M. Nishino, N. Gorokhov, 2 Y. Tanaka, 3 H. Yamagishi, 4 and T. Hansen s

Transcription

1 Radi Science, Vlume 34, Number 4, Pages , July-August 1999 Prbe experiment characterizing 30-MHz radi wave scatter in the high-latitude insphere M. Nishin, N. Grkhv, 2 Y. Tanaka, 3 H. Yamagishi, 4 and T. Hansen s Abstract. A prbe experiment, cnsisting f radi links between a cmmn 30-MHz transmitter lcated at Murmansk, Russia, and tw receivers used as the imaging rimeter (tw-dimensinal 64 multiple-beam antenna) lcated Ny. lesund, Svalbard, and Tjrnes, Iceland, was carried ut t characterize wave scatter in the high-latitude insphere. They are nearly aligned with and perpendicular t the gemagnetic meridian, respectively. In experiments cnducted in March-April 1994, the 30-MHz prbe signals were identified at nighttime mre frequently than during the day at bth receiver statins during perids f increased gemagnetic activity near the path midpints, indicating that a relatinship between the prpagatin path and the lcatin f the aurral val cntrls signal identificatin. Fr the nighttime prpagatin paths within r crssing thrugh the aurral val, duty cycles f the prbe signals were rughly crrelated with increases in gemagnetic activity. Their arrival directins shwed a spread with a dminant pwer n the lw elevatin and a nrmal distributin in azimuth. These results indicate that the prbe signals are characterized as nnmeteric "aurral E" scatter caused by irregular, large-scale prfiles f electrn density enhancements at the lwer edge f the insphere. Hwever, n 2 days f weak gemagnetic activity, strng prbe signals with bursty behavir were identified by an extremely high duty cycle (---98%) fr the nighttime meridian path nly, and their arrival directin shwed an istrpic spread in azimuth. Such nnmeteric prbe signals are characterized as "cherent" scatter caused by smallscale (---5 m) field-aligned irregularities in electrn density in the E regin insphere, related t "spradic E" ccurrence. 1. Intrductin gins t mnitr the inspheric effects f slar Aurral inspheric anmalies strngly affect high- prtn fluxes [Bailey and Pmerantz, 1965] and enerlatitude radi prpagatin as a functin f frequency. getic electrn precipitatin events [Bailey et al., 1966, The mst prbable causes f aurral inspheric 1970]. Subsequently, Weitzen et al. [1987] perfrmed a effects are inspheric absrptin f radi wave radi cmmunicatin system experiment using VHF prpagatin in the MF and HF bands and scatter and radi links (42.4 MHz) in the aurral regin f refractin frm aurral E and F regin irregularities Alaska, which characterize multipath and fading prin the lw VHF band [Hunsucker, 1992]. In rder t files f the high-latitude meter channel, including investigate the characteristics f scatter prpagatin the effects f aurral scatters and spradic E layers. in the high-latitude insphere, during the Interna- Ostergaard et al. [1991] investigated inspheric distinal Quiet Sun Year (IQSY) perid ( ), turbance effects n a 1200-km high-latitude plar cap meter scatter cmmunicatin link between Sndresix inspheric HF (between 23 and 24 MHz) frward strm and Thule in Greenland. The cmmunicatin scatter paths were installed in the high-latitude relink perated in the VHF band frm 35 t 147 MHz 1 Slar-Terrestrial Envirnment Labratry, Nagya University, during plar cap absrptin (PCA) events, alng with Tykawa, Japan. simultaneus rimeter (relative inspheric pacity 2 Applied Research Center, Academy' f Sciences, Murmansk, meter) bservatins. They fund that all frequencies Russia. 3 Yamaguchi University, Yamaguchi, Japan. lwer than 104 MHz were interrupted fr perids 4 Natinal Institute f Plar Research, Tky, Japan. ranging frm a few hurs t 1 day when the zenith 5 Aurra Observatry, University f TrmsO, TrmsO, Nrway. absrptin f the PCA event exceeded 10 db. Weitzen et al. [1993] further investigated in- Cpyright 1999 by the American Gephysical Unin. Paper number 1999RS /99/1999 RS $11.00 spheric disturbanc effects n scattering prpagatin at 45 MHz ver 2 years, using the plar cap radi link between Sndrestrm and Thule. Using autmatic 883

2 884 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER data prcessing and analysis techniques, they shwed in relatin t gemagnetic activity in the high-latitude that the mnthly average duty cycle fr received regin, and pssible mechanisms f radi wave scatsignals abve dbm was in the range f 2-3% fr ter are discussed. meter scatter bursts, while the duty cycle reached apprximately 20% fr nnmeteric scatter in the 2. Experimental Gemetry and Identificatin summer mnths and was less than 5% in ther f Prbe Signals mnths. They als revealed that the mnthly variatin f the nnmeteric mde was similar t that mea- Figure 1 shws the lcatins f the 30-MHz transsured fr blique prpagatin frm spradic E layers mitter system at Murmansk (MUR) (gegraphi cat temperate latitudes. Hunsucker et al. [1996] fund rdinates 69.0øN, 33.1øE) and the receivers at Ny that the received signals f 25.5-MHz ne-hp mde fklesund (NYA)(78.9øN, 11.9øE) and Tjrnes (TJO) prpagatin n a 950-km Alaskan path exhibited (66.2øN, 17.1øW) n a map f the Eurpean Arctic bursty behavir lasting frm 1 min t ver 3 hurs. regin, which shws the invariant latitudes. The parameters f the tw radi links are as fllws: They believed this behavir t be the first quantitative demnstratin f the "frward prpagatin charac- Fr the Murmansk t Ny desund path the great teristics" f a ne- r tw-hp E prpagatin path circle is apprximately 1260 km lng. The path frm within and parallel t the aurral val n a frequency Murmansk t Ny Alesund is deflected by 340 ø in at the high end f the HF band. Cannn et al. [1996] azimuth frm the gegraphic meridian and is nearly analyzed the duty cycles f VHF sunding signals parallel t the gemagnetic meridian (meridian path). caused by inspheric prpagatin fr bth the plar Fr the Murmansk t Tjrnes path the great circle cap (Sndrestrm t Thule) and aurral (Sndre- is apprximately 2080 km lng. The path frm Murstrm t Narssarssuaq) paths and fund that the mansk t Tjrnes is deflected by 286 ø in azimuth frm prpagatin at 35 and 45 MHz is ften sustained by the gegraphic meridian and is nearly perpendicular spradic E layers and ther nnmeteric mdes t the gemagnetic meridian (lngitudinal path). rather than by meteric scatter. Angling et al. [1998] The transmitting antenna at Murmansk is a hrireprted measurements f Dppler and multipath zntally plarized lg-peridic Yagi which radiates spread n fur high-latitude, high-frequency (3- t maximum pwer t an azimuth f 350 ø (half-pwer 22-MHz) paths in Scandinavia. In the highest-fre- width _+60 ø) at an elevatin angle f 10 ø. The antenna quency band the multipath spread ranged frm 1 t gain is abut 10 db at an elevatin angle f 7 ø, 11 ms, indicating dminant single-mde, single-hp crrespnding t ne-hp E prpagatin prpagatin n the Svalbard t Kiruna path (-1200 sund, and abut 5 db at an elevatin angle f 21 ø, km length). crrespnding t ne-hp F prpagatin. On the As described abve, the prpagatin characteristics ther hand, the antenna gain is 1.0 db at an elevatin f the VHF scatter mdes have been dcumented, angle f 1.2 ø, crrespnding t ne-hp E prpagabut the scatter mechanisms, particularly, fr the nn- tin t Tjrnes, and 1.1 db at an elevatin angle f meteric mde have nt been fully understd. 10 ø, crrespnding t ne-hp F prpagatin. The purpse f this study was t examine prpaga- The maximum blique frequency f which can be tin effects f 30-MHz wave scatter in the high- supprted by an inspheric layer critical frequency, latitude insphere. In this paper we present the fc, can be apprximated by characteristics (identificatin prbability, duty cycle, f = fc sec 4' and arrival directin) f 30-MHz prbe signals btained using tw radi links in the aurral and plar fr shrt grund ranges, where (b is the incident angle cap regins. The tw 30-MHz radi links cnsist f a f the equivalent ray path t the nrmal t the cmmn transmitter at Murmansk, Russia, and re- insphere at the reflectin pint [Milan et al., 1997]. ceivers at Ny 3,1esund, Svalbard, and Tjrnes, Ice- The rati f/fc is knwn as the secant factr. Table 1 land. They are nearly aligned with and perpendicular shws calculated secant factrs fr varius prpagat the gemagnetic meridian, respectively. Using an tin mdes n the paths t Ny fklesund imaging rimeter with a tw-dimensinal 64 multiple- Dav [1990] revealed that the layer critical frequenbeam antenna was a great advantage fr this prbe cies (fc) were apprximately 3 and 2.5 MHz fr experiment. The characteristics f the prbe signals ne-hp E and ne-hp F, respectively, frm the btained frm March t April in 1994 are presented mde structure diagrams fr Andya (66øN gemag- t Ny 3de- and Tjrnes.

3 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER ø N 70 ø N DMH BJN 60 ø N TJO 20 ø W 0 ø Figure 1. Lcatins f the 30-MHz transmitter at Murmansk (MUR), Russia, and the receivers used as the imaging rimeters at Ny lesund (NYA), Svalbard, and Tjrnes (TJO), Iceland, in the Eurpean Arctic regin. The gemagnetic statins Bjrnya (BJN) and Jan Mayen (JAN) are als displayed. netic latitude) Nrway, fr March, at 0000 UT n the MUR-TJO path). It is again evident that the 30 MHz gemagnetic index (Kp = 2.2), with a nrthward exceeds the maximum blique frequency. These valgrund range f apprximately 1200 km (which is ues are similar fr the daytime (1200 UT) mde similar t the MUR-NYA path). Frm the prducts structure diagrams. Therefre it is predicted that f these layer critical frequencies and the secant 30-MHz radi waves frm Murmansk seldm reach factrs, it is evident that the 30 MHz exceeds the either Ny lesund r Tjrnes by ne-hp E and F maximum blique frequency. On the ther hand, the laye reflectin under quiet gemagnetic cnditins. layer critical frequency was abut 3 MHz fr bth the The transmitter radiates cntinuus waves with a ne-hp E and ne-hp F, with a westward grund pwer f apprximately 20 kw and a special mdurange f rughly 2000 km (which is similar t the latin f 1-min pulse and 1-min pause s that the prbe signals can be reliably identified frm the backgrund radi nise and ther interference. The Table 1. Secant Factrs fr Varius Prpagatin Mdes transmissin was scheduled t last fr 2 hurs during n the Paths t Ny lesund and Tjrnes the day ( UT) and 2 hurs at night (2000- Reflectin Secant Factr 2200 UT) every day frm March 28 t April 29, Height, The imaging rimeter fr inspheric study (IRIS) Mde km Ny fklesund Tjrnes was first develped n a 38.2-MHz frequency at the 1E Suth Ple, t determine the tw-dimensinal in- 2E spheric absrptin f csmic nise (CNA) assciated 1F with aurral disturbances [Detrick and Rsenberg, 2F ]. The imaging rimeters (30.0 MHz) installed at

4 886 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER Main Lbe Main Lbe N6 Grating L ( ) (b) Figure 2. (a) Antenna beam pattern in the nrth-suth plane f the imaging rimeter. The main eight lbes are assigned t be N1, N2,..., N8 frm nrth t suth, and the three crrespndingrating lbes are assigned t be GN1, GN2, and GN3, directed suthward. (b) Antenna beam pattern in the east-west plane. The main eight lbes are assigned t be El, E2,..-, E8 frm east t west, and the three crrespndingrating lbes are assigned t be GE8, GE7, and GE6, directed eastward. Ny lesund and Tjrnes have almst identical per- statin preferentially. Hwever, it shuld be nted frmance characteristics. Each antenna cnsists f a that grating lbes frm utside f the eight main tw-dimensinal 8 x 8 array f half-wavelength di- lbes. Fr example, in the pattern in Figure 2a a ples which are nearly aligned with the gemagnetic grating lbe (GN1) directed suthward is frmed, nrth-suth (N-S) and east-west (E-W) planes. Each array cnsists f eight linearly plarized diples f a half wavelength lng spaced 0.65 wavelength apart hrizntally. The linear diple elements are aligned in the gemagnetic nrth-suth plane and elevated by a quarter wavelength. Many wires are stretched in the gemagnetic N-S directin n the grund under the diple elements as a reflectr. Tw-dimensinal 8 x 8 beams are frmed by the rthgnal cmbinatin f which crrespnds t the main lbe (N1). Likewise, GN2 crrespnds t N2, and s n. Similar grating lbes frm in the EW plane f the pattern in Figure 2b. This results in ambiguity, depending n whether the wave arrives at a main lbe r the crrespnding grating lbe. Hwever, since the transmitter lcatin is knwn in this prbe experiment, the grating lbes are available fr measuring wave arrival with lw elevatin angles utside the main lbes. the eight lbes using a tw-dimensinal Butler phas- Figure 3 shws an example f 30-MHz signals ing matrix. The technical details, including data pr- received at Ny.3desund between 2000 and 2200 UT cessing, are reprted by Yamagishi et al. [1992] and Nishin et al. [1993]. Figure 2 shws tw directinal beam patterns in the N-S (Figure 2a) and E-W (Figure 2b) planes frmed n April 21, The figure shws the time variatin in signal intensity n the specific beam N3E4 ver time. In this case, the sidereal variatin f the csmic radi nise is reduced t zer. A pulse/pause mduby the eight-element array with 0.65 wavelength spacing. The N-S pattern is btained assuming the grund latin with a 1-min interval is evident in the variatin reflectin cefficient f 0.5. Since the wires n the grund are stretched in the gemagnetic N-S directin, the grund is nt a perfect cnductr fr waves arriving in a nrth-suth directin. The E-W pattern is btained assuming a reflectin cefficient f 1.0 (perfect cnductr) fr waves arriving frm the eastwest. The spacing f 0.65 wavelength was determined t achieve a high spatial reslutin (half-pwer beam width f 11 ø) in the inspheric field f view ver the in intensity, identifying the prbe signals frm the 30-MHz transmitter. Fr 2 hurs, strng prbe signals shws bursty behavir, and rughly half f all pulsed signals are saturated due t limitatins in the receiver's dynamic range. The intensity f the backgrund csmic radi nise has a brightness temperature f apprximately 15,000 K at 30 MHz [Cane, 1978]. Therefre the saturating intensity is estimated t be abut 21 db higher than the detectin threshld (-120 dbm).

5 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER APRIL 21, 1994 Ny Alesund 150, ',, t i i I i i i i i i i i 20:00 20:30 21:00 21:30 22:00 Universal Figure 3. Time variatin f the signal intensity n the specific beam (N3E4) f the NYA antenna during UT n April 21, time Plate 1 shws a time series f the intensity images f the 30-MHz signals frm 2048:05 t 2117:57 UT n April 21 at Ny Alesund, which are prduced frm the 8 x 8 beam signals. Each image is averaged ver 128 s. The images f the backgrund csmic radi nise are seen in the nrthern sectr in the IRIS field f view, and lcal, artificial interference nise (rectangular shape) is seen in the suthern sectr. Enhanced and scattered images (the yellw clr) f the prbe signals are superpsed n the nise between 2054:29 and 2113:41 UT. As is illustrated by the beam pattern in Figure 2a, since the images in the nrthern sectr prbably crrespnd t the prbe signals received by the suthern grating lbes utside the main lbes, the dminant arrival directin f the prbe signals is suthward. On the ther hand, dt-like images, which shw the strngest intensity and a small-scale structure (the red spts), are seen near the central part in the IRIS field. These are islated frm the scattered images described abve. This suggests that the waves arrived frm the zenith thrugh the prpagatin within small-scale ducts r via field- aligned irregularities in inspheric electrn density prfiles ver the statin. Figure 4 shws an example f the 30-MHz signal intensity in a specific beam (N4E7) frm 2000 t 2200 UT n April 21 at Tjrnes. The strng pulse signals reached saturatin arund 2130 UT with a shrt- duratin (10- t 20-min) burst, while n pulse signal was recrded frm 2000 t 2026 UT. The intensity variatin belw zer level between rughly 2040 and 2123 UT crrespnds t the inspheric absrptin f csmic radi nise (CNA). The intensity images shw enhancement f the scattered structure in the western part f the IRIS field at 2056:17 UT, which crrespnds t the arrival f lwer elevatin angle signals n the eastern grating lbes, and als shw secndary enhancement n the real eastern beams. The dt-like images seen near the zenith in Plate 1 were nt seen at Tjrnes (nt shwn). In the examples frm the night f April 21 shwn in Figures 3 and 4, the gemagnetic activity was weak (Kp and 3- fr UT and UT, respectively). The prbe signal identificatin in this situatin will be discussed later. 3. Signal Identificatin Prbability and Gemagnetic Activity As described in the previus sectin, we have reasns t assume that the 30-MHz radi waves prpagate t the receiver statins by the ne-hp E mde. Therefre we examined the relatinship between the 30-MHz signals and the gemagnetic H

6 ß ~. ß 888 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER O.,.. (D N mw mm. J J (D.,...,.. (D.,.. (D (D (D (D

7 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER APRIL 21, 1994 Tjrnes :00 20:30 21:00 21:30 22:00 Universal Figure 4. Time variatin f the signal intensity n the specific beam (N4E7) f the TJO antenna during UT n April 21, time cmpnents at Bjrnya (BJN) (74.5øN, 19.2øE) fr the meridian path and at Jan Mayen (JAN) (70.9øN, 8.7øW) fr the lngitudinal path. These statins are lcated near the midpints f the respective path (Figure 1). The peak-t-peak value f the H variatin fr each 2-hur perid was used t characterize the gemagnetic activity. A perid was defined as quiet if this value is less than 200 nt and as active if it was abve 200 nt. The prbe signal was detected if it was twice as strng as the csmic nise fr at least tw successive pulses. The bserving perids have been srted int fur grups: (1) nighttime and quiet gemagnetically, (2) nighttime and active gemagnetically, (3) daytime and quiet gemagnetically, and (4) daytime and active gemagnetically. Table 2. Identificatin Prbability f the Prbe Signals fr Ny,3desund and Tjrnes as a Functin f the Gemagnetic H Deflectin at Bjrnya and Jan Mayen AH Ny 3Aesund Tjrnes Nighttime ( UT) <200 nt 47% 56% >200 nt 71% 81% Daytime ( UT) <200 nt 6% 0% >200 nt 43% 0% The signal identificatin prbability was calculated fr each grup. The identificatin prbability is the rati f the number f perids when the signal was detected at least nce t the ttal number f perids in the grup. The results are given in Table 2. The prbability increased during gemagnetic active perids, bth at night and during the day. At night in particular, the prbability is very high (70-80%) when cnditins fr bth the meridian and the ln- gitudinal paths are disturbed. This result indicates that the high identificatin prbability is strngly related t aurral disturbances near the path midpints. In the daytime the prbability als becmes high with increasing gemagnetic activity n the meridian path, althugh it is lwer than the nighttime value. The difference between the nighttime and daytime prbabilities is briefly discussed belw in relatin t the lcatin f the aurral val [Ostergaard et al., 1991; Angling et al., 1998]. Figure 5 shws the cnfiguratin f the tw radi links and aurral val lcatins at 0000 UT (Figure 5a) and 1200 UT (Figure 5b) [Akasfu, 1979]. The lcatin f the aurral val is apprximated t the psitins fr high gemagnetic activity (Kp = 5) at 0000 UT and lw gemagnetic activity (Kp = 0) at 1200 UT, as given byangling et al. [1998]. The duble circle shws the directin f the Sun. In the nighttime

8 890 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER the meridian and lngitudinal paths are lcated at the lwer latitudes utside the aurral val, prviding supprt fr the lwer (6%) and zer (0%) prbability in the daytime values in Table 2. The aurral val shifts equatrward with increasing gemagnetic activities [e.g., Feldstein and Starkv, 1967; Hlzwrth and Meng, 1975], and therefre the pleward part f the meridian path might enter the val, resulting in an increase in prbability f up t 43% under disturbed cnditins. This summary reveals that the mutual cnfiguratin f the 30-MHz prpagatin path and the aurral val lcatins essentially cntrls the identificatin prbability f the prbe signals. (b) Figure 5. The tw radi links (MUR-NYA and MUR- TJO) and the aurral vals fr (a) 0000 UT and (b) 1200 UT, sketched by Akasfu [1979]. at 0000 UT, mst f the meridian path (MUR-NYA) crsses thrugh the aurral val, and the lngitudinal path (MUR-TJO) lies entirely within the aurral val fr high gemagnetic activity. This relatinship prvides supprt fr the higher nighttime prbability shwn in Table 2. On the ther hand, in the daytime at 1200 UT, bth 4. Duty Cycle f the Prbe Signals With Gemagnetic Activity The identificatin prbability described in sectin 3 is statistically evaluated whether the 30-MHz radi waves culd prpagate ver 1000 km in the high latitudes depending n gemagnetic cnditins. In this sectin we examine the duty cycles f the prbe signals. These are the mst imprtant factrs fr quantitatively evaluating the feasibility f a radi link, particularly frm a radi cmmunicatin pint f view, in relatin t gemagnetic activity. The duty cycle is generally defined as the percentage f the transmissin time that the prbe signals are detected. Figure 6 shws day-t-day variatin in the Kp index during the nighttime experiment (Figure 6a) and duty cycles (slid bars) f the 30-MHz nighttime Prbe signal fr the meridian path (Murmansk-Ny Alesund), alng with peak-t-peak intensities f the gemagnetic H deflectins at Bjrnya (Figure 6b). The prbe signals are taken frm the N2E4 beam, whse grating lbe lks suthward. The required threshld f the duty cycle is twice the csmic nise level. It appears that the duty cycles were crrelated with increase in the H intensities and Kp index between April 2 and 8. Unfrtunately, IRIS data acquisitin failed between April 9 and 19. Hwever, the crrelatin cefficient is lw (---0.1) during all days. This results frm the fact that the duty cycles are extremely high (---98%) n April 21 and 28, in spite f weak gemagnetic activities (<100 nt). The circles in Figure 6 represent the percentage f the time that CNA was bserved simultaneusly. In the daytime the prbe signals fr the meridian path were nly identified n 7 days during the experiment, and their duty cycles were less than 10% (nt shwn). It shuld

9 NISHINO ET AL.' HIGH-LATITUDE VHF WAVE SCATTER (b) ; Z DATE Figure 6. Day-t-day variatins f Kp index Figure 6a) and duty cycles (in percent) f the prbe signals in the nighttime ( UT) at Ny Alesund (slid bars) and the peak-t-peak intensities f the gemagnetic H deflectins at Bjrnya (squares). The prbe signals are taken frm the N2E4 beam. Time rate (in percent) f inspheric absrptin (CNA) is als displayed by pen circles (Figure 6b). be nted that the prbe signals were nt identified in dinal path (Murmansk-Tjrnes), alng with the peakthe daytime n April 21 and 28. t-peak intensities f the H deflectins at Jan Mayen. Figure 7 shws the day-t-day variatin in the duty The prbe signals are taken frm the N3E7 beam, cycles f the nighttime prbe signals fr the lngitu- whse grating lbe lks eastward. The duty cycle was loo a = =,< < <,< <,< < < < <,<,< <,< DAYE Figure 7. Day-t-day variatin f duty cycles (in percent) f the prbe signals in the nighttime at Tjrnes and the peak-t-peak intensities f the gemagnetic H deflectins at jan Mayen. The prbe signals are taken frm the N3E7 beam.

10 892 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER (b) N APRIL 7, 1994 NY ALESUND E4-ARRAY db 3O --'-----'-- maximal (abut 63%) n April 18. The relatinship between the duty cycles and the H deflectins had a better crrelatin cefficient (-- 0.5) during the day than that fr the meridian path. A reasnable explanatin is that the lngitudinal path lies entirely within the aurral val. It shuld be nted that the duty cycles are apprximately 30% and 0% n April 21 and 28, respectively. These values are smaller than thse fr the meridian path. The crrelatin cefficients between the Kp index and the H deflectins during the prbe experime _ S ø0ø were 0.70 and 0.81 fr Bjrnya and Jan Mayen *'O respectively, indicating thathe increase the duty [e,, cycles is assciated with The bservatins large-scale gemagne n the meridian (au rral) disturbances. ' path n April 21 and 28 are excepti Arrival Directin f the Prbe Signals It is generally understd that irregular prfiles f electrn density in the E regin insphere scatter bliquely prpagated radi waves in VHF band [e.g., db Hunsucker, 1992]. Therefre the arrival directin f POWER INTENSITY APRIL 7, 1994 NY LESUND GN3-ARRAY / ß,!.1 2O db POWER INTENSITY Figure 8. Arrival directins f the nighttime prbe signals at NYA n April 7, Shwn are (a) the zenith angle distributin in the E4 array with pwer intensity and (b) the azimuth angle distributin in the GN3 array with pwer intensity. incming prbe signals is spread in multiple direc- tins. Measuring the arrival directin f radi waves is essential t discussin f wave scattering at the reflectin pint in the insphere. The imaging rimeter used in this prbe experiment has a tw-dimensinal antenna pattern f 64 pencil beams, as described in sectin 2, enabling it t directly measure the spread f the arrival directins. Examples f the measured arrival directins f the prbe signals n the meridian path during the strng and weak gemagnetic activity are given belw. Figure 8 shws the distributin f the arrival directins f the nighttime signals n April 7 at Ny fidesund. The upper plar diagram (Figure 8a) dis- plays a zenith distributin f the signal pwer intensities in the NS plane fr the E4 array, in which the intensities are nrmalized with a reference f the N5E4 beam near the zenith, based n the beam pattern in the NS plane shwn in Figure 2a. The pwer intensities at the respective beams are aver- aged values. On April 7 the gemagnetic intensity was large ( nt) at Bjrnya, and the duty cycle was 10% (see Figure 6). The lwer plar diagram (Figure 8b) displays an azimuth distributin f the signal pwer intensities in the EW plane fr the GN3 array. The intensities are nrmalized with a reference f the GN3E4 beam, based n the beam pattern in the EW plane shwn in Figure 2b. Frm these

11 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER 893 diagrams it is seen that the zenith distributin is widely spread frm the zenith in the suth directin, with the maximum intensity in the lwest elevatin beam (82 ø suth). The azimuth distributin is nearly a nrmal distributin, with the maximum intensity at the central beam within _+ 42 ø. Figure 9 shws the distributin f the arrival directins f the nighttime signals n April 21 at Ny Xlesund. The upper plar diagram (Figure 9a) dis- plays a zenith distributin in the NS plane fr the E4 array and an azimuth distributin in the EW plane fr the GN3 array (Figure 9b). On April 21 the prbe signals were the mst enhanced and had an extremely high duty cycle (Figure 6), with weak gemagnetic activity (<100 nt). In these diagrams the zenith distributin is widely spread, with the maximum intensity in the suthernmst beam, as als seen in Figure 8a. The azimuth distributin is als spread, but there is a significant difference frm Figure 8b. In Figure 9b the distributin has a near-istrpic spread within +_42 ø, nt a nrmal distributin. This spread suggests that a different wave scatter mechanism frm that seen n April 7 was at wrk in the E regin insphere. (a) N APRIL 21, 1994 NY ALESUND E4-ARRAY db db POWER INTENSITY 6. Discussin We cnducted a prbe experiment f the 30-MHz radi wave prpagatin using meridian and lngitudinal links in the high-latitude regins. We fund that the identificatin prbability f the prbe signal increased with the gemagnetic activity near the path midpints. Mrever, the duty cycles f the nighttime prbe signals tended t increase with the gemagnetic activity fr prpagatin paths within r crssing thrugh the aurral val. These characteristic suggest that the 30-MHz radi waves can prpagate by nnmeteric mde, prbably by "frward scatter" in the aurral-e insphere near the path midpint, where the electrn density is markedly enhanced and shws irregular prfiles by the precipitatin f aurral electrns (several t several tens f kev energies). An earlier experiment using HF frward scatter (between 23 and 24 MHz) t mnitr inspheric effects f slar prtn fluxes n the 1000-km aurral prpagatin path fund that a scattering prcess ccurred at an altitude f km depending n sunlight cnditins at the path midpint. Enhanced inizatin belw the nrmal scattering layer de- creased the intensity f the received signal [Bailey and Pmerantz, 1965]. Analysis f the frward scatter (b) APRIL 21, 1994 NY Jff, ESUND GN3-ARRAY W db POWER E INTENSITY Figure 9. Arrival directins f the nighttime prbe signals at NYA n April 21, Shwn are (a) the zenith angle distributin in the E4 array with pwer intensity and (b) the azimuth angle distributin in the GN3 array with pwer intensity.

12 894 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER Figure 10. Mdel f the sinusidal prfile f electrn density at the lwer edge f the aurral E regin [after Ktsuka, 1995]. recrds tgether with rimeter data led t the recg- between kh = 1.0 and 3.0, shwn in Figure 11, nitin f a daytime aurral absrptin event charac- resembles the measured zenith distributin (Figures terized by enhanced inizatin at lw altitudes (_-<75 8a and 9a). Hwever, since the scattering surface at km). These events were attributed t an influx f the lwer edge f the aurral E insphere is actually lcally precipitated electrns with higher energies irregular in tw dimensins and varius spatial-scale (_->400 kev) t penetrate t a height belw km prfiles, the distributin f the arrival directin is [Bailey et al., 1966]. Using the inspheric scatter cmplicated. Further analysis is required t mdel experiment (frequency MHz) and CNA bser- the scattering surface at the aurral insphere. vatins f energetic electrn precipitatin during daylight, Bailey et al. [1970] als determined that the effective scattering height in the aurral insphere was 75 _+ 2 km. During ur prbe experiment, hwever, there was n slar prtn event in the high-latitude regin. Therefre we believe that a nrmal scattering layer (-100 km altitude) was present in the E regin insphere. In the previus sectin we demnstrated the arrival directin distributin f the nighttime prbe signals fr the meridian path n tw specific days n strng and weak gemagnetic activities, fr which we btained scatter distributins in azimuth and zenith angles. In the fllwing we discuss scatter mechanisms f the 30-MHz radi waves frm the distribu- tins f the meridian path. As described in sectin 2, we predict that the 30-MHz wave prpagatin is predminantly ne-hp E mde fr the meridian path (-1200 km). At night in winter ne ften sees curtain-like aurras, in which the lwer edge appears irregular and has a variable structure with multiple rays. These structures represent spatial variability in electrn density prfiles. Here we cnsider the simplest case, a sinusidal prfile f electrn density variability at the lwer edge f the E insphere. In this case, enhanced and depressed areas f electrn density appear repeatedly alng the prpagatin path, as shwn in Figure 10, where A is the spatial wavelength, L is the ttal length f the sinusidal prfile, and h is the amplitude. Ktsuka [1995] calculated the scattered electric fields fr a wave incidence t this sinusidal prfile using the Kirchhff apprximatin methd. The radius f curvature (rc) f the sinusidal prfile is very much larger than the wavelength (-10 m) f the incident radi wave, that is, 2kr c cs 01 >> 1, where k is the wave number (k = 2rr/A) and 01 is the incident angle frm the nrmal t the curvature. He demnstrated varius directinal patterns f scattered electric fields fr the incident angle f 80 ø (apprximately the incident angle fr a ne-hp E reflectin n a 1200-km path) and a spatial wavelength A - 10A, as a parameter f kh. Of these patterns, ne pattern = A=IOA, 0=80 ø, kh ' _ 60 ø 70' p [db) A = 10 A, 0,=80 ø, kh =1.0 Figure 11. Directinal patterns (02) f scattered electric fields t the incident wave (0-80 ø) fr the spatial wavelength, A - 10X, and kh = 1.0 and 3.0 n sinusidal prfile [after Ktsuka, 1995].

13 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER 895 On the ther hand, the measured azimuth distributin had a nrmal distributin, with peak intensity at the central beam and spread f abut +_20 ø (Figure 8b). Using Dppler measurement f meter burst cmmunicatin channel at 42.4 MHz n a 430-km path in the aurral regin in Alaska, Weitzen et al. [1987] btained a multiple-path prfile f an average delay time f 806 +_ 174 /xs (mean _+ standard deviatin) fr direct prpagatin f apprximately 1.5 ms between the transmitter and receiver. This spread crrespnds t the average spread f abut 40 ø in azimuth angle. They als revealed that the aurral E scatter was crrelated with the standard gemagnetic indexes. Cnsequently, the inspheric effect f the frward scatter at the lwer edge f the aurral E insphere assciated with gemagnetic disturbances seen with the 30-MHz prbe signals is characterized as "aurral E" scatter. Hunsucker et al. [1996] characterized the 25.5-MHz ne-hp mde prpagatin inside and parallel t the aurral val as "frward prpagatin characteristics" by "aurral E" inizatin, and the waves seem t be refracted by aggregatins r "cluds" f inizatin irregularities. This is essentially the same with the aurral E scatter nted here. Hwever, they demnstrated a lng-duratin "aurral E burst" bserved in lcal daytime in the midsummer during quiet gemagnetic perids and indicated that this may pssibly be the "midlatitude spradic E" mde. Therefre we shuld pint ut that the nighttime prbe signals fr the meridian path n April 21 and 28 during weak gemagnetic activity had very strng intensities with bursty behavir, resulting in the extremely high duty cycles (98%). Furthermre, the azimuth distributin had an istrpic spread. These characteristicsuggest a different scatter mechanism frm aurral E scatter. Weitzen et al. [1993] investigated 45-MHz frward scatter waves using the plar cap radi link between Sndrestrm and Thule (Greenland) alng the meridian. They fund that the nnmeteric duty cycle exceeded the meteric cycles, with a prbability f % at all the times during June-August, except fr the time-perid UT. They inferred that the nnmeteric prpagatin with such high prbability is prbably due t a spradic E layer in the plar cap insphere. Cannn et al. [1996] reprted that a spradic E duty cycle at midsummer and at 35 MHz, reaching a peak clse t 100%, was btained fr the plar cap path. We discuss the causative characteristics f such high duty cycle signals belw. Haldupis et al. [1989] pinted ut that nndeviative inspheric absrptin ptentially attenuates the 50-MHz backscatter. Since inspheric absrptin takes place at the D regin altitudes primarily, the backscattered waves wuld be markedly attenuated because the radi wave spends a large part f its prpagatin time penetrating the D regin bliquely. They examined day-t-day variatins f 50-MHz backscattered amplitude, gemagnetic Kp index, and rimeter absrptin bserved at Cape Parry (gemagnetic latitude 75øN) during a 2-week campaign perid and fund a clse crrelatin between the ccurrence f strng VHF eches and the appearance f a prnunced spradic E type trace in the ingrams, indicating relatively weak nndeviative absrptin in the D regin. Cnsequently, they revealed that fairly strng scatter ccurred during perids f weak gemagnetic activity (lw Kp). This indicates a pr crrelatin between 50-MHz backscatter amplitude variatin and gemagnetic activity. In Figure 6 we als shw the variatin f the CNA time rate (in percent) bserved with the imaging rimeter at Ny Alesund fr 2 hurs at night. The CNA data frm Bjrnya, the path midpint, might be mre apprpriate, but unfrtunately we d nt have thse data. The relatively lw duty cycles (10-25%) f the prbe signals n April 3 and 6-8 might have been influenced by increased absrptin n the blique prpagatin in the D regin, assciated with increased gemagnetic activities ( nt). Hwever, there was n CNA n April 21 and 28 during the perids f weak gemagnetic activity. Furthermre, the duty cycles n the ther days f weak gemagnetic activity (<100 nt) were less than 20%, fr example, n April 20, 25, and 26. Therefre the characteristics f the strng signals n April 21 and 28 agree well with the strng backscatter eches bserved during perids f lw Kp by Haldupis et al. [1989]. Radar bservatins f aurral backscatter are based n the premise that the radar eches are received preferentially frm the regin where the line-f-sight wave vectr intersects the Earth's magnetic field at clse t right angles in the regin where scattering tk place [e.g., Haldupis, 1989]. In a theretical study, Villain et al. [1984] fund that the 24-MHz radi waves achieve magnetic field perpen- dicularity (aspect angle within 1 ø) in the E regin at an altitude f abut 100 km at a grund range f km. Then they are scattered back by large-scale (1- t 100-km) enhancement arrays f

14 896 NISHINO ET AL.' HIGH-LATITUDE VHF WAVE SCATTER 8O' N 75' N 10' E 20' E 30' E 7O' N Figure 12. Cnturs f mean aspect angles f +1.0 ø, _0.5 ø, and -1.0 ø t the Earth's magnetic fields in the area between Murmansk and Ny i, lesund. electrn density elngated alng the Earth's magnetic field. Here we examine the pssibility that the 30-MHz radi wave vectrs achieve perpendicularity with the Earth's magnetic field in the high-latitude regin between Murmansk and Ny desund. The regin within _+ 45 ø in azimuth at Ny Alesund is between 75 ø and 125 ø in azimuth in the suthward sectr (the dtted lines in Figure 12). The mean aspect angle between the angle at which the line-f-sight path frm the transmitter t a scatter pint at 100-km altitude [McNamara, 1971] intersects the magnetic field and the angle at which the line-f-sight path frm the scatter pint t the receiver intersects the magnetic field is calculated fr sme grid pints in the sectr. Dip angles in the Earth's magnetic fields at the grid pints are determined frm the T-96 mdel under quiet gemagnetic cnditins using the methd f Tsyganenk and Stem [1996]. Figure 12 shws the cnturs fr calculated mean aspect angles f ø, _+0.5 ø, and -1.0 ø. In this figure we see that magnetic field perpendicularity (aspect angles within 1 ø) is achieved in narrw gegraphic latitudes between 73øN and 76øN and extend frm apprximately 5 ø t 45øE lngitude. Villain et al. [1987] bserved highlatitude inspheric irregularities drifting alng L cnturs in the night sectr at E altitude using the E Gse Bay HF radar. Therefre their arrival directins f the prbe signals wuld shw a near-istrpic distributin, as seen in Figure 9b. The 50-MHz backscatter bservatins, made using tw bistatic CW radi links in the plar cap inspheric E regin in the Canadian Arctic, shwed that the scattering regin was cnfined t invariant magnetic latitudes f 76 ø and 77 ø. As a result, the magnetic aspect angles ranged frm 9.5 ø t 13.5 ø frm the perpendicular t the Earth's magnetic field [Haldupis et al., 1989]. In the aurral and subaurral regins ( ø gegraphic latitudes) f Canada the aspect angles fr the backscattered eches (50 MHz) btained with the Mint-Suhend-Saskatn radi link ranged frm 3.5 ø t 9.0 ø [Kehler et al., 1985]. Cnsidering the effects f n-perpendicular aspect angles, these experimental results indicate that the effects f the aspect angles are mre cmplex and that they may spread mre widely ver _+ 45 ø in azimuth. Cnsequently, the strng prbe signals with the extremely high duty cycles n April 21 and 28 are caused by "cherent" scatter due t small-scale (---5 m) field-aligned irregularities f electrn densities in the high-latitude E insphere. This cherent scatter is characterized by nnmeteric mde due t a spradic E insphere. 7. Cnclusin The prbe experiment cnducted in March and April using tw radi links at 30 MHz cnsisting f Murmansk-Ny Alesund and Murmansk-Tjrnes paths has prvided significant infrmatin n the characteristics f VHF wave prpagatin in the highlatitude insphere. In cnclusin, frm the prbe experiment we characterized tw different mechanisms f "aurral" and "cherent" scatter fr the nnmeteric mde f the 30-MHz waves in the high-latitude insphere. The experiment perids crrespnded t a phase f increasing mnthly variatin f nnmeteric duty cycles at 35 MHz [Cannn et al., 1996]. Cannn et al. [1996] als demnstrated that during the winter the plar cap nnmeteric duty cycle decreases as Kp increases, while during the summer there is n such variatin. The summer perid f nnmeteric transmissin is dminated by the wind shear-driven spradic E, fr which n Kp dependence is expected. Further experiments in ther mnths shuld prvide us definite infrmatin n the mechanisms driving the spradic E layer. Simultaneus Dppler measurements with the arrival direc-

15 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER 897 tin f 30-MHz prbe signals will be helpful in further studies f the scatter mechanisms related t field- aligned irregularities f electrn densities in the aurral and plar cap E regin [Haldupis et al., 1989; Sfk et al., 1987; Kustv et al., 1995]. Acknwledgments. This study was supprted by cllabrative research between Applied Research Center (ARC), Academy f Sciences, Murmansk, Russia, and Slar-Terrestrial Envirnment Labratry, Nagya University. We are grateful t ARC fr the peratin f the transmitter at Murmansk and t the Nrwegian Plar Research Institute fr the supprt f imaging rimeter data acquisitin at Ny lesund. A part f the data analysis was supprted by the JSPS fellwship fr research in Japan. We als appreciate T. Ota, Graduate Schl f Engineering f Nagya University (presently at Mitubishi Electric Cmpany in Japan), fr his supprt f the IRIS data analysis. We acknwledge T. Ogawa, Nagya University, fr his prvisin f useful cmments. References Akasfu, S.-I., Aurra brealis, Alaska Gegr., 6(2), 54-55, Angling, M. J., P.S. Cannn, N. C. Davies, T. J. Willink, V. Jdalen, and B. Lundbrg, Measurements f Dppler and multipath spread n blique high-latitude HF paths and their use in characterizing data mdem perfrmance, Radi Sci., 33(1), , Bailey, D. K., and M. A. Pmerantz, Relativistic electrn precipitatin int the messphere at subaurral latitudes, J. Gephys. Res., 70(23), , Bailey, D. K., M. A. Pmerantz, K. W. Sullivan, and C. C. Taieb, Characteristics f precipitated electrns inferred frm inspheric frward scatter, J. Gephys. Res., 71(21), , Bailey, D. K., R. R. Brwn, and M. H. Rees, Simultaneus frward-scatter, rimeter, and bremmstrahlung bservatins f a daytime electrn precipitatin event in the aurral zne, J. Atms. Terr. Phys., 32, , Cane, H. V., A 30 MHz map f the whle sky, Aust. J. Phys., 31, , Cannn, P.S., J. A. Weitzen, J. Ostergaard, and J. E. Rasmussen, Relative impact f meter scatter and ther lng-distance high-latitude prpagatin mdes n VHF cmmunicatin systems, Radi Sci., 31 (5), , Davfi, N., The use f mde structure diagrams in the predictin f high-latitude HF prpagatin, Radi Sci., 25(4), , Detrick, D. L., and T. J. Rsenberg, A phased-array radi wave imager fr studies f csmic nise absrptin, Radi Sci., 25(4), , Feldstein, Y. I., and G. V. Starkv, Dynamics f aurral belt and plar gemagnetic disturbances, Planet. Space Sci., 15, , Haldupis, C., A review f radi studies f aurral E-regin inspheric irregularities, Ann. Gephys., 7(3), , Haldupis, C., M. J. McKibben, J. A. Kehler, and G. L. Sfk, 50 MHz backscatter bservatins in the plar cap inspheric E regin, in Electrmagnetic Cupling in the Plar Clefts and Caps, edited by P. E. Sandhlt and A. Egeland, pp , Kluwer Acad., Nrwell, Mass., Hlzwrth, R. H., and C.-I. Meng, Mathematical representatin f the aurral val, Gephys. Res. Lett., 2(9), , Hunsucker, R. D., Mini-review. Aurral and plar-cap inspheric effects n radi prpagatin, Prc. IEEE, Antennas Prpag., 40(7), , Hunsucker, R. D., R. E. Rse, R. W. Adler, and G. K. Ltt, Aurral-E mde blique HF prpagatin and its dependence n aurral val psitin, IEEE Trans. Antennas Prpag., 44(3), , Kehler, J. A., G. J. Sfk, V. Mehta, A. G. MacNamara, and D. R. McDiamnd, Observatins f magnetic aspect effects in aurral radar backscatter, Can. J. Phys., 63, , Ktsuka, Y., Electrmagnetic Wave Analysis (in Japanese), pp , Crna, Tky, Kustv, A. V., G. C. Hussey, J. A. Kehler, G. L. Sfk, and J. Mu, Spectral width f type 2 cherent eches at large magnetic aspect angles, J. Gephys. Res., 100(A4), , McNamara, A. G., Survey f radi reflectins frm aurra, in The Radiating Atmsphere, edited by B. M. McCrmac, pp , D. Reidel, Nrwell, Mass., Milan, S. E., T. B. Jnes, and E. M. Warringtn, Enhanced MUF prpagatin f HF radi waves in the aurral zne, J. Atms. Sl. Terr. Phys., 59(2), , Nishin, M., Y. Tanaka, T. Oguti, H. Yamagishi, and J. A. Hltet, Initial bservatin results with imaging rimeter at Ny-Alesund (L = 16), Prc. NIPR Symp. Upper Atms. Phys., 6, 47-61, Ostergaard, J. C., J. A. Weitzen, P. A. Kssey, A.D. Bailey, P.M. Bench, S. W. Li, J. R. Katan, A. J. Criaty, and J. E. Rasmussen, Effects f absrptin n high-latitude meter scatter cmmunicatin systems, Radi Sci., 26(4), , Sfk, G. L., J. A. Kehler, C. Haldupis, J. McKibben, and A. G. McNamara, Dppler radi bservatins f 3-m irregularities in the plar cap E regin, J. Gephys. Res., 92(A2), , Tsyganenk, N. A., and D. P. Stern, Mdeling the glbal magnetic field f the large-scale Birkeland current systems, J. Gephys. Res., 101(A12), 27,187-27,198, Villain, J.P., R. A. Greenwald, and J. F. Vickrey, HF ray tracing at high latitudes using measured meridinal electrn density distributins, Radi Sci., 19(1), , Villain, J.P., R. A. Greenwald, K. B. Baker, and J. M. Ruhniemi, HF radar bservatins f E regin plasma

16 898 NISHINO ET AL.: HIGH-LATITUDE VHF WAVE SCATTER irregularities prduced by blique electrn streaming, J. Gephy. Res., 92(All), 12,327-12,342, Weitzen, J. A., M. J. Swa, R. O. Scfidi, and J. Quinn, Characterizing the multipath and Dppler spreads f the high-latitude meter burst cmmunicatin channel, IEEE Trans. Cmmun., 35(10), , Weitzen, J. A., P.S. Cannn, J. C. Ostergaard, and J. E. Rasmussen, High-latitude seasnal variatin f meteric and nnmeteric blique prpagatin at a frequency f 45 MHz, Radi Sci., 28(2), , Yamagishi, H., M. Nishin, M. Sat, Y. Kat, M. Kjima, N. Sat, and T. Kikuchi, Develpment f imaging rimeters (in Japanese with English abstract),antarct. Recrd, 36(2), , N. Grkhv, MDARC, P.O. Bx 12, Mscw- 321, Russia. (mdarc@dl.ru) T. Hansen, Nrthern Light Observatry, University f Trms0, N-9000 Trms0, Nrway. (truls.hansen@phys. uit.n) M. Nishin, Slar Terrestrial Envirnment Labratry, Nagya University, 3-13, Hnhara Tykawa, Aichi, , Japan. (nishin@stelab.nagya-u.ac.jp) Y. Tanaka, Schl f Educatin, Yamaguchi University, Yshida, Yamaguchi , Japan. (tanaka@p. cc.yamaguchi-u.ac.jp) H. Yamagishi, Natinal Institute f Plar Research, Kaga, Itabashi-ku, Tky , Japan. (yamagisi@ nipr.ac.jp) (Received July 15, 1998; revised January 19, 1999; accepted February 16, 1999.)