Ionospheric Drift Measurements Research Articles

Long-term trends of midlatitude horizontal mesosphere/lower thermosphere winds over four decades

Abstract. We analyse 43 years of mesosphere/lower thermosphere (MLT) horizontal winds obtained from a joint analysis of low frequency (LF) spaced receiver lower ionospheric drift measurements from late 1978 through 2008 and VHF meteor radar wind observations since summer 2004 at Collm (51.3∘ N, 13.0∘ E). Due to limitations of the earlier LF measurements, we restrict ourselves to the analysis of monthly mean winds near 90 km, which represents the meteor peak height as well as mean LF reflection heights in the MLT. We observe mainly positive trends of the zonal prevailing wind throughout the year, while the meridional winds tend to decrease in magnitude in both summer and winter. Furthermore, there is a change in long-term trends around the late 1990s, which is most clearly visible in summer MLT winds. We compare these measurements with long-term partial reflection radar observations of winds at 81–85 km over Juliusruh (54.6∘ N, 13.4∘ E) since 1990, and find general qualitative agreement of trends except for summer. The latter can be explained by the different altitudes considered, and by the latitude dependence of the summer mesospheric jet.

ПЕЛЕНГОВАНИЕ ВЧ СИГНАЛОВ, РАССЕЯННЫХ ИОНОСФЕРНЫМИ НЕОДНОРОДНОСТЯМИ, С ПОМОЩЬЮ МАЛОРАЗМЕРНЫХ АНТЕНН

PACS number: 94.20.Bb Purpose: A modification of the Doppler Interferometry Technique is suggested to enable estimating angles of arrival of comparatively broadband HF signals scattered by random irregularities of the ionospheric plasma with the use of small-size near-omnidirectional antennas. Desing/methodology/approach: The technique is based on the measurements of cross-spectra phases of the probe radiation recorded at least in three spaced points. Findings: The developed algorithm has been used to investigate the angular and time-and-frequency characteristics of HF signals propagating at frequencies above the maximum usable one for the direct radio path Moscow–Kharkiv. The received signal spectra show presence of three families of spatial components attributed, respectively, to scattering by plasma irregularities near the middle point of the radio path, ground backscatter signals and scattering of the sounding signals by the intense plasma turbulence associated with auroral activations. It has been shown that the regions responsible for the formation of the third family components are located on the equatorial slope of the maximum of the precipitating particle energy. The drift velocity and direction of the polar ionosphere plasma has been determined. Conclusions: The obtained estimates are consistent with the classical conception on the magnetospheric convection and plasma convection in the polar regions and do not contradict to the results of investigations of the auroral ionosphere dynamics using the SuperDARN network. Key words: ionospheric plasma, scattering, direction finding, polar ionosphere, auroral oval, velocity and direction of plasma drift Manuscript submitted 01.06.2016 Radio phys. radio astron. 2016, 21(3): 231-241 REFERENCES 1. Afraimovich, E. L., 1982. Interference methods of ionospheric radio sounding. Moscow: Nauka Publ. (in Russian). 2. Reinisch, B. W., 1996. Modern Ionosondes. In: H. Kohl, K. Schlegel, eds. Modern Ionosphere Science. Katleburg-Lindau: European Geophysical Society, pp. 440–458. 3. Reinisch, B. W. and Galkin, I. A., 2008. A new digisonde for research and monitoring applications. In: XXIX GA URSI, August 7-16, 2008: Abstracts. Chicago, Illinois, USA, p. 131. 4. REINISCH, B. W., SCALI, J. L. and HAINES, D. M., 1998. Ionospheric drift measurements with ionosondes. Ann. Geophys. vol. 41, no. 5-6, pp. 695–702. DOI: 10.4401/ag-3812. 5. GALUSHKO, V. G., 1997. Frequency-and-angular sounding of the ionosphere. 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Introduction to statistical radiophysics. Part 2: Random fields. Moscow: Nauka Publ. (in Russian). 15. Kashcheyev, S. B., Zalizovski, A. V., Sopin, A. A. and Pikulik, I. I., 2013. On the possibility of bistatic HF ionospheric sounding by exact time signals. Radio Phys. Radio Astron. vol. 18, no. 1, pp. 34–42 (in Russian). 16. Brunelli, B. E. and Namgaladze, A. A., 1988. Physics of the ionosphere. Moscow: Nauka Publ. (in Russian). 17. JAYACHANDRAN, P. T., Macdougall, J. W., ST-MAURICE,J.-P., MOORCROFT, D. R., NEWELL, P. T. and PRIKRYL, P., 2002. Coincidence of the ion precipitation boundary with the HF E-region backscatter boundary in the dusk-midnight sector of the auroral oval. Geophys. Res. Lett. vol. 29, is. 8, pp. 97-1–97-4. DOI: https://doi.org/10.1029/2001GL014184 18. JAYACHANDRAN, P. T., DONOVAN, E. F., MACDOUGALL, J. W., MOORCROFT, D. R., ST-MAURICE, J.-P. and PRIKRYL, P., 2002. 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Meteor radar measurements of mean winds and tides over Collm (51.3° N, 13° E) and comparison with LF drift measurements 2005–2007

Abstract. An all-sky VHF meteor radar (MR) has been continuously operated at Collm (51.3° N, 13° E) since summer 2004. The radar measures meteor parameters, diffusion coefficients, and horizontal winds in the mesopause region. There exists a temporal overlap of the MR wind measurements with co-located low-frequency (LF) ionospheric drift measurements until 2007. Comparison of MR and LF semidiurnal tidal phases allows to empirically determine the virtual height overestimation of LF reflection heights due to the group retardation of LF waves. LF reference heights have to be reduced by up to 20 km to match real heights. Correction of LF heights for group retardation allows to determine the wind underestimation by the LF method compared with meteor radar measurements and opens the possibility to continue long-term trend analysis using mesosphere/lower thermosphere winds.

Penetration characteristics of the interplanetary electric field to the daytime equatorial ionosphere

Using 8 years of ionospheric drift measurements from the low‐latitude JULIA (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere) radar and the solar wind and interplanetary magnetic field data from the ACE (Advance Composition Explorer) satellite, we study the characteristics of the prompt penetration of electric fields to the equatorial ionosphere. A large database allowed us to bring out statistically significant characteristics of electric field penetration as a function of frequency. The coherence between the interplanetary electric field (IEF) and the equatorial electric field (EEF) peaks around a 2‐hour period with a maximum magnitude squared coherence of 0.6. The coherence is slightly higher (0.7) on magnetically active (Ap > 20) days. The cross‐phase spectra between the ACE and JULIA variations, after elimination of the propagation delay, have negligible values. Correspondingly, the time shift between IEF and EEF is less than 5 minutes at all periods. We also find that the penetration efficiency is highest during local noon, as compared with that of morning and evening hours. The coherence is lower for days with high solar flux values. We find that the penetration of electric fields into the equatorial ionosphere has no significant dependence on season and on the polarity of IMF Bz. We propose a transfer function between IEF and EEF, which was validated on synthetic as well as observed IEF data. The use of this transfer function decreases the misfit of a climatological model with the measured equatorial electric field by 27%.

Ionospheric drift measurements: Skymap points selection

In this paper we concentrate on the plasma drift measurement performed by Digisonde Portable Sounder DPS 4 developed by the UMLCAR, Lowell. Special attention is paid to the quality control of data acquired prior to drift velocity calculations. Risks of carefree use of raw Doppler spectra for the plasma motion analysis have been recognized in the Digisonde community. In this paper we propose improvements of Digisonde drift data processing for velocity evaluation and provide a detailed step‐by step description: (1) robust height range selection, (2) setting limits on the Doppler frequency shift, and (3) setting limits on the echo arrival angle. Raw data that passed our quality control were then used to derive three velocity components of the bulk plasma drift in the ionosphere. In the paper we discuss, in detail, application of the method on the DPS measurements. However, this technique can be efficiently applied on any other digital ionosondes drift data processing.

F region drift observations from Athens Digisonde

Ionospheric drift measurements are of significant importance in the monitoring of the upper atmosphere dynamics and an important contributor toward the deeper understanding of the ionospheric electrodynamics. An interesting perspective for operational ionospheric drift measurements comes from the HF radars, such as the Digisondes. Nevertheless, when interpreting velocity measurements obtained from the Digisonde drift analysis method, care must be taken in relating the motion to the effects of photoionization, recombination, gravity waves, and plasma motion. In this paper we investigate the effect of all different mechanisms contributing to the apparent velocity measured by the Athens Digisonde. As a first step, the daily drift pattern over Athens under quiet magnetospheric conditions is preliminarily determined for the equinox months. Moreover, the substorm effect on the extracted pattern is also investigated. Our results provide some evidence for the direct effect of traveling atmospheric disturbances in the vertical component, while in the horizontal plane, the disturbance could be attributed to other substorm‐related mechanisms.

Recent advances in real-time analysis of ionograms and ionospheric drift measurements with digisondes

Reliable long distance RF communication and transionospheric radio links depend critically on space weather, and specifically ionospheric conditions. Modern ground-based ionosondes provide space weather parameters in real-time including the vertical electron density distribution up to ∼1000 km and the velocity components of the ionospheric F region drift. A global network of digisondes distributes this information in real-time via internet connections. The quality of the automatic scaling of the echo traces in ionograms was a continuous concern ever since first attempts have been reported. The modern low-power ionosonde with ∼100 W transmitters (compared to several kilowatt for the older ionosondes) relies on more sophisticated signal processing to enhance the signal-to-noise ratio and to retrieve the essential ionospheric characteristics. Recent advances in the automatic scaling algorithm ARTIST have significantly increased the reliability of the autoscaled data, making the data, in combination with models, more useful for ionospheric now-casting. Vertical and horizontal F region drift velocities are a new real-time output of the digisondes. The “ionosonde drift” is derived from the measured Doppler frequency shift and angle of arrival of ionospherically reflected HF echoes, a method similar to that used by coherent VHF and incoherent scatter radars.

Southern polar cap DPS and CADI ionosonde measurements: 2. F-region drift comparison

Digisonde Portable Sounder 4 (DPS-4) and Canadian Advanced Digital Ionosonde (CADI) digital ionosondes were operated concurrently at the polar cap station Casey (110.5°E, 66.3°S geographic, 81.0°S magnetic), Antarctica during 2000–2001. These two digital ionosondes employ different antenna arrays and digital signal processing techniques, yet nevertheless are closely related instruments. An interleaved operation schedule permitted concurrent recording of ionograms and E- and F-region ionospheric drift measurements. This paper presents observations of background ionospheric drift in the F-region polar cap ionosphere using these two different digital ionosondes. A preliminary comparison between DPS and CADI drift measurements revealed substantial differences between the vertical (Vz) and horizontal (Vh) drift velocities determined by the two digital ionosondes. The DPS has since been relocated to the polar cusp station Davis (78.0°E, 68.6°S geographic, 74.6°S magnetic), Antarctica.

The non-zonal effect in the dynamical structure of the midlatitude MLT-region

Abstract Ionospheric drift measurements in the LF range, which refer to altitudes between 80 and 100 km, make it possible to permanently monitor the circulation in the upper mesosphere/lower thermosphere region (MLT-region). A long-term (1975–1996) study of the MLT region dynamical structure at stations, well-spaced along 52°N latitude circle has revealed the nonzonality in the MLT dynamical structure. This non-zonality in the nighttime prevailing wind, tidal variability and planetary wave activity is considered including results obtained during the MLTCS, DYANA and CRISTA/MAHRSI Campaigns. The longitudinal variations could be interpreted as sign of “coupling from below” and may be important when empirical and physical upper atmosphere models are being formed.

Wind in the middle atmosphere with partial reflection measurements during winter and spring in middle europe

Results of partial reflection wind measurements in the lower inosphere at Juliusruh (54.6 N, 13.4 E) are described. Height-time cross sections of the prevailing wind and the semidiurnal tide in the height region between 73 and 90 km determined during winter and spring in 1989 and 1990 are discussed. Comparisons of the results with corresponding wind values observed by ionospheric drift measurements in the LF-range at Collm (about 370 km apart) show a good agreement in the 30 minutes mean values as well as in the prevailing wind and semidiurnal tide and allow the estimation of wind profiles from 75 km to about 110 km. Further, comparisons with wind results from meteorological rockets launched at Zingst (about 40 km apart) up to a height of about 70 km indicate the possibility of obtaining combined wind profiles over the height range from 25 to 90 km.

Response of the polar cap F region convection direction to changes in the interplanetary magnetic field: Digisonde measurements in northern Greenland

Results of ionospheric drift measurements with a Digisonde 256 digital ionospheric sounder located at Qaanaaq, Greenland (87°N, corrected geomagnetic latitude), are presented. Digisonde drift data have been related to the interplanetary magnetic field (IMF) measured by the IMP 8 satellite for 32 days during 1986, 1987, and 1988. Extremely good statistical agreement between these measurements and convection directions derived from satellite instrumentation is demonstrated when the IMF z component is negative. For Bz negative and By negative the daily average convection direction is centered on −12° (anticlockwise) from the antisunward direction. When Bz is negative but By is positive, the convection direction is centered on +36°. These directions differ by 3° and 9°, respectively, from the models of Heppner and Maynard (1987). The variation about these values is ∼ ±20°. The excellent agreement between the Digisonde measurements and models derived from satellite measurements demonstrates the utility of the Digisonde for making ground‐based measurements of the convection direction in the polar cap F region when Bz is south. The convection directions under conditions of positive Bz have also been examined, and we have measured three types of temporal variation in azimuth, namely, an ordered and slowly (OS) varying change in direction, an ordered and quickly (OQ) varying change in direction, and disordered (D) variations in direction. The latter are believed to result from a breakdown of the analysis technique due to velocity shears in the vicinity of polar cap arcs, and we estimate that they account for ∼25% of the measurements when Bz > 0. When Bz is positive and By is negative, our small subset of OS measurements supports the distorted two‐cell model of Heppner and Maynard (1987). The remainder of the measurements show no well‐defined daily average convection direction or diurnal variation. Likewise for Bz positive and By positive, no well‐defined convection direction can be discerned, nor can any diurnal variation. The existence of OQ variations when Bz > 0 suggests that meaningful average statistical convection patterns may be much harder to synthesize than similar patterns when Bz < 0.

Triangle size effect in spaced antenna wind measurements

MF radar ionospheric drift measurements are usually performed by correlating signals from spaced antennas. It has long been noted that the speeds obtained tend to increase to a limiting value when the antenna spacing is increased, and noise was indicated as a likely factor. This paper takes the view that noise is the only significant factor, and, with the help of some simplifying assumptions, shows that elimination of the noise term also eliminates the speed discrepancy on experimental data from two receiving arrays, each with two available spacings.

Polar cap plasma convection measurements and their relevance to the modeling of the high‐latitude ionosphere

Plasma convection measurements, using Digisonde ionospheric sounders, have been conducted in the central polar cap at Thule, Greenland (86° CGL) and more recently at Qaanaaq, Greenland (87° CGL), in the auroral oval at the Air Force Geophysics Laboratory Goose Bay Ionospheric Observatory (65° CGL), and at suboval latitudes at Argentia NAS (57° CGL). The plasma convection or ionospheric drift measurements conducted at Thule and Qaanaaq during campaigns from the 1983–1984 winter to the present provide evidence that antisunward convection dominates in the polar cap with velocities typically between 300 and 900 m s−1. Velocity reversals or shears were observed in association with polar cap F layer auroras during quiet magnetic conditions. Observations of the plasma drift at Goose Bay show, as expected, a drift reversal from westward to eastward around midnight CGLT, indicating the rotation of Goose Bay from the evening into the morning convection cell. Observations at Argentia, typically a suboval/trough station, provide evidence under magnetically disturbed conditions for the midnight reversal of the antisunward flow pattern. However, the data at Argentia are generally less consistent under magnetically quiet conditions. This likely indicates the proximity of Argentia to the boundary between corotating and convecting plasma. Recent theoretical calculations of electron density profiles within the high‐latitude/polar cap ionosphere demonstrate that the diurnal f0 F2 variation observed at Thule is controlled by the plasma convection pattern and the associated drift velocities. The model calculations for Bz < 0 and Bz ≈ 0 show factors of 2 to 3 differences in Nmax over Thule, supporting the stated importance of convection pattern and velocity measurements for the modeling of the high‐latitude ionosphere.

Data processing in a FM-CW radar system for ionospheric drift measurements by means of partial reflections

Data processing in a FM-CW radar system for ionospheric drift measurements by means of partial reflections

Moving random surfaces and correlation analysis

Two models of moving random surfaces with known velocity and known correlation function are described. The random surfaces are formed from an infinite set of plane waves with decorrelation arising from random temporal and spatial changes. For model 1 the random changes are independent of the wave frequency. For model 2 they increase with frequency. It is shown that if there are random spatial changes in the direction of motion, then speeds derived by the full correlation method will be less than the true speed of the surface. If temporal series based on the models are subjected to a high‐pass filter before correlation analysis, the correlation speeds increase in value. Correlation functions from one of these models (model 2) fail the requirements for full correlation analysis because the half widths of the cross‐correlation functions are greater than the half width of the autocorrelation function. However, from a survey of experimental ionospheric drift measurements it is argued that radio interference patterns appear to have the form of this model. For this model, (1) correlation speeds increase with increasing antenna spacing, (2) the correlation ellipse tends to line up with that antenna pair with the greatest spacing, (3) the straight line of the straight‐line method will not be straight, and as stated above, (4) the speeds will be low. These features have been found experimentally and are contrary to the assumptions of the correlation method. It is shown that an alternate analysis, the “similar fades average velocity method,” gives a better estimate of the surface speed than does the correlation method.

Long-period variations of wind parameters in the mesopause region and the solar cycle dependence

A solar dependence of wind parameters below 100 km was found by Sprenger and Schminder on the basis of long-term continuous ionospheric drift measurements (D1) in the l.f. range. For winter they obtained for the prevailing wind a positive correlation with solar activity and for the amplitude of the semi-diurnal tidal wind a negative correlation. Later on this result was confirmed by radar meteor wind measurements (D2) at Obninsk and further D1 measurements at KÜhlungsborn and Collm. However, after the years 1973–1974 a change in the behaviour of the zonal prevailing wind was observed. At this time we found a significant negative correlation with solar activity with an indication of a new change after 1983. This was obtained from D1 results in Collm and D2 results in Kühlungsborn not only for winter, but also for summer and even for annual averages. We conclude that this long-term behaviour points rather to a climatic variation with an internal atmospheric cause than to a direct solar control. The negative correlation with solar activity of the semi-diurnal tidal wind in winter remained unchanged (up to 1984) and also proved to be the same in summer and for annual averages. Recent satellite data of the solar u.v. radiation and the upper stratospheric ozone have shown that the possible variation of the thermal tidal excitation during the solar cycle amounts to only a few per cent. This is, therefore, insufficient to account for the 40–70% variation of the tidal amplitudes. Some other possibilities of explaining this result are discussed.

Estimation of ionospheric electric fields and currents from a regional magnetometer array

We investigate the technique of calculating ionospheric electric fields and currents and field‐aligned currents using an equivalent current function obtained from a regional array of ground‐based magnetometers at high latitudes (Scandinavian Magnetometer Array). Like similar calculations previously done on a global scale, the derived electric fields and currents are found to depend on the model of ionospheric conductivity assumed. Unlike the global scale calculations, the regional calculations are also dependent on the assumed boundary conditions. Additional information on the electric fields, currents, and/or conductivities is required, besides the ground magnetic variations, to obtain unique estimations of the ionospheric electrodynamic features. In the present case, ionospheric drift measurements from one of the Scandinavian Twin Auroral Radar Experiment radars help constrain the calculations. Analysis of an equivalent current function at 1835 UT on October 7, 1976, representing Harang discontinuity conditions, suggests that upward field‐aligned current probably existed near the boundary between eastward and westward electrojets.

Dynamics of the quiet thermosphere (a review)

A survey of experimental data on thermospheric motions obtained by radiophysical methods is presented. Hydrodynamical motions in the height range 80–120 km have a clearly pronounced meteorological origin (viz. seasonal variations, longitudinal effects, local properties, etc.). A systematic statistical description of horizontal ionospheric drift measurements allows an empirical global model of motions at ionospheric levels to be obtained by the method of spaced receivers in a concerted effort of the world-wide network of stations. The prevailing zonal wind in the mid-latitude E-region is westward in the winter hemisphere and eastward in the summer hemisphere. The semi-diurnal tides at mid-latitudes and the diurnal tides at low latitudes are very intensive. At high latitudes, perhaps there is possibly an independent circulation cell that is defined by the properties of interactions of the high-latitude ionosphere with the magnetosphere and low atmosphere. In the F-region, the seasonal variations are delicately pronounced, while the manner of drift is determined mainly by the latitude. The prevailing drift during solstices is southwestward at high- and mid-latitudes and southeastward in the near equatorial zone. The transition zone coincides with the Sq-current system focus. Drift measurements in the F-region allow information about the space-time variations in ionospheric electric fields to be attained and the neutral wind evaluated at these altitudes. The results are in good agreement with incoherent scatter measurements.

On the behaviour of wind systems in the upper mesopause region in winter and during the transition from winter to summer conditions

Ionospheric drift measurements in the L.F. range have been carried out to examine the phase changes of the semidiurnal tidal wind in the upper mesopause region during winter and during the transition from winter to summer conditions. The observed phase changes do not appear to be predetermined by the initial situation and there is no distinct reciprocal relation between the processes in autumn and in spring. These studies are of interest because certain typical phase changes in the semidiurnal tidal wind can be regarded as a sensitive indicator of the start-time and the character of the seasonal reversals of high-atmospheric circulation systems.

The influence of geomagnetic storms on the measured wind in the upper mesopause region deduced from ionospheric drift measurements

Ionospheric drift measurements in the LF range which normally permit reliable monitoring of circulation and tidal wind systems in the upper mesopause region may be misinterpreted during geomagnetic storms. These cause primary disturbances of the wind fields in the upper atmosphere by plasma heating and electric fields. A recording effect arises from an increase in the reflection level, so that the measurements relate to a different altitude with different wind conditions.