Scintillation Pattern Research Articles

Assessing the ionospheric scintillations occurrence on L-band in the southern Mediterranean sector

We assess the ionospheric scintillation occurrence on Global Navigation Satellite Signals (GNSSs) over the Mediterranean sector under the rising phase of the current solar cycle. To the scope, we leverage on a network of three Ionospheric Scintillation Monitoring Receivers (ISMRs), being part of the electronic Space Weather upper atmosphere (eSWua: eswua.ingv.it) system. Such ISMRs are located in Lampedusa Island (Italy, Lat: 35.52°N–Lon: 12.63°E), Chania (Crete, Greece, Lat: 35.52°N, Lon: 24.04°E) and Nicosia (Cyprus, Lat: 35.18°N–Lon: 33.38°E). To our knowledge, this is the first thorough assessment of the scintillation patterns in the Southern Mediterranean sector, aimed at depicting how small-scale irregularities that form in the area can potentially affect the GNSS-based positioning and related application and services. We analyse the period from January 2021 to December 2023, reporting that the bulk of the scintillation occurrence is due to small-scale irregularities forming in the southernmost area of the field of view of the network. Irregularities of such a scale are formed during the evolution of the Rayleigh-Taylor instability featuring the Equatorial Plasma Bubbles (EPB), which may spill-over in the field of view (FoV) of the receivers, i.e. at low elevation angles in the southernmost azimuthal range. As observations at low elevation angles are subject to multipath mimic weak to moderate scintillation conditions, we focus exclusively on severe amplitude scintillation occurrence (S4 > 0.7) in the azimuthal range 110–250°, w.r.t the FoV of the receivers, subsequently reaching down to the Saharian ionosphere. To further confirm the nature of the detected GNSS scintillation occurrence, we compare the results with the Swarm Level-2 Ionospheric Bubble Index (IBI) evaluated within the same period. In the context of the April 2023 geomagnetic storm, a worst-case scenario is also documented, illustrating the potential impact of ionospheric disturbances associated with EPBs in the Southern Mediterranean area.

Fine frequency structure of interstellar scintillation pattern in radio emission of the PSR B1133+16 at 111 MHz

The B1133+16 pulsar was observed at a frequency of 111 MHz with the LPA PRAO radio telescope from October 2022 to March 2023. Observations were made twice a week for two days in a row. In total 38 measurements of the scattering parameters were carried out with a high frequency resolution (up to 65 Hz). We used continuous recording of voltage in the frequency band 2.5 MHz with 8-bit digitization. The signal was reconstructed using the coherent dedispersion method. Dynamic spectra (DSP) were constructed for each observing session. Then, for each DSP, we calculated a two-dimensional autocorrelation function (2DACF) and analyzed its frequency and time sections. We have studied the fine frequency structure of pulsar scintillations by analyzing both the dynamic spectra, and the spectra of individual pulses. It has been found that the intrinsic shape of diffraction distortion on average can be represented by a sharp two-sided function with a characteristic frequency width of 1–2 kHz. The long-term variability of the following parameters has been carefully studied: characteristic scales in frequency and time (fdifandtdif), and rotation measureRM. Based on the analysis of long-term variations, it is suggested that the true frequency form of scintillations may be distorted by ionospheric effects. We also compared the scintillation parameters separately for the two mean profile components, and no differences between the parameters were found.

Timing and scintillation studies of PSR J1439−5501

Context. PSR J1439−5501 is a mildly recycled pulsar in a 2.12-day circular orbit around a heavy white dwarf. A white dwarf cooling model has estimated the companion mass to be between 1 and 1.3 M⊙ and the inclination angle to be greater than 55°. Such high mass and inclination are expected to induce a Shapiro delay, namely, a relativistic time delay in the signal propagation caused by the curved space-time induced by the companion. Until now, however, no Shapiro delay has been measured in this system. Aims. Our aim is to detect the Shapiro delay and, thus, to independently measure the mass and inclination of PSR J1439−5501 by using data from the Parkes and MeerKAT radio telescopes. Methods. The Shapiro delay parameters were measured through pulsar timing, which coherently accounts for every rotation of the pulsar. These measurements were then used to estimate the masses of the component stars and the inclination angle of the binary. A scintillation analysis was additionally performed by investigating the secondary spectra, which are the Fourier-transformed observed scintillation patterns. The obtained secondary spectral variations were analyzed in terms of the orbital motion and annual variation to estimate the ascending nodes, distance, and the location of the screen. Results. We obtained a highly significant measurement of the Shapiro delay, which allows estimates of the pulsar mass (1.57−0.26+0.30 M⊙), the white dwarf (WD) companion mass (1.27−0.12+0.13 M⊙), and inclination angle, (75(1)° or 105(1)°). These estimates assume that the companion mass cannot exceed the Chandrasekhar mass limit (1.48 M⊙), along with a lower limit of 1.17 M⊙ for NS masses. These results are consistent with previous studies, but the precision of the component masses has been improved significantly. The orbital and spin parameters and the large WD mass make this system very similar to that of PSR J2222−0137 and PSR J1528−3146, thereby suggesting a common evolutionary mechanism. The scintillation analysis suggests that the longitude of the ascending node is 16(7)° or −20(6)°, depending on the sense of the inclination angle. The screen distance is 260 ± 100 pc, potentially associated with the edge of the Local Bubble.

Fine Frequency Structure of Interstellar Scintillation Pattern in Radio Emission of the PSR B1133+16 at 111 MHz

Fine Frequency Structure of Interstellar Scintillation Pattern in Radio Emission of the PSR B1133+16 at 111 MHz

Effects of the Relative Dynamics of Ionospheric Irregularities and GPS Satellites on Receiver Tracking Loop Performance

AbstractTransionospheric satellite signals are exposed to perturbation, caused by irregularities generated in the ionosphere. However, the characteristics of the satellite motion can have an additional impact on signal perturbation, in addition to the effects of irregularity structures, that drift from west to east during geomagnetic quiet conditions. This paper reports the effect of Global Positioning System satellite geometry, on tracking loop performance of ground‐based receivers, during occurrence of ionospheric scintillations. Observations are made from station Calcutta (22.58°N, 88.38°E geographic; magnetic dip 34.54°), located near the northern crest of Equatorial Ionization Anomaly, during three different solar activity periods (March 2014, March 2015, and March 2022). Efforts have been made to study the correlation of east‐west component of satellite velocity at Ionospheric Pierce Point (IPP) with duration of loss‐of‐lock and rate of signal fading (<−10 dB), from ground scintillation pattern observations. Results of this study show 75%–78% correlation between duration of loss‐of‐lock and eastward component of satellite velocity, for all three period of observation. Subsequently, shorter duration of loss‐of‐lock has been observed corresponding to satellite velocity being westward. Signal fading rate is found to decrease with increasing satellite velocity, having median value of the fading rate cumulative distribution percentage, corresponding to satellite velocity of 16.69, 31.76, and 19.14 m/s respectively during March 2014, March 2015, and March 2022. Results of this study also indicate direction and component of satellite velocity at IPP to be a dominant cause of signal outage, even during periods of weak to moderate scintillations.

Interplanetary scintillation observation and space weather modelling

Interplanetary scintillation (IPS) refers to random fluctuations in radio intensity of distant small-diameter celestial object, over time periods of the order of 1 s. The scattering and scintillation of emergent radio waves are ascribed to turbulent density irregularities transported by the ubiquitous solar wind streams. The spatial correlation length of density irregularities and the Fresnel radius of radio diffraction are two key parameters in determining the scintillation pattern. Such a scintillation pattern can be measured and correlated between multi-station radio telescopes on the Earth. Using the “phase-changing screen” scenario based on the Born approximation, the bulk-flow speed and turbulent spectrum of the solar wind streams can be extracted from the single-station power spectra fitting and the multi-station cross-correlation analysis. Moreover, a numerical computer-assisted tomography (CAT) model, iteratively fit to a large number of IPS measurements over one Carrington rotation, can be used to reconstruct the global velocity and density structures in the inner heliosphere for the purpose of space weather modelling and prediction. In this review, we interpret the underlying physics governing the IPS phenomenon caused by the solar wind turbulence, describe the power spectrum and cross correlation of IPS signals, highlight the space weather application of IPS-CAT models, and emphasize the significant benefits from international cooperation within the Worldwide IPS Stations (WIPSS) network.

Modelling annual scintillation velocity variations of FRB 20201124A

ABSTRACT Compact radio sources exhibit scintillation, an interference pattern arising from propagation through inhomogeneous plasma, where scintillation patterns encode the relative distances and velocities of the source, scattering material, and Earth. In previous work, we showed that the scintillation velocity of the repeating fast radio burst (FRB) FRB20201124A could be measured by correlating burst spectrum pairs, with low values of the scintillation velocity and scattering time-scale suggesting scattering nearby the Earth at $\sim$0.4 kpc. In this work, we have measured the scintillation velocity at 10 epochs spanning 1 yr, observing an annual variation that strongly implies the screen is within the Milky Way. Modelling the annual variation with a 1D anisotropic or 2D isotropic screen results in a screen distance $d_{\mathrm{ l}} = 0.40\pm 0.04$ or $0.46\pm 0.06\,$ pc from the Earth, respectively, possibly associated with material outside of the Local Bubble or the edge of the Orion–Eridanus superbubble. Additional measurements particularly at times of low effective velocity will help probe changes in screen properties, and distinguish between screen models. Where scintillation of an FRB originates in its host galaxy or local environment, these techniques could be used to detect orbital motion, and probe the FRB’s local ionized environment.

Electron Density Variations in the Interstellar Medium and the Average Frequency Profile of a Scintle from Pulsar Scintillation Spectra

We observed the scintillation pattern of nine bright pulsars at 324 MHz and three at 1.68 GHz and analyzed the wavenumber spectrum, which is related to electron density variations of the plasma turbulence of the interstellar medium (ISM). For all pulsars the frequency section of the autocorrelation function (ACF) of the dynamic spectra to at least 45% of the maximum corresponds to predictions of scattering theories with a range of power-law exponents of the wavenumber spectrum of 3.56 ≤ α ≤ 3.97 with errors ≤0.05 and a mean with standard deviation of 3.76 ± 0.13. The range includes α = 3.67 for the Kolmogorov spectrum. Similar results, although with larger errors, were found from the Fourier transform of the ACFs down to ∼10−3 of the maximum. No clear case of a distinction between thin-screen and extended-medium scattering models was found. The average frequency profile of the scintles can be characterized for steep wavenumber spectra with α ≲ 4 by a cusp with a somewhat rounded peak. For flatter spectra, down to at least α ∼ 3.56 the cusp with its peak becomes more pronounced and its decay steepens. We discuss our findings in the context of the scattering characteristics of the ISM.

Electro-Optical Sensors for Atmospheric Turbulence Strength Characterization with Embedded Edge AI Processing of Scintillation Patterns

This study introduces electro-optical (EO) sensors (TurbNet sensors) that utilize a remote laser beacon (either coherent or incoherent) and an optical receiver with CCD camera and embedded edge AI computer (Jetson Xavier Nx) for in situ evaluation of the path-averaged atmospheric turbulence refractive index structure parameter Cn2 at a high temporal rate. Evaluation of Cn2 values was performed using deep neural network (DNN)-based real-time processing of short-exposure laser-beacon light intensity scintillation patterns (images) captured by a TurbNet sensor optical receiver. Several pre-trained DNN models were loaded onto the AI computer and used for TurbNet sensor performance evaluation in a set of atmospheric propagation inference trials under diverse turbulence and meteorological conditions. DNN model training, validation, and testing were performed using datasets comprised of a large number of instances of scintillation frames and corresponding reference (“true”) Cn2 values that were measured side-by-side with a commercial scintillometer (BLS 2000). Generation of datasets and inference trials was performed at the University of Dayton’s (UD) 7-km atmospheric propagation test range. The results demonstrated a 70–90% correlation between Cn2 values obtained with the TurbNet sensors and those measured side-by-side with the scintillometer.

The annual cycle in scintillation time-scale of PMN J1726+0639

ABSTRACT We discovered rapid intra-day variability in radio source PMN J1726+0639 at GHz frequencies, during a survey to search for such variability with the Australia Telescope Compact Array. Follow-up observations were conducted over 2 yr and revealed a clear, repeating annual cycle in the rate, or characteristic time-scale, of variability, showing that the observed variations can be attributed to scintillations from interstellar plasma inhomogeneities. The strong annual cycle includes an apparent ‘standstill’ in April and another in September. We fit kinematic models to the data, allowing for finite anisotropy in the scintillation pattern. The cycle implies a very high degree of anisotropy, with an axial ratio of at least 13:1, and the fit is consistent with a purely one-dimensional scintillation pattern. The position angle of the anisotropy, and the transverse velocity component are tightly constrained. The parameters are inconsistent with expectations from a previously proposed model of scattering associated with plasma filaments radially oriented around hot stars. We note that evidence for a foreground interstellar cloud causing anomalous Ca ii absorption towards the nearby star Rasalhague (α Oph) has been previously reported, and we speculate that the interstellar scintillation of PMN J1726+0639 might be associated with this nearby cloud.

Modelling annual scintillation arc variations in PSR J1643−1224 using the Large European Array for Pulsars

ABSTRACT In this work, we study variations in the parabolic scintillation arcs of the binary millisecond pulsar PSR J1643−1224 over five years using the Large European Array for Pulsars (LEAP). The two-dimensional (2D) power spectrum of scintillation, called the secondary spectrum, often shows a parabolic distribution of power, where the arc curvature encodes the relative velocities and distances of the pulsar, ionized interstellar medium, and Earth. We observe a clear parabolic scintillation arc, which varies in curvature throughout the year. The distribution of power in the secondary spectra is inconsistent with a single scattering screen, which is fully 1D or entirely isotropic. We fit the observed arc curvature variations with two models: an isotropic scattering screen and a model with two independent 1D screens. We measure the distance to the scattering screen to be in the range 114–223 pc, depending on the model, consistent with the known distance of the foreground large-diameter H ii region Sh 2-27 (112 ± 17 pc), suggesting that it is the dominant source of scattering. We obtain only weak constraints on the pulsar’s orbital inclination and longitude of ascending node, since the scintillation pattern is not very sensitive to the pulsar’s motion and the screen is much closer to the Earth than the pulsar. More measurements of this kind – where scattering screens can be associated with foreground objects – will help to inform the origins and distribution of scattering screens within our galaxy.

Long-lasting stagnant equatorial plasma bubble event and the related scintillation over the Brazilian region

Diverse studies about equatorial plasma bubble structures and their relation with ionospheric scintillation have been performed in the last decades. Among many findings, the investigations usually reported dominant plasma bubble eastward velocity with a magnitude of few tens of m/s and larger amplitude scintillation for transionospheric signals aligned with these depleted structures. However, an uncommon long-lasting event with negligible average zonal drift prevailing for hundreds of minutes was registered over the Brazilian region, allowing a case study of the scintillation pattern under this particular condition. Data from ionosondes, all-sky imager (6300 Å filter), geostationary satellites and Global Navigation Satellite System were used here, covering the eastern portion of the Brazilian ionosphere. The results show that the scintillation was less intense than in other nights around the event, suggesting that larger plasma density gradients and sequential bubble structures seem to cause more severe scintillation scenario than the alignment condition alone, even though the last also contributes to worsen the scintillation effects. Regarding the stagnant bubble pattern, the results suggest prevailing E region Hall conductivity and equatorward wind as potential agents.

Vela pulsar: single pulses analysis with machine learning techniques

ABSTRACT We study individual pulses of Vela (PSR B0833−45/J0835−4510) from daily observations of over 3 h (around 120 000 pulses per observation), performed simultaneously with the two radio telescopes at the Argentine Institute of Radioastronomy. We select four days of observations in 2021 January to March and study their statistical properties with machine learning techniques. We first use Density-Based Spatial Clustering of Applications with Noise clustering techniques, associating pulses mainly by amplitudes, and find a correlation between higher amplitudes and earlier arrival times. We also find a weaker (polarization dependent) correlation with the mean width of the pulses. We identify clusters of the so-called mini-giant pulses, with ∼10 times the average pulse amplitude. We then perform an independent study, with Self-Organizing Maps (SOM) clustering techniques. We use Variational AutoEncoder (VAE) reconstruction of the pulses to separate them clearly from the noise and select one of the days of observation to train VAE and apply it to the rest of the observations. We use SOM to determine four clusters of pulses per day per radio telescope and conclude that our main results are robust and self-consistent. These results support models for emitting regions at different heights (separated each by roughly a hundred km) in the pulsar magnetosphere. We also model the pulses amplitude distribution with interstellar scintillation patterns at the inter-pulses time-scale finding a characterizing exponent nISS ∼ 7–10. In the appendices, we discuss independent checks of hardware systematics with the simultaneous use of the two radio telescopes in different one-polarization/two-polarizations configurations. We also provide a detailed analysis of the processes of radio-interferences cleaning and individual pulse folding.

Scintillation time-scale measurement of the highly active FRB20201124A

ABSTRACT Scintillation of compact radio sources results from the interference between images caused by multipath propagation, and probes the intervening scattering plasma and the velocities of the emitting source and scattering screen. In FRB20201124A, a repeating fast radio burst (FRB) that entered a period of extreme activity, we obtained many burst detections in observations at the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Effelsberg 100-m Radio Telescope. Bursts nearby in time show similar scintillation patterns, and we measure a scintillation time-scale of 14.3 ± 1.2 and 7 ± 2 min at Effelsberg (1370 MHz) and uGMRT (650 MHz), respectively, by correlating burst pair spectra. The scintillation bandwidth scaled to 1 GHz is 0.5 ± 0.1 MHz, and the inferred scintillation velocity at Effelsberg is $V_{\mathrm{ISS}}\approx (59\pm 7) \sqrt{d_{\mathrm{ l}}/2\, \rm {kpc}}~{\rm km~s}^{-1}$, higher than Earth’s velocity for any screen beyond a lens distance of $d_{\mathrm{ l}} \gtrsim 400\,$ pc. From the measured scintillation bandwidth, FRB20201124A has comparatively lower scattering than nearby pulsars, and is underscattered by a factor of ∼30 or ∼1200 compared to the NE2001 and YMW16 model predictions, respectively. This underscattering together with the measured scintillation velocity is consistent with a scattering screen more nearby the Earth at $d_{\mathrm{ l}} \sim 400\,$ pc, rather than at 2 kpc spiral arm that NE2001 predicts to be the dominant source of scattering. With future measurements, the distance, geometry, and velocity of the scattering screen could be obtained through modelling of the annual variation in VISS, or through interstation time delays or interferometric observations. Scintillation/scattering measurements of FRBs could help improve Galactic electron density models, particularly in the Galactic halo or at high Galactic latitudes.

Scintillation of PSR B1508+55 – the view from a 10 000-km baseline

ABSTRACT We report on the simultaneous Giant Metrewave Radio Telescope (GMRT) and Algonquin Radio Observatory (ARO) observations at 550–750 MHz of the scintillation of PSR B1508+55, resulting in an ∼10 000-km baseline. This regime of measurement lies between the shorter few 100- to 1000-km baselines of earlier multistation observations and the much longer earth–space baselines. We measure a scintillation cross-correlation coefficient of 0.22, offset from zero time lag due to a ∼45-s traversal time of the scintillation pattern. The scintillation time of 135 s is 3× longer, ruling out isotropic as well as strictly one-dimensional scattering. Hence, the low cross-correlation coefficient is indicative of highly anisotropic but two-dimensional scattering. The common scintillation detected on the baseline is confined to low delays of ≲1μs, suggesting that this correlation may not be associated with the parabolic scintillation arc detected at the GMRT. Detection of pulsed echoes and their direct imaging with the Low-Frequency Array (LOFAR) by a different group enable them to measure a distance of 125 pc to the screen causing these echoes. These previous measurements, alongside our observations, lead us to propose that there are at least two scattering screens: the closer 125-pc screen causing the scintillation arc detected at GMRT, and a screen further beyond causing the scintillation detected on the GMRT-ARO baseline. We advance the hypothesis that the 125-pc screen partially resolves the speckle images on the screen beyond leading to loss of coherence in the scintillation dynamic spectrum to explain the low cross-correlation coefficient.

Examining the Tolerance of GNSS Receiver Phase Tracking Loop Under the Effects of Severe Ionospheric Scintillation Conditions Based on Its Bandwidth

AbstractThis study aims to evaluate the performance of phase‐locked loop (PLL) under the presence of distinct scintillation patterns in the signal. Scintillation is very common in low‐latitude regions due to the ionospheric dynamics. Under strong scintillation scenario the occurrence of deep fading events is usually registered and may cause severe degradation in the communication. The investigation conducted in this study uses the amplitude scintillation index S4, the decorrelation time, and the bandwidthas main parameters. The study evaluates 54 different combinations of these parameters. The results indicate that in order to minimize the occurrence of cycle slips in the output phase of the PLL, the most appropriate tracking loop bandwidthdepends on the values of both S4and, which characterize the scintillation fading pattern. Simulations showed that as the S4index increases, the automatic increment in the PLL bandwidth may not be the best choice as the performance depends on the temporal characteristics of the scintillating signal. The analysis showed that, among the configurations tested, the equivalent noise bandwidth of 10 Hz achieved the best performance overall. The investigation also showed that choosing the wrong parameter regarding the PLL bandwidth may increase the probability of cycle slip by up to 10 times during fading events.

Instantaneous Ionospheric Scintillation Mapping over the East African Region by use of GPS Derived Amplitude Scintillation Proxy

Ionospheric scintillation activity over the East African region is often monitored using measurements from the SCIntillation Network Decision Aid (SCINDA) receivers. Many of the SCINDA receivers over East Africa are currently not archiving data and therefore a large part of the region remain un sampled. We investigated the possibility to use dual frequency receivers of the Global Navigation Satellite System (GNSS) network for scintillation mapping. A proxy for amplitude scintillation was first derived by scaling the rate of change of total electron content index (ROTI). The proxy was validated against S4 derived from nearly collocated SCINDA receivers over the region. A good correlation was observed between the proxy and S4. The proxy was then used to map the occurrence of amplitude scintillation over East Africa based on semivariogram modeling and Kriging interpolation technique. The results indicate that the S4 values had a good positive correlation with *Corresponding author: E-mail: emirant.amabayo@gmail.com; Amabayo et al.; AJR2P, 4(2): 6-20, 2021; Article no.AJR2P.66815 the simulated S4p from the Kriging interpolation. This observation suggests that data from the dual frequency receivers of GNSS may be used to map scintillation over East Africa. These maps can in turn be used to study the evolution of ionospheric scintillation patterns over the region.

Atmospheric Turbulence Study with Deep Machine Learning of Intensity Scintillation Patterns

A new paradigm for machine learning-inspired atmospheric turbulence sensing is developed and applied to predict the atmospheric turbulence refractive index structure parameter using deep neural network (DNN)-based processing of short-exposure laser beam intensity scintillation patterns obtained with both: experimental measurement trials conducted over a 7 km propagation path, and imitation of these trials using wave-optics numerical simulations. The developed DNN model was optimized and evaluated in a set of machine learning experiments. The results obtained demonstrate both good accuracy and high temporal resolution in sensing. The machine learning approach was also employed to challenge the validity of several eminent atmospheric turbulence theoretical models and to evaluate them against the experimentally measured data.

Leveraging Geodetic GPS Receivers for Ionospheric Scintillation Science

AbstractWe demonstrate scintillation analysis from a network of geodetic Global Positioning System (GPS) receivers which provide data at 1‐second resolution. We introduce proxy phase (σTEC) and amplitude (SNR4) scintillation indices and validate them against the rate of change of TEC index (ROTI) and S4. Additionally, we validate scintillation observations against a Connected Autonomous Space Environment Sensor scintillation receiver. We develop receiver‐dependent scintillation event thresholding using hardware‐dependent noise variance. We analyze 6 days adjacent to the 7–8 September 2017 geomagnetic storm, using 169 receivers covering magnetic latitudes between 15° and 65° in the American longitude sector. We leverage the available spatial sampling coverage to construct 2‐D maps of scintillation and present episodic evolution of scintillation intensifications during the storm. We show that low‐latitude and high‐latitude scintillation morphology match well‐established scintillation climatology patterns. At midlatitudes, spatiotemporal evolution of scintillation partially agrees with known scintillation patterns. Additionally, the results reveal previously undocumented midlatitude scintillation‐producing structures. The results provide an unprecedented view into the spatiotemporal development of scintillation‐producing plasma irregularities and provide a resource to further exploit scintillation evolution at large spatial scales.

Interstellar scintillation of an extreme scintillator: PKS B1144−379

ABSTRACT The University of Tasmania Ceduna radio telescope has been used to investigate rapid variability in the radio flux density of the BL Lac object PKS B1144−379 at 6.7 GHz. High-cadence monitoring of this extreme scintillator was carried out over a period of approximately 9 yr, between 2003 and 2011. We have used structure functions created from the intensity time-series to determine the characteristic time-scale of the variability. The characteristic time-scale is consistently observed to increase during certain periods of each year, demonstrating the annual cycle expected for scintillation through an interstellar scattering screen. The best-fitting annual cycle model for each year suggests that the scintillation pattern has an anisotropic structure and that the upper limit of its scattering screen is at a distance of ∼0.84 kpc. Higher anisotropy in some of the annual cycle fits suggests that changes in the intrinsic source structure might be influencing the variability time-scale. We found a prominent annual cycle is only present in the variability time-scale for certain years, where other evidence suggests that the core is compact. From our measurements, we calculated that the core angular size varied between 5.65 and 15.90 μas (0.05–0.13 pc). The core component was found to be at its most compact during two flares in the total flux density, which were observed in 2005 and 2008. We conclude that the long-term variability in the radio flux density of PKS B1144−379 is due to intrinsic changes in the source and that these affect our ability to measure an annual cycle in its variability time-scale.