Average Time Scale Research Articles

Enhancing the frequency stability of national time scale using EMD

Almost all time keeping laboratories maintain the 5071A Cesium (Cs) frequency standard for generating their national time scales. The frequency stability of this standard is limited by different types of noise, especially the White Frequency Modulation noise. These noise types affect the stability of the resultant average Time Scale (TS). Kalman Filter (KF) is still applied until now within the time scale algorithm for de-noising and predicting improved resultant TS frequency stability. But, this method is very complicated and is based on difficult estimation techniques. In 2013, the Empirical Mode Decomposition (EMD) technique was applied for the first time on Cs atomic clock signal de-noising and frequency prediction. In this paper, the EMD technique is embedded in the TS generation algorithm for studying its effect on the stability of the resultant TS. Results show that the frequency stability of the resultant TS is improved for averaging times up to nearly 40 days due to EMD. These results are verified by comparing the effect of the EMD to that of the KF on the resultant TS frequency stability. Results show that the frequency stability of the resultant average TS is improved due to using EMD or KF for Cs clock signal de-noising for averaging times up to nearly 40 days. In addition, EMD is found to be more effective and simpler than the complicated KF.

Ultrafast Intersystem Crossing in Isolated Ag29(BDT)123- Probed by Time-Resolved Pump-Probe Photoelectron Spectroscopy.

The photophysics of the isolated trianion Ag29(BDT)123- (BDT = benzenedithiolate), a ligand-protected cluster comprising BDT-based ligands, terminating a shell of silver thiolates and a core of silver atoms, was studied in the gas phase by femtosecond time-resolved, pump-probe photoelectron spectroscopy. UV excitation at 490 nm populates one or more singlet excited states with significant charge transfer (CT) character in which electron density is shifted from shell to core. These CT states relax on an average time scale of several hundred femtoseconds by charge recombination to yield either the vibrationally excited singlet ground state (internal conversion) or a long-lived triplet (intersystem crossing). Our study is the first ultrafast spectroscopic probe of a ligand-protected coinage metal cluster in isolation. In the future, it will be interesting to study how cluster size, overall charge state, or heteroatom doping can be used to tune the corresponding relaxation dynamics in the absence of solvent.

Effect of Downscaling the Frequency Offset of Clock Ensemble on the Stability of the Time Scale

Improving the stability of national time scale (TS) is a main target for all time keepers. The stability of TS can be improved by either increasing the number of atomic clocks in the ensemble or by adding clocks of higher stability. In both cases, a large financial budget is needed which may be unaffordable by some developing economics. In this paper, we propose a new method for improving the stability of national TS by a factor approaching 50% with zero expenses by replacing the real clocks in the ensemble by virtual clocks (VCs). The VCs are obtained by downscaling (DS) the frequency offset of the real clocks. The DS process improves the frequency stability of the clocks and hence that of the resultant average TS. Since the downscaled clocks are correlated to the original ones, they replace the real clocks and not added to the ensemble. The performance of the proposed method is verified by applying it on the daily clocks comparison data of the Observatory of Paris (OP) published on the time department server of the Bureau International des Poids et Measures. The performance of the proposed method is compared with that of Kalman filter (KF) method presented in the literature. It is found that the proposed method is simpler and acting as a wider bandwidth de-noising filter than KF method for achieving the same performance.

Joint Timekeeping of Navigation Satellite Constellation with Inter-Satellite Links.

As a system of ranging and positioning based on time transfer, the timekeeping ability of a navigation satellite constellation is a key factor for accurate positioning and timing services. As the timekeeping performances depend on the frequency stability and predictability of satellite clocks, we propose a method to establish a more stable and predictable space time reference, i.e., inter-satellite link time (ISLT), uniting the satellite clocks through inter-satellite links (ISLs). The joint timekeeping framework is introduced first. Based on the weighted average timescale algorithm, the optimal weights that minimize the increment of the ISLT timescale are determined and allocated to the clock ensemble to improve the frequency stability and predictability in both the long and short term. The time deviations with respect to the system time of nine BeiDou-3 satellites through multi-satellite precise orbit determination (MPOD) are used for joint timekeeping evaluation. According to the Allan deviation, the frequency of the ISLT is more stable than the nine satellite clocks in the short term (averaging time smaller than 7000 s), and its daily stability can reach 6 × 10−15. Meanwhile, the short-term (two hours) and long-term (10 h) prediction accuracy of the ISLT is 0.18 and 1.05 ns, respectively, also better than each satellite clock. Furthermore, the joint timekeeping is verified to be robust against single-satellite malfunction.

Rotation and spots in normal A and Am/Fm stars

ABSTRACT Frequency analysis of long-term ultraprecise photometry can lead to precise values of rotation frequencies of rotating stars with ‘hump and spike’ features in their periodograms. Using these features, we computed the rotation frequencies and amplitudes. The corresponding equatorial rotational velocity ($V_{\rm{rot}}$) and spot size were estimated. On fitting the autocorrelation functions of the light curves with the appropriate model, we determined the star-spot decay time-scale. The $\rm \mathit{ V}_{rot}$ agrees well with the projected rotational velocity ($\rm \nu ~sin$ i) in the literature. Considering a single circular and black spot, we estimate its radius from the amplitude of the ‘spike’. No evidence for a significant difference in the average ‘spike’ amplitude and spot radius was found for Am/Fm and normal A stars. Indeed, we derived an average value of $\rm \sim\! 21\pm 2$ and $\rm \sim\! 19\pm 2$ ppm for the photometric amplitude and of $\rm 1.01\, \pm \, 0.13$ and $\rm 1.16\, \pm \, 0.12$ $R_\mathrm{ E}$ for the spot radius (where $\rm \mathit{ R}_E$ is the Earth radius), respectively. We do find a significant difference for the average spot decay time-scale, which amounts to 3.6 ± 0.2 and 1.5 ± 0.2 d for Am/Fm and normal A stars, respectively. In general, spots on normal A stars are similar in size to those on Am/Fm stars, and both are weaker than previously estimated. The existence of the ‘spikes’ in the frequency spectra may not be strongly dependent on the appearance of star-spots on the stellar surface. In comparison with G, K, and M stars, spots in normal A and Am/Fm stars are weak, which may indicate the presence of a weak magnetic field.

Intergyre Salt Transport in the Climate Warming Response

ABSTRACTRegional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input.

Ultrafast core-excited electron dynamics in model crystalline organic semiconductors.

Electron transfer is key to the operation of devices based on molecular (organic) semiconductors. Others have shown that electron transfer in the solid state often proceeds on sub-50 fs timescales, the details of which can be difficult to temporally resolve using pump-probe spectroscopy. A popular technique to measure average time scales for such rapid electron-transfer events is the core-hole clock implementation of resonant Auger electron spectroscopy at a single X-ray absorption edge. This is often done on relatively small molecules with core-excited states that are highly localized. We have used resonant Auger spectroscopy to probe sub-50 fs electron dynamics of two relatively large model organic semiconductors: Cu phthalocyanine (CuPc) along with its fluorinated analog, F16CuPc. We have interrogated electron dynamics simultaneously at N and C K-edges, along with calculations of initial and final states participating in the core-excited states. Our measurements show that the electron dynamics differ substantially across the two absorption edges for a given molecule, and that there are significant differences at a given edge between the two derivatives. X-ray spectroscopy calculations suggest that the extension of π-electron density onto peripheral F atoms in F16CuPc is implicated in the large change in ultrafast electron dynamics upon fluorination. We believe our results have important implications for analysis of core-hole clock measurements on relatively large organic semiconductors.

Attosecond delays in photoionization of diatomic and polyatomic molecules

SynopsisWe have investigated the photoionization dynamics of several molecular systems using attosecond in-terferometric techniques based on ultrashort extreme ultraviolet (XUV) pulses. For diatomic molecules such as N2, we can get a detailed picture of the photoionization process by measuring the attosecond delays. For polyatomic molecules such as naphthalene, such measurements can provide the average timescale over which photoionization processes at different electron energies can occur in complex quantum systems.

Self-similar characteristics of drought duration, total deficit, and intensity curves

Droughts are among the creeping extreme events with meteorological, hydrological, agricultural, and even social impacts. Due to their complex occurrences, drought identification and prediction need simple, effective, conceptual, logical, and rational innovative approaches that can help for practical applications. A new approach is being suggested at a set of threshold (or demand) levels to identify drought features such as duration, total deficit, and intensity relations, which depend on the run analysis and standardized precipitation index (SPI) methodology. This concept is applicable to identify mathematical relationships between the threshold values and the drought features. Exponential and power laws appear as straight lines on semi-logarithmic and double-logarithmic papers, respectively. In addition, power law implies self-similarity property with fractal geometrical patterns. As the threshold level increases, so does each one of the drought descriptors. Graphical representations help to identify climate change implications as well as practical design quantity determination. The applications of the suggested methodology are presented for monthly meteorological time series records. This methodology is also considered for the SPI over 6-month and 12-month moving average time scales at seven meteorology stations from different geographical parts of Turkey. The empirical mathematical expressions are obtained between the threshold values and the drought durations, total deficits, and drought intensities, which provide future drought characteristic predictions.

The Potential Role of Flexibility During Peak Hours on Greenhouse Gas Emissions: A Life Cycle Assessment of Five Targeted National Electricity Grid Mixes

On the path towards the decarbonization of the electricity supply, flexibility and demand response have become key factors to enhance the integration of distributed energy resources, shifting the consumption from peak hours to off-peak hours, optimizing the grid usage and maximizing the share of renewables. Despite the technical viability of flexible services, the reduction of greenhouse gas emissions has not been proven. Traditionally, emissions are calculated on a yearly average timescale, not providing any information about peak hours’ environmental impact. Furthermore, peak-hours’ environmental impacts are not always greater than on the base load, depending on the resources used for those time periods. This paper formulates a general methodology to assess the potential environmental impact of peak-hourly generation profiles, through attributional life cycle assessment. This methodology was applied to five different countries under the INVADE H2020 Project. Evaluation results demonstrate that countries like Spain and Bulgaria could benefit from implementing demand response activities considering environmental aspects, enhancing potential greenhouse gas reductions by up to 21% in peak hours.

Impact of atmospheric stability conditions on heat fluxes from Alexandria Eastern Harbor, Egypt

The fluxes of energy through the water surface influence the water balance and regulate heat storage. This study analyzed the impact of stability condition of the atmosphere on the fluxes of heat on daily average time scale at Alexandria Eastern Harbor (AEH), Egypt. The method used here is depending on the bulk aerodynamic transfer method using standard meteorological parameters. The stability conditions of the atmosphere above the surface water can influence the convective and evaporative heat fluxes from the surface water. Unstable conditions of the atmosphere occurred 96.09% of the time during the study period (from 6/6/2010 to 16/2/2011) and resulted in 24.55% increments in the calculated average evaporation, when compared to the neutral condition. The results showed that an unsteady condition of the atmosphere has a significant influence in increasing evaporation in addition to sensible heat flux of AEH surface water.

Two cross-validation techniques to comprehensively characterize global horizontal irradiation regression models: Single data-splitting is insufficient

Data-splitting is the most widely used method to cross-validate global horizontal irradiation regression models. An available dataset is split into two subsets, one to calibrate models and the other to validate them. This study investigated the sufficiency of this method within the ambit of two other cross-validation techniques—Monte Carlo cross-validation nested with double cross-validation and leave-one-year-out cross-validation. These techniques facilitated cross-validation in long and short term periods, respectively. They were applied to the De Souza and Hargreaves-Samani temperature-based regression models. Unlike data-splitting, the techniques promoted full characterization of the models by the averages and sensitivities (%) of their tuned parameters, the averages and spread of their predictive accuracies via root mean square errors, and their stability (Monte Carlo-determined). On a monthly average daily time scale, their fully characterized (less their average tuned parameters) Monte Carlo results were <6%, 0.56 ± 0.12 and 0.032 MJ m−2 day−1 for the De Souza model, and <1.5%, 0.94 ± 0.14 and 0.174 MJ m−2 day−1 for the Hargreaves-Samani model. Similarly, the leave-one-year-out results were <2% and 0.88 ± 0.28 MJ m−2 day−1 for the De Souza model and <1% and 1.31 ± 0.24 MJ m−2 day−1 for the Hargreaves-Samani model. The De Souza model performed better. We further demonstrated the erroneous assessments possible with models subjected to traditional data-splitting which proved inadequate. Consequently, we proposed an algorithm to implement our cross-validation techniques that reduces computational burden for multiple model evaluation. This was achieved by including a novel controlled data-splitting cross-validation subroutine.

Bivariate Luminosity Function of Galaxy Pairs

Abstract We measure the bivariate luminosity function (BLF) of galaxy pairs and use it to probe and characterize the galaxy–galaxy interaction between pair members. The galaxy pair sample is selected from the main galaxy sample of Sloan Digital Sky Survey and supplied with a significant number of redshifts from the LAMOST spectral and GAMA surveys. We find the BLFs depend on the projected distance d p between pair members. At large separation d p > 150 h −1 kpc, the BLF degenerates into a luminosity function of single galaxies, indicating few interactions between pair members. At 100 h −1 kpc ≤ d p ≤ 150 h −1 kpc, the BLF starts to show a correlation between pair members, in the sense that the shape of the conditional luminosity function (CLF) of one member galaxy starts to depend on the luminosity of the other member galaxy. Specifically, the CLF with a brighter companion has a steeper faint-end slope, which becomes even more significant at 50 h −1 kpc ≤ d p ≤ 100 h −1 kpc. This behavior is consistent with the scenario—and also is the observational evidence—that dynamic friction drives massive major merger pairs to merge more quickly. At close distance d p ≤ 50 h −1 kpc, aside from the merging timescale effect, the BLF also shows an overall brightening of ΔM r ≥ 0.04 mag, which reveals the enhanced star formation of the close-pair phase. By combining this with the statistical conclusion that the star formation rate of late-type galaxies in close pairs is enhanced at a level of about 40%, we further conclude that the average starburst timescale of close pairs is as long as 0.4 Gyr.

OGLE-2017-BLG-1186: first application of asteroseismology and Gaussian processes to microlensing

AbstractWe present the analysis of the event OGLE-2017-BLG-1186 from the 2017 Spitzer microlensing campaign. This is a remarkable microlensing event because its source is photometrically bright and variable, which makes it possible to perform an asteroseismic analysis using ground-based data. We find that the source star is an oscillating red giant with average time-scale of ∼9 d. The asteroseismic analysis also provides us source properties including the source angular size (∼27 $\mu$as) and distance (∼11.5 kpc), which are essential for inferring the properties of the lens. When fitting the light curve, we test the feasibility of Gaussian processes (GPs) in handling the correlated noise caused by the variable source. We find that the parameters from the GP model are generally more loosely constrained than those from the traditional χ2 minimization method. We note that this event is the first microlensing system for which asteroseismology and GPs have been used to account for the variable source. With both finite-source effect and microlens parallax measured, we find that the lens is likely a ∼0.045 M⊙ brown dwarf at distance ∼9.0 kpc, or a ∼0.073 M⊙ ultracool dwarf at distance ∼9.8 kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a $\sim 35{{\ \rm per\ cent}}$ probability for the lens to be a bulge object and $\sim 65{{\ \rm per\ cent}}$ to be a background disc object.

ПОСТРОЕНИЕ ТЕМПЕРАТУРНЫХ КОРРЕЛЯЦИОННЫХ ЗАВИСИМОСТЕЙ МЕЖДУ ГОРИЗОНТАМИ ПОЧВ ПРИ ПРОВЕДЕНИИ ИССЛЕДОВАНИЙ С ИСПОЛЬЗОВАНИЕМ МАЛОГЛУБИННЫХ СКВАЖИН

Актуальность исследования обусловлена необходимостью изучения геологических объектов путем прямых геофизических измерений их параметров и применения полученных данных и результатов математической обработки для решения задач рационального природопользования. Эффективное решение этих задач особенно важно при исследовании влияния метеорологических параметров на водно-тепловой режим горных пород в районах с глубоким сезонным промерзанием. Цель: разработать и провести испытания геофизического комплекса для определения пространственного распределения температуры в грунте, динамики изменения этого распределения во времени и корреляционные соотношения его промерзания на различных горизонтах в условиях Сибири и Крайнего Севера. Объекты: федеральная автомобильная дорога «Сургут–Салехард» в пределах Ямало-Ненецкого автономного округа и дорога территориального значения «Коротчаево–Уренгой». Методы: методы анализа статистических данных для изучения взаимосвязей между значениями переменных, включая исследование статистической взаимосвязи двух или более случайных величин (корреляционный анализ), методы функционального анализа для исследования статистических данных в частотно-временном диапазоне (вейвлет-преобразование). Результаты. Показано, что с увеличением глубины грунта, график значений температур промерзания принимает более плоский характер и с увеличением удаленности между горизонтами наблюдений уменьшается температурная корреляционная связь между ними. В частности, в слоях грунта 1,9–2,1 м значения коэффициентов температурной корреляции практически не изменяются на всех временных масштабах выборки. В слоях грунта 0,9–1,2 м, 1,2–1,5 м, 1,5–1,8 м на средних временных масштабах выборки наблюдается незначительное изменение коэффициентов температурной корреляции, а максимальное отклонение коэффициентов корреляции наблюдается на больших временных масштабах выборки. В слоях грунта 0,3–2,1 м, 0,3–1,8 м, 0,6–2,1 м на средних временных масштабах выборки наблюдается значительное отклонение коэффициентов температурной корреляции, а максимальное отклонение коэффициентов температурной корреляции также наблюдается на больших масштабах выборки. На базе полученных температурных рядов были построены спектрограммы вейвлет-преобразований для классических базисных вейвлетов (вейвлет Морле, DOG-вейвлет, вейвлет Пауля), косвенно демонстрирующие волнообразное «продвижение» фронта (фазы) холода в грунте.

A new delay time distribution for merging neutron stars tested against Galactic and cosmic data

The merging of two neutron stars (MNS) is thought to be the source of short gamma-ray bursts (SGRB) and gravitational wave transients, as well as the main production site of r-process elements like Eu. We have derived a new delay time distribution (DTD) for MNS systems from theoretical considerations and we have tested it against (i) the SGRB redshift distribution and (ii) the Galactic evolution of Eu and Fe, in particular the [Eu/Fe] vs [Fe/H] relation. For comparison, we also tested other DTDs, as proposed in the literature. To address the first item, we have convolved the DTD with the cosmic star formation rate, while for the second we have employed a detailed chemical evolution model of the Milky Way. We have also varied the DTD of Type Ia SNe (the main Fe producers), the contribution to Eu production from core-collapse SNe, as well as explored the effect of a dependence on the metallicity of the occurrence probability of MNS. Our main results can be summarized as follows: (i) the SGRB redshift distribution can be fitted using DTDs for MNS that produce average timescales of 300-500 Myr; (ii) if the MNS are the sole producers of the Galactic Eu and the occurrence probability of MNS is constant the Eu production timescale must be on the order of <30 Myr; (iii) allowing for the Eu production in core-collapse SNe, or adopting a metallicity-dependent occurrence probability, allow us to reproduce both observational constraints, but many uncertainties are still present in both assumptions.

Striations in Marginal Seas and the Mediterranean Sea

AbstractStriations, which are jet‐like structures comprising alternating ridges and furrows within large‐scale circulations, are widely studied in the open ocean, although it is unclear whether they exist in marginal or semiclosed seas. Based on an appropriate choice of averaging time scale and cutoff wavelength, our analysis reveals striations in marginal seas, including the South China Sea (SCS), Sea of Japan, and Gulf of Mexico, and in the Mediterranean Sea (MS). Compared with the open ocean, these striations have a narrower bandwidth (about 60 km in marginal seas and 100 km in the MS), fewer zonal orientations, stronger baroclinic structure, and deeper intensification. Striations in the open ocean are usually barotropic; however, within the South China Sea and MS, they are baroclinic in the upper 1,000 m and intensified at depths greater than 3,000 m.

Enhancement of secondary organic aerosol formation and its oxidation state by SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; during photooxidation of 2-methoxyphenol

Abstract. 2-Methoxyphenol (guaiacol) is derived from the lignin pyrolysis and taken as a potential tracer for wood smoke emissions. In this work, the effect of SO2 at atmospheric levels (0–56 ppbv) on secondary organic aerosol (SOA) formation and its oxidation state during guaiacol photooxidation was investigated in the presence of various inorganic seed particles (i.e., NaCl and (NH4)2SO4). Without SO2 and seed particles, SOA yields ranged from (9.46±1.71) % to (26.37±2.83) % and could be well expressed by a one-product model. According to the ratio of the average gas-particle partitioning timescale (τ‾g-p) over the course of the experiment to the vapor wall deposition timescale (τg−w), the determined SOA yields were underestimated by a factor of ∼2. The presence of SO2 resulted in enhancing SOA yield by 14.04 %–23.65 %. With (NH4)2SO4 and NaCl seed particles, SOA yield was enhanced by 23.07 % and 29.57 %, respectively, which further increased significantly to 29.78 %–53.43 % in the presence of SO2, suggesting that SO2 and seed particles have a synergetic contribution to SOA formation. The decreasing trend of the τ‾g-p/τg-w ratio in the presence of seed particles and SO2 suggested that more SOA-forming vapors partitioned into the particle phase, consequently increasing SOA yields. It should be noted that SO2 was found to be in favor of increasing the carbon oxidation state (OSC) of SOA, indicating that the functionalization or the partitioning of highly oxidized products into particles should be more dominant than the oligomerization. In addition, the average N∕C ratio of SOA was 0.037, which revealed that NOx participated in the photooxidation process, consequently leading to the formation of organic N-containing compounds. The experimental results demonstrate the importance of SO2 on the formation processes of SOA and organic S-containing compounds and are also helpful to further understand SOA formation from the atmospheric photooxidation of guaiacol and its subsequent impacts on air quality and climate.

The intraday variations of the polarization vector direction in radio source S5 0716+714

The bright radio source S5 0716+714, that is usually classified as BL Lac object, is one of the most intensively studied. S5 0716+714 demonstrates extremely peculiar properties such as the shortest time-scale of optical and polarimetric variations observed in blazars. In given paper, the results of 9-hour polarimetric monitoring of S5 0716+714 with the $\sim$70-second resolution, carried out by the 6-m telescope BTA of SAO RAS, are presented. The observation data analysis reveals the variability both in total and polarized light on the 1-1.5-hour time-scale. Since polarization is generated by plasma motion in the magnetic field, the variations of polarization vector are bounded with the magnetic field configuration and the average time-scales specify the size of the emitting region, unresolved directly. We suggest a processed numerically model of polarization in the jet with the precessing helical magnetic field. Fitting the model discovers that observed short-term variations with the complicated trajectory of polarization vector could be induced by the magnetic field precession with the period $T \approx 15$ days.

The MOSDEF Survey: Stellar Continuum Spectra and Star Formation Histories of Active, Transitional, and Quiescent Galaxies at 1.4 &lt; z &lt; 2.6

Abstract Using the MOSFIRE Deep Evolution Field (MOSDEF) rest-frame optical spectroscopic survey, we investigate the star formation histories (SFHs) of different galaxy types, ranging from actively star-forming to quiescent at 1.4 ≤ z ≤ 2.6. SFHs are constrained utilizing stellar continuum spectroscopy, specifically through a combination of Balmer absorption lines, the 4000 Å break, and the equivalent width of the Hα emission line. To attain a sufficiently high signal-to-noise ratio (S/N) to conduct these measurements we stack spectra of galaxies with similar spectral types, as determined from their rest-frame U − V and V − J colors. We bin the MOSDEF sample into five spectral types, subdividing the quiescent and star-forming bins to better explore galaxies transitioning between the two. We constrain the average SFHs for each type, finding that quiescent and transitional galaxies in the MOSDEF sample are dominated by an SFH with an average star formation timescale of τ ∼ 0.1–0.2 Gyr. These findings contrast with measurements from the low-redshift Universe where, on average, galaxies form their stars over a more extended time period (τ &gt; 1 Gyr). Furthermore, our spectral index measurements correlate with mass surface density for all spectral types. Finally, we compare the average properties of the galaxies in our transitional bins to investigate possible paths to quiescence, and speculate on the viability of a dusty post-starburst phase.