On the sources of Jovian hectometric radiation

Adv. Space Res. Vol. 26, No. 10, pp. 1541-1544.2000 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/00 $20.00 + 0.00

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ON THE SOURCES OF JOVIAN HECTOMETRIC F. Nakagawa, A. Morioka,

Planetary

RADIATION

and H. Misawa

Plasma and Atmospheric

Research Center, Tohoku University, Sendai, 980-8578,

Japan

ABSTRACT Jovian

hectometric

the information SWOOPS

radiation(HOM)

emitted

on the electro-magnetic

data, the occurrence

from the polar region of the Jovian magnetsphere

environment

characteristics

of the source region.

to the solar wind parameters

have different system III occurrence

and is observed

characteristics.

aurora, L values of each HOM source are inferred to be L>30, by Elsevier

and

(1) Solar wind HOM(sw-HOM),

are related to the solar wind dynamic pressure, and (2) non Solar wind HOM(nsw-HOM),

which is independent components

Using the Ulysses URAP

of HOM and its relation to the solar wind were investigated.

As a result, we found that HOM can be classified into two components; whose amplitudes

reflects

with large amplitudes.

Considering L~12,

the relationship

respective1y.O

These two with Jovian

2000 COSPAR. Published

Science Ltd. All rights reserved.

INTRODUCTION Jovian hectometric

radiation(HOM)

1973(Barrow,1974).

It is accepted

hemisphere

is discovered

by the RAE-l(Desch

with hollow cones of large opening half-angle(Ladreiter

observations,

and Carr,1974)

and the IMP-6 in

that HOM is emitted from polar regions both in northern and southern and Leblanql991).

is is known that HOM has a relation with the solar wind parameter(Desch

From the Voyager and Barrow, 1984;

Barrow and Desch, 1989). In this paper, using data observed by the Ulysses, we analyzed detail occurrence characteristics

of HOM, especially

relationship

with the solar wind.

DATA The Ulysses data were obtained

from the COordinated

Heliospheric

Observations(COH0)

data base pro-

vided by the National Space Science Data Center(NSSDC). Receiver(RAR) plasma

HOM s were measured by the Radio Astronomy in Unified Radio And Plasma wave investigation(URAP)(Stone et oz.,) and the solar wind

data were detected

aL,1992).

Analyzed

by Solar Wind

Observation

Over the Poles of the Sun(SWOOPS)(Bame

data period in this paper was from September

To analyze the Jovian HOM statistically,

we first eliminated

the observed intensity Ids was expressed by

1541

to December,

et

1991.

the solar Type3 bursts from the data.

Then,

F. Nakagawa et al.

1542

where, IHOM is the normalized intensity at an unit distance, r is the distance between the Ulysses and Jupiter, N is the constant noise intensity independent of r, such as galactic component and receiver noise. Here, radial dependence of Jovian radio waves has a form of l/r because intensities are in volts per root Hertz. The observed data were fitted to this equation by the least square method as shown in Figurel. The horizontal axis is the radial distance between the Ulysses and Jupiter. The circles are the observed signals averaged by Jovian 1 rotation. The solid and dashed lines are estimated Id8 and noise level by the least square method, respectively. Thus, we could eliminate the noise component and could normalize the observed intensity at an unit distance. To discuss the solar wind effect on HOM at the Jovian magnetosphere using the in-situ SWOOPS data, we considered the propagation time At of the solar wind from the Ulysses to Jupiter by

where A(p is the longitudinal separation between Jupiter and the Ulysses, V,, is the solar wind velocity, and s1 is solar rotation fiequency(14.1°/dat). In the following analysis, we use the normalized UHAP data and the revised SWOOPS data as mentioned above.

ANALYSIS Prom the spacecraft observation of Jovian HOM, it is known that HOM has a system III periodicity as well as the other Jovian radiations,DAM(Alexander et al.1981). This periodicity results corn variation of magnetic latitude at observer(Ladreiter and Leblanc,1989). Figure 2 shows the present result of HOM occurrence characteristics at 940kHz with respect to system III longitude. Solid line is average intensity during 4 months normalized to a distance of 1AU. It is evident that HOM is always observed at all system III longitude having two occurrence maxima at 110“ and 280”, and the lane structures(Higgins et al., 1995) at 140°, 250°, and 330” in system III longitude. Voyager observations also found that HOM is correlated with the ram pressure of the solar wind(Desch and Barrow, 1984; Barrow and Desch, 1989). To investigate the distribution of the correlation with respect to the system III longitude, HOM data were divided into 18 longitude bands, that is, every 20 degrees of system III longitude. We calculated cross correlation between each band HOM intensity and solar wind ram pressure. The cross correlation C(r) was calculated using following relation:

0.07 0.06

-0.2

0.4

0.6 0.6 diidinancebaturanUA~W

1.2

1.4

Fig . 1.Result of fitting by the least square method. Circles are observed intensity. Solid line and dashed line sre estimated Iobsand noise level, respectively.

0

60

120 180 240 SptemiiIlonpiWdsatLWns

300

360

Fig .2. Occuren ce characteristics of Jovian HOM in system III longitude at 94OkHz.

On the Sources of Jovian Hectometric

C(r) =

&

Radiation

1543

2 vi - (4) (pi+7- (P)) 1

where I and P are HOM intensities at each band and the solar wind ram pressure averaged around 1 Jovian rotation, err and op are deviations of I and P, respectively. Due to the normalization of amplitudes by azap, the resultant cross-correlation coefficient is not affected by the absolute amplitude of each phenomenon. The results is shown in Figure 3. In the figure, the horizontal axis is the system III longitude. The solid line is the average HOM intensity in system III longitude. Bar graphs show maximum cross-correlation coefficient at each longitude range. As can be seen, the correlation coefficient shows large value in the system III range from 130’ to 270”. On the other hand, the correlation is rather weak when HOM intensities are strong(ll0” and 280’). From this results, we can suggest that there are two components of HOM, one is a solar wind HOM(sw-HOM) which depends on solar wind, and the other is a non solar wind HOM(nsw-HOM) which is independent of solar wind. It is also pointed out that the lane structures in Figure 2 were located at boundary of SW-HOM and nsw-HOM. Next, we tried to separate two components of HOM. Assuming that SW-HOM is not observed when solar wind pressure is weak, the occurrence characteristics can be represented by

HOM Paw : strong) = nswHOM

+ swHOM

HOM (Pew: weak) = nswHOM

(5)

namely, both nsw-HOM and SW-HOM are observed when the solar wind ram pressure is strong, and only nsw-HOM is detectable only when the pressure is weak. From these relations, we can derive the occurrence characteristics of each HOM component with respect to system III. The SW-HOM is obtained after subtracting the HOM observed when solar wind ram pressure is weak (eq.(5)) from the HOM observed when solar wind ram pressure is strong(eq.(4)). Figure 4 shows the result at 940kHz. As can be seen in the figure, the nsw-HOM(solid line) has two strong peaks, and sw-HOM(dash line) has some weak peaks. DISCUSSION Here, we consider the energy sources of each HOM. We suspect there are three energy sources of radio waves. The first is the solar wind. In this case, the location of energy source for the radio emission is expected to be outer magnetsphere and the SW-HOM will be generated in the polar region where the field line is connected to this region. The second one is V x B field induced by the slipping of plasma in the middle magnetosphere and nsw-HOM should be generated on the field line connected to in this region. The third is V x B dynamo

0

60

120

180

240

300

360

System II h7qude

Fig .3. Cross correlation analysis of HOM in each system III longitude with solar wind ram pressure.

Fig .4. Occurrence characteristics of nsw-HOM (solid line) and sw-HOM (dashed line) in system III longitude, respectively.

1544

F. Nakagawa

at 10. Both HOMs are not dependent

et al

on 10 phase, so it should be concluded

that the energy source of HOM

is not related to IO. Thus we suppose that the energy sources of SW-HOM and nsw-HOM and slipping of plasma, respectively. to those of Jovian aurora observed

are the solar wind

The L values of these HOM source regions are expected by Hubble space telescope(Clarke

to correspond

et al., 1996), that is, L values greater

than 30 and about 12, respectively.

CONCLUSION We have investigated are obtained

detail relations between Jovian HOM and the solar wind using the Ulysses data which

from the Coordinated

divided into two components,

Heliospheric

a non solar wind HOM(nsw-HOM) region are supposed

Observations(COH0)

one is solar wind HOM(sw-HOM) which is independent

data base.

As a result, HOM was

which depends on solar wind, the other is

of solar wind. The L values of these HOM source

to be greater than 30 and about 12, respectively.

ACKNOWLEDGMENTS We are grateful National

to Prof.

H. Oya for his helpful discussions.

Space Science Data Center.

This study financially

The Ulysses data were obtained was supported

by the Ministry

from the

of Education,

Science, and Culture of Japan in the form of Grants-inAid(09440171).

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