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The position and height of auroral absorption deduced from VLF phase measurements
Joumslof Atmospheric andTerrestrial Physics, 1969,Vol.31,pp.187to 192.Pergamon Preys.Printed inNorthern Ireland
SHORT PAPER
The position and height of aurora1 absorption deduced from VLF phase measurements A. EGELAND, T. R. LARSEN* and E. NAUSTVIK Norwegian Institut,e of Cosmic Physics, Oslo, Norway (Received
15 &1arch1968; i?s vewised form 15 &1uy 1968)
Abstract-The main findings from VLF measurements during aurora1absorption are as follows: The geographical extent of aurora1 absorption seems to be at least a factor of two larger than found by HF riometers. The VLF reflection layer is markedly depressed during aurora1disturbances. At night large and rapid fluctuations in the reflection layer may occur simultaneously over distances of more than 500-1000 km. During day-time the depressed VLF reflection layer seems to be relatively homogeneous over the same distances. 1. INTRODUCTION UP TO now aurora1 absorption (AA) has been studied by sounding rockets and in particular by riometers operating at one or several frequencies in the HP range (cf. e.g. LERFALD et al., 1964; JESPERSEN et al., 1966, ECKLUND and HARGREAVES, 1968). In connection with solar X-ray events and polar cap absorptions VLF radio waves have in many aspects proved to be a more sensitive tool for investigation of the lowermost ionosphere than riometers (cf. e.g. EGELAND and NAUSTVIK, 1967; CROMBIE, 1967). This is specially the case during moderate disturbances. In this paper it will be demonstrated that VLF measurements may be of great importance in studying the geographical extent and lower boundary of AA. When discussing the height distribution and spatial extent of AA the following facts should be noted: (a) There is a controversy over the height distribution of AA, mainly due to insufficient knowledge of the electron density profile (cf. REID, 1965; HULTQVIST, 1966). (b) There seems to be a general agreement that the north-south extent of AA is of the order of 200-300 km. The ionization producing (c) A high correlation between AA, the AA normally shows large time fluctuations. precipitated electrons (energies above say 40 keV) and bremsstrahlung X-rays has been established (MAEHLUM and O’BRIEN, 1963; HULTQVIST, 1966; BROWN, 1966). 2. INSTRUMENTATIONAND OBSERVATIONS The VLF transmitter at Aldra, Norway (geographic coordinates 66*4”N, 13.2”E) has been received at the following three stations: Ny dlesund (geogr. lat. 79”N), Ha,mmerfest (geogr. lat. 71°N), and at Lillestrom (geogr. lat. 6O”N). One of the receivers is thus located within the aurora1 oval, while the other two are situated approximately 10’ further to the north and south. In addition, three or more different VLF transmitters were recorded at the receiving stations. * Sormogian Defence Research Establishment, Kjeller, Norway. 187
188
A. EGELAND,
T. R. LARSEN and E. NAUSTVIK
We have mainly concentrated on phase measurements, and for all data presented here rubidium standards (with long term stabilities better than 5 . lo-‘I) have been used as references. Amplitude recordings have also been carried out for some of the transmitters. Several upper atmospheric parameters have been monitored at Tromso (geogr. lat. 70”N, approximately midway between Aldra and Hammerfest) and at Ny Alesund. 3.
DATA
PRESENTATION
ANI)
DISCUSSION
VLF records obtained during three AA events will be presented: Event 1 The phase variations of Aldra at IO.2 kHz as observed at Hammerfest (650 km to the north-east of Aldra) and at Ny Alesund (1350 km due north of Aldra) for the night between 28 and 29 December 1967 are shown in Fig. 1, curves A and B. Curve C represents the phase variation of the NPG 18.6 kHz signal observed at Ny A. Aldro
t
- tiommerfest,
C NPG-Ny
hesund,
Qu~+tday,
12
UT
15
D
Fig. 1. VLF
16 Riometer
10 2 ktiz
186 kHz
29 - 30 DEC
21
276 MHz,Tiomso
2L
03
06
09
UT
12
i
phase p&tterns and riometer data for the interval 28 December 12 UT-29 December 12 UT, 1967.
The position and height of aurora1 absorption
189
Alesund (path length 5550 km). For comparison the normal quiet phase curves are also plotted (cf. the dotted curves). These three paths (except for a small portion of the NPG path) are located north of 60” geomagnetic latitude. Curve D shows the riometer absorption on 27.6 MHz at Tromso, 350 km north of Aldra. Other VLF phase and amplitude curves at Hammerfest and Ny Alesund also On the Aldra records obtained at exhibit marked deviations from normal. Lillestrsm no pronounced anomalous behaviour is observed. The multiple frequency riometers at Ny Alesund showed absorption only between 1925 and 2100 UT on 28 December, with a maximum of 3.3 dB at 20 MHz and 1.8 dB at 27.6 MHz. The riometer data at Tromso and Ny Alesund (separated by a distance of The magnetic records from Tromss exhibit 900 km) thus show little similarity. fluctuations between 2000 and 0300 UT, but no close similarity to the 10.2 kHz phase curves is found. By comparing the phase curves with riometer data (cf. Fig. 1) the foIlowing experimental findings should be stressed: This weak absorption event at Tromso (less than about 0.5 dB in average) coincided in time with the very marked 10.2 kHz phase anomalies. The duration of the phase advances on the two short paths is roughly the same as found for the aurora1 absorption at Tromsra, while for NPG the period of anomalous records is shorter, cf. Fig. 1. The marked phase deviation as seen on curves A and B represents closely 100 per cent of total, normal diurnal variation, while the maximum of 10 ps as observed on the NPG path (curve C) corresponds to about 50 per cent of the diurnal variation. A striking similarity is seen between curves A and B, even though the distance between Hammerfest and Ny Alesund is about 900 km. Furthermore, the 10.2 kHz phase curves show more fluctuations than seen on the riometer curves at Tromser and Ny Alesund. Event 2 The VLF records obtained between 0700 and 1900 UT on 10 December show effects from an AA event which was observed at Tromsar, cf. Fig. 2, curve D. Maximum absorption was about 2 dB on 274 MHz. No riometer absorption was found at Ny Alesund at this time. For the 10.2 kHz path between Aldra and Ny Alesundamarkedphaseadvancewithamaximumofabout lOys,isfound (cf. curveB). For the short path from Aldra to Hammerfest the amplitude, plotted in Curve A, shows a pronounced decrease of about 10 dB. In addition, the NPG phase curve for this event, together with the average phase variation is drawn in Fig. 2 curve C. Also for this event other VLF results support the observation presented above. One important finding for this event should be stressed. Even though the AA shows strong fluctuation the anomalous 10.2 kHz phase and amplitude curves are very smooth. For the 10.2 kHz curve it looks as if the sunset occurs about 2-3 hr later than normal, whiIe the sunrise for the NPG occurs at normal time. Furthermore, the phase advance of NPG (night-time condition) is more marked than on the 10.2 kHz curve (compare curve B with curve C). Event 3 Finally, we will present the data for 31 December between 1000 and 2000 UT (cf. Fig. 3), which is an extremely interesting event. The sunset at 80 km for the
,4. EGELAND, T. R. LARSEN and E. KAUSTVIK
190
uoLr-----A. Amplitude,
- Hammerfest,
12
09
06
IT I 1
Aldra
_
6.
Phase,
Aldra-Ny
_
C
Phase,
NPG-Ny
10.2
15 &und,
ilesund,
UT
10.2
186
kHz
18
kHz
kHz
*
10 DEC
Fig. 2. VLF
1967
phase and amplitude patterns together with riometer data for the interval 10 December 07.00-19.00 U.T.. 1967.
midpoint of the paths Aldra-Hammerfest and Aldra-Ny Alesund is close to 1400 UT at which time the riometer absorption starts at Tromso, cf. curve E. Thus, this is essentially also a night-time event. The riometer curve is of a somewhat unusual character, with four periods of very rapid changes in the absorption at about 1400, 1430, 1630 and 1830 UT. This event is rather intense with more than 5 dB absorption between 1630 and 1700 as well as between 1820 and 1850 UT. In addition, a very strong magnetic bay is observed at Tromser. At the above mentioned periods intense magnetic deflections (maximum 700 y on the H-component) superposed on No absorption at all was recorded on three riothe bay activity were observed. meters (20.0, 27.6 and 40.0 MHz) at Ny Alesund. Notice that during precisely the same four periods with riometer absorption at Tromso very strong phase fluctuations are observed on the 10.2 kHz curves both at Hammerfest and Ny Alesund. The fluctuations seen in curves A and B are almost
The position and height of ituroral absorption
# h.
I91 I
A~dra-Hammertes~,
10 2
1 _4___---6
tar
_____-_----“------~;,~,-,-,
*9_3oDcr.
____----C
B
Aldro-Ny
hsund,
10 2 kHr _
t 9I= 1 c*
C.
NWC-Ny
D.
Ampistudc.
.’
,_
ifcsund.
0 ____”
-_____---_--
D_ --
a
.,=
196
NWC-Ny
ktiz
ftesund,
Average 23-26 Dtc ______” _------
o Avcygc
-----
e
29-30
(’
Dee
f9 6 kfit
“tI.
I
0
t
#O t
I
ii
r
IL E.
Riomrtw
tb 27.6 MHz,
16
20
UT
Tromsb
Jf DEE
IS67
Fig. 3. VLF phase and amplitude patterns together with riometcr data for the interval 31 December 12.00-21.00 U.T., 1967.
identical. The amplitude and phase for NWC at 19.8 kHz (propagated over a path length of 12600 km) are drawn in curves C and D, respectively. It is very interesting to note that both the phase and amplitude curves of NWC show much of the same fine structure as found from the short paths. This is especially true for the two periods with maximum absorption. For this event no marked anomalies were
192
A. EGELAND,
T. R. LARSEN and E. NAUSTVIK
Hitherto, two more examples of such an found for the path Aldra-Lillestrram. unexpectable high correlation between phase fluctuations on different paths have been observed. 4. CONCLUDING REMARKS From the data presented above together with other VLF phase and amplitude records obtained during AA, the following conclusions may be drawn: (i) For every AA event observed at Tromso anomalous VLF phase and amplitude behaviour has been seen on all propagation paths which have an appreciable part north of 60” geomagnetic latitude. This is true both for day- and night-conditions. (ii) Anomalous propagation conditions for VLF radio waves have also been observed even if no absorption has been found on riometers close to the paths. (iii) During night-time the VLF phase in general shows more fluctuations than found on the riometer. The opposite seems to be the ease during day-time. (iv) There seems to be a very close correlation between the Aldra 10.2 kHz phase fluctuations observed at Hammerfest and Ny Alesund, which are separated by 900 km. It has not been possible to find a similar correlation between the riometer curves from Tromss and Ny Alesund which are the same distance apart. Even the phase ~uctuations for XPG and NWC at Ny Alesund show a fine structure similar to the 10.2 kHz curves discussed above. A preliminary interpretation of these findings may be as follows: (a) Simultaneous phase measurements both on short and long paths (cf. Point IV above) indicate that the reflection layer is markedly depressed both at Thus the lower boundary night and at day during moderate aurora1 disturbances. of AA must be below 85 km at night and 70 km at day. (b) The depressed VLF reflecting layer has a geographical extent which is considerably larger (at least a factor of two) than found for AA with riometers. (c) During day-time the depressed VLY reflecting layer seems to be relatively homogeneous over large distance (order of500 to 1000 km). At night great and rapid changes may occur simultaneously over comparable areas. REFERENCES BROWN R. R. CROMBIE D. D.
1966 1967
ECKX.UND W. L. and HARGREA~ES I. K. EGELAND A. and KAUSTWK E. HULTQVIST B . JESPERSEN M., HATJGA. and LANDMARK B.
1968 1967 1966 1966
LERFUD G. M., LITTLE C. C. and PARTHASARATEY R. MAEFILUM B. LandO’BRIEN B. J. REID G. C.
1964 1963 1965
space Sci. Rev. 5, 3 11. M.F., L.F. and V.L.F. Radio Propagation IEE Symposium, London, Nov. 8-10, 1967. .J. Atmosph. Terr. Phys. 30, 265. Ra&o Scz:. 2, 659. Space Sci. Rev. 5, 771. Electron Density Profiles iva Ionosphere a&Exosphere (EditedbyJ.Fs~~~a~N) p. 27, North-Holland, Amsterdam. J. Geophya. Res. 69, 2857. J. Geophye. Res. 68, 997. High Latitude Particles snd the Ionosphere (Edited by B. MAEHLUM) p. 22 1, Lagos Press and Academic Press, New York.
SHORT PAPER
The position and height of aurora1 absorption deduced from VLF phase measurements A. EGELAND, T. R. LARSEN* and E. NAUSTVIK Norwegian Institut,e of Cosmic Physics, Oslo, Norway (Received
15 &1arch1968; i?s vewised form 15 &1uy 1968)
Abstract-The main findings from VLF measurements during aurora1absorption are as follows: The geographical extent of aurora1 absorption seems to be at least a factor of two larger than found by HF riometers. The VLF reflection layer is markedly depressed during aurora1disturbances. At night large and rapid fluctuations in the reflection layer may occur simultaneously over distances of more than 500-1000 km. During day-time the depressed VLF reflection layer seems to be relatively homogeneous over the same distances. 1. INTRODUCTION UP TO now aurora1 absorption (AA) has been studied by sounding rockets and in particular by riometers operating at one or several frequencies in the HP range (cf. e.g. LERFALD et al., 1964; JESPERSEN et al., 1966, ECKLUND and HARGREAVES, 1968). In connection with solar X-ray events and polar cap absorptions VLF radio waves have in many aspects proved to be a more sensitive tool for investigation of the lowermost ionosphere than riometers (cf. e.g. EGELAND and NAUSTVIK, 1967; CROMBIE, 1967). This is specially the case during moderate disturbances. In this paper it will be demonstrated that VLF measurements may be of great importance in studying the geographical extent and lower boundary of AA. When discussing the height distribution and spatial extent of AA the following facts should be noted: (a) There is a controversy over the height distribution of AA, mainly due to insufficient knowledge of the electron density profile (cf. REID, 1965; HULTQVIST, 1966). (b) There seems to be a general agreement that the north-south extent of AA is of the order of 200-300 km. The ionization producing (c) A high correlation between AA, the AA normally shows large time fluctuations. precipitated electrons (energies above say 40 keV) and bremsstrahlung X-rays has been established (MAEHLUM and O’BRIEN, 1963; HULTQVIST, 1966; BROWN, 1966). 2. INSTRUMENTATIONAND OBSERVATIONS The VLF transmitter at Aldra, Norway (geographic coordinates 66*4”N, 13.2”E) has been received at the following three stations: Ny dlesund (geogr. lat. 79”N), Ha,mmerfest (geogr. lat. 71°N), and at Lillestrom (geogr. lat. 6O”N). One of the receivers is thus located within the aurora1 oval, while the other two are situated approximately 10’ further to the north and south. In addition, three or more different VLF transmitters were recorded at the receiving stations. * Sormogian Defence Research Establishment, Kjeller, Norway. 187
188
A. EGELAND,
T. R. LARSEN and E. NAUSTVIK
We have mainly concentrated on phase measurements, and for all data presented here rubidium standards (with long term stabilities better than 5 . lo-‘I) have been used as references. Amplitude recordings have also been carried out for some of the transmitters. Several upper atmospheric parameters have been monitored at Tromso (geogr. lat. 70”N, approximately midway between Aldra and Hammerfest) and at Ny Alesund. 3.
DATA
PRESENTATION
ANI)
DISCUSSION
VLF records obtained during three AA events will be presented: Event 1 The phase variations of Aldra at IO.2 kHz as observed at Hammerfest (650 km to the north-east of Aldra) and at Ny Alesund (1350 km due north of Aldra) for the night between 28 and 29 December 1967 are shown in Fig. 1, curves A and B. Curve C represents the phase variation of the NPG 18.6 kHz signal observed at Ny A. Aldro
t
- tiommerfest,
C NPG-Ny
hesund,
Qu~+tday,
12
UT
15
D
Fig. 1. VLF
16 Riometer
10 2 ktiz
186 kHz
29 - 30 DEC
21
276 MHz,Tiomso
2L
03
06
09
UT
12
i
phase p&tterns and riometer data for the interval 28 December 12 UT-29 December 12 UT, 1967.
The position and height of aurora1 absorption
189
Alesund (path length 5550 km). For comparison the normal quiet phase curves are also plotted (cf. the dotted curves). These three paths (except for a small portion of the NPG path) are located north of 60” geomagnetic latitude. Curve D shows the riometer absorption on 27.6 MHz at Tromso, 350 km north of Aldra. Other VLF phase and amplitude curves at Hammerfest and Ny Alesund also On the Aldra records obtained at exhibit marked deviations from normal. Lillestrsm no pronounced anomalous behaviour is observed. The multiple frequency riometers at Ny Alesund showed absorption only between 1925 and 2100 UT on 28 December, with a maximum of 3.3 dB at 20 MHz and 1.8 dB at 27.6 MHz. The riometer data at Tromso and Ny Alesund (separated by a distance of The magnetic records from Tromss exhibit 900 km) thus show little similarity. fluctuations between 2000 and 0300 UT, but no close similarity to the 10.2 kHz phase curves is found. By comparing the phase curves with riometer data (cf. Fig. 1) the foIlowing experimental findings should be stressed: This weak absorption event at Tromso (less than about 0.5 dB in average) coincided in time with the very marked 10.2 kHz phase anomalies. The duration of the phase advances on the two short paths is roughly the same as found for the aurora1 absorption at Tromsra, while for NPG the period of anomalous records is shorter, cf. Fig. 1. The marked phase deviation as seen on curves A and B represents closely 100 per cent of total, normal diurnal variation, while the maximum of 10 ps as observed on the NPG path (curve C) corresponds to about 50 per cent of the diurnal variation. A striking similarity is seen between curves A and B, even though the distance between Hammerfest and Ny Alesund is about 900 km. Furthermore, the 10.2 kHz phase curves show more fluctuations than seen on the riometer curves at Tromser and Ny Alesund. Event 2 The VLF records obtained between 0700 and 1900 UT on 10 December show effects from an AA event which was observed at Tromsar, cf. Fig. 2, curve D. Maximum absorption was about 2 dB on 274 MHz. No riometer absorption was found at Ny Alesund at this time. For the 10.2 kHz path between Aldra and Ny Alesundamarkedphaseadvancewithamaximumofabout lOys,isfound (cf. curveB). For the short path from Aldra to Hammerfest the amplitude, plotted in Curve A, shows a pronounced decrease of about 10 dB. In addition, the NPG phase curve for this event, together with the average phase variation is drawn in Fig. 2 curve C. Also for this event other VLF results support the observation presented above. One important finding for this event should be stressed. Even though the AA shows strong fluctuation the anomalous 10.2 kHz phase and amplitude curves are very smooth. For the 10.2 kHz curve it looks as if the sunset occurs about 2-3 hr later than normal, whiIe the sunrise for the NPG occurs at normal time. Furthermore, the phase advance of NPG (night-time condition) is more marked than on the 10.2 kHz curve (compare curve B with curve C). Event 3 Finally, we will present the data for 31 December between 1000 and 2000 UT (cf. Fig. 3), which is an extremely interesting event. The sunset at 80 km for the
,4. EGELAND, T. R. LARSEN and E. KAUSTVIK
190
uoLr-----A. Amplitude,
- Hammerfest,
12
09
06
IT I 1
Aldra
_
6.
Phase,
Aldra-Ny
_
C
Phase,
NPG-Ny
10.2
15 &und,
ilesund,
UT
10.2
186
kHz
18
kHz
kHz
*
10 DEC
Fig. 2. VLF
1967
phase and amplitude patterns together with riometer data for the interval 10 December 07.00-19.00 U.T.. 1967.
midpoint of the paths Aldra-Hammerfest and Aldra-Ny Alesund is close to 1400 UT at which time the riometer absorption starts at Tromso, cf. curve E. Thus, this is essentially also a night-time event. The riometer curve is of a somewhat unusual character, with four periods of very rapid changes in the absorption at about 1400, 1430, 1630 and 1830 UT. This event is rather intense with more than 5 dB absorption between 1630 and 1700 as well as between 1820 and 1850 UT. In addition, a very strong magnetic bay is observed at Tromser. At the above mentioned periods intense magnetic deflections (maximum 700 y on the H-component) superposed on No absorption at all was recorded on three riothe bay activity were observed. meters (20.0, 27.6 and 40.0 MHz) at Ny Alesund. Notice that during precisely the same four periods with riometer absorption at Tromso very strong phase fluctuations are observed on the 10.2 kHz curves both at Hammerfest and Ny Alesund. The fluctuations seen in curves A and B are almost
The position and height of ituroral absorption
# h.
I91 I
A~dra-Hammertes~,
10 2
1 _4___---6
tar
_____-_----“------~;,~,-,-,
*9_3oDcr.
____----C
B
Aldro-Ny
hsund,
10 2 kHr _
t 9I= 1 c*
C.
NWC-Ny
D.
Ampistudc.
.’
,_
ifcsund.
0 ____”
-_____---_--
D_ --
a
.,=
196
NWC-Ny
ktiz
ftesund,
Average 23-26 Dtc ______” _------
o Avcygc
-----
e
29-30
(’
Dee
f9 6 kfit
“tI.
I
0
t
#O t
I
ii
r
IL E.
Riomrtw
tb 27.6 MHz,
16
20
UT
Tromsb
Jf DEE
IS67
Fig. 3. VLF phase and amplitude patterns together with riometcr data for the interval 31 December 12.00-21.00 U.T., 1967.
identical. The amplitude and phase for NWC at 19.8 kHz (propagated over a path length of 12600 km) are drawn in curves C and D, respectively. It is very interesting to note that both the phase and amplitude curves of NWC show much of the same fine structure as found from the short paths. This is especially true for the two periods with maximum absorption. For this event no marked anomalies were
192
A. EGELAND,
T. R. LARSEN and E. NAUSTVIK
Hitherto, two more examples of such an found for the path Aldra-Lillestrram. unexpectable high correlation between phase fluctuations on different paths have been observed. 4. CONCLUDING REMARKS From the data presented above together with other VLF phase and amplitude records obtained during AA, the following conclusions may be drawn: (i) For every AA event observed at Tromso anomalous VLF phase and amplitude behaviour has been seen on all propagation paths which have an appreciable part north of 60” geomagnetic latitude. This is true both for day- and night-conditions. (ii) Anomalous propagation conditions for VLF radio waves have also been observed even if no absorption has been found on riometers close to the paths. (iii) During night-time the VLF phase in general shows more fluctuations than found on the riometer. The opposite seems to be the ease during day-time. (iv) There seems to be a very close correlation between the Aldra 10.2 kHz phase fluctuations observed at Hammerfest and Ny Alesund, which are separated by 900 km. It has not been possible to find a similar correlation between the riometer curves from Tromss and Ny Alesund which are the same distance apart. Even the phase ~uctuations for XPG and NWC at Ny Alesund show a fine structure similar to the 10.2 kHz curves discussed above. A preliminary interpretation of these findings may be as follows: (a) Simultaneous phase measurements both on short and long paths (cf. Point IV above) indicate that the reflection layer is markedly depressed both at Thus the lower boundary night and at day during moderate aurora1 disturbances. of AA must be below 85 km at night and 70 km at day. (b) The depressed VLF reflecting layer has a geographical extent which is considerably larger (at least a factor of two) than found for AA with riometers. (c) During day-time the depressed VLY reflecting layer seems to be relatively homogeneous over large distance (order of500 to 1000 km). At night great and rapid changes may occur simultaneously over comparable areas. REFERENCES BROWN R. R. CROMBIE D. D.
1966 1967
ECKX.UND W. L. and HARGREA~ES I. K. EGELAND A. and KAUSTWK E. HULTQVIST B . JESPERSEN M., HATJGA. and LANDMARK B.
1968 1967 1966 1966
LERFUD G. M., LITTLE C. C. and PARTHASARATEY R. MAEFILUM B. LandO’BRIEN B. J. REID G. C.
1964 1963 1965
space Sci. Rev. 5, 3 11. M.F., L.F. and V.L.F. Radio Propagation IEE Symposium, London, Nov. 8-10, 1967. .J. Atmosph. Terr. Phys. 30, 265. Ra&o Scz:. 2, 659. Space Sci. Rev. 5, 771. Electron Density Profiles iva Ionosphere a&Exosphere (EditedbyJ.Fs~~~a~N) p. 27, North-Holland, Amsterdam. J. Geophya. Res. 69, 2857. J. Geophye. Res. 68, 997. High Latitude Particles snd the Ionosphere (Edited by B. MAEHLUM) p. 22 1, Lagos Press and Academic Press, New York.