1
To appear in the Journal of Geophysical Research, 2001.

A comparison study of the auroral lower thermospheric neutral winds
derived by the EISCAT UHF radar and the Troms MF radar.
S. Nozawa,1 A. Brekke,2 A. Manson,3 C. M. Hall,2 C. Meek,3 K. Morise,1 S. Oyama,4 K.
Dobashi,5 and R. Fujii1

Abstract

A comparative study of the neutral wind in the polar upper mesosphere/lower thermosphere isconducted using two radars, the European Incoherent Scatter (EISCAT) UHF radar ( =931
MHz) and the Troms MF radar ( =2.8 MHz) co-located in northern Scandinavia. Comparisonsof winds are made in case studies as well as on a statistical basis. Based on simultaneous
observations obtained by the two radars, comparisons over a height range from 90 to 100 km
are made for about 20 days between February and October, 1999. In these comparisons, we
directly compare temporal wind variations. On the other hand, mean winds as well as diurnaland semidiurnal components are derived using 1-month-averaged wind data obtained by the MF
radar for 1999. They are compared with those derived from 56 days of EISCAT data studied by
Nozawa and Brekke 1999a,b]. In the case studies, we have also utilized altitude proles ofelectron density obtained by the EISCAT radar at and above 62 km height to determine thetotal reection height as well as to estimate the eect of group retardation for 2.8 MHz radio
wave. It can be seen that the eect of particle precipitation sometimes penetrates into the Dregion (down to 84 km). Generally, wind velocities derived by EISCAT with the monostatic
method exhibit signicant scatter with time below 100 km, while the number of the MF radar
wind data sharply decreases above 90 km except for summer. These facts make the comparisondicult, but no signicant departures between dierent radar winds are identied on an
instantaneous basis. In the statistical studies, generally both altitude proles are well connected,
or continuous, but signicant disagreements are observed for the mean wind and the tidal
components for summer. Based on these comparisons, we raise strong concerns regarding the useof summer wind data above 91 km obtained by MF radar located at high latitude under highsolar activity conditions. The wind values during the winter and equinox observed by the 2radars are complementary. Thus here is a good opportunity to utilize wind data from the twodierent radars through a year, which then enables us to cover a wide height range(approximately 70-120 km). This wide range gives us good opportunities to examine themesosphere/lower thermosphere coupling processes based on observational data.

1. Introduction
The atmospheric dynamics in the lower thermosphere and upper mesosphere is strongly controlled byatmospheric waves such as gravity, tidal, and planetary waves which mainly propagate upward from below. In these regions, substantial amount of gravity wave energy must reach the lower thermospherefrom below see e.g., Hocking, 1996]. Hocking 1996]
pointed out the importance of comparative studies ofwinds at di erent heights in these regions. Nozawa
and Brekke 1999a,b] studied seasonal and solar cycle
variations of the mean and tidal winds in the lower
thermosphere between 95 and 119 km. They showed,
for example, a strong eastward mean wind (40 ms;1 ) in summer maximizing at 101 km, and suggested
that this feature in the altitude wind prole is produced by gravity wave drag. It is, however, impossible to prove this idea quantitatively by using observational data in the lower thermosphere only. To
understand the lower thermospheric dynamics more
completely, therefore, it is necessary to make simultaneous observations of the wind both in the lower
thermosphere and the upper mesosphere. There arevarious techniques to observe (or derive) wind velocity
in the lower thermosphere as well as the mesosphere.
Regarding the ground-based observations, radar techniques are generally superior to optical techniques interms of tidal studies because radars can make measurement continuously for 24 hours a day with relatively good height resolution (3 km or so). However, no radar techniques can be applied to derivewind measurements simultaneously both in the upper mesosphere and the lower thermosphere. For example, a medium frequency (MF) radar can observe
winds regularly from 60 km to 100 km e.g., Reid,
1996], while an incoherent scatter (IS) radar can derive wind velocity from 90 km to 120 km e.g.,
Nozawa and Brekke, 1999a,b]. Therefore, we need toutilize at least two radars to cover the whole region.
In recent years several papers e.g., Hocking, 1997
Reid, 1996] have questioned the validity of MF radarobservations above 90 km. Problems, for example,
due to receiver saturation, strong reection from Elayer and group retardation are pointed out. Namboothiri et al. 1993] investigated the di erence between real and virtual heights for 2.2 MHz radio
wave using standard electron density proles fromrocket observations as well as the International Reference Ionosphere (IRI) model Rawer et al., 1978]
for application to the Saskatoon (52N, 107W) MF
radar. They concluded that in winter no corrections

2
for group retardation are necessary up to 100 km
height, while during summer such corrections are necessary above 95-97 km. Hall 1998] estimated groupretardation for 2.8 MHz radio wave by using the empirical electron density model Friedrich and Torkar,
1981] which is based on a substantial number of rocket
soundings from the Andya Rocket Range (69 N,
16E) situated only 120 km to the west of Troms.
Hall 1998] discussed virtual to real height correction
for the Troms (69.58N, 19.23E) MF radar ( =2.8
MHz) e.g., Hall, 2001]. Hall 1998] supported theconclusions by Namboothiri et al. 1993] and notedthat the upper limit was 94 km for reliable observations for the Troms MF radar. These two studies,
however, were based on models for electron density
proles. At high latitudes during geomagnetically
disturbed periods, particles originating in the magnetosphere penetrate into the lower thermosphere andeven into the upper mesosphere, lowering the total reection height of 2-3 MHz radio wave down to 100 km
on some occasions and thereby causing severe groupretardation e ects below 100 km.
Hines et al. 1993] made a comprehensive comparison of winds in the mesosphere and lower thermosphere obtained during the Arecibo Initiative inDynamics of the Atmosphere (AIDA) '89 campaign.
They compared 14 proles of wind data derived by
the MF/HF partial-reection technique with thosefrom the Arecibo IS radar at heights of 65-95 km
and meteor radar at heights of 80-100 km colocatedin Arecibo (18 N, 67W). They concluded that windvelocities derived by the MF/HF radar are good representations of ambient winds only up to about 80
km. Contrary to the conclusion by Hines et al.
1993], Turek et al. 1995] examined 208 proles of
the AIDA campaign wind data and concluded thatboth the imaging Doppler interferometer (IDI) andthe Arecibo IS radar winds are normally distributed
over the height range from 70 to 97 km and the IDI
wind compares well with winds derived from other
MF/HF radar wind measuring techniques such as fullcorrelation analysis (FCA) and three dimensional interferometry analysis (INT). Furthermore Turek et al.
1998] again compared the Arecibo IS radar wind datawith MF/HF wind data derived from several techniques (IDI, FCA and INT). They found in generalvery good agreement (within 15 m s;1 ) up to 90
km between the zonal winds determined from all thetechniques. Furthermore, in the last decade comparisons of winds from MF radar and ground-based optical instruments have been widely carried out e.g.,

3Phillips et al., 1994 Manson et al., 1996 Meek et al.,1997], and generally good agreement was illustrated
at a height of 97 km. For example, Manson et al.
1996] made comparison between instantaneous measurements made at Saskatoon (52 N, 107W) using
the colocated MF radars and Fabry-Perot interferometer (FPI) instruments over 1988-1992. They foundgood agreement (the speed ratios of winds from two
instruments are close to unity the vector angle di erences are centered about 0 , with medians less than+5) on the climatological comparison. Meek et al.
1997] made comparisons of winds derived from several instruments such as Canadian prairie MF radars,
FPI, and the Upper Atmosphere Research Satellite(UARS) high resolution Doppler imager (HRDI) systems. On the statistical comparisons of hourly mean
winds obtained over 1988-1992 for the Saskatoon andCalgary MF radars and FPIs (OH and 557.7 nm greenline), they found excellent agreement (e.g., speed ratio as well as phase di erence are close to 1.0 and 0
degree, respectively) for Saskatoon at two heights of
88 km and 98 km. However, Meek et al. 1997]
also showed some discrepancy that the Sylvan Lake
MF radar found values larger than the FPI (88 km
layer) by a factor of 1.2 and Saskatoon MF radar
speeds were less than HRDI values by factors 0.7-0.85.
Burrage et al. 1996] made comparisons of winds derived from the HRDI on UARS and MF radars, nding reasonable agreement (when considering the very
di erent sampling conditions of a remote sensing technique and a localized system) between 60 and 95 km,
but discrepancy in the relative magnitudes above 95
km. Therefore, the validity of MF radar wind measurements above 90 km is still an open question.
At high latitude, there are only few studies made tocompare winds derived by MF radar and other instruments. Manson et al. 1992] made the rst comparison of winds at high latitudes based on the TromsMF radar, VHF radar at Andenes, rockets launchedfrom Andenes, and the European Incoherent Scatter (EISCAT) Folkestad et al., 1983 Rishbeth andWilliams, 1985] Common Program One (CP-1) data
for six short IS campaigns during 1987-1989. MF
radar winds were shown to be 0.65 of those fromthe Andenes data (rocket and VHF radar). However,until 1988 neutral wind velocities could not be derived below 100 km using the CP-1 mode with thetristatic method e.g., Williams et al., 1994]. Thus,
during these campaigns too few events occurred forcomparing winds derived from simultaneous EISCATand MF radar data. They concluded, however, thatafter modication by the 1.5 factor there was, in general, very favorable agreement in phase and amplitude
between the EISCAT and MF radars (e.g., in caseof June 1987, the semidiurnal amplitude and phasenear 100 km derived from two radars are very similar: the di erences of amplitude and phase are a few
m s;1 and 1 hour, respectively). Di erences were
attributed to low signal to noise (S/N) conditions for
either radar, lack of complete data overlap, and strong
wind gradients in the lower thermosphere.
In this paper, we have made comparisons of winds
derived by the these two radars which are colocatedin Ramfjordmoen, nearby Troms in Norway. Section
2 is devoted to descriptions of these radars and their
methods of observations, and comparisons of winds
obtained simultaneously by the both radars are presented in Section 3. In Section 4, mean and tidal
winds derived from 1-month averaged wind data obtained by the Troms MF radar for 1999 are compared with those of the statistical study by Nozawa
and Brekke 1999a]. Discussions are given in Section
5, and summary and conclusions will be given in Section 6.

2. Observational methods
2.1. The Troms MF radar

The Troms MF radar has been in operation for
more than 10 years in a spaced antenna (wind measuring) mode. A recent specication of this radar canbe found in Hall 2001]. In summer 1998 its transmitter antenna system was refurbished, and in earlyNovember in 1998 a new solid state transmitter (50kW) was installed. After these improvements, theTroms MF radar has furnished wind data more routinely in the height region from 70 to 100 km (invirtual height) on a diurnal basis. Since then, thisradar is operated under collaboration of the Universities of Saskatchewan and Troms and Nagoya University. We have assembled all data obtained by the
MF radar in 1999. The time resolution was 5 min
until February 15, 1999 and since then it has been
2 min. The Troms MF radar receives signals (partially) reected in the mesosphere and lower thermosphere with thirty-two 3 km wide gates starting at 40km, and its analysis is based on the Saskatoon Full
Correlation Analysis (FCA) technique of Spaced Antenna (SA) data Briggs, 1984 Meek, 1980]. The SA
technique is most commonly applied to MF radars
see e.g., Reid, 1996]. The Saskatoon FCA technique4
involves sampling coherent fading signals simultaneously at three or more, non-colinear, spaced antennas
at a set of xed height gates (40-133 km with a step of
3 km). The resulting complex time sequences at eachheight are correlated (cross and auto) and tted witha Gaussian correlation function Meek, 1980] which
includes ground-pattern spatial anisotropy and decay
terms, as well as the velocity vector. The groundpattern is assumed to be moving twice the speedof the scattering irregularities. Because the system
is coherent, Doppler velocity and angle of arriving
phase fronts are also calculated. Generally, errors ofwind velocities from the MF radar based on the FCA
method is estimated to be less than 10 m s;1 see
Meek and Manson, 2001].
The MF radar operates continually. For the comparative analysis of simultaneously obtained MF and
EISCAT radar data, we use the MF wind data with 2
min or 5 min resolution, while for statistical comparisons, we calculated monthly averages. We only use
the MF wind data obtained below the height where
the receiving power maximizes see e.g., Namboothiri
et al., 1994]. Owing to this criterion for acceptanceof wind data, the number of data is signicantly reduced above 90 km and no wind data remain at andabove 106 km over the year 1999. In order to securedata reliability, this criterion is necessary and commonly applied to the Saskatoon MF radar data inthese days. Hereafter we refer to this as "MF criterion". Based on the 1-month averaged data obtainedby the Troms MF radar, we have derived mean windand diurnal and semidiurnal amplitudes and phases
for each month in 1999. The tidal components are derived by using the Lomb-Scargle periodogram method
which is based on least-squares frequency analysis ofunequally spaced data cf. Hocke, 1998].

2.2. The EISCAT UHF radar

Among the EISCAT CP modes e.g., Collis, 1995],
CP-1 and CP-2 modes have been used for deriving
neutral wind velocity vectors in the E region. In theCP-1 mode the line of sight of the combined transmitter and receiver antenna is xed along the magnetic
eld line at Troms at an elevation angle of 77.5,
while in the CP-2 mode the line of sight of the antenna
is pointed into four consecutive positions with a dwell
time of 1 min in each position, resulting in a full cycle time of the antenna of 6 min. There exist threeobservational methods to derive neutral wind velocities such as the tristatic method (applied to earlier

CP-1 mode), the monostatic method (CP-2 mode),and the eld-aligned method (mainly CP-1 mode) see
Williams et al., 1994 Nozawa and Brekke, 2000]. In
this study, we have derived meridional wind velocities
from data obtained by the CP-1 mode based on theeld-aligned method. On the other hand, from dataobtained by the CP-2 mode (monostatic method) wehave derived both meridional and zonal wind velocities. Here we briey describe the ways to derive neutral wind velocities from ion velocity measurementssee details in Williams et al., 1994].
Like other IS radars, the EISCAT radar measures
ion velocities, not the wind velocity directly, in theionosphere. In the lower thermosphere due to thestrong coupling between ions and neutrals one can derive the neutral wind velocity vector (u) using bothmeasured ion velocity (v) in the E region and derived
E-eld (E) from F region ion velocity measurement
by using the following equation Rino et al., 1977]:
u = v ; i (E + v  B)
(1)

jBjinwhere B is the earth's magnetic eld, in is ionneutral collision frequency, and i is ion gyrofrequency. We base our analysis on an International Geomagnetic Reference Field (IGRF) model (IAGA Division I working Group 1, 1987) and a formula for theion-neutral collision frequency introduced by Schunkand Walker 1973]. The model neutral atmosphere isgiven by MSIS86 model Hedin, 1987]. This method(equation 1) was rstly introduced by Brekke et al.
1973], evaluated by Comfort et al. 1976], improved
by Rino et al. 1977]. From this technique, which canbe applied to data obtained by both the tristatic andmonostatic methods, one can derive tri-dimensional
neutral wind velocity vectors in the lower thermosphere Nozawa and Brekke, 1999a].
Another way is the eld-aligned method. Thismethod takes the component of the ion velocity vector (v// ) measured along the local magnetic eld line,
which is at an inclination of I = 77.5 and azimuthof 182.6 (counting from north to east) at Troms.
In the lower thermosphere, where ion di usion velocity along the magnetic eld line is signicantly small(less than 1 m s;1) e.g., Kofman et al., 1996], theion motion along the magnetic eld is independentof electric eld and dominated by neutral wind drag.
Thus the eld-aligned ion velocity (v// ) is expressed
as follows:5v// = ;uhcosI + uv sinI(2)
where uh and uv are the horizontal and vertical neu-

tral wind velocity, respectively. It follows that if thevertical velocity of the neutral wind (uv ) is considerably smaller than uh cotI, then the horizontal neutral
wind velocity is given by Williams et al., 1994]:

uh = ;v// =cosI(3)
At the EISCAT Troms site, where the local magnetic eld is directed almost south, the relative contributing factor of zonal component to uh , comparedto that of the meridional component, is 0.045 (= tan(182:6)). Thus, the meridional component of neutral wind can be determined in this way from eq.
(3). Although the eld-aligned method can furnish
only meridional wind velocity, it has proved very effective in identifying tidal modes over the height range
100-160 km Virdi et al., 1986 Huuskonen et al.,1991] and is the best one among the methods for themeridional component Nozawa and Brekke, 2000].
Comparisons of derived winds from di erent modes
and/or methods were conducted on a statistical basis
Nozawa and Brekke, 1999a] and on a case study basis
Nozawa and Brekke, 2000], and showed that the derived winds agreed reasonably well in terms of longer
temporal variations (>3 hours). Nozawa and Brekke
2000], however, showed that derived wind data by themonostatic method had a large variance compared tothose derived from the tristatic method and the eldaligned method.
Table 1 summarizes data obtained by the CP-1 andCP-2 modes in 1999, which are used in this study. In
addition, averaged system noise temperature as well
as averaged transmitter power for each experiment arelisted in Table 1 to give a general idea of conditionsof the EISCAT UHF radar. As Table 1 shows, there
are more than 20 days when temporal wind variations
can be directly compared with the MF radar winds.
Nozawa and Brekke 1999a,b] studied seasonal and solar cycle variations based on 56 days of EISCAT CP-1
and CP-2 data obtained from 1987 to 1996. We areutilizing this separate data base for computing themean wind and diurnal and semidiurnal components
in this work.
It should be pointed out that for IS radars (basedon the normal IS theory e.g., Schlegel, 1995]) measurement is less reliable below 100 km than above.
This is because the electron density decreases significantly below 100 km, and particularly a ect windvelocities derived by the monostatic method. In contrast, the eld-aligned method is more reliable thanthe monostatic method, since longer integration timefor each IS spectrum can be applied see Nozawa andBrekke, 2000].

3. Comparisons of wind velocities
based on simultaneous data
Here we present comparisons of temporal windvariations based on simultaneous observations by the
EISCAT and MF radars co-located in Troms. Table 2 summarizes geomagnetic activity (3-h Kp anddaily Ap indices) and the solar 10.7 cm ux (F 10:7
index) for these events. The dates for comparison are
distributed from quiet to moderate geomagnetic activities under high solar activity conditions.
We rst discuss the horizontal resolution of the
EISCAT and MF radars. The EISCAT UHF radarobserves a column volume (diameter = 1 km andheight = 3 km) in the E region. In the CP-2 mode,to derive tri-dimensional ion velocity vectors, we needto combine three line of sight measurements madeat three column volumes which are 3050 km apartfrom each other in the E region. On the other hand
it is a bit dicult to make a similar estimate for the
MF radar. If angular spread is estimated from FCA
ground pattern scales, values for scattering volume
extent of the order of 10 km are found for the TromsMF radar. We think the di erence in resolution introduces disagreement of wind velocities for short-time
periods into the wind comparisons, but the variationsof winds should be similar for a few hours or longer
period (especially for mean and tidal winds).
On the basis of the propagation theory of error,
each error of the neutral wind velocity derived by the
EISCAT radar is calculated from errors determinedoriginally by the IS spectrum analysis. Concerning
error bars of tidal components, we rstly calculate astandard deviation for each point of time when winddata are averaged. Then we calculate an averaged
root-mean-square of the standard deviations for each
averaged data set, such as seasonal averaged data for
EISCAT radar and monthly averaged data for MF
radar. Using the averaged root-mean-square of standard deviations (we refer to this as averaged standard
deviation, hereafter) we nally estimate standard deviations for all mean and tidal components from EISCAT and MF radars and shown them as error bars(1) in corresponding gures. The way of calculation

Table 2

6
is as follows:Amean =rrA = 2 ns582 km for long pulse Collis, 1995]. Ambiguity of
2

(4)

n

2

i

i = 2

(5)

2

(6)

Ai 2 nwhere Amean , Ai , i are standard deviations formean wind, tidal amplitude and phase (diurnal andsemidiurnal components), respectively, and n is thenumber of data points with time,  is the averaged
2
standard deviation derived from  =  nk , and kis the standard deviation at point k (of time). A detailed description of this calculation is presented in
Nozawa and Brekke 1995].
It is important for radars using medium frequency
to identify where the transmitted radio wave is (partially) reected. Generally, electron density increases
with increasing height in the mesosphere and lower
thermosphere, group delay of the radio wave becomes
more signicant with increasing height. To clarify theobserved height for MF radar wind data, we present
di erences between true and virtual heights calculated using electron density proles obtained simultaneously by the EISCAT radar. The EISCAT electron density used in this study is calibrated by using
dynasonde "fof2" data. The di erence dh in km as afunction of height is give by:qs 80:5Ne(z)Zz1;dh(z ) = z ;
dz

(7)

fr 2
where z is the true height in km, Ne is the electron
0

density in m;3 as a function of (true) height given
by EISCAT observations, and fr (= 2.8 MHz in thiscase) is the radar frequency in Hz Hall, 1998 Namboothiri et al., 1993]. EISCAT CP-1 and CP-2 modes
make measurements of electron density from 62.3 km
to 582 km. Thus, in practice, the di erence is calculated by:z s
X
1 ; 80:5Ne(z ) z ) (8)h(z ) = z ; (62:3 +
62:3

fr 2

where z is a gate interval which is 3.15 km from 62
to 269 km for both short pulse (power prole) andalternating code (ACF) and 22.5 km from 142 km to

EISCAT electron density measurement is expected to
be about 10% or so. When electron density is higher(> 31010 m;3 ), its ambiguity of measurement becomes lower. Therefore we estimate an ambiguity inh due to 10 % ambiguity in electron density. Although we can derive the di erence between true andvirtual heights by using Eq. (8), we present MF radar
wind data as a function of virtual height in gures
to avoid possible confusions. Readers should keep inmind that there would be di erence between true andvirtual heights (especially above 90 km) when comparing the wind data. Also, it should be noted that
there is a height di erence up to 1 km between thecenters of each gate of both radars. Since their heightresolutions are about 3 km, this mismatch of the center gate probably does not cause any serious problems
with the comparison. On instantaneous wind comparison, EISCAT wind data with errors greater than 50
m s;1 are always rejected.
As shown in Table 1, on case study basis we madecomparison of wind data derived with the EISCATand MF radars for ve cases. Here we present threecases from each season (winter, summer and equinox)
in detail, and for the other two cases brief summaries
are given.

3.1. Case 1: February 9-12, 1999

Figure 1a shows meridional wind variations as afunction of universal time obtained by the EISCAT
CP-1 mode (plus) from 10 UT on February 9 to 16
UT on February 12, 1999 for four heights from 90 km
to 99 km. The wind data were derived by the eldaligned method with post-integration time of 5 min.
Meridional and zonal components of the electric eld
as well as 3-h Kp index are also shown in the upper two panels and in the bottom panel, respectively,
for examining possible relationship between winds below 100 km and geomagnetic activity. Corresponding
wind data (open circle) obtained by the Troms MF
radar are shown for comparison.
Geomagnetic activity was generally quiet (Kp <3) on February 9 and 10, and was moderate (Kp =
3 ; 4) on February 11 and 12. The amplitude of thezonal component of the electric eld was small (lessthan 10 mV m;1 ) over the whole period except between approximately 0100 and 0300 UT on February
12 when it increased to 26 mV m;1 eastward. The
meridional component of the electric eld showed enhancements (up to 60 mV m;1 northward) between

Figure 1a1b7
approximately 1700 and 2300 UT on February 9 and10. On February 11 it again increased to 70 mV
m;1 northward at around 1430 UT, and turned from
northward to southward at around 2200 UT. The electric eld amplitude reached 80 mV m;1 southward
at around 0200 UT, and the electric eld again turned
northward and was strong between 1100 and 1500 UT
on February 12. A comparison of the MF radar winddata with the strength of the electric eld during theperiods showed no clear relationship. For example,
between 0100 and 0300 UT on February 12, when Eeld is strongly enhanced, the variation of MF radar
wind at 90 km did not show large departure from thatfor other periods, and the MF radar wind velocity (except for one) ranged from -50 to 50 m s;1 over theheight region.
Although data gaps often occurred, it is clear that
the EISCAT wind data ranged from approximately-150 to 150 m s;1 and showed semidiurnal variation
over the height region. At 90 km the MF radar winddata showed small uctuations which increased withheight. In addition, the number of MF radar winddata tended to decrease with height. When comparing variations of wind velocity derived by EISCATand MF radars over the height region from 90 to 99
km, a smoother time variation is seen for the MF
radar wind data than for the EISCAT radar data.
The variance of the MF radar wind data was lessthan that of the EISCAT wind data. EISCAT windcan be derived during sunlit hours as well as for particle precipitation periods, while MF radar wind dataobtained for particle precipitation periods are often
discarded by the MF criterion (see section 2.1). Between 90 and 96 km, wind velocities from two radars
appear to correspond well, and at 99 km no strong departures of the wind velocities between EISCAT and
MF radar data can be found in this case.
Figure 1b illustrates di erences between real andvirtual heights for 2.8 MHz radio wave for the experiment period between 84 and 99 km. When totalreection occurs at and below the height, a value of-1 is shown. Furthermore, at the bottom of Figure
1b, temporal variation of height integrated Hall conductivity (conductance) calculated according to thework by Brekke and Hall 1988] is shown, since theconductance could be a good indicator of particle precipitation. Generally, the di erences are small up to
90 km. When no particle precipitation occurred, thedi erences were less than 3 km (= 1 gate interval) up
to 105 km over the experiment period. However, MF
wind data was a ected by group retardation or totalreection due to particle precipitation for almost half
the time at and above 99 km, and below 99 km suchinuence can be often seen. Even at 90 km, the inuence is found for time periods between 0030 UT and
0200 UT as well as 0600 UT and 1000 UT on February
11, and between 0600 UT and 1300 UT on February
12, indicating care is necessary in selecting MF radar
wind data above 90 km at high latitude station.

3.2. Case 2: July 1-9, 1999

This EISCAT experiment was conducted partly asa CP and partly as a special program Collis, 1995]
during this period. Its observational mode was identical to the CP-2 mode in which tri-dimensional winddata are furnished at every 6 min. The EISCAT data,
unfortunately, were a ected badly by the high system
noise temperature (156 K), approximately twice ashigh as normal. This makes the comparison more difcult. Figure 2a and Figure 2b show comparisons of
the winds for the meridional and zonal components,
respectively. As in Figure 1a, variations of meridionaland zonal components of the electric eld and 3-h
Kp index are illustrated at the top 2 panels and thebottom, respectively. It was geomagnetically quiet(Kp <3) almost whole the experiment period except
for July 2. The amplitude of the meridional component of the electric eld was enhanced northward (upto 40 mV m;1 ) between approximately 1600 and
2100 UT on July 2 and between approximately 1300
and 2000 UT on July 8. The electric eld was alsoenhanced southward (up to 30 mV m;1 ) between
approximately 2200 and 0300 UT on July 5-6 and between approximately 2100 and 0200 UT on July 8-9.
For other periods the amplitude of electric eld was
generally small (less than 10 mV m;1 ). During theperiod when the electric eld was enhanced, MF radar
wind velocity of the horizontal components over theheight region ranged approximately between -50 and
50 m s;1 except for the period between 1600 and 2100
UT on July 2 when the meridional wind data at 90km height ranged approximately 20 m s;1 to -100 ms;1 . Here again no clear relationship can be foundbetween strength of electric eld and MF radar windvelocity.
Very low numbers of wind data were obtained by
EISCAT at and below 92 km in this experiment. Even
though wind data with errors of more than 50 m s;1were eliminated, the EISCAT wind data still exhibit
much scatter over the height region. This is probably due to several causes such as low electron density

Figure 2a
Figure 2b

2c

3a

8
below 100 km, high system noise temperature, andshortcomings of the monostatic method Nozawa andBrekke, 2000]. For the MF radar wind data, the number of values is high compared with that in other seasons and they again show smoother variations withtime than those of EISCAT wind data. At 89 km MF
radar wind data have almost no data gaps, while windderivation by EISCAT failed for almost the whole period. Again the number of the MF radar wind data
decrease with height. At 98 km MF radar wind data
were only obtained for one third of the experimentperiod, but they still showed smaller scatter than the
EISCAT wind data. These indicate that poor agreement for this case between 89 and 98 km for the horizontal component can be primarily attributed to thehigher ambiguity of the EISCAT wind measurements
due to the high system noise temperature.
Figure 2c shows the di erence between true andvirtual heights as well as the Hall conductance. Fromthe variation of the Hall conductance, it is apparent
that during night time particle precipitation often occurred. As a result the 2.8 MHz radio wave was often
totally reected at and below 93 km. When no particle precipitation occurred, the di erence is about halfa gate interval (1.5 km) below 90 km almost all thetime. However, the di erence becomes larger with increasing height. At and above 96 km, the di erenceshows a clear diurnal variation likely due to solar insolation. Sometimes total reection occurred at 99 km
regardless of particle precipitation. These facts imply
that although the number of values is high, special
care is needed in selecting MF radar data above 90
km in summer under high solar activity conditions athigh latitudes.

3.3. Case 3: September 15-17, 1999

Figure 3a compares meridional wind data obtainedby the EISCAT and MF radars between 90 and 99 km
for equinox season. Again variations of the meridionaland zonal components of the electric eld and 3-h Kp
index are presented altogether with the wind data.
The meridional component of the electric eld haslarge time variations from about 0000 UT on September 16 and to 0300 UT on September 17, ranging fromapproximately -80 mV m;1 and 70 mV m;1 . It was
small between 0300 and 1200 UT on September 17
and again increased up to about 50 mV m;1 northward at the end of the experiment. The zonal component of the electric eld also showed enhancements(up to 30 mV m;1 eastward) between about 0500 UT

and 0600 UT on September 16, and between about
0200 UT and 0300 UT on September 17. The 3-h Kp
index was high (up to 6) on September 16. Again,
no clear relationship between MF radar wind velocity
and strength of the electric eld can be found. In thiscase, the number of MF wind data is relatively low
over the height region, and until 07:40 UT on September 16, no data were obtained by the MF radar due toa system failure. The EISCAT wind data show a clear
semidiurnal variation ranging from approximately 130 to 130 m s;1 over the height region with a gooddata coverage. MF radar wind data obtained for particle precipitation periods appear to be discarded dueto the MF criterion. Although the number of the MF
radar wind data is low, both experiments agree relatively well between 90 and 96 km. At some times MF
radar wind velocity signicantly departed from thatof the EISCAT wind velocity at 99 km between 0400
and 0500 UT on September 17, but these are likely
due to e ects of group retardation or total reection
as shown in Figure 3b (i.e., true height of those MF
wind data should be lower).
Figure 3b shows the di erence between true andvirtual height for the 2.8 MHz radio wave between 84
and 99 km as well as the Hall conductance during theexperiment period. From a plot of the Hall conductance it appears that particle precipitation was almost
continually present on September 16 and 17, resulting in MF group delays as well as total reection. On
September 15, during no particle precipitation or soft
particle precipitation (no particle reached to the Eregion) period, the di erence is less than 3 km up to
105 km. However, on September 16 and 17, during
the particle precipitation period, the total reection
height went down below 90 km (down to 84 km oreven below) on some occasions. During the period
between 0300 and 0900 UT on September 16, totalreection occurred below 90 km almost all the time.
On September 17 at 96 km and 99 km, the di erence
ranged from 1.5 km to 9 km (or more), and it uctuated with time. These facts again imply that special
care is necessary with the MF radar wind data obtained at high latitude stations.

3.4. Other two cases

We have also made comparisons for March 8-12
and October 12-15, 1999. In the case of March 8-12,both the EISCAT and MF radar wind data have longdata gaps. Owing to system failures, low numbers of
the wind data were assembled for March 1999. Al-

Figure 3b4a9though the occasions for comparison are few, at leastno signicant departure of wind velocity between two
radars are identied. Geomagnetic activity was highon March 9 and 10, when MF radar data were a ectedby group retardation or total reection for more than
one third of the observing period at 90 km. In thecase of October 12-15, the number of MF radar windderivation is very low at and above 90 km. E ects
due to likely particle precipitation can be seen at thelower height of 84 km. At and above 90 km the MF
radar su ers from group retardation or total reection for more than 40 % of the period.
Finally, it should be pointed out that over all the
instantaneous wind comparisons presented in this section, it is likely that the MF criterion led to data rejection for periods of strong group retardation or totalreection due to particle precipitation.

4. Comparison of mean and tidal
winds
Based on 1-month-averaged wind data obtained by
the Troms MF radar, we have derived mean winds aswell as amplitudes and phases of diurnal and semidiurnal components for each month for 11 heights from70 to 100 km (virtual height) where numbers permitted. Resulting values cover 70 to 91 km for March,and 70 to 94 km for October. In 1999 the solar activity was high (see Table 2), and the averaged valueof F10:7 index for 1999 is 16010;22 W m;2 Hz;1 .
Nozawa and Brekke 1999a] collected 56 days of EISCAT CP-1 and CP-2 wind data between 95 and 119
km obtained under geomagnetically quiet conditions(Ap < 16) over a solar cycle from 1987 to 1996. The
averaged F10:7 index over those 56 days was 142
10;22 W m;2 Hz;1 . As a preliminary step to combining EISCAT and MF radar wind data for climatological study, we compare these statistical EISCAT
wind data with the MF radar wind data in terms ofmean wind and tidal (diurnal and semidiurnal) amplitudes and phases, and then we examine the upper
limit for reliable wind observations of the MF radarfrom a statistical viewpoint. In the study of Nozawa
and Brekke 1999a], the wind data were sorted into
three seasons: summer (May 3 - August 18), equinox(February 24 - April 14, and August 30 - October 20)
and winter (October 21 - February 20).
Figure 4a compares meridional and zonal mean
winds derived by the EISCAT and MF radars foreach of these seasons. For the height of the MF radar

data no correction to the group retardation is made,meaning the wind values derived by the MF radar are
shown as a function of virtual height. Readers should
keep in mind that di erences between true and virtual
height are generally 3-6 km above 90 km for the summer data. Concerning the altitude prole obtainedby the MF radar, month-to-month variation in thesame season appears less prominent except for thezonal component in the equinox season when considerable changes occur in the tidal forcing as well asback ground wind and temperature e.g., Manson etal., 1999]. Relatively good agreement (di erences areless than 10 m s;1 ) is found in the meridional component for equinox. The altitude prole of the zonal
mean wind in summer, which is the most structured inaltitude, obtained by the MF radar is fairly constantwith height above 91 km. In summer the electron density in the ionosphere is high, and this assures morereliable IS radar measurements than other seasons.
In contrast, high electron density causes group retardation and total reection below 100 km as shown in
Figure 2c. Therefore, it appears that reasonable variations as a function of height are seen up to 91 km for
the MF radar wind data. In winter, the tendency thatthe eastward wind amplitude reduces gradually withincreasing height appears to be reasonable, but at 95
km a di erence of more than 20 m s;1 is found between the wind data obtained by the di erent radars.
It is worth noting that winds modeled by Canadian
Middle Atmosphere Model (CMAM) e.g. McLandress, 1998] and provided from meteor wind radarat similar latitudes (Mitchell, private communication)
do show the reversal height near 100 km. The EISCAT winds below 100 km thus might be questionable,
but we cannot neglect the fact that the MF radar
wind data above 90 km appear more noisy than thosebelow 90 km.
Figure 4b illustrates altitude proles of the diurnal amplitudes and phases derived by EISCAT and
MF radars. Fairly good agreement (the di erence being less than 10 m s;1 ) is seen for meridional andzonal amplitudes except for winter where large di erences (30 m s;1 ) exist at 95 and 98 km for the zonal
component. The EISCAT values for winter below 100
km are much larger than those at 100 km, suggesting a problematic result. These cannot be separated
from the large mean winds in Figure 4a. Concerning phases, good agreement (<3 hours) is found formeridional components for summer and equinox. The
meridional phase values obtained in January and December exhibit large (up to 11 hours) di erences be-

Figure 4b4c10tween them. The di erence of the zonal phase component between EISCAT and MF radar data is large forsummer (<10 hours) (again perhaps related to themean wind values in Figure 4a), while relatively good
agreement (<6 hours) is found for equinox and winter (except for January data) considering their largechange with month. The winter values are again noisy
in appearance.
Figure 4c compares semidiurnal amplitudes andphases. Signicant di erences for summer and winterare identied in the amplitudes. Especially in summer there are large (>20 m s;1 ) disagreements at
and above 95 km. Concerning phases, they appear
to be in fairly good agreement (<3 hours) for almost all the seasons. However, the phase values ofthe meridional component in summer months tend to
be constant with height at and above 91 km in Juneand between 88 and 94 km in July, implying the occurrence of group retardation.
Finally, if we combine altitude proles of winds derived from the MF radar between 70 and 91 km andthe EISCAT radar between 95 and 120 km, it seems
like that they can smoothly connect to each other(except for zonal mean wind and diurnal components
both in winter). This feature is very encouraging for
future studies.

5. Discussion
In this paper, we have shown comparisons of winds
between 90 and 100 km derived from two radars:
the EISCAT and MF radars. The most important
fact is to examine how well we can utilize the winddata obtained from the di erent radars in order tocover a wide height range, which enable a detailed
research on the mesosphere/lower-thermosphere coupling process on an observational basis. Generally, IS
radar measurement under geomagnetically quiet conditions is reliable between 100 and 120 km regardless of season, while MF radar measurements work
well between approximately 70 and 90 km. As shown
before, however, the wind measurements (or derivations) from the two radars are relatively dicult between 90 and 100 km especially on an instantaneousbasis. In summer, the high ionization assures the reliable IS radar measurement even below 100 km, while
it causes group retardation or total reection of the
MF radio wave. Thus, the IS radar measurement islikely more reliable than the MF radar measurement
between 90 and 100 km in summer. On the other
hand, in winter the IS radar measurement is dicultbelow 100 km due to low electron density, especiallyfor night-time. In contrast, low electron densities allow the MF radio wave to penetrate to the E region
heights.
In Figures 1a, 2a, 2b, and 3a, in addition to temporal wind variations, we also present the 3-h Kp index
to examine the e ects of geomagnetic disturbance on
the MF radar wind measurement as well as the relationship with the occurrence of the group retardationand total reection. From Table 2, the daily Ap index
varied between 3 and 46 for the events studied. As wehave shown, under these conditions, an e ect of auroral particle precipitation sometimes penetrated into
the D region. In the case of February 1999, the particle precipitation occurred when Kp was less than 2.
Since particle precipitation and Kp index show no direct relationship, it is dicult to assess the validity of
MF wind data using only Kp index. However, since
we use the wind data obtained below the height where
receiving power maximizes (MF criterion), the winddata which would su er from possible contamination
due to particle precipitation are likely rejected.
It is clearly shown in Figure 2c that in summer
when no particle precipitation occurred, the group retardation became signicant at and above 93 km. At
99 km, the di erence ranged from 1 to 6 km. Fromthese facts, we can conclude that in summer underhigh solar activity conditions the upper limit for reliable MF radar wind observations is 91 km at this latitude. This conclusion agrees well with that of Hall
1998], who calculated the group retardation basedon an the empirical electron density model. Furthermore, high latitude comparisons suggest a greater virtual height correction and a more restricted usable
MF height range than that found by Namboothiri etal. 1993] at mid latitudes. For equinox season, when
no particle precipitation occurred, it appears that the
MF radar can furnish reliable wind data up to 96
km under high solar activity conditions. However,when the particle precipitation occurs, the wind measurement at and above 90 km is no longer reliable.
In winter, during periods at no particle precipitation,
the e ect of group retardation is small (less than 1gate interval) up to 105 km. Fairly good agreement isfound between winds by the two methods up to 96
km when no particle precipitation occurred as shown
Figure 1a. Therefore, it is concluded that concerning
instantaneous wind measurement during no particle
precipitation period, the MF radar can furnish reliable data up to 96 km except for summer. During
periods of particle precipitation, it is unlikely the MF

11wind measurements are reliable above 91 km.
In the statistical study, as shown in Figures 4a, 4b
and 4c some discrepancies between the EISCAT and
MF radar winds are found at and above 95 km. By
comparing altitude proles of the zonal mean wind
as well as the semidiurnal amplitude, it appears that
MF radar wind measurements are questionable above
91 km in summer, although the semidiurnal phasevalues from two radars are in good agreement (difference being <3 hours) at and above 95 km. This
good agreement of the phase should be a coincidence.
These amplitude disagreements could be caused by
the e ect of group retardation and/or an e ect suchas strong reection from the E layer as pointed by
Hocking 1997]. For equinox, considering its variability from month-to-month as well as the limited number of EISCAT wind data (averaged spring and fall
data together), the agreement of mean and semidiurnal components can be concluded to be relatively
good. The EISCAT tidal wind parameters are often
quite variable below 100 km.
On the other hand, the disagreement of altitude
proles of the winter mean wind can suggest an important fact. Eastward mean wind amplitude derived
by the MF radar at 70 km is about 20 m s;1 andit decreases with increasing height and close to zero
above 90 km. In contrast, EISCAT observations show
eastward mean wind with amplitudes of 25 m s;1
at 95 and 98 km and then decreasing with increasingheight. Both altitude proles appear to be reasonable,
but a large di erence (15 ms;1 ) between EISCATand MF radar winds is found at 100 km height. The
EISCAT wind data are averaged over 17 days, obtained from a period 1987-1996, while the MF radar
wind are monthly means in January and December
1999, when the solar activity was high. These facts
suggest that winter zonal mean wind may be morevariable with month, year and/or solar activity conditions than that in summer.
The semidiurnal amplitudes derived by the MF
radar in winter months are about 10-20 m s;1 between 90 and 100 km. This feature is similar tothat of the earlier study by Manson et al. 1999],
who compared tidal components derived from the MF
wind data with those predicted by Global Scale Wave
Model (GSWM) see e.g., Hagan et al., 1999]. In
their comparison, they showed a general tendency
at all locations for observed values in winter above
85-90 km to be smaller than the model, while inall other months the MF wind amplitudes were often considerably larger than GSWM-95. Bearing in

mind the 1.5 amplitude factor found by Manson etal. 1992] for the MF wind amplitude, and applyingthat to these proles, the semidiurnal amplitudes become much stronger. This plus the excellent phaseagreements, suggest that tidal MF radar 12-h datanear 100 km for winter are a valuable product. In
contrast, concerning the diurnal tide, the amplitude
and phase from the 2 radars are not in agreement for
winter. There are some large amplitudes from EISCAT below 100 km. The amplitude proles of the
MF radar wind is again similar to that of Manson etal. 1999] who showed a relatively good agreement(the modeled amplitude laying within the range of
observed values over much of the height range) of thediurnal amplitude between MF radar wind observations and GSWM-95 predictions for all months. If
we exclude these large values from the EISCAT radar
wind proles and combine the two altitude proles at
97-100 km from the di erent radar observations, thealtitude prole is reasonably well behaved from 70 to
116 km. Therefore those large amplitudes from the
EISCAT radar might be wrong, but nal judgment
should be left for future studies.
Electric elds are also shown in Figures 1a, 2a, 2b,
and 3a to examine a possible relationship with windvelocity obtained by the MF radar. The MF radar
wind measurements depend upon scatter from electron density irregularities associated with turbulenceor sharply bounded layers Thrane et al., 1987]. However, in the lower thermosphere and above 95 km,
there is a tendency for electrons to decouple from theneutrals, a process countered by ambi-polar di usion
within various turbulence and irregularity structures
Kaiser, 1969 Reid, 1983]. In the latter paper, thedecoupling process was shown to be enhanced by Eelds associated with auroral activity. It appears,
however, that no such e ect can be identied between the E-eld and the MF radar wind data inthese events we have examined. Thus, it is probable that up to 100 km, the MF radar observes theneutral wind proper without a contamination of theEB drift velocity of electrons.
Finally Plate 1 shows seasonal variations of thehorizontal mean winds. The wind data obtained by
the MF radar in 1999 and the EISCAT radar in 19871996 are shown for the height range from 70 to 91 km
and from 95 to 119 km, respectively. Although thewind data were obtained at di erent times under different solar activity conditions, they show clear variations in season and altitude from the mesosphere to
the lower thermosphere. For the zonal mean wind,

Plate 1

12strong westward ow is seen in the summer mesosphere, while strong eastward ow is found in thelower thermosphere below 110 km in summer. On
the other hand, the meridional mean wind in summer
blows southward in the mesosphere and northward
in the lower thermosphere. It is interesting to notice that the amplitude of the meridional mean windin summer maximizes at 10 km higher than thatof the zonal mean wind both in the mesosphere andthe lower thermosphere in agreement with zonal andmeridional wind contour plots from a range of radars(35S - 70N) shown by Manson et al. 1991]. The
Coriolis torque on the summer southward ow (acting
westward) was therefore shown to maximize in the region of maximum eastward shear it was argued thatduring a time of relatively constant mean zonal wind
the gravity wave drag would be equal and opposite(eastward). Similar relationships are therefore appropriate here. Plate 1 implies that our understandingsof thermospheric dynamics at high latitude will besignicantly improved by combining wind data fromthe EISCAT and MF radars.

6. Summary and conclusions
Using wind data obtained by the EISCAT UHF
radar and MF radar co-located in Troms, Norway,
we have made a comparison study of winds obtained
both as case studies and based on statistical treatment. For more than 20 days from February to October in 1999, simultaneous wind data obtained by EISCAT and MF radars are compared in terms of temporal wind variations. Based on electron density proles obtained simultaneously by the EISCAT radar,the di erence between real and virtual heights causedby the e ect of group retardation for 2.8 MHz radio
wave is calculated. Solar EUV e ect on the group retardation is likely signicant above 90 km in summer,
but only at and above 105 km in winter. It is shownthat at a high latitude station like Troms the e ectof particle precipitation is important and a ects windobservations based on the MF radar. Sometimes, thee ect is seen at 84 km or lower.
We have also made comparisons of mean, diurnaland semidiurnal winds. From 1-month-averaged winddata obtained by the Troms MF radar, we have derived mean, diurnal and semidiurnal winds for eachmonth in 1999. These are compared with the statistical wind proles obtained by Nozawa and Brekke
1999a] that were based on 56 days of the EISCAT
wind data obtained from 1987 to 1996 under geomag-

netically quiet conditions (Ap < 16). From the statistical comparison, we showed a good possibility for
combining wind proles from the two radars. Finally,it should be noticed that care is necessary for MF
radar wind data above 91 km for summer obtained
at a high latitude station. In the height range 90-100
km for winter and equinoxes, the two systems appear complementary. There is some variability withheight for both systems in winter in the region, which
is probably associated with reduced ionization andthe relatively decreased signal to noise ratio for the
scatter from both radars.

Acknowledgments. S. N. thanks for Dr. K. Hockefor kindness to allow us use the Lomb-Scargle periodogram
method routines, and also for Dr. M. Rietveld for providing dynasonde data. We wish to thank the EISCAT sta.
EISCAT is jointly funded by the Particle Physics and Astronomy Research Council (U.K.), Centre National de laRecherche Scientique (France), Max-Planck Gesellschaft(F.R.G.), Suomen Akatemia (Finland), National Instituteof Polar Research (Japan), Norges Allmennvitenskapelige
Forskningsrad (Norway), and Naturvetenskapliga Forskningstradet (Sweden). This research was nancially supported by the Grant-in-Aid for Scientic Research A(12373002), B (11440144) and C (11640441), and a Centerof Excellence (COE) Leadership grant by the Ministry of
Education, Science, Sports and Culture, Japan. This research has also been supported by Norges Forskningstradthrough grant 129283/431. The Canadian authors recognize the granting council (NSERC) and the University of
Saskatchewan (through ISAS).

References

Brekke, A., J. R. Doupnik, and P. M. Banks, A preliminary study of the neutral wind in the aurora E region,
J. Geophys. Res., 78, 8235-8250, 1973.
Brekke, A. and C. Hall, Auroral ionospheric quiet summer
time conductances, Ann. Geophys., 6, 361-376, 1988.
Brekke, A., S. Nozawa, and T. Sparr, Studies of the E region neutral wind in the quiet auroral ionosphere, J.
Geophys. Res., 99, 8801-8825, 1994.
Briggs, B. H., The analysis of spaced sensor records by correlation techniques, Handb. MAP, 13, 166-186, 1984.
Burrage, M. D., et al., Validation of mesosphere and lower
thermosphere winds from the high resolution Doppler
imager on UARS, J. Geophys. Res., 101, 10,365-10,392,
1996.
Collis, P. N., EISCAT data base for ionospheric modelling:
F-region and topside ionosphere, Adv. Space Res., 16,(1)37-(1)46, 1995.
Comfort, R. H., S. T. Wu, and G. R. Swenson, An analysis
of auroral E region neutral winds based on incoherent

13scatter radar observations at Chatanika, Planet Space
Sci., 24, 541-560, 1976.
Folkestad, K., T. Hagfors, and S. Westerlund, EISCAT:
An updated description of technical characteristics andoperational capabilities, Radio Sci., 18, 867-879, 1983.
Friedrich, M. and K. M. Torkar, High latitude plasma densities and their relation to riometer absorption, Rep.
INW 8109, pp. 1-18, Techn. Univ. Graz, Graz, Austria, 1981.
Hagan, M. E., D. M. Burrage, J. M. Forbes, J. Hackney,
W. J. Randel, X. Zhang, GSWM-98: Results for migrating solar tides, J. Geophys. Res., 104, 6813-6828,
1999.
Hall, C. M., The Ramfjordmoen MF radar (69 N, 19 E):
Application development 1990-2000 J. Atmos. Sol.Terr. Phys., 63, 171-179, 2001.
Hall, C. M., Virtual to true height correction for high latitude MF radar, Ann. Geophysicae, 16, 277-279, 1998.
Hedin, A. E., MSIS-86 Thermospheric Model, J. Geophys.
Res., 92, 4649-4662, 1987.
Hocke, K., Phase estimation with the Lomb-Scargle periodogram method, Ann. Geophysicae, 16, 356-358,
1998.
Hines, C. O., G. W. Adams, J. W. Brosnahan, F. T.
Djuth, M. P. Sulzer, C. A. Tepley and J. S. Van Baelen,
Multi-instrument observations of mesospheric motions
over Arecibo: comparisons and interpretations, J. Atmos. Terr. Phys., 55, 241-287, 1993.
Hocking, W. K., Dynamical coupling processes between
the middle atmosphere and lower ionosphere, J. Atmos.
Terr. Phys., 58, 735-752, 1996.
Hocking, W. K., Strengths and limitations of MST radarmeasurements of middle-atmosphere winds, Ann. Geophysicae, 15, 1111-1122, 1997.
Huuskonen, A., T. S. Virdi, G. O. L. Jones, and P. J. S.
Williams, Observations of day-to-day variability in themeridional semi-diurnal tide at 70 N, Ann. Geophys.,
9, 407-415, 1991.
IAGA Division I Working Group 1, International geomagnetic reference eld revision 1987, J. Geomag. Geoelectr., 39, 773-779, 1987.
Kaiser, T. R., W. M. Pickering, and C. D. Watkins, Ambipolar diusion and motion of ion clouds in the earth's
magnetic eld, Planet. Space Sci., 17, 519, 1969.
Kofman, W., C. Lathuillere and B. Pibaret, Neutral dynamics of the high latitude E region from EISCAT measurements: a new approach, J. Atmos. Terr. Phys., 58,
121-138, 1996.
Manson, A. H., C. E. Meek, A. Brekke, and J. Moen,
Mesosphere and lower thermosphere (80-120 km) winds
and tides from near Troms(70 N, 19 E): comparisons
between radars (MF, EISCAT, VHF and rockets, J.
Atmos. Terr. Phys., 54, 927-950, 1992.
Manson, A., et al., Comparisons between Satellite-derived
Gradient Winds and Radar-derived Winds from the
CIRA-86, J. Atm. Sci., 48, 411-428, 1991.

Manson, A., et al., Seasonal variations of the semi-diurnaland diurnal tides in the MLT: multi-year MF radar observations from 2 to 70 N, and the GSWM tidal model,
Ann. Geophys., 61, 809-828, 1999.
Manson, A. H., F. Yi, G. Hall, and C. E. Meek, Comparisons between instantaneous wind measurements
made at Saskatoon (52 N, 107 W) using the colocatedmedium frequency radars and Fabry-Perot interferometer instruments: Climatologies (1988-1992) and casestudies, J. Geophys. Res., 101, 29,553-29,563, 1996.
McLandress, C., On the importance of gravity waves
in the middle atmosphere and their parameterizationin general circulation models, J. Atmos. Sor.-Terr.
Phys., 60, 1357-1381, 1998.
Meek, C. E., An ecient method for analyzing ionosphericdrifts data, J. Atmos. Terr. Phys., 42, 835-839, 1980.
Meek, C. E., and A. H. Manson, MF Radar spaced antenna experiment: wind variance versus record length,
J. Atmos. Sor.-Terr. Phys., 63, 181-191, 2001.
Meek, C. E., A. H. Manson, M. D. Burrage, G. Garbe, andL. L. Cogger, Comparisons between Canadian prairie
MF radars, FPI (green and OH lines) and UARS HRDI
systems, Ann. Geophys., 15, 1099-1110, 1997.
Namboothiri, S. P., A. H. Manson, and C. E. Meek, E
region real heights and their implications for MF radarderived wind and tidal climatologies, Radio Sci., 28,
187-202, 1993.
Namboothiri, S. P., A. H. Manson, and C. E. Meek, Extension of MF radar tidal measurements to E-region
heights (95-125 km): Saskatoon (52 N, 107 W), Canada,Ann. Geophys., 12, 333-341, 1994.
Nozawa, S. and A. Brekke, Studies of the E region neutral
wind in the disturbed auroral ionosphere, J. Geophys.
Res., 100, 14,717-14,734, 1995.
Nozawa, S. and A. Brekke, Studies of the auroral E region neutral wind through a solar cycle: Quiet days, J.
Geophys. Res., 104, 45-66, 1999a.
Nozawa, S. and A. Brekke, Seasonal variation of the auroral E region neutral wind for dierent solar activities,
J. Atmos. Sol.-Terr. Phys., 61, 585-605, 1999b.
Nozawa, S. and A. Brekke, A case study of the auroral E
region neutral wind on a quiet summer day: Comparison of three methods of the EISCAT UHF radar forderiving the E region wind, Radio Sci., 35, 845-863,
2000.
Phillips A., A. H. Manson, C. E. Meek, and E. J. Llewellyn,
A long term comparison of middle atmosphere winds
measured at Saskatoon (52 N, 107 W) by a mediumfrequency radar and a Fabry-Perot interferometer, J.
Geophys. Res., 99, 12,923-12,935, 1994.
Rawer, K., D. Bilitza, and S. Ramakrishnan, Goals andstatus of the International Reference Ionosphere, Rev.,
Geophys., 16, 177-181, 1978.
Reid, The inuence of electric elds on radar measurements of winds in the upper mesosphere, Radio Sci.,
18, 1028-1034, 1983.

14
Reid, I. M., On the measurement of gravity waves, tides
and mean winds in the low and middle latitude mesosphere and thermosphere with MF radar, Adv. SpaceRes., 18, (3)131-(3)140, 1996.
Rino, C. L., A. Brekke, and M. J. Baron, High-resolution
auroral zone E region neutral wind and current measurements by incoherent scatter radar, J. Geophys.
Res., 82, 2295-2304, 1977.
Rishbeth, H., and P. J. S. Williams, The EISCAT Ionospheric Radar: the System and its Early Results, Q. Jl
R. astr. Soc., 26, 478-512, 1985.
Schlegel, K., EISCAT and the EISCAT data base - A toolfor ionospheric modeling (E- and D-region), Adv. SpaceRes., 16, (1)147-(1)154, 1995.
Schunk, R. W. and J. C. G. Walker, Theoretical ion densities in the lower ionosphere, Planet. Space Sci., 21,
1875-1896, 1973.
Thrane, E. V., et al., Small scale structure and turbulencein the mesosphere and lower thermosphere at high latitudes in winter, J. Atmos. Terr. Phys., , 49, 751-762,
1987.
Turek, R. S., K. L. Miller, R. G. Roper and J. W. Brosnahan, Mesospheric wind studies during AIDA Act '89:
morphology and comparison of various techniques, J.
Atmos. Terr. Phys., 57, 1321-1343, 1995.
Turek, R. S., R. G. Roper and J. W. Brosnahan, Further
direct comparisons of incoherent scatter and medium
frequency radar winds from AIDA '89, J. Atmos. Sol.Terr. Phys., 60, 337-347, 1998.
Virdi, T. S., G. O. L. Jones, and P. J. S. Williams, EISCAT observations of the E-region semidiurnal tide, Nature, 324,, 354-356, 1986.
Williams, P. J. S., T. S. Virdi, G. O. L. Jones and A. Huuskonen, A comparison of three methods of measuring
tidal oscillations in the lower thermosphere using EISCAT common programmes, J. Atmos. Terr. Phys., 56,
1347-1359, 1994.

A. Brekke, C. M. Hall, Faculty of Science, University of Troms, N-9037, Troms, Norway. (e-mail:Asgeir.Brekke@phys.uit.no chris.hall@phys.uit.no)
K. Dobashi, Department of Astronomy and Earth
Sciences, Tokyo Gakugei University, Japan. (e-mail:dobashi@u-gakugei.ac.jp)
A. Manson, C. Meek, Institute of Space and Atmospheric Studies, University of Saskatchewan, Canada.
(e-mail: manson@skisas.usask.cameek@dansas.usask.ca)
S. Nozawa, R. Fujii, K. Morise Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho,Nagoya 464-8601, Japan. (e-mail: nozawa@stelab.nagoyau.ac.jp rfujii@stelab.nagoya-u.ac.jpmorise@stelab.nagoyau.ac.jp)
S. Oyama, Communications Research Laboratory,
Japan. (e-mail: oyama@endo.crl.go.jp)
Received November 21, 2000 revised October 11, 2001

1 Solar-Terrestrial Environment Laboratory, Nagoya University, Japan.
2 Faculty of Science, University of Troms , Norway.
3 Institute of Space and Atmospheric Studies, University of
Saskatchewan, Canada.
4 Communications Research Laboratory, Japan.
5 Department of Astronomy and Earth Sciences, Tokyo Gakugei
University, Japan.

This preprint was prepared with AGU's LATEX macros
v5.01. File p9_cr formatted October 11, 2001.

15
Figure 1a. Upper two panels electric eld of meridional (positively north) and zonal (positively east) components
are shown from 10 UT on February 9 to 16 UT on February 12, 1999 obtained by the EISCAT CP-1 mode with error
bars (1). Next four panels wind data obtained by EISCAT (pluses) and MF (lled circles) radars are illustrated asa function of time for four heights between 90 and 99 km. The vertical bars represent averaged errors for EISCAT
velocity values. At the bottom, 3-h Kp index is also shown over the experiment period.
Figure 1b. The di erence in km between virtual and real heights for 2.8 MHz radio wave are shown as a functionof UT for 8 heights from 84 to 99 km from 10 UT on February 9 and 16 UT on February 12, 1999. The di erenceis calculated by using altitude proles of electron density obtained by the EISCAT radar (see text). Solid circles
with a value of -1 mean that total reection occurs below the height, and solid circle with a value of 9 means thatthe di erence is greater than 9 km. Vertical bar associated with each value represents possible ambiguity whenelectron density is higher and lower by 10% . At the bottom, height integrated Hall conductivities (conductances)are shown over the experiment period.
Figure 2a. Upper two panels, northward and eastward components of electric eld obtained by EISCAT CP-2
mode from 15 UT on July 1 to 16 UT on July 9 are shown. Next four panels, meridional wind velocity (positively
northwards) obtained by EISCAT (pluses) and MF (dots) radars are shown for 4 heights from 89 km to 98 km.
Bottom panel: 3-h Kp index is also shown.

Figure 2b. Same as Figure 2a except for zonal wind data (positively eastwards).
Figure 2c. Same as Figure 1b except for the period from 15 UT on July 1 to 16 UT on July 9, 1999.
Figure 3a. Same as Figure 1a except for the period from 15 UT on September 15 to 16 UT on September 17,
1999.

Figure 3b. Same as Figure 1b except for the period from 15 UT on September 15 to 16 UT on September 17,
1999.

Figure 4a. Mean winds derived by the MF radar and EISCAT radar observations (solid circle) are compared.
The values of the MF radar are based on 1-month averaged wind data obtained in 1999, while the EISCAT winddata are based on 56 days obtained from 1987 to 1996. Since EISCAT data are sorted to 3 seasons, wind databy MF radar obtained in June and July, March, April, September and October, and January and December arecompared with EISCAT wind data for summer, equinox and winter, respectively. Following work of Nozawa andBrekke 1995] to estimate ambiguities of derived values, a standard deviation (1) associated with each data value
is also shown.
Figure 4b. Same as Figure 4a except for diurnal amplitudes and phases.
Figure 4c. Same as Figure 4a except for semidiurnal amplitudes and phases.

Plate 1. Seasonal variation of meridional and zonal mean winds are shown at upper and lower panels respectively
from 70 km to 119 km. From 70 km to 91 km, MF radar wind data are used and at and above 95 km EISCAT
radar wind data are shown. The MF radar wind data were obtained over 1999 and averaged for 1-month, while
the EISCAT wind data were obtained from 1987 to 1996 and were divided onto 3 seasons.

16

Table 1. List of EISCAT datasets
DatasetStartdate, time (hr)Enddate, time (hr)Observation
period (hr)ModeAveraged
Tsys 1(K)Averaged Tx 2
power (kW)990209990308990701990915991012

Feb 9, 10
Mar 8, 10
July 1, 15
Sep 15, 15
Oct 12, 10

Feb 12, 16
Mar 12, 12
July 9, 16
Sep 17, 16
Oct 15, 16

78
98
193
49
78

CP-1
CP-2
CP-23
CP-1
CP-1

96
102
156
108
99

1122
1216
1086
1092
1143

1 Tsys stands for the system noise temperature.
2 Tx stands for transmitter.
3 Conducted as a Special Program by the seven EISCAT associated countries for almost half of time.

17

Table 2. Geomagnetic activity.
Date
990209990210990211
990212990308990309990310990311
990312990701990702990703990704990705990706990707990708990709990915990916990917991012991013991014
991015

0-3

3-6

6-9

3-h Kp index in UT
9-12
12-15
15-18

18-21

21-24

index

index

21
44345
3+
1+
1
43
0
0
31+
2
2+
43+
3
5
53+
5

1+
1
3444
6+
3+
2+
13+
2+
0
0
1+
1+
1
1
46
2+
5
3
4+
5

12
3
4+
24
53+
2+
1+
5
40+
0
211
15
5
2
44
44+

0
0+
3
4
23+
5
3
3
1+
4
20+
0+
10+
2164+
2
5
344

12+
4
4
2+
3+
32
3+
34
0+
0+
1+
32
213
3
45+
4
4+
3+

12+
3
2+
4+
22+
2334+
1+
122+
22
1232+
4+
3+
4+
3-

3
6
20
24
12
21
34
15
11
6
26
9
2
3
8
4
6
4
27
31
16
46
23
31
31

125.9
148.4
159.3
183.6
125.0
125.3
133.6
135.3
138.5
202.0
193.4
203.5
191.9
180.0
173.5
163.7
154.1
155.9
156.6
159.9
159.1
182.9
190.1
198.7
197.1

0+
14
3+
1+
4+
2+
431+
2+
11110+
2+
143+
4
5+
454-

124
4+
2
3+
33
2+
1
4
2
0+
1
3
1
20+
344+
5+
4
5
5-

Ap

F10:7