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1

Low-Frequency Sound Absorption Potential of Subwavelength Absorbers Based on Coupled Micro-Slit Panels

Qian Yujie, Gao Zhengyuan, Zhang Jie

Archives of Acoustics

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2024

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Vol. 49, nr 1

121--128

EN

Due to space limitations during installation, reducing low-frequency noise has always been a challenging area. Sub-wavelength structures are typically favored in such scenarios for noise reduction. This paper explores the potential of micro-slit panels (MSP) for low-frequency sound absorption. To further optimize the panel thickness, coupled MSPs (CMSP) with a distance between two MSPs of less than 1 mm are proposed. Firstly, the low-frequency absorption performances of a single MSP based on two optimized schemes – the cavity-depth optimal scheme (COS) and the panel thickness optimal scheme (TOS) – are examined and compared with those of existing ultrathin metamaterials. The results demonstrate that MSP has significant potential for low frequency sound absorption, and COS allows for a smaller overall structural thickness but a larger panel thickness than TOS. Secondly, to reduce the panel thickness, the CMSP is developed and the theoretical model of its acoustic impedance is established and validated by experiments. Then, based on the theoretical model, the low-frequency absorption potential of CMSP is optimized using COS. The results show that both the overall thickness and the panel thickness of the CMSP absorber are reduced while maintaining better performance. Furthermore, the proposed absorber achieves a subwavelength scale since its total thickness can be as small as 0.138λ.

2

Study on Chinese Speech Intelligibility Under Different Low-Frequency Characteristics of Reverberation Time Using a Hybrid Method

Huang Wuqiong, Peng Jianxin, Xie Tinghui

Archives of Acoustics

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2023

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Vol. 48, No. 2

151--157

EN

Reverberation time (RT) is an important indicator of room acoustics, however, most studies focus on the mid-high frequency RT, and less on the low-frequency RT. In this paper, a hybrid approach based on geometric and wave methods was proposed to build a more accurate and wide frequency-band room acoustic impulse response. This hybrid method utilized the finite-difference time-domain (FDTD) method modeling at low frequencies and the Odeon simulation at mid-high frequencies, which was investigated in a university classroom. The influence of the low-frequency RT on speech intelligibility was explored. For the low-frequency part, different impedance boundary conditions were employed and the effectiveness of the hybrid method has also been verified. From the results of objective acoustical parameters and subjective listening experiments, the smaller the low-frequency RT was, the higher the Chinese speech intelligibility score was. The syllables, consonants, vowels, and the syllable order also had significant effects on the intelligibility score.

3

An experimental study on the complex resistivity of fractured rock under different saturation conditions

Pan Baozhi, Ruhan A., Guo Yuhang, Zhang Lihua, Wei Boyang, Zhou Weiyi

Acta Geophysica

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2022

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Vol. 70, no 3

1061--1081

EN

The fracture can be a good channel for oil and gas migration, which has a great influence on the permeability of the reservoir. Therefore, it is of major significance to identify fractures and determine the characterization parameters and physical proper ties of fractured reservoirs. In this study, homogeneous sandstone was used to simulate different artificially fractured rocks. The fractured rock samples had different fracture widths, fracture numbers, and fracture dip angles. In addition, the complex impedance and weights of the rock samples were measured during the process of natural evaporation, and the relationships between the water saturation and the complex resistivity values at different frequencies were examined. The frequency range is 100 Hz–10 kHz. It was found that the influence effects of frequency on the resistivity, dielectric constant, and loss factor had differed among the homogeneous samples and the fractured rock sample. The fracturing had led to the resistivity index and the water saturation curves separating under the different frequencies, and the degree of the dielectric constant index and water saturation curve separation became larger. Furthermore, the influencing effects of the fracture widths, fracture numbers, and fracture dip angles mainly occurred in terms of three aspects. The first was the slope of the resistivity index and water saturation curves (IR–Sw). The second was the slope of the dielectric constant index and water saturation curves (Iε–Sw), and the third was the loss tangent D and water saturation Sw curves.

4

Approximate formulation of the rigid body motions of an elastic rectangle under sliding boundary conditions

Şahin Önur, Erbaş Barış, Wilson Brent

Acta Mechanica et Automatica

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2021

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Vol. 15, no. 2

82--90

EN

Low-frequency analysis of in-plane motion of an elastic rectangle subject to end loadings together with sliding boundary conditions is considered. A perturbation scheme is employed to analyze the dynamic response of the elastic rectangle revealing nonhomogeneous boundary-value problems for harmonic and biharmonic equations corresponding to leading and next order expansions, respectively. The solution of the biharmonic equation obtained by the separation of variables, a consequence of sliding boundary conditions, gives an asymptotic correction to the rigid body motion of the rectangle. The derived explicit approximate formulae are tested for different kinds of end loadings together with numerical examples demonstrating the comparison against the exact solutions.

5

Frequency extension and robust full waveform inversion based on nth power operation

Zhang Pan, Han Liguo, Gao Rui, Zhang Fengjiao, Xing Zhenzhen

Acta Geophysica

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2020

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Vol. 68, no. 5

1317--1333

EN

The conventional full-waveform inversion (FWI) often minimizes the objective function using some local optimization algorithms. As a result, when the initial model is not good enough, the inversion process will drop into a local minimum. The low-frequency components contained in seismic data are of vital importance for reducing the initial model dependence and mitigating the cycle-skipping phenomenon of FWI. In this research, a frequency extension method using the nth power operation is proposed, which compresses the seismic data in time domain and extends their frequency band. Based on this, we construct a new objective function using the nth power wavefeld and derive the corresponding gradient formula. The new objective function shows better property to overcome local minimum than the conventional one. When conduct inversion, we can invert from high-order to low-order successively, which is a new multiscale strategy. Since seismic data is more sensitive to source wavelet errors after high-order operation, we make the method more robust by proposing a source-independent method to mitigate the efects of source wavelet inaccuracy. After that, we extend the proposed method to encoded multisource waveform inversion. The numerical examples on the Marmousi model demonstrate that the proposed method can efectively mitigate the cycle-skipping of FWI, and it also has good anti-noise property.

6

Denoising of desert seismic signal based on synchrosqueezing transform and Adaboost algorithm

Sun Xiaofu, Li Yue

Acta Geophysica

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2020

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Vol. 68, no. 2

403--412

EN

Seismic data in desert area generally have low signal-to-noise ratio (SNR) due to special surface conditions. Desert noise is characterized as low-frequency, non-Gaussian and non-stationary noise, which makes the noise suppression in desert area more challenging by conventional methods. Conventional methods are efective for the signal with high SNR, but in desert seismic signal, the SNR is low and the signal can easily be obliterated in desert noise. In this paper, we propose an approach that operates in synchrosqueezing transform (SST) domain and use classifcation techniques obtained from supervised machine learning to identify the coefcients associated with signal and noise. First of all, we transform the real desert seismic data into time–frequency domain by SST. Secondly, we select features by calculating the SST coefcients of signal and noise. And then, we train them in the Adaboost classifer. Finally, when the training is completed, we can obtain the fnal classifer that can efectively separate the signal from noise. We perform tests on synthetic and feld records, and the results show great advantages in suppressing random noise as well as retaining efective signal amplitude.

7

Low Frequency Electromagnetic Interferences Impact on Transport Security Systems Used in Wide Transport Areas

Dziula P., Pas J.

TransNav : International Journal on Marine Navigation and Safety of Sea Transportation

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2018

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Vol. 12, no. 2

251--258

EN

The article presents the impact of electromagnetic interferences of low frequency range, on transport security systems used in wide transport areas. Intended and unintended (stationary and mobile), electromagnetic interferences, impacting on items and components constituting transport system at wide area (seaport, railway, etc.), cause changes of its vulnerability, resistance and durability. Diagnostics of interferences sources (amplitude, frequency range, radiation characteristics, etc.), appearing within transport environment, and usage of appropriate technical solutions of systems (i.e. shielding, reliability structures), allows for safe implementation of safety surveillance of human beings, properties and communication means.

8

Comparative Analysis of Various Types of Filters for the Processing of Biological Signals in the Low Frequency Range

Żołubak M., Kolańska-Płuska J., Bauer W., Grochowicz B., Ozana S., Kawala-Janik A.

Elektronika : konstrukcje, technologie, zastosowania

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2018

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Vol. 59, nr 11

22--25

EN

The aim of this work is to present various types of filters, their capabilities and effectiveness for filtering of low voltage signals. A comparative analysis of examples was made for EEG signals using several filters, and then fitted with the best filter parameters for filtering electroencephalographic signals recorded with sampling frequency of 500 Hz.

PL

Celem niniejszej pracy jest przedstawienie różnych typów filtrów, ich możliwości i skuteczności w filtrowaniu sygnałów niskonapięciowych. Przeprowadzono analizę porównawczą przykładowych sygnałów EEG przy użyciu kilku filtrów, a następnie dobrano najlepsze parametry filtrów do filtrowania sygnałów elektroencefalograficznych rejestrowanych z częstotliwością próbkowania 500 Hz.

9

Analyzing effects of ELF electromagnetic fields on removing bacterial biofilm

Karaguler T., Kahraman H., Tuter M.

Biocybernetics and Biomedical Engineering

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2017

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Vol. 37, no. 2

336--340

EN

Use of extremely low frequency electromagnetic field (ELF-EMF) to prevent and/or remove bacterial biofilm formation is rather a new research area. However the technique is becoming popular as the conventional methods such as antimicrobial surfaces, quorum-sensing and usage of phage are mostly insufficient. In this work, 1 mT–50 Hz and 100 Hz magnetic fields are applied on bacterial biofilm produced locally in our MOBGAM laboratory and found reasonable level of success of its removal.

10

Field models in low-frequency bioelectromagnetics

Mognaschi M. E.

Przegląd Elektrotechniczny

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2016

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R. 92, nr 12

1--4

EN

In the paper, a review of the state of the art on numerical models of the electromagnetic field in biological entities is proposed. In particular, the field produced by cells and the one in which cells and biological tissues are exposed to, is considered; low frequency problems are investigated. Issues and drawbacks of field models in bioelectromagnetics with respect to field models for industrial applications are discussed.

PL

W artykule dokonano przeglądu stanu wiedzy na temat numerycznych modeli pola elektromagnetycznego w biologicznych komórkach. W szczególności, rozważane jest pole wytwarzane przez komórki i to, na które narażone są komórki i tkanki biologiczne; badane są problemy niskiej częstotliwości.. W artykule omówiono problemy i wady spotykane w adaptacji modeli polowych w bioelektromagnetyzmie w odniesieniu do modeli polowych w zastosowaniach przemysłowych.

11

An Analysis of Eruption of the Sun Detected by Solar Radio Burst Type I

Hamidi Z. S., Shariff N. N. M., Arifin S., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2015

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Vol. 47

139--146

EN

Type I solar burst were identified based on data recorded by CALLISTO BLEIN, Switzerland in the period of 17th of January, 2011. Solar Radio Burst Type I is one of the main type of solar burst which is believed to provide a diagnostic of electron acceleration in the corona. This noise storm burst is associated with emerging and growing active regions and last from hours to days. It can be observed that solar radio burst type I is formed within four minutes, although the number of sunspots is just 15. The results of the recent time indicate that Sunspot group 1147 has been mostly quiet since it rounded the eastern limb, but previous week's far side activity shows it is capable of significant eruptions. In 2011, only one day has been detected with spotless day, which means that it is about 7% of overall cases. Probabilities for significant disturbances in Earth's magnetic field are given for three activity levels: active, minor storm, severe storm. From the current conditions in the space weather website on 16th January 2011 that is the first event was shown that the solar wind occurred with a speed of 433.2 km/second while its density about 3.2 protons/cm3. Besides the solar wind, X-ray solar flare with 6 hours maximum: B1 at 1846 UT and 24 hours: B2 at 1544 UT were detected. While type I seem to be an indicator of pre-solar flare and CMEs, on the observational analysis, we could not directly confirmed that this is the only possibility, and we need to consider other processes to explain in detailed the injection, energy loss and the mechanism of the acceleration of the particles. We could conclude one active region will not produce a huge explosion of solar phenomena.

12

Analysis and measure of novel stereo-garage driven by linear induction motor

Lu Q., Ye Y., Shen J.

Archives of Electrical Engineering

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2015

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Vol. 64, nr 4

669--678

EN

The car access time is a key parameter, especially in a huge stereo-garage, where this one should be decreased as much as possible. This paper proposes a novel stereo-garage. Adopting the linear induction motors (LIMs), the system has a simple structure and rapid response capability. In the stereo-garage, several LIMs are installed below the crossbeam on a lifting platform, and several LIMs are fixed on the top of a moving frame. During the operation of LIMs, the moving frame moves forward and backward to reach the required parking place, whereas the crossbeam moves horizontally in order to take or store the vehicle rapidly. All these LIMs are the same and should be designed at a low frequency. The influences of key structure parameters and dynamic performances are investigated, based on FEM. The predicted results are validated by a prototype. Finally, the designed LIMs are successfully applied in two 8-layer stereo-garages.

13

Type II Solar Radio Burst with a Split and Herring − Bones During a Minimum Solar Activity

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 13, iss. 2

104--111

EN

A preliminary correlation study of the herring − bone type II with a type III solar burst of has been made. On the basis of this study and in combination with the observation in radio emission, an interpretation of the mechanism of the occurrence of this event has been proposed. The type II solar radio burst with a split and herring bone is occurring at the same time from 36 MHz till 50 MHz. We have noted that an individual type III burst also can be observed at 13:23 UT from 45-50 MHz. During that day, a stream of solar wind from a coronal hole on the Sun has disturbing Earth's magnetosphere creating a minor geomagnetic storm, G1 on the NOAA scale of G1-G5. In this case, the solar flare is not very high, but CME is responsible to form a solar radio burst type II. Overall, based on seven days observation beginning from 25th March 2013, the solar activity is considered as very low. The highest solar flare can be observed within 7 days is only a class of B8 flare. There was no CMEs event that directed to the Earth is detected. The geomagnetic field activities are also at minimum level. Although the solar flare event is at a lower stage, it is still possible to form the solar radio burst type II which is associated with CME event. From the selected event, although theoretically solar radio burst type II is associated with CMEs, there is no compelling solar radio burst type II without a flare. The only difference is the dynamic structure and the intensity and speed of both phenomena (solar flares and CMEs) which depend on the active region. Nevertheless, understanding how energy is released in solar flares is one of the central questions in astrophysics. This solar radio burst type II formation is the first event that successfully detected by e-CALLISTO network in 2013.

14

The Tendencies and Timeline of the Solar Burst Type II Fragmented

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 12

84--102

EN

We report the timeline of the solar radio burst Type II that formed but fragmented at certain point based on the eruption of the solar flare on 13th November 2012 at 2:04:20 UT. The active region AR 1613 is one of the most active region in 2012. It is well known that the magnetic energy in the solar corona is explosively released before converted into the thermal and kinetic energy in solar flares. In this work, the Compound Astronomical Low-frequency, Low-cost Instrument for Spectroscopy Transportable Observatories (CALLIISTO) system is used in obtaining a dynamic spectrum of solar radio burst data. There are eight active regions and this is the indicator that the Sun is currently active. Most the active regions radiate a Beta radiation. The active regions 1610, 1611 and 1614 are currently the largest sunspots on the visible solar disk. There is an increasing chance for an isolated M-Class solar flare event. It is also expected that there will be a chance of an M flare, especially from AR 1614 and 1610. Although these two observations (radio and X-rays) seem to be dominant on the observational analysis, we could not directly confirmed that this is the only possibility, and we need to consider other processes to explain in detailed the injection, energy loss and the mechanism of the acceleration of the particles. In conclusion, the percentage of energy of solar flare becomes more dominant rather than the acceleration of particles through the Coronal Mass Ejections (CMEs) and that will be the main reason why does the harmonic structure of type II burst is not formed. This event is one fine example of tendencies solar radio burst type III, which makes the harmonic structure of solar radio burst type II fragmented.

15

Occurrences Rate of Type II and III Solar Radio Bursts at Low Frequency Radio Region 45 − 870 MHz

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 18

103--112

EN

Observations of type II and III solar bursts indicate that while type III bursts may appear at any altitude, from the very low corona into interplanetary space, type II solar bursts do not act the same way. This work focuses on recent observations in the radio region on the low frequency region from 45 MHz to 870 MHz. Our analysis employed the accuracy of the daily solar burst measurements of e-CALLISTO network. It was found that solar burst type II explode quite minimum with 1-2 events from 2006 - 2010. However, the data 2011 for solar burst type II increases drastically with 16 events has been recorded. The occurrences of Coronal Mass Ejections (CMEs) events are also increasing up to four times in 2011. Most of the both events can be observed in the range of 150 MHz till 500 MHz. Overall, we can say that the range of photon energy for solar burst type III is between 7.737 x 10-7 eV to 1.569 x 10-6 eV. In the case of solar burst type II, the distribution of energy is much smaller with 1.596 x 10-6 eV to 6.906 x 10-6 eV. Detailed investigation of solar burst will concern the 2011 data seem to show a significant trend for both types. We showed that the increasing of both solar burst events via years implies directing an increasing of solar activities including sunspot number, solar flare and Coronal Mass Ejections (CMEs) events. It is expected that both types will increase gradually in the beginning of 2014.

16

Observations of Different Type of Bursts Associated with M 6.3 Solar Flares

Hamidi Z. S., Shariff N. N. M.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 4

29--36

EN

Variation of solar bursts due to solar flares such as type an isolated type III , a complex type III, U is being highlighted. These bursts occurred on 9th March 2012 at the National Space Centre, Sg. Lang, Selangor, Malaysia Here, we study a unique case with a combination of two types burst associated with solar flare and CMEs. Our observation is focused on the low frequency region starting from 150 MHz till 400 MHz. We found that a solar flare type solar flare type M 6.3 which occurred in active region AR 1429 starting from 3:32 UT and ending at 05:00 UT. The flare has been confirmed to be the largest flare since 2005. Some physical parameters will be measured. We then compared our results with X-ray data from NOAA Space Weather Prediction Centre (SWPC).

17

Investigation on a Broken Solar Burst Type II during High Activities in AR1613 on 13th November 2012

Hamidi Z. S., Shariff N. N. M., Monstein C., Zulkifli W. N. A. W., Ibrahim M. B., Arifin N. S., Amran N. A.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 9

8--15

EN

The present article is an attempt to analyze the solar burst Type II observations based on solar flare and Coronal Mass Ejections (CMEs) events. We choose an intriguing type II radio burst with a velocity of 1193 kms-1 that occurred on 2012 November 13 at 2:04:20 UT. In this case, the study of solar radio burst type III is of paramount importance because of the fact that it helps to gain an insight of generation mechanisms of solar flare and Coronal Mass Ejections (CMEs) phenomena. Here, we have got a reasonably clear idea of the various forms under which the type III continuum emission may appear and potentially form a type II burst. However, in this case, the Type II solar burst only successfully forms a fundamental structure within the first few minute period, but broken suddenly before evolve a harmonic structure. This phenomenon is very interesting to be tackled and study. How the burst suddenly broken is still ongoing research seems the event is very rare and hard to be proved. There are a few questions that cause this unique situation which related to: (i) the intensity and duration of type III burst which also related to the classification of solar flare (ii) the probabilities CMEs to occur during that time and also the factor of the total amount of massive burst that exploded, Thus, we can conclude that the solar burst type III event still tells us an enigmatic characteristic from time to time due to the relationship of energetic particles and streams of particles with coronal magnetic fields and the pattern of Sun activity due to the 24th solar cycle. It might an interesting to study in detail the main factor that caused the Type II solar burst broken. Indirectly, it might because of the very intense of solar flares that make the percentage of energy of solar flare become more dominant rather than the acceleration of particles through the Coronal Mass Ejections. Thus, we realize that the potential energy during this event is higher than the kinetic energy of the particles.

18

First Light Detection of A Single Solar Radio Burst Type III Due To Solar Flare Event

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 11, iss. 1

51-58

EN

The eruption mechanism of solar flares and type III are currently an extremely active area of research, especially during the solar cycle is towards maximum. In this case, the total energy of solar burst type III is of the order of solar flare with the explosion of the energy can up to 1015 ergs. The solar flare event is one of the most spectacular explosions that still be on-going study in the solar physics world. This event occurred at 2:000 UT on 15th April 2012 is due to the explosion of the magnetic energy in from the chromosphere and converted into the heating, mass motion and particle acceleration which can be detected by solar radio burst type III. In this work, we will highlight our first light detection of very tiny solar radio burst type III, which has been observed at the National Space Centre, Banting, Selangor detected by the Compound Low Cost Low Frequency Transportable Observatories (CALLISTO) system at 5:53:23. The region of the data is from 150 − 400 MHz in radio region. This burst is drifted from 150 MHz till 260 MHz. It represents a total energy of 6.2035 × 10-7 eV − 1.0753 × 10-6 eV. This fast drift burst is a continuity of the acceleration of the particles which is intermittent, and can be observed since the explosion of the solar flare. Although the burst is very tiny, it is still significant because this burst is the first detection of a single type III burst from our site. Still, the acceleration of the particles can be detected from Earth in the radio region within 3 hours period of observation at the post stage of solar flare.

19

Comparison of the Radio Frequency Interference (RFI) in the Region of Solar Burst Type III Data At Selected CALLISTO Network

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 10

38--45

EN

Compact Astronomical Low-frequency, Low-cost Instrument for Spectroscopy in Transportable Observatories (CALLISTO) is a global network of spectrometer system with the purpose to observe the Sun’s activities. There are 25 stations that are used for this purpose. Radio Frequency Interference (RFI) is a major obstacle when performing observation with CALLISTO. We have confirmed at least 2 stations out of 10 stations with a complete overview spectral (OVS) made available to us showed clear detection of these consistent types of RFI for each specific region. In Malaysia, these RFI are also clearly detected. The major RFI affecting CALLISTO within radio astronomical windows below 1 GHz are local electronic system specifically radio navigation (at 73.1 MHz and 75.2 MHz), broadcasting (at (i) 151 MHz, (ii) 151.8 MHz and 152 MHz), aeronautical navigation (at (i) 245.5 MHz, (ii) 248.7 MHz and (iii) 249 MHz and fixed mobile at (i) 605 MHz, (ii) 608.3 MHz, (iii) 612.2 MHz, (iv) 613.3 MHz). It is obviously showed that all sites within this region are free from interference at 320-330 MHz and is the best specific region to be considered for solar burst monitoring. We also investigate the effect of RFI on detection of solar burst. We have considered type III solar bursts on 9th March 2012 in order to measure the percentage of RFI level during the solar burst. The RFI level is as low as 6.512 % to 80.769 % above solar burst detection.

20

Characterization of Selected Solar Radio Bursts Based on Solar Activity Detected by e- CALLISTO (Malaysia)

Hamidi Z. S., Shariff N. N. M., Monstein C.

International Letters of Chemistry, Physics and Astronomy

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2014

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Vol. 13, iss. 2

144--159

EN

One of the main reasons to study more about the dynamics of solar radio bursts is because solar these bursts can interfere with the Global Positioning System (GPS) and communications systems. More importantly, these bursts are a key to understand the space weather condition. Recent work on the interpretation of the low frequency region of a main solar burst is discussed. Continuum radio bursts are often related to the solar activities such as an indication of the formation of sunspot, impulsive phase of solar flares and Coronal Mass Ejections (CMEs) and their frequencies correspond to the densities supposed to exist in the primary energy release volume. Specifically, solar burst in low frequency play an important role in interpretation of Sun activities. In this work, we have selected few solar bursts that successfully detected by our station at the National Space Centre, Banting Selangor. Our objective is to correlate the solar burst with Sun activities by looking at the main sources that responsibility with the trigger of solar burst. It is found that type II burst is dominant with Coronal Mass Ejections (CMEs), type III burst associated with solar flare, IV burst with the formation of active region and type U burst high solar flare. We believed that this work is a good start to monitor Sun’s activities in Malaysia as equatorial country.