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The Madden–Julian Oscillation (MJO)/Boreal Summer Intraseasonal Oscillation (BSISO) has been considered an important climate mode of variability on subseasonal timescales for East Asian summer. However, it is unclear how well the MJO/BSISO indices would serve as guidance for subseasonal forecasts. Using a probabilistic forecast model determined through multiple linear regression (MLR) with MJO, ENSO, and long-term trend as predictors, we examine lagged impacts of each predictor on East Asia extended summer (May–October) climate from 1982 to 2015. The forecast skills of surface air temperature (T2m) contributed by each predictor is evaluated for lead times out to five weeks. We also provide a systematic evaluation of three commonly used, real-time MJO/BSISO indices in the context of lagged temperature impacts over East Asia. It is found that the influence of the trend provides substantial summertime skill over broad regions of East Asia on subseasonal timescales. In contrast, the MJO influence shows regional as well as phase dependence outside the tropical band of the main action centers of the MJO convective anomalies. All three MJO/BSISO indices generate forecasts that yield high skill scores for week 1 forecasts. For some initial phases of the MJO/BSISO, skill reemerges over some regions for lead times of 3–5 weeks. This emergence indicates the existence of windows of opportunity for skillful subseasonal forecasts over East Asia in summer. We also explore the dynamics that contribute to the elevated skills at long lead times over Tibet and Taiwan–Philippine regions following the initial state of phases 7 and 5, respectively. The elevated skill is rooted in a wave train forced by the MJO convective heating over the Arabian Sea and feedbacks between MJO convection and SSTs in Taiwan–Philippine region. Two out of the three commonly used MJO/BSISO indices tend to identify MJO events that evolve consistently in time, allowing them to serve as reliable predictors for subseasonal forecasts for up to 5 weeks.

Although the impedance/admittance model of modular multilevel converter (MMC) considering internal harmonic is discussed extensively in the literature, few attentions have been focused on how to determine the number of harmonics to be included in the model. This article fills this gap by developing the MMC signal-flow graph (SFG), providing the analytical transfer function to quantitatively estimate the attenuation degree of MMC harmonics, so as to guide the model truncation. Besides, SFG offers an intuitive understanding on the mathematical characteristic of MMC, including the conjugate symmetry and the effect of phase reference on admittance, which helps to reduce the modeling calculation. Moreover, conversion formula of MMC multiple input multiple output and single input single output admittance is derived based on the truncated model to further simplify the stability analysis. The correctness of this article has been verified by simulation and experiment.

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

The stationary-frame complex-valued frequency-domain modeling has been applied to characterize the frequency-coupling dynamics of three-phase converters. Yet, those models are generally derived through mathematical transformations of the linearized time-invariant models in the rotating dq -frame. A step-by-step modeling method with clear physical insight in the stationary frame is still missing. This article attempts to fill in the void by introducing a general stationary ( $\alpha \beta$ )-frame, three-port equivalent circuit model for the converter power stage, based on the direct linearization around time-periodic trajectories. The model not only reveals the frequency-coupling effect of the ac–dc dynamic interaction but also provides an explicit theoretical basis for incorporating the control dynamics. Moreover, the dependence of the frequency-coupling terms on the initial phase of the input voltage is pointed out. Considering the phase-dependent feature, a frequency scan method that can accurately measure the $\alpha \beta $ -frame converter model is proposed. The measured frequency responses in both the nonlinear time-domain simulations and experimental tests validate the effectiveness of the frequency scan method and the theoretical analysis.

The problem of energetics of flare activity of dwarf stars in the lower main sequence of the Hertzsprung-Russell diagram is relevant in the context of the detection of powerful flares (superflares) and the possibility of assessing such phenomena on the Sun in the past and future. A brief historical review of the EV Lac original researches is presented, and the modern databases containing information on photometric observations of the star are described. The main attention is paid to the observations of EV Lac made by the TESS observatory. Results of the analysis of flare activity of the star according to the data of the TESS project are presented. The most typical flares have been demonstrated. The dependence of flare activity of EV Lac on the phase of its axial rotation has been studied. The dependence of the number of flares on the phase and energy is illustrated. The segment of the phase of the EV Lac light curve, which contains flares with the highest energies, is indicated., Таврический астрофизический журнал, Выпуск 3 (2) 2022, Pages 39-45

This article presents a model of conjugated Gaussian random particles, which are convenient for simulation of irregular particles that constitute cometary dust. Computer simulation is conducted for the polarimetric properties of these particles; phase dependences are calculated for the linear polarization degree. The calculated results are used to interpret cometary polarimetric observations and determine possible physical and chemical characteristics of comets as well as the variation range within which the model adequately describes the observed data. The model calculations are used to refine the empirical formula that describes the phase dependence of the linear polarization degree for the cometary continuum.

Fluctuations in cortical excitability are a candidate mechanism involved in the trial-to-trial variation of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). We explore whether infraslow EEG activity (NEW & NOTEWORTHY Cortical excitability measures are highly variable during transcranial magnetic stimulation. Although ongoing brain oscillations are assumed to modulate excitability, no consistent associations are found for the traditional frequency bands. We focus on the role of infraslow EEG activity, defined as rhythms with frequencies < 0.1 Hz. We provide experimental evidence suggesting that infraslow activity most likely modulates corticomotor excitability and that response variation could be reduced when stimulation is targeted at a specific infraslow phase.

The Tibetan Plateau (TP), referred to as the “Asian water tower”, contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals a distinct out-of-phase connection between the February North Atlantic Oscillation (NAO) and March TPSAT, which is non-stationary and regulated by the warm phase of the Atlantic Multidecadal Variability (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO does not persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO−TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low-frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the enhanced and southward-shifted storm track, help maintain the NAO anomaly pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic−TP connections in late winter−early spring.

Using the Cosmology Large Angular Scale Surveyor, we measure the disk-averaged absolute Venus brightness temperature to be 432.3 $\pm$ 2.8 K and 355.6 $\pm$ 1.3 K in the Q and W frequency bands centered at 38.8 and 93.7 GHz, respectively. At both frequency bands, these are the most precise measurements to date. Furthermore, we observe no phase dependence of the measured temperature in either band. Our measurements are consistent with a CO$_2$-dominant atmospheric model that includes trace amounts of additional absorbers like SO$_2$ and H$_2$SO$_4$., Comment: 7 pages, 3 figures, published in PSJ

Correction: 10.1007/s00442-021-04856-4, Oecologia (2021) Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Contains fulltext : 218355.pdf (Publisher’s version ) (Open Access) Different inputs from a multisensory object or event are often integrated into a coherent and unitary percept, despite differences in sensory formats, neural pathways, and processing times of the involved modalities. Presumably, multisensory integration occurs if the cross-modal inputs are presented within a certain window of temporal integration where inputs are perceived as being simultaneous. Here, we examine the role of ongoing neuronal alpha (i.e. 10-Hz) oscillations in multimodal synchrony perception. While EEG was measured, participants performed a simultaneity judgement task with visual stimuli preceding auditory ones. At stimulus onset asynchronies (SOA’s) of 160–200 ms, simultaneity judgements were around 50%. For trials with these SOA’s, occipital alpha power was smaller preceding correct judgements, and the individual alpha frequency was correlated with the size of the temporal window of integration. In addition, simultaneity judgements were modulated as a function of oscillatory phase at 12.5 Hz, but the latter effect was only marginally significant. These results support the notion that oscillatory neuronal activity in the alpha frequency range, which has been taken to shape perceptual cycles, is instrumental in multisensory perception. 8 p.

Essential tremor manifests predominantly as a tremor of the upper limbs. One therapy option is high-frequency deep brain stimulation, which continuously delivers electrical stimulation to the ventral intermediate nucleus of the thalamus at about 130 Hz. Constant stimulation can lead to side effects, it is therefore desirable to find ways to stimulate less while maintaining clinical efficacy. One strategy, phase-locked deep brain stimulation, consists of stimulating according to the phase of the tremor. To advance methods to optimise deep brain stimulation while providing insights into tremor circuits, we ask the question: can the effects of phase-locked stimulation be accounted for by a canonical Wilson–Cowan model? We first analyse patient data, and identify in half of the datasets significant dependence of the effects of stimulation on the phase at which stimulation is provided. The full nonlinear Wilson–Cowan model is fitted to datasets identified as statistically significant, and we show that in each case the model can fit to the dynamics of patient tremor as well as to the phase response curve. The vast majority of top fits are stable foci. The model provides satisfactory prediction of how patient tremor will react to phase-locked stimulation by predicting patient amplitude response curves although they were not explicitly fitted. We also approximate response curves of the significant datasets by providing analytical results for the linearisation of a stable focus model, a simplification of the Wilson–Cowan model in the stable focus regime. We report that the nonlinear Wilson–Cowan model is able to describe response to stimulation more precisely than the linearisation.

The large difference of dielectric functions between the amorphous and crystalline phases of Ge–Sb–Te based phase-change materials (PCMs) used in memory storage devices also affects their Schottky barrier heights (SBHs) and thus their electrical device properties. Here, the SBHs of each phase of Ge2Sb2Te5, GeTe, GeSe, and SnTe are found by density functional supercell calculations. The Fermi level pinning factor S calculated for the crystalline phases (with a larger dielectric constant) is smaller than their amorphous phases, agreeing well with the empirical relationship linking SBH to a dielectric constant. The relatively large dielectric constant of crystalline PCMs arises from their resonant bonding (metavalent bonding), but their pinning factor is not always as small as empirically expected. The results are useful for optimizing the design of metal contacts for Ge–Sb–Te type phase-change memory devices.

A growing number of studies suggest the phase of ongoing alpha oscillations in the brain influences visual perception. However, it remained largely unconsidered if this is associated with a phase dependence of neurophysiological processes especially in the visual cortex. Therefore, this study investigated the link between the pre-stimulus oscillatory alpha phase and neural correlates of early visual perception. In 64 subjects a 64-channel EEG system was used to examine the phase dependence of pattern-reversal visual evoked potentials (VEP) in a visual perception experiment. The pre-stimulus oscillatory phase over the primary visual cortex was determined for the individual alpha peak frequency (iAPF) as well as the frequency of maximal phase locking (PLFfmax). The phase dependence of VEP latency was determined using single-trial phase sorting. The results indicate a significantly shorter latency for the N75 and P100 components of the VEP between 40°–100° (p The study indicates a link between the pre-stimulus alpha phase and neural correlates of early visual perception. These results extend previous behavioral findings to the neurophysiological level and support current models suggesting visual perception is modulated by ongoing alpha oscillations.

Temperature and phase dependences of Raman spectra (RS) and molecular relaxation characteristics of nitrate-ion vibrations in (1 – x)RbNO3 + xAl2O3 composites with various concentrations of Al2O3 nanopowder were studied. It was shown that the ν 1(A) vibrational relaxation time with filler concentration x = 0.6 was essentially constant in high-temperature phase RbNO3-II as compared with RbNO3-III, which indicated that the cubic RbNO3-III phase was stabilized. The salt subsystem was highly disordered at high filler concentrations so that structurally distinct states were difficult to detect at the macro level.

The human body is mechanically unstable during walking. Maintaining upright stability requires constant regulation of muscle force by the central nervous system to push against the ground and move the body mass in the desired way. Activation of muscles in the lower body in response to sensory or mechanical perturbations during walking is usually highly phase-dependent, because the effect any specific muscle force has on the body movement depends upon the body configuration. Yet the resulting movement patterns of the upper body after the same perturbations are largely phase-independent. This is puzzling, because any change of upper-body movement must be generated by parts of the lower body pushing against the ground. How do phase-dependent muscle activation patterns along the lower body generate phase-independent movement patterns of the upper body? We hypothesize that when a sensory system detects a deviation of the body in space from a desired state that indicates the onset of a fall, the nervous system generates a functional response by pushing against the ground in any way possible with the current body configuration. This predicts that the changes in the ground reaction force patterns following a balance perturbation should be phase-independent. Here we test this hypothesis by disturbing upright balance in the frontal plane using Galvanic vestibular stimulation at three different points in the gait cycle. We measure the resulting changes in whole-body center of mass movement and the location of the center of pressure of the ground reaction force. We find that the magnitude of the initial center of pressure shift in the direction of the perceived fall is larger for perturbations late in the gait cycle, while there is no statistically significant difference in onset time. These results contradict our hypothesis by showing that even the initial CoP shift in response to a balance perturbation depends upon the phase of the gait cycle. Contrary to expectation, we also find that the whole-body balance response is not phase-independent. Both the onset time and the magnitude of the whole-body center of mass shift depend on the phase of the perturbation. We conclude that the central nervous system recruits any available mechanism to generate a functional balance response by pushing against the ground as fast as possible in response to a perturbation, but that the different mechanisms available at different phases in the gait cycle are not equally strong, leading to phase-dependent differences in the overall response.

In the most conventional pulsed nuclear quadrupole resonance (NQR) systems, radio frequency (RF) pulses for irradiation are produced by chopping a continuous RF wave with a DC pulse. Therefore, RF pulses have phase coherency but NQR signals after each RF pulse have different initial phase. In this case, a phase-sensitive detection (PSD) using the same continuous RF wave is necessary to accumulate NQR signal. Meanwhile, recent improvement of device technology enables a phase controlled RF pulse and a direct sampling of NQR signal in the laboratory frame. We developed a new pulse NQR system using phase controlled RF pulse, in which RF pulses with the same initial phase are produced and NQR signal after each pulse is directly acquired in the laboratory frame, and then the accumulated signal is processed by digital quadrature detection and fast Fourier transform. This paper provides and compares the feature of NQR signals from both types of pulse NQR system. Main findings are that NQR signal obtained by a train of phase coherent pulses and PSD shows the periodical variation of the intensity depending on the offset frequency of the irradiation RF pulse from the resonance frequency but the signal obtained by the phase controlled RF pulse train does not. This feature would be important for the application of NQR to detection system for such as explosives.

Geostationary-satellite anomalies are mainly caused by extreme space environment such as flux enhancements of relativistic electrons. A prediction of the relativistic electrons at geostationary orbit (GEO) is crucial to alert of the flux enhancements before it reaches a harmful condition. This work investigates contribution of solar activities at various phases in the daily electron flux (E > 2 MeV) at GEO measured by GOES satellites during the solar cycles 23-24. An alternative method based on the solar activity and feed forward neural network (NN) scheme with back-propagation learning in Waikato Environment for Knowledge Analysis is adopted to predict the electron fluxes in GEO by sorting the solar activity into maximum, descending, and ascending phases. As the model inputs, the historical daily sum K p and the > 2 MeV electron fluxes are used. Results indicate that the prediction capability of the NN model is dependent on the solar activity in relation to the sum K p. Crossing the test over the similar solar phases gives a better prediction result than crossing over different phases. The NN model based on the lagged sum K p and log-flux input are more suitable for the forecasting during the descending and ascending phases, while the lagged log-flux input is more suitable during the maximum phase.

Neural oscillations appear important for perception and attention processes because stimulus detection is dependent upon the phase of 7–11 Hz oscillations before stimulus onset. Previous work has examined stimulus detection at attended locations, but it is unknown whether unattended locations are also subject to phasic modulation by ongoing oscillatory activity, as would be predicted by theories proposing a role for neural oscillations in organizing general neural processing. Here, we recorded brain activity with EEG while human participants of both sexes detected brief visual targets preceded by a spatial cue and determined whether performance for cued (attended) and uncued (unattended) targets was influenced by oscillatory phase across a range of frequencies. Detection of both attended and unattended targets depended upon an ∼5 Hz theta rhythm and an ∼11–15 Hz alpha rhythm. Critically, detection of unattended stimuli was more strongly modulated by the phase of theta oscillations than was detection of attended stimuli, suggesting that attentional allocation involves a disengagement from ongoing theta sampling. There was no attention-related difference in the strength of alpha phase dependence, consistent with a perceptual rather than attentional role of oscillatory phase in this frequency range. These results demonstrate the importance of neural oscillations in modulating visual processing at both attended and unattended locations and clarify one way in which attention may produce its effects: through disengagement from low-frequency sampling at attended locations.SIGNIFICANCE STATEMENTPast work on the interaction between oscillatory phase and neural processing has shown the involvement of posterior ∼7–11 Hz oscillations in visual processing. Most studies, however, have presented stimuli at attended locations, making it difficult to disentangle frequencies related to attention from those related to perception. Here, we compared the oscillatory frequencies involved in the detection of attended and unattended stimuli and found that ∼11–15 Hz oscillations were related to perception independently of attention, whereas ∼5 Hz oscillations were more prominent for the detection of unattended stimuli. This work demonstrates the importance of neural oscillations for mediating stimulus processing at both attended and unattended locations and clarifies the different oscillatory frequencies involved in attention and perception.

We study the entropy dynamics of a dephasing model, where a two-level system (TLS) is coupled with a squeezed thermal bath via non-demolition interaction. This model is exactly solvable, and the time dependent states of both the TLS and its bath can be obtained exactly. Based on these states, we calculate the entropy dynamics of both the TLS and the bath, and find that the dephasing rate of the system relies on the squeezing phase of the bath. In zero temperature and high temperature limits, we prove that both the system and bath entropy increases monotonically. Moreover, we find that the dephasing rate of the system relies on the squeezing phase of the bath, and this phase dependence cannot be precisely derived from the Born-Markovian approximation which is widely adopted in open quantum systems., Comment: 9.3 pages, 2 figures

Using the classical ensemble method, we have investigated double ionization (DI) of diatomic molecules driven by elliptically polarized few-cycle laser pulses. The results show that DI channel depends strongly on internuclear distances ( R ), which is dominated by nonsequential double ionization (NSDI) for small and large R , while sequential double ionization (SDI) for mediate R . By tracing NSDI trajectories, we find that NSDI mainly originates from recollision process for small R and collision process for large R . Moreover, the correlated momentum distributions along the long axis strongly depend on the carrier-envelope-phase (CEP), and this phase dependence is affected by R .

With the help of the quantum theory of radiation, it is shown that the stimulated radiation from an atom has the same direction and phase dependence as the incident stimulating radiation.

We investigate femtosecond double-pump single-molecule signals in the strong-field regime, which is characterized by nonlinear scaling of the signal with the intensity of the pump pulses. The signals can be decomposed into population and coherence contributions. In contrast to the weak-field regime (in which only the coherence contribution is important) both contributions are relevant in the strong-field regime and reveal the vibrational dynamics of the chromophore. Other than in the weak-field regime, the detection of vibrational beatings is not limited by the electronic dephasing time of the chromophore. Moreover, the signals in the strong-field regime are more robust with respect to the environment-induced modulation of the chromophore parameters. It is shown that excited-state absorption in chromophores with three electronic states is reflected in the phase dependence of single-molecule signals. The simulations reveal that the information content of femtosecond double-pump single-molecule signals is enhanced in the strong-coupling regime.

With exactly forty years since the discovery of the first cyclotron line in Her X-1, there have been remarkable advancements in the field related to study of the physics of accreting neutron stars -- cyclotron lines have been a major torchbearer in this regard, from being the only direct estimator of the magnetic field strength, a tracer of accretion geometry and an indicator of the emission beam in these systems. The main flurry of activities have centred around studying the harmonic separations, luminosity dependence, pulse phase dependence and more recently the shapes of the line and the trend for long term evolution in the line energy. This article visits the important results related to cyclotron lines since its discovery and reviews their significance. An emphasis is laid on pulse phase resolved spectroscopy and the important clues a joint timing and spectral study in this context can provide, to build a complete picture for the physics of accretion and hence X-ray emission in accreting neutron stars., Has appeared in Journal of Astrophysics and Astronomy special issue on 'Physics of Neutron Stars and related objects', celebrating the 75th birth-year of G. Srinivasan

We report on the first multi-color precision light curve of the bright Wolf-Rayet binary $\gamma^2$ Velorum, obtained over six months with the nanosatellites in the BRITE- Constellation fleet. In parallel, we obtained 488 high-resolution optical spectra of the system. In this first report on the datasets, we revise the spectroscopic orbit and report on the bulk properties of the colliding winds. We find a dependence of both the light curve and excess emission properties that scales with the inverse of the binary separation. When analyzing the spectroscopic properties in combination with the photometry, we find that the phase dependence is caused only by excess emission in the lines, and not from a changing continuum. We also detect a narrow, high-velocity absorption component from the He I $\lambda$5876 transition, which appears twice in the orbit. We calculate smoothed-particle hydrodynamical simulations of the colliding winds and can accurately associate the absorption from He I to the leading and trailing arms of the wind shock cone passing tangentially through our line of sight. The simulations also explain the general strength and kinematics of the emission excess observed in wind lines such as C III $\lambda$5696 of the system. These results represent the first in a series of investigations into the winds and properties of $\gamma^2$ Velorum through multi-technique and multi-wavelength observational campaigns., Comment: 16 pages, 14 figures, additional measurements to be included in online dataset. Accepted for publication in MNRAS

For the search for charginos and neutralinos in the Minimal Supersymmetric Standard Model (MSSM) as well as for future precision analyses of these particles an accurate knowledge of their production and decay properties is mandatory. We evaluate the cross sections for the chargino and neutralino production at ee colliders in the MSSM with complex parameters (cMSSM). The evaluation is based on a full one-loop calculation of the production mechanisms e+e-cc′∓ and e+e-n0χ~n′0 including soft and hard photon radiation. We mostly restricted ourselves to a version of our renormalization scheme which is valid for | M| < | M| , | μ| and M≠ μ to simplify the analysis, even though we are able to switch to other parameter regions and correspondingly different renormalization schemes. The dependence of the chargino/neutralino cross sections on the relevant cMSSM parameters is analyzed numerically. We find sizable contributions to many production cross sections. They amount to roughly ± 15 % of the tree-level results but can go up to ± 40 % or higher in extreme cases. Also the complex phase dependence of the one-loop corrections was found non-negligible. The full one-loop contributions are thus crucial for physics analyses at a future linear ee collider such as the ILC or CLIC., Funded by SCOAP3. The work of S.H. is supported in part by CICYT (Grant FPA 2013-40715-P), in part by the MEINCOP Spain under contract FPA2016-78022-P, in part by the “Spanish Agencia Estatal de Investigación” (AEI) and the EU “Fondo Europeo de Desarrollo Regional” (FEDER) through the project FPA2016-78645-P, and by the Spanish MICINN’s Consolider-Ingenio 2010 Program under Grant MultiDark CSD2009-00064.

Results of polarimetric observations of a high-albedo asteroid 64 Angelina obtained in the phase-angle range from 0.8° to 24.3° are presented. The observations were carried out in the period from September 28 to October 9, 2008, and on November 15, 2011, and September 18, 2012, with the 1.25-m and 2.6-m telescopes of the Crimean Astrophysical Observatory equipped with a five-color double-beam photopolarimeter and a single-channel photometer-polarimeter, respectively. Our observations confirm the polarimetric opposition effect in asteroid 64 Angelina at small phase angles and well agree with the other observations. The obtained results are discussed in terms of the currently available models of the light scattering by regolith surfaces.

The resonance fluorescence of a four-level atom in J = 1/2 to J = 1/2 transition driven by two coherent fields is studied. We find that the incoherent fluorescence spectrum shows a direct indication of vacuum-induced coherence in the atomic system. We show that such coherence manifests itself via an enhancement or suppression of the spectral peaks in the $\pi$-polarized fluorescence. The effect of the relative phase of the driving fields on the spectral features is also investigated. We show that phase-dependent enhancement or suppression of the fluorescence peaks appears in the incoherent spectrum emitted along the $\sigma$ transitions. It is found that this phase dependence occurs because of the polarization-detection scheme employed for the observation of the fluorescence light. We present an analytical explanation, based on dressed-states of the atom-field system, to interpret the numerical results., Comment: 10 pages, 7 figures