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Spectral characteristics of ionospheric scintillations of VHF radiosignal near magnetic zenith
Journal of Atmospheric and Solar-Terrestrial Physics 160 (2017) 48–55
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Spectral characteristics of ionospheric scintillations of VHF radiosignal near magnetic zenith Roman Vasilyev *, Mariia Globa, Dmitry Kushnarev, Andrey Medvedev, Konstantin Ratovsky Institute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences, 664033, Irkutsk P/O Box 291, Lermontov St., 126a, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: VHF ionospheric scintillations Ionospheric irregularities Scintillation spectrum Geomagnetic storm
We present the results of observing the Cygnus-A radio source scintillation in the Earth's ionosphere under quiet and disturbed geomagnetic conditions by using the Irkutsk incoherent scattering radar (IISR). The scintillation method applied for ionosphere testing at IISR confidently determines the Fresnel frequency and power cutoff, the spectral characteristics usually related to the velocities and spatial spectra of ionospheric plasma irregularities. We also use the IGRF magnetic field model to show the relation between the shape of discrete radio source scintillation spectra and the direction to the radio source with respect to the geomagnetic field. The S4 index increase within the magnetic zenith is observed to be conditioned by the scintillation spectrum widening. We also evaluate the zonal velocity of observed ionospheric irregularities as ~10 m/sec assuming the irregularity height to be equal to the F2-layer maximum height in the ionosphere.
1. Introduction Variations in the celestial radio source intensity on the time scale from seconds to hundreds of seconds that appear due to ionospheric irregularities frequently arise at radio astronomical observations or satellite radio sounding and data transmission. The phenomenon, also referred to as scintillation of radio signal in the ionosphere, was well studied in the last century and widely described in scientific literature [Hunsucker and Hargreaves, 2015; Kung and Chao-Han, 1982; Priyadarshi, 2015; Rytov et al., 1989]. Typical indicators used in scintillation technique are the S(1–4) indices reflecting the degree (measure) of ionospheric irregularities. Also, used is the scintillation spectrum shape reflecting the spatial spectrum of ionospheric irregularities and the relative velocity of these irregularities and of the radio source [Guozhu et al., 2007; Bezrodny et al., 2007]. Observation of the magnetic zenith effect in ionospheric scintillations has also had a long history. Scintillation amplification near the magnetic zenith is observed both in the polar ionosphere [Gola et al., 1992] and in equatorial bubbles [Kintner and Ledvina 2004], and is believed to be caused by field-aligned irregularities. The same field-aligned ionospheric irregularities are also responsible for the magnetic zenith effect in the mid-latitude ionosphere [Afraimovich et al., 2011]. Heating experiments with high power HF radiowaves also demonstrate the developing magnetic field co-aligned ionospheric plasma density bunch structures that stimulate scintillations of VHF radiosignal [Tereshchenko et al., 2004]. The ionospheric * Corresponding author. E-mail address: [email protected] (R. Vasilyev). http://dx.doi.org/10.1016/j.jastp.2017.05.016 Received 5 August 2016; Received in revised form 30 March 2017; Accepted 29 May 2017 Available online 30 May 2017 1364-6826/© 2017 Elsevier Ltd. All rights reserved.
scintillations discussed in our study are caused by natural reasons in the mid-latitude ionosphere. We apply spectral analysis of scintillation data to show some features that could be useful for studying the disturbed ionosphere. 2. Observation technique and geometry The Irkutsk incoherent scattering radar (IISR) is located at 52 520 N, 103 150 E. The radar performs observations of the discrete cosmic radio sources in a continuous regime during long time intervals, up to several hours per day for several months. Scintillations of a radiosignal from a discrete space radiosource were also observed and studied, preliminarily, with the IISR [Vasilyev et al., 2013]. The investigations showed the expected seasonally-diurnal variation in the scintillation parameters, as well as the relation between the observed scintillations with an ionogram blurring. Unfortunately, in this study, we were restricted in the measuring system temporal resolution to perform a quality spectral analysis of the data. By now, we have improved the temporal resolution of observations from 18 to 4.5 s. As a result, the frequency resolution in the spectral presentation of the recorded signal has considerably increased. This allows us to distinguish radio source signal from ambient noise more precisely and get spectral parameters of ionospheric scintillations at a better resolution. We performed passive observations for 14 days, 2015 June 18 through July 01, and extracted the variations in the Cygnus-A radio galaxy intensity during that period. Fig. 1 shows the
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Journal of Atmospheric and Solar-Terrestrial Physics 160 (2017) 48–55
Ni ¼
ðSi Li Þ Li
(1)
where N is the processed data set, S is the initial data set, L is the data set obtained from the initial one by the low frequency filtration procedure, i is the number of readings. We used the running median with a 4-min window (60 data points) to build L. The running median filter has shown better results among other types of filters (running average, digital finite impulse response filter, etc.) Normalizing the subtracted data in (1) is necessary to exclude the amplitude dependence of variations. Fig. 3 shows an example of filtered data sets and scintillation spectra obtained from them. Each spectrum was computed by using the following procedure: the entire dataset was split into 20-min subsets (300 data points), and, after applying the Fourier transform to each subset, the transform results were reduced to one spectrum. One can see the Fresnel frequency peak fF and the power law behavior of the resulting spectra for both cases in Fig. 3. The noise floor starting frequency (fN) is clearly defined only for June 23, whereas for June 24, this value is difficult to mark due to the limited time resolution.
Fig. 1. Geometry of the observations (schematically). Green arrows are the direction to the radio source from IISR. Blue arrows show the direction of the geomagnetic field at the ionosphere height. Red dots are ionosphere pierce (penetration) points (IPP). Red arrows are the IPP velocity vector, α is the angle between the geomagnetic field vector and the direction (line of sight) to the radio source. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3. Ionosphere scintillation near the magnetic zenith From our observations, we extract S4 index data for each day (Fig. 4). S4 is determined through the common procedure via the dispersion relation to the mean value in 1 min square window (15 data points). We found that S4 tends to smoothly increase until ~20:00 local stellar time (LST) and smoothly decrease after. At the same time, there are several cases of strong scintillations occurring at random times. In [Jiao et al., 2013], the scintillation amplitude was shown to increase around the magnetic zenith. We suppose the same mechanism to be responsible for the S4 background behavior in our observations. Fig. 4 shows the angle between the magnetic field and the line of sight to the radio source (α) for our observation geometry (black line). The magnetic field direction at the IISR coordinates was calculated with the expansion in spherical harmonics using the WMM2015 [Chulliat et al., 2014] coefficients set. We use the “Geomag 7.0” Software [https://www.ngdc.noaa.gov/IAGA/ vmod/geomag70_license.html] and input data files with exact coordinates of point where line of sight to the Cygnus-A from IISR penetrate through ionosphere. The index slowly increases as α decreases, and has the maximum value, when α is close to zero, i.e., at the magnetic zenith. It is also interesting to extract the spectral characteristics of scintillations near the magnetic zenith. We process our data by using (1) and get the dynamic spectrum (square 20 min window, 300 data points) from the obtained dataset. Fig. 5 presents the dynamic spectrum of ionospheric scintillations accumulated for all the observational days (overall spectrum with the superposed α behavior). The maximum frequency persists
geometry of our observations. Fig. 2 presents a typical behavior for the Cygnus-A signal power (during one observation session). The radio source is moving through the scan view during the observation time and the power of the recorded signal is affected by the antenna beam pattern and by the recording system properties. IISR guiding (controlling DP for tracking the radio signal source over the sky) is based on the frequency scanning principle. Therefore, we use a set of several frequency bands to overlap the entire coverage sector. As compared with the previous observations, the number of bands decreased from 88 to 11 (temporal resolution improvement), and the width of each band increased by 4 times (frequency resolution improvement). Each frequency range features its own uneven amplitude-response curve with the maximum at the central frequency. These curves are responsible for a smooth variation in the recorded signal power in Fig. 2. The envelope of smooth power variations is defined by the antenna system directivity pattern. Despite such a complex and variable structure of the recorded power, one can clearly distinguish a quiet observational day and a disturbed one, because the scintillation frequency is significantly higher than the frequency of variations corresponding to the directivity pattern and the recording system properties. To eliminate low frequency variations showed in Fig. 2, we used a simple procedure:
Fig. 2. Example of signals from the Cygnus-A discrete radio source obtained by a new IISR technique for 2015 June 18 (left) and June 22 (right). Scintillations of radio signal are stronger for 2015 June 22 due to the geomagnetic storm. The high frequency scintillations are clearly different from the smooth variations conditioned by the amplitude-response curve and the antenna system directivity pattern (antenna beam pattern).
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Fig. 3. Example of normalized signals from the Cygnus-A discrete radio source (right panels), and the corresponding accumulated spectra (left panels). The 2015 June 23 Fresnel frequency is ~5 mHz, for 2015 June 24, it is ~20 mHz, the cut-off indices are 1.33 and 0.71, respectively. The 2015 June 23noise floor starting frequency is ~40 mHz, for 2015 June 24, it is fN > 100 mHz.
Fig. 5. Accumulated dynamic spectrum of scintillations for the entire observation period. Black line corresponds to α variation over local stellar time.
observation time from 19:00 to 22:00 LST. The increasing spectral power and additional peaks appearing in the spectrum 21:00 through 21:30 LST are due to the 2015 June 22 geomagnetic storm, the spot of maximum spectral power 19:30 through 20:00 LST is due to strong scintillations not related to the geomagnetic activity. One can see that the same features are responsible for several sharp bounds of S4 in Fig. 4 at the same LST. One can perform a simple parameterization of the scintillation spectrum widening rate in time. Obviously, the rate has the exponential law, so we can propose the equation describing the fN value as a function of time:
Fig. 4. Gray lines are the S4 indices vs local stellar time for the IISR location for all the observational days. Black line is the angle between the magnetic field and the line of sight to the radio source (α).
relatively constant (taking into account the data dispersion) during all the observation period, while the spectrum width changes significantly. The width slowly increases as the source approaches the magnetic zenith, and then slowly decreases. The power spectrum expanding is, apparently, the reason for the background S4 increase during our observations. Also, the second local diffused peak appears in the scintillation spectrum near log10(fF) ~ 1.75. The peak was observed during all the effective
fN ¼ f0 expfrðt t0 Þg
(2)
where f0 is the noise floor starting frequency near the magnetic zenith, t0
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Fig. 6. Accumulated dynamic spectrum of scintillations for the entire observation period. Black lines are the fN values from (3) with parameters: f0 ~ 100 mHz, t0 ~ 20.2 LST, and r ~ 1.5 sec1 positive for the time before the magnetic zenith (20.2 LST) and negative after.
Fig. 7. Dynamic spectra of the 2015 June 21–24scintillations.
positive before the magnetic zenith and negative after the latter (see Fig. 6). Separate days can have parameters notably distinguished from the ones obtained by the accumulated data (Fig. 7). The spread of parameters between several separate days reflects a significant instability of
is the time of approaching the magnetic zenith, r is the rate of fN changing. The accumulated data have the initial frequency f0 ~ 100 mHz, the time for reaching the maximal frequency t0 ~ 20.2 LST (local star time), and the frequency variation tempo r ~ 1.5 sec1. This tempo is 51
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the UHF radio wave scintillation process during a day, except e.g. June 24.
Table 1 Zonal velocity of ionospheric irregularities.
4. Zonal velocity of ionospheric irregularities Another interesting feature of our study (observation) is the comparison between the scintillation spectrum Fresnel frequency and a frequency derived from the velocity of motion of ionospheric pierce point (IPP) through ionospheric irregularities at a fixed height. The height also determines the size of the irregularities through the Fresnel zone size for the IISR wavelength. The IPP velocity vector for our geometry is almost perpendicular to the magnetic field for the entire observation period, and the frequency can be determined as in [Guozhu et al., 2007]:
V⊥ fF ¼ pffiffiffiffiffiffiffiffi 2λR
fF
log10(fF)
V ¼ V┴ ± Vdis
Vdis ¼ VV┴
0.018 0.010 0.006
1.75 2 2.25
19.56 11.00 6.186
0 8.561 13.38
Note that we have not included the motion of irregularities in the described pattern of the observed phenomena yet. In our geometry, IISR observe a part of the Cygnus-A trajectory that, predominantly, lays along the parallel. Due to this fact, in case of the meridional motion of irregularities at a velocity less than V┴, the Fresnel frequency will correspond only to IPP motion through ionospheric irregularities as in (3). If the meridional velocity of irregularities increases to the values significantly more than V┴, the observed Fresnel frequency starts to grow and exhibit rather the motion of the ionospheric irregularities, than the ionosphere piers point (IPP) motion. The zonal motion of the ionospheric irregularities for the IISR observation geometry can significantly increase or decrease the Fresnel frequency, depending on the motion direction. Irregularities travelling East to West lead to a decrease in the observed Fresnel frequency, whereas the opposite direction of motion increases it. In case of Fig. 8, one can say that most of the observed irregularities exhibit the East-West direction, because the spectrum main maximum shifts down to smaller frequencies. We can modify (3) to account for the velocity of irregularities in our case:
(3)
where fF is the (calculated) theoretical frequency, V┴ is the component of IPP velocity normal to the magnetic field, λ is the wavelength of the received electromagnetic radiation (~2 m), and R is the distance from IISR to IPP. Fig. 8 shows the fF behavior for different heights (gray lines). If we treat the spectrum Fresnel frequency as a spectral power maximum, it lays close to the black line for the 100-km height, and the second spectral power local maximum lays at the values corresponding the 400km height. We assume that the maximal intensity of ionospheric irregularities corresponds to the maximal plasma density. The plasma density vertical distribution real values can be obtained from independent instruments. The electron concentration value in the ionospheric plasma is provided by the Irkutsk DPS-4 Digisonde [Oinats et al., 2006; Reinisch et al., 1997] located ~100 km away from the IISR. By using the maximal concentration height from the DPS-4 data, we put the corresponding points of fF on the dynamic spectrum (black empty circles in Fig. 8). The position of the circles is close to fF corresponding to 400 km, and they are located in the area of the second local maximum, so we can conclude that, here, we observe motionless irregularities located at the maximum of ionospheric plasma density.
V⊥ ±Vdis fF ¼ pffiffiffiffiffiffiffiffi 2λR
(4)
Vdis is the velocity of irregularities, sign “þ” corresponds to the West-East irregularities, sign “–“ corresponds to the East-West irregularities. From the scintillation intensity in Fig. 8, we can evaluate the maximal and minimal of the shifted log10(fF) as 2.25 and 2, respectively. Table 1 contains the values of the maximal and minimal zonal velocities of irregularities calculated from (4) assuming that the height of irregularities
Fig. 8. Accumulated dynamic spectrum of scintillations for the entire observation period. Grey lines show the frequency behavior from (3), the top line corresponds to R ¼ 400 km, the bottom line corresponds to R ¼ 100 km. Open black circles correspond to R obtained from ionosonde data. 52
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density of particles. But, at the same time, one can note a correlation between the S4 shape of the relatively strong scintillations and the shape of the relatively weak proton flux density of June 23 and 24.
is ~300–350 km (from the Digisonde data). The zonal velocities are V┴ ¼ 19.56 m/sec. 5. Disturbed geomagnetic conditions
6. Discussion and conclusion All of the processed data 2015 June 18 through 2015 July 01 show a similar behavior. There is a weak spectral width dependence on the angle between the magnetic field and the line of sight to the radio source, and the scintillation process is mostly non-stationary. The 2015 June 22 geomagnetic storm (Fig. 7) is different from other scintillation events only in stronger high frequency scintillations. The beginning of the 2015 June 22 dynamic spectrum possesses the same behavior as the accumulated spectrum does. However, starting with ~19:45 UT (~18:45 LST), the 2015 June 22 dynamic spectrum significantly grows in intensity and expands toward higher frequencies. The Fresnel frequency moves down possibly due to the East – West motion of the irregularities. This fact confuses in a way. Obviously, the source of the ionospheric irregularities lays in the North direction in this case. An explanation may be a phenomenon, when large-scale (hundreds and thousands of kilometers) ionospheric irregularities generate small-scale (meters through kilometers) irregularities [Astafyeva et al., 2008] that move independently from their parent large-scale disturbances. Figs. 9 and 10 present the comparison between the S4 data and the Bz component of the interplanetary magnetic field, as well as the proton flux density. Bz and the proton density were retrieved from the OMNI database (ftp://spdf.gsfc.nasa.gov) [Mathews and Towheed, 1995]. A significant increase in the S4 appears both under the disturbed geomagnetic conditions (June 22, Fig. 8) and under the quiet ones (June 24, Fig. 8). The maximal S4 values for those days differ by a factor of two, but the median values are similar. By using the obtained data, we cannot conclude that the mid latitude ionosphere irregularities are strongly conditioned by the disturbances in the geomagnetic field only. One can see the same patterns with the proton flux. The amount of charged particles at the bow shock does not determine the scintillation intensity. The comparison of different days in Fig. 10 shows that S4 can vary significantly at the comparable
The presented spectral analysis of the UHF-radio wave ionospheric scintillations is based on 14 days of the IISR data. Scintillations appear predominantly during summer nights and have significantly nonstationary nature, both for disturbed and quiet geomagnetic conditions. Also, one can see that the scintillations presented in the ionosphere during the entire described period differ only in the intensity. The several-day averaged data exhibit a dependence on the geomagnetic field. The behavior of the “stationary” scintillation intensity is strongly associated with the geomagnetic field relative directions and with the line of sight to the source: the scintillation intensity maximum observed at the magnetic zenith, when the observer looks at the radio source along the geomagnetic field. The reason for intensification of the scintillation signal is a significant widening of the scintillation spectrum near the magnetic zenith. The widening rate can be described by the moving noise bound of scintillation spectra, fN, with time, and it has the exponential law with an about 1.5 sec1 rate. A similar behavior of the scintillation spectra was demonstrated in [Tereshchenko et al., 2004], with ionosphere modification experiments by powerful HF radio waves. This value can be used as a simple marker to study diurnal, seasonal, etc. dependences of scintillations, and the morphology of ionospheric irregularities causing scintillations. The comparison between the ionosonde and the IISR data exhibits the East-West zonal motion of the irregularities during the observation period. Assuming that ionospheric irregularities causing scintillations develop at the ionization maximum height, one can evaluate the velocity of the irregularities: the value is around 10 m/sec. The obtained velocity of irregularities is significantly lower than the neutral wind velocity by HWM14: ~30 m/sec, westward. This discrepancy can be explained by possible complex mechanism for the irregularity formation. That
Fig. 9. S4 index (black) and magnetic field (gray) variation over 2015 June 21–24. 53
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Fig. 10. S4 index (black) and proton flux (grey) variation over 2015 June 21–24.
The Irkutsk Incoherent Scatter Radar can contribute significantly to the ionosphere monitoring by radio astronomical observations. The IISR observation time window slides over a day due to the fixed scan view. Unfortunately, this closes the possibility to make seasonal observation of the ionospheric radio wave scintillations that mostly appear during a local night at the IISR latitudes. Nevertheless, operations aggregately with other instruments (GPS stations, ionosondes, optical monitoring) open extensive possibilities, in particular, for searching mid-latitude radio wave scintillations sources in the ionosphere.
mechanism should involve the MHD theory approach. Similar velocities of irregularities were also obtained in [Bezrodny et al., 2007] and in [Bezrodny et al., 2010], and the result agrees with the velocity data obtained through an independent spectral method in [Fallows et al., 2015]. The velocity values obtained in [Guozhu et al., 2007] are higher than the presented results. A possible reason may be both a relatively short period of observations in our case and the systematic error intrinsic the instrument applied: we use the data obtained by about 1-m wavelength radio emission, whereas in [Guozhu et al., 2007] used were the GPS data obtained by the 0.1-m emission. The comparison between the S4 indices and the solar wind parameters exhibits an ambiguous dependence of mid-latitude scintillations on the solar wind. Apparently, the 2015 June 22 scintillations are directly related to the geomagnetic storm and to the consequent disturbances that traveled down from the magnetosphere to the ionosphere. Whereas the 2015 June 23 and 24 scintillations, despite a relatively high intensity, do not correlate with the geomagnetic field disturbances. A possible reason for scintillations, in this case, is variations in the proton density, because the proton flux for those days slightly correlates with S4. But the 2015 June 21 low-intensity scintillations accompanied by a high particle density reject the proposed dependence. The described pattern requires an additional reason for mid-latitude scintillations that should perform an energy deposition at the upper atmosphere comparable with the geomagnetic storm effects. Possible agents able to stimulate mid-latitude scintillations are the troposphere events: storms, cyclones, atmospheric fronts, etc., transporting enough energy (e.g. via wave processes) through the stratosphere and the mesosphere to the 200–400 km heights. The described analysis of the scintillation data requires more attention than was paid. It is based on the assumption that scintillations appear due to field-aligned irregularities, but this limits the height range: we should take into account only plasma without significant collisions. Irregularities appearing at the heights, where collisions play a significant role, would form another kind of the scintillations spectra. To clarify possibilities of the method, one should perform a simulation with a set of known ionospheric plasma parameters and different directions of irregularity motions.
Acknowledgements The experimental data were recorded by the Incoherent scatter radar, the No. 01–28 unique scientific facility. The work was supported by the Russian Foundation for Basic Research through Grant 15-05-03946 А and the basic research project II.12.2.1 “Development of new methods for experimental radiophysical studies of the upper atmosphere of the Earth and near-Earth space”. References Afraimovich, E.L., Ishin, A.B., Tinin, M.V., Yasyukevich, Yu.V., Jin, S.G., 2011. First evidence of anisotropy of GPS phase slips caused by the mid-latitude field-aligned ionospheric irregularities. Adv. Space Res. 47, 1674–1680. Astafyeva, E.I., Afraimovich, E.L., Voeykov, S.V., 2008. Generation of secondary waves due to intensive large-scale AGW traveling. Adv. Space Res. 41 (9), 1459–1462. http://dx.doi.org/10.1016/j.asr.2007.03.059. Bezrodny, V.G., Watkins, B., Galushko, V.G., Groves, K., Kashcheyev, A.S., Charkina, O.V., Yampolski, Yu. M., 2007. Observation of ionospheric scintillations of discrete cosmic sources with the use of an imaging riometer. Radio Phys. Radio Astron. 12 (3), 242–260. Bezrodny, V.G., Charkina, O.V., Yampolsky, Yu.M., et al., 2010. Research into stimulated ionospheric scintillation and absorption of emission of discrete space sources using panoramic HF riometer. Radiofizika i radioastronomiya. Radio Phys. Radio Astron. 15 (2), 151–163 (in Russian). Chulliat, A., Macmillan, S., Alken, P., Beggan, C., Nair, M., Hamilton, B., Woods, A., Ridley, V., Maus, S., Thomson, A., 2014. The US/UK World Magnetic Model for 20152020. NOAA National Geophysical Data Center, Boulder, CO. http://dx.doi.org/ 10.7289/V5TH8JNW.
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Journal of Atmospheric and Solar-Terrestrial Physics 160 (2017) 48–55 Oinats, A.V., Kotovich, G.V., Ratovsky, K.G., 2006. Comparison of the main ionospheric characteristics measured by the Digisonde at Irkutsk in 2003 with IRI 2001 model data. Adv. Space Res. 37 (5), 1018–1022. Priyadarshi, S.A., 2015. Review of ionospheric scintillation models. Surv. Geophys. 36 (2), 295–324. http://dx.doi.org/10.1007/s10712-015-9319-1. Reinisch, B.W., Haines, D.M., Bibl, K., Galkin, I., Huang, X., Kitrosser, D.F., Sales, G.S., Scali, J.L., 1997. Ionospheric sounding support of OTH radar. Radio Sci. 32 (4), 1681–1694. Rytov, S.M., Kravtsov, Yu. A., Tatarskii, V.I., 1989. Principles of Statistical Radiophysics 4 Wave Propagation through Random Media. Springer-Verlag Berlin Heidelberg, ISBN 978-3-642-72684-2. Original Russian edition published by Nauka, Moscow 1978. Tereshchenko, E.D., Khudukon, B.Z., Gurevich, A.V., Zybin, K.P., Frolov, V.L., Myasnikov, E.N., Muravieva, N.V., Carlson, H.C., 2004. Radio tomography and scintillation studies of ionospheric electron density modification caused by a powerful HF-wave and magnetic zenith effect at mid-latitudes, 24 May 2004 Phys. Lett. A 325 (5–6), 381–388. http://dx.doi.org/10.1016/j.physleta.03.055, 03759601. Vasilyev, R.V., Kushnarev, D.S., Lebedev, V.P., Medvedev, A.V., Nevidimov, N.I., Ratovsky, K.G., 2013. Perspectives of usage of Irkutsk incoherent scatter radar (IISR) as an imaging riometer and radio-heliograph. J. Atmos. Solar-Terr. Phys. 105, 273–280. http://dx.doi.org/10.1016/j.jastp.2013.06.012.
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Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Spectral characteristics of ionospheric scintillations of VHF radiosignal near magnetic zenith Roman Vasilyev *, Mariia Globa, Dmitry Kushnarev, Andrey Medvedev, Konstantin Ratovsky Institute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences, 664033, Irkutsk P/O Box 291, Lermontov St., 126a, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: VHF ionospheric scintillations Ionospheric irregularities Scintillation spectrum Geomagnetic storm
We present the results of observing the Cygnus-A radio source scintillation in the Earth's ionosphere under quiet and disturbed geomagnetic conditions by using the Irkutsk incoherent scattering radar (IISR). The scintillation method applied for ionosphere testing at IISR confidently determines the Fresnel frequency and power cutoff, the spectral characteristics usually related to the velocities and spatial spectra of ionospheric plasma irregularities. We also use the IGRF magnetic field model to show the relation between the shape of discrete radio source scintillation spectra and the direction to the radio source with respect to the geomagnetic field. The S4 index increase within the magnetic zenith is observed to be conditioned by the scintillation spectrum widening. We also evaluate the zonal velocity of observed ionospheric irregularities as ~10 m/sec assuming the irregularity height to be equal to the F2-layer maximum height in the ionosphere.
1. Introduction Variations in the celestial radio source intensity on the time scale from seconds to hundreds of seconds that appear due to ionospheric irregularities frequently arise at radio astronomical observations or satellite radio sounding and data transmission. The phenomenon, also referred to as scintillation of radio signal in the ionosphere, was well studied in the last century and widely described in scientific literature [Hunsucker and Hargreaves, 2015; Kung and Chao-Han, 1982; Priyadarshi, 2015; Rytov et al., 1989]. Typical indicators used in scintillation technique are the S(1–4) indices reflecting the degree (measure) of ionospheric irregularities. Also, used is the scintillation spectrum shape reflecting the spatial spectrum of ionospheric irregularities and the relative velocity of these irregularities and of the radio source [Guozhu et al., 2007; Bezrodny et al., 2007]. Observation of the magnetic zenith effect in ionospheric scintillations has also had a long history. Scintillation amplification near the magnetic zenith is observed both in the polar ionosphere [Gola et al., 1992] and in equatorial bubbles [Kintner and Ledvina 2004], and is believed to be caused by field-aligned irregularities. The same field-aligned ionospheric irregularities are also responsible for the magnetic zenith effect in the mid-latitude ionosphere [Afraimovich et al., 2011]. Heating experiments with high power HF radiowaves also demonstrate the developing magnetic field co-aligned ionospheric plasma density bunch structures that stimulate scintillations of VHF radiosignal [Tereshchenko et al., 2004]. The ionospheric * Corresponding author. E-mail address: [email protected] (R. Vasilyev). http://dx.doi.org/10.1016/j.jastp.2017.05.016 Received 5 August 2016; Received in revised form 30 March 2017; Accepted 29 May 2017 Available online 30 May 2017 1364-6826/© 2017 Elsevier Ltd. All rights reserved.
scintillations discussed in our study are caused by natural reasons in the mid-latitude ionosphere. We apply spectral analysis of scintillation data to show some features that could be useful for studying the disturbed ionosphere. 2. Observation technique and geometry The Irkutsk incoherent scattering radar (IISR) is located at 52 520 N, 103 150 E. The radar performs observations of the discrete cosmic radio sources in a continuous regime during long time intervals, up to several hours per day for several months. Scintillations of a radiosignal from a discrete space radiosource were also observed and studied, preliminarily, with the IISR [Vasilyev et al., 2013]. The investigations showed the expected seasonally-diurnal variation in the scintillation parameters, as well as the relation between the observed scintillations with an ionogram blurring. Unfortunately, in this study, we were restricted in the measuring system temporal resolution to perform a quality spectral analysis of the data. By now, we have improved the temporal resolution of observations from 18 to 4.5 s. As a result, the frequency resolution in the spectral presentation of the recorded signal has considerably increased. This allows us to distinguish radio source signal from ambient noise more precisely and get spectral parameters of ionospheric scintillations at a better resolution. We performed passive observations for 14 days, 2015 June 18 through July 01, and extracted the variations in the Cygnus-A radio galaxy intensity during that period. Fig. 1 shows the
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Ni ¼
ðSi Li Þ Li
(1)
where N is the processed data set, S is the initial data set, L is the data set obtained from the initial one by the low frequency filtration procedure, i is the number of readings. We used the running median with a 4-min window (60 data points) to build L. The running median filter has shown better results among other types of filters (running average, digital finite impulse response filter, etc.) Normalizing the subtracted data in (1) is necessary to exclude the amplitude dependence of variations. Fig. 3 shows an example of filtered data sets and scintillation spectra obtained from them. Each spectrum was computed by using the following procedure: the entire dataset was split into 20-min subsets (300 data points), and, after applying the Fourier transform to each subset, the transform results were reduced to one spectrum. One can see the Fresnel frequency peak fF and the power law behavior of the resulting spectra for both cases in Fig. 3. The noise floor starting frequency (fN) is clearly defined only for June 23, whereas for June 24, this value is difficult to mark due to the limited time resolution.
Fig. 1. Geometry of the observations (schematically). Green arrows are the direction to the radio source from IISR. Blue arrows show the direction of the geomagnetic field at the ionosphere height. Red dots are ionosphere pierce (penetration) points (IPP). Red arrows are the IPP velocity vector, α is the angle between the geomagnetic field vector and the direction (line of sight) to the radio source. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3. Ionosphere scintillation near the magnetic zenith From our observations, we extract S4 index data for each day (Fig. 4). S4 is determined through the common procedure via the dispersion relation to the mean value in 1 min square window (15 data points). We found that S4 tends to smoothly increase until ~20:00 local stellar time (LST) and smoothly decrease after. At the same time, there are several cases of strong scintillations occurring at random times. In [Jiao et al., 2013], the scintillation amplitude was shown to increase around the magnetic zenith. We suppose the same mechanism to be responsible for the S4 background behavior in our observations. Fig. 4 shows the angle between the magnetic field and the line of sight to the radio source (α) for our observation geometry (black line). The magnetic field direction at the IISR coordinates was calculated with the expansion in spherical harmonics using the WMM2015 [Chulliat et al., 2014] coefficients set. We use the “Geomag 7.0” Software [https://www.ngdc.noaa.gov/IAGA/ vmod/geomag70_license.html] and input data files with exact coordinates of point where line of sight to the Cygnus-A from IISR penetrate through ionosphere. The index slowly increases as α decreases, and has the maximum value, when α is close to zero, i.e., at the magnetic zenith. It is also interesting to extract the spectral characteristics of scintillations near the magnetic zenith. We process our data by using (1) and get the dynamic spectrum (square 20 min window, 300 data points) from the obtained dataset. Fig. 5 presents the dynamic spectrum of ionospheric scintillations accumulated for all the observational days (overall spectrum with the superposed α behavior). The maximum frequency persists
geometry of our observations. Fig. 2 presents a typical behavior for the Cygnus-A signal power (during one observation session). The radio source is moving through the scan view during the observation time and the power of the recorded signal is affected by the antenna beam pattern and by the recording system properties. IISR guiding (controlling DP for tracking the radio signal source over the sky) is based on the frequency scanning principle. Therefore, we use a set of several frequency bands to overlap the entire coverage sector. As compared with the previous observations, the number of bands decreased from 88 to 11 (temporal resolution improvement), and the width of each band increased by 4 times (frequency resolution improvement). Each frequency range features its own uneven amplitude-response curve with the maximum at the central frequency. These curves are responsible for a smooth variation in the recorded signal power in Fig. 2. The envelope of smooth power variations is defined by the antenna system directivity pattern. Despite such a complex and variable structure of the recorded power, one can clearly distinguish a quiet observational day and a disturbed one, because the scintillation frequency is significantly higher than the frequency of variations corresponding to the directivity pattern and the recording system properties. To eliminate low frequency variations showed in Fig. 2, we used a simple procedure:
Fig. 2. Example of signals from the Cygnus-A discrete radio source obtained by a new IISR technique for 2015 June 18 (left) and June 22 (right). Scintillations of radio signal are stronger for 2015 June 22 due to the geomagnetic storm. The high frequency scintillations are clearly different from the smooth variations conditioned by the amplitude-response curve and the antenna system directivity pattern (antenna beam pattern).
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Journal of Atmospheric and Solar-Terrestrial Physics 160 (2017) 48–55
Fig. 3. Example of normalized signals from the Cygnus-A discrete radio source (right panels), and the corresponding accumulated spectra (left panels). The 2015 June 23 Fresnel frequency is ~5 mHz, for 2015 June 24, it is ~20 mHz, the cut-off indices are 1.33 and 0.71, respectively. The 2015 June 23noise floor starting frequency is ~40 mHz, for 2015 June 24, it is fN > 100 mHz.
Fig. 5. Accumulated dynamic spectrum of scintillations for the entire observation period. Black line corresponds to α variation over local stellar time.
observation time from 19:00 to 22:00 LST. The increasing spectral power and additional peaks appearing in the spectrum 21:00 through 21:30 LST are due to the 2015 June 22 geomagnetic storm, the spot of maximum spectral power 19:30 through 20:00 LST is due to strong scintillations not related to the geomagnetic activity. One can see that the same features are responsible for several sharp bounds of S4 in Fig. 4 at the same LST. One can perform a simple parameterization of the scintillation spectrum widening rate in time. Obviously, the rate has the exponential law, so we can propose the equation describing the fN value as a function of time:
Fig. 4. Gray lines are the S4 indices vs local stellar time for the IISR location for all the observational days. Black line is the angle between the magnetic field and the line of sight to the radio source (α).
relatively constant (taking into account the data dispersion) during all the observation period, while the spectrum width changes significantly. The width slowly increases as the source approaches the magnetic zenith, and then slowly decreases. The power spectrum expanding is, apparently, the reason for the background S4 increase during our observations. Also, the second local diffused peak appears in the scintillation spectrum near log10(fF) ~ 1.75. The peak was observed during all the effective
fN ¼ f0 expfrðt t0 Þg
(2)
where f0 is the noise floor starting frequency near the magnetic zenith, t0
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Journal of Atmospheric and Solar-Terrestrial Physics 160 (2017) 48–55
Fig. 6. Accumulated dynamic spectrum of scintillations for the entire observation period. Black lines are the fN values from (3) with parameters: f0 ~ 100 mHz, t0 ~ 20.2 LST, and r ~ 1.5 sec1 positive for the time before the magnetic zenith (20.2 LST) and negative after.
Fig. 7. Dynamic spectra of the 2015 June 21–24scintillations.
positive before the magnetic zenith and negative after the latter (see Fig. 6). Separate days can have parameters notably distinguished from the ones obtained by the accumulated data (Fig. 7). The spread of parameters between several separate days reflects a significant instability of
is the time of approaching the magnetic zenith, r is the rate of fN changing. The accumulated data have the initial frequency f0 ~ 100 mHz, the time for reaching the maximal frequency t0 ~ 20.2 LST (local star time), and the frequency variation tempo r ~ 1.5 sec1. This tempo is 51
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the UHF radio wave scintillation process during a day, except e.g. June 24.
Table 1 Zonal velocity of ionospheric irregularities.
4. Zonal velocity of ionospheric irregularities Another interesting feature of our study (observation) is the comparison between the scintillation spectrum Fresnel frequency and a frequency derived from the velocity of motion of ionospheric pierce point (IPP) through ionospheric irregularities at a fixed height. The height also determines the size of the irregularities through the Fresnel zone size for the IISR wavelength. The IPP velocity vector for our geometry is almost perpendicular to the magnetic field for the entire observation period, and the frequency can be determined as in [Guozhu et al., 2007]:
V⊥ fF ¼ pffiffiffiffiffiffiffiffi 2λR
fF
log10(fF)
V ¼ V┴ ± Vdis
Vdis ¼ VV┴
0.018 0.010 0.006
1.75 2 2.25
19.56 11.00 6.186
0 8.561 13.38
Note that we have not included the motion of irregularities in the described pattern of the observed phenomena yet. In our geometry, IISR observe a part of the Cygnus-A trajectory that, predominantly, lays along the parallel. Due to this fact, in case of the meridional motion of irregularities at a velocity less than V┴, the Fresnel frequency will correspond only to IPP motion through ionospheric irregularities as in (3). If the meridional velocity of irregularities increases to the values significantly more than V┴, the observed Fresnel frequency starts to grow and exhibit rather the motion of the ionospheric irregularities, than the ionosphere piers point (IPP) motion. The zonal motion of the ionospheric irregularities for the IISR observation geometry can significantly increase or decrease the Fresnel frequency, depending on the motion direction. Irregularities travelling East to West lead to a decrease in the observed Fresnel frequency, whereas the opposite direction of motion increases it. In case of Fig. 8, one can say that most of the observed irregularities exhibit the East-West direction, because the spectrum main maximum shifts down to smaller frequencies. We can modify (3) to account for the velocity of irregularities in our case:
(3)
where fF is the (calculated) theoretical frequency, V┴ is the component of IPP velocity normal to the magnetic field, λ is the wavelength of the received electromagnetic radiation (~2 m), and R is the distance from IISR to IPP. Fig. 8 shows the fF behavior for different heights (gray lines). If we treat the spectrum Fresnel frequency as a spectral power maximum, it lays close to the black line for the 100-km height, and the second spectral power local maximum lays at the values corresponding the 400km height. We assume that the maximal intensity of ionospheric irregularities corresponds to the maximal plasma density. The plasma density vertical distribution real values can be obtained from independent instruments. The electron concentration value in the ionospheric plasma is provided by the Irkutsk DPS-4 Digisonde [Oinats et al., 2006; Reinisch et al., 1997] located ~100 km away from the IISR. By using the maximal concentration height from the DPS-4 data, we put the corresponding points of fF on the dynamic spectrum (black empty circles in Fig. 8). The position of the circles is close to fF corresponding to 400 km, and they are located in the area of the second local maximum, so we can conclude that, here, we observe motionless irregularities located at the maximum of ionospheric plasma density.
V⊥ ±Vdis fF ¼ pffiffiffiffiffiffiffiffi 2λR
(4)
Vdis is the velocity of irregularities, sign “þ” corresponds to the West-East irregularities, sign “–“ corresponds to the East-West irregularities. From the scintillation intensity in Fig. 8, we can evaluate the maximal and minimal of the shifted log10(fF) as 2.25 and 2, respectively. Table 1 contains the values of the maximal and minimal zonal velocities of irregularities calculated from (4) assuming that the height of irregularities
Fig. 8. Accumulated dynamic spectrum of scintillations for the entire observation period. Grey lines show the frequency behavior from (3), the top line corresponds to R ¼ 400 km, the bottom line corresponds to R ¼ 100 km. Open black circles correspond to R obtained from ionosonde data. 52
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density of particles. But, at the same time, one can note a correlation between the S4 shape of the relatively strong scintillations and the shape of the relatively weak proton flux density of June 23 and 24.
is ~300–350 km (from the Digisonde data). The zonal velocities are V┴ ¼ 19.56 m/sec. 5. Disturbed geomagnetic conditions
6. Discussion and conclusion All of the processed data 2015 June 18 through 2015 July 01 show a similar behavior. There is a weak spectral width dependence on the angle between the magnetic field and the line of sight to the radio source, and the scintillation process is mostly non-stationary. The 2015 June 22 geomagnetic storm (Fig. 7) is different from other scintillation events only in stronger high frequency scintillations. The beginning of the 2015 June 22 dynamic spectrum possesses the same behavior as the accumulated spectrum does. However, starting with ~19:45 UT (~18:45 LST), the 2015 June 22 dynamic spectrum significantly grows in intensity and expands toward higher frequencies. The Fresnel frequency moves down possibly due to the East – West motion of the irregularities. This fact confuses in a way. Obviously, the source of the ionospheric irregularities lays in the North direction in this case. An explanation may be a phenomenon, when large-scale (hundreds and thousands of kilometers) ionospheric irregularities generate small-scale (meters through kilometers) irregularities [Astafyeva et al., 2008] that move independently from their parent large-scale disturbances. Figs. 9 and 10 present the comparison between the S4 data and the Bz component of the interplanetary magnetic field, as well as the proton flux density. Bz and the proton density were retrieved from the OMNI database (ftp://spdf.gsfc.nasa.gov) [Mathews and Towheed, 1995]. A significant increase in the S4 appears both under the disturbed geomagnetic conditions (June 22, Fig. 8) and under the quiet ones (June 24, Fig. 8). The maximal S4 values for those days differ by a factor of two, but the median values are similar. By using the obtained data, we cannot conclude that the mid latitude ionosphere irregularities are strongly conditioned by the disturbances in the geomagnetic field only. One can see the same patterns with the proton flux. The amount of charged particles at the bow shock does not determine the scintillation intensity. The comparison of different days in Fig. 10 shows that S4 can vary significantly at the comparable
The presented spectral analysis of the UHF-radio wave ionospheric scintillations is based on 14 days of the IISR data. Scintillations appear predominantly during summer nights and have significantly nonstationary nature, both for disturbed and quiet geomagnetic conditions. Also, one can see that the scintillations presented in the ionosphere during the entire described period differ only in the intensity. The several-day averaged data exhibit a dependence on the geomagnetic field. The behavior of the “stationary” scintillation intensity is strongly associated with the geomagnetic field relative directions and with the line of sight to the source: the scintillation intensity maximum observed at the magnetic zenith, when the observer looks at the radio source along the geomagnetic field. The reason for intensification of the scintillation signal is a significant widening of the scintillation spectrum near the magnetic zenith. The widening rate can be described by the moving noise bound of scintillation spectra, fN, with time, and it has the exponential law with an about 1.5 sec1 rate. A similar behavior of the scintillation spectra was demonstrated in [Tereshchenko et al., 2004], with ionosphere modification experiments by powerful HF radio waves. This value can be used as a simple marker to study diurnal, seasonal, etc. dependences of scintillations, and the morphology of ionospheric irregularities causing scintillations. The comparison between the ionosonde and the IISR data exhibits the East-West zonal motion of the irregularities during the observation period. Assuming that ionospheric irregularities causing scintillations develop at the ionization maximum height, one can evaluate the velocity of the irregularities: the value is around 10 m/sec. The obtained velocity of irregularities is significantly lower than the neutral wind velocity by HWM14: ~30 m/sec, westward. This discrepancy can be explained by possible complex mechanism for the irregularity formation. That
Fig. 9. S4 index (black) and magnetic field (gray) variation over 2015 June 21–24. 53
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Fig. 10. S4 index (black) and proton flux (grey) variation over 2015 June 21–24.
The Irkutsk Incoherent Scatter Radar can contribute significantly to the ionosphere monitoring by radio astronomical observations. The IISR observation time window slides over a day due to the fixed scan view. Unfortunately, this closes the possibility to make seasonal observation of the ionospheric radio wave scintillations that mostly appear during a local night at the IISR latitudes. Nevertheless, operations aggregately with other instruments (GPS stations, ionosondes, optical monitoring) open extensive possibilities, in particular, for searching mid-latitude radio wave scintillations sources in the ionosphere.
mechanism should involve the MHD theory approach. Similar velocities of irregularities were also obtained in [Bezrodny et al., 2007] and in [Bezrodny et al., 2010], and the result agrees with the velocity data obtained through an independent spectral method in [Fallows et al., 2015]. The velocity values obtained in [Guozhu et al., 2007] are higher than the presented results. A possible reason may be both a relatively short period of observations in our case and the systematic error intrinsic the instrument applied: we use the data obtained by about 1-m wavelength radio emission, whereas in [Guozhu et al., 2007] used were the GPS data obtained by the 0.1-m emission. The comparison between the S4 indices and the solar wind parameters exhibits an ambiguous dependence of mid-latitude scintillations on the solar wind. Apparently, the 2015 June 22 scintillations are directly related to the geomagnetic storm and to the consequent disturbances that traveled down from the magnetosphere to the ionosphere. Whereas the 2015 June 23 and 24 scintillations, despite a relatively high intensity, do not correlate with the geomagnetic field disturbances. A possible reason for scintillations, in this case, is variations in the proton density, because the proton flux for those days slightly correlates with S4. But the 2015 June 21 low-intensity scintillations accompanied by a high particle density reject the proposed dependence. The described pattern requires an additional reason for mid-latitude scintillations that should perform an energy deposition at the upper atmosphere comparable with the geomagnetic storm effects. Possible agents able to stimulate mid-latitude scintillations are the troposphere events: storms, cyclones, atmospheric fronts, etc., transporting enough energy (e.g. via wave processes) through the stratosphere and the mesosphere to the 200–400 km heights. The described analysis of the scintillation data requires more attention than was paid. It is based on the assumption that scintillations appear due to field-aligned irregularities, but this limits the height range: we should take into account only plasma without significant collisions. Irregularities appearing at the heights, where collisions play a significant role, would form another kind of the scintillations spectra. To clarify possibilities of the method, one should perform a simulation with a set of known ionospheric plasma parameters and different directions of irregularity motions.
Acknowledgements The experimental data were recorded by the Incoherent scatter radar, the No. 01–28 unique scientific facility. The work was supported by the Russian Foundation for Basic Research through Grant 15-05-03946 А and the basic research project II.12.2.1 “Development of new methods for experimental radiophysical studies of the upper atmosphere of the Earth and near-Earth space”. References Afraimovich, E.L., Ishin, A.B., Tinin, M.V., Yasyukevich, Yu.V., Jin, S.G., 2011. First evidence of anisotropy of GPS phase slips caused by the mid-latitude field-aligned ionospheric irregularities. Adv. Space Res. 47, 1674–1680. Astafyeva, E.I., Afraimovich, E.L., Voeykov, S.V., 2008. Generation of secondary waves due to intensive large-scale AGW traveling. Adv. Space Res. 41 (9), 1459–1462. http://dx.doi.org/10.1016/j.asr.2007.03.059. Bezrodny, V.G., Watkins, B., Galushko, V.G., Groves, K., Kashcheyev, A.S., Charkina, O.V., Yampolski, Yu. M., 2007. Observation of ionospheric scintillations of discrete cosmic sources with the use of an imaging riometer. Radio Phys. Radio Astron. 12 (3), 242–260. Bezrodny, V.G., Charkina, O.V., Yampolsky, Yu.M., et al., 2010. Research into stimulated ionospheric scintillation and absorption of emission of discrete space sources using panoramic HF riometer. Radiofizika i radioastronomiya. Radio Phys. Radio Astron. 15 (2), 151–163 (in Russian). Chulliat, A., Macmillan, S., Alken, P., Beggan, C., Nair, M., Hamilton, B., Woods, A., Ridley, V., Maus, S., Thomson, A., 2014. The US/UK World Magnetic Model for 20152020. NOAA National Geophysical Data Center, Boulder, CO. http://dx.doi.org/ 10.7289/V5TH8JNW.
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