Periodogram Analysis Research Articles

Photometry, rotation period determination and dust coma numerical study of comet C/2017 K2 (PanStarrs)

C/2017 K2 (Pan-STARRS) is an Oort cloud comet discovered in May 2017. Ground observations have revealed that this long-period comet was active at a heliocentric distance beyond 20 au. Several studies observed this object when it was far from the Sun and proposed that, at such a distance, its activity is primarily driven by the sublimation of super-volatile ices such as CO and CO2.The aim of this paper is to analyze the comet’s dust behavior when it was much closer to the Sun. A series of images were taken on different observation routines between July and August 2022 with the 0.6 m Helen Sawyer Hogg (HSH) telescope at the Complejo Astronómico El Leoncito (CASLEO), using broadband V and R filters. The objective was to conduct a morphological, photometric, and numerical analysis of this comet.Digital filters were applied to enhance contrast in the cometary images, revealing an active region embedded within an isotropic coma. Analysis of the magnitudes and dust production rate from the A(0°)fρ parameter suggests a steady-state dust behavior, consistent with findings from other authors.Moreover, thanks to the images covering a significant time span, it was possible to determine the comet’s rotation period using a periodogram analysis with the Phase Dispersion Minimization method (PDM) and to calculate the coordinates of the rotation axis, yielding a rotation period of 14.24 h with a pole situated at the ecliptic longitude and latitude of (244°,−20°).Finally, to gain deeper insights into C/2017 K2’s dust behavior, observations were fitted to a newly developed theoretical model for studying dust comas. The analysis suggests that the comet’s dust coma was principally formed by large dust particles emitted at a velocity of 180ms−1.

Occurrence of sudden storm commencement in interplanetary space

Sudden storm commencement (SSC) events detected in interplanetary space from 1869 to 2023, are subjectedto wavelet and Lomb-Scargle periodogram analyses to determine the potential frequency of their recurrence. This period includes SSC events from the preceding fourteen solar cycles. Using Lomb-Scargle periodogram analysis, we have identified SSC occurrences independently during the odd and even solar activity cycles. We also used the wavelet analysis approach throughout the positive (A > 0) and negative (A < 0) polarity states of the heliospheric magnetic fields, in addition to solar activity cycles. This study reveals some notable short-term periodic variations in the SSC time series. The findings indicate that SSC occurrences have a conspicuous and important ∼ 44.0-day period. We have also observed solar and extended solar rotation periods in the time series, along with some intermittent variations (e.g., ∼22.0-, ∼19.0-days). The observed periodicities in the SSC time series are discussed and compared with the previously detected periodicities of solar wind and plasma, geomagnetic activity, and cosmic rays’ intensity indicators.

A versatile recording device for the analysis of continuous daily external activity in colonies of highly eusocial bees.

As pollinators, bees are key to maintaining the biodiversity of angiosperm plants, and for agriculture they provide a billion-dollar ecosystem service. But they also compete for resources (primarily nectar and pollen), especially the highly social bees that live in perennial colonies. So, how do they organize their daily temporal activities? Here, we present a versatile, low-cost device for the continuous, automatic recording and data analysis of the locomotor activity in the colony-entrance tube of highly eusocial bees. Consisting of an in-house built block containing an infrared detector, the passage of bees in the colony entrance tunnel is registered and automatically recorded in an Arduino environment, together with concomitant recordings of temperature and relative humidity. With a focus on the highly diverse Neotropical stingless bees (Meliponini), we obtained 10-day consecutive recordings for two colonies each of the species Melipona quadrifasciata and Frieseomelitta varia, and also for the honey bee. The Lomb-Scargle periodogram analysis identified a predominant circadian rhythmicity for all three species, but also indications of ultradian rhythms. For M. quadrifasciata, which is comparable in size to the honey bee, we found evidence for a possibly anticipatory activity already before sunrise. As all three species also presented activity at night in the colony entrance tube, this also raises questions about sleep organization in social insects. The cost and versatility of the device and the open-source options for data analysis make this an attractive system for conducting studies on circadian rhythms in social bees under natural conditions, complementing studies on flower visits by these important pollinators.

ANALISIS POLA CURAH HUJAN DI WILAYAH SUMATERA BAGIAN UTARA MENGGUNAKAN FAST FOURIER TRANSFORM (FFT)

This research was conducted in North Sumatra, namely at Iskandar Muda, Malikussaleh and Deli Serdang stations in the period 1885 to 2019. The purpose of the study was to analyze seasonal and interannual rainfall patterns. The method used is Fast Fourier Transform (FFT) and correlation between rainfall, sea surface temperature and wind. The results of the study showed that the rainfall pattern at Iskandar Muda, Malikussaleh and Deli Serdang stations had two peaks of rainfall. The peak rainfall of the three stations occurs in May and the second peak has differences between Deli Serdang and Iskandar Muda and Malikussaleh stations. Where the peaks of high periodization are only in the 6-month and 12-month periods. There is also a peak periodogram from each station seen in 18-30 months. Rainfall periodogram analysis shows the magnitude of data frequency for each phase cycle and its frequency. The correlation between rainfall, wind and sea surface temperature (SST) at the three stations has a different pattern. At Deli Serdang station, there is an influence of ENSO and IOD, while at Iskandar Muda and Malikussaleh stations there is no influence of ENSO and IOD. Then in the September, October, November (SON) season at the three stations there is no visible influence of ENSO and IOD.

The CARMENES search for exoplanets around M dwarfs. Revisiting the GJ 581 multi-planetary system with new Doppler measurements from CARMENES, HARPS, and HIRES

GJ\,581 is a nearby M dwarf known to host a packed multiple planet system composed of two super-Earths and a Neptune-mass planet. We present new orbital analyses of the GJ\,581 system, utilizing recent radial velocity (RV) data obtained from the CARMENES spectrograph combined with newly reprocessed archival data from the HARPS and HIRES spectrographs. Our aim was to analyze the post-discovery spectroscopic data of GJ\,581, which were obtained with CARMENES. In addition, we used publicly available HIRES and HARPS spectroscopic data to seek evidence of the known and disputed exoplanets in this system. We aimed to investigate the stellar activity of GJ\,581 and update the planetary system's orbital parameters using state-of-the-art numerical models and techniques. We performed a periodogram analysis of the available precise CARMENES, HIRES, and HARPS RVs and of stellar activity indicators. We conducted detailed orbital analyses by testing various orbital configurations consistent with the RV data. We studied the posterior probability distribution of the parameters fit to the data and we explored the long-term stability and overall orbital dynamics of the GJ\,581 system. We refined the orbital parameters of the GJ\,581 system using the most precise and complete set of Doppler data available. Consistent with the existing literature, our analysis confirms that the system is unequivocally composed of only three planets detectable in the present data, dismissing the putative planet GJ\,581\,d as an artifact of stellar activity. Our N-body fit reveals that the system's inclination is $i = $\,deg, which implies that the planets could be up to 30&lt;!PCT!&gt; more massive than their previously reported minimum masses. Furthermore, we report that the GJ\,581 system exhibits long-term stability, as indicated by the posterior probability distribution, characterized by secular dynamical interactions without the involvement of mean motion resonances.

An open-access simultaneous electrocardiogram and phonocardiogram database

Objective. The EPHNOGRAM project aimed to develop a low-cost, low-power device for simultaneous electrocardiogram (ECG) and phonocardiogram (PCG) recording, with additional channels for environmental audio to enhance PCG through active noise cancellation. The objective was to study multimodal electro-mechanical activities of the heart, offering insights into the differences and synergies between these modalities during various cardiac activity levels. Approach. We developed and tested several hardware prototypes of a simultaneous ECG-PCG acquisition device. Using this technology, we collected simultaneous ECG and PCG data from 24 healthy adults during different physical activities, including resting, walking, running, and stationary biking, in an indoor fitness center. The data were annotated using a robust software that we developed for detecting ECG R-peaks and PCG S1 and S2 components, and overseen by a human expert. We also developed machine learning models using ECG-based, PCG-based, and joint ECG-PCG features, like R–R and S1–S2 intervals, to classify physical activities and analyze electro-mechanical dynamics. Main results. The results show a significant coupling between ECG and PCG components, especially during high-intensity exercise. Notable micro-variations in S2-based heart rate show differences in the heart’s electrical and mechanical functions. The Lomb-Scargle periodogram and approximate entropy analyses confirm the higher volatility of S2-based heart rate compared to ECG-based heart rate. Correlation analysis shows stronger coupling between R–R and R-S1 intervals during high-intensity activities. Hybrid ECG-PCG features, like the R-S2 interval, were identified as more informative for physical activity classification through mRMR feature selection and SHAP value analysis. Significance. The EPHNOGRAM database, is available on PhysioNet. The database enhances our understanding of cardiac function, enabling future studies on the heart’s mechanical and electrical interrelationships. The results of this study can contribute to improved cardiac condition diagnoses. Additionally, the designed hardware has the potential for integration into wearable devices and the development of multimodal stress test technologies.

Optimal sampling interval for characterisation of the circadian rhythm of body temperature in homeothermic animals using periodogram and cosinor analysis.

Core body temperature (T c) is a critical aspect of homeostasis in birds and mammals and is increasingly used as a biomarker of the fitness of an animal to its environment. Periodogram and cosinor analysis can be used to estimate the characteristics of the circadian rhythm of T c from data obtained on loggers that have limited memory capacity and battery life. The sampling interval can be manipulated to maximise the recording period, but the impact of sampling interval on the output of periodogram or cosinor analysis is unknown. Some basic guidelines are available from signal analysis theory, but those guidelines have never been tested on T c data. We obtained data at 1-, 5- or 10-min intervals from nine avian or mammalian species, and re-sampled those data to simulate logging at up to 240-min intervals. The period of the rhythm was first analysed using the Lomb-Scargle periodogram, and the mesor, amplitude, acrophase and adjusted coefficient of determination (R 2) from the original and the re-sampled data were obtained using cosinor analysis. Sampling intervals longer than 60 min did not affect the average mesor, amplitude, acrophase or adjusted R 2, but did impact the estimation of the period of the rhythm. In most species, the period was not detectable when intervals longer than 120 min were used. In all individual profiles, a 30-min sampling interval modified the values of the mesor and amplitude by less than 0.1°C, and the adjusted R 2 by less than 0.1. At a 30-min interval, the acrophase was accurate to within 15 min for all species except mice. The adjusted R 2 increased as sampling frequency decreased. In most cases, a 30-min sampling interval provides a reliable estimate of the circadian T c rhythm using periodogram and cosinor analysis. Our findings will help biologists to select sampling intervals to fit their research goals.

On the Periodic Variation of the Ion Density in the Martian Dayside Ionosphere During the Regional Dust Storm in September 2016

AbstractDust storms and the atmospheric waves can play a significant role in the dynamics in the upper neutral atmosphere of Mars. Recent observations found that a periodic variation of neutral H and O exists in the upper atmosphere, which is likely associated with atmospheric waves that occurred during the regional dust storm on 4 September 2016. However, such periodic variations accompanying the dust storm are far from understood in terms of the Martian ionized particles (i.e., ionosphere). Here we investigated the periodic variation of the ion density in the Martian ionosphere during the regional dust storm in September 2016 based on the MAVEN/STATIC observations. Assuming a simple Chapman layer model, we implemented numerical fitting for the ion density altitude profile to retrieve the peak ion densities in the Martian ionosphere. We then applied periodogram analysis to these peak ion densities in order to identify the peak periodicities together with their confidence levels. We identified several distinct peak periodicities with a scale from ∼1 day up to ∼20 days. The peak periodicities around 6–9 days are comparable to those seen in the Martian neutral atmosphere. In addition, the other peak periodicities likely correspond to the periodic crossings of the local crustal fields and periodic variation of the upstream solar wind, indicating a strong regional coupling between the lower atmosphere and the upstream solar wind.

Radio pulse profile evolution of magnetar Swift J1818.0−1607

ABSTRACT The shape and polarization properties of the radio pulse profiles of radio-loud magnetars provide a unique opportunity to investigate their magnetospheric properties. Gaussian process regression analysis was used to investigate the variation in the total intensity shape of the radio pulse profiles of the magnetar Swift J1818.0–1607. The observed profile shape was found to evolve through three modes between MJDs 59104 and 59365. The times at which these transitions occurred coincided with changes in the amplitude of modulations in the spin-down rate. The amount of linear and circular polarization was also found to vary significantly with time. Lomb–Scargle periodogram analysis of the spin-down rate revealed three possibly harmonically related frequencies. This could point to the magnetar experiencing seismic activity. However, no profile features exhibited significant periodicity, suggesting no simple correlations between the profile variability and fluctuations of the spin-down on shorter time-scales within the modes. Overall, this implies that the mode changes seen are a result of local magnetospheric changes, with other theories, such as precession, less able to explain these observations.

Spectral Investigation of the Relationship between Seismicity and Water Level in the Enguri High Dam Area (Georgia)

A spectral analysis of the time dynamics of seismicity occurring in the Enguri area of Georgia from 1978 to 2021 is performed by means of Schuster’s spectrum analysis, periodogram analysis, and empirical mode decomposition. The results of our analysis suggest that earthquakes around the reservoir (within a 50 km radius from the center of the dam) may be due to changes in water level, featured by the yearly cycle of loading and unloading operations of the reservoir. It is observed that the impacts of water fluctuations are more pronounced in shallower strata (down to 10 km) than deeper ones (down to 20 km); this could indicate that earthquakes occurring at deeper levels may primarily result from tectonic forces, whereas those at shallower depths may be predominantly triggered by reservoir-induced factors.

Photometric Variability of Low Mass Stars and Brown Dwarfs in IC 348 and Taurus Star-Forming Regions

Low-mass stars belonging to the M spectral type are the most numerous stars in our Galaxy, amounting to about two-thirds in number, and are found at the bottom of the main sequence in the H-R diagram. Photometric studies of low-mass stars, including brown dwarfs (Bds), provide several important evolutions of their atmosphere, magnetic flares and chromospheric activity. This paper highlights a few interesting results from our optical I-band observations of 2MASS J03435638+3209591 in the young star-forming IC 348 region and three BDs in Taurus star-forming regions using ground-based telescopes as well as a space-based telescope. We estimated the fast periodicities in the range of 1.5 to 3 hours in Taurus BDs. Furthermore, using the long-term photometry from the Transiting Exoplanet Survey Satellite (TESS), we have conducted a time-resolved variability analysis of CFHT-BD-Tau 4. The periodogram analysis of TESS data reveals an orbital period of ∼ 3 days. We found two flare events in TESS sector 43 data for this BD and estimated the flared energies as 4.59×1035 erg and 2.64×1036 erg, which sit in the superflare range.

Underwater Object Identification by Means of Reflected Sonar Signals

The paper presents the research results of the underwater object identification method on the basis of periodogram analysis of hydroacoustic signals reflected from them with the application of voting algorithm. The identification of an object in the article means making a decision to distinguish it from other objects. Hydroacoustic signals are the main means of obtaining information about the underwater environment, they are also widely used to detect and identify underwater objects, including ships, submarines and underwater structures. The identification of underwater objects is an important task to ensure the safety of underwater exploration and the efficient utilization of marine resources in a noisy and complex underwater environment, including the presence of several natural objects and the use of various underwater exploitation techniques. The study proposes an approach that uses periodogram analysis followed by a voting algorithm to solve the problem of underwater object identification. Periodogram analysis is a method that identifies characteristic time and frequency features in hydroacoustic signals reflected from different types of objects. The proposed approach takes into account different characteristics of the signal periodogram to identify the unique features of each underwater object. The main purpose of this work is to develop and investigate a set of features using periodograms that take into account the specificity of hydroacoustic signals and the unique properties of different types of underwater objects under study. This approach allows to achieve a sufficiently high accuracy of identification. The paper presents the results of experiments conducted on real hydroacoustic signals. These results allow us to conclude about the effectiveness of the underwater object identification method by reflected hydroacoustic signals on the basis of periodogram analysis. The results of the study have practical significance and can be applied in applications related to underwater research.

Wavelet analysis for quantifying NavIC-IR multipath signal frequency with the aim of determining soil moisture in agricultural fields

ABSTRACTThis paper explores the potential of Global Navigation Satellite Systems (GNSS) multipath signals for soil moisture retrieval, specifically focusing on Navigation with Indian Constellations (NavIC). Utilizing GNSS-Interferometric Reflectometry (IR) techniques, wavelet analysis is employed to extract the low-frequency signal sensitive to soil moisture from the reflected multipath signals. The reconstructed low-frequency multipath signal is then subjected to Lomb-Scargle Periodogram (LSP) analysis to determine multipath frequency. A comprehensive sensitivity analysis reveals the dependency of multipath frequency on field soil moisture, particularly in the presence of vegetation. The sensitivity varies across different crop growth stages, categorized by crop height. The Pearson correlation coefficient (r) for crop height less than one wavelength was 0.81 and for greater than one wavelength it was 0.67. The sensitivity of multipath frequency for soil moisture was 0.48 rad s−1 cm3 cm−3 and 0.47 rad s−1 cm3 cm−3 for crop height less than one wavelength and crop height greater than one wavelength respectively.This research underscores the potential of GNSS-IR techniques, incorporating wavelet analysis and LSP, for soil moisture sensitivity, especially in areas with varying vegetation and crop growth stages.

A Blind Search for Transit Depth Variability with TESS

The phenomenon of transit depth variability offers a pathway through which processes such as exoplanet atmospheric activity and orbital dynamics can be studied. In this work we conduct a blind search for transit depth variations among 330 known planets observed by the Transiting Exoplanet Survey Satellite within its first four years of operation. Through an automated periodogram analysis, we identify four targets (KELT-8b, HAT-P-7b, HIP 65 Ab, and TrES-3b) that appear to show significant transit depth variability. We find that KELT-8b’s transit depth variability likely comes from contaminating flux from a nearby star, while the apparent variabilities of HIP 65 Ab and TrES-3b are probable artifacts due to their grazing orbits. HAT-P-7b indicates signs of variability that possibly originate from the planet or its host star. A population-level analysis does not reveal any significant correlation between transit depth variability and the effective temperature and mass of the host star; such correlation could arise if stellar activity was the cause of depth variations via the transit light source effect. Extrapolating our ∼1% detection rate to the upcoming Roman mission, predicted to yield of order 100,000 transiting planets, we expect that ∼1000 of these targets will be found to exhibit significant transit depth variability.

Ionosphere Heterogeneities at Dawn−Dusk Terminator Related to the Starlink Satellites Launch Disaster on 3−8 February 2022

AbstractWe present an analysis of the evolution of the ionospheric Total Electron Content (TEC) and its variability index Vσ during the Starlink launch disaster on 3–8 February 2022 two‐phase geomagnetic storm. JPL GIM‐TEC global maps with increased spatial (1° in latitude and longitude) and temporal (15 min cadence) resolution as well as SWARM satellite data have been used in the analysis. Comparison of data for five time periods including Starlink launches with geomagnetic Dst index demonstrates thermospheric mass densities increasing by about 50% during the development of the storm on 3–5 February. A spectrum periodogram analysis is applied on TEC at sunrise and sunset solar terminator (STh) at an altitude h = 300 km. Periodicities in the solar terminator TEC generated waves are estimated as 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min. A significant morning−evening asymmetry resulted in the density anomalies concentrated in a certain—evening—sector of terminator when plasma is shifted to the dusk side forming a global asymmetry.

A hot mini-Neptune and a temperate, highly eccentric sub-Saturn around the bright K-dwarf TOI-2134

ABSTRACT We present the characterization of an inner mini-Neptune in a 9.2292005 ± 0.0000063 d orbit and an outer mono-transiting sub-Saturn planet in a 95.50$^{+0.36}_{-0.25}$ d orbit around the moderately active, bright (mv = 8.9 mag) K5V star TOI-2134. Based on our analysis of five sectors of TESS data, we determine the radii of TOI-2134b and c to be 2.69 ± 0.16 R⊕ for the inner planet and 7.27 ± 0.42 R⊕ for the outer one. We acquired 111 radial-velocity (RV) spectra with HARPS-N and 108 RV spectra with SOPHIE. After careful periodogram analysis, we derive masses for both planets via Gaussian Process regression: 9.13$^{+0.78}_{-0.76}$ M⊕ for TOI-2134b and 41.89$^{+7.69}_{-7.83}$ M⊕ for TOI-2134c. We analysed the photometric and RV data first separately, then jointly. The inner planet is a mini-Neptune with density consistent with either a water-world or a rocky core planet with a low-mass H/He envelope. The outer planet has a bulk density similar to Saturn’s. The outer planet is derived to have a significant eccentricity of 0.67$^{+0.05}_{-0.06}$ from a combination of photometry and RVs. We compute the irradiation of TOI-2134c as 1.45 ± 0.10 times the bolometric flux received by Earth, positioning it for part of its orbit in the habitable zone of its system. We recommend further RV observations to fully constrain the orbit of TOI-2134c. With an expected Rossiter–McLaughlin (RM) effect amplitude of 7.2 ± 1.3 $\rm m\, s^{-1}$, we recommend TOI-2134c for follow-up RM analysis to study the spin–orbit architecture of the system. We calculate the Transmission Spectroscopy Metric, and both planets are suitable for bright-mode Near Infrared Camera (NIRCam) atmospheric characterization.

Stochastic star formation in early galaxies: Implications for the James Webb Space Telescope

Contact. The star formation rate (SFR) in high-redshift galaxies is expected due to competing physical processes. This stochastic variability might boost the luminosity of galaxies and might explain the over-abundance seen at z ≳ 10 by the James Webb Space Telescope. Aims. We quantify the amplitude and timescales of this variability and identify the key physical processes. Methods. We selected 245 z = 7.7 galaxies with stellar mass 5 × 106 ≲ M⋆/M⊙ ≲ 5 × 1010 from SERRA, which is a suite of high-resolution radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, ⟨SFR⟩, we quantified the time-dependent variation, δ(t)≡log[SFR/⟨SFR⟩], for each system and performed a periodogram analysis to search for periodicity modulations. Results. We find that δ(t) is distributed as a zero-mean Gaussian, with standard deviation σδ ≃ 0.24 (corresponding to a UV magnitude s.d. σUV ≃ 0.61) that is independent of M⋆. However, the modulation timescale increases with stellar mass: tδ ∼ (9, 50, 100) Myr for M⋆ ∼ (0.1, 1, 5)×109 M⊙, respectively. These timescales are imprinted on the SFR by different processes: (i) photoevaporation, (ii) supernova explosions, and (iii) cosmological accretion/merging dominating in low-, intermediate-, and high-mass systems, respectively. Conclusions. The predicted SFR variations cannot account for the required z ≳ 10 UV luminosity function boost. Other processes, such as radiation-driven outflows clearing the dust, must then be invoked to explain the enhanced luminosity of super-early systems.

Peculiar Wave Structure of the Mesospheric Sporadic Sodium Layer Observed by Lidars in Hefei (31.8°N, 117.3°E) and Wuhan (30.5°N, 114°E), Central China

AbstractLidar observations are an effective tracker for identifying atmospheric wave signals. In this research, observations through three lidars are utilized to study the wave structures. A broadband sodium fluorescence lidar and a narrowband Temperature/Wind (T/W) lidar located at Hefei (31.8°N, 117.3°E), while the third lidar system, a broadband lidar system, is located in Wuhan (30.5°N, 114.4°E), about 310 km from Hefei. The three lidars can observe sodium density, temperature and wind profiles simultaneously. Joint sodium density measurements by the fourth lidar beams from Wuhan and Hefei yield a peculiar NaS with distinguished C‐structure and a large horizontal scale of wave structure over more than 310 km, on 21 December 2014. The NaS event occurring at 14:20 to 17:50 UT corresponds well to the region where the Richardson Number (Ri) is larger than 10, which in turn indicates that the NaS was formed in an extremely stable atmosphere. Results from the Lomb‐Scargle periodogram analysis suggest that the occurrence and evolution of this NaS event may have been caused by a gravity wave with a period of about 1.25 hr, and its end is closely related to the gravity wave fragmentation caused by convective instability.

The space weather around the exoplanet GJ 436b

Context. The space environment in which planets are embedded mainly depends on the host star and impacts the evolution of the planetary atmosphere. The quiet M dwarf GJ 436 hosts a close-in hot Neptune which is known to feature a comet-like tail of hydrogen atoms that escaped from its atmosphere due to energetic stellar irradiation. Understanding such star-planet interactions is essential to shed more light on planet formation and evolution theories, in particular the scarcity of Neptune-sized planets below a 3 d orbital period, also known as the ‘Neptune desert’. Aims. We aimed to characterise the stellar environment around GJ 436, which requires accurate knowledge of the stellar magnetic field. The latter is studied efficiently with spectropolarimetry, since it is possible to recover the geometry of the large-scale magnetic field by applying tomographic inversion on time series of circularly polarised spectra. Methods. We used spectropolarimetric data collected in the optical domain with Narval in 2016 to compute the longitudinal magnetic field, examine its periodic content via Lomb-Scargle periodogram and Gaussian process regression analysis, and finally reconstruct the large-scale field configuration by means of Zeeman-Doppler imaging. Results. We found an average longitudinal field of −12 G and a stellar rotation period of 46.6 d using a Gaussian process model and 40.1 d using Zeeman-Doppler imaging, which are both consistent with the literature. The Lomb-Scargle analysis did not reveal any significant periodicity. The reconstructed large-scale magnetic field is predominantly poloidal, dipolar, and axisymmetric, with a mean strength of 16 G. This is in agreement with magnetic topologies seen for other stars of a similar spectral type and rotation rate.

Model Hibrid Harmonik, ARMA dan Outlier Curah Hujan di Surabaya, Malang dan Banyuwangi

The main factors affecting the climate in Indonesia are monsoons, El Nino Southern Oscillation (ENSO) and sunspot cycles. Another influence is local characteristics, namely the geography and topography of each region. The harmonic model is an applied statistical model of Fourier series analysis. Because the harmonic model is a deterministic model with residuals that often still have autocorrelation, it is necessary to add a stochastic Autoregressive Integrated Moving Average (ARIMA) model. With the existence of 3 main factors that influence the climate in Indonesia with different seasonal periods, plus local influences, the bulk modeling with Seasonal ARIMA, Double S ARIMA is not enough so that a combination of harmonics, ARMA and Additive Outliers (AO) appears. The purpose of this study is to find the periodicity of rainfall and model rainfall time series data with hybrid harmonics, ARMA outliers. Periodogram analysis of monthly rainfall data for three cities in East Java shows a 1-year seasonal return period influenced by monsoons, an 11-year return period influenced by sunspots that cannot be detected. The monthly rainfall model for the city of Surabaya is the 2nd order Harmonic Hybrid Model, ARMA([1,9,11],0,0) and the outlier type , for the city of Banyuwangi is the 1st order Harmonic Hybrid Model, ARMA (1,0,0) and 2 outliers of type , , Malang city are first order Harmonic Hybrid models, ARMA([6,7],0,0) and outliers of type . First order Harmonic Hybrid models, ARMA and Outlier are better than ARIMA and Outlier models.&#x0D; Key words: Monsoon, ENSO, sunspot, harmonics, ARMA and outliers.