Global Pulsations Research Articles

Experimental and theoretical study on the burning and pulsation behaviors of hydrogen-containing and hydrocarbon jet flames

Hydrogen-containing and hydrocarbon fuels have been widely used in industrial production. Jet fires involving these fuels emerge frequently after leakage or explosion accidents for gas pipelines and processing equipments. To reveal the influencing mechanisms of inclination angle on the pulsation behaviors of jet fires, the burning experiments of hydrogen, syngas, methane and propane jet fires with a nozzle diameter (D) of 15 mm, inclination angles (θ0) of 0°∼90°, fuel flow rates (Qf)of 2.5–40 L/min were conducted. The variations of the global and spatial pulsation frequencies with inclination angle and fuel flow rate are sensitive to the fuel types. Subharmonic pulsation was found at large fuel flow rates (Qf>Qcri). For hydrogen-containing jet flame, a transition from natural pulsation to subharmonic pulsation occurs at a critical inclination angle (θcri). In this case, the transition height where the local pulsation frequency begins to be dominated by subharmonic pulsation, increases with the inclination angle. The local Richardson number (Riy) at the transition height is close to 1. The interaction between the inner and outer vortices increases with the decrease of inclination angle, resulting in decreased natural frequency at the nozzle exit. With the theoretical velocity and flame width at y/D = 2, a unified dimensionless model of natural pulsation frequency was developed, Sty=0.411/Fry0.5, reasonably predicting the natural pulsation frequency of jet flames under different conditions.

Self-Acceleration and global pulsation of unstable laminar Hydrogen-Air flames

The self-acceleration and global pulsation of spherically expanding laminar hydrogen-air flames were studied in a spherical explosion vessel, over a wide range of equivalence ratios (0.4–2.0), initial temperatures (300–400 K), and initial pressures (0.1–0.7 MPa). Comprehensive quantitative data on the self-acceleration propagation of unstable laminar hydrogen-air flames were obtained. The results show that the self-similarity appears after the onset of instability, and the previous assertion of acceleration exponent α = 1.5 is not valid for hydrogen-air flames. The derived values of α for such flames vary from 1.125 and 1.39 in the present work. For the self-accelerating flame, the temporal evolution of the flame radius is described by ru=Atα, and temporal flame propagation speed is described by Sn=Aαtα-1. A modified theoretical expression of the constant A is proposed and validated against the present experimentally derived results. The acceleration phase after flame instability presents a global pulsating acceleration pattern. The frequency of global pulsation rises as the pressure or temperature of the flame increases. A pulsing flame speed equation can be used to reliably estimate flame speed after the onset of cellular instability. Self-acceleration observed in small laboratory explosions can serve as a predictive indicator for flame behaviour on a larger atmospheric scale.

Effect of inclination angle on the pulsation frequency of syngas jet fire

The pulsation frequency of inclined jet fire was systematically studied in this study. Experiments of syngas jet fire with a nozzle diameter of 15 mm at different fuel flow rates (2.5 L/min∼20 L/min) and inclination angles (0°∼90°) were conducted. The pulsation frequency of inclined jet flame is quantified by applying FFT to the time variations of image correlation coefficient, flame height and flame width, corresponding to the global, vertical and spatial pulsation frequencies, respectively. Experimental results indicate that the vertical pulsation frequency can be derived only for jet flames with significant clip-off. The effect of inclination angle on the pulsation frequency depends on the fuel flow rate. For Qf≤7.5L/min, the natural frequency is dominant and increases slightly at small inclination angles. For larger fuel flow rates, a transition from subharmonic frequency to natural frequency occurs at a critical inclination angle θcri, which is sensitive to fuel flow rate. The pulsation behaviors of inclined jet flame are driven by three different instability modes, R-T instability at small fuel flow rates, and Extended R-T and K-H instabilities at larger fuel flow rates. The increase of global pulsation frequency with an increasing inclination angle for Qf≥10L/min results from the increased natural frequency at the nozzle exit. By defining a characteristic diameter D∗, a dimensionless model is developed to predict the global pulsation frequency of inclined jet fires with large fuel flow rates, St=0.284(1/Fr)0.473 for natural frequency and St=0.516(1/Fr)0.473 for subharmonic frequency.

Study on the global and spatial pulsation frequencies of rectangular syngas jet fire with different aspect ratios

Flame pulsation is a significant feature for both laminar and turbulent jet fires. In this study, the pulsation behaviors and the dominating mechanism of rectangular jet fires were explored. A series of experiments were conducted on the rectangular syngas jet fires with fuel flow rates ranging from 5 L/min to 45 L/min and aspect ratios ranging from 2 to 8. Both the global and spatial pulsation frequencies of syngas jet flame were obtained based on the time variations of image correlation coefficient and flame width. The local pulsation frequency of jet flame near the nozzle exit, which is defined as the natural frequency, increases slightly with the fuel flow rate due to the enhanced interaction of the inner vortex with the flame sheet and outer vortices. A transition from natural pulsation to subharmonic pulsation was found to happen when the local Richardson number (RiZ) of the jet flame is close to 1. The global pulsation frequency is dominated by the natural frequency at the nozzle exit for Qf<20 L/min, however, the subharmonic frequency for higher fuel flow rates. Non-axisymmetrical development of buoyancy-induced instability is considered to play a significant role in the rectangular jet fire, leading to the varied spatial pulsation frequency and the elongated flame height at side edges. In addition, the dimensionless global pulsation frequency (St) was found to correlate well with the Froude number (Fr) at the nozzle exit, St=0.42(1/Fr)0.46 for Fr > 10 and St=0.40(1/Fr)0.56 for Fr < 10.

Semi-regular red giants as distance indicators

Context. Semi-regular variables (SRVs) are similar to Miras in brightness, and they also follow one or more period–luminosity relations (PLRs), though not necessarily the same one as Miras. As potential standard candles they are more challenging than Miras because of their smaller variability amplitudes and less regular light curves, but they are substantially more numerous and especially promising for probing old stellar populations. Aims. We aim to characterise the variability of SRVs, specifically focusing on their connection with Miras, in order to prepare the ground for investigating their potential as distance indicators. Methods. We examine SRVs and Miras in the Magellanic Clouds from OGLE-III observations, with data from Gaia and 2MASS. After cleaning the sample of variability periods unrelated to pulsation, we classify each source by chemical type and combination of pulsation modes. We examine the results in terms of global photometric and pulsation properties. Results. We identify four SRV groups that fit the general evolutionary scenario predicted by theory. SRVs dominated by fundamental-mode pulsation are very similar to Miras, especially if mono-periodic. They further split into two subgroups, one of which follows the same sequence as Miras in the period–luminosity and period–amplitude diagrams, without discontinuity. Conclusions. The similarities between Miras and SRVs suggest that the latter can be adopted as distance indicators in a way that is complementary to the use of the former, thereby at least doubling the available number of long-period variables (LPVs) suitable for use as distance indicators. The traditional amplitude-based separation between Miras and SRVs is not necessarily appropriate, and a more physically sound criterion should also involve pulsation periods. While this would require comparatively longer time-series, they are expected to become accessible in the coming years even for weak sources thanks to current and future large-scale surveys. The table of reclassified LPVs is made public.

Global Pc5 Pulsations From the Polar Cap to the Equator: Wave Characteristics, Phase Variations, Disturbance Current System, and Signal Transmission

AbstractWe present observations of global Pc5 pulsations by ∼180 ground magnetometers during the recovery phase of an intense geomagnetic storm. Measurements of magnetic fields along four latitudinal magnetometer chains and one longitudinal magnetometer chain are used to determine the wave characteristics of Pc5 pulsations. Many new features of Pc5 pulsations are identified from a single event for the first time. The Pc5 pulsations occur simultaneously at all latitudes and longitudes/local times, and the period of the Pc5 pulsations is about 10 min and does not depend on latitude and longitude. The latitudinal distributions of the northward (H) component and magnetic pressure of Pc5 perturbations are derived. The phase of the H component reverses across 50–70° magnetic latitude (MLat) but is nearly the same at middle and low latitudes. The phase of the H component at low latitudes does not vary much with longitude/local time. The phase of the D component magnetic field reverses near the equator in the latitudinal direction and also reverses around dawn, noon, dusk, and midnights in the longitudinal direction. The distribution and evolution of global ionospheric equivalent disturbance currents during Pc5 pulsations are derived. A coupled magnetospheric‐ionospheric current system is proposed to explain the observed characteristics of global Pc5 pulsations. It is suggested that the variations of the phase of Pc5 pulsations with latitude and local time are determined by the global distribution of the coupled magnetospheric‐ionospheric current system but not caused by propagation delay of the signals.

A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

Context. Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant in shedding some light on the internal structure of these stars through asteroseismology, similarly to what occurs with other classes of pulsating white dwarfs. Aims. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the ε mechanism is able to excite gravity-mode pulsations. We assess the ranges of unstable periods and the corresponding instability domain in the log g − Teff plane. Methods. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models with stellar masses between 0.2025 and 0.3630 M⊙ going through CNO flashes during their early cooling phases. Results. We found that the ε mechanism due to stable hydrogen burning can excite low-order (ℓ = 1, 2) gravity modes with periods between ∼80 and 500 s for stars with 0.2025 ≲ M⋆/M⊙ ≲ 0.3630 located in an extended region of the log g − Teff diagram, with effective temperature and surface gravity in the ranges 15 000 ≲ Teff ≲ 38 000 K and 5.8 ≲ log g ≲ 7.1, respectively. For the sequences that experience multiple CNO flashes, we found that with every consecutive flash, the region of instability becomes wider and the modes are more strongly excited. The magnitudes of the rate of period change for these modes are in the range of ∼10−10–10−11 [s/s]. Conclusions. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. Given the current problems in distinguishing some stars that populate the same region of the log g − Teff plane, the eventual detection of short-period pulsations may help in the classification of such stars. Furthermore, if a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the ε mechanism. In addition, confirming a rapid rate of period change in these pulsations would support the idea that these stars actually experience CNO flashes, as has been predicted by evolutionary calculations.

Periodic Modulation of the Upper Ionosphere by ULF Waves as Observed Simultaneously by SuperDARN Radars and GPS/TEC Technique

AbstractRecent work has demonstrated that global Pc5 pulsations observed by ground‐based magnetometers may be accompanied by periodic oscillations in the total electron content (TEC) of the ionosphere measured by GPS receivers. These TEC observations may provide new insights into magnetosphere‐ionosphere coupling mechanisms, especially when combined with other ground‐based observational techniques. Presented in this paper is a large‐scale morning sector Pc5 event which was observed simultaneously by ground‐based magnetometers, two high‐frequency Super Dual Auroral Radar Network (SuperDARN) radars, and several Global Positioning System (GPS)/TEC receivers. The transient 2.6 mHz pulsations observed by the ground magnetometers and radars are accompanied by periodic fluctuations in the time rate‐of‐change of TEC (ROT) at the same frequency. To investigate possible mechanisms for the TEC modulation by ultralow frequency (ULF) waves, we determine the ratios between the spectral amplitudes of the magnetic, ionospheric Doppler velocity and ROT oscillations. The relationship between the simultaneous magnetic field and ionospheric Doppler velocity oscillations can be reasonably well interpreted using the theory of Alfven wave interaction with the thin ionospheric layer. Though the observed ratio between ROT and Doppler velocity amplitudes can be explained by the occurrence of local steep gradient of the topside ionosphere plasma at auroral latitudes, the responsible modulation mechanism cannot be considered as firmly established.

White-dwarf asteroseismology: An update

AbstractThe vast majority of stars that populate the Universe will end their evolution as white-dwarf stars. Applications of white dwarfs include cosmochronology, evolution of planetary systems, and also as laboratories to study non-standard physics and crystallization. In addition to the knowledge of their surface properties from spectroscopy combined with model atmospheres, the global pulsations that they exhibit during several phases of their evolution allow spying on the deep interior of these stars. Indeed, by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable white dwarfs and the periods predicted by representative theoretical models, we can infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile and strength of magnetic fields. In this article, we review the current state of the area, emphasizing the latest findings provided by space-mission data.

Self-acceleration and global pulsation in expanding laminar H2−O2−N2 flames

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Constant-Speed Oscillation of a Pump Turbine Observed on a Pumped-Storage Model System

The hydraulic characteristics of pump turbines in off-design conditions, especially the S-shaped characteristics, are crucial for the safety and stability of the unit. To explore the S-characteristics of pump turbines through a transient method, an experimental investigation was conducted based on a pumped-storage model system at Wuhan University. By shutting down the circulating pump, a special transient process was triggered, forcing the pump turbine to operate in turbine mode, turbine brake mode, and reverse rotational pump mode. As the rotational speed of the pump turbine was maintained almost constant in the oscillation process with a maximum deviation of 0.6%, this transient operation was named as constant-speed oscillation (CSO). The parameters for global performance and pressure pulsations in the vaneless gap were measured and analyzed. In addition, the one-dimensional rigid column theory was used to establish a mathematical model for simulation. The results from simulation were quantitatively compared with the experimental results. Finally, the reason for the CSO was theoretically explained based on stability analysis through the established mathematical model. It was observed that the positive slope of ned–Qed characteristic curves at no-flow resulted in this oscillation. In contrast, the simulation was performed under the same conditions with a modified ned–Qed characteristic curve, which had a negative slope at no-flow. However, the results showed that, with the modified characteristic curve, the pump turbine would stabilize at no-flow.

Purple Auroral Rays and Global Pc1 Pulsations Observed at the CIR‐Associated Solar Wind Density Enhancement on 21 March 2017

AbstractThis paper reports two unique auroral features: postmidnight purple auroral rays and global Pc1 geomagnetic pulsations, observed before the onset of the corotating interaction region (CIR) storm of 21 March 2017, at the beginning of the first campaign of the new Particles and Waves in the Inner magnetosphere using Ground‐based network observation (PWING) longitudinal ground network with the Arase satellite. The purple auroral rays were observed from ~0315 to 0430 UT (~03–04 magnetic local time) in the northeastern sky at Husafell, Iceland (magnetic latitude: 64.9°N). We newly propose that the entry of high‐density CIR plasma into the magnetotail created purple auroral rays in the sunlit ionosphere. Pc1 geomagnetic pulsations at frequencies of 0–0.5 Hz were observed after ~00 UT over a wide local time range, of 13 hr, from midnight to afternoon sectors at subauroral latitudes associated with CIR arrival. These results indicate preconditioning of the magnetosphere due to crossing of a CIR.

Self-acceleration and global pulsation in hydrodynamically unstable expanding laminar flames

The cellularly unstable laminar flame is of fundamental and practical interest because of its intrinsic ability to accelerate without any external sources, as demonstrated previously for the spherically expanding flame. In this work, enriched physical insights and useful quantitative data on the subject phenomena are obtained by employing stoichiometric H2/O2/N2 flames and focusing on the individual roles of pressure, P, and the burned flame temperature, Tb, on the flame speed acceleration. Specifically, we first demonstrate that the propagation speed of the self-accelerating cellularly unstable flames at various pressures, including the state of the critical radius at which the flame becomes unstable, can be collapsed by plotting the flame speed, normalized by the planar flame speed, versus the normalized radius, or the Peclet number. Furthermore, through experiments with reduced burned flame temperature achieved by increasing the amount of N2 in air, the normalized flame speed with lower Tb is found to be larger than that with higher Tb. We have also found that instead of continuing with steady acceleration after its attainment, the unstable flames exhibit a global pulsatory acceleration mode in that stages of strong and weak acceleration are cyclically repeated. Mechanistically and confirming results from previous studies, while the flame is globally expanding, the local cascading process in the cellular structure could cause phases of faster and slower growth in the surface area. Such an intermittent local burning rate or flame speed consequently translates to oscillatory global propagation, even after global averaging, due to nonlinear coupling. The frequency and acceleration exponent of the pulsatory multi-stage acceleration are also determined, with the latter slightly smaller than the critical value of 1.5 suggested for self-turbulization.

PEPSI deep spectra

Context. Full-disk solar flux spectra can be directly compared to stellar spectra and thereby serve as our most important reference source for, for example stellar chemical abundances, magnetic activity phenomena, radial-velocity signatures or global pulsations. Aim. As part of the first Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) key-science project, we aim to provide well-exposed and average-combined (viz. deep) high-resolution spectra of representative stellar targets. Such deep spectra contain an overwhelming amount of information, typically much more than what could be analyzed and discussed within a single publication. Therefore, these spectra will be made available in form of (electronic) atlases. The first star in this series of papers is our Sun. It also acts as a system-performance cornerstone. Methods. The Sun was monitored with PEPSI at the Large Binocular Telescope (LBT). Instead of the LBT we used a small robotic solar disk integration (SDI) telescope. The deep spectra in this paper are the results of combining up to ≈100 consecutive exposures per wavelength setting and are compared with other solar flux atlases. Results. Our software for the optimal data extraction and reduction of PEPSI spectra is described and verified with the solar data. Three deep solar flux spectra with a spectral resolution of up to 270 000, a continuous wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio (S/N) of between 2000–8000:1 depending on wavelength are presented. Additionally, a time-series of 996 high-cadence spectra in one cross disperser is used to search for intrinsic solar modulations. The wavelength calibration based on Th-Ar exposures and simultaneous Fabry–Pérot combs enables an absolute wavelength solution within 10 m s−1 (rms) with respect to the HARPS laser-comb solar atlas and a relative rms of 1.2 m s−1 for one day. For science demonstration, we redetermined the disk-average solar Li abundance to 1.09 ± 0.04 dex on the basis of 3D NLTE model atmospheres. We detected disk-averaged p-mode RV oscillations with a full amplitude of 47 cm s−1 at 5.5 min. Conclusions. Comparisons with two solar FTS atlases, as well as with the HARPS solar atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are being taken with a sampling of one deep spectrum per day, and are supposed to continue a full magnetic cycle of the Sun.

Computer Simulation of Turbulent Flow Phenomena in Nuclear Fuel Pin Subassemblies

This paper focuses on the turbulence phenomena that have been identified by computational fluid dynamics (CFD), and that are peculiar to the flows in tight lattice nuclear fuel subassemblies, eccentric annuli and two-rectangular channels interconnected by a narrow gap. The CFD methods employed are the Direct Numerical Simulation of turbulence (DNS) and Large Eddy Simulation (LES) techniques. Highlighted phenomena include turbulence-driven secondary flows inside a subchannel, local turbulent-laminar transition in the narrow gap region between two adjacent fuel pins, and global pulsation of the flow across the length of a fuel subassembly. Special attention will be paid on linkage to the physics, limitation of the computational capability of the CFD methods, and the usefulness of the technology in reference to the foreseeable future.

Late Cenozoic Global Pulsations in Hotspot Magmatism and their Possible Interplay with Plate Tectonics, Earth's Core and Climate

A study of the Earth's main hotspots indicates an increase in magmatism during the last 1.5 myr, supporting a previous hypothesis on global magmatic copulsation on c. 10 myr scale through the Cenozoic. A similar pattern is found for magmatism dominantly related to plate tectonics since 15 Ma. It is suggested that the inferred syncronicity in magmatism is related to two large areas under Africa and the central Pacific expressing anomalously slow S-wave velocities in the lower mantle. Heterogeneous heat flux in the fluid outer core related to the lower mantle anomalies may cause spatially related aspherical inner core growth and rotation of the inner core with respect to the lower mantle. It is speculated that the heterogeneous heat flux in the outer core can be modulated by inner core processes of aspherical growth and rotation on 10 myr scale, subsequently leading to syncronized plume pulses from the rim of the hot lower mantle anomalies. Some of the magmatic pulses appear to coincide with abrupt drops in eustatic sea level, and it is suggested that these were caused by glaciations triggered by the volcanism.

Modulation of the ionosphere by Pc5 waves observed simultaneously by GPS/TEC and EISCAT

Earlier studies demonstrated that the monitoring of the ionospheric total electron content (TEC) by global satellite navigation systems is a powerful method to study the propagation of transient disturbances in the ionosphere, induced by internal gravity waves. This technique has turned out to be sensitive enough to detect ionospheric signatures of magnetohydrodynamic waves as well. However, the effect of TEC modulation by ULF waves is not well examined as a responsible mechanism has not been firmly identified. During periods with intense Pc5 waves distinct pulsations with the same periodicity were found in the TEC data from high-latitude GPS receivers in Scandinavia. We analyze jointly responses in TEC variations and EISCAT ionospheric parameters to global Pc5 pulsations during the recovery phase of the strong magnetic storms on October 31, 2003. Comparison of periodic fluctuations of the electron density at different altitudes from EISCAT data shows that main contribution into TEC pulsations is provided by the lower ionosphere, up to ~150 km, that is the E-layer and lower F-layer. This observational fact favors the TEC modulation mechanism by field-aligned plasma transport induced by Alfven wave. Analytical estimates and numerical modeling support the effectiveness of this mechanism. Though the proposed hypothesis is basically consistent with the analyzed event, the correspondence between magnetic and ionospheric oscillations is not always perfect, so further studies need to be conducted to understand fully the TEC modulations associated with Pc5 pulsations.

Coupled pulsating and cellular structure in the propagation of globally planar detonations in free space

The globally planar detonation in free space is numerically simulated, with particular interest to understand and quantify the emergence and evolution of the one-dimensional pulsating instability and the two-dimensional cellular structure which is inherently also affected by pulsating instability. It is found that the pulsation includes three stages: rapid decay of the overdrive, approach to the Chapman-Jouguet state and emergence of weak pulsations, and the formation of strong pulsations; while evolution of the cellular structure also exhibits distinct behavior at these three stages: no cell formation, formation of small-scale, irregular cells, and formation of regular cells of a larger scale. Furthermore, the average shock pressure in the detonation front consists of fine-scale oscillations reflecting the collision dynamics of the triple-shock structure and large-scale oscillations affected by the global pulsation. The common stages of evolution between the cellular structure and the pulsating behavior, as well as the existence of shock-front pressure oscillation, suggest highly correlated mechanisms between them. Detonations with period doubling, period quadrupling, and chaotic amplitudes were also observed and studied for progressively increasing activation energies.

Global frequency distributions of pulsations driven by sharp decrease of solar wind dynamic pressure

On 24 August 2005, an impulse of solar wind dynamic pressure ( P sw) hit the magnetosphere. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail and TC-1, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of P sw at the trailing edge of the impulse. The results show that the sharp decrease of P sw can excite a global pulsation in the frequency range 4.3–11.6 mHz. The pulsation has a reversal of polarization between two auroral latitude stations, a larger power spectral density (PSD) close to resonant latitude and increasing frequency with decreasing latitude. All these features indicate that the pulsations are associated with field line resonance (FLR). The fundamental resonant frequency (the peak frequency of PSD between 4.3 and 5.8 mHz) is dependent on magnetic local time and is largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is dependent on local time and smallest at noon. A second harmonic wave at about 10 mHz is also observed, which is strongest in the daytime sector, and becomes heavily attenuated in the night sector. The comparison of the PSDs of the pulsations driven by sharp increase and sharp decrease of P sw shows that the frequency of pulsations is negatively proportional to the size of magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, the above results indicate that the resonant frequency in the magnetospheric cavity/waveguide is controlled not only by solar wind parameters but also by magnetic local time of observation point.

Modulation of total electron content by ULF Pc5 waves

AbstractAn intriguing effect was found while analyzing the small‐scale variations of total electron content (TEC) derived from global positioning system (GPS) signals. We found a response in TEC variations to intense global Pc5 pulsations with periods of a few millihertz covering the corrected geomagnetic latitudes ~58°–75° during the recovery phase of the strong magnetic storms on 31 October 2003. Earlier studies demonstrated that the GPS‐TEC technique is a powerful method to study the propagation pattern of transient disturbances in the ionosphere, generated by seismic or internal gravity waves. This technique has turned out to be sensitive enough to ULF waves as well. During periods with intense Pc5 geomagnetic wave activity, distinct pulsations with the same periodicity were found in the TEC data from high‐latitude GPS receiving stations in Scandinavia. Wavelet and cross‐spectral analysis showed a high coherence (~0.9) between the periodic geomagnetic and TEC variations. Moreover, the relative amplitude of TEC periodic fluctuations ΔTEC/TEC was about or even larger than the relative amplitude of geomagnetic variations ΔB/B. So far, the effect of TEC modulation by Pc5 waves is not well understood and is still a challenge for the MHD wave theory. Various possible modulation mechanisms have been estimated, but no mechanism has been firmly identified.