collisions[Winser et al., 1989]. Modelers have demonstrated that this anisotropic ion frictional heating
can cause ion upflow [Köeröesmezey et al., 1992]. Around the dayside polar ionosphere,
Ogawa et al. [2001] have shown ion upflow associated with frictional heating using dataobtained from the EISCAT Svalbard radar (ESR) and the CUTLASS HF radars.
Wahlund et al. [1992a] also reported ion upflow associated with auroral precipitation,
which showed an enhanced electron temperature and field-aligned currents with the bulk ion
population moving upward and the bulk electron population moving downward. They argued
that this ion upflow may be due to enhanced field-aligned electric fields caused by anomalous
resistively due to low-frequency plasma turbulence (e.g., ion-acoustic turbulence). While thistype of ion upflow occurs, anomalously enhanced IS signals often received toward the fieldaligned direction. We describe the plausible physical mechanisms of the enhanced IS signals
in the next section in this chapter.
Although both ion upflow and TAIs seem to occur in the vicinity of the auroral zone andare related to the soft electron precipitations, no ion anisotropy has been reported in this typeof ion upflow. It is hence a clear question as to how the ion upflows induced by particle
precipitation are related to heating associated with TAIs, which are often observed at
somewhat higher altitudes as well as at higher transverse energies. Generally, IS radarobservations of ion upflow phenomena are difficult to correlate with satellite measurements.

9

1.5

Naturally Enhanced Ion-Acoustic Lines

IS radar cannot observe plasma waves, which are strongly associated with ion accelerationand ion outflow in high altitudes. However, powerful spectra with strongly asymmetricfeatures, which may be caused by plasma instabilities, have been frequently observed with IS
radars. Figure 1.7 shows the typical examples of powerful spectra. The main characteristics ofthe observations are that one of the ion-acoustic shoulders of the IS spectra is strongly
enhanced. At certain altitudes, both sides of the spectra can be simultaneously enhanced
during the time the radar takes to integrate its signal. These characteristic ion lines have beencalled the naturally enhanced ion-acoustic lines (NEIALs). The statistics seem to indicate thatthe upper shoulder (ion-acoustic waves going downward) is more frequently enhanced thanthe lower shoulder (waves going upward) at altitudes below 300 km. Conversely the lower
shoulder is enhanced more frequently at altitudes exceeding 450 km [Rietveld et al., 1991,
1996]. At altitudes between 300 and 450 km, neither of the shoulders dominated the statistics.
A theoretical explanation for the generation of NEIALs is still a matter of debate. Twomain classes of physical processes have been advanced so far. The first class relies on relative
drifts between the thermal species in the plasma to excite the unstable waves. Foster et al.
[1988], Rietveld et al. [1991], and St.-Maurice et al. [1996] proposed the ion-acoustic current
driven instability [Kindel and Kennel, 1971] as the physical process producing NEIALs.
However, the required current densities would have to be of the order of 1 mA m-2, which has
never been reported with direct observations. Although a relative drift between two ion
populations can also reproduce NEIALs theoretically [Wahlund et al., 1992b], the required
drift velocity greatly exceeds the ion sound speed.
The second mechanism producing NEIALs is enhanced ion-acoustic fluctuations due toparametric decay of Langmuir waves [Forme, 1993]. In this case, the ion-acoustic waves
would be produced by the non-linear cascade of plasma waves. Low-energy precipitating
electrons would first excite electron plasma waves, which by cascade would then produce
secondary plasma and ion-acoustic waves. The parametric decay of beam-induced Langmuir
waves has been observed in-situ using the Freja satellite [Stasiewicz et al., 1996]. Using the
weak turbulence approximation, Forme [1999] modeled the spectral signature in which theparametric decay of beam driven Langmuir waves can enhance the right, the left, or both ionlines simultaneously as observed. In addition, Cabrit et al. [1996] showed that enhanced
power due to NEIALs can be seen by the EISCAT VHF radar (224 MHz and kVHF ≈ 9.4 m-1)

10

while it is not observed by the EISCAT UHF radar (931 MHz and kUHF ≈ 39.1 m-1) when bothradars simultaneously view the same ionospheric regions. In this parametric decay model, therelative spread of the ion-acoustic k spectrum is determined essentially by the spread of theelectron beam. Hence this parametric decay model has a possibility to explain the result fromthe VHF/UHF radar. However, we have not known whether the characteristics of the kdependence is frequently observed or not, because the previous result was only a case study.

1.6

Purpose of Thesis

In this thesis, we investigate three aspects of ion upflow phenomena in order to understand thegeneration mechanisms of ion upflow. The first is to investigate regions where ion upflow
occurs. The second is to investigate the relation between ion upflow and heating. The third is
to investigate the characteristics of NEIALs, which are strongly related to ion upflow and may
be caused by plasma instabilities. It is again noted that the ion upflow in the polar topside
ionosphere is not a continuous outflow phenomenon of ionospheric light ions, called the polarwind, but an outflow phenomenon of ionospheric heavy ions such as NO+ and O2+
accompanied by their transient heating. The heating mechanisms of ion upflow are likely todiffer from those of ion outflow observed in the magnetosphere, although it is still not wellunderstood how the ion upflow at high latitudes relates to the magnetospheric ion outflow.
Recently, it has been suggested that frictional heating inputs are usually of secondary
importance, and the predominant source driving ion upflows seems to be soft (< 0.5 keV)electron precipitation [Horwitz et al., 1997, Kundsen et al., 1998]. The production mechanismof the ion upflow should be related to the free energy supplied by particle precipitation.
Typical energy and flux density of the particle precipitation are different depending on themagnetospheric regions. In order to understand the ion upflow phenomena, it is important todetermine in which regions of the dayside magnetosphere ion upflow occurs. Nilsson et al.
[1994] found from simultaneous observations by the Søndre Størmfjord IS radar and the Frejasatellite that some ion upflow events occur in the topside ionosphere of the cusp. These ion
upflows were seen in the cusp under several conditions, such as the stable cusp, boundary
cusp and storm cusp [Nilsson et al., 1996]. However, very little is currently known about
whether ion upflow can occur in other dayside plasma regions or not. For example, theanswer to the question of whether the ion upflow occurs in the mantle or LLBL is not known
as is either the question of whether it also occurs in the dayside closed field line regions. A

11systematic study of the dayside ion upflow is thus necessary to assess the regions where ion
upflows occur.
The EISCAT Svalbard radar (ESR), which was constructed in Svalbard (invariant latitude
75.2˚ N) in 1996, allows us to study the high latitude ionosphere connected to the several
magnetospheric regions. In order to understand the generation mechanisms of ion upflow
phenomena in the dayside ionosphere, we determine, using approximately 170 ESR andDMSP satellite simultaneous data samples, in which regions of the magnetosphere and underwhat conditions the ion upflow occurs. This will be presented in Chapter 3.
Previous studies with the IS radars investigated the relation between ion anisotropy andion upflow around the F region where ions start to flow up. However, in order to understandheating mechanisms associated with ion upflow, it is essential to investigate the velocity
distribution of ions at the topside ionosphere where ions flow fully upward. Combined ESR
and EISCAT VHF radar observations provide us with an opportunity to investigate the ion
upflows and anisotropy at the topside of the F region. We hence investigate the relation
between ion upflow and the heating process using data obtained from the simultaneous ESR
and EISCAT VHF radar observations. They will be presented in Chapter 4.
Plasma instabilities as well as soft electron precipitations are considered to drive ion
upflow. As related phenomena, NEIALs are seen in IS signals and the causes of these signals
are proposed using various plasma instabilities. To know whether and how the cross section of
NEIALs depends on the wave vector k may enable us to distinguish between such plasma
instabilities. Cabrit et al. [1996] first discussed the k-dependence of the NEIAL cross section
using the EISCAT VHF and UHF systems simultaneously. Buchert et al. [1999] made use ofdata recorded separately from different receiver channels of the ESR (500 MHz and kESR ≈ 21m-1), which are spaced over frequency intervals of 2 MHz. They suggested from one casestudy that the power of NEIALs varies strongly with wavelength. Such a strong dependenceof k may be explainable by a model involving the parametric decay of beam-driven Langmuir
waves [Forme, 1993]. In order to investigate the characteristics of the NEIALs statistically,
we utilize a larger database of NEIALs events and also compare their power characteristics
with those of real satellite echoes. This will be presented in Chapter 5.

12Figure 1.1: Height profiles of the number density of different neutral species(a) and electron and different ion species (b) below 1000 km altitude above
Longyearbyen, Norway (78.2˚ N, 16.0˚ E) at 0900 UT on July 6, 1998. The
height profiles of different neutral species are calculated using the MSIS 90
model [Hedin, 1991] and electron and different ion species using the IRI 95
model [Bilitza, 1997].
13

Figure 1.2: Height profiles of the temperature of neutral particles, ions, andelectrons below 1000 km altitude above Longyearbyen, Norway (78.2˚ N,
16.0˚ E) at 0900 UT on July 6, 1998. The height profiles are calculated using
the IRI 95 model [Bilitza, 1997].

14Figure 1.3: Schematic diagram of the Earth's magnetosphere showing the various regions
and boundaries [After Roederer, 1981].

15

MLT

MLAT
Figure 1.4: A map of the ionosphere to the magnetosphere based on plasma characteristics
as functions of magnetic latitude (MLAT) and magnetic local time (MLT) [After Newelland Meng, 1992].

16

Figure 1.5: Parallel ion velocity of the polar wind as a function of altitude,
averaged over all magnetic local times, all Kp levels, and all invariant latitudes
above 80˚. (open circles indicate) H+ (crosses) He+, (and pluses) O+. (Left)
Averaged ion velocity; (right) standard velocity deviation (bottom abscissa)
and the number of data samples (top abscissa) [After Abe et al., 1993].

17

Figure 1.6: Different types of ion heating events observed with the Freja satellite near
1700 km height during about 1300 passes as a function of magnetic local time (MLT):(a) number of events and (b) the number of seconds spent in each type of event. Type
1 and 2 are both associated with broadband low-frequency waves, type 3 is connectedwith auroral electron and waves near half of the hydrogen gyro frequency or waves
around the frequency of the lower hybrid wave, while type 4 is associated with
precipitating H+ or O+ and around the frequency of the lower hybrid wave [After
André et al., 1998].

18

Figure 1.7: Examples of IS spectra at the EISCAT UHF radar. A sequence of 5 successive
10 sec data intervals showing how the long-pulse spectra change from normal to anomalous
and back again. Each spectra is normalized [After Rietveld et al., 1991].

19

20

Chapter 2

Instruments

In this thesis, we mainly utilized datasets obtained from the EISCAT radars and the DMSP
satellites. In this chapter, these instruments are briefly described.

2.1

EISCAT Radars

The European Incoherent Scatter (EISCAT) scientific association is a cooperative project
between seven countries: Germany, Finland, France, Japan, Norway, Sweden, and the United
Kingdom. EISCAT is administered from the Headquarters in Kiruna, Sweden and has
operational sites at Tromsø (Norway), Kiruna (Sweden), Sodankylä (Finland) andLongyearbyen (Svalbard). Geographical positions of the EISCAT radar sites and operating
parameters of the EISCAT radars are shown in Figure 2.1 and Table 2.1, respectively. The
EISCAT experiment has been divided into 2 groups: Common Programmes (CP), where thedata are immediately available to all, and Special Programmes (SP), where the use of the data
is restricted to particular users for one year. Both programs are conducted by all sevenassociates.
The facility at Tromsø consists primarily of two radars (at UHF and VHF frequencies).
The UHF radar is a tristatic radar operating at 931 MHz with its transmitter at Tromsø andreceivers at Tromsø, Kiruna, and Sodankylä. The VHF radar operates at 224 MHz with boththe transmitter and receiver at Tromsø. The UHF radars have parabolic 32m antennas that are
fully steerable both in azimuth and elevation, while the VHF radar has four 40 m × 30 m
segments of a cylindrical antenna that are mechanically steerable between a vertical and 60˚21zenith angle to the north. Additionally, the VHF segments can be individually phased up to
14.8˚ from the bore-sight direction. The UHF radar has been operating since 1981 and the
VHF radar since 1987.
The EISCAT Svalbard radar (ESR), located at Longyearbyen, consists of two antennas.
The 32m steerable antenna has been operating since 1996, followed by the installation of 42m
antenna which was completed in 1999. The 42m antenna is permanently directed parallel tothe local geomagnetic field. Both antennas operate at 500 MHz and the output of thetransmitter can be switched between the two antennas at 3.2 sec at the minimum, allowing
experimenters to make almost simultaneous measurements on both the 32m steerable antenna
and the 42m fixed dish.
The ESR has mainly used three radar code designs, called GUP0, GUP3, and TAU0 inorder. We describe these radar code designs in detail, because we have utilized data obtained
from these different radar code designs together in this thesis, mainly in Chapter 3.
First, the GUP0 is a basic long pulse (LP) code and mainly intended for the Fregion/topside observations, between 123 km and 921 km. Figure 2.2a shows the timing
diagram of 8 channels for the GUP0. In every cycle, long pulses are transmitted on 4
frequencies. The pulse length of the transmitted three pulses is 320 µ sec, which corresponds
to a range resolution of 54 km. The pulses provide range coverage from 180 to 921 km. The
last pulse is shortened to 150 µ sec in an effort to reduce ground clutter effects. The range
resolution of the last pulse is approximately 23 km and the pulse provides range coverage
from 123 to 750 km. The gate separation of the GUP0 is 3 km in length, correspond to thereceiver’s sampling rate of 20 µ sec. The pulses and channels for the GUP0 are also
summarized in Chapter 5. This GUP0 had been used for test experiments and CP of the ESRfrom March 1996 until May 1998.
Secondly, the GUP3 is designed for both the E and F region observations. Figure 2.2bshows the timing diagram of 4 channels for the GUP3. In every cycle, one LP and one
alternating code (AC), by use of a 16 × 40 µ sec code, are transmitted on 2 frequencies. The
pulse length of the LP is 640 µ sec, corresponding to a range resolution of 108 km. The range
coverage of the LP is between 150 and 900 km. The range resolution of the AC, derived fromrange ambiguity functions, is approximately 3 km at maximum. The range coverage of the AC
is between 20 and 750 km. The gate separation of both the LP and AC of the GUP3 is 3 km in
length as well as that of the GUP0. This GUP3 had been used for CP of the ESR from August
1998 until March 2000.

22

Finally, the TAU0 has been used for CP of the ESR since the GUP3, March 2000. The
TAU0 is also designed for both the E and F region observations, but it utilizes two alternating
codes (ACs), by use of a 15 × 64 µ sec code. Figure 2.2c shows the timing diagram of the ACs
for the TAU0. One AC has a range coverage from 212 to 1306 km and the other one from 58
to 1153 km. The range resolution of both the ACs for TAU0 is 2.4 km at maximum, which isthe same as the gate separation of the TAU0 (2.4 km in length).
These incoherent scatter (IS) radars use powerful transmitters, large antennas and sensitive
receivers to measure the spectral characteristics of electromagnetic radiation scattered fromelectrons in the ionized parts of the Earth’s atmosphere at altitudes between approximately 80
and 1500 km. A range of ionospheric parameters is recovered by iteratively fitting theoretical
spectra to the received data [e.g., Lehtinen and Huuskonen, 1996]. The basic parameters
measured are electron density and temperature, ion temperature, and ion drift velocity.

2.2 DMSP Satellites
The Defense Meteorological Satellite Program (DMSP) satellites are in a near polar orbit, andsun synchronous at an altitude of approximately 840 km. Each satellite has an orbital periodof approximately 101 minutes. Each DMSP satellite is loaded with a particle energy
spectrometer (called SSJ/4), visible and infrared sensors, an ion drift meter and other related
equipment.
The DMSP SSJ/4 particle detectors were designed to measure the flux of precipitating
electrons and ions as they enter the Earth's upper atmosphere. They consist of fourelectrostatic analyzers that record electrons and ions between 30 eV and 30 keV. The two lowenergy detectors consist of 10 channels centered at 34, 49, 71, 101, 150, 218, 320, 460, 670,
and 960 eV. The high-energy detector measures particles in 10 channels centered at 1.0, 1.4,
2.1, 3.0, 4.4, 6.5, 9.5, 14.0, 20.5 and 29.5 keV. Each detector pauses at each channel for 0.09
seconds while switching from a high-energy channel to a low one. A complete cycle issampled each second, which corresponds to a spatial resolution of approximately 7 km. The
nominal response efficiency is 50% at a value of 10% of the central energy for that channel.

23

Table 2.1: Parameters of the EISCAT Radars

Location

Tromsø

KirunaSodankylä

Longyearbyen

Geographic Coordinates (˚)

69.6 N

67.9 N

67.4 N78.2 N

19.2 E

20.4 E

26.6 E

16.0 E

Geomagnetic Inclination (˚)

77.5 N

76.8 N

76.7 N

81.5 N

Invariant Latitude (˚)

66.2 N64.5 N63.6 N75.2 N

Band

VHF

UHF

UHF

UHF

UHF

Center Operating

224

931

931

931

500

Max. Bandwidth (MHz)

3

8

8

8

10

Channels

8

8

8

8

8

Peak Power (MW)

3.0

2.6

-

-

1.0

Average Power (MW)

0.3

0.3

-

-

0.25

Pulse Duration (msec) 0.001-2.0 0.001-1.0--

0.001-2.0

Gain (dB)

48

48

48

42.5

90-110

30-35

30-35

80-85

Frequency (MHz)46

System Temperature (K) 250-350

2480˚ NLongyearbyen (ESR)

Tromsø

70˚ N

Sodankylä

Kiruna
30˚ E

60˚ N
0˚

20˚ E

10˚ E

Figure 2.1: Geographical locations of the EISCAT radar sites.

25

(a)

GUP0

Figure 2.2: Timing diagram for (a) GUP0, (b) GUP3, and (c) TAU0.

26

(b)

GUP3

GUP3L120 Timing Diagram

4

channel number

AC2

3

LP2

2

AC1

1

LP1
0

2

3

4

5

6

ms

7

8

TAU0

channel number(c)1

Figure 2.2: (continued)

27

9

10

11

12

13

28

Chapter 3

Occurrence Regions of Ion Upflow in theDayside Ionosphere

The aim of this chapter is to understand the generation mechanisms of ion upflow phenomenain the dayside ionosphere. In particular, this chapter tries to determine, using simultaneous
data between the EISCAT Svalbard radar (ESR) and DMSP satellites, in which regions of themagnetosphere and under what conditions the ion upflow occurs. Section 3.1 describes the
instruments and data. Section 3.2 describes the observational results. The occurrence regionsof ion upflow in the dayside ionosphere are discussed in Section 3.3.

3.1

Instruments and Data

In order to examine magnetic field-aligned (FA) ion upflow in the polar ionosphere, we haveused only FA data obtained from ESR experiments of several modes, Common Program 1 L(CP-1L) mode, Common Program 2 L (CP-2L) mode and several Special Program (SP)modes where the antenna is either continually or occasionally pointed to the magnetic fieldaligned direction. The total number of observations used in this study is approximately 170
days occurring from January 1998 to November 2001. The integration time period is 2 min,except for 1 min integrated CP2L data during September 21 - 23, 1998, and 4 min integrated
CP2L data during February 5 - 26, 2001. Although pulse codes for the topside ionosphere aredifferent between the long pulse (LP) of GUP0 and GUP3 and the alternating code (AC) of
TAU0, we have used the same gate interval of 36 km for both LP and AC. The differences29between the pulse codes are described in Chapter 2. Spectra in the incoherent scatter (IS)radar data sometimes include echoes from satellites or space debris, and naturally enhanced
echoes [Rietveld et al., 1991], from which correct physical parameters cannot be obtained. We
have hence excluded these data from the present study. We have also excluded poor quality
data whose error values of the ion velocity and electron density exceed certain values.
Plasma particle data from the DMSP satellite provide information about whichmagnetospheric region –the cusp, low-latitude boundary layer (LLBL), mantle, Boundary
Plasma Sheet (BPS), or Central Plasma Sheet (CPS)– the satellite is located in. Since
statistically the cusp has a fairly wide longitudinally elongated shape [Newell et al., 1992;
Maynard et al., 1997], we have selected simultaneous ESR and DMSP data where a DMSP
satellite passed by within 350 km (equivalently within a one hour magnetic local time (MLT)
span) from the magnetic field line of the ESR during the time period when the ESR is locatedbetween 0800 and 1600 MLT.

3.2

Results

We have investigated, by using simultaneous events between the ESR and DMSP satellites, inwhich regions of the magnetospheric plasma regions ion upflow occurs. The dayside specificmagnetospheric plasma regions here are the cusp, LLBL, mantle, BPS, and CPS, which are
distinguishable from plasma properties measured from the DMSP satellites based on themethods proposed by Newell et al. [1988, 1991a, 1991b]. The classification of each region
adopted in this study are summarized as follows:
1. “Cusp” can be detected by very high fluxes, particularly for ions. The ions typically have aspectral peak of differential energy fluxes of more than 108 eV cm-2 s-1 sr-1 eV-1, which ishigher than anywhere else in the auroral oval. Furthermore, low energy ion cutoff drop
exists at least at 1 - 3 keV and below. Electron temperature is below 100 eV and theaverage energy of electron precipitation is below approximately 200 eV.
2. “LLBL” can be distinguished from those in the cusp by their hotter ions and electrons thanthose in the cusp. Electron temperature is typically less than 200 eV and the average energyof electron precipitation is less than approximately 400 eV. The ion fluxes are 106 - 107 eV
cm-2 s-1 sr-1 eV-1, which are smaller than those in the cusp by approximate factor of 10.

30

3. “Mantle” can be distinguished by the de-energized magnetosheath ions observablepoleward of the dayside auroral oval. The ion temperature ranges from a few tens of a eV
up to approximately 100 eV, and ion average energy is no more than a few 100 eV. The ion
fluxes as well as those in the LLBL are smaller than those in the cusp.
4. “BPS” is classified as any soft region of electron precipitation except for the cusp, LLBL,
mantle, and polar rain in the polar cap. The use of the term “soft” means electron averageenergy below 1 keV. This region resembles the nightside BPS; i.e., the ion precipitationshows no clear sign of a magnetsheath origin, electron temperatures are a few 100 eV, andspatially and spectrally structured electron spectra are common.
5. “CPS” is classified by spectrally smooth clouds of more than 1 keV electrons and ions,which is the probability of observing the extension of the nightside zones of hard
precipitation.
Figure 3.1 shows the typical examples of each magnetospheric region observed from the
DMSP satellites. The particle data are obtained from electrostatic analyzers that measure
precipitating electrons and ions from 32 eV to 30 keV. Note that the ion energy scale isinverted. Figure 3.1a shows a conjunction event in the cusp at 0735 UT on July 6, 1998. The
red arrow denotes the geomagnetic latitude of the ESR (75.2˚ N). The cusp, a 75.8˚ - 75.1˚geomagnetic latitude (MLAT), is characterized by a strong peak in ion energy flux atapproximately 1 keV and high number fluxes of ions. The electron spectral peak in the cusp isapproximately 100 eV which is similar to the magnetosheath value. Immediately equatorwardof the cusp is a region of LLBL. The LLBL is distinguished from the cusp by higher than
average particle energies, and it has ion densities which are a few times smaller than those in
the cusp. In Figure 3.1b, a conjunction event in the LLBL is seen at 1103 UT on February 12,
1999. The average energies of ions and electrons in the LLBL are 3 keV and 300 eV,
respectively. The number fluxes of ions in the LLBL are reduced by a factor of approximately
5, but those of the electrons are similar to those of the cusp. Figure 3.1c shows a conjunction
event in the mantle at 1054 UT on March 25, 1998. The mantle is recognized by lower
energies when compared with the cusp. The ion densities in the mantle are a few times
smaller than those in the cusp. The poleward low energy ion “plume” results from its entrance

31

into the magnetosphere until precipitating to undergo significant poleward E × B deflection[Shelley et al., 1976; Reiff et al., 1977]. As is also the case with the LLBL, soft electron
precipitation (below 200 eV) has occurred in the mantle, although the number flux is smaller
than that in the cusp around 74.5˚ MLAT. Figure 3.1d shows a conjunction event in the BPS at
0815 UT on February 8, 2001. In the BPS, a few keV and a few hundred eV electron
precipitations coexist. Hence soft electron precipitation occasionally occurs in the BPS as
shown in Figure 3.1d. Figure 3.1e shows a conjunction event in the CPS at 0905 UT on April8, 1999. The CPS is recognized by a few keV electron precipitation. The electrons havedrifted from the nightside plasma sheet. There is no soft electron precipitation in the CPS.
Characteristic energies of ion precipitation such as those in the cusp, LLBL and mantle are notseen in the BPS and CPS.
Figure 3.2 shows height profiles of the electron temperature obtained from the ESR. Eachplot from (a) to (e) corresponds to each conjunction event shown in Figure 3.1. The thick lines
indicate electron temperature during the conjunction events marked as red arrows in Figure
3.1. The thin lines denote the background levels of the electron temperature. Each backgroundlevel is different by time, season and solar cycle. We hence have defined the backgroundelectron temperature as a level of electron temperature under a quiet (no precipitation)
condition that occurred at a time close to each conjunction event. Figure 3.2a shows a heightprofile of the electron temperature in the cusp shown in Figure 3.1a. The electron temperaturein the cusp increases up to 5000 K at 500 km altitude, whereas the background electron
temperature is approximately 3000 K at the same altitude. The electron temperatureenhancement is also clearly seen in the LLBL (Figure 3.2b) and mantle (Figure 3.2c),
although differences from the background levels are smaller than those in the cusp in Figure
3.2a. Figure 3.2d shows no difference between the electron temperature in the BPS and thebackground levels. The electron temperature is approximately 2000 K at 500 km altitude. The
CPS in Figure 3.2e also shows no difference of electron temperatures.
Figure 3.3 shows height profiles of the field-aligned (FA) ion velocity obtained from the
ESR. Each plot from (a) to (e) again corresponds to each conjunction event shown in Figure
3.1. The background FA ion velocity in each plot is also obtained as quiet (no precipitation)
conditions and is approximately 0 m s-1, but some fluctuations due to the low SNR exist above
550 km altitude. Figure 3.3a indicates an existence of a significant upward flow in the cusp.
The upward flow commences from 350 km altitude and the FA ion velocity increases with ahigher altitude, reaching 200 m s-1 at 600 km altitude. In the LLBL (Figure 3.3b) and mantle32

(Figure 3.3c), height profiles of the FA ion velocity are similar to those in the cusp. The FA
ion velocities in both the LLBL and mantle are more than 100 m s-1 at 600 km altitude. In the
BPS (Figure 3.3d), ions hardly move toward the field-aligned direction below 550 km altitude.
Above 550 km altitude, ions flow slightly but not continuously upward. In the CPS (Figure3.3e), FA ion velocity is approximately 0 m s-1 below 700 km altitudes.
Figure 3.4 shows FA ion velocity distributions in these magnetospheric regions. We havedefined one “event” as a set of 2 min integrated ESR data, which is obtained within ± 5 minfrom a conjunction time. A satellite pass conjunction thus provides us with 5 events in thecase of the CP1L experiment. In the following, we use a unit of “events” to count the ion
upflow phenomena. This unit is defined as follows: Assume that a conjunction between the
ESR and DMSP happens at t0, and t0 - 5 min < t < t0 + 5 min as the conjunction time. This isequivalent to the assumption that the ionospheric locations of the plasma regions observed by
DMSP do not change during this 10 min time period. We determine the FA ion velocities ofion upflow within each of the 2 min intervals.
The abscissa in Figure 3.4 shows the FA ion velocity averaged over altitudes between 400
and 700 km observed from the ESR. The quantified bin of the FA velocity used here is 50 m
s-1. The ordinate is the number of events in each FA ion velocity bin. First, there are 23
conjunction events in the cusp (Figure 3.1a). For all of these events, ion upflows occur, whose
velocity ranges from approximately 100 to 300 m s-1. Secondly, there are more than 80 eventsin the LLBL (Figure 3.1b). Figure 3.1b clearly shows that most of the events in the LLBL (orcleft) have an upward FA ion velocity. In particular, half of the events have an upward FA ion
velocity of more than 100 m s-1. It should be noted that a few events have a negative FA ion
velocity showing that ion upflow does not always occur in the LLBL. The peak of the
distribution of FA ion velocity in the LLBL is at approximately 100 m s-1. Thirdly, 70
conjunction events are found in the mantle (Figure 3.1c). The distribution of FA ion velocity
in the mantle is similar to that in the LLBL. Ion upflow with the FA ion velocity of more than
100 m s-1 is frequently observed in the mantle. Downflow with FA ion velocity of less than 200 m s-1 is also observed in this region. On the contrary, the FA ion velocity in the BPS andCPS is fairly distributed around 0 m s-1, implying that there is a smaller probability of having(intense) ion upflow in these two regions.
Figure 3.5 also shows the FA ion velocity distributions in these magnetospheric regions,and the figure adds the results of the case in which soft electron precipitation occurs to Figure
3.4. When the differential energy flux of electrons at 100 eV is more than 107 eV cm-2 s-1 sr-1

33

eV-1, the FA ion velocity distributions are shown in black color. In the cusp (Figure 3.5a), allevents are associated with soft electron precipitation. The FA ion velocities are distributed tothe upward direction. Almost all events in the LLBL (Figure 3.5b) are also associated with
soft electron precipitation. But some events with the precipitation have small upward ion
velocity (less than 100 m s-1). In the mantle (Figure 3.5c), FA ion velocities with the soft
electron precipitations are clearly distributed to the upward direction. In particular, FA ion
velocities of more than 150 m s-1 are always associated with the soft electron precipitations.
On the contrary, FA ion velocities without soft electron precipitations are distributed to thedownward direction or to nearly 0 m s-1. In the BPS (Figure 3.5d), there are many events inwhich soft electron precipitation exists and the FA ion velocity is small. The distribution of
FA ion velocity in the BPS is obviously different from that in the mantle, although both the
BPS and mantle often have soft electron precipitations. There is little precipitation of soft
electrons in the CPS (Figure 3.5e), by definition.

3.3

Discussion

Ion upflow in the dayside is frequently observed from the ESR because of its latitudinallocation. The simultaneous observations between the ESR and DMSP satellites show that ion
upflow frequently occurs in the cusp, LLBL and mantle, but much less frequently in theclosed field line regions such as the CPS/BPS precipitation. The characteristics do not changeeven when different (more strict) longitudinal criteria are adopted as a simultaneous
observation between the ESR and DMSP satellites. Nilsson et al. [1996] have reported the ion
upflows in several cusp types including equatorward boundary of the cusp. However, thispaper for the first time demonstrates that ion upflow occurs in the regions connected not only
to the cusp and cleft (LLBL) but also to the plasma mantle, and it less frequently occurs in theregion connected to the BPS and CPS.
Almost all of the events in which average FA ion velocity is more than 100 m s-1 areassociated with soft electron (< 0.5 keV) precipitation. In particular, there is good correlation
between ion upflow and soft electron precipitation in the mantle. The soft electron
precipitation is short burst electron precipitation [Veselovsky et al., 1995] or continuous
precipitation with no gap between the cusp and mantle regions. The characteristics of the ion
upflow in the mantle are the same as those in the cusp/cleft region. Hence the ion upflow inthe mantle cannot be explained by heated ions originally flowing out of the cusp/cleft region

34then drifting into the mantle by E × B drift. We conclude that the ionospheric region
corresponding to the mantle region is also the occurrence region of ion upflow.
An analysis of DE 2 observations by Seo et al. [1997] found that at approximately 800 km
altitude a quasi-inverse relationship is seen between the observed FA ion upflow velocities
and the characteristic energy of the precipitating soft electrons (< 0.5 keV) for restricted
ranges of electron energy flux. Several simulation models also show that soft particle
precipitation can cause ion upflow in the topside F region [e.g., Caton et al., 1996; Su et al.,1999]. Soft particle precipitation causes neutral particles around the F region/topside
ionosphere to ionize [e.g., Millward et al., 1999]. The precipitation also causes electrons
around the F region/topside ionosphere to heat through Coulomb collisions with primary andsecondary electrons. As a result, the ambipolar electric field, which is proportional to both thedensity and temperature gradients, becomes largely upward and works on ions as an upward
force. We have described the effect of the ambipolar electric field on ion motions in detail in
Appendix A.
However, there are little upflow events associated with soft precipitation in the BPS. Even
if the criterion of the differential energy flux of electrons at 100 eV is changed from 107 to 108
eV cm-2 s-1 sr-1 eV-1, the characteristics of the distributions of FA ion velocities in each region,
especially the BPS, are not changed. The present results indicate that the relation of soft
particle precipitation to ion upflow is different between the corresponding magnetosphericregions. DE 2 observations also show that the distribution of FA ion velocity associated with
soft electron precipitation around 50 eV is scattered, and there are many cases with small FA
ion velocity (in Figure3c in Seo et al. [1997]). Our results indicate that the small upwardvelocity events with soft particle precipitation are associated with the BPS region in thedayside. Thus the effect of soft particle precipitation on ion upflow may be different between
the corresponding magnetospheric regions. The existence of heat fluxes by thermal electrons
from the magnetosphere also plays a role in causing the difference between the regions,because electron temperature enhancement is not seen in the BPS (see Figure 3.2). Caton et al.
[1996] shows that electron heat fluxes from the magnetosphere, as well as the soft particle
precipitation, are the primary driver of ion upflow.
Ion upflow is rarely found in the regions corresponding to the CPS. A reason for thisinfrequent occurrence of the ion upflow in the CPS may be that the energy of electron
precipitation is generally higher than that in the cusp, LLBL and mantle, and hence theelectron precipitation in the CPS provides less energy at the higher altitude ionosphere where

35the ion upflow is initiated, but rather it ionizes the E region atmosphere.
It may be worth pointing out that Kozolovsky et al. [2001] have shown that mean FA ion
motions at 300 - 550 km were upward in a region equatorward of a postnoon arc around 16 17 MLT. They considered that the source of the postnoon arc might be located inside the
LLBL or probably in the BPS, implying that the ion upflow they observed was located in the
BPS. We leave it to a future work to establish how the occurrence regions of ion upflow in thedayside ionosphere continue in the dawn and dusk, and even in the nightside region.

3.4

Chapter Summary

We have examined the correspondence of dayside ion upflows to the various magnetosphericregions based on a statistical analysis using more than 100 simultaneous observations between
the ESR and the DMSP satellites. The results obtained are summarized below.
1. Ion upflows occur not only in the cusp and cleft (the low-altitude portion of the LLBL)
which have been considered as ion upflow regions, but also in the topside ionosphereconnected to the mantle region. The ion upflows seldom occur either in the BPS or in the
CPS at the dayside high latitudes. These results are important to understand the processesof ions escaping from the ionosphere to the magnetosphere.

2. Almost all events in which the average field-aligned (FA) ion velocity is more than 100 m
s-1 are associated with soft electron precipitation (differential energy flux of electrons at 100
eV > 107 eV cm-2 s-1 sr-1 eV-1). Although soft electron precipitation also exists in the BPS,
the ion velocities are mostly less than 100 m s-1. The present results indicate that soft
particle precipitation is the predominant source driving ion upflow in the topside
ionosphere, but it may work on ion upflow effectively in the higher latitudinal regions in
the dayside and not in the BPS.

36

F13

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GLAT
79. 1
GLONG
28. 1
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11:01

F14

cusp

07:35:11
75. 7
78. 7
25. 4
10:51

07:35:23
75. 6
78. 3
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llbl

07:35:35
75. 3
77. 8
20. 0
10:30

07:35:47
75. 1
77. 3
17. 6
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bps

07:35:59
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E FLUXELEC ION
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cps

07:36:23
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75. 2
9.6
09:43

07:36:47
73. 5
74. 6
7.9
09:34

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09:26

Jul 6

12

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00:00:00
MLAT
0. 0
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0. 0
GLONG
0.0
MLT
00:00

11:03:21
75. 6
77. 0
8.2
13:21

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75. 3
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bps

11:03:45
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4.1
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359.0
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8

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72. 8
72. 4
354.9
12:12

11:05:09
72. 4
71. 8
353. 7
12:05

Feb 12
JHU/APL

Figure 3.1: DMSP spectrograms including conjunction events with the ESR.
Each conjunction event shows (a) cusp, (b) LLBL, (c) mantle, (d) BPS, and(e) CPS.
37

3

void

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UT
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78. 6
GLONG
17. 1
MLT
13:49

F13

10:54:11
76. 1
78. 1
14. 6
13:39

10:54:23
75. 9
77. 7
12.0
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77. 2
9.7
13:18

cusp

10:54:47
75. 4
76. 7
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opll

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UT
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MLAT
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GLONG
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11:30

08:14:11
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79. 0
19. 5
11:20

bps

08:14:23
76. 3
78. 6
16. 6
11:09

08:14:35
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cps

08:14:59
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9.3
10:37

08:15:11
75. 3
76. 6
7.4
10:28

08:15:23
75. 0
76. 0
5.4
10:18

Figure 3.1: (continued)

38

08:15:35
74. 7
75. 5
3.6
10:10

08:15:47
74. 3
74. 9
1.9
10:01

08:15:59
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Feb 8
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MLT
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79. 7
35. 2
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cps

09:05:13
75. 9
79. 4
31. 8
12:47

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75. 7
79. 0
28. 7
12:36

09:05:37
75. 5
78. 5
25. 7
12:25

09:05:49
75. 3
78. 1
23. 0
12:14

09:06:01
75. 1
77. 6
20. 5
12:04

09:06:13
74. 9
77. 1
18. 2
11:54

Figure 3.1: (continued)

39

09:06:25
74. 6
76. 6
16. 1
11:45

09:06:37
74. 2
76. 0
14. 1
11:36

09:06:49
73. 9
75. 5
12. 2
11:27

Apr 8
JHU/APL

Figure 3.2: Height profiles of electron temperature at corresponding magnetospheric regions.
Each conjunction event shows (a) cusp, (b) LLBL, (c) mantle, (d) BPS, and (e) CPS.

40

Figure 3.3: Height profiles of field-aligned (FA) ion velocity at corresponding
magnetospheric regions. Each conjunction event shows (a) cusp, (b) LLBL, (c) mantle,(d) BPS, and (e) CPS. Positive ion velocity is in an upward direction.

41

Figure 3.4: Ion velocity distributions at corresponding magnetospheric regions.
Positive ion velocity is in an upward direction.

42

Figure 3.5: Ion velocity distributions at corresponding magnetospheric regions. Ion
velocity distribution is shown in black when the differential energy flux of electrons
at 100 eV is more than 107 eV cm-2 s-1 sr-1 eV-1.

43

44

Chapter 4

Ion Temperature Anisotropy Associated With
Ion Upflow

In this chapter, we investigate the relations between ion anisotropy and ion upflow by using
the EISCAT Svalbard radar (ESR) and EISCAT VHF radar simultaneously. The simultaneous
ESR and VHF radar observations provide us with an opportunity to investigate the ion
upflows at higher latitudes and anisotropy at higher altitudes than previous observations did
using the EISCAT VHF and UHF systems in Northern Scandinavia. Section 4.1 describes theexperiment. Section 4.2 describes the data set. Section 4.3 describes the observational results.
The causes of ion anisotropy and ion upflow in the dayside topside ionosphere are discussed
in Section 4.4. This chapter is mostly a reproduction of Ogawa et al. [2000a].

4.1

Experiment

We have conducted a simultaneous incoherent scatter (IS) radar experiment (Special
Programme SP-NI-IOF) using both the ESR in Longyearbyen and the EISCAT VHF radar inTromsø. This combination lets us particularly focus on the temperature anisotropies. Figure
4.1 illustrates the design of the experiment. The ESR, located at 78.2˚ N and 16.0˚ E, points
towards the magnetic field-aligned (FA) direction and measures the electron density ne, themagnetic FA ion velocity Vi//, and ion and electron temperatures Ti// and Te// at altitudes
above 150 km with an altitude resolution of 36 km. Complementarily, the EISCAT VHF radar,
located at 69.6˚ N and 19.2˚ E, is operated in the Common Programme 4 mode: Two split

45

beams looking at a low elevation angle of 30˚ into two directions to the north; one beam is
directed due north and the other is steered 14.8˚ to the west of north. The former beam crosses
point C in Figure 4.1 located approximately on the magnetic field line of Longyearbyen at analtitude of 665 km. The range resolution is 65 km. The inclination of the magnetic field line atLongyearbyen is 81.5˚. The aspect angle between the radar beam from Tromsø and theLongyearbyen magnetic field is approximately 59˚.

4.2

Data

It is known that IS radar signals sometimes show spectra which cannot be explained bynormal IS theory [Foster et al., 1988]. One is an artifact; a reflection from satellites. The otheris considered to be a natural phenomenon, the so-called naturally enhanced ion-acoustic lines(NEIALs) [e.g., Rietveld et al., 1991; Collis et al., 1991; Forme and Fontaine, 1999]. Intense
echoes, often with asymmetrical spectra, are a frequently observed feature of the latter one.
Using these criteria, we carefully inspected all raw IS spectra pre-integrated over 10 s during
the time period of the experiment, between 0700 and 1100 UT on July 6, 1998, andconstructed a 1 min post-integrated dataset after excluding all the abnormal spectra from thedataset. The FA ion velocity, electron density, and electron/ion temperatures were then derived
assuming that the ions are oxygen ions (O+) with a molecular composition at the lowest
altitudes. From the catalogue of the NEIAL events (a subset of the abnormal spectra), we haveobtained their occurrence frequency, which will be compared with the occurrence of ion
upflows.

4.3

Observations

Figure 4.2 shows the color-coded height-time profiles of 1 min integrated data obtained fromthe ESR between 0700 and 1100 UT on July 6, 1998. These include, from top to bottom, theelectron density ne, the FA ion velocity Vi//, and the electron and ion temperatures Te// and Ti//,
respectively. Magnetic Local Time (MLT) ≈ UT + 3 hours ≈ Geographical Local Time (LT) +2 hours.
In the top panel, the electron density (a) shows two kinds of enhancements. One iscontinuous in time from 0810 until 1010 UT, which increases ionization by the solar
illumination. The other is more short-lived and is seen between 0700 and 0800 UT and after

46

1010 UT. These enhancements are due to particle precipitation in the dayside high latitude
region. The precipitation apparently contained low energy particles, which ionized the neutral
atmosphere in the F region. The FA ion velocity Vi// (b) shows FA upflow events between
0700 and 0740 UT, at 0805 UT, and after 1010 UT. These FA upflow events are a cluster ofshort-lived (or a burst of) ion upflows. Each upflow starts at approximately 400 km in height.
The velocity of the ion upflows tends to become faster with increasing altitude andoccasionally reaches 500 m s-1 at 700 km. The electron temperature, shown in the third panel(c), also shows enhancements. Both the envelope and each segment of the enhancements very
well coincide with those of FA upflows as well as with electron density increases. The FA ion
temperature Ti// (d) also increases during roughly the same time periods as the enhancementsof both the FA upflows and electron density. There are, however, periods of poorcorrespondence between these two. For example, around 0720 UT there are FA ion upflows,
but no significant Ti// increases.
Figure 4.3 shows the color-coded height-time profiles of 20 sec integrated data obtained
from the EISCAT VHF radar between 0718 and 1115 UT on July 6, 1998. They include, from
top to bottom, the electron density ne, the ion velocity Vi,VHF toward the geographic north,and electron and ion temperatures Te,VHF and Ti,VHF, respectively. The higher altitude
corresponds to the higher geographic latitude, because the VHF radar is pointed at a low
elevation angle of 30˚ to the north. Hence the altitude of 700 km corresponds to the 78˚geographic north, while the altitude of 300 km corresponds to the 74˚ geographic north.
In the top panel, the electron density (a) shows a continuous increasing due to the solar
illumination during the time from 0810 until 1010 UT. More short-lived enhancements seen
with the ESR are not clearly observed, which is due more to a longer VHF radar pulse length(900 µ sec) in comparison to that of the ESR (360 µ sec). The ion velocity Vi,VHF (b) shows
northward flow events (> 500 m s-1) between 0730 and 0737 UT, at 0805 UT, at 1010 UT, andafter 1030 UT. Southward flow (< -100 m s-1) is also seen between 0718 and 0720 UT at
around 700 km altitudes, which corresponds to the latitude located near the ESR. Northward
flow regions after 1030 UT gradually extend to the low latitudes, up to the 75˚ geographic
north. Around 0800 to 1000 UT, fluctuations of the northward ion velocity with the period of
approximately 5 min are seen. The electron temperature Te,VHF, shown in the third panel (c),also shows enhancements. The enhancements continue to the high altitude (high latitude)
regions from the large northward flow regions. The ion temperature Ti,VHF (d) also increases
during roughly the same time periods as the electron temperature enhancement. The ion

47temperature enhancements around 0735, 1014 and 1050 UT correspond to the large northward
flow events and occur at the same time as the FA ion temperature Ti// (in Figure 4.2d)enhancements. They are probably due to the result of the frictional heating process between
ions and neutral particles. Other ion temperature enhancements are seen from the VHF radarobservation above 500 km altitude around 0725 - 0730 and 1025 - 1030 UT. They neither
correspond to the large northward flow events nor the FA ion temperature Ti//.
The dayside ionosphere around the cusp is directly and/or indirectly affected by theinterplanetary magnetic field (IMF) and by the solar wind conditions. We hence show thesolar wind and IMF conditions on July 6, 1998, related to the SP we conducted. Figure 4.4
shows the IMF conditions observed from the ACE satellite (a) and the WIND satellite (b). The
ACE satellite was located close to the L1 point, approximately 232 Earth’s radii upstream
from the Earth on the Sun-Earth line, while WIND was located approximately 60 Earth’s radii
upstream from the Earth on July 6, 1998. Based on these locations and the observed solar
wind flow speed, the expected delay times for the IMF observed by ACE and WIND to reach
the Earth would have been approximately 45 min and 8 min, respectively. The IMF Bz
observed WIND became negative between 0710 and 0755, and after 0905 UT. In these time
periods except for a time period between 0905 and 1010 UT, enhancements of the electron
temperature are seen from both the ESR and the VHF radar. The enhancements are due to theparticle precipitation, which tends to become active in the dayside when IMF Bz is negative.
The ACE and WIND IMF data hence provide a consistent explanation for the enhancementsof the electron temperature observed with the ESR and the VHF radar. The solar windconditions observed from the ACE and WIND satellites are shown in Figure 4.5. The solar
wind velocity is roughly stable around 630 ± 30 km s-1, but the ion density in the solar windsuddenly changes from approximately 4 to 12 cc-1 around 0550 UT by ACE and then around0630 UT by WIND. Our SP observation from 0700 to 1100 UT is hence conducted after thesolar wind shock.
Particle data from the DMSP F13 satellite shows a cusp signature near the ESR sitearound 0735 UT (see Figure 3.1a). The equatorward boundary of the cusp signature is seen at
75.1˚ N, and the equatorward boundary of soft electron precipitations is at approximately
74.4˚ N. This is consistent with the equatorward boundary of the electron temperatureenhancements at the same time period 0735 UT observed with the EISCAT VHF radar. Hencethe electron temperature enhancement seems to be caused by soft electron precipitations andheat flux from the magnetosphere in the vicinity of the cusp.

48

Figure 4.6 shows time variations of several physical parameters obtained from the ESR
and the VHF radar at point C (in Figure 4.1), which is located approximately on the magnetic
field line of Longyearbyen at an altitude of 665 km. In Figure 4.6a, ion upflows are seen at
this altitude between 0710 and 0745 UT and after 1010 UT. The maximum velocities are 350
and 500 m s-1 during these two time periods, respectively.
Figure 4.6b shows the relative occurrence frequency of the naturally enhanced ion line
(NEIAL) events that were excluded before post-integration and were not used for deriving Vi//,
Ti//, Te//, and ne. The upflow events are strongly correlated with high occurrences of naturally
enhanced ion line events.
Figure 4.6c compares the ion temperatures measured with the ESR and with the VHF
radar. As mentioned before, the ESR observes the FA temperature Ti//, and the VHF radar thetemperature Vi,VHF at an aspect angle of approximately 59˚ from the magnetic field. The ion
temperature is strikingly anisotropic when an ion upflow occurs, while otherwise bothobserved temperatures are approximately equal. Associated with the ion upflows around 0730
UT, Ti,VHF shows significant enhancements reaching values up to 3000 K. This isapproximately 800 K higher than the background ion temperature (2000 - 2200 K). Ti//, quite
differently, is even decreased by a few 100 K compared to the temperature (1900 K) before
and after. The anisotropy factor, Ti,VHF / Ti//, shown in Figure 4.6d, becomes larger (1.4 - 1.7)
when ion upflows occurred, while it stayed around 1.1 when no clear ion upflows wereobserved.
Figure 4.6e shows Te// (open circles) and Te,VHF (solid circles) measured with the ESR
and VHF radar, respectively. Both temperatures are strongly enhanced during the ion upflow
events and reach more than 5500 K. There are no significant differences between Te// andTe,VHF. This suggests that isotropic electron temperatures are associated with ion upflows.
Figure 4.6f displays the electric field strength derived from the VHF radar measurement.
Here we assume that the two directional VHF radar beams were measuring the same electricfield from different view angles. The electric field is expected to cause enhanced frictional
heating in the F region. The correspondence between enhancements of upflows and theelectric field strength is at best only partial.
So far, it is clear that ion upflows correlate closely with particle precipitation. In order toinvestigate relations between ion upflow and wave activity due to the particle precipitation indetail, we focus on ion velocities and NEIALs at the time periods around 0730 and 1036 UT
when the effect of frictional heating due to enhancements of electric field is relatively small.

49

Figure 4.7 and Figure 4.8 show the relation between the FA ion velocity and the NEIALs
using high time resolution (10 sec) data. Figure 4.7 indicates the relation between the FA ion
velocity and the NEIALs at altitudes from 380 km to 665 km between 0700 and 0800 UT on
July 6, 1998. Vertical lines indicate the occurrence time and altitude of the NEIALs. Note thatplasma parameters such as ion velocity and plasma temperature cannot be obtained while the
NEIALs are received. The received powers of the NEIALs denoted by the thickest lines are
1000 times stronger than those of normal IS echoes. The thickest NEIALs are found around0722, 0730, and 0733 UT. FA ion velocities just after the thickest NEIALs are largely positiveand reach 400 m s-1 above 600 km altitude, while FA ion velocity is less than 200 m s-1 in any
altitude around 0718 UT before one of the thickest NEIALs around 0722 UT. The strongly
upward ion velocities immediately decrease with time. The ion velocities at 630 and 665 km
altitudes at 0725 UT decrease to nearly 0 m s-1. These characteristics are clearly seen in Figure
4.8, which also shows the relation between ion velocity and the NEIALs, but from 1000 to1100 UT. The thickest and relatively strong NEIALs are found around 1034, 1037, and 1044
UT. There are large velocity differences above 500 km between before and after the NEIALs
at 1034 UT. The increased velocities immediately decrease with time around 1040 UT and
1045 UT, or gradually decrease with time after 1046 UT.
Let us, for the moment, consider other examples. Upward ion velocities (of more than 100
m s-1 above 600 km altitude) are also shown around 0703 and 0736 UT in Figure 4.7 andaround 1012 and 1025 UT in Figure 4.8, when enhancements of the electric field and bothparallel and perpendicular ion heating are seen. These ion upflows seem to be also affected by
frictional heating, as described in the introduction. Since the frictional heating input isconsidered to be of secondary importance to ion upflow in this event, we are not concerned
here with ion upflow at these time periods as mentioned before.

4.4

Discussion

The simultaneous ESR and EISCAT VHF radar experiment has shown that in addition toisotropic (Te// ≈ Te,VHF) enhancements of the electron temperature, significant ion
temperature anisotropy (increases of the ion temperature obliquely to the magnetic field line
Ti,VHF and small decreases of the FA ion temperature Ti//) are associated with FA ion upflows.
The anisotropic ion temperature could indicate the presence of toroidal or nearly toroidal
ion velocity distribution functions. As one possibility, such distributions are produced by50

charge exchange reactions between ions and neutrals, when their relative drift/the electricfield is high (≥ 70 mV m-1). These charge exchange reactions cause frictional heating whichheat ions in both the parallel and perpendicular directions to the magnetic field. The
perpendicular ion temperature enhancement is slightly (approximately 1.5 – 2.0 times) larger
than the parallel ion temperature enhancement. The point of this frictional heating is thatparallel ion temperature Ti// also increases when ion anisotropy occurs. This mechanism
hence cannot explain the anisotropy observed around 0730 UT in our experiment, because
parallel ion temperature Ti// does not increase around the same time period of 0730 UT.
Furthermore, the characteristic feature of IS spectra for non-Maxwellian ion distribution
produced by the charge exchange reactions, a central peak, was observed with the EISCATUHF radar by Lockwood et al. [1989] and Winser et al. [1989]. In our ESR/VHF experiment a
central peak in the spectra did not occur and a bi-Maxwellian distribution seems to fit the datawell. The EISCAT UHF measurements were at approximately 300 km where ion-neutral
reactions play a crucial role in the ion kinetics below F peak [Lee et al., 1998]. Our
measurements are at 665 km, where ion-neutral reactions play a less important role for the ion
kinetics. The sum of the observations indicates that here microphysical processes different
from the charge exchange under high relative drifts cause the anisotropy.
The good correlation of upflow with enhancements of Te and ne suggests that it is caused
by precipitation related processes rather than by high convection. The total energy of electron
precipitation around the cusp (typically, ne,beam ≈ 107 m-3 and kBTe,beam ≈ 100 eV) isapproximately 1.6 × 10-11 J m-3, which is roughly equal to the total kinetic energy of ion upflow
around 350 km (typically, ne ≈ 1011 m-3 and Vi// ≈ 100 m s-1). As already mentioned in Chapter
3, in this kind of scenario previous models have identified enhanced ambipolar diffusion andheat flux as the main drivers of upflow. Our new data suggest that wave activity, which isprobably also caused by precipitation, plays an additional role. The waves seem to heat ions at
the energy of approximately 0.3 eV transversely up to a ratio Ti⊥ Ti // ≈ 2. In-situ
measurements have shown that the ion temperature remains isotropic below approximately
0.4 eV in quiet time (non-auroral) regions [Norqvist et al., 1998]. This partial inconsistency
might be explained by the lower altitude, perhaps also by the rather different instrumentation.
Using our data, we have calculated the resulting hydrodynamic mirror force [Suvanto etal., 1989]. It accelerates the plasma upward, but is smaller by a factor of roughly five times incomparison to the FA ion and electron pressure gradients, which were also calculated from thedata (see Appendix A). Therefore it remains unclear whether there is some additional, still51

unknown, FA bulk ion acceleration mechanism due to the wave activity. For example, lower
hybrid waves, generated in the events of electron precipitation, could effectively accelerate
electrons and ions [Lynch et al., 1994]. Furthermore, the upward parallel electric field due toanomalous resistively produced by plasma turbulence also has a possibility to drive ion
upflow ([Wahlund et al., 1992a] and see Appendix A).
The FA ion velocities immediately change around strong NEIALs, which may be causedby plasma instabilities. Although we cannot obtain the FA ion velocities from IS signals while
the NEIALs are received, ions are also accelerated upward while the NEIALs are received[Forme and Fontaine, 1999]. Thus generation mechanisms of ion upflow must havetransversely ion heating and upward acceleration by wave-particle interaction.
We hence finally suggest that the combined effects of the wave-induced accelerations andof precipitation enhanced ambipolar diffusion and heat conduction are predominant drivers of
ion upflow in the topside ionosphere. The mirror force effect associated with transversely ion
heating becomes more important as altitude increases. Our suggestion could be checked by
combining and extending existing hydrodynamic and kinetic models.

4.5

Chapter Summary

Plasma heating associated with ion upflow in the dayside F region/topside ionosphere has
been presented using data obtained simultaneously with the ESR and the EISCAT VHF radaron July 6, 1998. The results obtained are summarized below.
1. Our observation is the first experiment using two IS radars for targeting plasma temperature
anisotropies at the topside ionosphere where field-aligned (FA) ion upflow occurs. Plasma
temperature obtained along the magnetic field line and nearly perpendicular to the magnetic
field have sufficient quality with a time-resolution of 5 min and can be used to investigate
plasma temperature anisotropies continuously.
2. The FA ion upflows observed at an altitude of 665 km in the dayside cusp are associatedwith significant anisotropy of ion temperature, isotropic increases of electron temperatureand enhancements of electron density. The anisotropy factor, Ti,VHF / Ti//, reaches
maximum values of 1.7. There is no clear correspondence between the enhancements of theelectric field strength and the occurrence of the ion upflows. This suggests that the ion

52

upflow is driven primarily by precipitation.
3. The energy of soft particle precipitation is supplied to the ions in the F region/topside
ionosphere via wave-particle interaction, such as wave-induced transverse ion heating, andupward parallel electric field due to anomalous resistively produced by plasma turbulence.
The data support that in addition to direct precipitation effects, namely enhanced ambipolar
diffusion and heat flux, the wave-particle interaction may play an important role in ion
upflow in the F region/topside ionosphere.
4. The FA ion velocities immediately changed around strong NEIALs. This implies thatplasma waves associated with the NEIALs play a significant role in heating ionstransversely and driving FA bulk ion upflows. In order to specify plasma waves associatedwith ion upflow, the characteristics of NEIALs, particularly the dependence of the wave
number k, are investigated in the next chapter.

53

Figure 4.1: Sketch of the geometry and antenna pointing during the EISCAT special
programme used for this study. The ESR is pointed in the magnetic field-aligned (FA)direction. The EISCAT VHF radar is simultaneously pointed at a low elevation angleof 30˚ into two directions far to the north. One of the beams crossed a point C locatedapproximately on the magnetic field line of Longyearbyen at the altitude of 665 km.

54

July 6 , 1998
log Ne [m-3]

700
600
500
400
300
200

)

(a)

12.0

10.0
7

)

(c)

Altitude [km]

)

(b)

)

9

10

11

Vi [m/s]

700
600
500
400
300
200

400

-400
7

8

9

10

11

Te [K]

700
600
500
400
300
200

6000

500
7

(d)

8

8

9

10

Ti [K]

700
600
500
400
300
200

0700

11

3000

500

0800

0900
UT [hour]

1000

1100

Figure 4.2: Color-coded height-time profiles of 1 min integrated data obtained fromthe ESR between 0700 and 1100 UT on July 6, 1998. They are, from top to bottom,
the electron density ne, the magnetic field-aligned (FA) ion velocity Vi//, andelectron and ion temperatures Te// and Ti//.

55

July 6 , 1998

800

log Ne [m-3]

700

12.0

600

(a))

500
400
300
800

10.0
8

9

10

11

Vi [m/s]

700

Altitude [km]

(b))

(c))

1000

600
500
400
300
800

-1000
8

9

10

11

Te [K]

700

6000

600
500
400
300
800

500
8

9

10

11

Ti [K]

700

(d))

600

3000

500
400
500

300

0800

0900
UT [hour]

1000

1100

Figure 4.3: Color-coded height-time profiles of 20 sec integrated data obtained from the
EISCAT VHF radar (directed to the geographic north) between 0718 and 1115 UT on
July 6, 1998. They are, from top to bottom, the electron density ne, the ion velocity
Vi,VHF, and electron and ion temperatures Te,VHF and Ti,VHF.

56

(a)

Figure 4.4: (a) Components of the Interplanetary Magnetic Field (IMF) in GSM
coordinates on July 6 1998, as measured with the ACE satellite near the L1 point.
(b) The format is the same as (a), but showing the IMF components measured with
the WIND satellite near the Earth.

57

(b)

Figure 4.4: (continued)

58

(a)

Figure 4.5: (a) Components of the ion density and velocity in the solar wind in GSM
coordinates on July 6 1998, as measured with the ACE satellite near the L1 point.
(b) The format is the same as (a), but measured with the WIND satellite near the Earth.

59

(b)

Figure 4.5: (continued)

60

July 6 , 1998(a) Field-aligned ion velocity

[m/s]

600
400
200
0
-200
100

[%]

80

(b) Naturally enhanced ion acoustic lines

60
40
20
0

(c) Ion temperature

[K]

3000

ESR
VHF

2500
2000
1500

(d) Ti VHF / Ti //

2.0
1.5
1.0
0.5

(e) Electron temperature

[K]

6000
5000

ESR
VHF

4000

[mV/m]

3000
140
120
100
80
60
40
20
0

0700

(f) Electric field strength

0800

0900
1000
1100
UT [hour]
Figure 4.6: Time variations of several physical parameters obtained from the ESR
and the EISCAT VHF radar at an altitude of 665 km (point C in Figure 4.1) along theLongyearbyen magnetic field line. The parameters shown are (a) Vi//; (b) the relative
occurrence frequency of the naturally enhanced ion-acoustic line; (c) Ti// and Ti,VHF ,measured with the ESR (open circles) and with the EISCAT VHF radar (solid circles);(d) the ion temperature anisotropy Ti,VHF / Ti// ; (e) Te// (open circles) and Te,VHF (solid
circles); (f) the electric field strength derived from the VHF radar measurement.
61

+400
Vi//
[m s-1]-400

Figure 4.7: Relation of the magnetic field-aligned (FA) ion velocity Vi// to naturally
enhanced ion-acoustic lines (NEIALs) at altitudes from 380 to 665 km between
0700 and 0800 UT on July 6, 1998. Received powers denoted by thickest lines are
1000 times stronger than those of normal incoherent scatter (IS) signals.

62

+400
Vi//
[m s-1]
-400
1000

1012

1024

1036

1048

1100

Figure 4.8: Relation of the magnetic field-aligned (FA) ion velocity Vi// to naturally
enhanced ion-acoustic lines (NEIALs) at altitudes from 380 to 665 km between
1000 and 1100 UT on July 6, 1998. Received powers denoted by thickest lines are
1000 times stronger than those of normal incoherent scatter (IS) signals.

63

64

Chapter 5

k-Dependence of the Naturally Enhanced IonAcoustic Lines

In this chapter, the focus is on the phenomena driven by plasma instabilities, in particular on
the wave number (k)-dependence of the naturally enhanced ion-acoustic lines (NEIALs). In
order to specify the causes of the NEIALs, it is important to statistically examine theexistence of the k-dependence of the NEIALs. In this study we use a database of NEIAL
events obtained from 0700 to 0800 UT on July 6, 1998 and compare their power
characteristics with those of real satellite echoes. Section 5.1 describes the reason for this in
more detail. Section 5.2 describes the analysis methods which use data recorded separatelyfrom different receiver channels of the EISCAT Svalbard radar (ESR). Section 5.3 and 5.4
describe the observational results of NEIALs and of real satellite echoes, respectively. The kdependence of NEIALs is discussed in Section 5.5. This chapter is mostly a reproduction of
Ogawa et al. [2000b].

5.1

PurposeFigure 5.1 shows power profiles of incoherent scatter signals from the GUP0 program
obtained from the ESR at 0724:10 UT on July 6, 1998. In the left panels the thick line
indicates data from channel 1 using the lowest frequency of the ESR, and the thin line
indicates data from channel 6 using the highest frequency of the ESR. In the case of normal
incoherent signals (upper left panel), there seems to be no significant difference between the65powers from both channels. The right panels show the normalized power difference between
the powers from either of the channels. This normalized difference increases with range,
because the signal-to-noise ratio (SNR) decreases with range; thus the statistical fluctuations
increase, for low SNR statistical power differences between the two channels are relatively
large and randomly fluctuate. This is expected because the GUP0 receiver settings produce
only small correlations between subsequent samples for a noise-dominated input. For high
SNR the power differences are small and seem somewhat correlated, which is also expected
for an IS signal with a poor range resolution of 54 km.
On the other hand, in the lower left panel (at 0723:20 UT), NEIALs enhance the power inthe range between 400 and 800 km. In the case of the NEIALs, the received power from bothchannels is relatively high, causing a high SNR. Note that the power scale (x axis) in thelower panel is 10 times larger than that in the upper panel. Normal IS signals are seen aroundthe F peak, and the normalized power difference from the two channels (right panel) is within
statistical fluctuations. However, around the power peak (500 km), the powers from bothchannels seem to differ more than we expected, taking into account that the SNR is very highcompared to the upper panel. For such a high SNR we expect rather small relative power
differences, smaller than in the same profile at around 300 km. At the largest ranges around
850 km there is still significant power in the noise-subtracted profile with NEIALs (lower
panel in Figure 5.1). We have observed that this is a typical feature of profiles with NEIALs,
and it is absent in profiles with only IS signals. The NEIALs seem to produce a distinct power
peak but also to extend over a wider range consistent with earlier studies [Rietveld et al.,1991]. They are rather soft radar targets filling the volume illuminated by the radar pulses as
opposed to real satellites which are hard targets. If the power differences of a soft target are
fluctuating uncorrelated along the profile, the differences should cancel each other out whenlong pulses (54 km in this experiment) are used, although some correlation between samples
arises from the uneven power level along the illuminated volume, which is similar to thesituation in the upper panel for the IS signals. However the relatively smooth variation and thelarge power difference as seen in Figure 5.1 between approximately 400 and 700 km cause us
to suspect that NEIALs not only exhibit larger power differences than expected, but that thedifferences seem to be systematic along the altitude.
In the following we investigate whether these power differences are related to theoccurrence of a high SNR or perhaps an inherent feature of NEIALs. Furthermore, we assess
how often these frequency dependences occur.

66

5.2

Methods

The relatively low peak but high average power (25% duty cycle) of the ESR transmitter canbe utilized efficiently only through a systematic use of multi-frequency modulations[Wannberg et al., 1997]. Among the basic radar codes constructed for the ESR, GUP0 is themost suitable to compare power differences between channels. The GUP0 is a multifrequency long-pulse code, and mainly intend for F region and topside observations.
Table 5.1 shows a summary of the GUP0. The maximum frequency difference between 8channels around 500 MHz is 2 MHz. In this study, we used channels 1, 2, 3, 5, 6, and 7,
because they receive pulses of equal length (360 µ sec) with a range resolution of 54 km. The
other two channels use a pulse length of 150 µ sec. For our investigation, we have used dataobtained between 0700 and 0800 UT on July 6, 1998, because in this period a number of the
NEIALs samples were seen (see Figure 4.6 and 4.7 in Chapter 4). The antenna was set along
the field-aligned (FA) direction in this experiment. In a sophisticated radar like the ESR thesignals pass through numerous stages. It is difficult to calculate the expected relative gainfrom two different channels. Therefore we use the data themselves to calibrate and assume thefollowing relation between channels:
Powern = gn ⋅ Power1 ,gn( t) =backgroundn( t)background1( t)

(5.1)

,

(5.2)

r

where n indicates the channel number, and gn is an estimate of the relative gain during one
pre-integration period which may depend on time t. The subscript r indicates that averaging is
made over a pre-integrated power profile using the SNR to weight the data.
The five panels in Figure 5.2 show the relative gains gn( t ) from 0700 to 0800 UT on July
6, 2000. gn seems to depend only weakly on time. The histograms in Figure 5.3 show the
distributions of the relative gains. All distributions are consistent with the assumption that therelative gains have simply statistical fluctuations. We use the average of each distribution tocompare the measured power at different channels/frequencies.

67

5.3

Results

In Figure 5.4, five scatter plots show the relative power differences over SNR. The thin points
are from normal IS signals, and the thick ones at the high SNR are from ranges with power
enhancements from NEIALs. NEIALs were always seen on all channels. Assuming a
Gaussian distribution of both the noise and the scatter signal, data should scatterapproximately according to the relation∆power=power1

1 

,
⋅ 1+
N ( IPP)  SNR 
1

(5.3)

N(IPP) is the number of samples. One effective interpulse period (IPP) lasts 13.4 msec. The
auto-correlated data are integrated over 744 IPPs or 10 sec. We have also estimated thestandard deviation of the relative differences in 8 SNR bins and found empirically that therelation (5.3) represents reasonably well the actual standard deviation. Most data points, inparticular at SNR < 3, scatter not more than expected from normal distributions with standard
deviation given by the curve in Figure 5.4. Enhanced (thick) data points connected by a line
originate from the same profile (pre-integration period). It can be seen that the data points
with large power differences come from the same profiles. In other words, where a power
difference is larger than statistically expected, it also tends to vary systematically with analtitude over one post-integration period. If all the enhanced powers of NEIALs were
produced in a narrow layer, similarly as for a hard target, the long pulse length combined withthe receiver filter would lead to horizontally rather than vertically connected data points aswill be shown in the next section for real satellite data. It should be noted that our NEIALs
sometimes last 30 sec and longer, consistent with previous EISCAT UHF observations[Rietveld et al., 1991]. NEIALs typically seem to come and go over seconds or tens ofseconds. If they were made of conglomerations of very short-lived (< 1 sec) bursts, each one
producing a much higher SNR than the observed, average SNR, a random frequency
dependency of the cross section should tend to cancel when integrating over 10 sec.
In order to obtain a statistically meaningful quantity for the difference between NEIAL
and normal IS data, we have mapped all data points in Figure 5.4 to a reference SNR of 1
using equation (5.3). Thus two distributions of power differences are created, one for normal
IS signals and one for NEIALs. They are shown in Figure 5.5. These distributions are not68composed of completely independent points because the long pulse length causes correlations
along a profile as discussed qualitatively in section 5.1. We have assumed that only every 18th
point is independent, which is pessimistic, since data points are obviously not correlated overa complete pulse length. Using the test described by Press et al. [1992] there is 5% or lower
probability that both distributions in Figure 5.5 are the same or that NEIALs statistically do
not have larger power differences than IS signals. Next we investigate the relative power inreal satellites which also cause high, often even higher SNR than NEIALs.

5.4

Comparison with Echoes From Satellites

In order to explain the spectral difference between normal echoes, NEIALs, and echoes fromreal satellites, their IS spectra are shown in Figure 5.6, 5.7, and 5.8, respectively. Formats ofthese figures are the same as those in Rietveld et al. [1991] (see also in Figure 1.7 in thisthesis), Sedgemore-Schulthess et al. [1999], and Forme et al. [2001]. The right (left) ion line
indicates downgoing (upgoing) ion-acoustic waves. In the following we use the terms“upshifted” and “downshifted” to refer to downgoing and upgoing ion-acoustic waves,respectively. The black lines in these panels follow the positions of each ion-acoustic peak.
Figure 5.6 shows the normal IS spectra measured with the ESR at 0724:10 UT. Usually
double humped peaks are seen in the IS spectra, and the spectral intensities decrease withincreasing altitude above the F peak. Spectra including NEIALs shown in Figure 5.7 havestrong peaks around 470 km altitudes. The spectral intensities are more than 10 times larger
than those in normal IS echoes. Both downshifted and upshifted ion-acoustic lines haveenhancement, but the downshifted ion-acoustic line is more intense than the upshifted one.
The spectral signatures resemble those exhibited for the EISCAT UHF radar in Figure 1.7.
Figure 5.8 shows IS spectra including echo from a real satellite. The enhanced spectral peak isseen in narrower ranges than NEIALs. However, it may be difficult to distinguish between
echoes from satellites and NEIALs by using only spectral signatures, because the enhanced
spectral peak is shifted, by the flight speed of the satellite, into 4 kHz, which is near the ionacoustic frequency.
Figure 5.9 shows the same example of echoes from a real satellite, and the power profiles
and power difference between the two channels. The satellite return is seen over a range of 54km, which is the same as the pulse length. The received power level around 600 km altitude isalmost the same as NEIALs, but the power difference between the two channels is nearly69constant. The feature is clearly seen in the right panel (b). The normalized power difference
increases with increasing altitude, but the power difference from a real satellite is steady atapproximately -0.18 at altitudes between 570 and 620 km. We hence used not only spectral
signatures but also power profiles in order to distinguish between the NEIALs and echoesfrom real satellites.
We found two real satellites in the data period, and used them for the NEIAL study. Figure5.10 shows the relative power differences between channels for real satellites in the sameformat as Figure 5.4. The power differences follow approximately a horizontal line for eachprofile containing a satellite echo mainly due to the small correlations induced by the digital
ESR receiver filters. Most satellite echoes are in the right tips of the approximate horizontal
lines. This means that the power difference is constant over an integration period, even if the
SNR becomes very high. This is obviously different from the behavior for NEIALs. We thus
conclude that the relative gains in the receiver do not vary systematically over a pulse cycle
within a few interpulse periods, but is rather constant over such short time scales (13.4 msec).
The relative power in real satellites, although constant over a pre-integrated profile, varies
from event to event by a large factor, much more than expected as can be seen in Figure 5.2.
In GUP0 the pulses from channels 1, 2, and 3 are transmitted in one IPP, and from channels 5,6, and 7 they are transmitted in the next IPP (see Figure 2.2a). A satellite may enter or leave
the radar beam rapidly within 100 msec or so between pulses from different channels, which
explains large power difference between channel 1 and 5, 6, and 7, respectively. This,
however, leaves a large unexpected and unexplained difference between the two events whencomparing channels 1 and 2 with in time adjacent transmitter pulses as shown in the upper left
panel of Figure 5.10.

5.5Discussion

We have studied the relative powers between channels using different frequencies in high
SNR situations. In the case of NEIALs, we found that variations of the relative power
between channels well above both the estimated and expected 1-sigma level occur over signal
pre-integrated profiles, and a few profiles within 1 hour of data where the power radiated by
NEIALs varies systematically with wavelength/wave number (k). This feature of NEIALs
does not seem to be caused by the ESR system. Furthermore, power differences along one
profile with NEIALs are obviously different from those of echoes from satellites at a similarly70high SNR. Therefore the NEIAL cross-section itself seems to vary strongly with frequency. If
the NEIAL power is highly frequency dependent, why are the differences usually in the noiselevel? It may be due to the very small frequency spacing. The relative frequency difference isapproximately 0.3%. We believe that the study of a k-dependence of the NEIALs will requirea large statistical database and dedicated experimental modes using a wider frequency spacing
between channels.
In addition to this k-dependence, the amplitude enhancement of upgoing and downgoingion-acoustic waves occurs at somewhat different power levels and different altitudes (also see
Figure 1 in Forme et al. [2001]). These observational facts are modeled using the parametricdecay of beam driven Langmuir waves in an inhomogeneous medium [Forme et al., 2001]. It
is shown that the density gradient with altitude as well as the collisions contribute to an
altitude variation of ion-acoustic spectra and may account for the observed signatures. It isalso responsible for the transition from upshifted to downshifted enhanced ion lines. The
model shows that the power radiated by enhanced spectra varies with the wavelength and thatthe power difference between two wavelengths varies with altitude.
Lower hybrid (LH) waves as well as ion-acoustic waves are theoretically considered as
signatures of the low frequency modulation of Langmuir waves. The evidence for interactionsof Langmuir waves with LH waves are found with the Freja satellite around 1500 km[Stasiewicz et al., 1996]. While Freja observations from approximately 1700 km and models
show the lower hybrid waves heat ions transversely above approximately 40 eV [André et al.,1994], observations by sounding rockets at up to approximately 1000 km show a goodcorrelation between waves above lower hybrid waves and transverse ion energization tocharacteristic energies of up to approximately 10 eV [Kintner et al., 1992; Chang, 1993;
Singh, 1994; Lynch et al., 1996]. Although it is still unknown how plasma waves heat thermal
ions (< 0.4 eV) transversely, it seems reasonable to suppose that the heating effects by plasma
waves, which may be related to parametric decay instabilities of Langmuir waves driven by
precipitating soft energy electron beams, play an important role in ion upflow in the topside
ionosphere. The heating is considered to occur in the vicinity of the cusp, according to theinvestigation discussed in Chapter 3 and 4.

5.6

Chapter Summary

The k-dependence of the received power in high signal-to-noise ratio (SNR) conditions,71

occurring for naturally enhanced ion-acoustic lines (NEIALs) and for real satellites, has been
investigated by using the EISCAT Svalbard radar (ESR), where the data are recorded in 8
separate channels using different frequencies/wave numbers. The results obtained aresummarized below.
1. For the real satellites we found large variations of the relative powers from event to event,which is probably due to a different number of pulses received by the satellite over the
integration period. However, the large power difference remains unexpected in one case.
Over a short time scale (≤ 10 sec) the relative power difference seems to be highly stable.
2. For NEIAL events we found that variations of the relative power between channels well
above both the estimated and expected 1-sigma level occur over signal pre-integrated
profiles, and a few profiles within 1 hour of data where the power radiated by NEIALs
varies systematically with wavelength/wave number (k).
3. The amplitude enhancement of upgoing and downgoing ion-acoustic waves occurs at
somewhat different altitudes and different power levels. These observational facts aremodeled using the parametric decay of beam driven Langmuir waves in an inhomogeneous
medium [Forme et al., 2001]. We suppose that the heating effects by plasma waves, which
may be related to parametric decay instabilities of Langmuir waves driven by precipitating
soft energy electron beams, play an important role in ion upflow in the topside ionosphere.

72

Table 5.1: List of Channels of GUP0 Used at the ESR

ChannelFrequency (MHz)

Pulse Length ( µ sec)

Range (km)1498.875360

294 - 9212

500.125360

237 - 8643

499.375360

180 - 8074500.375150

123 - 7505

499.625360

294 - 9216

500.625360

237 - 8647

499.875360

180 - 8078

500.875150

123 - 75073

(a) 0724:10 UT

900

800

800

700

700
range [ km ]range [ km ]

900

600
500
400

(b) 0724:10 UT

600
500
400

CH6 (500.625 [MHz])

300

300

CH1 (498.875 [MHz])
200

0

power

2

200
0. 6 0. 4 0. 20
0.2 0.4
power difference (P6-P1)/P1

3

6

x 10

(c) 0723:20 UT

900

900

800

800

700

700
range [ km ]range [ km ]

1

600
500
400

0.6

(d) 0723:20 UT

600
500
400

CH6 (500.625 [MHz])CH1 (498.875 [MHz])

300
200

0

1

power

2

300
200
0. 6 0. 4 0. 20
0.2 0.4
power difference (P6-P1)/P1

3

7

x 10

0.6

Figure 5.1: Power profiles from two different receiver channels obtained from the ESR.
The power of channel 6 has been corrected for the relative gain difference (see text in
Section 5.2). (a) Normal IS signals and (c) naturally enhanced ion-acoustic lines (NEIALs).
The right panels ((b) and (d)) show normalized power differences. Dotted lines indicate
estimated 1-sigma level derived from equation (5.3).

74

<CH2/CH1>

1.3

g2(t)

1.2
1.1
1
0.9
0700
0700

0706
0706

0712
0712

0718
0718

0724
0724

0730
0736
0730
0736
<CH3/CH1>

0742
0742

0748
0748

0754
0754

0800

0706
7.1

0712
7.2

0718
7.3

0724
7.4

0730
0736
7.5
7.6
<CH5/CH1>

0742
7.7

0748
7.8

0754
7.9

0800
8

7.1
0706

7.2
0712

7.3
0718

7.4
0724

7.5
7.6
0730
0736
<CH6/CH1>

7.7
0742

7.8
0748

7.9
0754

8
0800

7.1
0706

7.2
0712

7.3
0718

7.4
0724

7.5
7.6
0730
0736
<CH7/CH1>

7.7
0742

7.8
0748

7.9
0754

8
0800

0706
7.1

0712
7.2

0718
7.3

0724
7.4

0742
7.7

0748
7.8

0754
7.9

0800
8

1.3

g3(t)

1.2
1.1
1
0.9
0700
7
1.3

g5(t)

1.2
1.1
1
0.9
7
0700
1.3

g6(t)

1.2
1.1
1
0.9
7
0700
1.3

g7(t)

1.2
1.1
1
0.9
0700
7

0730
7.5
UT

0736
7.6

Figure 5.2: Estimated relative gains between channels over time between 0700
and 0800 UT on July 6, 1998.

75

(a) <CH2/CH1>50(b) <CH3/CH1>

50

count no.

average = 1.08

average = 1.01

40

40

30

30

20

20

10

10

0
0.95

1

1.05

1.1
g2

1.15

0
0.95

1.2

(c) <CH5/CH1>

50

1

1.05

1.1
g3

count no.

average = 0.99

average = 1.21
40

30

30

20

20

10

10

1

1.05

1.1
g5

1.15

1.2

(d) <CH6/CH1>

50

40

0
0.95

1.15

0
0.95

1.2

1

1.05

1.1
g6

1.15

1.2

(e) <CH7/CH1>50

average = 1.05

count no.

40
30
20
10
0
0.95

1

1.05

1.1
g7

1.15

1.2

Figure 5.3: Distributions of the relative gains over the time period shownin Figure 5.2.

76

(a) (CH2-CH1)/CH1

0.8

0.7

0.7

0.6

0.6

|power difference||power difference|

0.8

0.5
0.4
0.3
0.2
0.1
0

0.5
0.4
0.3
0.2
0.1

0

10

0

2

SNR

10

(c) (CH5-CH1)/CH1

0.8

0.7

0.7

0.6

0.6

|power difference||power difference|

0.8

0.5
0.4
0.3
0.2
0.1
0

0.8

(b) (CH3-CH1)/CH1

0

10

2

SNR

10

(d) (CH6-CH1)/CH1-

0.5
0.4
0.3
0.2
0.1

0

10

0

2

SNR

10

0

10

2

SNR

10

(e) (CH7-CH1)/CH1|power difference|

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

10

2

SNR

10

Figure 5.4: Relative power differences between channels over signal-to-noise ratio (SNR).
Vertical bars indicate the 1-sigma levels that are calculated from the distribution of data
points falling within corresponding SNR bins. The boundaries of the SNR bins are 0.2, 0.5,
1, 1.5, 3, 7, 20 and 100. Gray dots are from normal IS signals, and thin vertical bars indicate
the respective 1-sigma levels. Black dots (connected by lines) are from NEIAL data, and thethick vertical bars indicate their respective 1-sigma levels. The curve represents equation (5.3).
77

Normal IS signals

200
180
160

count no.

140
120
100
80
60
40
20
0
-0. 8

-0. 6

-0. 4

-0. 2
0
0.2
0.4
power difference between channel 1 and 6

0.6

0.8

0.6

0.8

NEIAL signals

40
35
30

count no.

25
20
15
10
5
0
-0. 8

-0. 6

-0. 4

-0. 2
0
0.2
0.4
power difference between channel 1 and 6

Figure 5.5: Distributions of the power differences between two channels (1 and 6)
in normal IS signals (upper panel) and in NEIAL signals (lower panel). Each power
difference is mapped by using equation (5.3) to a SNR = 1.

78

0724:10 UT July 6, 1998
750
700

altitude [km]

650
600
550
500
450
400
350
300
-10

-5

0
frequency [kHz]5

10

Figure 5.6: IS spectra measured with the ESR. (Left panel); The range corrected spectral
power, coded with a gray scale, over the frequency on the x axis and range on the y axis.
The black lines follow the positions of each of the ion-acoustic peaks. (Right panel);
The same spectra as line plots. Each spectral intensity is normalized.

790723:20 UT July 6, 1998
750
700

altitude [km]

650
600
550
500
450
400
350
300
-10

-5

0
frequency [kHz]5

10

Figure 5.7: Same format as Figure 5.6, but for NEIAL event.

80

0711:50 UT July 6, 1998
750
700

altitude [km]

650
600
550
500
450
400
350
300

-10

-5

0
frequency [kHz]5

10

Figure 5.8: Same format as Figure 5.6, but for echoes from a real satellite.

81

(a) 0711:50 UT

900

800

800

700

700
range [ km ]range [ km ]

900

600
500
400

(b) 0711:50 UT

600
500
400

CH6 (500.625 [MHz])CH1 (498.875 [MHz])

300
200

0

1

power

2

300
200
0. 6 0. 4 0. 20
0.2 0.4
power difference (P6-P1)/P1

3

7

x 10

0.6

Figure 5.9: Same format as Figure 5.1, but for echoes from a real satellite.

82

(a) (CH2-CH1)/CH1

0.8

0.7

0.7

0.6

0.6

|power difference||power difference|

0.8

0.5
0.4
0.3
0.2
0.1
0

0.5
0.4
0.3
0.2
0.1

0

10

0

2

SNR

10

(c) (CH5-CH1)/CH1

0.8

0.7

0.7

0.6

0.6

|power difference||power difference|

0.8

0.5
0.4
0.3
0.2
0.1
0

0.8

(b) (CH3-CH1)/CH1

0

10

2

SNR

10

(d) (CH6-CH1)/CH1

0.5
0.4
0.3
0.2
0.1

0

10

0

2

SNR

10

0

10

2

SNR

10

(e) (CH7-CH1)/CH1|power difference|

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0

10

2

SNR

10

Figure 5.10: Relative power differences for real satellites in the same format as Figure 5.4.
Black lines at the high SNR are from ranges with power enhancements from real satellites.

83

84

Chapter 6

Conclusions and Suggestions for Future Studies

Three investigations on the generation mechanisms of ion upflow in the ionosphere werecarried out using data from the EISCAT Svalbard radar (ESR), the EISCAT VHF radar, andthe DMSP satellites.
The simultaneous observations with the ESR and the DMSP satellites provide, for the firsttime, a systematic investigation on the occurrence regions of ion upflows in the dayside
topside ionosphere. The distributions of field-aligned (FA) ion velocities in each of several
magnetospheric regions suggest that ion upflows occur not only in the cusp and cleft (the lowaltitude portion of the low-latitude boundary layer (LLBL)) which have been considered as
ion upflow regions, but also in the topside ionosphere connected to the mantle region. The ion
upflows seldom occur either in the Boundary Plasma Sheet (BPS) or in the Central Plasma
Sheet (CPS) in the dayside high latitudes. Almost all the events where the average ion
velocity is more than 100 m s-1 are associated with soft electron precipitation (differential
energy flux of electrons at 100 eV > 107 eV cm-2 s-1 sr-1 eV-1). Although soft electron
precipitation also exists in the BPS, the associated ion velocities are mostly less than 100 m s-1.
The present results indicate that soft particle precipitation is the predominant source driving F
region ion upflow, but it works on ion upflow effectively only in the higher latitude regions in
the dayside and not in the BPS.
A case study using the ESR and the EISCAT VHF radar simultaneously has shown that FA
ion upflows observed at an altitude of 665 km in the cusp are associated with significant
anisotropy of ion temperature, isotropic increases of electron temperature and enhancementsof electron density. There is no clear correspondence between the enhancements of the electric85

field strength and the occurrence of the ion upflows. This suggests that the ion upflow isdriven primarily by precipitation. The energy of soft particle precipitation is supplied to theions in the F region/topside ionosphere via wave-particle interaction, such as wave-induced
transverse ion heating, and an upward parallel electric field due to anomalous resistively
produced by plasma turbulence. The data support that in addition to direct precipitation effects,
namely enhanced ambipolar diffusion and heat flux, wave-particle interaction may play animportant role in ion upflow in the F region/topside ionosphere. The difference of theoccurrence frequency of ion upflow at each region may hence depend on the existence of theeffect of the wave-particle interaction.
In order to understand the plasma waves related to the ion upflow, the k-dependence of thereceived power in high signal-to-noise ratio (SNR) conditions, occurring for naturally
enhanced ion-acoustic lines (NEIALs) and for real satellites, is investigated by using dataobtained from the ESR, where the data are recorded in 8 separate channels using different
frequencies. We found that variations of the relative power between channels well above boththe estimated and expected 1-sigma level occur over signal pre-integrated profiles. The mostplausible explanation we can suggest is that the frequency/wave number (k) dependence of thepower in NEIAL events has its origin in the scattering medium itself. The observational facts
are at least modeled using the parametric decay of beam driven Langmuir waves in an
inhomogeneous medium [Forme et al., 2001].
The conclusions from these three investigations are that wave-particle interactions are
significantly important for ion upflow in the polar topside ionosphere, as well as ion heatingin the high-energy part seen in ion conics and TAIs in the bottomside magnetosphere.
Although the energy of soft electron precipitation is considered as the main source of the ion
upflow, this thesis shows that strong ion upflow (≥ 100 m s-1) is not directly driven by soft
electron precipitation. Strong ion upflows during and after the NEIALs occur via interaction
between ions and plasma waves induced by soft electron precipitation. From the result
mentioned above, generation mechanisms of ion upflow must have transverse ion heating andupward acceleration by wave-particle interaction while the induced plasma waves decay. Theinteraction is considered to frequently occur in the vicinity of the cusp. Other approaches willcertainly be needed to further understand the proposed mechanism. We propose here threeexperiments through which one can more extensively understand the relations between ion
upflow and plasma instabilities.

861. Statistical studies of NEIALs observed with the ESR
NEIALs observed with the EISCAT UHF radar in Tromsø have been statistically investigatedby Rietveld et al. [1996]. The distribution of the occurrence of NEIALs as a function ofuniversal time (UT) shows a general period of maximum occurrence in the 12 to 24 UT(approximately 14 to 02 magnetic local time (MLT)). The distribution probably reflects thetimes when the auroral oval is above Tromsø which is most likely in the evening. On thecontrary, NEIALs observed with the ESR are frequently received in the cusp around noon,
due to its latitudinal location. Since plasma waves associated with ion heating are slightly
different between the dayside and nightside polar regions [André et al., 1998], thecharacteristics of NEIALs observed with the ESR may be different from those obtained fromthe EISCAT UHF radar. It should be also noted that NEIALs are more frequently observedwith the EISCAT VHF radar (224 MHz and kVHF ≈ 9.4 m-1) than with the EISCAT UHF radar(931 MHz and kUHF ≈ 39.1 m-1). The difference is probably due to the wave number
dependence of the ion-acoustic waves. Statistical studies of the occurrence frequency of
NEIALs with the ESR (500 MHz and kESR ≈ 21 m-1), whose wave number dependence is
probably approximately between the EISCAT UHF and VHF radars, will also help us tounderstand the physical mechanism responsible for NEIALs.
2. Simultaneous observations with the EISCAT radar system and in-situ measurements
The IS radars enable us to examine time and height distributions of several plasma parameters,
but they are not capable of providing direct information on plasma waves associated withplasma instabilities. On the contrary, in-situ measurements such as satellites and sounding
rockets carrying plasma wave instruments enable us to examine the characteristics of plasma
instabilities associated with plasma waves, but they cannot provide time and height
distributions of several plasma parameters. Hence simultaneous observations with the
EISCAT radar system and in-situ measurements complement each weak point to aid inunderstanding these relations between ion upflow and plasma instabilities. Yoshida et al.
[2000] have shown the relation between ELF waves, ion conics, and ion upflow around thenightside inverted-V region, by using simultaneous observations between the Akebono
satellite and the EISCAT radars in Tromsø. Since EISCAT observations have been recently
coordinated with Cluster II satellite observations, a relatively high number of simultaneous87events including ion upflow phenomena can be found in the dayside around the cusp. Cluster
II is composed of 4 simultaneous satellite measurements hundreds to thousands of kilometersapart, and also the magnetic conjunction situation is different from that of most other satellites
because of the polar orbits with an apogee of approximately 20 and a perigee ofapproximately 4 Earth’s radii. Those simultaneous observations allow for the investigations of
the influence of plasma waves to ions in both the ionosphere and the magnetosphere.
3. Comparison between the results of modeled and observed dynamic behavior of ion upflow
Statistical and simultaneous observations as described above can be used to estimate thephysical mechanism of plasma instability associated with ion upflow. The relation of theplasma instability to ion acceleration will be examined from plasma parameters obtained fromthe ESR just before and after NEIALs. It is, however, difficult to estimate how plasma
instability affects ion upflow from the observational data. Although previous models indicatethat precipitation effects, particularly enhanced ambipolar diffusion, are sufficient to explain
the results of ion upflow from the IS radar data [Liu et al., 1995; Caton et al., 1996; Blelly etal., 1996], additional physical processes are required to explain the observed ion upflow
around NEIALs [e.g., Wahlund et al., 1992a]. Hence a model including microscopic processes
such as wave-particle interactions is needed to explain such ion upflow phenomena. Usingsuch a model, we will be able to understand interactions between plasma waves and ionsaround NEIALs, which should in turn be confirmed by observations.
Finally, the ion upflow and outflow phenomena that we focus on in this thesis are
considered to occur not only on the Earth, but also on other planets such as Venus and Mars.
These physical mechanisms obtained on the Earth will also help researchers understand thegeneration mechanisms of ion outflow occurring on other planets.

88

Appendix A

Ion Motions in the Polar Topside Ionosphere

In this appendix, we quantitatively examine the effect of ion accelerations due to the external
field on the ion motions in the polar topside ionosphere, where local magnetic field lines arenearly vertical and ions start to flow up along the magnetic field line. At the beginning, we
explain a general equation of momentum and investigate the relative importance of each termin the equation. If the momentum equation is correct, we can determine unknown parameterssuch as the ion-neutral collision frequency ν in and neutral velocity Vn from observational
results. However, it should be noted that the momentum equation cannot be applied to the ion
upflow phenomena accompanied with the naturally enhanced ion lines (NEIALs), which we
mainly treat in this thesis. In Section A.1, we estimate the contributions of each force inmomentum equation of ions to ion motions in the topside ionosphere. In Section A.2, wecompare the theoretical estimation described in Section A.1 with observational results using
the EISCAT Svalbard radar (ESR).

A.1

Theoretical Formulation

The main ion and neutral species in the topside ionosphere, around an altitude between 400
km and 700 km, are the oxygen ion (O+) and atomic oxygen (O), respectively. The number
density of O is approximately 1012 - 1014 m-3 and a few 10 to a few 100 times larger than thatof O+ (see Figure 1.1). It means that the topside ionosphere is partially ionized plasma, as
mentioned in Section 1.1.
The topside ionosphere includes the boundary between collision and diffusion dominant89

regions. The mean free path of ions λ i , which is related to the collision between O+ and O, iswritten as

λi =

1
,
nnσ in(A.1)

where nn is number density of neutrals (O) and σ in is the cross section between O+ and O.
Because the cross section σ in is approximately 3 × 102 × (Bohr radii) 2 ≈ 10−18 m 2 for both
the resonance charge exchange and the momentum transfer [Pesnell et al., 1993], the mean
free path of ions is approximately 10 km to 1000 km in the topside ionosphere. This indicatesthat the collisions between O+ and O lessen with an increase in altitude. The scale height ofions H i is related to the thermal diffusion, and is written ask (T + T )Hi = B i e .
mi g

(A.2)

In the case of the total temperature of ions and electrons being approximately 3000 K in thetopside ionosphere, the scale height of the ions H i is approximately 200 km, which isintermediate between the minimum and maximum length of the mean free path. This meansthat ions can freely move with the thermal velocity at the upper part of the topside ionosphere(called the exosphere), whereas collisions between O+ and O still dominate ion motions at thelower part of the topside ionosphere (called the barosphere). For that reason we shall discussboth thermal and collision forces in the topside ionosphere in detail.
A general momentum equation for the topside ionospheric ions is written as

ni mi

∂ Vi
+ ni mi Vi ⋅ ∇ Vi∂t

( )= −∇Pi + ni kB (Ti // − Ti⊥ )

(

)

1 ∂ B
+ ∇ ⋅τ i − ni mi g + ni e E + Vi × B
B ∂s
,

(

(

)

(

− mi (Qi − Li ) Vi − Vn − ni miν in Vi − Vn − ni miν ie Vi − Venm
nmm z ν 
 3 mieν ie 
qi − i i qe + in in in qi − i i qn  +kBTin nnmn  5 kBTie 
ne me 

90

)

)

(A.3)

wheremi mn≈ mimi + mn
mi me
mie =
≈ me
mi + me

min =

(A.4)

and

mnTi + miTn Ti + Tn≈
mi + mn2,
meTi + miTe
Tie =
≈ Te
mi + me
Tin =

(A.5)

Q and L are the production and loss ratios, τ and q are the stress tensor and the heat flow

vector.

∂indicates a differential toward parallel to the magnetic field lines. The subscripts i,∂se, and n are for ions (O+), electrons, and neutral particles (O), respectively. When we consider
ion motions only in the direction parallel to the magnetic field lines, the Lorentz force
ni e Vi × B , related to a cyclotron gyration, disappears from equation (A.3). Hence the fieldaligned (FA) momentum equation (A.3) can be written as∂ Vi // ∂ + Vi // ⋅  Vi //
∂t 
∂ s
e
1 ∂Pi // kB
1 ∂ B
1 ∂τ i //E//=−+ (Ti // − Ti⊥ )+− g// +
ni mi ∂ s mi
ni mi ∂ s
mi
B ∂s
−

(Qi − Li ) V+

z inν in nmnm
 3 meν ie 
qi // − i i qn //  +qi // − i i qe // 
 5 ni mi kBTie // 
ni kBTin // 
nnmn
ne me

ni

(

) (

) (

i // − Vn // − ν in Vi // − Vn // − ν ie Vi // − Ve //

91

)

.

(A.6)

In order to investigate ion acceleration per one particle, equation (A.3) is divided by ni mi . We
describe, in sequence, the contributions of each term of equation (A.6) to the ion motions in
the polar topside ionosphere.
The first term on the left hand side of equation (A.6) is the time differential of FA ion
velocity. For the time scale of the IS radar observations (typically 1 min), this time differential
term is sufficiently smaller than, for example, the gravity force. The second term on the left
hand side of equation (A.6), called the advection term, is also negligibly small when FA ion
velocity is subsonic. Hence ion motions are usually considered in the steady state condition,
which means that the upward forces must balance the downward forces.
The first term on the right hand side of equation (A.6) is the ion pressure gradient force.
Parallel ion pressure can be described, using the ion density and the parallel ion temperature,as
Pi // = ni kBTi // .

(A.7)

Since electron density decreases with increasing altitude in the topside ionosphere, thispressure gradient force acts on ions as an upward force. This force is the same order as thegravity force. For example, when ion temperature increases through frictional heating, as
given by [e.g., Schunk et al., 1975; St-Maurice and Laneville, 1998]

Ti = Tn +miVi − Vn ,
3kB

(A.8)

the upward pressure force increases with the ion temperature enhancements. To be exact, thisfrictional heating is due to resonant charge exchange collisions between O+ and O, and thecontribution perpendicular to the magnetic field lines is larger than that parallel to themagnetic field lines [e.g., Schunk and Nagy, 1980]. However, the ion temperature anisotropy
due to the frictional heating becomes weak with higher altitudes in the topside ionosphere,
because the influence of Coulomb collisions, which cause isotropic heating, increases withincreasing altitude [e.g., McCrea et al., 1993].
The second term on the right hand side of equation (A.6) is the magnetic mirror force dueto the ion temperature anisotropy. The temperature anisotropy Ti⊥ Ti // ≈ 2 was found in thetopside ionosphere [Ogawa et al., 2000a]. In the case of Ti⊥ = 4000 K and Ti // = 2000 K at92

665 km altitude, for example, the magnetic mirror force is approximately 0.5 m s-2, which is 1
order smaller than the gravity force. Hence this term can be usually neglected in the topside
ionosphere.
The third term on the right hand side of equation (A.6) is the height differential of thestress tensor. A general expression for the parallel stress tensor is given by Schunk [1975] asτ i // ≈

2

1
1
.
ni mi Vi − Vn 2 − Vi − Vn


4
3
//

(

)

(

)

(A.9)

For the neutrals, the main flows are horizontal, at speeds from 100 - 800 m s-1, while thevelocity gradients are in the FA direction. These conditions yield a large viscous stress effect.
The viscosity associated with FA ion velocity is small. Large horizontal ion flows can occur,with speeds up to several km s-1, but they are E × B drifts, and they exhibit little variation withaltitude. The lack of a velocity gradient implies that viscous stress is not important even forthese large drifts. The height differential of the stress tensor term is important if there are large
relative drifts in the horizontal direction.
The fourth term on the right hand side of equation (A.6) is the gravity force, which
accelerates ions downward along the vertical direction. The gravity force is approximately 9m s-2 around 600 km altitudes. The gravity force is the main downward force in the polar
topside ionosphere. The dip angle of the magnetic field line at Longyearbyen is 81.5˚ (seeTable 2.1 and Figure 4.1), so the gravity force toward the magnetic field lines is written as
g// = g vertical sin( I ) .

(A.10)The magnitude of the gravity force toward the magnetic field lines at Longyearbyen is nearly
the same as that along the vertical direction.
The fifth term on the right hand side of equation (A.6) is an electric force by the electricfield. Ions interact with electrons through the electric field as well as Coulomb collision andheat conductance. Here we also consider the electron momentum equation in order to evaluate
the electric force working ions. The FA momentum equation of electrons is given as93

ne me∂ Ve //∂
+ ne me Ve // ⋅  Ve //

∂t
∂s 

=−

∂ Pe //
1 ∂ B ∂τ e //+− ne me g// − ne eE//+ ne kB (Te // − Te⊥ )
∂s
∂s
B ∂s

(

)

(

)

(

− me (Qe − Le ) Ve // − Vn // − ne meν en Ve // − Vn // − ne meν ei Ve // − Vi //−.

(A.11))

1 meν en 
n m
n m

 3 meν ei 
qe // − e e qn //  +qe // − e e qi // 
5 kBTen // 
 5 kBTei // 
nnmn
ni mi

The magnetic mirror force on electrons can be neglected, based on the fact that there is noelectron temperature anisotropy obtained from the IS radar observations described in Chapter
4. The force caused by the electron stress tensor is also small. The collision frequency
between the electrons and neutrals (O) in the topside ionosphere is written as [Schunk andNagy, 2000]()

ν en = 8.9 × 10−17 nn Te 1 + 5.7 × 10−4 Te ,(A.12)

while the collision frequency between the electrons and ions (O+) in the topside ionosphere is
written as [Schunk and Nagy, 2000]

ν ei = 54.5 × 10−6ni
Tei 3 / 2.

(A.13)

ν en is at least one order smaller than ν ei , whose value is a few 10 s-1 in the topside
ionosphere. We can hence ignore the electron-neutral collision term and electron-neutral heat
flux term in equation (A.11). Since the other terms multiplied by electron mass me can beignored, except for the electron-ion collision and heat flux terms, the electric field derived
from the FA electron momentum equation (A.11) is written as94

1 ∂ Pe // meν ei
3 meν ei−
Ve // − Vi // +qe //5 ne ekB Tei //ne e ∂ se,
1 ∂ Pe // miν ie
3 miν ie=−−
Ve // − Vi // +qe //5 ne ekB Tei //ne e ∂ se((

E// = −))

(A.14)

where ne meν ei = ni miν ie and ne ≈ ni . This electric field is called “the ambipolar electricfield.” The value is approximately 1 µ V m-1 in the topside ionosphere. The fifth term on theright hand side of equation (A.6), for which is substituted this ambipolar electric field, isusually one of the main upward forces in the topside ionosphere.
The sixth term on the right hand side of equation (A.6) indicates a force due to chemical
reactions. Regarding the production ratio Qi , the ionization caused by particle precipitation isapproximately 109 m-3 per second at maximum in the topside ionosphere even if the cusp-like(burst) soft electron precipitation occurs, which is smaller than the background electron
density (approximately 1011 m-3) in the topside ionosphere. On the other hand, the loss ratio
Li , can be derived from the following chemical reactions [e.g., Schunk and Walker, 1970;

Schunk and Nagy, 2000][ ] [ ] [ ]≈ (α n + k [ N ] + k [O ] ) [O ]Li = α r O + + α1 NO + + α 2 O2+  ne
r e

1

2

2

2

+

,

(A.15)≈ (α r ne + k 1[ N 2] + k 2[O2] ) ne

where
α

α r = 3.7 × 10−18( 250 Te )0.7

[ m3s−1 ]1NO + + e − 
→ N +Oαα 1 = 4.0 × 10−13( 300 Te )0.5

[ m3s−1 ]2
O2+ + e − →O + Oαα 2 = 7.4 × 10−14 (1200 Te )0.56

[ m3s−1 ] .

kk 1 = 1.2 × 10−18( 300 Ti )kk 2 = 2.0 × 10−17( 300 Ti )r
O + + e − 
→ N + hν1
O + + N 2 
→ NO + + N
2
O + + O2 →O2+ + O(A.16)[ m3s−1 ]0.5[ m3s−1 ]It should be noted that we assume equilibrium conditions for the nitrogen monoxide ion(NO+) and molecular oxygen ion (O2+) when neglecting the production of NO+ and O2+ due to95solar radiation in the topside ionosphere. Below approximately 600 km altitude, loss
processes through the rearrangements of O+ into NO+ and O2+ are still dominate in equation(A.15), because the number density of nitrogen molecules (N2) and oxygen molecules (O2) arelarger than electron density ne below the altitude (see Figure 1.1) although each reaction rate
given by αr , k1, and k2 has almost the same order. At any altitude, however, this loss ratio isless than 10-4 ne in the topside ionosphere. Hence the force regarding the chemical reactions
can be neglected in the topside ionosphere.
The seventh term on the right hand side of equation (A.6) is the ion-neutral collision force.
This force works on ions as the resistance due to the neutral motion. The ion-neutral collision
frequency is written as [Schunk and Nagy, 2000]2

ν in = 3.67 × 10−17 nn Tin (1 − 0.064 log10 Tin) ,(A.17)and the value is between 0.01 and 1 s-1 in the topside ionosphere. Because parallel ion velocity
Vi// can take a large value (more than 100 m s-1) in the topside ionosphere, this force is one of
the main forces in the topside ionosphere as well as in the F region ionosphere.
The eighth term on the right hand side of equation (A.6) is the ion-electron collision force.
This collision is the Coulomb interaction between ions and electrons. The Coulomb collision
frequency between O+ and electron is written asν ie = 18.5 × 10−10ni
Tie 3 / 2,

(A.18)and this value is a few 10 times smaller than ν in in the topside ionosphere. This Coulomb
collision effect becomes important at a higher altitude. This term is eliminated through the
ambipolar electric field described in equation (A.14), which is derived from electron
momentum equation (A.11).
The ninth term on the right hand side of equation (A.6) is a force due to heat flux of ionsand neutrals, which interact through the collision between them. Parameter z in, for the
resonant charge exchange collision is described in Schunk [1975] in detail. This force can bedivided into two factors and written by using ion and neutral temperature gradients [Conrad
and Schunk, 1979]. When the ion (neutral) temperature increases with increasing altitude, theterm associated with the ion (neutral) temperature gradient works as an upward (downward)

96

force. Although the coefficient of the neutral temperature gradient term takes a large value forTn ≈ Ti , the neutral temperature gradient is generally small in the F region/topside

ionosphere; therefore, the O+ thermal diffusion associated with this factor is not likely to besignificant under normal ionospheric conditions [Conrad and Schunk, 1979]. On the other
hand, the O+ temperature gradient can be large throughout the dayside F region/topside
ionosphere; however, the coefficient of the ion temperature gradient is small [Conrad andSchunk, 1979]. Hence this force can be usually neglected in the topside ionosphere.
The last term on the right hand side of equation (A.6) is a force due to heat flux ofelectrons and ions, which interact through the collision between them. The first factor related
to the ion heat flux qi // can be negligible because the term is multiplied by me mi . When thevelocity difference between ions, electrons and neutrals can be negligible, the electron heat
flux is written as [Conrad and Schunk, 1979]




25
25 2 kB Pe // ∂ Te //kB Pe //
 ∂ Te //qe // = −,=−
13   ∂ s
8  meν ei ∂ s
8132+m ν + ν e  ee 8 ei  ()(A.19)

where ν ei is written in equation (A.13) andν ee =

54.5
ni
× 10−62
Tei 3 / 2

 ν ei .
=2

(A.20)

The force associated with the electron heat flux is thus written as−k ∂T
3 ν ie15 2 kBν ie Te // ∂ Te //qe // =
≈ 0.8 B e // .
mi ∂ s
5 ni kBTie //8 + 13 2 meν ei Tie // ∂ s()(A.21)

When the electron temperature strongly increases with increasing altitude, for example, underconditions of particle precipitation, this force becomes as important as the ion pressure
gradient force. Although this term, as well as the ion-electron collision term, is eliminatedthrough the ambipolar electric field described in equation (A.14), it does not mean that theeffect of the electron heat flux can be negligible, but it means that the effect of the ambipolar97

electric field becomes small when the effect of the electron heat flux from the magnetosphereto the topside ionosphere becomes large.
Consequently ion and electron diffusion terms, due to those pressure gradient and gravity
forces, and a collision term between ions and neutrals are considered to be the main forces in
the polar topside ionosphere. Hence the FA ion momentum equation (A.6) is usuallysimplified as
 1 d( ne kBTi //)1 d( ne kBTe //)−−− g//  = ν in Vi // − Vn // ne mi

ds
ne mi
ds

(

)

(A.22)

in the polar ionosphere, using the ambipolar electric field derived from equation (A.12) andni ≈ ne . The effects of particle precipitation, electron heat flux and flectional heating are

included on the left hand side of equation (A.22) as functions of the electron density ne, theFA electron temperature Te//, and the FA ion temperature Ti//.

A.2

Observational Results

Next we investigate the validity of equation (A.22), using observational results obtained fromthe ESR. Figure A.1 shows the diffusion term on the left hand side in equation (A.22) and thefield-aligned (FA) ion velocity at an altitude of 452 km on December 8, 1999. It is clearlyseen in Figure A.1 that the fluctuations of the diffusion term correlate with those of the ion
velocity. When ion-neutral collision frequency ν in and FA neutral velocity Vn // are assumed
constants, the relation between the diffusion term and the ion velocity is handled as a linear
function according to equation (A.22). Conversely, if the relation between the diffusion term
and the ion velocity, which are derived from observational data, can be expressed with a linear
function, the inclination in the linear function should be interpreted as the ion-neutral collision
frequency ν in. Furthermore, the intercept of the axis of the ion velocity should be interpreted
as the FA neutral velocity Vn // . In the following, the validity of equation (A.22) is discussed
using the ion-neutral collision frequency ν in derived from the inclination and modeled onequation (A.17).
Figure A.2 and Figure A.3 show the relation between the diffusion term and the FA ion
velocity at four altitudes, 417, 452, 488, and 523 km, respectively. Every 2 min data for 2
hours are over-plotted in each panel. The ion velocity increases with the diffusion term at any

98altitude. Line (A) indicates the results of a least square approximation from the x axis. Broken
line (B) indicates the results of a least square approximation from the y axis. The difference
between line (A) and line (B), associated with an ambiguity of the inclination, is small, andthe larger inclination value is within 2 times that of the smaller one in many cases. This meansthat ν in and Vn // , which are unknown quantities in equation (A.22), are almost constant for 2
hours. Therefore the inclination is regarded as the ion-neutral collision frequency ν in at each
altitude. Then, the intercept of the x axis is regarded as the FA neutral velocity Vn // .
Figure A.4 shows the results of the inclinations derived from the ESR data at different
altitudes. An example of the top left panel in the Figure A.4 shows that reliable inclinations
are obtained from observational data between 400 km and 550 km altitudes. This indicatesthat the ion velocities are well correlated with the diffusion terms for 2 hours. The larger
inclination values at each altitude are within 3 times those of smaller ones. Below the altitudesof 400 km, only a few reliable inclinations are obtained. The black line indicates a heightprofile of the ion-neutral collision frequency ν in obtained from the model described inequation (A.17). The neutral number density nn and temperature Tn are derived from MSIS–E
90 [Hedin, 1991], and the ion temperature Ti is derived from the ESR data. Around 412 km
and 452 km altitudes in the left upper panel in Figure A.4, the collision frequency derived
from the observations corresponds to that obtained from equation (A.17). Above 488 km
altitude, the collision frequency derived from the observations is approximately 2 times
higher than that obtained from equation (A.17). The same features above 488 km are seen inother examples in Figure A.4. On the other hand, the collision frequency derived from theobservations is lower than that from equation (A.17), when the inclinations are exactly
decided below 400 km altitude in the lower left panel. Hence the derivation of ion-neutral
collision frequency ν in from the ESR data and equation (A.22) is possible only within altitude
limits between approximately 400 and 600 km. However, this derivation from only
observational data may also be helpful for constructing a realistic model, because theoretical
ion-neutral (O+ - O) collision frequency models remain a matter for debate [e.g., Salah, 1993].
The collision frequency obtained from equation (A.17) is in proportion to the neutral
number density. The theoretical collision frequencies around 500 km altitude on November 26,
2000 (in the lower right panel of Figure A.4) are approximately 4 times larger than that on
December 21, 1998 (in the upper right panel of Figure A.4). This is because the neutral
density on November 26, 2000, near the solar maximum, is higher than those on December 21,
1998. The observational collision frequencies also show similar differences. The collision99frequencies at 488 km and 523 km on November 26, 2000 (also in the lower right panel of
Figure A.4) are approximately 3 times higher than those on December 21, 1998 (also in theupper right panel of Figure A.4). It is likely that the collision frequencies obtained from theinclination also have a solar cycle dependence.
Figure A.5 shows an example of time variations of the collision frequency ν in and the FA
neutral velocity Vn // at an altitude of 452 km, derived from the ESR data. Magnetic Local
Time (MLT) ≈ UT + 3 hours ≈ Geographical Local Time (LT) + 2 hours. The upper panel in
Figure A.5 shows the collision frequency derived from the observation at each 2 hours. The
collision frequency at noon (around 11 UT) is higher than that at night (around 21 UT). Whenthe diffusion term (in the middle panel of Figure A.5) and the ion velocity (in the lower panelof Figure A.5) have small fluctuations (e.g., those between 16 and 18 UT), the ambiguity of
the collision frequency ν in becomes relatively large. The collision frequency derived fromequation (A.17), plotted as a solid line in the same upper panel, also has a similar change asthat derived from the ESR data. It is mainly due to the variation of the neutral number density;
the maximum is around noon and the minimum is around night.
The red line in the lower panel of Figure A.5 shows the FA neutral velocity Vn // derived
from the ESR data. The velocity changes from downward around noon to strongly upward
around night. When we regard the FA neutral velocities at every 2 hours as offsets of the FA
ion velocities, the change of direction of the relative velocities in the lower panel coincides
with that of the diffusion term in the middle panel.
The neutral velocity Vn // should generally be reflected by the meridional neutral winddescribed as

Vn // = Vn ( South− North) cos( I ) ,(A.23)

where I is the dip angle of the magnetic field line at Longyearbyen (see Figure 4.1). The
ordinate in the right hand of the lower panel in Figure A.5 indicates the meridional neutral
wind velocity. The positive (negative) velocities indicate motion to the southward (northward).
According to equation (A.23), the FA neutral velocities observed on December 7, 1999 areinterpreted as neutrals moving poleward with the velocity of 100 m s-1 around daytime andequatorward with the velocity of 700 m s-1 around nighttime. These trends agree with thedirections and amplitudes of the average meridional neutral wind in the polar F region/topside100

ionosphere [Thayer et al., 1987]. The neutral velocities derived from observations at different
altitudes between 400 km and 600 km also have the same trends, although they are not shown.
These observational results lead to the conclusion that the approximation of the FA ion
momentum equation (A.22) is greatly useful in understanding ion motions in the polar topside
ionosphere. Ion upward flows are often explained by thermal diffusion using equation (A.22).
As an exception, when the flectional heating is considerably strong or naturally enhanced
ion lines are often seen, observational results rarely satisfy equation (A.22). In the former case,
we must consider transportation of ions and electrons to the perpendicular direction as well as
to the parallel direction. In the latter case, additional effects such as “anomalous” resistively
produced by plasma turbulence [e.g., Papadopolous, 1977]
E// = η*// j// ,(A.24)

may be needed to explain the observational results, because the left hand side in equation(A.22) becomes clearly smaller than the right hand side in equation (A.22) [Wahlund et al.,1992a].

101

Figure A.1: Time variations of diffusion term (upper panel) and field-aligned (FA) ion
velocity (lower panel) at an altitude of 452 km obtained from the ESR between 02 and14 UT on December 8, 1999. Positive diffusion term and FA ion velocity are in an
upward direction.

102

(B)
(A)
(A)
(B)

(A)
(B)

(A)
(B)

(A)
(B)

Figure A.2: Relation between diffusion term and field-aligned ion velocity obtained
from the ESR between 06 and 08 UT on December 21, 1998. Numbers at lower leftof each panel indicate inclinations of line (A) and (B).

103

(B)

(A)

(A)
(B)

(A)
(B)

(A)
(B)

(A)
(B)

Figure A.3: Same format as Figure A.2, but between 02 and 04 UT on December 8, 1999.

104

Figure A.4: Height variations of ion-neutral collision frequencies derived from observations
and models. Red lines indicate that larger inclination value is within 3 times that of thesmaller one. Black line indicates a collision frequency obtained from model described inequation (A.17).

105

Figure A.5: Time variations of the collision frequency (upper panel), diffusion term
(middle panel), and field-aligned (FA) ion and neutral velocities (lower panel) at analtitude of 452 km obtained from the ESR between 10 and 22 UT on December 7, 1999.
Horizontal line and each vertical line in the upper panel indicate collision frequencies
obtained from model described in equation (A.17) and from ESR observations, respectively.
The red line and each green point in the lower panel indicate FA neutral velocity derived
from observations and FA ion velocity from observations. The ordinate on the right handside of the lower panel indicates the reflected meridional neutral velocity. The positive(negative) velocities indicate motion to the southward (northward).

106

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