Drift Echoes Research Articles

Characteristic times for radiation belt drift phase mixing

Impulsive radial transport events occurring in the radiation belts leave lasting marks in the form of drift echoes, that is, energy-dependent drift phase structures in the radiation belts that evolve at the drift frequency. Drift echoes are known to be transient structures that dissipate due to phase mixing. The objective of this paper is to discuss how much time it takes for drift echoes to dissipate, and what drives this phase-mixing process. While any uncertainty or perturbation in the variables controlling trapped particles’ drift frequency contributes to phase mixing, we highlight two main drivers: the observational uncertainty associated with the finite size of the instrument energy channels, and the natural field fluctuations driving perturbations in trapped particles’ drift frequency. It is the combination of both instrumental and natural sources of phase mixing that determines the observed dissipation and lifetime of drift echoes. This means that the observed magnitude and lifetime of a drift echo are always underestimations of the natural magnitude and lifetime of the structure. This calls into question the applicability of the standard, drift-averaged formulation of radial diffusion. The three key points of the study are the following: First, the time it takes for particles initially localized in local time to phase-mix is measured in hours in the Earth’s radiation belts. Second, phase mixing at the drift scale is primarily due to uncertainties in measured kinetic energy and field perturbations. Third, our analysis can be utilized to set an energy resolution requirement for future particle instruments.

Non‐Adiabatic Acceleration of Injected Electrons in the Inner Magnetosphere: Joint Observations by the Van Allen Probe and the BeiDa Imaging Electron Spectrometer

AbstractWe present joint observations of a substorm injection event on 5 February 2016. The enhancements of energetic electron (∼30 to ∼300 keV) fluxes, the drift echoes of the injected electrons, and the quasi‐monochromatic ultralow frequency oscillations embedded in the flux variations were simultaneously observed by the BeiDa Imaging Electron Spectrometer and the Van Allen Probe B. These vivid features of substorm injection were concretely captured by multiple spacecraft within the geosynchronous distance for the first time. The radial separation of the spacecraft (∼0.3 RE) allowed for a quantitative comparison of the electron flux and phase space density (PSD) across different drift shells. It is found that a more intense increase of equatorial PSD occurred at the lower L‐shell (L* ∼5.5), which indicated a non‐adiabatic acceleration of the injected electrons in the inner magnetosphere.

Drift Phase Structure Implications for Radiation Belt Transport

AbstractWe examine drift phase structure in the electron radiation belt observations to differentiate radial transport mechanisms. Impulsive electrostatic or electromagnetic fields can cause radial transport and produce drift echoes (periodic drift phase structures with energy‐dependent period). Narrow‐band standing electromagnetic wave fields can also cause radial transport, while producing energy‐independent periodic drift phase structures. Broad‐band, random‐phase electromagnetic wave fields can cause radial transport, but do not necessarily produce drift phase structure. We present results of three case studies showing little association between drift phase structure and approximately MeV electron flux enhancements in the outer belt. We estimate the amplitude of drift phase structures expected for impulsive or narrow‐band interactions to compete with broad‐band, random‐phase waves. We show that the observed drift phase structure is typically much smaller than would be present if either impulses or narrow‐band waves were the dominant cause of radial transport. We conclude that radial transport is primarily consistent with the broad‐band, random‐phase, small perturbations assumed in quasilinear diffusion theory, although we cannot rule out the unlikely possibility that radial transport plays little role in radiation belt dynamics.

Energetic Electron Enhancement and Dropout Echoes Induced by Solar Wind Dynamic Pressure Decrease: The Effect of Phase Space Density Profile

AbstractThe interplanetary (IP) shock with the sudden increase of solar wind dynamic pressure can cause the acceleration and transportation of energetic electrons in the inner magnetosphere. In this study, we further examine the influence of the sudden drop of solar wind dynamic pressure on magnetospheric energetic particles, through case study of energetic electron drift echoes and statistics with 25 cases from 2012 to 2017. We found that there were both dropout and enhancement drift echoes of electrons in some dynamic pressure decrease events, while other events only show electron dropout drift echoes. In addition, we analyzed the electron phase space density (PSD) and found that the radial profile of electron PSD were different for the two types of events, suggesting that the electron PSD plays an important role in the variations of energy electron fluxes induced by the dynamics pressure decrease. The results of this study, combined with previous studies of energetic electron dynamics caused by IP shocks, can provide a more general picture of both solar wind dynamic pressure decrease and increase effects on the energetic electrons.

Drift echoes and flux oscillations: A signature of prompt and diffusive changes in the radiation belts

Prompt creation of new multi-MeV electron and proton radiation belts in the normally benign slot region between inner and outer Van Allen zones was observed by the short-lived Combined Release and Radiation Effects Satellite (CRRES) mission almost 30 years ago. Flux oscillations at lower energies coincident with ULF wave measurements and consistent with drift resonance were also a feature of the CRRES observations. The next mission designed to study the radiation belts in similar detail from a near equatorial plane orbit was the Van Allen Probes launched in August 2012. These twin spacecraft in similar orbits observed many milder events which produce drift echoes indicative of injection over a limited range of longitude on less than an electron drift time. Related substorm injection drift echoes were also observed by Van Allen Probes with much higher energy resolution than previously. Along with other flux oscillations associated with Ultra Low Frequency (ULF) waves, these events form a class which contributes to the overall radial transport and energization of radiation belt electrons as well as protons. This paper focuses on the greatly improved measurements from the Van Allen Probes relative to the CRRES era. These measurements illuminate our understanding of the importance of flux oscillations as a signature of radial transport relative to other energization and loss processes affecting the radiation belts.

Global‐Scale ULF Waves Associated With SSC Accelerate Magnetospheric Ultrarelativistic Electrons

AbstractWe study electron behavior in the outer radiation belts during the 16 July 2017 storm sudden commencement (SSC), in which prompt intensification of ultrarelativistic electron fluxes was observed at around L = 4.8 by Van Allen Probe B immediately after an interplanetary shock. The electron fluxes in multiple energy channels show clear oscillations in the Pc5 frequency range, although the oscillation characteristics are quite different in different energy channels. At energies above ∼1 MeV, the oscillation periods were very close to the electron drift period, which resembles an energy spectrogram evolution expected for an energetic particle injection event and its drift echoes. At lower energies, however, the oscillation periods hardly depended on the energy: They were very close to the ultralow frequency (ULF) wave period derived from electric field measurements (about 250 s according to wavelet analysis). These complex signatures are consistent with the picture of drift resonance between electrons and short‐lived ULF waves with low azimuthal wave numbers. Good agreement between the observations and numerical simulations confirms that shock‐induced global‐scale ULF waves can efficiently accelerate outer belt ultrarelativistic electrons up to 3.4 MeV over a time scale shorter than 1 hr.

Simulated Prompt Acceleration of Multi‐MeV Electrons by the 17 March 2015 Interplanetary Shock

AbstractPrompt enhancement of relativistic electron flux at L = 3–5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by ∼1 MeV is inferred on less than a drift timescale as seen in prior shock compression events, which launch a magnetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at >1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression.

On the use of drift echoes to characterize on‐orbit sensor discrepancies

AbstractWe describe a method for using drift echo signatures in on‐orbit data to resolve discrepancies between different measurements of particle flux. The drift period has a well‐defined energy dependence, which gives rise to time dispersion of the echoes. The dispersion can then be used to determine the effective energy for one or more channels given each channel's drift period and the known energy for a reference channel. We demonstrate this technique on multiple instruments from the Van Allen Probes mission. Drift echoes are only easily observed at high energies (100 s keV to multiple MeV), where several drift periods occur before the observing satellite has moved on or the global magnetic conditions have changed. We describe a first‐order correction for spacecraft motion. The drift echo technique has provided a significant clue in resolving substantial flux discrepancies between two instruments measuring fluxes near 2 MeV.

Interactions of energetic electrons with ULF waves triggered by interplanetary shock: Van Allen Probes observations in the magnetotail

AbstractWe present in situ observations of a shock‐induced substorm‐like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm‐like electron injections with energy of 30–500 keV were observed in the region from L∼5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90° phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is ∼14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex‐like flow structure‐driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L∼3 to 6 on the nightside, while the time profile of the wave is different from dayside cases.

Association of substorm chorus events with drift echoes

Over recent years, substorm chorus events (SCEs) have been proposed as a useful indicator of substorm onset. The events are regularly seen in the data from the VELOX (VLF/ELF Logger Experiment) instrument at Halley, Antarctica, which has provided over a decade of near continuous observations. SCEs are generally thought to be excited by the injection of electrons near midnight as they gradient‐curvature drift toward dawn. On close one‐to‐one inspection of SCEs seen at Halley and energetic electron signatures seen with the LANL geostationary spacecraft, we have found that many events are associated with the drift echo of the injected electrons rather than the initial injection. In this paper we present some example events as well as the relative statistics. We find that approximately 1/4 of SCEs where a clear signature can be seen in the LANL data are associated with drift echoes rather than the initial substorm injection. We argue that rather than being a direct signature of substorm onset, SCEs are a signature of enhanced electron fluxes in the chorus generation region, which are often, but not exclusively, associated with substorm injections.

Simulation of energetic particle injections associated with a substorm on August 27, 2001

Discrete injections of energetic electrons and ions and their subsequent drift echoes were observed shortly after 0410 UT on August 27, 2001 by three geosynchronous satellites which were arrayed on the dayside. The GOES‐8 spacecraft at geosynchronous orbit near local midnight, the four‐satellite CLUSTER constellation also near local midnight. Concurrent and POLAR was at two hours after midnight. The observations from this suite of satellites provide the basis of information and constraints for us to model the particle injections. The simulations with test‐particle simulations, reproduce both the observed electron and ion injections and subsequent drift echoes. Our simulation results support the idea that the energetic particle injections into the inner magnetosphere are a consequence of near‐Earth magnetic reconnection.

Simulation of dispersionless injections and drift echoes of energetic electrons associated with substorms

The term “dispersionless injection” refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test‐particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

Substorm onsets observed by CRRES: Determination of energetic particle source regions

We examine data from the Electron/Proton Angle Spectrometer instrument, also known as Medium Energy B, flown on the CRRES satellite (July 1990 to August 1991). Because of its strongly eccentric orbit, CRRES can detect substorm onsets in the region L = 4 to L = 8, covering the time sectors 1600–0800 MLT. Substorm onsets which are virtually dispersionless in energy have commonly been associated with acceleration processes which occur in the immediate vicinity of the satellite. Data from CRRES show that such virtually dispersionless onsets can occur far in the inner magnetosphere down to L = 4.3 and are distributed up to ±5 hours around local magnetic midnight. Results from a statistical study are presented. We develop a method of using single‐satellite measurements of near‐dispersionless substorm onsets, together with the observed “drift echoes” of these injections, to establish a “source region” for the energetic particles observed at onset. This method is here applied to electrons only. A database of model drift paths has been set up and is used to calculate the possible interceptions between drift paths of various energies from a source region with the satellite trajectory. Our results indicate that the source region can have a sharp boundary in MLT and can extend over several RE. For some events it is possible to deduce an outer radial limit for the injection region, favoring a more local substorm onset mechanism. Our modeling further shows that the injection and drift echo signature observed at the satellite is quite insensitive to the actual shape of the injection boundary.

Large amplitude electric and magnetic field signatures in the inner magnetosphere during injection of 15 MeV electron drift echoes

Electric and magnetic fields were measured by the CRRES spacecraft at an L‐value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross‐L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field.

Electron drift echoes in the inner magnetosphere

A magnetospheric shock event on 24 March 1991 produced major effects in the energetic particle population in the inner magnetosphere. Electron drift echoes were observed in the range of L=2.6 to 2.1 by instrumentation on the CRRES satellite. It is inferred from the drift rate that the energy of the electrons which were observed was in the range of 12 to 25 MeV and perhaps higher.

High‐energy proton drift echoes: Multiple peak structure

Many high‐energy proton drift echoes observed in association with magnetospheric substorms exhibit multiple fine‐structure peaks during the course of one or more of the echoes. Because of the difficulty in distinguishing between temporal, positional, pitch angle selection, and drift shell effects with instruments on a single satellite the identification of these as individual entities (as opposed to statistical variations in proton intensity or other effects) has not been convincingly established heretofore. We have examined energetic ion data from similar instruments on three widely spaced geosynchronous satellites, with the goal of establishing whether or not these multiple peaks are a persistent feature of the injection process. Our results suggest that multiple peaks are indeed a real feature of drift echoes. At least three quarters of the ion enhancements that result in drift echoes are made up of multiple‐peaked events. A very good correlation between the onset of the various peaks and detailed features observed in high‐latitude magnetograms is observed for some of the multiple peaked events. In fact, in their relation to the magnetograms, there seems to be a pairing of pulses; that is, with some dips in the magnetogram there appears to be two associated pulses. Various possible models for the production of multiple peaks, including the ion tearing mode instability in the plasma sheet, magnetic mirroring effects, temporal and/or positional effects of multiple neutral line formation, and effects due to different ion species are discussed.

A determination of the charge state of energetic magnetospheric ions by the observation of drift echoes

The mean charge states of heavy ions trapped in the earth's magnetosphere are important parameters for distinguishing among magnetospheric theories. No direct measurements have been made at energies greater than ∼ 50 keV. A technique, which uses the E/q dependence of ion drift velocity, is described in this letter. A drift‐echo event, which followed a substorm that occurred at 2100‐2200 UT on 25 February 1979, is discussed to illustrate the application of this technique. In this event the charge state of helium ions was +2 and CNO ions was > +2 and probably ≥ +5. This result implies that the drift‐echo particles were from freshly injected solar‐wind plasma.

High‐resolution energetic particle measurements at 6.6 RE, 2. High‐energy proton drift echoes

Drifting bunches of protons in the 0.14‐ to 1.0‐MeV energy range have been observed with good energy resolution by the charged particle analyzer instruments on the 1976‐059A and 1977‐007A satellites in geostationary orbit. Well‐defined flux peaks are observed corresponding to as many as six circuits of the earth by these proton bunches. Some 52 clear events have been recorded by the instrument on 76‐059 in 8 months of data, and several cases have been observed by both spacecraft. In the twenty‐five cases studied in detail, the proton bunches appear to be injected at locations near midnight. All injections are closely associated with substorm onsets, but fewer than 10% of all substorms have associated high energy proton drift echoes. After injection the magnetic field Structure at ∼6.6 RE often appears to be stable for 1–2 hours, as evidenced by the constant periods between the drift echoes. Observations made using the two spacecraft separated by ∼4 hours of local time show many events which fit a picture of very localized injection and Simple drift in a stable magnetospheric field. The majority of events observed, however, show a more complex behavior than the simple injection‐drift picture would imply.

High-energy electron drift echoes at the geostationary orbit

Approximately 60 high energy (0.7–2.6 MeV) electron drift echoes, observed with the UCSD omni-directional detectors on board ATS-5, have been analyzed. The period for longitudinal drift around the Earth is a function of energy and verifies the now classical law for relativistic electrons. Empirical relationships have been established, which statistically relate the characteristic period T 0 (the drift period for ~320 keV electrons) with the AE and Kp indices. These results are well interpreted if we adopt a Mead and Williams model (with special coefficients) for the magnetic field, and if we assume that the subsolar distance of the magnetopause R b is mainly a function of Kp, the tail field B T being mainly a function of AE. Evidence is given that these functional dependences agree with other experimental data, such as Coleman and McPherron's (1976) measurements of the midnight magnetic field intensity at the geostationary orbit. It is demonstrated that the origin of drift echoes is less likely to be an injection of high-energy particles, but more likely to be the consequence of a redistribution of particles among different L-shells after a sudden compression or expansion of the magnetosphere, as originally proposed by Brewer et al. (1969).

Experimental study of high-energy electron drift echoes observed on board ATS 5

The high-energy electron data (0.7 ≲ E ≲ 2.6 MeV) which were obtained by the University of California at San Diego experiment on board ATS 5 during the period from September 1969 to January 1971 present a large number of drift echo events. A detailed analysis of ∼60 of these events has been performed in order to study their main characteristics. In all cases, the drift period at each energy follows the classical law T = T0(1 + x)/x(2 + x), where x = E/m0c². An event is therefore characterized by T0 (the drift period at E = 320 keV), by the amplitude of the modulation A equal to (Jmax − Jmin)/(Jmax + Jmin), and by its starting time t0. The events may occur at any local time, although there is a noticeable decrease of their occurrence between ∼2300 and ∼0200 LT. There is no clear relationship between the occurrence of these events and the parameters which characterize the magnetic activity (Kp, AE, and Dst); however, in 80% of the cases, ssc's are followed (within 1 min) by drift echoes. Empirical laws have been obtained which relate T0 with the AE index (at constant Kp) or with Kp (for small values of AE). These laws are in good agreement with the theoretical values of T0 which are obtained by using a Mead-Williams magnetospheric model in which the subsolar distance Rb is a function of Kp and the tail field BT is a function of AE. This model is consistent with recent experimental data (Coleman and McPherron, 1976) relating the midnight magnetic field intensity at the geostationary orbit and the AE index, provided one chooses the right coefficients of the Mead-Williams development. There is evidence that the large majority of drift echoes are not the consequence of a direct particle injection. Their quasi-sinusoidal flux variation is consistent with a redistribution of particle drift shells after a compression or an expansion of the magnetosphere. The amplitude of the modulation is of the order of the one which was predicted by Brewer et al. (1969). During the course of this study we found an empirical relation between BT and AE, valid for AE ≳ 40 γ: BT = 2 + 23 log (AE/33), where both BT and AE are expressed in γ (1 γ = 1nT).