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Features of downshifted maximum spectra during a dual-pump ionospheric heating experiment
Available online at www.sciencedirect.com
ScienceDirect Advances in Space Research 64 (2019) 1078–1084 www.elsevier.com/locate/asr
Features of downshifted maximum spectra during a dual-pump ionospheric heating experiment Libin Lv a,b, Zhensen Wu a,b, Qingliang Li b,⇑, Shuji Hao b, Guanglin Ma a,b, JuTao Yang b, Jian Ding b, Jian Wu b b
a School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China National Key Laboratory of Electromagnetic Environment, China Research Institute of Radiowave Propagation, Qingdao 266107, China
Received 18 March 2019; received in revised form 7 June 2019; accepted 8 June 2019 Available online 20 June 2019
Abstract An ionospheric heating experiment was conducted using the dual-pump mode at the EISCAT/HEATING facility in Tromsø, Norway. Some new features were found in the downshifted maximum (DM) component of the stimulated electromagnetic emission (SEE) spectra. During the experiment, the DM1 generated by pump 1 was enhanced under the action of pump 2 with the peak intensity being increased by 4.8–9.8 dB to achieve maximum value, when the frequency of pump 2 was 4.100 MHz. The gyro resonance at the upper hybrid altitude played an important role in this phenomenon. It was also observed that the development time of DM2 generated by pump 2 was greatly shorter than that of DM1 due to the precondition provided by the artificial field-aligned irregularities (AFAIs) stimulated by pump 1. Additionally, the frequency offset and peak intensity of the DM1 spectrum showed a significant negative correlation, where the correlation coefficient reached a value of 0.91. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Dual-pump heating; Stimulated electromagnetic emission; Downshifted maximum; Artificial field-aligned irregularities
1. Introductions High power ordinary-mode (O-mode) high-frequency (HF) pump waves transmitted from the ground interact with the local plasma of the ionosphere and can produce spectral sidebands in the upper hybrid (UH) resonance region called stimulated electromagnetic emission (SEE; Thide´ et al., 1982). The SEE spectra contain rich spectral features in a bandwidth of approximately 100 kHz (Stubbe and Kopka, 1990), which are a result of different nonlinear processes taking place during the interactions between HF pump waves and ionospheric plasma. One of the most common SEE spectral features is the downshifted maximum (DM; Stubbe et al., 1984) with a frequency offset ⇑ Corresponding author.
E-mail address: [email protected] (Q. Li). https://doi.org/10.1016/j.asr.2019.06.009 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
from the pump wave frequency by 8–12 kHz (Thide´ et al., 1982). It is suggested that parametric instability plays a key role in the excitation of the DM, although there is still some argument on its excitation (Wang et al., 2016, 2018; Blagoveshchenskaya et al., 2017). Because of the close relationship between DM and artificial field-aligned irregularities (AFAIs), it is also used to study the excitation mechanism of AFAIs in the ionosphere (Borisova et al., 2014; Norin et al., 2008; Sergeev et al., 2018). Experimental studies have shown that the features of the SEE spectrum vary greatly with pump wave frequency and mode. For example, when the O-mode pump wave frequency is close to an integer multiple of the electron cyclotron frequency, the electron cyclotron resonance affects the nonlinear interaction between the pump and the ionospheric plasma, which, in turn, affects the excitation of the SEE spectrum, and the SEE spectrum characteristics
L. Lv et al. / Advances in Space Research 64 (2019) 1078–1084
differ near different integer multiples of the electron cyclotron frequency (Fu et al., 2018; Leyser, 2001; Mahmoudian et al., 2017; Sergeev et al., 2006; Stocker, 1998; Tereshchenko et al., 2012). Additionally, researchers have studied SEE using a variety of pump wave modes, one being the dual-pump mode (Bernhardt et al., 1994; Trakhtengerts et al., 1996). By observing the influence of additional pump waves on the characteristics of SEE spectra, the excitation mechanism of SEE has been studied. In an experiment at the Sura Ionospheric Heating Facility in Russia, the DM1 generated by pump 1 was enhanced by 2 dB under the action of pump 2 at a frequency of f2 = f1 + 423.6 kHz (Bernhardt et al., 1994). According to the correlation between the heating height of the two pump waves and the degree of DM enhancement, the spatial range of the AFAI was obtained (Trakhtengerts et al., 1996). In this study, a dual-pump ionospheric heating experiment was designed and performed at the European Incoherent Scatter Scientific Association (EISCAT) heating station near Tromsø, Norway. Continuously changing the frequency of the additional pump (pump 2) led to variations in the intensity, development time, and frequency offset of the DM1 generated by the pump 1, which were then observed and recorded. The spectral variation characteristics were used to study the DM1 excitation mechanism using the dual-pump waves.
f
1079
MHz 4.180 f1 f2
4.115 4.100 4.085 4.070
4.055 4.040 12:14 12:15 12:17 12:19 12:21 12:23 12:25 12:27
Heating Off 12:29
t
UTC
Fig. 1. Heating sequence: Part 1 worked for 15 min at frequency f1 from 12:14 UTC, and Part 2 was turned on at 12:15 UTC; the latter operated for 2 min at frequencies of 4.180 MHz, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, and 4.115 MHz. At 12:29 UTC, both arrays were turned off.
The dynasonde located within the heating facility was employed to monitor the ionospheric conditions by measuring the ionogram at intervals of two minutes. The experiment was conducted from 12:14–12:30 UTC. During this time period, there was no significant change in the background ionospheric conditions, and the maximum plasma frequency (foF2) of the ionosphere was 4.200 MHz as obtained from the background ionograms shown in Fig. 2. 3. Results
2. Experimental setup 3.1. Effect of the additional pump on the intensity The EISCAT heating facility is located at Ramfjordmoen near Tromsø in northern Norway (69.58°N, 19.23°E; Rietveld et al., 1993; Rietveld et al., 2016). During the ionospheric heating experiments performed on November 13, 2017, array 2, with a frequency range of 3.85– 5.65 MHz, was split into two 3 6 sub-arrays (Part 1 and Part 2) for dual-pump heating. Part 1 operated with a fixed frequency: f1 = 4.180 MHz, while Part 2 operated at several frequencies: 4.180 MHz, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, and 4.115 MHz, successively, where each frequency lasted for 2 min. During the experiment, the two arrays transmitted electromagnetic O-mode HF waves into the ionosphere toward the magnetic zenith with maximum effective radiation power (ERP) of 170 MW. The heating sequence is shown in Fig. 1. An SEE receiver was installed near Breivikeidet, Norway (69.64°N, 19.49°E), 13 km E–NE of the EISCAT site (Fu et al., 2015). The antenna was a broadband with resistively loaded folded dipole. The receiver was an Ettus Research (USA) USRP N210 fitted with a Global Positioning Satellite- (GPS)-disciplined oscillator to provide precise time and frequency references, which was tuned to receive at 4.150 MHz and to record at a 500-kHz bandwidth to cover all heater frequencies. The data sampled at a 500kHz sampling rate were processed with a fast Fourier transform (FFT) to yield low-frequency spectra.
Fig. 3 shows the variations in electromagnetic radiation signals with time and frequency received by the SEE receiver between 12:14 and 12:30 UTC. The center frequency was set at 4.180 MHz. The frequency offsets of the additional pump relative to the center frequency were df = 0, 140 kHz, 125 kHz, 110 kHz, 95 kHz, 80 kHz, and 65 kHz. Starting from 12:14 UTC, pump 1 was turned on, and a significant DM feature (denoted by DM1) at f1 9.4 kHz was observable, which is consistent with the conclusion of Thide´ et al (1982) that the DM usually occurs at Df 8–12 kHz range. At the same time, the 2DM1 appeared near f1 17.8 kHz, and even a weak 3DM1 appeared near f0 27.7 kHz. Moreover, the DM component (denoted by DM2) was also observed near the frequency of the additional pump, and with the gradual increase of f2, DM2 gradually increased, along with the appearance of 2DM2 and 3DM2. According to the results shown in Fig. 3, the DM1 and 2DM1 were enhanced significantly after pump 2 was turned on at 12:15 UTC. Fig. 4 shows the intensity variations of nDM1 (n = 1, 2, 3) with the increase in f2. When f2 = 0 (pump 2 was turned off), the peak intensity of DM1 was 109.7 dB m. After pump 2 was turned on, the intensity of DM1 increased notably, where the enhancement had a trend of increasing at first, followed by a decrease with
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Fig. 2. Ionograms of Tromsø at 12:12 UTC and 12:32 UTC. The maximum plasma frequency (foF2) of the ionosphere was 4.200 MHz.
Pump f1
DM1
2DM1
3DM1 Pump f2
3DM2 2DM2 DM2
Fig. 3. Variations in stimulated electromagnetic emission (SEE) spectra with time and frequency. The arrows point out the pump signals and the downshifted maximum components.
an increase in f2. Under the action of the additional pump with different frequencies, the peak intensity of DM1 increased by 4.8–9.8 dB and reached a maximum of 99.9 dB at f2 = 4.100 MHz. Additionally, the intensities of 2DM1 and 3DM1 increased by 2.3–7.9 dB and 0.2– 3.4 dB, respectively, and had the same trend as DM1. 3.2. Effect of the additional pump on the development time Pump 2 produced fluctuations by interacting with ionospheric plasma, which was equivalent to changing the background ionospheric state of the interactions, thus affecting the generation of DM1. In contrast, pump 1 also played the same role and had an impact on the excitation of DM2. Fig. 5 shows the variations in the peak intensities (1-second average (mean)) of DM1 and DM2 with time. The red curve depicts the variation in the peak intensity of DM1 with time from 12:13–12:15 UTC, while the blue curve is for DM2 from 12:26–12:28 UTC. We believe that
when pump 1 was turned on at 12:14 UTC, AFAIs were produced in the ionospheric interaction region through non-linear mechanisms, such as thermal parametric instability (TPI). Electromagnetic radiation was then stimulated by direct mode conversion and the parametric decay process, and DM1 was formed on the SEE spectral line. When pump 2 was turned on at 12:27 UTC, the AFAIs generated by pump 1 extended to the UH region of pump 2, which could then directly interact with pump 2, stimulating DM2, and thus, its generation time was greatly shortened. 3.3. Effect of the additional pump on the frequency offset According to the experimental data, it is found that an additional pump can also cause changes in the frequency offset of DM1. Fig. 6 show the variations in nDM1 (1-min average) with the frequency of the additional pump. The peaks of different DM1 components inclined toward the center frequency, whose peak value changed with the
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Fig. 4. Effects of the additional pump on nDM1 (n = 1, 2, 3). Each spectral line here is a 1-min average (mean), and the width is 8 kHz. The top, middle, and bottom parts represent DM1, 2DM1, and 3DM1, respectively. From left to right, the frequencies of additional pump f1 are 0, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, 4.115 MHz, and 4.180 MHz.
DM2 ( f2 = 4.100 MHz ) DM1 ( pump 2 off )
Fig. 5. Development time of DM. The red curve depicts the peak intensity of DM1 and how it changes with time from 12:13–12:15 UTC; the blue curve is the peak intensity of DM2 and how it changes with time from 12:26–12:28 UTC.
wave frequency of the additional pump. As shown in Fig. 7, the maximum variations in the peak frequency offset were about 0.9 kHz, 0.6 kHz, and 0.3 kHz. Interestingly, the maximum variation in the peak frequency offset occurred at f2 = 4.100 MHz, which is similar to the variation observed in the peak intensity of DM1. Fig. 8 shows a comparison of the results for DM1 intensities and frequency offsets with f2. The intensities and frequency offsets of DM1 essentially showed opposite trends to the frequency of the additional pump, and the correla-
tion coefficient reached a value of 0.91. This phenomenon is similar to what has been reported for single-pump heating experiments with stepping heating power (Leyser et al., 1994). 4. Discussion and conclusions In this study, a dual-pump ionospheric heating experiment was performed using an additional pump, whose frequency was lower than f1, and obvious enhancement of the
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f2 = 0 f2 = 4.040 MHz f2 = 4.055 MHz f2 = 4.070 MHz f2 = 4.085 MHz f2 = 4.100 MHz f2 = 4.115 MHz f2 = 4.180 MHz Heating off
DM1 2DM1
3DM1
Fig. 6. Variations in nDM1 with the wave frequency of the additional pump. The DM1, 2DM1, 3DM1 curves are presented in the left, middle, and right of the figure, respectively.
Fig. 7. Variations in the peak frequency offset of nDM1 with the frequency of the additional pump wave (left panel) DM1; (middle panel) 2DM1; (right panel) 3DM1.
DM1 was observed with the peak intensity being increased by 4.8–9.8 dB. We have shown from our experiment that when the frequency difference between the two pumps is small enough, such as 140 kHz f2 f1 0, the AFAIs generated by the additional pump f2 can extend into the UH resonance region of the pump f1, which enhances the conversion efficiency of the pump f1 to a UH wave, and ultimately leads to the enhancement of DM1. In contrast, the peak intensity of the DM1 spectrum observed in the Sura experiments increased by only 2 dB under the action of an additional pump with a frequency of f2 = f1 + 423.6 kHz (Bernhardt et al., 1994). This may be due to two reasons. Firstly, in our experiment, the fre-
quency difference between the two pumping waves was smaller, and the degree of interaction between the AFAIs was greater. Secondly, in the Sura experiment, since f2 > f1, the additional pump wave had to pass through the heating region of f1, suffering from abnormal absorption effects caused by the AFAIs that weakened its ability to produce AFAIs by its interaction with plasma. Another interesting phenomenon is the variation in DM1 peak intensity that has a maximum at f2 = 4.100 MHz. Considering the frequencies that were used, a gyro resonance effect may have occurred with the variation in f2. During the experiment, the dynasonde recorded the background ionospheric state. According to
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Fig. 8. Variation in the DM1 intensity (blue curve) and frequency offset (red curve) with f2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Reflection and UH heights corresponding to different pump frequencies. Frequency (MHz) Reflection height (km) UH height (km)
4.040 220.5 207.2
4.055 221.4 208.1
4.070 222.2 209.0
the background ionospheric parameters shown in Fig. 1, the reflection heights of pump waves with different frequencies were estimated to be in the range of 220.5–228.5 km, while the UH heights were in the range of 207.2– 215.6 km as shown in Table 1. Using the International Geomagnetic Reference Field (IGRF), the local magnetic dip I = 78.2° and the electron cyclotron frequency fce 1.360 MHz were obtained. Fig. 3 shows that when f1 = 4.100 MHz, it exceeded the third electron gyro-harmonic frequency (3fce 4.080 MHz) by 20 kHz. The excitation of the 2DM component, and even the 3DM component, was observed in the spectrum of stimulated electromagnetic emission. This is consistent with Blagoveshchenskaya’s conclusions (Blagoveshchenskaya et al, 2009, 2011). They also suggested that the combined effect of upper hybrid resonance and gyro resonance gives rise to strong electron heating and the enhancement of AFAIs. Therefore, the AFAIs produced by the two pumps in our experiment are the greatest, ultimately leading to the highest intensity of DM1. Altitude parameters were, however, estimated from the dynasonde data and may not be accurate enough. There is a need for further experiments to obtain precise altitude parameters using the VHF incoherent scattering radar. This will help analyze the explicit relationship between the heating frequency and electronic cyclotron frequency and verify the phenomena observed in our experiment.
4.085 223.1 209.9
4.100 223.9 210.8
4.115 224.8 211.7
4.180 228.5 215.6
Acknowledgements The authors are very grateful for the technical support provided by EISCAT in designing and conducting this experiment, and also for the SEE receiver provided by Lancaster University in SEE measurement. This work was supported by the Innovation Foundation of the China Electronics Technology Group Corporation, China (grant no. KJ1602004). References Bernhardt, P.A., Wagner, L.S., Goldstein, J.A., Trakhtengerts, V.Y., Ermakova, E.N., Rapoport, V.O., Komrakov, G.P., Babichenko, A. M., 1994. Enhancement of stimulated electromagnetic emission during two frequency ionospheric heating experiments. Phys. Rev. Lett. 72, 2879–2882. https://doi.org/10.1103/PhysRevLett. 72.2879. Blagoveshchenskaya, N.F., Carlson, H.C., Kornienko, V.A., Borisova, T. D., Rietveld, M.T., Yeoman, T.K., Brekke, A., 2009. Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency. Ann. Geophys. 27, 131–145. https://doi.org/ 10.5194/angeo-27-131-2009. Blagoveshchenskaya, N.F., Yeoman, T.K., Rietveld, M.T., 2011. The effects of modification of a high-latitude ionosphere by high-power HF radio waves. Part 1. Results of multi-instrument ground-based observations. Radiophys. Quantum Electron. 53, 512–531. https:// doi.org/10.1007/s11141-011-9247-y. Blagoveshchenskaya, N.F., Borisova, T.D., Yeoman, T.K., 2017. Comment on ‘‘Parametric Instability Induced by X-Mode Wave Heating at EISCAT” by Wang et al. (2016): Comment on Wang et al. (2016). J.
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Geophys. Res. Space Phys. 122 (12), 12,570–12,585. https://doi.org/ 10.1002/2017JA023880. Borisova, T.D., Blagoveshchenskaya, N.F., Kalishin, A.S., Kosch, M., Senior, A., Rietveld, M.T., Hagstrom, I., 2014. Phenomena in the high-latitude ionospheric F region induced by a HF heater wave at frequencies near the fourth electron gyroharmonic. Radiophys. Quantum Electron. 57, 1–19. https://doi.org/10.1007/s11141-0149489-6. Fu, H.Y., Scales, W.A., Bernhardt, P.A., Briczinski, S.J., Kosch, M.J., Senior, A., Ruohoniemi, J.M., 2015. Stimulated Brillouin scattering during electron gyro-harmonic heating at EISCAT. Ann. Geophys. 33, 983–990. https://doi.org/10.5194/angeo-33-983-2015. Fu, H.Y., Scales, W.A., Bernhardt, P.A., Jin, Y.Q., Briczinski, S.J., 2018. Asymmetry in stimulated emission polarization and irregularity evolution during ionospheric electron gyroharmonic heating. Geophys. Res. Lett. 45, 9363–9371. https://doi.org/10.1029/ 2018GL078957. Leyser, T.B., 2001. Stimulated electromagnetic emissions by high-frequency electromagnetic pumping of the ionospheric plasma. Space Sci. Rev. 98, 223–328. https://doi.org/10.1023/a:1013875603938. Leyser, T.B., Thide´, B., Waldenvik, M., Veszelei, E., Frolov, V.L., Grach, S.M., Komrakov, G.P., 1994. Downshifted maximum features in stimulated electromagnetic emission spectra. J. Geophys. Res. 99 (A10), 19555–19568. https://doi.org/10.1029/94ja01399. Mahmoudian, A., Scales, W.A., Watkins, B.J., Bernhardt, P.A., Isham, B., Vega-Cancel, O., Ruohoniemi, J.M., 2017. Investigation of third gyro-harmonic heating at HAARP using stimulated radio emissions and the MUIR and Kodiak radars. Adv. Space Res. 59, 337–350. https://doi.org/10.1016/j.asr.2016.09.029. Norin, L., Grach, S.M., Leyser, T.B., Thide´, B., Sergeev, E.N., Berlin, M., 2008. Ionospheric plasma density irregularities measured by stimulated electromagnetic emission. J. Geophys. Res. Space Phys. 113 (A9). https://doi.org/10.1029/2008JA013338. Rietveld, M.T., Kohl, H., Kopka, H., Stubbe, P., 1993. Introduction to ionospheric heating at Tromsø—I. Experimental overview. J. Atmos. Terr. Phys. 55, 577–599. https://doi.org/10.1016/0021-9169(93)90007L. Rietveld, M.T., Senior, A., Markkanen, J., Westman, A., 2016. New capabilities of the upgraded EISCAT high-power HF facility. Radio Sci. 51, 1533–1546. https://doi.org/10.1002/2016RS006093.
Stubbe, P., Kopka, H., 1990. Stimulated electromagnetic emission in a magnetized plasma: a new symmetric spectral feature. Phys. Rev. Lett. 65, 183–186. https://doi.org/10.1103/physrevlett.65.183. Stubbe, P., Kopka, H., Thide´, B., Derblom, H., 1984. Stimulated electromagnetic emission: a new technique to study the parametric decay instability in the ionosphere. J. Geophys. Res. Space Phys. 89, 7523–7536. https://doi.org/10.1029/JA089iA09p07523. Sergeev, E.N., Grach, S.M., Shindin, A.V., Mishin, E.V., Bernhardt, P.A., Briczinski, S.J., McCarrick, M., 2018. Spatiotemporal characteristics of HF-induced ionospheric turbulence revealed by diagnostic stimulated electromagnetic emission and test radio waves at HAARP. Radiol. Sci. 53, 1506–1520. https://doi.org/10.1029/2018RS006719. Stocker, A.J., 1998. An overview of experimental observations made at Tromsø during heating at frequencies near harmonics of the electron gyrofrequency. Adv. Space Res. 21, 653–661. https://doi.org/10.1016/ S0273-1177(97)84004-5. Sergeev, E.N., Frolov, V.L., Grach, S.M., Kotov, P.V., 2006. On the morphology of stimulated electromagnetic emission spectra in a wide pump wave frequency range. Adv. Space Res. 38, 2518–2526. https:// doi.org/10.1016/j.asr.2005.02.046. Thide´, B., Kopka, H., Stubbe, P., 1982. Observations of stimulated scattering of a strong high-frequency radio wave in the ionosphere. Phys. Rev. Lett. 49, 1561–1564. https://doi.org/10.1103/PhysRevLett. 49.1561. Tereshchenko, E.D., Yurik, R.Y., Rietveld, M.T., Isham, B., Belyey, V., 2012. The spatial features of the up-and downshifted maxima in stimulated electromagnetic emissions. Adv. Space Res. 50, 619–622. https://doi.org/10.1016/j.asr.2012.05.013. Trakhtengerts, V.Y., Rapoport, V.O., Yermakova, Y.N., Komrakov, G. P., Bernkhard, P.A., Vagner, L.S., Goldshteyn, I.A., 1996. Artificial radio emission in the two-frequency heating regime of the ionosphere. Geomag. Aeron. 35, 818–822. Wang, X., Zhou, C., Liu, M., Honary, F., Ni, B., Zhao, Z., 2016. Parametric instability induced by X-mode wave heating at EISCAT. J. Geophys. Res. Space Phys. 10, 10–536. https://doi.org/10.1002/ 2016JA023070. Wang, Xiang, Zhou, Chen, Honary, Farideh, 2018. Reply to Comment on the Article ‘‘Parametric Instability Induced by X-Mode Wave Heating at EISCAT” by Wang et al. (2016). J. Geophys. Res. Space Phys. 123 (9), 8051–8061. https://doi.org/10.1029/2018JA025808.
ScienceDirect Advances in Space Research 64 (2019) 1078–1084 www.elsevier.com/locate/asr
Features of downshifted maximum spectra during a dual-pump ionospheric heating experiment Libin Lv a,b, Zhensen Wu a,b, Qingliang Li b,⇑, Shuji Hao b, Guanglin Ma a,b, JuTao Yang b, Jian Ding b, Jian Wu b b
a School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China National Key Laboratory of Electromagnetic Environment, China Research Institute of Radiowave Propagation, Qingdao 266107, China
Received 18 March 2019; received in revised form 7 June 2019; accepted 8 June 2019 Available online 20 June 2019
Abstract An ionospheric heating experiment was conducted using the dual-pump mode at the EISCAT/HEATING facility in Tromsø, Norway. Some new features were found in the downshifted maximum (DM) component of the stimulated electromagnetic emission (SEE) spectra. During the experiment, the DM1 generated by pump 1 was enhanced under the action of pump 2 with the peak intensity being increased by 4.8–9.8 dB to achieve maximum value, when the frequency of pump 2 was 4.100 MHz. The gyro resonance at the upper hybrid altitude played an important role in this phenomenon. It was also observed that the development time of DM2 generated by pump 2 was greatly shorter than that of DM1 due to the precondition provided by the artificial field-aligned irregularities (AFAIs) stimulated by pump 1. Additionally, the frequency offset and peak intensity of the DM1 spectrum showed a significant negative correlation, where the correlation coefficient reached a value of 0.91. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Dual-pump heating; Stimulated electromagnetic emission; Downshifted maximum; Artificial field-aligned irregularities
1. Introductions High power ordinary-mode (O-mode) high-frequency (HF) pump waves transmitted from the ground interact with the local plasma of the ionosphere and can produce spectral sidebands in the upper hybrid (UH) resonance region called stimulated electromagnetic emission (SEE; Thide´ et al., 1982). The SEE spectra contain rich spectral features in a bandwidth of approximately 100 kHz (Stubbe and Kopka, 1990), which are a result of different nonlinear processes taking place during the interactions between HF pump waves and ionospheric plasma. One of the most common SEE spectral features is the downshifted maximum (DM; Stubbe et al., 1984) with a frequency offset ⇑ Corresponding author.
E-mail address: [email protected] (Q. Li). https://doi.org/10.1016/j.asr.2019.06.009 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
from the pump wave frequency by 8–12 kHz (Thide´ et al., 1982). It is suggested that parametric instability plays a key role in the excitation of the DM, although there is still some argument on its excitation (Wang et al., 2016, 2018; Blagoveshchenskaya et al., 2017). Because of the close relationship between DM and artificial field-aligned irregularities (AFAIs), it is also used to study the excitation mechanism of AFAIs in the ionosphere (Borisova et al., 2014; Norin et al., 2008; Sergeev et al., 2018). Experimental studies have shown that the features of the SEE spectrum vary greatly with pump wave frequency and mode. For example, when the O-mode pump wave frequency is close to an integer multiple of the electron cyclotron frequency, the electron cyclotron resonance affects the nonlinear interaction between the pump and the ionospheric plasma, which, in turn, affects the excitation of the SEE spectrum, and the SEE spectrum characteristics
L. Lv et al. / Advances in Space Research 64 (2019) 1078–1084
differ near different integer multiples of the electron cyclotron frequency (Fu et al., 2018; Leyser, 2001; Mahmoudian et al., 2017; Sergeev et al., 2006; Stocker, 1998; Tereshchenko et al., 2012). Additionally, researchers have studied SEE using a variety of pump wave modes, one being the dual-pump mode (Bernhardt et al., 1994; Trakhtengerts et al., 1996). By observing the influence of additional pump waves on the characteristics of SEE spectra, the excitation mechanism of SEE has been studied. In an experiment at the Sura Ionospheric Heating Facility in Russia, the DM1 generated by pump 1 was enhanced by 2 dB under the action of pump 2 at a frequency of f2 = f1 + 423.6 kHz (Bernhardt et al., 1994). According to the correlation between the heating height of the two pump waves and the degree of DM enhancement, the spatial range of the AFAI was obtained (Trakhtengerts et al., 1996). In this study, a dual-pump ionospheric heating experiment was designed and performed at the European Incoherent Scatter Scientific Association (EISCAT) heating station near Tromsø, Norway. Continuously changing the frequency of the additional pump (pump 2) led to variations in the intensity, development time, and frequency offset of the DM1 generated by the pump 1, which were then observed and recorded. The spectral variation characteristics were used to study the DM1 excitation mechanism using the dual-pump waves.
f
1079
MHz 4.180 f1 f2
4.115 4.100 4.085 4.070
4.055 4.040 12:14 12:15 12:17 12:19 12:21 12:23 12:25 12:27
Heating Off 12:29
t
UTC
Fig. 1. Heating sequence: Part 1 worked for 15 min at frequency f1 from 12:14 UTC, and Part 2 was turned on at 12:15 UTC; the latter operated for 2 min at frequencies of 4.180 MHz, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, and 4.115 MHz. At 12:29 UTC, both arrays were turned off.
The dynasonde located within the heating facility was employed to monitor the ionospheric conditions by measuring the ionogram at intervals of two minutes. The experiment was conducted from 12:14–12:30 UTC. During this time period, there was no significant change in the background ionospheric conditions, and the maximum plasma frequency (foF2) of the ionosphere was 4.200 MHz as obtained from the background ionograms shown in Fig. 2. 3. Results
2. Experimental setup 3.1. Effect of the additional pump on the intensity The EISCAT heating facility is located at Ramfjordmoen near Tromsø in northern Norway (69.58°N, 19.23°E; Rietveld et al., 1993; Rietveld et al., 2016). During the ionospheric heating experiments performed on November 13, 2017, array 2, with a frequency range of 3.85– 5.65 MHz, was split into two 3 6 sub-arrays (Part 1 and Part 2) for dual-pump heating. Part 1 operated with a fixed frequency: f1 = 4.180 MHz, while Part 2 operated at several frequencies: 4.180 MHz, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, and 4.115 MHz, successively, where each frequency lasted for 2 min. During the experiment, the two arrays transmitted electromagnetic O-mode HF waves into the ionosphere toward the magnetic zenith with maximum effective radiation power (ERP) of 170 MW. The heating sequence is shown in Fig. 1. An SEE receiver was installed near Breivikeidet, Norway (69.64°N, 19.49°E), 13 km E–NE of the EISCAT site (Fu et al., 2015). The antenna was a broadband with resistively loaded folded dipole. The receiver was an Ettus Research (USA) USRP N210 fitted with a Global Positioning Satellite- (GPS)-disciplined oscillator to provide precise time and frequency references, which was tuned to receive at 4.150 MHz and to record at a 500-kHz bandwidth to cover all heater frequencies. The data sampled at a 500kHz sampling rate were processed with a fast Fourier transform (FFT) to yield low-frequency spectra.
Fig. 3 shows the variations in electromagnetic radiation signals with time and frequency received by the SEE receiver between 12:14 and 12:30 UTC. The center frequency was set at 4.180 MHz. The frequency offsets of the additional pump relative to the center frequency were df = 0, 140 kHz, 125 kHz, 110 kHz, 95 kHz, 80 kHz, and 65 kHz. Starting from 12:14 UTC, pump 1 was turned on, and a significant DM feature (denoted by DM1) at f1 9.4 kHz was observable, which is consistent with the conclusion of Thide´ et al (1982) that the DM usually occurs at Df 8–12 kHz range. At the same time, the 2DM1 appeared near f1 17.8 kHz, and even a weak 3DM1 appeared near f0 27.7 kHz. Moreover, the DM component (denoted by DM2) was also observed near the frequency of the additional pump, and with the gradual increase of f2, DM2 gradually increased, along with the appearance of 2DM2 and 3DM2. According to the results shown in Fig. 3, the DM1 and 2DM1 were enhanced significantly after pump 2 was turned on at 12:15 UTC. Fig. 4 shows the intensity variations of nDM1 (n = 1, 2, 3) with the increase in f2. When f2 = 0 (pump 2 was turned off), the peak intensity of DM1 was 109.7 dB m. After pump 2 was turned on, the intensity of DM1 increased notably, where the enhancement had a trend of increasing at first, followed by a decrease with
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Fig. 2. Ionograms of Tromsø at 12:12 UTC and 12:32 UTC. The maximum plasma frequency (foF2) of the ionosphere was 4.200 MHz.
Pump f1
DM1
2DM1
3DM1 Pump f2
3DM2 2DM2 DM2
Fig. 3. Variations in stimulated electromagnetic emission (SEE) spectra with time and frequency. The arrows point out the pump signals and the downshifted maximum components.
an increase in f2. Under the action of the additional pump with different frequencies, the peak intensity of DM1 increased by 4.8–9.8 dB and reached a maximum of 99.9 dB at f2 = 4.100 MHz. Additionally, the intensities of 2DM1 and 3DM1 increased by 2.3–7.9 dB and 0.2– 3.4 dB, respectively, and had the same trend as DM1. 3.2. Effect of the additional pump on the development time Pump 2 produced fluctuations by interacting with ionospheric plasma, which was equivalent to changing the background ionospheric state of the interactions, thus affecting the generation of DM1. In contrast, pump 1 also played the same role and had an impact on the excitation of DM2. Fig. 5 shows the variations in the peak intensities (1-second average (mean)) of DM1 and DM2 with time. The red curve depicts the variation in the peak intensity of DM1 with time from 12:13–12:15 UTC, while the blue curve is for DM2 from 12:26–12:28 UTC. We believe that
when pump 1 was turned on at 12:14 UTC, AFAIs were produced in the ionospheric interaction region through non-linear mechanisms, such as thermal parametric instability (TPI). Electromagnetic radiation was then stimulated by direct mode conversion and the parametric decay process, and DM1 was formed on the SEE spectral line. When pump 2 was turned on at 12:27 UTC, the AFAIs generated by pump 1 extended to the UH region of pump 2, which could then directly interact with pump 2, stimulating DM2, and thus, its generation time was greatly shortened. 3.3. Effect of the additional pump on the frequency offset According to the experimental data, it is found that an additional pump can also cause changes in the frequency offset of DM1. Fig. 6 show the variations in nDM1 (1-min average) with the frequency of the additional pump. The peaks of different DM1 components inclined toward the center frequency, whose peak value changed with the
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Fig. 4. Effects of the additional pump on nDM1 (n = 1, 2, 3). Each spectral line here is a 1-min average (mean), and the width is 8 kHz. The top, middle, and bottom parts represent DM1, 2DM1, and 3DM1, respectively. From left to right, the frequencies of additional pump f1 are 0, 4.040 MHz, 4.055 MHz, 4.070 MHz, 4.085 MHz, 4.100 MHz, 4.115 MHz, and 4.180 MHz.
DM2 ( f2 = 4.100 MHz ) DM1 ( pump 2 off )
Fig. 5. Development time of DM. The red curve depicts the peak intensity of DM1 and how it changes with time from 12:13–12:15 UTC; the blue curve is the peak intensity of DM2 and how it changes with time from 12:26–12:28 UTC.
wave frequency of the additional pump. As shown in Fig. 7, the maximum variations in the peak frequency offset were about 0.9 kHz, 0.6 kHz, and 0.3 kHz. Interestingly, the maximum variation in the peak frequency offset occurred at f2 = 4.100 MHz, which is similar to the variation observed in the peak intensity of DM1. Fig. 8 shows a comparison of the results for DM1 intensities and frequency offsets with f2. The intensities and frequency offsets of DM1 essentially showed opposite trends to the frequency of the additional pump, and the correla-
tion coefficient reached a value of 0.91. This phenomenon is similar to what has been reported for single-pump heating experiments with stepping heating power (Leyser et al., 1994). 4. Discussion and conclusions In this study, a dual-pump ionospheric heating experiment was performed using an additional pump, whose frequency was lower than f1, and obvious enhancement of the
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f2 = 0 f2 = 4.040 MHz f2 = 4.055 MHz f2 = 4.070 MHz f2 = 4.085 MHz f2 = 4.100 MHz f2 = 4.115 MHz f2 = 4.180 MHz Heating off
DM1 2DM1
3DM1
Fig. 6. Variations in nDM1 with the wave frequency of the additional pump. The DM1, 2DM1, 3DM1 curves are presented in the left, middle, and right of the figure, respectively.
Fig. 7. Variations in the peak frequency offset of nDM1 with the frequency of the additional pump wave (left panel) DM1; (middle panel) 2DM1; (right panel) 3DM1.
DM1 was observed with the peak intensity being increased by 4.8–9.8 dB. We have shown from our experiment that when the frequency difference between the two pumps is small enough, such as 140 kHz f2 f1 0, the AFAIs generated by the additional pump f2 can extend into the UH resonance region of the pump f1, which enhances the conversion efficiency of the pump f1 to a UH wave, and ultimately leads to the enhancement of DM1. In contrast, the peak intensity of the DM1 spectrum observed in the Sura experiments increased by only 2 dB under the action of an additional pump with a frequency of f2 = f1 + 423.6 kHz (Bernhardt et al., 1994). This may be due to two reasons. Firstly, in our experiment, the fre-
quency difference between the two pumping waves was smaller, and the degree of interaction between the AFAIs was greater. Secondly, in the Sura experiment, since f2 > f1, the additional pump wave had to pass through the heating region of f1, suffering from abnormal absorption effects caused by the AFAIs that weakened its ability to produce AFAIs by its interaction with plasma. Another interesting phenomenon is the variation in DM1 peak intensity that has a maximum at f2 = 4.100 MHz. Considering the frequencies that were used, a gyro resonance effect may have occurred with the variation in f2. During the experiment, the dynasonde recorded the background ionospheric state. According to
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Fig. 8. Variation in the DM1 intensity (blue curve) and frequency offset (red curve) with f2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Reflection and UH heights corresponding to different pump frequencies. Frequency (MHz) Reflection height (km) UH height (km)
4.040 220.5 207.2
4.055 221.4 208.1
4.070 222.2 209.0
the background ionospheric parameters shown in Fig. 1, the reflection heights of pump waves with different frequencies were estimated to be in the range of 220.5–228.5 km, while the UH heights were in the range of 207.2– 215.6 km as shown in Table 1. Using the International Geomagnetic Reference Field (IGRF), the local magnetic dip I = 78.2° and the electron cyclotron frequency fce 1.360 MHz were obtained. Fig. 3 shows that when f1 = 4.100 MHz, it exceeded the third electron gyro-harmonic frequency (3fce 4.080 MHz) by 20 kHz. The excitation of the 2DM component, and even the 3DM component, was observed in the spectrum of stimulated electromagnetic emission. This is consistent with Blagoveshchenskaya’s conclusions (Blagoveshchenskaya et al, 2009, 2011). They also suggested that the combined effect of upper hybrid resonance and gyro resonance gives rise to strong electron heating and the enhancement of AFAIs. Therefore, the AFAIs produced by the two pumps in our experiment are the greatest, ultimately leading to the highest intensity of DM1. Altitude parameters were, however, estimated from the dynasonde data and may not be accurate enough. There is a need for further experiments to obtain precise altitude parameters using the VHF incoherent scattering radar. This will help analyze the explicit relationship between the heating frequency and electronic cyclotron frequency and verify the phenomena observed in our experiment.
4.085 223.1 209.9
4.100 223.9 210.8
4.115 224.8 211.7
4.180 228.5 215.6
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