Loss Timescales Research Articles

Mass-Loss Timescale of Star Cluster in External Tidal Field

AbstractWe investigate evolution of star clusters in steady external tidal field by means of N-body simulations. We followed several sets of cluster models whose strength and Coriolis's contribution of the external tidal field are different. We found that the mass loss timescale due to the escape of stars, t_mloss, and its dependence on the two-body relaxation timescale, trh,i, are determined by the strength of the tidal field. The logarithmic slope [≡ dln(tmloss)/dln(trh,i)] approaches unity for the cluster models in weaker tidal fields. We also found that stronger Coriolis force against others, produced by parent galaxy whose density profile is shallower, makes the mass loss timescale longer. This is due to the fact that a fraction of stars whose orbit are nearly regular increases as the Coriolis force becomes stronger.

Slot region electron loss timescales due to plasmaspheric hiss and lightning‐generated whistlers

Energetic electrons (E > 100 keV) in the Earth's radiation belts undergo Doppler‐shifted cyclotron resonant interactions with a variety of whistler mode waves leading to pitch angle scattering and subsequent loss to the atmosphere. In this study we assess the relative importance of plasmaspheric hiss and lightning‐generated whistlers in the slot region and beyond. Electron loss timescales are determined using the Pitch Angle and energy Diffusion of Ions and Electrons (PADIE) code with global models of the spectral distributions of the wave power based on CRRES observations. Our results show that plasmaspheric hiss propagating at small and intermediate wave normal angles is a significant scattering agent in the slot region and beyond. In contrast, plasmaspheric hiss propagating at large wave normal angles and lightning‐generated whistlers do not contribute significantly to radiation belt loss. The loss timescale of 2 MeV electrons due to plasmaspheric hiss propagating at small and intermediate wave normal angles in the center of the slot region (L = 2.5) lies in the range 1–10 days, consistent with recent Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) observations. Wave turbulence in space, which is responsible for the generation plasmaspheric hiss, thus leads to the formation of the slot region. During active periods, losses due to plasmaspheric hiss may occur on a timescale of 1 day or less for a wide range of energies, 200 keV < E < 1 MeV, in the region 3.5 < L < 4.0. Plasmaspheric hiss may thus also be a significant loss process in the inner region of the outer radiation belt during magnetically disturbed periods.

Parameterization of radiation belt electron loss timescales due to interactions with chorus waves

Wave‐particle interactions lead to the loss of relativistic electrons from the outer radiation belt on timescales ranging from hours to weeks. For a fixed value of chorus wave amplitudes pitch‐angle diffusion coefficients are computed for a range of energies and L, and are related to the loss rates of radiation belt electrons. By analyzing the dependence of the loss rates on L‐value and energy we find functional dependencies for the lifetime of the radiation belt electrons. Parameters of the functional dependences are obtained using a linear regression technique. To create parameterizations of loss rate as a function of geomagnetic indices, we also analyzed the statistical data from day‐side lower band chorus observations in the range of geomagnetic latitudes from 20° to 30°. The combined parameterizations of the wave amplitudes and scattering rates indicate that electron loss due to chorus waves strongly depends on energy and geomagnetic activity. During storm‐time conditions the lifetimes of relativistic electrons, in the heart of the outer zone are on the order of a day and are on the scale of hours at lower energies. Pitch‐angle scattering by chorus waves thus plays an important role in radiation belt dynamics. The developed parameterizations may be used in particle tracing codes and radial diffusion codes. The limitations of the parameterization, effect of scattering by other waves, and local acceleration processes are also discussed.

Timescales for radiation belt electron acceleration and loss due to resonant wave‐particle interactions: 2. Evaluation for VLF chorus, ELF hiss, and electromagnetic ion cyclotron waves

Outer zone radiation belt electrons can undergo gyroresonant interaction with various magnetospheric wave modes including whistler‐mode chorus outside the plasmasphere and both whistler‐mode hiss and electromagnetic ion cyclotron (EMIC) waves inside the plasmasphere. To evaluate timescales for electron momentum diffusion and pitch angle diffusion, we utilize bounce‐averaged quasi‐linear diffusion coefficients for field‐aligned waves with a Gaussian frequency spectrum in a dipole magnetic field. Timescales for momentum diffusion of MeV electrons due to VLF chorus can be less than a day in the outer radiation belt. Equatorial chorus waves (∣λW∣ < 15 deg) can effectively accelerate MeV electrons. Efficiency of the chorus acceleration mechanism is increased if high‐latitude waves (∣λW∣ > 15 deg) are also present. Our calculations confirm that chorus diffusion is a viable mechanism for generating relativistic (MeV) electrons in the outer zone during the recovery phase of a storm or during periods of prolonged substorm activity when chorus amplitudes are enhanced. Radiation belt electrons are subject to precipitation loss to the atmosphere due to resonant pitch angle scattering by plasma waves. The electron precipitation loss timescale due to scattering by each of the wave modes, chorus, hiss, and EMIC waves, can be 1 day or less. These wave modes can separately, or in combination, contribute significantly to the depletion of relativistic (MeV) electrons from the outer zone over the course of a magnetic storm. Efficient pitch angle scattering by whistler‐mode chorus or hiss typically requires high latitude waves (∣λW∣ > 30 deg). Timescales for electron acceleration and loss generally depend on the spectral properties of the waves, as well as the background electron number density and magnetic field. Loss timescales due to EMIC wave scattering also depend on the ion (H+, He+, O+) composition of the plasma. Complete models of radiation belt electron transport, acceleration and loss should include, in addition to radial (cross‐L) diffusion, resonant diffusion due to gyroresonance with VLF chorus, plasmaspheric hiss, and EMIC waves. Comprehensive observational data on the spectral properties of these waves are required as a function of spatial location (L, MLT, MLAT) and magnetic activity.

Energetic outer zone electron loss timescales during low geomagnetic activity

Following enhanced magnetic activity the fluxes of energetic electrons in the Earth's outer radiation belt gradually decay to quiet‐time levels. We use CRRES observations to estimate the energetic electron loss timescales and to identify the principal loss mechanisms. Gradual loss of energetic electrons in the region 3.0 ≤ L ≤ 5.0 occurs during quiet periods (Kp < 3−) following enhanced magnetic activity on timescales ranging from 1.5 to 3.5 days for 214 keV electrons to 5.5 to 6.5 days for 1.09 MeV electrons. The intervals of decay are associated with large average values of the ratio fpe/fce (>7), indicating that the decay takes place in the plasmasphere. We compute loss timescales for pitch‐angle scattering by plasmaspheric hiss using the PADIE code with wave properties based on CRRES observations. The resulting timescales suggest that pitch angle scattering by plasmaspheric hiss propagating at small or intermediate wave normal angles is responsible for electron loss over a wide range of energies and L shells. The region where hiss dominates loss is energy‐dependent, ranging from 3.5 ≤ L ≤ 5.0 at 214 keV to 3.0 ≤ L ≤ 4.0 at 1.09 MeV. Plasmaspheric hiss at large wave normal angles does not contribute significantly to the loss rates. At E = 1.09 MeV the loss timescales are overestimated by a factor of ∼5 for 4.5 ≤ L ≤ 5.0. We suggest that resonant wave‐particle interactions with EMIC waves, which become important at MeV energies for larger L (L > ∼4.5), may play a significant role in this region.

The Outburst of HST‐1 in the M87 Jet

The X-ray intensity of knot HST-1, 085 from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 yr. The optical increase is similar, and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-Ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a modest beaming model as indicated in our earlier papers. Based on this model, the decay of the X-ray light curve appears to be dominated by the light-travel time across the emitting region of HST-1, rather than synchrotron loss timescales.

Timescale and magnitude of plasma thermal energy loss before and during disruptions in JET

In this paper we analyse and discuss the thermal energy loss dynamics before and during JET disruptions that occurred between 2002 and 2004 in discharges which reached >4.5 MJ of thermal energy. We observe the slow thermal energy transients with diamagnetic loops and the fast ones with electron cyclotron emission and soft x-ray diagnostics. For most disruption types in JET, the plasma thermal energy at the time of the thermal quench is substantially less than that of the full performance plasma, typically in the range of 10–50% depending on plasma conditions and disruption type. The exceptions to this observation are disruptions in plasmas with a strong internal transport barrier (ITB) and in discharges terminating in a pure vertical displacement event, in which the plasma conserves a very high energy content up to the thermal quench. These disruption types are very sudden, leaving little scope for the combined action of soft plasma landing strategies and intrinsic performance degradation, both requiring >500 ms to be effective, to decrease the available thermal energy. The characteristic time for the loss of energy from the main plasma towards the PFCs in the thermal quench of JET disruptions is in the range 0.05–3.0 ms. The shortest timescales are typical of disruptions caused by excessive pressure peaking in ITB discharges. The available thermal energy fraction and thermal quench duration observed in JET can be processed (with due caution) into estimates for the projected PFC lifetime of the ITER target.

Drizzle in Stratiform Boundary Layer Clouds. Part I: Vertical and Horizontal Structure

Abstract Detailed observations of stratiform boundary layer clouds on 12 days are examined with specific reference to drizzle formation processes. The clouds differ considerably in mean thickness, liquid water path (LWP), and droplet concentration. Cloud-base precipitation rates differ by a factor of 20 between cases. The lowest precipitation rate is found in the case with the highest droplet concentration even though this case had by far the highest LWP, suggesting that drizzle can be severely suppressed in polluted clouds. The vertical and horizontal structure of cloud and drizzle liquid water and bulk microphysical parameters are examined in detail. In general, the highest concentration of r > 20 μm drizzle drops is found toward the top of the cloud, and the mean volume radius of the drizzle drops increases monotonically from cloud top to base. The resulting precipitation rates are largest at the cloud base but decrease markedly only in the upper third of the cloud. Below cloud, precipitation rates decrease markedly with distance below base due to evaporation, and are broadly consistent in most cases with the results from a simple sedimentation–evaporation model. Evidence is presented that suggests evaporating drizzle is cooling regions of the subcloud layer, which could result in dynamical feedbacks. A composite power spectrum of the horizontal spatial series of precipitation rate is found to exhibit a power-law scaling from the smallest observable scales to close to the maximum observable scale (∼30 km). The exponent is considerably lower (1.1–1.2) than corresponding exponents for LWP variability obtained in other studies (∼1.5–2), demonstrating that there is relatively more variability of drizzle on small scales. Singular measures analysis shows that drizzle fields are much more intermittent than the cloud liquid water content fields, consistent with a drizzle production process that depends strongly upon liquid water content. The adiabaticity of the clouds, which can be modeled as a simple balance between drizzle loss and turbulent replenishment, is found to decrease if the time scale for drizzle loss is shorter than roughly 5–10 eddy turnover time scales. Finally, the data are compared with three simple scalings derived from recent observations of drizzle in subtropical stratocumulus clouds.

Timescale for MeV electron microburst loss during geomagnetic storms

Energetic electrons in the outer radiation belt can resonate with intense bursts of whistler‐mode chorus emission leading to microburst precipitation into the atmosphere. The timescale for removal of outer zone MeV electrons during the main phase of the October 1998 magnetic storm has been computed by comparing the rate of microburst loss observed on SAMPEX with trapped flux levels observed on Polar. Effective lifetimes are comparable to a day and are relatively independent of L shell. The lifetimes have also been evaluated by theoretical calculations based on quasi‐linear scattering by field‐aligned waves. Agreement with the observations requires average wide‐band wave amplitudes comparable to 100 pT, which is consistent with the intensity of chorus emissions observed under active conditions. MeV electron scattering is most efficient during first‐order cyclotron resonance with chorus emissions at geomagnetic latitudes above 30 degrees. Consequently, the zone of MeV microbursts tends to maximize in the prenoon (0400–1200 MLT) sector, since nightside chorus is more strongly confined to the equator.

Dynamic relationship between the outer radiation belt and the plasmapause during March–May 2001

The dynamic spatial relationship between the outer radiation belt electrons and the plasmapause during the two‐month period 30 March–30 May 2001 is determined using SAMPEX daily averaged fluxes of 2–6 MeV electrons and IMAGE EUV 10‐min global images of the plasmasphere. In two events, severe erosion moved the plasmapause inside L = 2 and 2–3 days later the outer belt was located in the normal slot region. We determine on what time scale the outer belt location responds to changes in plasmapause location and find the inner extent of the outer belt matched (to within ΔL ≈ 0.1) with a 3.5‐day running average of the plasmapause location, implying a ≥3.5‐day loss time scale from pitch‐angle scattering by plasmaspheric hiss and EMIC waves.

Meridional variations of temperature, C 2H 2 and C 2H 6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C 2H 2 and C 2H 6 abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C 2H 6 in Saturn's middle and lower stratosphere ( ∼ 40 – 10 , 000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C 2H 2 ( ∼ 1 – 7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13–14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a L S ≈ 270 ° , corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm −1 with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈ 10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C 2H 2 at − 1 ° and − 83 ° planetocentric latitude to be 9.2 −3.8 +6.4 × 10 −7 and 2.5 −1.0 +1.8 × 10 −7 , respectively. The C 2H 2 mixing ratio at 0.12 mbar is found to be 1.0 −0.3 +0.5 × 10 −5 at − 1 ° planetocentric latitude and 2.6 −0.9 +1.3 × 10 −6 at − 83 ° planetocentric latitude. The 2.3 mbar mixing ratio of C 2H 6 inferred from the data is 7.5 −1.7 +2.3 × 10 −6 and 1.0 −0.2 +0.3 × 10 −5 at − 1 ° and − 83 ° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C 2H 2 and C 2H 6 are affected by seasonal forcing and/or stratospheric circulation.

Time scale for loss of massive vector hair by a black hole and its consequences for proton decay

It has long been known that matter charged under a broken U(1) gauge symmetry collapsing to form a black hole will radiate away the associated external (massive) gauge field. We show that the timescale for the radiation of the monopole component of the field will be on the order of the inverse Compton wavelength of the gauge boson (assuming natural units). Since the Compton wavelength for a massive gauge boson is directly related to the scale of symmetry breaking, the timescale for a black hole to lose its gauge field "hair" is determined only by this scale. The timescale for Hawking radiation, however, is set by the mass of the black hole. These different dependencies mean that for any (sub-Planckian) scale of symmetry breaking we can define a mass below which black holes radiate quickly enough to discharge themselves via the Hawking process before the gauge field is radiated away. This has important implications for the extrapolation of classical black hole physics to Planck-scale virtual black holes. In particular, we comment on the implications for protecting protons from gravitationally mediated decay.

Gas stripping by radiation drag from an interstellar cloud

We perform two-dimensional hydrodynamic simulation of gas stripping by radiation drag from interstellar clouds moving in uniform radiation fields. To include relativistic radiation drag properly, the radiation hydrodynamic equation is solved taking into account the dilution of radiation fields by the optical depth of the cloud. As a result, it is found that the optically thin surface layers are effectively stripped by radiation drag from an optically thick gas cloud, and, simultaneously, the stripped gas loses momentum. The momentum loss time-scale is found to be on the order of 10 8 yr under the intensive radiation fields expected in the early phase of galaxy evolution. The present results show that the radiation drag is an effective mechanism for extracting angular momentum from the interstellar medium and allows it to accrete on to the galactic centre. Mass accretion driven by radiation drag may lead to the formation of a central supermassive black hole.

Lognormal form of the ring current energy content

It is shown that the stormtime ring current energy content, from kinetic simulations, has a lognormal distribution. This type of functional form naturally arises from the superposition of many processes with a common initiation event but with differing growth and decay timescales. For the ring current, such a situation occurs from the disparate timescales of energization and decay for the hot ions at various energies, pitch angles, and spatial locations. The summation of this plethora of small currents results in a single current system (the ring current, both partial and symmetric) that has a loss time scale that decreases and then increases during every storm. The consequence is that the stormtime D st index, which also has a lognormal shape, can be (and in fact is) dominated by this single current system.

Flaring X-Ray Emission from HST-1, a Knot in the M87 Jet

We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that the intensity of HST-1, an optical knot 08 from the core, increased by a factor of 2 in 116 days and a factor of 4 in 2 yr. There was also a significant flux decrease over 2 months, with suggestive evidence for a softening of the spectrum. From this variability behavior, we argue that the bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of the other conceivable emission processes can match the range of observed characteristics. By estimating synchrotron model parameters for various bulk relativistic velocities, we demonstrate that a model with a Doppler factor δ in the range 2-5 fits our preliminary estimates of light-travel time and synchrotron loss timescales.

Cataclysmic Variables: An Empirical Angular Momentum Loss Prescription from Open Cluster Data

We apply the angular momentum loss rates inferred from open cluster stars to the evolution of cataclysmic variables (CVs). We show that the angular momentum prescriptions used in earlier CV studies are inconsistent with the measured rotation data in open clusters. The timescale for angular momentum loss () above the fully convective boundary is ~2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in behavior at the fully convective boundary. This provides evidence against the hypothesis that the period gap is caused by an abrupt change in the angular momentum loss law when the secondary becomes fully convective. Furthermore, the empirical angular momentum loss law implies a timescale for CV evolution that is comparable to a Hubble time; for the same reason, it will be more difficult to produce CVs from the products of common envelope evolution, and it implies a lower space density of CVs. The predicted loss rate for short-period CVs is consistent with the observed period minimum (1.3 hr). We infer the time-averaged mass accretion rate and derive the mass-period relation for different evolutionary states of the secondary. The steady-state accretion rates are significantly lower than the claimed observational rates; we discuss some possible explanations. The mass-period relationship is more consistent with evolved secondaries than with unevolved secondaries above the period gap. Implications for the CV period gap are discussed, including the possibility that two populations of secondaries could produce the gap.

Neutral Atom Diffusion in a Partially Ionized Prominence Plasma

The support of solar prominences is normally described in terms of a magnetic force on the prominence plasma that balances the solar gravitational force. Because the prominence plasma is only partially ionized, this support needs to be understood in terms of the frictional coupling between the neutral and ionized components of the prominence plasma, the efficacy of which depends directly on the ion density. More specifically, the frictional force is proportional to the relative flow of neutral and ion species, and for a plasma with a sufficiently small vertical ion column density, this flow must be relatively large to produce a frictional force that balances gravity. A large relative flow, of course, implies significant draining of neutral particles from the prominence. We evaluate the importance of this draining effect for a hydrogen-helium plasma and consider the variation of the draining with a variety of prominence parameters. Our calculations show that the loss timescale for hydrogen is much longer than that for helium, which for typical prominence parameters is about one day.

The ultraviolet line spectrum of the soft X-ray transient XTE J1118+480: a CNO-processed core exposed

We compare ultraviolet (UV) spectra of the recent soft X-ray transients XTE J1118+480 and XTE J1859+226. The emission line strengths in XTE J1118+480 strongly suggest that the accreting material has been CNO processed. We show that this system must have come into contact with a secondary star of about 1.5 M, and an orbital period 15 h, very close to the bifurcation value at which the nuclear and angular momentum loss time-scales are similar. Subsequent evolution to the current period of 4.1 h was driven by angular momentum loss. In passing through a period of 7.75 h the secondary star would have shown essentially normal surface abundances. XTE J1118+480 could thus represent a slightly later evolutionary stage of A0620-00. We briefly discuss the broad Lyα absorption wings in XTE J1118+480.

Adiabatic energization in the ring current and its relation to other source and loss terms

The influence of adiabatic energization and deenergization effects, caused by particle drift in radial distance, on ring current growth rates and loss lifetimes is investigated. Growth and loss rates from simulation results of four storms (5 June 1991, 15 May 1997, 19 October 1998, and 25 September 1998) are examined and compared against the y component of the solar wind electric field (Ey,sw). Energy change rates with and without the inclusion of adiabatic energy changes are considered to isolate the influence of this mechanism in governing changes of ring current strength. It is found that the influence of adiabatic drift effects on the energy change rates is very large when energization and deenergization are considered separately as gain and loss mechanisms, often about an order of magnitude larger than all other source or loss terms combined. This is true not only during storm times, when the open drift path configuration of the hot ions dominates the physics of the ring current, but also during quiet times, when the small oscillation in L of the closed trajectories creates a large source and loss of energy each drift orbit. However, the net energy change from adiabatic drift is often smaller than other source and loss processes, especially during quiet times. Energization from adiabatic drift dominates ring current growth only during portions of the main phase of storms. Furthermore, the net‐adiabatic energization is often positive, because some particles are lost in the inner magnetosphere before they can adiabatically deenergize. It is shown that the inclusion of only this net‐adiabatic drift effect in the total source rate or loss lifetime (depending on the sign of the net‐adiabatic energization) best matches the observed source and loss values from empirical Dst predictor methods (that is, for consistency, these values should be compared between the calculation methods). While adiabatic deenergization dominates the loss timescales for all Ey,sw values, there is a characteristic decrease in the loss lifetime for higher Ey,sw, seen computationally and observationally. It is shown that these shorter loss lifetimes at higher Ey,sw are caused primarily by enhanced dayside flow out. The simulation results show that, on average, flow out is a larger energy loss term than charge exchange (the next biggest loss term for the ring current) for all Ey,sw values.

De Gustibus: time scale of loss and recovery of tastes caused by radiotherapy

Purpose: To quantify the prevalence and distress of taste loss at different intervals after radiotherapy (RT) for head and neck cancer. Materials and methods: In four different groups of head and neck cancer patients (73 patients in total), taste loss and distress due to taste loss were evaluated by taste acuity tests and taste questionnaires. Group 1 ( n=17) was analyzed prior to RT. Groups 2 ( n=17), 3 ( n=17) and 4 ( n=22) were at 2, 6 and 12–24 months after treatment, respectively. A cross-sectional analysis was performed between these four groups. Results: Prior to initiation of RT (group 1), partial taste loss was observed in 35, 18 and 6% of patients for bitter, salt and sweet, respectively. At 2 months after RT (group 2), taste loss (partial or total) was seen in 88, 82, 76 and 53% for bitter, salt, sweet and sour, respectively. At 6 months (group 3), partial taste loss was seen in 71, 65, 41 and 41% (bitter, salt, sweet, sour) and after 1–2 years (group 4) in 41, 50, 27 and 27% (bitter, salt, sweet, sour). Distress caused by taste loss was most frequent in group 2 (82%). Conclusions: In this study, loss of taste after RT was found to be most pronounced after 2 months. Bitter and salt qualities were most impaired. Gradual recovery was seen during the first year after treatment. Partial taste loss still persisted 1–2 years after treatment and was responsible for slight to moderate discomfort.