Low-frequency Method Research Articles

Diagnosing ratio of electron density to collision frequency of plasma surrounding scaled model in a shock tube using low-frequency alternating magnetic field phase shift

A non-contact low-frequency (LF) method of diagnosing the plasma surrounding a scaled model in a shock tube is proposed. This method utilizes the phase shift occurring after the transmission of an LF alternating magnetic field through the plasma to directly measure the ratio of the plasma loop average electron density to collision frequency. An equivalent circuit model is used to analyze the relationship of the phase shift of the magnetic field component of LF electromagnetic waves with the plasma electron density and collision frequency. The applicable range of the LF method on a given plasma scale is analyzed. The upper diagnostic limit for the ratio of the electron density (unit: m−3) to collision frequency (unit: Hz) exceeds 1 × 1011, enabling an electron density to exceed 1 × 1020 m−3 and a collision frequency to be less than 1 GHz. In this work, the feasibility of using the LF phase shift to implement the plasma diagnosis is also assessed. Diagnosis experiments on shock tube equipment are conducted by using both the electrostatic probe method and LF method. By comparing the diagnostic results of the two methods, the inversion results are relatively consistent with each other, thereby preliminarily verifying the feasibility of the LF method. The ratio of the electron density to the collision frequency has a relatively uniform distribution during the plasma stabilization. The LF diagnostic path is a loop around the model, which is suitable for diagnosing the plasma that surrounds the model. Finally, the causes of diagnostic discrepancy between the two methods are analyzed. The proposed method provides a new avenue for diagnosing high-density enveloping plasma.

Unveiling low-to-high-frequency data sampling caveats for aquaculture environmental monitoring and management

Aiming at the sustainability of aquaculture production, producers should adopt tools and protocols for environmental monitoring and management of these enterprises. There are currently issues concerning the efficacy of data collection procedures and limnological sampling at low frequency, which is widely used by managers and aquaculture surveillance agencies. In this context, the present study evaluated the effectiveness of high-frequency (HF) and low-frequency (LF) limnological monitoring. For the HF, autonomous data collection platforms (ADCP) were installed in a tropical reservoir, four ADCP in areas with fish production (WFP), and one ADCP in an area free of fish production (FFP, control). For both sampling methods, the temperature (Temp), pH, dissolved oxygen (DO), turbidity (Tbt), electrical conductivity (EC), and chlorophyll-a (Chl-a) were sampled at a depth of 1.5 m from the water surface. While the HF used a multiparameter probe, recording each parameter every 10 min, the LF method consisted of monthly data acquisitions of the same settings using water sampling techniques for further measurement in the laboratory. The comparison of the sampling frequency and methods revealed different profiles for all measured parameters during the monitored period. The average values of the daily amplitude of variation differed between the two monitoring strategies for water temperature, pH, DO, Tbt, and Chl-a. Comparison of the results showed that the HF limnological monitoring allowed us to create a more accurate variation profile of the water quality variables measured. The ADCP is a useful strategy that can be used to capture the influences of fish production and to bring essential water quality changes for fish management. The evaluations with LF did not demonstrate the natural variability of the measured parameters, being an ineffective tool for environmental monitoring of fish production.

An Improved Audio Based Key Frame Detection Model for Movie Summarization

In the recent scenario, the amount of multimedia content mainly videos is increasing at a high rate. This makes the browsing, retrieval and delivery of videos a very slow and difficult task. Video summarization helps in faster browsing and content indexing of large videos by producing an abstract overview of the whole video. Movie summarization condenses a full length movie into a summary which includes the relevant contents in the movie. The proposed framework implements a movie summarization method based on the audio and video features of a movie. The method comprises of three phases. In the pre-processing phase, audio and frames of the video are extracted and stored. Next in the audio feature analysis phase, audio data having high frequency value are extracted. Three techniques are proposed for audio frame analyses which are high frequency method (HFM), Low frequency method (LFM) and threshold frequency method (TFM). In summary generation phase, key frames are extracted which is further refined in the post processing phase. The experimental results indicate that in majority of the cases HFM achieves better performance than LFM and TFM in both qualitative and quantitative analysis.

Meteor radar measurements of mean winds and tides over Collm (51.3° N, 13° E) and comparison with LF drift measurements 2005–2007

Abstract. An all-sky VHF meteor radar (MR) has been continuously operated at Collm (51.3° N, 13° E) since summer 2004. The radar measures meteor parameters, diffusion coefficients, and horizontal winds in the mesopause region. There exists a temporal overlap of the MR wind measurements with co-located low-frequency (LF) ionospheric drift measurements until 2007. Comparison of MR and LF semidiurnal tidal phases allows to empirically determine the virtual height overestimation of LF reflection heights due to the group retardation of LF waves. LF reference heights have to be reduced by up to 20 km to match real heights. Correction of LF heights for group retardation allows to determine the wind underestimation by the LF method compared with meteor radar measurements and opens the possibility to continue long-term trend analysis using mesosphere/lower thermosphere winds.

A deterministic (non-stochastic) low frequency method for geoacoustic inversion

It is well known that multiple frequency sources are necessary for accurate geoacoustic inversion. This paper presents an inversion method which uses the low frequency (LF) spectrum only to estimate bottom properties even in the presence of expected errors in source location, phone depths, and ocean sound-speed profiles. Matched field processing (MFP) along a vertical array is used. The LF method first conducts an exhaustive search of the (five) parameter search space (sediment thickness, sound-speed at the top of the sediment layer, the sediment layer sound-speed gradient, the half-space sound-speed, and water depth) at 25 Hz and continues by retaining only the high MFP value parameter combinations. Next, frequency is slowly increased while again retaining only the high value combinations. At each stage of the process, only those parameter combinations which give high MFP values at all previous LF predictions are considered (an ever shrinking set). It is important to note that a complete search of each relevant parameter space seems to be necessary not only at multiple (sequential) frequencies but also at multiple ranges in order to eliminate sidelobes, i.e., false solutions. Even so, there are no mathematical guarantees that one final, unique "solution" will be found.