HF Doppler Sounder Research Articles

Periodic oscillations of Doppler frequency excited by the traveling ionospheric disturbances associated with the Tonga eruption in 2022

The explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 min. In this study, such periodic oscillations were examined by comparing Doppler frequency data with Total Electron Content data obtained by Global Navigation Satellite System. The observed periodic oscillations in the Doppler frequency were characterized by a sawtooth or S-letter shaped variation, implying the passage of the traveling ionospheric disturbances through the reflection points of the HF Doppler sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the total electron content data, the traveling ionospheric disturbances causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HF Doppler sounding system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north–south direction. This suggests that the traveling ionospheric disturbance has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the propagation of the traveling ionospheric disturbances and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that 4-min period variations are sometimes observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere is a plausible source of the traveling ionospheric disturbances detected in the northern hemisphere.Graphical

Simultaneous observations of equatorial plasma bubbles with an all-sky airglow imager and a HF Doppler sounding system in Taiwan

High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area.Graphical

Instantaneous Achievement of the Hall and Pedersen–Cowling Current Circuits in Northern and Southern Hemispheres During the Geomagnetic Sudden Commencement on 12 May 2021

The present article aims at a consistent understanding of observation, theoretical model, and simulation with the geomagnetic sudden commencement (SC) observed in the morning and afternoon at high and middle latitudes in the northern and southern hemispheres and at the noontime equator on 12 May 2021. The SC in Bx- and By-components of the geomagnetic field, SCx,y, was composed of the positive/negative preliminary (PI) and main impulses (MI) as SCx (+ -) and SCy (- +) in the morning and SCx (- +) and SCy (+ -) in the afternoon at middle latitudes in the northern hemisphere. SCx in the southern hemisphere is in the same polarity as those in the northern hemisphere, except for SCx (+ +) in the morning. SCy in the southern hemisphere has reverse polarity to those in the northern hemisphere. The PIx in the northern hemisphere matches the well-established two-cell Hall current vortices with anti-clockwise and clockwise directions in the morning and afternoon, respectively, and the MIx matches reverse Hall current vortices. The PIx and MIx in the southern hemisphere meet the Hall currents that are mirror images of those in the northern hemisphere with respect to the equator except for the positive MI in the morning. The PIy in the northern hemisphere is shown to meet the northward and southward Pedersen currents in the morning and afternoon, respectively, and the MIy meets reverse Pedersen currents. The PIy and MIy in the southern hemisphere are found to meet the Pedersen currents that are mirror images of those in the northern hemisphere. At the equator, typical SCx (- +) is observed, meeting the Cowling currents that should be supplied by the Pedersen currents responsible for the observed midlatitude SCy in the northern and southern hemispheres. The electric fields of the PI and MI observed by the HF Doppler sounders at the middle latitudes in the northern hemisphere are westward and eastward, respectively, in both the morning and afternoon, meeting the conventional dusk-to-dawn PI and dawn-to-dusk MI electric fields. The onset of the PI is found to be simultaneous with the resolution of a few seconds from high latitude to the equator in both the northern and southern hemispheres, indicating instantaneous achievement of the Pedersen–Cowling currents from high latitude to the equator. The instantaneous achievement of the energy-consuming Pedersen–Cowling currents is explained by the TM0/TEM mode wave in the Earth-ionosphere waveguide/transmission line rather than the compressional waves in the magnetosphere and F-region ionosphere. REPPU (REProduce Plasma Universe) global simulation model equipped with a potential solver at the inner boundary of the model magnetosphere reproduces the PI and MI electric fields at middle latitudes and SCx (- +) at the dayside equator. The simulation results are found to be consistent with most features of observations, such as the time scale of PI and MI, direction of the midlatitude electric field and generation of the Cowling currents. The simulation proves that the electric fields and FACs are generated in the outer magnetosphere, transmitted to the polar ionosphere and then to the equator in the Pedersen–Cowling current circuit.

Propagation characteristics of sporadic E and medium-scale traveling ionospheric disturbances (MSTIDs): statistics using HF Doppler and GPS-TEC data in Japan

We carried out a statistical analysis of the propagation characteristics of Es and Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) by combining data of HF Doppler (HFD) sounder and Total Electron Content (TEC) obtained from the GPS receivers of GEONET (GPS-TEC) for 4 years from 2014 to 2017. We made use of Es reflection data from the HFD receivers in Sugito, Saitama (36.0°N, 139.7°E), Fujisawa, Kanagawa (35.3°N, 139.5°E), and Sugadaira, Nagano (36.4°N, 138.3°E) in Japan. By using this triangle observation, we succeeded in deriving the horizontal speed and direction of the motion of Es. In addition, we estimated the phase velocity of MSTIDs observed in the simultaneously obtained maps of GPS-TEC with the same triangle observation procedure. The speeds of Es and MSTIDs were commonly less than 100 m/s in most cases and their propagation direction was predominantly southwestward. This result is consistent with the statistical characteristics of nighttime MSTIDs observed in the previous studies. More importantly, good correspondence between the propagation characteristics of the two phenomena at two different altitudes confirms that Es and MSTIDs move in tandem with each other, further suggesting that Es in the E region plays an important role in the generation and propagation of MSTIDs in the F region.Graphical

Vertical Propagation of Coseismic Ionospheric Disturbances Associated With the Foreshock of the Tohoku Earthquake Observed Using HF Doppler Sounding

AbstractThe vertical propagation of coseismic ionospheric disturbances (CIDs) associated with the foreshock of the Tohoku earthquake was examined using an HF Doppler sounding system. The sounding system used in this study received radio waves at four different frequencies (5.006, 6.055, 8.006, and 9.595 MHz), indicating that ionospheric disturbances can be observed at up to four altitudes. CIDs were observed at these four frequencies in association with the foreshock of the Tohoku earthquake. From the propagation speed of the coseismic disturbances, it is assumed that the ground motion caused by the Rayleigh wave propagating from the epicenter generates the acoustic mode wave propagating upward, causing the neutral atmospheric particles to move vertically. To examine the characteristics of the acoustic wave, we determined the ratio of the vertical speed of the neutral particles to that of the ground motion observed by a seismometer. The vertical profiles of the ratio are roughly explained in terms of the characteristics of the acoustic mode wave resulting from the ground motion. However, the absolute values of the ratio are smaller than the theoretical ratio estimated by the atmospheric mass density and the attenuation model considering the viscosity, thermal conductivity, and relaxation losses. This result implies that the CIDs in this event may be affected by the nonlinear behavior of the acoustic mode wave.

Penetration of the electric fields of the geomagnetic sudden commencement over the globe as observed with the HF Doppler sounders and magnetometers

Using the HF Doppler sounders at middle and low latitudes (Prague, Czech Republic; Tucuman, Argentina; Zhongli, Republic of China, and Sugadaira, Japan), we observed the electric fields of the geomagnetic sudden commencement (SC) propagating near-instantaneously (within 10 s) over the globe. We found that the electric fields of the preliminary impulse (PI) and main impulse (MI) of the SC are in opposite direction to each other and that the PI and MI electric fields are directed from the dusk to dawn and dawn to dusk, respectively, manifesting the nature of the curl-free potential electric field. We further found that the onset and peak of the PI electric field are simultaneous on the day and nightsides (0545, 1250, 1345 MLT) within the resolution of 10 s. With the magnetometer data, we confirmed the near-instantaneous development of the ionospheric currents from high latitudes to the equator and estimated the location of the field-aligned currents that supply the ionospheric currents. The global simultaneity of the electric and magnetic fields does not require the contribution of the magnetohydrodynamic waves in the magnetosphere nor in the F-region ionosphere. The global simultaneity and day–night asymmetry of the electric fields are explained with the ionospheric electric potentials transmitted at the speed of light by the TM0 mode waves in the Earth-ionosphere waveguide.

Penetration electric fields observed at middle and low latitudes during the 22 June 2015 geomagnetic storm

Prompt penetration electric fields were detected during the intense geomagnetic storm on 22 June, 2015 by the HF Doppler sounders at middle and low latitudes; Prague (Czech Republic), Iitate, Oarai, Onna (Japan), and Zhongli (Republic of China). The storm was initiated by the storm sudden commencement (SC) at 1833 UT, immediately followed by the main phase with the minimum SYM-H of − 139 nT at 2017 UT. The convection electric field was found to be up to 5.1 mV/m at low latitudes, which is enough to cause ionospheric disturbances such as the enhancement of the total electron content, equatorial ionization anomaly and so on. The electric fields of the SC and main phase are westward in the night (0330 MLT), while eastward in the evening (1930 MLT). The main phase electric field intensified the eastward equatorial electrojets (EEJ) at Huancayo in South America on the dayside (1330 MLT) and the westward EEJ on the nightside at Guam in the western Pacific (0330 MLT) and Tirunelveli in India (00 MLT). The eastward direction of the evening-time electric field agrees with a feature of the evening anomaly of the penetration electric field, while the intensity is suppressed because of the sunlit condition in the evening in the summer solstice. The sudden northward turning of the interplanetary magnetic field (IMF) caused a transition from the main phase to the recovery phase, when the EEJ turned into the counterelectrojet (CEJ). It was found that the HF Doppler sounders detected an impulsive eastward overshielding electric field at 2005 UT on the nightside during the northward IMF-driven CEJ with the duration of 17 min and intensity of 8.0 mV/m at Iitate (0500 MLT) and 11.8 mV/m at Prague (2130 MLT). The impulsive overshielding electric field was accompanied by the anti-sunward plasma flows at middle latitude (50°–60° GML) equatorward of the dusk sunward convection as observed with SuperDARN, which signifies intensification of the Region-2 field-aligned currents at the flow reversal. Substorm signatures are identified with intensified westward auroral electrojet and magnetic dipolarization as detected by the geosynchronous orbit satellite, ETS-VIII. Consequently, the stormtime substorm may provide strong overshielding electric fields to the middle latitude ionosphere on the nightside.

Combinational frequencies of HF doppler radar signals in the PC1 geomagnetic pulsation range

Urgency. The urgency of this work is determined by the necessity of studying MHD waves originating from various sources within the Earth–atmosphere–ionosphere– magnetosphere system and arriving at ionospheric heights. The object of research. A matter of this study is ionospheric disturbances that accompanied geomagnetic pulsations during the dawn terminator on 23–24 March 2010. Purpose of Work. The present work was aimed at revealing short-period ionospheric disturbances in the Pc1 micropulsation frequency range (1–5-Hz) and at investigating their spectral content. Techniques and Methotology. The dynamic spectra of the variations under study were obtained with the HF Doppler radar. Results. The ionospheric disturbances have been shown to arise mainly at combinational frequencies. The durations of such disturbances have been estimated to be of the order of one minute, and the disturbance frequencies 0.7 Hz, 1.5 Hz, and 2.5 Hz. The quasi-periodic interference in the 1–5-Hz frequency band has been detected to persist for over one-half hour to a few hours. Based on the model of the signal modulated by ULF waves in the ionosphere, the appearance of constructive interference at combinational frequencies has been validated. The intercomparisons of the variations obtained using the spectrograms and the known models for the phase-modulated signals have been made. A model for the amplitude- and phase-modulated signal reflected from the ionosphere has been developed, and the signal basic parameters have been determined. Conclusions. The HF Doppler sounding can be a means for studying ionospheric disturbances in the Pc1 geomagnetic pulsation range. During the study of time variations of the Doppler frequency shift during the spring equinox, the following results were found. The period of ionospheric perturbations reached 0.2–1 s, their duration varied from 1 min to more than 10 min. A noticeable increase in the amplitude of the beat is detected, which may be a sign of the appearance of lateral maxima in the Doppler spectra. The presence of oscillations in the frequency range of the first harmonic of the spectral resonance system of the ionospheric Alfvén resonator is established. Long-term ionospheric perturbations have a linearly increasing frequency of filling the wave packet. The rate of frequency change is close to 10–3 Hz/s.

Substorm Overshielding Electric Field at Low Latitude on the Nightside as Observed by the HF Doppler Sounder and Magnetometers

AbstractThe convection electric field increases during the growth phase of substorms, driving the DP 2 ionospheric currents at high‐to‐equatorial latitudes, intensifying the eastward equatorial electrojet (EEJ) on the dayside. During the expansion phase, the electric field is often reversed; i.e., overshielding occurs at subauroral‐to‐equatorial latitudes where the EEJ turns to the westward counterelectrojet (CEJ). In this paper, we show that the HF Doppler sounders detected the eastward overshielding electric field at low latitudes on the nightside simultaneously with the CEJ on the dayside. We also show that the overshielding often occurs during the substorm recovery due to the convection reduction, resulting in a two‐step form in both the dayside CEJ and nightside electric field. The opposite direction of the electric field on the dayside and nightside is consistent with the dusk‐to‐dawn potential electric field associated with the region 2 field‐aligned currents intensified by the substorm. The overshielding electric field was found to drive an eastward electrojet with appreciable magnitude in the nighttime equatorial ionosphere, which in turn causes an equatorial enhancement of the midnight positive bay.

Medium-scale traveling ionospheric disturbances over Taiwan observed with HF Doppler sounding

We analyzed horizontal velocities of medium-scale traveling ionospheric disturbances using HF Doppler sounding over Taiwan from January 2014 to January 2016. Meridional components of the observed horizontal velocities are in most cases larger than zonal components. In summer, poleward propagation prevails, whereas in winter, disturbances primarily propagate southward. Zonal components of the analyzed events are mostly eastward. Seasonal behaviors of the observed propagation directions are similar at other locations with HF Doppler sounding at middle and low latitudes. Horizontal velocities of the observed events are in the 50–400-ms−1 range. The upper limit is determined using the spacing between reflection points, making the uncertainty in velocity very large for events with velocities larger than this limit. The mean horizontal velocity is 210 ms−1. The range between the 20th and 80th percentile of horizontal velocities is 144–281 ms−1. Graphical abstract.

Observations of ULF waves on the ground and ionospheric Doppler shifts during storm sudden commencement

AbstractUsing data from ground‐based magnetometers and HF Doppler sounder, we study ultralow frequency (ULF) waves excited during the storm sudden commencement (SSC) on 8 March 2012 and find possible evidence on the link between ULF waves and ionospheric Doppler shifts. Pc1–Pc2 ULF waves are observed from 11:04 to 11:27 UT after the SSC by ground stations of L shell ranging from 1.06 to 2.31, mapping to the topside ionosphere. There are weak responses in this frequency range in the power spectra of ionospheric Doppler shift. From 11:19 to 11:23 UT, oscillations of magnetic field in a lower frequency range of Pc3–Pc4 are observed and are well correlated with the trace of Doppler shift. It is thus suggested that ionospheric Doppler shift can response to ULF oscillations in magnetic field in various frequency ranges, especially in the frequency range of Pc3–Pc4 and below. This paper demonstrates a new mechanism of magnetosphere‐ionosphere coupling.

Response of the incompressible ionosphere to the compression of the magnetosphere during the geomagnetic sudden commencements

AbstractThe ionospheric plasma in midlatitude moves upward/downward during the geomagnetic sudden commencement causing the HF Doppler frequency changes; SCF (+ −) and (− +) on the dayside and nightside, respectively, except for the SCF (+ −) in the evening as found by Kikuchi et al. (1985). Although the preliminary and main frequency deviations (PFD, MFD) of the SCF have been attributed to the dusk‐to‐dawn and dawn‐to‐dusk potential electric fields, there still remain questions if the positive PFD can be caused by the compressional magnetohydrodynamic (MHD) wave and what causes the evening anomaly of the SCF. With the HF Doppler sounder, we show that the dayside ionosphere moves upward toward the Sun during the main impulse (MI) of the SC, when the compressional wave is supposed to push the ionosphere downward. The motion of the ionosphere is shown to be correlated with the equatorial electrojet, matching the potential electric field transmitted with the ionospheric currents from the polar ionosphere. We confirmed that the electric field of the compressional wave is severely suppressed by the conducting ionosphere and reproduced the SC electric fields using the global MHD simulation in which the potential solver is employed. The model calculations well reproduced the preliminary impulse and MI electric fields and their evening anomaly. It is suggested that the electric potential is transmitted from the polar ionosphere to the equator by the zeroth‐order transverse magnetic (TM0) mode waves in the Earth‐ionosphere waveguide. The near‐instantaneous transmission of the electric potential leads to instantaneous global response of the incompressible ionosphere.

HF radar detection of infrasonic waves generated in the ionosphere by the 28 March 2005 Sumatra earthquake

Surface waves generated by earthquakes create atmospheric waves detectable in the ionosphere using radio waves techniques: i.e., HF Doppler sounding, GPS and altimeter TEC measurements, as well as radar measurements. We present observations performed with the over-the-horizon (OTH) radar NOSTRADAMUS after the very strong earthquake (M=8.6) that occurred in Sumatra on March 28, 2005. An original method based on the analysis of the RTD (Range-Time-Doppler) image is suggested to identify the multi-chromatic ionospheric signature of the Rayleigh wave. The proposed method presents the advantage to preserve the information on the range variation and time evolution, and provides comprehensive results, as well as easy identification of the waves. In essence, a Burg algorithm of order 1 is proposed to compute the Doppler shift of the radar signal, resulting in sensitivity as good as obtained with higher orders. The multi-chromatic observation of the ionospheric signature of Rayleigh wave allows to extrapolate information coherent with the dispersion curve of Rayleigh waves, that is, we observe two components of the Rayleigh waves with estimated group velocities of 3.8km/s and 3.6km/s associated to 28mHz (T~36s) and 6.1mHz (T~164s) waves, respectively. Spectral analysis of the RTD image reveals anyway the presence of several oscillations at frequencies between 3 and 8mHz clearly associated to the transfer of energy from the solid-Earth to the atmosphere, and nominally described by the normal modes theory for a complete planet with atmosphere. Oscillations at frequencies larger than 8mHz are also observed in the spectrum but with smaller amplitudes. Particular attention is pointed out to normal modes 0S29 and 0S37 which are strongly involved in the coupling process. As the proposed method is frequency free, it could be used not only for detection of ionospheric perturbations induced by earthquakes, but also by other natural phenomena as well as volcanic explosions and particularly tsunamis, for future oceanic monitoring and tsunami warning systems.

Characteristics of traveling ionospheric disturbances observed by the TIDDBIT sounder

HF Doppler sounders represent a low‐cost and low‐maintenance solution for monitoring wave activity in theF region ionosphere. HF Doppler sounders together with modern data analysis techniques can provide comprehensive traveling ionospheric disturbance (TID) characteristics, including both horizontal and vertical TID velocities and wavelengths across the entire spectrum from periods of 1 min to over an hour. Atmospheric and Space Technology Research Associates LLC has developed a new system called “TIDDBIT” (TID Detector Built in Texas), and data will be presented from a TIDDBIT system deployed in Virginia. These results reinforce the relationship between atmospheric gravity waves (AGWs) and TIDs. The TID propagation azimuths rotate through 360° in 24 h, mimicking the rotation of the thermospheric winds but with approximately a 90° offset. The rotation of TID azimuths and thermospheric winds in Virginia is similar to that observed previously by other Northern Hemisphere systems and opposite from the direction observed in Antarctica. These results illustrate the filtering effects that thermospheric neutral winds can have on the propagation of AGW. The completeness of the wave information obtained from the TIDDBIT system makes it possible to reconstruct the vertical displacement of isoionic contours over the ∼200 km horizontal dimension of the sounder array. Such information will be relevant for understanding the seeding of irregularities, as well as for several operational needs involving navigation, communication, and surveillance systems.

Observations and modelling of the wave mode evolution of an impulse-driven 3 mHz ULF wave

Abstract. A combination of an HF Doppler sounder, a network of ground magnetometers, upstream solar wind monitors and a numerical model is used to examine the temporal evolution of an Ultra Low Frequency (ULF) wave. The event occurred on 16 April 1998 and followed a solar wind density and pressure increase seen in the upstream ACE spacecraft data. The magnetometer and HF Doppler sounder data show that the event develops into a low-m (−6) field line resonance. HF signals that propagate via the ionosphere exhibit Doppler shifts due to a number of processes that give rise to a time-dependent phase path. The ULF electric and magnetic fields are calculated by a one-dimensional model which calculates the wave propagation from the magnetosphere, through the ionosphere to the ground with an oblique magnetic field. These values are then used to determine a model HF Doppler shift which is subsequently compared to HF Doppler observations. The ULF magnetic field at the ground and Doppler observations are then used to provide model inputs at various points throughout the event. We find evidence that the wave mode evolved from a mixture of fast and Alfvén modes at the beginning of the event to an almost purely shear Alfvénic mode after 6 wavecycles (33 min).

Nostradamus: The radar that wanted to be a seismometer

Surface waves emitted after large earthquakes are known to induce, by dynamic coupling, atmospheric infrasonic waves propagating upward through the neutral and ionized atmosphere. Those waves have been detected in the past at ionospheric heights using a variety of techniques, such as HF Doppler sounding or GPS receivers. The HF Doppler technique, particularly sensitive to the ionospheric signature of Rayleigh waves is used here to show ionospheric perturbations consistent with the propagation of Rayleigh wave phases R1 and R2 following the Sumatra earthquake on the 28 March 2005 (M = 8.6). This is in our knowledge the first time that the phase R2 is detected by ionospheric sounding. In addition, we prove here that the ionospheric signature of R2 is also observed by over‐the‐horizon (OTH) Radar. The latter was never used before to detect seismic signature in the ionosphere. Adding the OTH Radar to the list of the “ionospheric seismometers” we discuss and compare the performances of the three different instruments mentioned above, namely HF Doppler sounding, GPS receivers and OTH radar.

Observations of acoustic-gravity waves in the ionosphere generated by severe tropospheric weather

Atmospheric waves influence the dynamics and energetic budget of the upper atmosphere. Using the continuous HF Doppler sounder, we study the wave activity in the ionosphere during tropospheric convective storms in western and central part of the Czech Republic. The study is focused on acoustic-gravity waves in the period range 2–30 minutes. We discuss possible methods of distinguishing the waves emitted by meteorological sources from waves of different origin, particularly waves of geomagnetic origin. In two cases out of twenty-five analysed, we found waves in the infrasonic period range which might be generated by exceptionally intense meteorological activity in the troposphere. The results differ considerably from those previously obtained in North America. In the central part of the United States, infrasonic waves were frequently observed during convective storms. As a possible reason, we discuss different intensity and dynamics of weather systems in both regions.

Doppler observations of infrasonic waves of meteorological origin at ionospheric heights

Ionospheric effects of meteorological origin observed by the continuous HF Doppler sounder over the Czech Republic are reported in this paper. We focused on detection of waves of periods 1–10 min. We discuss the influence of dynamics and intensity of active weather systems on the occurrence of short period waves and dependence of the observed ionospheric effects on the height of reflection of the sounding radio wave. We observed 3–5 min waves during a severe weather event in summer and 2.5–4 min waves during a severe weather event in winter. We excluded possible geomagnetic origin of these oscillations by the analysis of fluctuations of the local geomagnetic field. In eight cases of 10, wave activity in the analysed period range was not significantly increased comparing to quiet days. The intensity of weather systems as well as the location of potential sources of waves towards the points of HF Doppler shift observation influence significantly the occurrence of infrasonic waves in the ionosphere. The results in Central Europe differ considerably from those previously obtained in North America. As a possible reason, we discuss different intensity and dynamics of weather systems in both regions.

Peculiar transient phenomena observed by HF Doppler sounding on infrasound time scales

Compared to investigations of the influence of gravity and planetary waves on the ionosphere, the effects of infrasound (periods from about 0.01 s to several minutes) variations have not been studied very much in the last 20 years. Here we present some recent results on peculiar transient phenomena occurring at infrasound timescales, as observed by HF Doppler sounding in the Czech Republic. After a brief description of the measuring equipment for continuous HF Doppler sounding of the ionosphere, we deal with the observations of short-time transient changes that are observed in the Doppler spectrograms in time intervals of a minute or less, and therefore cannot be observed by ionosondes. First, we present examples of S-shaped traces and examine the diurnal and seasonal variation of their occurrence. We show that S-shape phenomena appear to be concentrated near sunset and sunrise. We also discuss the possible source of these disturbances and their relationship to gravity and infrasound waves. Then we show rare patterns with Doppler shifts corresponding to quasi-linear shape (QLS) phenomena in the time–frequency space. Their slope may be positive or negative. We present some of their properties and discuss the possible origin of such a phenomenon. Several potential sources of QLSs were excluded, such as aircrafts, satellites, bolides, meteors, meteorites, thunderstorms or geomagnetic storms. We speculate that QLSs may correspond to the radio waves in the Z-mode reflected at the upper hybrid resonance frequency.

HF doppler sounder measurements of the ionospheric signatures of small scale ULF waves

Abstract. An HF Doppler sounder, DOPE (DOppler Pulsation Experiment) with three azimuthally-separated propagation paths is used to provide the first statistical examination of small scale-sized, high m waves where a direct measurement of the azimuthal wavenumber m, is made in the ionosphere. The study presents 27 events, predominantly in the post-noon sector. The majority of events are Pc4 waves with azimuthal m numbers ranging from –100 to –200, representing some of the smallest scale waves ever observed in the ionosphere. 4 Pc5 waves are observed in the post-noon sector. The fact that measurements for the wave azimuthal m number and the wave angular frequency are available allows the drift-bounce resonance condition to be used to hypothesise potential particle populations which could drive the waves through either a drift or drift-bounce resonance interaction mechanism. These results are compared with the statistical study presented by Baddeley et al. (2004) which investigated the statistical likelihood of such driving particle populations occurring in the magnetospheric ring current. The combination of these two studies indicates that any wave which requires a possible drift resonance interaction with particles of energies >60 keV, is statistically unlikely to be generated by such a mechanism. The evidence presented in this paper therefore suggests that in the pre-noon sector the drift-bounce resonance mechanism is statistically more likely implying an anti-symmetric standing wave structure while in the post-noon sector both a drift or drift-bounce resonance interaction is statistically possible, indicating both symmetric and anti-symmetric standing mode structures. A case study is also presented investigating simultaneous observations of a ULF wave in ground magnetometer and DOPE data. The event is in the lower m range of the statistical study and displays giant pulsation (Pg) characteristics. Keywords. Ionosphere (Ionosphere-magnetosphere interactions) – Magnetospheric physics (MHD waves and instabilities) – Space plasma physics (Wave-particle interactions)