Occurrence Of Chorus Research Articles

The Effects of Substorm Injection of Energetic Electrons and Enhanced Solar Wind Ram Pressure on Whistler‐Mode Chorus Waves: A Statistical Study

AbstractWhistler‐mode chorus waves in the inner magnetosphere are typically excited by an electron temperature anisotropy. The anisotropy can be driven by two sources: particle injections from the tail (such as during substorms) and the solar wind ram pressure on the dayside magnetosphere. Based on 5 years of data from Van Allen Probe A, we have separately studied the effects of substorm injection of energetic ∼10–100 keV electrons and their gradient and curvature drifts () and enhanced solar wind ram pressure () on the generation of whistler‐mode chorus waves. We use time‐modified and indices to take into account time delays. We find that during the period of large but small , chorus waves are mainly observed in the midnight through dawn to noon sectors ( MLT 13) near the magnetic equator (|MLAT| < 10°) at L = 4.5–6.5. With an increase in both the chorus occurrence rates and the wave amplitudes increase. While under the condition of enhanced but small , chorus waves are preferentially detected on the dayside ( MLT 14) in a wide range of latitudes (|MLAT| < 20°) at outer L‐shells (L = 5.5–6.5). With the increase of , chorus occurrence rates also increase, while the amplitude remains relatively constant. Our study supports the two mechanisms for chorus excitation in the Earth’s magnetosphere and provides a better understanding of the global distribution and properties of chorus waves.

Temporal and spectral patterns of fish choruses in two protected areas in southern Atlantic

The Laje de Santos Marine State Park (LSMSP) and Xixová-Japuí State Park (XJSP) are two protected areas (PAs) located in the southern Atlantic Brazilian coast. One month of continuous underwater sound recordings allowed determining that durable fish choruses was a major contributor of LSMSP and XJSP soundscapes. The choruses showed well defined daily periodicities and exceeded background ambient noise for some hours every day, principally at dawn and dusk. This work describes the daily temporal patterns and spectral characteristics of the studied PAs choruses. Annotations of some chorus temporal descriptors (start, peak and end time instants) allow obtaining a precise statistical representation of diel chorus occurrence. Average Power Spectral Density and cepstral analysis were used to determine quantitative values for choruses spectral characterization. Results show that the passive acoustic technique could be a cost-effective method for management and fish monitoring in PAs.

Survey of whistler mode chorus intensity at Jupiter

AbstractWhistler mode chorus emission is important in the acceleration of electrons and filling of the radiation belts at Jupiter. In this work chorus magnetic intensity levels (frequency‐integrated spectral density, PB) at Jupiter are comprehensively binned and parameterized. The frequency range of chorus under study extends from the lower hybrid frequency, flh, to fceq/2 and fceq/2 < f < 0.8 fceq, where fceq is the cyclotron frequency mapped to the magnetic equator. The goal is to obtain a quantized distribution of magnetic intensity for use in stochastic modeling efforts. Parametric fits of magnetic plasma wave intensity are obtained, including PB versus frequency, latitude, and L shell. The results indicate that Jupiter chorus occurrence probability and intensity are higher than those at Saturn, reaching values observed at Earth. Jovian chorus is observed over most local times, confined primarily to the range 8 < L < 15, outside the high densities of the Io torus. The largest intensity levels are seen on the dayside; however, the sampling of chorus on the nightside is much less than on the dayside. Peak intensities occur near the equator with a weak dependence on magnetic latitude, λ. We conclude that Jovian chorus average intensity levels are approximately an order of magnitude lower than those at Earth. In more isolated regions the intensities are comparable to those observed at Earth. The spatial range of the chorus emissions extends beyond that assumed in previous Jovian global diffusive models of wave‐particle electron acceleration.

Hit Songs Deconstructed

Hit Songs Deconstructed. [New York, New York]: HSD Media Group, 2011--. http://www.hitsongsdeconstructed.com/ (Accessed 17 June 2016). [Requires: A Web browser and an Internet connection. Hit Songs Deconstructed academic pricing is calculated by FTE (full-time equivalent) as follows: FTE = Up to 10,000: $2,500/year; FTE = 10,001-15,000: $3,000/year; FTE = 15,001 - 20,000: $3,500/year; FTE = 20,000+: $4,000/year. Academic subscriptions include unlimited use by faculty and students and the rights to use Hit Song Deconstructed reports in the classroom. Individual subscriptions are $249.99 (annual billing cycle) with a free month trial.] Introduction Hit Songs Deconstructed (HSD) is a database featuring in-depth reports on hit songwriting trends and techniques used to craft today's hits. Founded in 2011 by David Penn (Editor-in-Chief) and Yael Penn (CEO) this database provides musicians, songwriters, producers, educators and music industry professionals with the tools they need to stay in tune with the most modern hit song musical vocabulary and production techniques. Unlike other music industry databases which focus on song sales or airplay, HSD focuses on the songs themselves by tracking their compositional characteristics. A Hit Songs Deconstructed annual subscription includes twelve monthly Hit Song Deconstructed reports, four quarterly Top Ten Trend reports, four quarterly Top Ten Hits reports and four quarterly Top Ten Genre reports. In addition, a subscription provides access to the weekly HSD Music charts. Layout and Navigation The Hit Songs Deconstructed Web site has a simple, clean, and well organized layout that makes it easy to find reports. Reports feature detailed analysis supported by snippets of notated music, as well as colorful charts and graphs illustrating trends, song structure, instrumentation and more. Links to each section of the report make it easy navigate. Official song videos and playlists are also available. There is a keyword search for reports at the top; this search function seems fairly basic, it does not highlight words in results or offer Boolean searching. The HSD Web site divides its information into two main sections: HSD Music Charts, and HSD Reports. HSD Music Charts The HSD Music Charts spotlight the characteristics driving the Billboard Hot 100 top 10 on a weekly basis. Each of the thirteen charts, ranging from First Chorus Occurrence to Sub-Genres & Influencers to Song Length, show the percentage of songs that possess a particular characteristic. For example, the Song Length chart shows what percentage of songs are three minutes versus four minutes long; this area also connects to Spotify playlists of top ten songs by week, which are curated by HSD as the Spotify user hitsongsdeconstructed. HSD Reports The HSD Reports section on the Web site houses all of the Hit Songs Deconstructed reports. David Penn manages the editorial team and spearheads all song analysis. Reports are authored by Hit Songs Deconstructed. The monthly Hit Song Deconstructed report takes a deep dive into the what, how and why behind a current chart-topping hit. The quarterly Top Ten Trend report provides a snapshot of the key hit songwriting trends that define all of the songs that land in the Billboard Hot 100 top 10, quarter by quarter, over the period of one year. The Top Ten Hits report provides an in-depth look at the specific techniques being utilized to craft the hits and the Top Ten Genre report provides a comprehensive overview of the characteristics and trends that define each genre represented in the Hot 100 top 10 each quarter. A Hit Song Deconstructed report averages approximately 120 pages when printed. Each report is divided into about twenty-two smaller sections with charts and graphs, which makes the reports easy to navigate. Sections such as Word and Syllable Count and Rhyming Schemes treat the song like a complex math problem that can be simplified by breaking it down into smaller manageable pieces. …

Effects of magnetic field configuration on the day‐night asymmetry of chorus occurrence rate: A numerical study

AbstractEffects of magnetic field configuration on the day‐night asymmetry of chorus occurrence rate are investigated using a recently developed hybrid code. Previous observational studies show that dayside chorus occurs in regions where the background magnetic field is nearly homogeneous; these observations suggest that the magnetic field configuration might play a significant role in the dayside chorus excitation. In this work, we use a hybrid code to numerically study the threshold conditions of hot electron distributions required to excite chorus waves for different background magnetic field inhomogeneities. Our numerical experiments demonstrate that in a magnetic field with smaller inhomogeneity, smaller temperature anisotropy and smaller number density of hot electrons are needed to excite chorus waves. This conclusion could help explain the higher occurrence rate of chorus waves on the dayside compared with that on the nightside especially in quiet times.

Statistical analysis of ground‐based chorus observations during geomagnetic storms

AbstractChorus observations from two ground‐based, Antarctic receiving stations are analyzed for a set of geomagnetic storms from 2000 to 2010. Superposed epoch analysis is performed together with statistical hypothesis testing to determine whether the observed quantities (geomagnetic indices, outer belt energetic electron fluxes, and chorus properties) are statistically significantly different as functions of storm phase, storm size, and storm type. Waves generated in the outer dayside magnetosphere and observed on the ground at South Pole Station are suppressed during main phase and are statistically unchanged from random intervals during recovery phase. Waves generated in the inner magnetosphere and observed on the ground at Palmer Station are significantly enhanced during storm main phase and for about 3 days into recovery. During main phase, there are larger enhancements in chorus occurrence, amplitude, and frequency extent as observed at Palmer during larger storms. During recovery phase, there are larger enhancements in chorus occurrence, amplitude, and frequency extent as observed at Palmer during larger storms and during storms where the average rate of electron flux increase, averaged across the outer belt, is higher.

Survey analysis of chorus intensity at Saturn

AbstractIn order to conduct theoretical studies or modeling of pitch angle scattering of electrons by whistler mode chorus emission at Saturn, a knowledge of chorus occurrence and magnetic intensity levels, PB, as well as the distribution of PB relative to frequency and spatial parameters is essential. In this paper an extensive survey of whistler mode magnetic intensity levels at Saturn is carried out, and Gaussian fits of PB are performed. We fit the spectrum of wave magnetic intensity between the lower hybrid frequency and fceq/2 and for frequencies in the interval fceq/2 < f < 0.9 fceq, where fceq is the cyclotron frequency mapped to the equator. Saturn chorus is observed over most local times, but is dominant on the nightside in the range of 4.5 < L <7.5, with minimum power at the equator and peak power in the range of 5° < λ < 10°. Saturn wave magnetic intensity averaged in frequency bins peaks in the range of 10−5 < PB < 10−4 nT2 for 0.4 < β < 0.5 (β = f/fceq). Gaussian fits of PB with frequency and latitude are obtained for lower band chorus. Plasma injection regions are occasionally encountered with significant chorus power levels. Upper band chorus is seen almost exclusively within plasma injection regions, and the number of events is very limited, but when present, the average levels of PB can be higher than the lower band chorus. The overall magnetic intensity contribution of the upper band, however, is insignificant relative to the lower band.

Environmental and social determinants of anuran lekking behavior: intraspecific variation in populations at thermal extremes

Environmental and social factors are critical to determine the timing and duration of lekking behavior since they provide species with signs to maximize benefits over costs in sexual displays. However, these factors have rarely been studied under different environmental conditions, and thus, it remains unclear whether exogenous factors affecting group displays show a general species-specific pattern or whether they are population-specific. Using audio-trapping techniques, we compared factors influencing the daily occurrence and duration of lekking behavior in two populations of Hyla molleri and two populations of Hyla meridionalis located at the thermal extremes (coldest vs. hottest) of their Iberian distribution range. From 12,240 hourly recordings over one season, multimodel inference revealed that the major determinants of chorus occurrence were similar between populations and species (i.e., chorus size the previous day, daytime air temperature, relative humidity, and barometric pressure), and accounted for 51–79 % of its deviance. In contrast, the major determinants of chorus duration differed between populations and species (i.e., chorus size, number of day, and air temperature and relative humidity at the onset of the chorus), and accounted for 38–69 % of its variance. Our findings suggest that the decision making related to lek attendance is environment-dependent, takes place at time between lekking events, and is associated with exogenous factors that may be both stable across species ranges and population-specific when populations are under different climatic conditions. This intraspecific variation might be underlain by plasticity mechanisms providing tree frogs with means to cope with changing environments. Moreover, social facilitation related to male-male acoustic competition seems to play a relevant role on the daily time invested by males in lek attendance.

Off‐equatorial chorus occurrence and wave amplitude distributions as observed by the Polar Plasma Wave Instrument

Determining the global distribution of chorus wave power in the off‐equatorial region (i.e., magnetic latitude λ > 15°) is a crucial component of understanding the contribution of chorus to radiation belt acceleration and loss. In this paper we use a database of chorus power spectral density observations from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on the Polar spacecraft to generate separate distributions of wave occurrence rate and magnetic field amplitude as a function of space and geomagnetic activity. Previous studies focused on a band‐integrated and time‐averaged data product to characterize the global distribution of wave power. Using a slightly different technique, we first estimate the wave amplitude from the peak wave power spectral density for times when chorus is observed. The mean wave amplitude at a given location is then multiplied by the wave occurrence rate to yield the time‐averaged amplitude. We present the spatial distributions of wave occurrence rate, mean amplitude, and time‐averaged amplitude in the region of maximum statistics, λ > 15° and R = 4−8 RE. We find that waves of significant amplitude (>10 pT) can be observed in all local time sectors, but significant wave occurrence (>20%) is confined to the dawn and noon local time sectors. Wave mean and time‐averaged amplitudes are also highest in the dawn and noon sectors. The spatial extent of regions with high time‐averaged amplitude is primarily defined by regions of high occurrence rate. Time‐averaged amplitudes exceeding ∼6 pT are observed up to a magnetic latitude of 40° at dawn and 50° at noon, while at midnight and dusk the time‐averaged amplitude tends to be below that value. We also examine the geomagnetic and solar wind dependence of the spatial distribution of wave occurrence, mean amplitude, and time‐averaged amplitude. In the off‐equatorial region (λ > 15°), wave amplitude and occurrence on the nightside increase dramatically during disturbed geomagnetic and solar wind conditions. In contrast, waves on the dayside occur over a wider range of activity, and even during quiet conditions, mean and time‐averaged amplitudes at noon significantly exceed amplitudes at midnight for disturbed times. In the dusk sector, observation of waves is mostly limited to quiet conditions, and during those times, mean amplitudes at dusk exceed those at midnight and approach amplitudes observed in the dawn sector. Parallel investigation of the independent variability of occurrence and amplitude provides a more complete picture of the chorus wave environment, particularly for application to modeling radiation belt dynamics on both short and long time scales.

Role of the plasmapause in dictating the ground accessibility of ELF/VLF chorus

This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground‐based observations of chorus via a 3 month study of ground‐based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50°S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L ∼ 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L ∼ 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode.

Connection between the upper ionospheric region of VLF chorus occurrence and the plasmapause position

The occurrence zone of the VLF chorus in the upper ionosphere appears at L-shells lower than plasmapause position Lpp; with increasing geomagnetic activity the spatial dimension of the zone diminishes, its upper boundary being shifted in correspondence with the plasmapause position, the lower remaining practically without change(L=2.0÷2.5). Calculations of propagation paths have shown that the similarity of the VLF chorus spectrum at different upper-ionospheric latitudes as well as the large spatial dimension of the zone of observation can be explained as special features in the propagation of VLF waves from an equatorial source, starting in the vicinity of the plasmapause with different initial normal angles.

Analysis of chorus emissions at Jupiter

On the Voyager 1 inbound pass through the Jovian magnetosphere, the frequent acquisition of wide band data from the plasma wave instrument has permitted a quasi‐continuous survey of the waves which occur in the frequency band below the electron cyclotron frequency, the chorus band. Structured, rising frequency chorus was observed just outside the dense Io plasma torus but inside the low‐density middle magnetosphere. A quasi‐continuous, very narrow band emission occurred just above one‐half the electron cyclotron frequency. Power spectra, which were constructed from the wide band data, showed that the half‐cyclotron emission is unaffected by the simultaneous occurrence of chorus at lower frequencies and is separated from the chorus by a deep spectral gap just below one‐half the cyclotron frequency. Within the rising chorus band, a hisslike emission was observed which consisted of two quasi‐continuous, very narrow band tones located near the upper and lower frequency limits of the rising chorus band. Sequential power spectra showed that these twin‐frequency tones persisted during rising chorus bursts. Chorus occurs in the spatial regions where whistlers have cyclotron resonant interactions with the suprathermal population of ∼keV electrons. A possible interpretation of the twin tones is that these signals are electrostatic or resonance cone whistlers. Growth rate calculations show that the electrostatic whistler is destabilized by a loss cone distribution with the same pitch angle anisotropy which is needed to excite the electromagnetic chorus whistlers. A speculative hypothesis is presented to explain the half‐cyclotron emission. The Landau absorption of oblique chorus whistlers results in the spectral gap just below one‐half the cyclotron frequency and in the formation of a quasi‐linear plateau in the parallel velocity distribution of the suprathermal electrons. The plateau alters the Landau and cyclotron resonant interactions and permits the unstable growth of electrostatic and electromagnetic whistlers in a narrow range just above one‐half the cyclotron frequency.

A qualitative explanation of the diurnal variation of chorus

Diurnal variation of chorus. One of the most regular and consistent features of the very low frequency (VLF) emissions known as chorus is the diurnal variation of occurrence. Storey [1953] noted that chorus occurrence peaked at dawn at Cambridge; later Allcock [1957] and Pope [1957] showed that the diurnal peak varied almost linearly with geomagnetic latitude, being near dawn at the lower latitudes and near noon at the high latitudes. Attempts have been made to reduce the scatter in the data by measuring the peak occurrence in auroral time [Brice and Ungstrup, 1963].

Chorus, hiss, and other audio-frequency emissions at stations of the whistlers-east network

Ionospherics, electromagnetic waves originating somewhere in the ionosphere or the magnetosphere of the earth, have been recorded for more than six years in the audio-frequency band at the Whistlers-East chain of ground-based receiving stations which extend approximately along the 0° geomagnetic meridian from Labrador to Antarctica. This has made it possible to study the latitude dependence of: 1) the local time of maximum occurrence of the various types of audio-frequency emissions, 2) the shape of the diurnal occurrence curves, 3) the rate of occurrence, and 4) the effects of geomagnetic disturbances upon the occurrence. Some of the results and conclusions of a comprehensive analysis of these synoptic data are: 1) At middle- and high-latitude stations the occurrence or nonoccurrence of audio-frequency emissions appears to be controlled mainly by the emission processes rather than by the subsequent propagation of the emitted wave through the magnetosphere and the ionosphere. This is contrary to what we have concluded about the occurrence of whistlers. Propagation conditions apparentally do become controlling at low latitudes, and at all latitudes at times of severe geomagnetic disturbances. 2) At middle-latitude stations, audio-frequency emissions are essentially a geomagnetic storm-time phenomenon. At high latitudes, the emissions are more likely to be observed under geomagnetically quiet conditions. 3) At a given station, the correlation between the occurrence of audio-frequency emissions and the level of geomagnetic activity is practically the same for all major types of emissions, although different types of emissions tend to occur at different times of the day. The correlation shows no significant seasonal variation (except possibly at Knob Lake). 4) At some stations, the local time of maximum occurrence of chorus remains surprisingly constant from season to season and from year to year. Moreover, when seasonal data of these stations are averaged over a long period of time, it is found that chorus shows no preference for any season (Dartmouth and Ellsworth). 5) At the two stations of the Whistlers-East network closest to the equator (Bermuda and Port Lockroy), chorus is almost absent in local summer but in local winter the activity is relatively high, displaying a maximum of occurrence appproximately at the same time as at stations more than ten degrees closer to the poles. 6) At high latitudes, on the other hand, more chorus is observed in local summer than in local winter. The local time of maximum occurrence of chorus at high latitudes depends on the level of geomagnetic activity, quiet-day chorus peaking later than disturbed-day chorus. 7) The diurnal variation of hiss also depends on latitude, season, and geomagnetic activity. It peaks near midday in latitudes above Dartmouth but develops a deep midday depression at Dartmouth in local summer and fall and in all seasons in lower latitudes. 8) In the northern hemisphere at sunspot maximum, in the longitude of the Whistlers-East chain, the activity of audio-frequency emissions appears to be highest at a geomagnetic latitude of about 60°. The activity decreases only slowly toward the poles but rapidly below about 50°. As the solar activity has subsided, the region of maximum activity has moved farther north. A corresponding situation is found in the magnetically conjugate region of the southern hemisphere.

V. L. F. Emissions and Geomagnetic Disturbances at the Auroral Zone

Some studies of the relation between the chorus activity and geomagnetic activity have shown that the strength or the occurrence of chorus is maximum at the auroral zone on days of moderate geomagnetic activity, and that the region of maximum strength or occurrence shifts toward lower latitude on stormy days.In this paper we have examined a relation between the chorus and geomagnetic conditions at the auroral zone in details, using the chorus indices, normal-run magnetograms, and the auroral echoe indices observed at College, Alaska from August 1959 to December 1960.The result has indicated that most of the intensity increases of chorus occurred at magnetically quiet times and followed the negative bay-type magnetic disturbances. Moreover, a good correlation has been found between the magnitudes of the preceding bay-type disturbances and the rising times of strength increases of choruses.