Premidnight Sector Research Articles

A study on the evolution of plasma bubbles using the single station‐multisatellite and multistation‐single satellite techniques

AbstractThe characteristics of plasma bubble‐induced ionospheric irregularities were analyzed by estimating the drift velocity of plasma bubbles, their percentage of occurrence, seasonal/LT distributions, and east‐west spatial width using multifrequency Global Navigation Satellite Systems (GNSS) receiver systems at Trivandrum, Bangalore, Hyderabad, Delhi, and Hanle. Two techniques, namely, single station‐multisatellite and multistation‐single satellite observations, were used in the analysis by tracking two geostationary Indian satellites GSAT‐8 (PRN 127) and GSAT‐10 (PRN 128) for the year 2014. In addition, GNSS receiver data from a recently commissioned near‐equatorial station at Changanacherry, Kerala, were used to evaluate 15 plasma bubble events during the period of 29 October 2015 to 31 October 2016. Using, single station‐multisatellite technique, the distribution of drift velocity measurements from plasma bubble scintillations lies in the range of 120–140 m/s for Trivandrum, Changanacherry, and Hyderabad. For Bangalore, and Delhi, the most probable ranges of drift velocity were 120–160 m/s and 80–100 m/s, respectively. The highest probable value of east‐west width of plasma bubble evolution for Trivandrum and Changanacherry was within the range of 100–150 km. However, the same at Bangalore and Hyderabad was in the range of 150–200 km and 100–200 km for Delhi. The maximum probable LT of occurrence of plasma bubble is premidnight sector over the equator. The comparison of the characteristics of plasma bubble evolution over equatorial/low‐latitude Indian sector considering six stations by employing two unique techniques and an objective correlation method is the first time attempt.

Second harmonic poloidal waves observed by Van Allen Probes in the dusk‐midnight sector

AbstractThis paper presents observations of ultralow‐frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single‐spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be ∼100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (∼0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy‐dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift‐bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift‐bounce resonance mechanism with free energy fed by the inward radial gradient of ∼80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Detailed study of Pi2 damped oscillations from low latitude magnetic observatory

The study of low latitude damped Pi2 oscillations (40–150s) are investigated using archived data from Choutuppal (CPL), geomagnetic observatory, operated by CSIR-NGRI, Hyderabad, India. The period of investigation is during solar cycle 21 (1975–1983). All the Pi2 events identified during this period are subjected to detailed analysis for their association with substorm and non-substorm events. It is interesting to note that there is equal probability of occurrence of Pi2s with or without a substorm. The Pi2 frequencies associated with substorms showed an increased value in the post-midnight sector than compared to the Pre-midnight sectors. The non-substorm Pi2s are seen to be associated with lower levels of geomagnetic activity. The corresponding period of Pi2s decreases with increasing level of activity. While the generation of Pi2s are generally attributed to substorm onset, it is seen that the quiet time non-substorm events are related to plasmaspheric cavity mode resonances at low latitudes.

SAMI3‐RCM simulation of the 17 March 2015 geomagnetic storm

AbstractWe present a self‐consistent modeling study of the ionosphere‐plasmasphere system response to the 17 March 2015 geomagnetic storm using the coupled SAMI3‐RCM code. The novel feature of this work is that we capture the important storm time dynamics of the ionosphere on a global scale and its manifestation in the plasmasphere. We find that the penetration electric fields associated with the magnetic storm lead to a storm time enhanced density in the low‐ to middle‐latitude ionosphere. We compare the modeled total electron content (TEC) with GPS‐measured TEC in the American sector. Additionally, we observe the development of polar cap “tongues of ionization” and the formation of subauroral plasma streams in the postsunset, premidnight sector, and its impact on the plasmasphere. However, we did not see the development of plasmaspheric plumes during this event which we attribute to the long main phase of the storm (∼18 h).

Void structure of O+ ions in the inner magnetosphere observed by the Van Allen Probes

AbstractThe Van Allen Probes Helium Oxygen Proton Electron instrument observed a new type of enhancement of O+ ions in the inner magnetosphere during substorms. As the satellite moved outward in the premidnight sector, the flux of the O+ ions with energy ~10 keV appeared first in the energy‐time spectrograms. Then, the enhancement of the flux spread toward high and low energies. The enhanced flux of the O+ ions with the highest energy remained, whereas the flux of the ions with lower energy vanished near apogee, forming what we call the void structure. The structure cannot be found in the H+ spectrogram. We studied the generation mechanism of this structure by using numerical simulation. We traced the trajectories of O+ ions in the electric and magnetic fields from the global magnetohydrodynamics simulation and calculated the flux of O+ ions in the inner magnetosphere in accordance with the Liouville theorem. The simulated spectrograms are well consistent with the ones observed by Van Allen Probes. We suggest the following processes. (1) When magnetic reconnection starts, an intensive equatorward and tailward plasma flow appears in the plasma lobe. (2) The flow transports plasma from the lobe to the plasma sheet where the radius of curvature of the magnetic field line is small. (3) The intensive dawn‐dusk electric field transports the O+ ions earthward and accelerates them nonadiabatically to an energy threshold; (4) the void structure appears at energies below the threshold.

Postmidnight ionospheric troughs in summer at high latitudes

AbstractIn this article we identify possible mechanisms for the formation of postmidnight ionospheric troughs during summer, in sunlit plasma. Four events were identified in measurements of European Incoherent Scatter and ESR radars during CP3 experiments, when the ionosphere was scanned in a meridional plan. The spatial and temporal variation of plasma density, ion, and electron temperatures were analyzed for each of the four events. Super Dual Auroral Radar Network plasma velocity measurements were added, when these were available. For all high‐latitude troughs the ion temperatures are high at density minima (within the trough), at places where the convection plasma velocity is eastward and high. There is no significant change in electron temperature inside the trough, regardless of its temporal evolution. We find that troughs in sunlit plasma form in two steps: the trough starts to form when energetic electron precipitation leads to faster recombination in the F region, and it deepens when entering a region with high eastward flow, producing frictional heating and further depleting the plasma. The high‐latitude plasma convection plays an important role in formation and evolution of troughs in the postmidnight sector in sunlit plasma. During one event a second trough is identified at midlatitudes, with different characteristics, which is most likely produced by a rapid subauroral ion drift in the premidnight sector.

Effects of auroral potential drops on plasma sheet dynamics

AbstractThe reaction of the magnetosphere‐ionosphere system to dynamic auroral potential drops is investigated using the Lyon‐Fedder‐Mobarry global model including, for the first time in a global simulation, the dissipative load of field‐aligned potential drops in the low‐altitude boundary condition. This extra load reduces the field‐aligned current (j||) supplied by nightside reconnection dynamos. The system adapts by forcing the nightside X line closer to Earth, with a corresponding reduction in current lensing (j||/B = constant) at the ionosphere and additional contraction of the plasma sheet during substorm recovery and steady magnetospheric convection. For steady and moderate solar wind driving and with constant ionospheric conductance, the cross polar cap potential and hemispheric field‐aligned current are lower by approximately the ratio of the peak field‐aligned potential drop to the cross polar cap potential (10–15%) when potential drops are included. Hemispheric ionospheric Joule dissipation is less by 8%, while the area‐integrated, average work done on the fluid by the reconnecting magnetotail field increases by 50% within |y| < 8 RE. Effects on the nightside plasma sheet include (1) an average X line 4 RE closer to Earth; (2) a 12% higher mean reconnection rate; and (3) dawn‐dusk asymmetry in reconnection with a 17% higher rate in the premidnight sector.

On the links between the radio flux and magnetodisk distortions at Jupiter

AbstractUsing measurements of the Galileo magnetometer and plasma wave instrument, it is shown that the flux of the Jovian auroral radio emissions is correlated with the azimuthal component of the magnetic field (Bφ) measured in the plasma disk, the situations of large magnetic twist of the disk (large ΔBφ, the difference between the measured and the model field) corresponding to enhanced radio intensities (frequency > 300 kHz). For the four orbits discussed here (three in the postmidnight and one in the premidnight sector), representing ~44 days of observations, from 25 to 85 Jovian radius in the magnetodisk, the radio intensity observed during periods of small radial current (ΔBφ < 1 nT) is typically a factor of 5 to 10 smaller than that observed at large radial current (ΔBφ > 5–6 nT). It is proposed that these variations are the direct consequences of enhanced magnetosphere/ionosphere coupling current systems linked to episodes of larger outward mass outflows in the disk, resulting in larger parallel currents and, thus, in enhanced auroral activity. The application of Hill's model shows that the observed variations of Bφ can be explained by increasing the mass outflow rate from ~150 kg/s (quiet periods) to more than 2 t/s (“energetic” events), for Pedersen conductance ranging from 0.1 to 1 S. This is consistent with the canonical values given in the literature. It is estimated that these modulations of the mass flow rates lead to variations of the power dissipated in the disk from ~1014 to 1015 W due to the torque exerted by the magnetic coupling with Jupiter's ionosphere, with a conversion rate into the power radiated by the radio waves of the order of 10−6.

Relativistic electron precipitation as seen by NOAA POES

AbstractWe performed a survey of relativistic electron precipitation (REP) events revealed by the Medium Energy Proton and Electron Detector instrument on board NOAA Polar‐orbiting Operational Environmental Satellites during a 38 day interval. We have divided the observed REP events into three groups with respect to the simultaneous observations of energetic (>30 keV) electron and proton precipitation. The first group consists of REP enhancements forming the isotropy zone at the poleward edge of trapped relativistic electron fluxes. These REP events are observed on the nightside, and they are, apparently, produced by isotropization process related to nonadiabatic motion of particles in the stretched magnetic field. The second group are the REP events related to simultaneous enhancements of energetic >30–300 keV electrons. These events have a wider magnetic local time range of occurrence with a maximum in the premidnight sector. They can be related to the interaction of electrons with waves whose possible nature is briefly discussed on the basis of comparison with the cold plasma density in the conjugated region of the equatorial plane. The third group consists of the REP events correlated with the burst‐like precipitation of >30–keV protons within an anisotropy zone, where the trapped flux dominates. These events are found in the dusk sector in association with enhanced cold plasma density in the conjugate equatorial magnetosphere. As is known, proton bursts within the anisotropy zone indicate the location of the electromagnetic ion cyclotron (EMIC) wave source. Such REP events can be due to scattering of the relativistic electrons by EMIC waves. However, we noted that some of these REP events are associated with precipitation of energetic electrons with low‐energy cutoff below 100 keV. We suggest that in such cases the electrons within a wide energy range are precipitated by other waves (probably, by plasmaspheric hiss).

Storm time impulsive enhancements of energetic oxygen due to adiabatic acceleration of preexisting warm oxygen in the inner magnetosphere

AbstractWe examine enhancements of energetic (>50 keV) oxygen ions observed by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on board the Van Allen Probes spacecraft in the inner magnetosphere (L ~ 6) at 22–23 h magnetic local time (MLT) during an injection event of the 6 June 2013 storm. Simultaneous observations by two Van Allen Probes spacecraft located close together (~0.5 RE) indicate that particle injections occurred in the premidnight sector (< ~24 h MLT). We also examine the evolution of the proton and oxygen energy spectra at L ~ 6 during the injection event. The spectral slope did not significantly change during the storm. The oxygen phase space density (PSD) was shifted toward higher PSD in a wide range of the first adiabatic invariant. The spectral evolution manifests the characteristics of adiabatic acceleration and density increase of oxygen ions. Warm (0.1–10 keV) oxygen measured by the Helium, Oxygen, Proton, and Electron (HOPE) instrument was enhanced prior to the storm mostly in magnetic field‐aligned directions. The most reasonable scenario of this event is that warm oxygen ions that preexisted in the inner magnetosphere were picked up and adiabatically transported and accelerated by spatially localized, temporarily impulsive electric fields.

Initial deflection of middle‐latitude Pi2 pulsations in the premidnight sector: Remote detection of oscillatory upward field‐aligned current at substorm onset

AbstractIn this study, we examined middle‐ and low‐latitude Pi2 events to address the following two issues regarding the well‐known substorm current wedge (SCW) model for Pi2 pulsations: (1) the center of the SCW, which is estimated using the Pi2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern; and (2) although ideally Pi2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultraviolet Imager data. We assumed that the ionospheric footprint of the upward field‐aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle‐latitude station Zyryanka (ZYK; GMLAT = 59.6°) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle‐latitude Pi2. Our results suggest that the circular shapes of Pi2 polarization curves are caused by the delayed driven Alfvénic waves that are superimposed on the geomagnetic northward components of SCW oscillations.

Statistics of the field‐aligned currents at the high‐latitude energetic electron boundaries in the nightside: Cluster observation

AbstractMagnetic field disturbances with a clear bipolar signature are frequently observed when the Cluster spacecraft fleet passes through both southern and northern high‐latitude energetic electron boundaries at the nightside magnetosphere. The dominant variation of the bipolar signature is in the azimuthal direction of the local mean field‐aligned coordinate, indicating a field‐aligned current. From 2001 to 2008, we have examined 110 events with the magnetic field and energetic electron measurements. The main results can be summarized as follows: (1) The density and thickness of the field‐aligned current, calculated under the assumption of the one‐dimensional sheet, are in order of tens of nA/m2 and hundreds of kilometers, respectively. (2) Currents flowing into and away from the ionosphere tend to be observed in the postmidnight and premidnight sector, respectively, which have the same polarity as the region 1 current system. (3) These currents mainly distribute in the 60°–75° magnetic latitude region after mapping to the ionosphere. We also find that the current density and corresponding magnetic field variation are positively correlated with the Kp index and solar wind pressure, but almost independent of the AE index.

Duskside enhancement of equatorial zonal electric field response to convection electric fields during the St. Patrick's Day storm on 17 March 2015

AbstractThe equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand‐Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM‐A and SWARM‐C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.

The dawn‐dusk length of the X line in the near‐Earth magnetotail: Geotail survey in 1994–2014

AbstractThe dawn‐dusk length of the X line is investigated for magnetic reconnection in association with substorms in the near‐Earth magnetotail on the basis of the 21 year Geotail plasma sheet observations. The X line is identified as a simultaneous plasma flow Vx and magnetic field Bz reversal inside the ion‐electron decoupling region of magnetic reconnection. Forty‐four X lines can be found at XGSM = −20 to −31 RE in the magnetotail. The X line length is estimated on the basis of occurrence of magnetic reconnection. Characteristics of flows and fields are investigated for magnetic reconnection and tailward flow events to verify the obtained X line length. The dawn‐dusk length of the X line is most likely 6 RE with its center in the premidnight sector for moderate substorms. Hence, the magnetic reconnection site is mapped to approximately 1 h local time sector in the auroral ionosphere. The dawn‐dusk length of the X line is extended mainly dawnward for larger substorms.

An interpretation of spacecraft and ground based observations of multiple omega band events

The source of the auroral phenomenon known as omega bands is not yet known. We examine in detail five different intervals when omega bands were observed on March 9th, 2008 between 0400UT and 1100UT over central Canada using both ground and space-based instrumentation. The THEMIS all sky imagers show the development of some of the omega bands from north–south streamers. Spherical elementary currents derived from ground magnetometer data indicate that the omega bands lie near the interface between the region 1 and region 2 currents in the post-midnight sector. THEMIS spacecraft data from the pre-midnight sector display multiple high speed flows and dipolarization features associated with high levels of geomagnetic activity, whereas four GOES geosynchronous spacecraft show multiple injections and dipolarization features. Magnetic field line tracing suggests that the magnetospheric location of the omega bands is at or just beyond geosynchronous orbit. We discuss in detail two potential source mechanisms for the omega bands: plasma sheet velocity shears and high speed flows in the magnetotail and relate the available data to those mechanisms. Our data and a magnetohydrodynamic (MHD) simulation support high speed flows in the magnetotail as the most likely generation mechanism, although the distribution of the magnetotail spacecraft does not provide unambiguous support for our interpretation of the source mechanism.

Near-Earth injection of MeV electrons associated with intense dipolarization electric fields: Van Allen Probes observations.

Substorms generally inject tens to hundreds of keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeVelectron injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the premidnight sector at L ∼ 5.5, Van Allen Probes (Radiation Belt Storm Probes)‐A observed a large dipolarization electric field (50 mV/m) over ∼40 s and a dispersionless injection of electrons up to ∼3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL‐GEO, THEMIS‐D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by 1 order of magnitude in less than 3 h in the outer radiation belt (L > 4.8). Our observations provide evidence that deep injections can supply significant MeV electrons.

Van Allen Probe observations of drift-bounce resonances with Pc 4 pulsations and wave–particle interactions in the pre-midnight inner magnetosphere

Abstract. We present Van Allen Probe B observations of azimuthally limited, antisymmetric, poloidal Pc 4 electric and magnetic field pulsations in the pre-midnight sector of the magnetosphere from 05:40 to 06:00 UT on 1 May 2013. Oscillation periods were similar for the magnetic and electric fields and proton fluxes. The flux of energetic protons exhibited an energy-dependent response to the pulsations. Energetic proton variations were anticorrelated at medium and low energies. Although we attribute the pulsations to a drift-bounce resonance, we demonstrate that the energy-dependent response of the ion fluxes results from pulsation-associated velocities sweeping energy-dependent radial ion flux gradients back and forth past the spacecraft.

Substorm evolution of auroral structures

AbstractAuroral arcs are often associated with magnetically quiet time and substorm growth phases. We have studied the evolution of auroral structures during global and local magnetic activity to investigate the occurrence rate of auroral arcs during different levels of magnetic activity. The ground‐magnetic and auroral conditions are described by the magnetometer and auroral camera data from five Magnetometers — Ionospheric radars — All‐sky cameras Large Experiment stations in Finnish and Swedish Lapland. We identified substorm growth, expansion, and recovery phases from the local electrojet index (IL) in 1996–2007 and analyzed the auroral structures during the different phases. Auroral structures were also analyzed during different global magnetic activity levels, as described by the planetary Kp index. The distribution of auroral structures for all substorm phases and Kp levels is of similar shape. About one third of all detected structures are auroral arcs. This suggests that auroral arcs occur in all conditions as the main element of the aurora. The most arc‐dominated substorm phases occur in the premidnight sector, while the least arc‐dominated substorm phases take place in the dawn sector. Arc event lifetimes and expectation times calculated for different substorm phases show that the longest arc‐dominated periods are found during growth phases, while the longest arc waiting times occur during expansion phases. Most of the arc events end when arcs evolve to more complex structures. This is true for all substorm phases. Based on the number of images of auroral arcs and the durations of substorm phases, we conclude that a randomly selected auroral arc most likely belongs to a substorm expansion phase. A small time delay, of the order of a minute, is observed between the magnetic signature of the substorm onset (i.e., the beginning of the negative bay) and the auroral breakup (i.e., the growth phase arc changing into a dynamic display). The magnetic onset was observed to precede the structural change in the auroral display. A longer delay of a few minutes was found between the beginning of the growth phase and the first detected auroral structure.

Solar wind control of ionospheric equivalent currents and their time derivatives

AbstractA solid understanding of the solar wind control of ground magnetic field disturbances is essential for utilizing the existing long time series of ground data to obtain information on solar wind‐magnetosphere‐ionosphere coupling. We have used 20 years of International Monitor for Auroral Geomagnetic Effects magnetometer data (54°–76° magnetic latitude) to study the solar wind control of the ionospheric equivalent current density and its time derivative (<|dJeq/dt|>). We found that <|dJeq/dt|> peaks at the premidnight and prenoon ends of the westward electrojet. The prenoon <|dJeq/dt|> peak was most intense during fast solar wind and radial interplanetary magnetic field (IMF). The location of the peak was not affected by the IMF orientation but persisted at 8–10 magnetic local time and 70°–75° latitude, near the boundary between the westward and eastward electrojets. Sensitivity of this boundary to disturbances was suggested as a possible explanation for the persistent prenoon location of the peak. The premidnight peak was most intense during southward IMF orientation. While faster solar wind mainly resulted in more intense <|dJeq/dt|> in the premidnight sector, stronger IMF caused the region of intense <|dJeq/dt|> to spread to the postmidnight, dawn, and dusk sectors. A good correspondence was found between development of the nightside <|dJeq/dt|> intensification and average substorm bulge and oval aurora as determined by Gjerloev et al. (2007). The bulge aurora covered the western end of the westward electrojet where the equivalent current also had a significant poleward component. The substorm oval aurora, on the other hand, extended eastward along the westward electrojet.

Magnetic phase structure of Saturn's 10.7 h oscillations

AbstractA source of Saturn's magnetic 10.7 h period oscillations has yet to be identified. The oscillations are known to consist of signals with slightly different periods from separate northern and southern sources. Here we present a novel way of examining observations, focusing on signal phase. We show that although the signals are highly periodic they are usually not sinusoidal and that there are differences in both phase structure and polarization between the outer magnetosphere (on the nightside) and the inner dipolar region. Paying particular attention to the deep midtail passes of 2006, the contrasting behavior between the inner and outer regions is clear with approximate sinusoidal behavior in the dipolar region and a pulse‐like signal once per cycle in the tail. The latter structure seems to indicate that tail magnetic stress is released impulsively once per cycle in the tail. After equinox, in 2010–2011, we find a different picture in the premidnight sector. The predetermined northern and southern frequencies are closer together and apparently show sudden shifts. Our signal reconstruction approach finds instances where it is likely that the narrow band filtering is not able to track completely the basic north and south periods as we find phase jumps indicating unpredicted beats.