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The effect of geomagnetic disturbance on the duct propagation of low-latitude whistlers
Pergamon Press.Printed inI?orthern Ireland Journal ofAtmospheric andTerre&is1 Physics, 1973, Vol.35,pp.18994703.
SHORT PAPER
The effect of geomagnetic ~st~b~ce on the duct propagation of low-latitude whistlers Y. TANAKA and M. HAYAICAWA The Research Institute of Atmospherics, Nagoya University, Toyokawa, Aichi, Japan
Abstraot-A study has been made of the storm-time dependence of the frequency of occurrence, dispersion and diffuseness of whistlers in the period January 1967 to December 1970 at Moshiri (geom. lat. 34’) and Sakushima (24.1’). At Moshiri whistler occurrence and dXuseness have a maximum end the dispersion a minimum for 1 or 2 days fo?.lowing a disturbed day (#IX I 30). At Sakushima there is a similar increase in the frequency of ocourrence, however, there is a less evident dependenae of dispersion on storm-time. The enhanced diffusenessis attributed to an increase in the effective width of the duet region consisting of many elemental duets.
1, INTRODUCTION is a marked latitudinal dependence in the storm-time variation of whistler activity. At low latitudes (34”-35” geom. lat.) there is a strong correlation with the local K-index (K.IMPARA, 1960) and enhanced whistler activity and reduced dispersion occur in the 1 and 2 days following a severe geomagnetic storm (OHTSU and I~AI, 1962; HAYAKAWA eb al., 1969). At much lower latitudes (24”-26”) a similar enhancement of whistler activity is observed but dispersion increases slightly during storm periods (SOMAYAJULU and TANTEY, 1968; OKUZAWA et al., 1971). The whistler dBuseness has been attributed to the difference in travel time for whistlers propagating along field lines at the near and far boundaries of the duct (CROUCHLEY and FINN, 1961), which pre-supposes elemental ducts within the duct region. On the other hand, SOMAYAJULU and TANTRY (1968) interpreted whistler diffuseness in terms of the difference in travel time for whistler energies propagating snake-like within a single duct. THERE
2. DATA USED IN THIS STUDY
There were 19 isolated geomagnetic disturbances observed at Memanbetsu (geom. lat. 34’) and Kakioka (26’) in the period January 1967-December 1970 such that the daily sum of the local K-index was greater than or equal to 30 for the disturbed day and less than 30 for the days before and after. WhistIer data was available at ~oshiri and Sakushima on 1S and 8 occasions, respectively. To exclude seasonal effects the occurrence rates and dispersions of the short whistlers were normalized in relation to their monthly mean values. The diffuseness is defined as the width of the whistler trace on the sonagram at 5 kHz and was measured in milliseconds. 1699
Fig. f . Average variations of the normalized occurrence rates, dispersions and of diffuseness-dispersion ratios (#“I for the 18 disturbances a$ Moshiri.
!I?& ~~~~~t~~~ variation in the normalized occurrence pate and dispersion at Moshiri are shown in Fig. 1. Also plotted is the diffuseness-dispersion ratio, P’ = lOOOJ’[D (CEOU~ELEYand FINN, 1961) where the reduced diffuseness (B’) of a whistler is measured on the sonag~am at 5 kEIz in units of O.Ofisec. Whistler activity increases sharply 1 day after the disturbed day, whilst the dispersion is reduced by aboA X5 per cent in relation to its mean monthly value. At -+ 1 to j-2 days P is doubled. In Fig, 2 the whistler diB%senessis found to increase from 15 to sevxai tens of milliseconds during the disturbance. These storm-time effects endure up to 4 days. At Sakushima, perhaps because of the limited data, the only significant effect was a similar increase in the occ~once rate. Ionosonde data from Wakkanai (geom. lat. 35”), which illustrate the behaviuur of the P-region fur these disturbed periods, are shown in Fig. 3. The typical minimum in f,B’Z coincides with a decrease in whistler dispersion,
The effect of geomagnetic disturbance
on
the duct propagation
100
Moshiri
cc $
-
January-March I970
----I
2 days after
50
$ a
disturbed
the
day
0 50
150
100
Diffuseness at
5 kHz,
200
msec
Fig. 3. Diffuseness defined by the width of the whistler trace on the sonagram at .5 kHz in units of msec.
.
I
I
I
I
1
1
I
1
-4 -3
-2
-I
0
I
2
3
4
I
No. of days: before t-9, after(t), disturbed
(0)
Fig. 3. Average variations of daily means of hourlyf,FZ and h’F values normalized by monthly medium values and average K-index variation during the 19 disturbances. 9
1701
1702
Y.
T~AKA and M.
HAYAKAWA
4. THE WHISTLERDIFFUSENESS We have calculated the spread in travel time to be expected for snake-like propagation of a ray at VLF in a single field-aligned duct of varying width and enhancement factor for a range of initial wave normal angles to the magnetic field above the F2 maximum. The electron density profile is assumed to be given by -
(b - bLd2 2AD2
1)
when Nbl . exp{ --k(r - rM)} = background electron density N_M= electron density at F2 maximum k = gradient b = r/sin20
b ,%I= r Jsin2 0_+f r = geocentric distance (in Earth radii) rM = height of F2 maximum (in Earth radii) 0 = co-latitude 8,
= co-latitude at rM
C = enhancement factor AD = duct semi-thickness. The duct width is defined as 2AD at the apex of the field line. A standard model of the low-latitude nocturnal ionosphere was used. The results are presented in Table 1 where it may be seen that, because the range of initial wave normal angles for trapping decreases with increasing duct width, the maximum time difference decreases. These small time differences are themselves probably upper limits because of the smallness of the transmission cone at the whistler exit point. Therefore, we suggest that the most probable interpretation of whistler diffuseness is that it is due to the difference in travel time for whistlers propagating along elemental ducts lying on the inner and outer field lines through a duct region. This small difference in travel time, At, may be expressed in terms of the corresponding difference in path length, As:
”
At = 2c(j&)“2
As
*
If we assume f, = 1 MHz andfE, = 300 kHz, we may now obtainvalues of As for the diffuseness, At, at 6 kHz and then, from the field line geometry, deduced the width of the duct region at the apex of the field line. This was about 50 km (field line anchored at Moshiri) for the normal diffuseness of 15 ms, increasing to about 200 km (At -N 50 ms) during disturbed periods. The elemental ducts included in a duct region seem to distribute continuously or lie very closely because some diffused whistlers show a diffused trace and some are composed of many components superposed on a background diffused trace. On this reasoning, the elemental duct
The effectof geomagnetic disturbance on the duct propagation
1703
Table 1. Some results of ray tracing for trapped waves in enhanced ducts along 34’ field line at the Earth’s surface. NH = 4.46 x 106/cm* (fop2 = GMHz.), r M= 1.0471 (300 km), k = 10.631 as the gradient fit to the mean dispersion 50 set”* of the nighttime normal short whistlers at Moshiri, xnr is the initial wave normal angle to the vertical at rI. Circle shows that a wave is trapped in a duct,. Maximum travel time difference (At,,) arising for the trapped wave with the minimum wave normal angle and the longitudinal wave (xX = 38.7’), and its ratio to the longitudinal propagation time of whistler energy ( Atma&& are represented Enhancement factor 0.25 25
0.50
50
Duct width (km) 200 25
50
0 0 0 0 0 0 0 0
0 0 0 0 0 0
200
O0
2.5’ 5O 10° 12.5’ 15O 2o” 25O 3o” 38.7’ A&,x At,,,
0 0 0 0 0 0
0 0 0 0 0
0 0 0
0 0 0 0 0 0 0 0 0
13 msec
6
2
31
20
4
0.9
0.3
3.9
2.5
0.5
1.8%
b=iL
width is probably order of 10 km and its upper limit may be about 50 km for the normal diffuseness. Acknowledgements-The authors thank Profs. A. IWAI and J. OHTSU of their institute for their encouragement. Thanks are also due to the referee for his kind criticisms and useful comments. REFERENCES CROUCHLEYJ.L~~~ FINN R. J. HAYARAWAM.,OETSUJ.~~~IWAI KIMPARAA. OHTSU J.and IWAI A.
A.
OEUZAWA T., YAMANAEA K. and YOSHINO T. SOMAYAJULUV. V. and TANTRY B. A.P.
1961 1969 1960 1962 1971
Au&. J. Phya. 14, 40. Rep. Ionosph. Space Ree., Japan 23, 9. Nature, Lond. 186, 230. Proc. Ree. Inst. Atmos. Nagoya Univ. 9, 19. Rep. Ionosph. Space Res., Ja/pan25, 17.
1968
J. Geomagn. Ceoelect. 20, 21.
SHORT PAPER
The effect of geomagnetic ~st~b~ce on the duct propagation of low-latitude whistlers Y. TANAKA and M. HAYAICAWA The Research Institute of Atmospherics, Nagoya University, Toyokawa, Aichi, Japan
Abstraot-A study has been made of the storm-time dependence of the frequency of occurrence, dispersion and diffuseness of whistlers in the period January 1967 to December 1970 at Moshiri (geom. lat. 34’) and Sakushima (24.1’). At Moshiri whistler occurrence and dXuseness have a maximum end the dispersion a minimum for 1 or 2 days fo?.lowing a disturbed day (#IX I 30). At Sakushima there is a similar increase in the frequency of ocourrence, however, there is a less evident dependenae of dispersion on storm-time. The enhanced diffusenessis attributed to an increase in the effective width of the duet region consisting of many elemental duets.
1, INTRODUCTION is a marked latitudinal dependence in the storm-time variation of whistler activity. At low latitudes (34”-35” geom. lat.) there is a strong correlation with the local K-index (K.IMPARA, 1960) and enhanced whistler activity and reduced dispersion occur in the 1 and 2 days following a severe geomagnetic storm (OHTSU and I~AI, 1962; HAYAKAWA eb al., 1969). At much lower latitudes (24”-26”) a similar enhancement of whistler activity is observed but dispersion increases slightly during storm periods (SOMAYAJULU and TANTEY, 1968; OKUZAWA et al., 1971). The whistler dBuseness has been attributed to the difference in travel time for whistlers propagating along field lines at the near and far boundaries of the duct (CROUCHLEY and FINN, 1961), which pre-supposes elemental ducts within the duct region. On the other hand, SOMAYAJULU and TANTRY (1968) interpreted whistler diffuseness in terms of the difference in travel time for whistler energies propagating snake-like within a single duct. THERE
2. DATA USED IN THIS STUDY
There were 19 isolated geomagnetic disturbances observed at Memanbetsu (geom. lat. 34’) and Kakioka (26’) in the period January 1967-December 1970 such that the daily sum of the local K-index was greater than or equal to 30 for the disturbed day and less than 30 for the days before and after. WhistIer data was available at ~oshiri and Sakushima on 1S and 8 occasions, respectively. To exclude seasonal effects the occurrence rates and dispersions of the short whistlers were normalized in relation to their monthly mean values. The diffuseness is defined as the width of the whistler trace on the sonagram at 5 kHz and was measured in milliseconds. 1699
Fig. f . Average variations of the normalized occurrence rates, dispersions and of diffuseness-dispersion ratios (#“I for the 18 disturbances a$ Moshiri.
!I?& ~~~~~t~~~ variation in the normalized occurrence pate and dispersion at Moshiri are shown in Fig. 1. Also plotted is the diffuseness-dispersion ratio, P’ = lOOOJ’[D (CEOU~ELEYand FINN, 1961) where the reduced diffuseness (B’) of a whistler is measured on the sonag~am at 5 kEIz in units of O.Ofisec. Whistler activity increases sharply 1 day after the disturbed day, whilst the dispersion is reduced by aboA X5 per cent in relation to its mean monthly value. At -+ 1 to j-2 days P is doubled. In Fig, 2 the whistler diB%senessis found to increase from 15 to sevxai tens of milliseconds during the disturbance. These storm-time effects endure up to 4 days. At Sakushima, perhaps because of the limited data, the only significant effect was a similar increase in the occ~once rate. Ionosonde data from Wakkanai (geom. lat. 35”), which illustrate the behaviuur of the P-region fur these disturbed periods, are shown in Fig. 3. The typical minimum in f,B’Z coincides with a decrease in whistler dispersion,
The effect of geomagnetic disturbance
on
the duct propagation
100
Moshiri
cc $
-
January-March I970
----I
2 days after
50
$ a
disturbed
the
day
0 50
150
100
Diffuseness at
5 kHz,
200
msec
Fig. 3. Diffuseness defined by the width of the whistler trace on the sonagram at .5 kHz in units of msec.
.
I
I
I
I
1
1
I
1
-4 -3
-2
-I
0
I
2
3
4
I
No. of days: before t-9, after(t), disturbed
(0)
Fig. 3. Average variations of daily means of hourlyf,FZ and h’F values normalized by monthly medium values and average K-index variation during the 19 disturbances. 9
1701
1702
Y.
T~AKA and M.
HAYAKAWA
4. THE WHISTLERDIFFUSENESS We have calculated the spread in travel time to be expected for snake-like propagation of a ray at VLF in a single field-aligned duct of varying width and enhancement factor for a range of initial wave normal angles to the magnetic field above the F2 maximum. The electron density profile is assumed to be given by -
(b - bLd2 2AD2
1)
when Nbl . exp{ --k(r - rM)} = background electron density N_M= electron density at F2 maximum k = gradient b = r/sin20
b ,%I= r Jsin2 0_+f r = geocentric distance (in Earth radii) rM = height of F2 maximum (in Earth radii) 0 = co-latitude 8,
= co-latitude at rM
C = enhancement factor AD = duct semi-thickness. The duct width is defined as 2AD at the apex of the field line. A standard model of the low-latitude nocturnal ionosphere was used. The results are presented in Table 1 where it may be seen that, because the range of initial wave normal angles for trapping decreases with increasing duct width, the maximum time difference decreases. These small time differences are themselves probably upper limits because of the smallness of the transmission cone at the whistler exit point. Therefore, we suggest that the most probable interpretation of whistler diffuseness is that it is due to the difference in travel time for whistlers propagating along elemental ducts lying on the inner and outer field lines through a duct region. This small difference in travel time, At, may be expressed in terms of the corresponding difference in path length, As:
”
At = 2c(j&)“2
As
*
If we assume f, = 1 MHz andfE, = 300 kHz, we may now obtainvalues of As for the diffuseness, At, at 6 kHz and then, from the field line geometry, deduced the width of the duct region at the apex of the field line. This was about 50 km (field line anchored at Moshiri) for the normal diffuseness of 15 ms, increasing to about 200 km (At -N 50 ms) during disturbed periods. The elemental ducts included in a duct region seem to distribute continuously or lie very closely because some diffused whistlers show a diffused trace and some are composed of many components superposed on a background diffused trace. On this reasoning, the elemental duct
The effectof geomagnetic disturbance on the duct propagation
1703
Table 1. Some results of ray tracing for trapped waves in enhanced ducts along 34’ field line at the Earth’s surface. NH = 4.46 x 106/cm* (fop2 = GMHz.), r M= 1.0471 (300 km), k = 10.631 as the gradient fit to the mean dispersion 50 set”* of the nighttime normal short whistlers at Moshiri, xnr is the initial wave normal angle to the vertical at rI. Circle shows that a wave is trapped in a duct,. Maximum travel time difference (At,,) arising for the trapped wave with the minimum wave normal angle and the longitudinal wave (xX = 38.7’), and its ratio to the longitudinal propagation time of whistler energy ( Atma&& are represented Enhancement factor 0.25 25
0.50
50
Duct width (km) 200 25
50
0 0 0 0 0 0 0 0
0 0 0 0 0 0
200
O0
2.5’ 5O 10° 12.5’ 15O 2o” 25O 3o” 38.7’ A&,x At,,,
0 0 0 0 0 0
0 0 0 0 0
0 0 0
0 0 0 0 0 0 0 0 0
13 msec
6
2
31
20
4
0.9
0.3
3.9
2.5
0.5
1.8%
b=iL
width is probably order of 10 km and its upper limit may be about 50 km for the normal diffuseness. Acknowledgements-The authors thank Profs. A. IWAI and J. OHTSU of their institute for their encouragement. Thanks are also due to the referee for his kind criticisms and useful comments. REFERENCES CROUCHLEYJ.L~~~ FINN R. J. HAYARAWAM.,OETSUJ.~~~IWAI KIMPARAA. OHTSU J.and IWAI A.
A.
OEUZAWA T., YAMANAEA K. and YOSHINO T. SOMAYAJULUV. V. and TANTRY B. A.P.
1961 1969 1960 1962 1971
Au&. J. Phya. 14, 40. Rep. Ionosph. Space Ree., Japan 23, 9. Nature, Lond. 186, 230. Proc. Ree. Inst. Atmos. Nagoya Univ. 9, 19. Rep. Ionosph. Space Res., Ja/pan25, 17.
1968
J. Geomagn. Ceoelect. 20, 21.