Studying of the spatial–temporal structure of wavelike ionospheric disturbances on the base of Irkutsk incoherent scatter radar and Digisonde data

Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 350–357

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Studying of the spatial–temporal structure of wavelike ionospheric disturbances on the base of Irkutsk incoherent scatter radar and Digisonde data A.V. Medvedev n, K.G. Ratovsky, M.V. Tolstikov, S.S. Alsatkin, A.A. Scherbakov Institute of Solar-Terrestrial Physics, Siberian Branch, Russian Academy of Sciences (ISZF SO RAN), P.O. Box 4026, Irkutsk 664033, Russia

art ic l e i nf o

a b s t r a c t

Article history: Received 15 February 2012 Received in revised form 16 July 2013 Accepted 5 September 2013 Available online 13 September 2013

In this paper the spatio-temporal structure of traveling ionospheric disturbances characteristics is studied on the base of the electron density profiles measured by two beams of the Irkutsk incoherent scatter radar and the Irkutsk Digisonde. The technique for determination of spatial–temporal structure of wavelike ionospheric disturbances was developed using cross-correlation and spectrum analysis of electron density. The automated method for extracting ionospheric disturbances including both longperiod day-to-day variability and short-period variations has been developed. Full analyses of January 15–February 17, 2011 data, including total velocity vector, was carry out for 1–6 h ionospheric disturbances, corresponding to internal gravitational waves. The propagation characteristics agree with those obtained from the known studies of the wavelike ionospheric disturbances. An automated method of ionospheric disturbances analysis was created on the basis of regular continuous measurements with the Irkutsk Digisonde. The statistical analysis of electron density disturbances was carried out for 2004– 2009 period. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Incoherent scatter radar Traveling ionospheric disturbances Internal gravitational waves Ionosphere

1. Introduction The study of wave disturbances (in particular internal gravity waves (IGW)) in the Earth's upper atmosphere is an important and actual problem of the modern solar-terrestrial physics. Now, researchers have understood that wave phenomena contribute greatly to the general circulation of the atmosphere, the formation of its global structure and dynamics (Holton, 1983; Fritts and Alexander, 2003; Alexander et al., 2008), carry out an efficient transfer of energy and momentum in the vertical direction (Drobyazko and Gavrilov, 2001; Pancheva et al., 2002; Lastovicka, 2006), provide connection between the lower, middle and upper atmospheres. Various factors (e.g. temperature stratification, vertical gradients of the background winds, and dissipative phenomena) determining the wave propagation conditions in the upper atmosphere are not investigated enough. The level of current experimental researches requires not only a wide spatial coverage and high temporal resolution but also height structure of disturbances characteristics. Only the observation of three-dimensional pattern and the determination of the horizontal and vertical wavelengths allows estimating the contribution of IGWs in the atmospheric dynamics and determining IGW energy and momentum flux (Alexander et al., 2008). The ionospheric responses

n

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to IGW are traveling ionospheric disturbances (TID), which have been studied for many years (Francis, 1975; Hunsucker, 1982; Williams et al., 1993; Hocke and Schlegel, 1996). Such characteristics of the TID as quasi-periods, wavelengths and amplitudes of virtual height h'F2 and critical frequency foF2 variations were obtained using vertical and oblique ionosondes. The dependences of these characteristics on solar and magnetic activity, season, and local time have been studied for different regions of the Earth. The numerical models were used to study the TID propagation features (Akhmedov and Kunitsyn, 2004) and the relation between the IGW and TID characteristics (Kirchengast et al., 1995). However, many problems are still of current interest. The key problems are the following: (1) identification of IGW sources, (2) transformation of large-scale IGW into small-scale waves, (3) wave–wave and wave–wind interaction mechanisms, and (4) effect of IGW on the development of ionospheric plasma instabilities and generation of plasma irregularities. These problems can be studied using new methods, which are able to measure TID complex spatial– temporal structure. There are very few tools and systems, which can provide this data. Incoherent scatter radars (ISR) give the most complete information about the TID height structure (Oliver et al., 1988; Ma et al., 1998; Vadas and Nicolls, 2008). It is necessary to measure the TID parameters along three directions outside one plane in order to determine the full vector of the TID velocity. Large fully rotatable antenna systems of available radars require much time for changing the direction of sounding and, consequently, do not

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give high enough time resolution. ISR using electronic scanning have a restricted scan sector and, correspondingly, an insufficient spatial base for similar studies. A joint analysis of the data from several tools can considerably improve the situation. We designed the technique for determination of spatial–temporal structure of wavelike ionospheric disturbances using the Irkutsk incoherent scatter radar (IISR) (52.9N, 103.3E) and the Irkutsk Digisonde (DPS-4) located  100 km of the radar (Ratovsky et al., 2008, Medvedev et al., 2009).

2. Method for determining TID propagation characteristics

from the expressions similar to (5) and (6): 8 k ðx x Þ þ ky ðy1 y2 Þ ¼ Δφ12 > < x 1 2 kx ðx2 x3 Þ þ ky ðy2 y3 Þ ¼ Δφ23 > : k ðx x Þ þ k ðy y Þ ¼ Δφ x

3

1

y

3

1

kz ¼ Δφz =Δz

methods for measuring the TID velocity full vector. It is clear that these values are not velocity projections onto the corresponding axis or plane, and these velocities are always larger or equal to V, being related to the velocity magnitude by the following expressions:

! ! ΔNeð R ; tÞ ¼ ΔN 0 ðzÞAðtτð R ÞÞ

3. Statistical analysis of the characteristics of TID based on data of ISR and ionosonde DPS-4

! !! τð R Þ ¼ ð e R Þ=V

ð2Þ

! ! R ¼ fRx ; Ry ; Rz g is radius-vector of observation point, e ¼ fex ; ey ; ez g is unit vector specifying the wave propagation direction, V is wave velocity. As a coordinate system we chose the Cartesian system with the origin in ISR location, where the z-axis is upward, ! the x-axis is northward, y-axis is eastward. In this system the e vector has the coordinates {cosθcosψ, cosθsinψ, sinθ}, where θ is the elevation angle over the horizon with upward wave propagation direction as a positive, ψ is azimuth angle with respect to north, taking clockwise as a positive. The delay (or time difference) between the Ne-TIDs observed at ! ! the points with radius-vectors R 1 and R 2 is ! ! !! ! Δτð R 1 ; R 2 Þ ¼ ð q ð R 1  R 2 ÞÞ

ð3Þ

where ! ! q ¼ e =V

ð4Þ

Using the mutual delays between the Ne-TIDs observed by two beams of ISR and DPS-4 at the same heights we obtain the linear system of equations for determination of qx and qy 8 q ðx x Þ þ qy ðy1 y2 Þ ¼ Δτ12 > < x 1 2 qx ðx2 x3 Þ þ qy ðy2 y3 Þ ¼ Δτ23 ð5Þ > : q ðx x Þ þ q ðy y Þ ¼ Δτ x

3

1

y

3

1

31

where (x1, y1), (x2, y2) and (x3, y3) are the observing points coordinates in xy-plain for two beams of ISR and DPS-4, respectively. Set of Eq. (5) is redundant and can produce three sets for determining qx and qy. This redundancy was used to decrease the error of measurements by averaging the results. Using the delays Δτz between the Ne-TIDs observed by DPS-4 at different heights we can determinate qz qz ¼ Δτz =Δz

ð6Þ

The phase difference approach for determining TID motion parameters can be used to extract one dominant harmonic from the entire TID spectrum. This method consists in determining the harmonic phase difference observed at different spatial points. ! Phase differences can be used to calculate the full wave vector k

ð8Þ

We should mention one important circumstance: V h ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=ðq2x þ q2y Þ and Vz ¼ 1/qz values can also be determined by both

and

where ΔN0(z) is height profile of TID, A(t) is its temporal form

ð7Þ

31

The initial data are the electron density (Ne) profiles measured with two beams of IISR (Potekhin et al., 2008) and DPS-4. Two approaches were used depending on the disturbance character. The cross-correlation approach is more universal and consists in determining delays between TID at spaced points using a correlation analysis. The shift corresponding to the crosscorrelation function maximum is considered as a delay. We assume that Ne-TID have the form of planar wave: ð1Þ

351

V z ¼ V = sin θ V h ¼ V= cos θ

ð9Þ ð10Þ

Continuous simultaneous observations of electron density with the IISR and DPS-4 were made during the winter stratospheric warming from January 15 to February 17, 2011. Full analyses of the data, including total velocity vector, was carried out for most pronounced ionospheric disturbances with the periods from 1 to 6 h, corresponding to IGW. The automated method is based on selecting the dominant harmonic from all spectrum of a wave disturbance. The data from all beams were reduced to one point of time in 15 min increments by interpolation. The spectral analysis was carried out for each beam and at each height in the running 12-hour window. To reduce the effect of sidelobes the 12 h Blackman window was used. The coincidence of spectral maxima at three neighbor heights as a minimum for each tool (DPS-4, and two IISR beams) was a criterion for the presence of a wave-like disturbance. The measurement time was assigned to the middle of the current 12-hour window. Prolonged disturbances occurring in several neighbor windows are taken into account several times in the overall statistics. The DPS-4 ionosonde Ne-profiles were constructed from the ionogram traces using the Reinisch and Huang method (Reinisch and Huang, 1983) with the extrapolation above a peak height (Reinisch and Huang, 2001). Only the heights below the peak height at middle of the current 12-hour window were included in the spectral analysis. The lower limit of the analyzed height range was 170 km due to IISR clutter obscures below this height. Fig. 1 demonstrates an example of the 12-h window spectral analysis. Left column shows Ne variations at three tools, the black line indicates the peak height variations. Spectra presented in middle column show two local maxima satisfying the above criterion at 0.33 h  1 and 0.73 h  1 frequencies. Results of calculations of TIDs propagation characteristics are displayed in right column (solid for 0.33 h  1 and dotted for 0.73 h  1 frequencies). Five disturbances were included in the statistical analysis from the demonstrated case: three at  0.33 h  1 frequency (200, 210, and 220 km heights) and two at 0.73 h  1 frequency (210 and 220 km heights). The processing of all data set revealed 2579 cases of TID corresponding to the criterion. Preliminary analysis showed that the number of detectable disturbances is noticeably higher at nighttime than at daytime which agrees with theoretical estimation (Ivanov and Tolstikov, 2003). It is known that day- and

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Number of disturbances

Fig. 1. An example of the 12-h window spectral analysis. Left column is Ne and peak height variations; middle column is spectra; and right column is TIDs propagation characteristics.

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0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

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(h-1)

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Frequency (h-1)

Fig. 2. Frequency distributions of wave disturbances. Gray is day, black is night.

nighttime wave propagation conditions in the upper atmosphere may be significantly different due to the differences in neutral temperatures and prevailing background wind direction. Therefore, day- and nighttime disturbances were studied separately. The frequency distributions of wave disturbances for the dayand nighttimes are shown in Fig. 2 (the day and the night were separated by the terminator on the ground surface). There is absolute maximum in the distribution corresponding TID with period about 2.5 h both for the day- and nighttimes. In addition, there is a local maximum at 1.25 h period for the daytime. The phase difference approach was used to obtain TID propagation

characteristics. The distribution of azimuths is shown in Fig. 3. Here and below, the relative frequency is ratio of number of disturbances with fixed parameter (azimuth for Fig. 3) to total number of disturbances. As can be seen from the distribution, the dominant direction of TID propagation is from north to south. The most probable azimuth is  1351 for the daytime and  2051 for the nighttime. There is also local maximum around northward direction. Fig. 4 shows distribution of elevation angles. Most of the TID have downward phase velocity (negative elevation angle), that corresponds to the IGW propagating from a source that lies below the observed area. However, at night many TID have zero elevation

Relative frequency

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Azimuth (degree)

Relative frequency

Fig. 3. Distribution of azimuths. Gray is day, black is night.

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Elevation (degree)

Relative frequency

Fig. 4. Distribution of elevation angles. Gray is day, black is night.

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Fig. 5. Distribution of velocity magnitudes. Gray is day, black is night.

angles, and this fact fits badly in our knowledge about laws of IGW propagation in the atmosphere. Most likely this is due to an interference of waves with the close frequencies. Distribution of velocity magnitudes is shown in Fig. 5. The velocity distribution has a global maximum at  35 m/s for daytime and  65 m/s for nighttime. Representative nighttime disturbances statistics may be used for an estimation of the neutral wind effect on the TID dynamic characteristics. It is known that nighttime neutral wind is mainly southward. All night disturbances were divided into two groups

according to their azimuths. The nighttime wave–wind interaction should reduce northward TID velocity and increase velocity of southward TID. Thus we expect to find the difference between southward (180 7451) and for northward (360 7451) TID velocities. Comparison showed that the southward TID velocity is only 10 m/s greater on average than the northward velocity. Fig. 6 shows the wave lengths distribution which has a global maximum at 150 km wave length. This distribution agrees with the Vadas (2007) model for the upper atmosphere filtering properties. Under the conditions of existence of thermospheric

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Relative frequency

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0

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Wave length (km)

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Wave length (km)

Tbv(min)

Fig. 6. Distribution of wave lengths. Gray is day, black is night.

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f (h-1)

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f (h-1)

Fig. 7. Frequency dependence of the Brunt–Vaisala period.

T bv ¼ cos ðθÞ=f

ð11Þ

Fig. 7 shows frequency dependence of the calculated Brunt– Vaisala period. It is seen that the calculated period strongly varies with frequency and this does not look reasonable, because the Brunt–Vaisala period is a characteristic of environment. The realistic values (10–30 min) are seen only for the highest frequencies. Thereby, the simplified Hines dispersion equation seems to be applicable only for high frequency AGWs. Generally, we can note that the period from January 15 to February 17, 2011 is characterized by high wave activity. On the average we obtained  76 wave-like 0.5–6 h period disturbances for a day. The main disturbance sources are below the observed area, and the probable cause of increased wave activity during this period are the dynamic processes in the stratosphere and mesosphere, related with the development of the sudden stratospheric warming (SSW). To illustrate the SSW influence on the wave activity we superimpose stratospheric temperature above Irkutsk and the number of TIDs for a day. As seen from Fig. 8, the local stratospheric warming (Medvedeva et al., 2012) ( 301 K increase of stratospheric temperature above Irkutsk during January 24–29, 2011) is accompanied by  20% wave activity enhancement.

0.024

250

0.022 0.02

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0.018 230

0.016 0.014

T (K)

Relative frequency

winds and a realistic temperature profile the TID with wavelength 100–200 km should be most often observed at ionospheric heights (Vadas, 2007). Having the elevation angle θ and the frequency f, we can calculate the Brunt–Vaisala period Tbv from the simplified Hines dispersion equation for atmospheric gravity waves

220

0.012 0.01

210 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Day Fig. 8. Superimpose stratospheric temperature above Irkutsk (black line) and the distribution relative frequency of TIDs (gray bars) for a day of 2011.

4. Statistical research of the wave disturbances in the period of low solar activity using DPS-4 ionosonde data Unlike ISR, an ionosonde makes regular observations. Of course, the full set of TID propagation characteristics cannot be determined on the basis of the only vertical electron density profile, but the representative statistics allows obtaining the seasonal and solar activity pattern of the wave activity and the apparent vertical velocity statistic. On the basis of regular, continuous observations from the Irkutsk DPS-4, the automated method of TID studies was developed and the full analysis of TID events was conducted for 2004–2009 period (phase of decreasing and minimum of solar activity). The spectral analysis with the 12 h Blackman window

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was used. If we have the only instrument, we must use stronger criteria for wavelike TID detection. So, we used four criteria to select wave-like disturbances.

Relative frequency

(1) The local spectral maxima coincide at three heights as a minimum. (2) The amplitude of local maximum is more than 3% of zero harmonic amplitude. (3) The amplitude of local maximum is more than 20% greater than neighbor amplitudes. (4) The apparent vertical velocity Vz is less then 1000 km/h.

The criteria 2–3 serve to exclude wideband noise variations. The last criterion is due to the fact that large apparent vertical velocities correspond to elevation angles close to zero (Eq. (9)). These cases disagree with IGW propagation theory and probably are due to interference of several TID. Fig. 9 shows the year-by-year distribution of wave disturbances. The level of daytime wave activity monotonically decreases from 2004 to 2008 and weakly increases to 2009 and this is true for the nighttime level except for 2005. Fig. 10 shows dependences of the disturbances number on both solar and geomagnetic activities (annual means of F10.7 and AE indexes

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0

0 2004 2005 2006 2007 2008 2009

2004 2005 2006 2007 2008 2009

Years

Years

Fig. 9. Year-by-year distribution of wave disturbances. Gray is day, black is night.

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120 160 200 240 AE (nT)

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40

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120 160 200 240 AE (nT)

Fig. 10. Dependences of the relative disturbances number on solar (a—day, b—night) and geomagnetic (c—day, d—night) activity.

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from Boulder WDC (http://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_ DATA/INDICES/)). The solar activity dependence is monotonic except for the nighttime of 2005. Also the wave activity level tends to increase with AE index, but the dependence is not monotonic. A similar dependence was found by Deminova et al. (1988) on the basis of the critical frequencies and the peak heights analysis from a large amount of the Northern hemisphere ionosondes data. Seasonal distribution of wave disturbances is shown in Fig. 11. The seasonal distribution shows the existence of asymmetry of wave activity between winter and summer. It should be noted that this result has good agreement with the wave activity at stratospheric and mesospheric heights (Alexander et al., 2010) and the seasonal distribution of IGW events in the lower ionosphere (Oleynikov et al., 2007). Both daytime and nighttime distribution shows the maxima in January and November. Daytime distribution is characterized by a local peak in May. Fig. 12 shows the distribution of apparent vertical velocities. It is seen that most of TID have downward phase velocity. The distribution has a global maximum at   22 m/s for the daytime and 42 m/s for the nighttime.

full wave vector of the TID velocity and to investigate their heighttime profile. Regular ionosonde observations provide a long series of wave disturbances data to study their statistical properties. The developed algorithm using a limited set of simple criteria for the wave-like disturbances separation makes it possible to automatically process the long series of data. Joint analyses of continuous simultaneous January 15–February 17, 2011 observations of electron density with the Irkutsk incoherent scatter radar and the Irkutsk Digisonde gave the following results:

The electronic scanning capabilities of the Irkutsk incoherent scatter radar coupled with conventional ionosonde data represent a large opportunity for detailed study of the internal gravity waves in the upper atmosphere. This radio complex allows us to get the

Results of analysis of Irkutsk ionosonde 2004–2009 data agree with the wave activity at strato-mesospheric altitudes and the seasonal distribution of IGW events in the lower ionosphere. The

Disturbances per hour

5. Conclusion

(1) The obtained TID parameters show predominant southward and downward phase propagation. The distribution of wave lengths and velocities is correlated with results of other researchers. (2) The most of TID have downward phase velocity, suggesting that the TID sources are below the observed area. For the considered Jan 15–Feb 17, 2011 period, the sources are probably related to convective processes developing in the lower and middle atmosphere during the SSW. (3) The dependence of the night TID on their propagation direction was found. Average southward TID velocity is 10 m/s greater than the northward one. This effect can be explained by the interaction of TID with the background neutral wind.

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Fig. 11. Seasonal distribution of wave disturbances. Gray is day, black is night.

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Fig. 12. Distribution of apparent vertical velocities. Left is day, right is night.

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