vlf observations by the akebono(exos-d)satellite

J.Geomag. Geoelectr.,42,459-478,1990

VLF Observations by the Akebono(EXOS-D)Satellite

Iwane KIMURA1,9, Kozo HASHIMOTO2, Isamu NAGANO3, Toshimi OKADA4,Masayuki YAMAMOTO1, Takeo YosHINO5, Hiroshi MATSUMOTO6, Masaki EJIRl7,

and Kanji HAYASHI8

1Department of Electrical Engineering II, Kyoto University, Kyoto 606, Japan2Department of Electrical Engineering, Tokyo Denki University, Tokyo 101, Japan3Department of Electrical Engineering, Kanazawa University, Kanazawa 920, Japan4Research Institute of Atmospherics, Nagoya University, Toyokawa 442, Japan

5Department of Communication, Electro-Communication university, Tokyo 182, Japan6Raclio Atmospheric Science Center, Kyoto University, Uji 611, Japan

7National Institute of Polar Research, Tokyo 173, Japan

8Geophysics Research Laboratory, University of Tokyo, Tokyo 113, Japan9Institute of Space and Aastronautical Science, Sagamihara 229, Japan

(Received August 1,1989;Revised December 15,1989)

The VLF wave instruments on board Akebono(EXOS-D)involve a wide bandanalyzer(WBA), multi-channel analyzers(MCA), Poynting flux analyzers(PFX), ELFfrequency analyzers and a vector impedance probe(VIP)and cover a frequency rangefrom a few Hz to 17.8kHz for electric and magnetic field components. The mainobjectives of the wave observations are to investigate the wave phenomena closelyassociated with energetic particle precipitation in the auroral zone and the physics ofwave-wave and wave-particle interaction phenomena. In order to clarify these wavecharacteristics, the determination of the wave normal direction and the direction of thePoynting vectors are essential. Our VLF instruments, especially PFX and ELF, aredesigned to satisfy such requirements. The present paper introduces all the subsystems ofthe VLF instruments and some preliminary results of observations to show the charac-teristic features of each subsystem.

1. Introduction

The EXOS-D satellite was launched at 23:30 UT on February 21,1989 in order toinvestigate energy flow from the magnetospheric tail to the auroral region, and is now

named Akebono, which means dawn in Japanese. The VLF instruments on board

Akebono are designed to investigate the behavior of plasma waves associated with

accelerated auroral particles, wave particle interaction mechanisms and propagation

characteristics of whistler mode, ion cyclotron mode, and electrostatic mode waves in the

magnetospheric plasma.

Specific wave phenomena of interest in the auroral region are: i)AKR, ii)auroralhiss, iii)saucers, and iv)broad-band electrostatic noise. Item i)is a free space mode

phenomenon, ii)and iii)are whistler mode phenomena and iv)is an electrostatic mode

phenomenon. These phenomena are qualitatively understood. However, the sourceregion for each phenomenon and the generation mechanisms have not yet been clearly

understood.

Auroral hiss is generated in a loes-altitude auroral region and electrons in the keV

energy range appear to be responsible for the emissions. AKR and auroral hiss appear to

459

460 I.KIMURA et al.

be generated on the same field line(GURNETT et al.,1983;CALVERT and HASHIMOTO,

1989).

Saucer observations by the ISIS satellites suggest that the phenomenon is generated

in the 500 to 3500km altitude region. It is, however, unclear why the saucer is generated

in such a region. Absorption or intensified bands at the harmonics of the ion cyclotron

frequency are also observed in saucer spectra(HORITA and JAMES,1982). This

phenomenon is also not yet completely understood.

The broad band electrostatic noise observed by the Hawkeye 1 and Imp 6 satellites

suggests that this noise occurs on the same magnetic field lines as the inverted V electron

events(GURNETT and FRANK,1977). Some propagation characteristics of the noise

observed by the S3-3 satellite were investigated by TEMERIN(1978).

Another electrostatic mode, electrostatic hydrogen cyclotron waves found by S3-3

near 1 RE(MOZER et al.,1977), has been shown to be consistent with the electrostatic ion

cyclotron dispersion relation(KINTNER et al.,1978).

As for the so-called wave-particle interaction phenomenon, represented by ASE

(artificially stimulated emissions), it has been found by the Jikiken observations of Siple

station transmissions that ASE's were generated by intense fluxes of resonant electrons at

small pitch angles(KIMURA et al.,1983). However, the wave intensities associated with

the ASE phenomenon observed by several satellites seem to be much lower than those

theoretically required for the wave particle interaction(INAN et al.,1977). ISIS satellite

observations of Aldra Omega signals and emissions triggered by these signals revealed

that the wave normal angle at the magnetic equatorial plane was 10-30°, but was

sometimes much larger(MATSUO et al.,1985).

Information on the wave normal directions of the triggering wave for triggered

emission events is very important for the study of the mechanism of wave-particle

interactions.

For a study of the propagation characteristics of whistler mode signals transmitted

from the ground-based VLF transmitters, such as Omega signals, the delay time and the

direction of the wave normal vectors of the signals are measured on board Akebono.

Electron density at the satellite orbit, which is needed for theoretical confirmation of the

characteristics, e.g. by the ray tracing technique(KIMURA et al.,1985;HASHIMOTO et al.,

1987),is determined by PWS(high frequency plasma wave analyzer).

In the present paper, first the subsystems of the VLF instruments are briefly

described and then some preliminary results of observations are introduced.

2. Description of the Subsystems of the VLF Instruments

In order to satisfy the above mentioned requirements, instruments are needed to

measure the wave normal and Poynting flux directions of these wave phenomena in

addition to their dynamic spectra. Vector electric as well as magnetic field components in

the frequency range from a few Hz to MHz must also be measured. The frequency range

above tens of kHz is covered by the PWS, the other wave instrument of Akebono for

high frequency plasma wave phenomena.

The VLF is responsible for the frequency range below 17.8kHz down to 3.16Hz,

and is composed of loop and dipole antennas, common preamplifiers directly connected

with the sensors, and the following subsystems: WBA(wide band analyzer), MCA(multi-

channel analyzers), PFX(Poynting flux analyzers), ELF(ELF range analyzers)and VIP

VLF Observations by the Akebono (EXOS-D) Satellite 461

Fig. 1. Block diagram of VLF instrument.

Fig. 2. Configuration of wave sensors and coordinate systems.

462 I.KIMURA et al.

(vector impedance probe). The block diagram of the VLF instruments is shown in Fig.1.

In this figure, the so-called DPU common is comprised of buffer amplifiers between the

sensor sections and the above-mentioned subsystems.

2.1 Sensors and their coordinate systems

To detect the electric component of the plasma wave fields, a pair of crossed dipole

wire antennas,60m tip-to-tip shielded for 3m nearest to the spacecraft, are used. For the

magnetic field sensors, three orthogonal open loop antennas installed on the tip of a 1.5

m mast are used for frequencies higher than 800Hz. For frequencies lower than this

frequency limit, three orthogonal search coils are used, which are mounted on the tip of

the search-coil mast of 3m length.

The coordinate systems for the various antennas and search coils are shown in Fig.

2. First of all the spin axis of the satellite, which is always directed to the sun with an

accuracy within 2.5°, is taken to be the Z axis. The mast of the loop antennas is directed

in the positive Y direction and the two masts of 3m and 5m for magnetic flux gate

magnetometers and search coils are directed in the -X and -Y directions respectively as

shown in Fig.2. The three orthogonal flux gate magnetometers and search coils measure

the X, Y, and Z components of the earth's magnetic field and their induction in the above

satellite coordinate system, and the geomagnetic aspect sensors(GA)also refer to this

coordinate system. The outputs of the three search coils are named sBx, sBY, and sBz.

The plane normal directions for the three loop antennas are orthogonal to each

other, but are directed in different directions from the X, Y, and Z axes so that the output

of the loop antennas are called B1, B2, and B3. The relation between the magnetic

components(B1, B2, B3)and those components in the satellite coordinates,(BX, BY, BZ)is

given by

The directions of the crossed dipole wire antennas are directed about 45° off from

the X and Y directions and the two components of the electric fields are called Ex and Ey

where the positive x and positive y axes are in the first and second quadrants in the X-Y

plane with the x axis 35° off from the X axis.

The equivalent noise level of the preamplifier for electric field measurement is about

4.5×10-8V/√Hz in the major part of the VLF band when the input terminal of the

preamplifier is grounded to the chassis of the satellite.

As to the sensitivity of the magnetic sensors, the open loop antennas are rectangular

(60cm×60cm)coils with 10 turns each, and the frequency dependence of the effective

length of each loop including stepup by a transformer and a preamplifier is shown in Fig.

3.These loop antennas were designed to be used for VLF as well as PWS(for the higher

frequency)by using two independent transformers whose primary coils are connected in


Fig.3. Frequency dependence of the effective length of the loop antennas and search coils.

series with the loop antenna circuit and whose secondary coils are independently

connected to the two preamplifiers for VLF and for PWS(OKADA et al.,1987). The

magnetic field detection threshold level at 5kHz is 0.005pT/√Hz.

The search coils are composed of 105 turns of 50μm polyurethane wire on a super-

permalloy rod of 3×3×300(mm); the sensitivity of the coils is 0.2pT/√Hz at 100Hz.

The frequency dependence of the effective length is also shown in Fig. 3.

2.2 WBA(wide band receiver for observation of VLF spectra)

Wide band analog VLF signals received in the frequency range from 50Hz to 14

kHz or 50Hz to 7kHz(selectable by command)are transmitted directly to the ground by

analog telemetry. The field components to be measured(Ex, Ey, sBY or B2)are selectable

by command. The gain of the WBA receiver is changed in 25 dB steps from 0 dB up to 75

dB by checking the averaged signal level every 0.5s. This procedure is automatically

executed by the WIDA hybrid IC, specially designed for the VLF instruments of this

satellite, by which more than 80 dB of dynamic range is secured. Signal intensity can also

be determined within the above dynamic range as long as the amplifier does not saturate.

2.3 MCA(multi-channel analyzers)

Sixteen MCA channels for Ex or Ey(selectable by command)and 16 channels for

sBY and B2(chs.1-10 for sBY and chs.11-16 for B2)are included in the VLF electronics.

The center frequency for each channel is 3.16,5.62,10.0 and 17.8[Hz]×1, ×10, ×100 and

×1,000 with a band width of 30% of each center frequency. Relative filter response curves

for 16 channels are shown in Fig.4, which is very similar to the ISEE multi-channel

analyzer(SCARF et al.,1978). The maximum allowable input voltage is 1 Vrms and a

dynamic range of more than 80 dB is obtained by the WIDA IC. The detector output is

converted to an 8 bit word and PCM-transmitted with sampling rates of 2 per s for the

high(H)bit rate,4 per s for the medium(M)bit rate, and 1 per s for the low(L)bit rate

transmission.

464 I. KIMURA et al.

Fig.4. Frequency response characteristics of MCA channels.

2.4 PFX(measurement of wave normal direction and Poyntingflux)Five components of electric and magnetic fields, Ex, Ey,(B1 or sBX),(B2 or sBY), and

(B3 or sBZ)are 12-bit-A/D-converted and log-compressed to 8 bits at a sampling rate of320Hz and sent to the ground by the PCM telemetry. The wave normal direction and thePoynting flux of received signals are calculated on the ground. The PFX is composed offive channels of triple-super-heterodyne receivers with an output band-width of 50Hz, asshown by a block diagram in Fig.5. The local oscillator frequency is either automaticallystepped by the minimum step of 50Hz or kept fixed at a constant frequency in thefrequency range from 100Hz to 12.75kHz. The selection of stepped or fixed mode ismade by command while the selection of B or sB is made automatically according to theselected center frequency. The dynamic range of the PFX is also more than 80dB, byusing the WIDA IC, which is illustrated by WIDA's in the block diagram shown inFig.5.

2.5 ELF(ELF range receiver)The ELF range receiver can be operated in either the 4 channel or 2 channel mode,

selectable by command. In the 4 channel mode, waves in a frequency range less than 50Hz are observed and are sampled by 160 Hz to 8 bit words. The four channels arecomposed of ch.1; Ex or Ey(selectable by command), ch. 2; sBX, ch.3; sBY, and ch.4;sBZ). In the 2 channel mode, the upper limit frequency is increased up to 100Hz with asampling frequency of 320Hz with 8 bit resolution, but only two components selected bycommand can be observed. The dynamic range for either case is 80dB. The wave normal

direction and the Poynting flux is then calculated by the ground data processing.

2.6 VIP(measurements of vector impedance of the wire antennas)In order to determine the electric field intensity of a signal as accurately as possible,

the antenna impedance must be known. Figure 6 shows the principle of the measurement.Aconstant current source is applied to the input terminals of the balanced preamplifier


Fig.6. Block diagram and equivalent circuit of VIP.

(Fig.6(a))with the center of the preamplifier being grounded to the chassis of thesatellite, and the relative output amplitude and phase with respect to the applied signalare measured with the VIP electronics to determine the vector impedance of the antenna.Figure 6(b)illustrates one side of the antenna and preamplifier system relative to thechassis of the satellite. Then RS and Cs+Cin, where RS and CS are the sheath resistance andcapacitance of the antenna respectively, and Cin is the input capacitance of the pre-amplifier relative to the ground, can be measured. Cin measured before launch was about100pF. The applied source intensity is controlled by feed-back so as to keep the outputvoltage constant. This makes it possible to measure the impedance in a wide range.Figure 6(c)is an equivalent circuit for signal reception. E is the electric field intensity ofthe signal and heir is the effective length of the antenna. The input signal to the pre-amplifier is divided by the sheath impedance and the input capacitance as shown in Fig.6(c).One of the antenna pairs, that is Ex or Ey, and the frequency for the impedancemeasurement is selected by command.

2.7 DPU(data processing unit)The main function of DPU is to provide an interface between the spacecraft data

handling unit(DHU)and the VLF instruments. The DPU also provides the controlsignals for the subsystems of the VLF instrument.

VLF Observations by the Akebono(EXOS-D)Satellite 467

3. Results of Initial Observations

Our VLF observations started when the wire antennas, loop and search coils were

deployed in the first week of March, about 10 days after launch, while most otherscientific instruments using high voltages were turned on at the end of March. WBA data

via U band telemetry have been taken at Kagoshima Space Center(KSC)and Prince

Albert(PA)in Canada. Other VLF data via PCM telemetry have been taken at KSC,

PA, Syowa in Antarctica, and Esrange in Sweden.

In the following, some of the analyzed results of each VLF subsystem are introduced

to show the characteristics of all subsystems described in the previous section.

3.1 Wide band spectra of various VLF phenomena including emissions triggered byOmega signals

Wide band VLF spectra taken by WBA are found to be of high quality with a

sufficient S/N ratio realized by the WIDA IC.

A couple ofexamples of spectra taken by WBA are shown in Panels 1 and 2. Panel 1

illustrates an example of f-t spectra including whistlers, LHR noise, ground-based VLF

signals at 10.2,11.05 and 11.9kHz, observed by one pair of the 60m tip-to-tip wire

antennas. The location of the satellite was at an altitude of 2800km, a geomagnetic

latitude and longitude of 40.0°N and 163.5°W respectively, and an L value of 2.7.

Panel 1. An example of wide band VLF spectrum of whistlers, LHR noise, and ground based VLFtransmitter signals.

468 I.KIMURA et al.

Signal strength of wave phenomena at any particular frequency and time can be read

on the f-t display on the CRT after FFT processing. In the case shown in panel 1, the

maximum signal intensity at the input terminal of the preamplifier was 7.85mV for the

most intensified portion of LHR noise with a whistler at 6.1kHz and at 10s after the

start of the spectrogram. lf we assume that the effective length of the antenna is 27 m and

the pickup factor is 0.75, the electric field is 0.39mV/m. As seen on the panel, the signal

intensity is strongly modulated by the satellite spin motion with a period of 8s, because

of the electrostatic characteristics of the LHR noise. Fortunately, the spin axis is directed

to the sun to make the solar cells always face the sun, so that there is no shadowing effect

to cause additional electrostatic noise at multiple harmonics of the spin frequency.

Panel 2 illustrates one example of spectrograms of observed Omega signals

transmitted from Australia(geomagnetic latitude of 47°S), which shows a sequence of

Australian Omega format; 11.05,13.0,10.2,13.6,11.333,13.0kHz. This spectrogram

was obtained by the loop antenna and VLF-WBA on April 5,1989. As seen on the panel,

the Omega pulses at frequencies of 10.2,11.05 and 11.333 are accompanied by strong

triggered emissions(so-called ASE). On this pass, the Omega signals were observed in the

period from 07:30 UT to the end of telemetry reception, i.e. 07:40 UT, while from 07:35

UT until 07:40 UT, strong emissions (ASE's)associated with Omega signals were

observed.

The delay of the leading edge of the received Omega pulse at 10.2kHz, from that of

the signal transmitted can be measured on the spectra with an accuracy of 20ms. A delay

time of the telemetry signal to the tracking station, which is about 20ms for a range of

6,000km, must be compensated by subtraction to obtain the virtual delay time of Omega

signals. The delay times(not corrected)for the above-mentioned period are in the range

from 0.5 to 0.7s, as shown in Fig.7. It was found by ray tracing that these delay times are

reasonable values for a direct single path to the satellite from the Omega transmitter in

Australia.

Fig. 7. The location of Akebono when Australian Omega signals and their triggered emissions were observed.


Panel 2. VLF wide band spectrum of ASE's triggered by Australian Omega signals.

Panel 3. One example off-t spectrogram obtained by MCA, showing a funnel type auroral hiss emission in the auroral region (PCM data acquisition made at Prince Albert).

470 I.KIMURA et al.

The location of Akebono corresponding to the above time interval is illustrated in

Fig.8. After O7:35 UT for every pulse at 10.2kHz, the Omega signals are followed by

strong triggered emissions, with a delay of about 450ms from the leading edge of the

pulse. It is very interesting to note that the signal intensity at the transmitter frequency of10.2kHz suddenly increases after the delay of 450ms from the leading edge of each pulse.

Fig.8. Spatial variation of the delay time and the signal intensity of Australian Omega signals at 10.2kHzobserved on April 5, 1989.

In order to compare the signal intensity before the time when the emissions aretriggered with that after the triggered emissions, we have measured the peak signalintensity over 200ms just after the leading edge of the pulse(denoted by Ai)and the peakintensity over the interval on the pulse after the delay of 450ms(denoted by Am)while theAustralian Omega signals were detected, i.e. from 07:30 to 07:40 UT, including thenontriggering period(from 07:30 to 07:35 UT).

Figure 7 shows a time variation of Ai and Am, where the ordinate indicates therelative signal intensity, but the 0 dB on this figure corresponds to 1.1 pT in the magneticfield intensity. In the figure, Ai shows a tendency to slowly increase with time orincreasing latitude. This tendency may not be due to amplification, but may be mainlydue to a spatial variation of signal strength by a difference in propagation path from thesource to the satellite location. After 07:35 UT, Am, the signal strength after thetriggering, remarkably increases compared with Ai, implying that an amplification effectis functioning due to wave-particle interaction. The difference between Aiand Am is a


measure of total amplification, which is about 24 dB at 07:40 UT. Actually, the telemetry

reception was terminated at the maximum triggering activity, so that the total amplifica-

tion could be larger than 24 dB, which is consistent with the results of ground

observations for the Siple transmitter(HELLIWELL and KATSUFRAKIS,1974).

on board Akebono we have already detected five other triggering events for the

satellite location in the northern hemisphere, in the interval of 09 to 10 UT on March 10,

11,21,and 27 for Australian Omega signals, and around 12:45 UT on March 27 for a

ground VLF signal at a frequency of 11.9kHz, which is not a frequency of any of the

Omega transmitters. These six events are concentrated within a month after March 10, so

that these triggering activities might be attributed to a period of high solar activity in

March and also to the local time of the satellite trajectory around evening or night, which

was a favorable condition for the signal to penetrate through the bottom of the

ionosphere at the source region.

3.2 MCA spectrogram

Each of the data from the 16 channels of MCA for E and B field components yields a

peak value every 1s and an average value every 0.5s. The MCA data are able to provide

us with general characteristics of the VLF wave activity, although the frequency

resolution is not good. In Panel 3, colored MCA spectrograms for E and B components

are shown for the data taken on March 11,1989 at Prince Albert, Canada, one of the

tracking stations for Akebono. The spectrogram is produced by drawing a contour map

based on the maximum of the averaged values for 4-s duration in time at 16frequencies.

Strong emissions, which are red-colored on the panel, covering all the observed

frequency range for the E and only down to 2kHz for the B component around 06:39 UT

correspond to a typical example of funnel-type auroral hiss emissions(GURNETT et al.,

1983).As shown in the panel, the emission was observed at an invariant latitude of 69°,

an altitude of 2,700km, and a magnetic local time of 20h.

3.3 Determination of wave normal directions

The PFX instrument can measure the five components consisting of three magnetic

fields and two electric fields in order to obtain the wave normal direction and Poynting

vector finally. It is not necessary to measure all six components to calculate the wave

characteristics for the electromagnetic waves because any one component can be

obtained from the measurement of the other five components, if the local electron density

is known(SHAWHAN,1970).

In the following, we will show one example of the determination of the wave normal

direction for whistlers, by analyzing the PCM data of the three entire magnetic

components. The wave normal direction can be calculated by a simple method based on

the facts that the wave normal direction of the electromagnetic waves is always

perpendicular to the polarization plane of their magnetic field, and that the observed

wave packet is composed of a plane wave with a single wave normal direction.

Figure 9(a)is an f-t diagram observed by WBA using one of the loop antennas. A

couple of whistlers shown were observed in the northern hemisphere at 10:53:18 UT on

March 24, 1989, at an altitude of 4396km and at a geomagnetic latitude of 21.8°,the first

one with a dispersion of 40s1/2 and the second about 60s1/2. Figure 9(b)indicates wave

forms of the five components(2 components of E fields and 3 components of B fields in

their coordinate systems of the antennas)of the whistlers at 6kHz with a bandwidth of 50


Panel 4. ELF emissions observed in the magnetic equatorial region. The red lines represent the helium

cyclotron frequency at the satellite orbit.


Fig.9.(a)f-t diagram of whistlers to be analyzed.(b)Wave form of the whistlers.(c)Direction of the wave

normal of a whistler.

Hz. The full length of the horizontal line corresponds to 0.5s. The wave form

corresponding to a whistler lasts about 30 ms. We can recognize two whistlers within the

above 0.5s. Figure 9(c)shows the wave normal direction with reference to the direction

of the earth's magnetic field which was simultaneously measured by the MGF instrument

on board the satellite. The calculated wave normal angle is nearly constant with an angle

of 60° during the period of strong signal strength, which is about 30ms. For the second

whistler in Fig.9(b)the wave normal angle was about 75～90°. The azimuth direction of

the wave normal vector can also be calculated if the satellite attitude is definitely

determined. Since the present paper is intended to show the preliminary results, only the

wave normal angle with reference to the earth's magnetic field is shown.

The source of whistlers with a dispersion larger than 20s1/2 is considered to be

located in the southern hemisphere. The large wave normal angle suggests that the wave

474 I.KIMURA et al.

has propagated to the satellite from the southern hemisphere in the non-ducted mode.

The above second whistler with a slightly larger dispersion than the first whistler is

interpreted in such a way that after passing through the satellite the first whistler

propagated down and was observed again after reflection either at the bottom of the

ionosphere or LHR reflection somewhere below the satellite.

We can get other characteristics of the wave from the five field components. In the

above whistler events, the polarization was of course right-handed circularly polarized

and the intensity of the magnetic component was 0.6pT for the band width of 50Hz. On

the other hand the electric field intensity was 10.2mV/m for the same band width, under

the assumptions that the effective length of the wire antenna is 27m, half of the tip-to-tip

length, and that the pickup factor calculated by using the sheath capacitance of the

antenna(see Subsection 3.5)is 0.75. The value of c|B|/|E| calculated by the above

quantities is l5.6, which should be the refractive index, ifthe |E| is the magnitude of

electric field perpendicular to both the wave magnetic field and the wave normal

direction. On the other hand, according to the on board observation by PWS and MGF,

the plasma and the cyclotron frequencies are 566kHz and 200kHz respectively. The

refractive index based on these parameters becomes 23 for the above mentioned wave

normal angle of 60°.The refractive index deduced from the field ratio and that obtained

from calculation is not exactly the same, but the difference is within a reasonable range if

we take account ofthe facts that we do not observe the Z component ofthe electric field

and the assumption of the effective length of the antenna is not always definitely

acceptable(SONWALKAR and INAN,1986).

3.4 ELF emissions observed near the magnetic equatorial plane

In the ELF range, which is below a frequency of 100Hz, there are many interesting

wave phenomena associated with ion constituents. One such phenomenon is the ELF

emissions that are observed only in the near vicinity of the magnetic equatorial plane.

Panel 4 shows one example observed on May 27,1989, when Akebono was approaching

the geomagnetic equator from the southern hemisphere. On the panel, four f-t spectro-

grams are shown for Ex and sBX, sBY, and sBZ components respectively. Strong

horizontal lines at frequencies of 32 and 64Hz are due to interference caused by other

instruments on board the satellite.

On each panel, the cyclotron frequency of helium at the satellite location is plotted

with a red line for reference. It is clearly seen that the frequency of emissions is always

above the helium cyclotron frequency(fHe)and is much below the proton cyclotron

frequency (fH). Moreover, there is a sharp frequency gap between the bottom of the

emissions and fHe. From this point, with the fact that this phenomenon has a magnetic

field component as well as an electric field component, it is conjectured that this emission

may be identified as one of the electromagnetic ion cyclotron modes existing above the

lower hybrid frequency by helium ions as indicated by the class 3 mode(according to the

classification by RAUCH and ROUX,1982)in the ω-k diagram as shown in Fig.10. This

mode is known to be confined to a narrow region around the magnetic equatorial plane

by LHR reflection(RAUCH and ROUX,1982), which was observed by GEOS 1 and 2

(YOUNG et al., 1981; ROUX et al., 1982).

In July,1989 several spectra were newly found in which two frequency bands coexist

between the local proton and helium ion cyclotron frequencies and between the local

helium and oxygen ion cyclotron frequencies. This phenomenon can be interpreted in a


Fig.10. ω-k diagram of the electromagnetic ion cyclotron modes. The abscissa and the ordinate are

normalized by the proton cyclotron angular frequency(ΩH).

similar way as those having one frequency band above the helium cyclotron frequency, as

mentioned previously. If we can assume that there is a sufficient content of oxygen ions,

say a few percent, in addition to proton and helium ions, one more LHR resonance

frequency appears above the oxygen cyclotron frequency and below the helium cyclotron

frequency in the ω-k diagram in the presence of the three ion constituents.

Our calculation has shown that the presence of high temperature ions can amplify

these modes, so that the appearance of such ELF emissions in the magnetic equatorial

region may be an indication of the existence of high temperature ions there. Unfortunately,

only a limited number of measurements of energetic ion spectra could have been made in

the low latitude region, because the radiation belt particles cause high erroneous counts

to the detectors. Much more statistical study of this phenomenon and the measurements

of energetic ions are needed.

3.5 Vector impedance of the wire antennas in the VLF range

So far several VIP measurements have been analyzed. Typically CS=250 pF and

RS=500kΩ at altitudes higher than four thousand km. The antenna becomes resistive

below the frequency f0=1/(2πCSRS)=1300Hz, and capacitive at frequencies above f0.

Near this frequency, effects of both RS and CS must be taken into consideration. Observed

impedances did not vary so much. The capacity of the co-axial capacitor formed by the

antenna and the boundary of the plasma sheath is(AGGSON and KAPETANAKOS,1966)

(2)

where λ is the sheath thickness, a is the antenna radius(0.19mm), and d is the antenna

length(30m). C is about 270pF if the sheath thickness is assumed to be 10cm(typical

476 I.KIMURA et al.

Debye length in these regions). Therefore the above results are reasonable.

4. Discussion and Conclusions

We have so far confirmed that all subsystems of the VLF instruments on boardAkebono are working satisfactorily. The three components of the magnetic field and thetwo components of the electric field of electromagnetic plasma waves in the magnetospherecan be measured by the PFX subsystem using the crossed loop antennas or search coils atafixed frequency or by staircase sweeping the frequency. WBA uses one of the wiredipole antennas, or one of the crossed loop or search coil antennas.

In any subsystem the magnetic field intensity can be measured very accurately,because there is no ambiguity in the pickup factor of the sensors. For the determinationof electric field intensity, the E field channels provide us with signals with sufficientlyhigh S/N ratio, and we can also measure the sheath impedance of wire antennas asmentioned in Subsection 3.5. We therefore determine the electric field intensity indepen-dently from the magnetic field component. However, we know that there is an ambiguityin determining the electric field, such as an ambiguity of the effective length of the wireantennas(SONWALKAR and INAN,1986). We have, therefore, to collect enough paireddata of E and B fields, before we can use the electric field intensity with much confidence.

Aphase difference between any two channels for the measurement of E and B fieldsis also very important data necessary to be compensated in the calculation of the wavenormal direction. It was found that the phase difference between any two B channels isnegligible within a few degrees. The phase differences between E and B channels are notalways negligible but we are able to calibrate them by on board calibration.

It will take time for us to reach the situation where we can calculate the direction ofthe wave normal and the Poynting vector by using five components of the E and B fields.In the present paper, we have, therefore, introduced only one result of the wave normaldirection for whistlers based on the simple method, as mentioned in Subsection 3.3.

For the study of wave particle interaction phenomena, we have detected severalOmega triggering events, some of which are analyzed in the present paper. We still havemuch work to do to know the characteristics of the triggering effect. Moreover it is, ofcourse, necessary for us to observe energetic particles at the same time. However, so farwe have had not much chance for simultaneous observations in the middle and low

geomagnetic latitudes,. because in these regions high energetic particles in the radiationbelt cause tremendous error in measuring low energetic particle fluxes and deteriorate thechannel electron multiplier quickly. But such joint observations will be made in the nearfuture for our study.

In the ELF range, we have found very interesting ELF emissions which are observedin a limited range around the geomagnetic equatorial plane with frequencies above thehelium ion cyclotron frequency and sometimes above the oxygen cyclotron frequency aswell. These phenomena appear to be of the electromagnetic ion cyclotron modes existingabove the lower hybrid frequencies by helium ions and oxygen ions and trapped in thelower latitude zone by LHR reflection. So far there have not been many observations of

energetic electrons and ions in the geomagnetic equatorial region, unfortunately. Wecould not have confirmed the mechanism of generation mentioned in the present paper.The results of the detailed analyses of these phenomena will be published elsewhere in thenear future.


We would like to thank Prof. K. Tsuruda, the Project manager, Profs. H. Oya and A. Nishida,Project Scientists, and the ISAS engineering team headed by Drs. M. Natori, I. Nakatani and J.Onoda at the project engineers, especially Mr. T. Oshima for their great effort in assembling andlaunching the satellite Akebono. We also have to acknowledge the staff of the overseas trackingstations at Syowa in Antarctica, Prince Albert in Canada and Esrange in Sweden for continualdata acquisition of Akebono telemetry.

We are also greatly indebted to NEC for their perfect arrangement of all satellite systems andtheir fabrication of the common part of the satellite, to Meisei Electric Co. Ltd. for theirfabrication of all subsystems of VLF, and to Nihon Hikoki Co. Ltd. for their fabrication of themasts for the search coils. We are also grateful to all colleagues of the Akebono scientificinstrument team for their cooperation and great effort in minimizing RF interference to our VLFinstruments.

In our data analyses, the electron density measured by PWS group and the magnetic fieldmeasured by MGF group were indispensable. We are grateful to these groups for their data used inour analyses.

Finally we acknowledge the cooperation and contribution of graduate students of Kimura'slaboratory and Nagano's laboratory in the preparation of software for satellite data handling andin the data analyses, especially Messrs. Akira Sawada, Yoshihiko Ito, Yoji Kishi and YoshiyaKasahara of Kyoto University and Eiichi Kennai of Kanazawa University.

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vlf observations by the akebono(exos-d)satellite

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