A. J. Smith

Wave acceleration of electrons in the Van Allen radiation belts

The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earths magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤<L≤4.5 in the Northern Hemisphere at 78°–106° magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226° magnetic longitude, for several days in October and November 1990. The temporal and spatial variation in plasma mass density was thus determined and compared with VLF whistler measurements of electron densities at similar times and locations. The plasma mass loading was estimated and found to be low, corresponding to 5–10% He+ on the days examined. The plasma mass density is described by a law of the form (R/Req)−p, where p is in the range 3–6 but shows considerable temporal variation, for example, in response to changes in magnetic activity. Other features that were observed include diurnal trends such as the sunrise enhancement in plasma density at low latitudes, latitude-dependent substorm refilling effects, shelves in the plasma density versus L profile, and a longitudinal asymmetry in plasma density. We can also monitor motion of the plasmapause across the station array. Properties of the resonance were examined, including the resonance size, Q, and damping. Finally, we note the appearance of fine structure in power spectra at these latitudes, suggesting that magnetospheric waveguide or cavity modes may be important in selecting wave frequencies.

Solar wind‐magnetosphere coupling leading to relativistic electron energization during high‐speed streams

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfven waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfven waves give multi-day periods of intermittent significantly enhanced convection.

The annual variation in quiet time plasmaspheric electron density, determined from whistler mode group delays

Abstract Whistler mode group delays from the VLF Doppler experiment at Faraday, Antarctica (65°S, 64°W) show an annual variation that has a maximum in December and a minimum in June/July. Assuming signal propagation at constant L (L = 2.5), this implies an annual equatorial electron density (Neq) variation, with December values 3 times higher than in June (during solar minimum—1986). This annual variation in Neq can be modelled from the combined ƒ o F2 medians at each end of the field line (Argentine Islands and Wallops Island), by assuming that diffusive equilibrium is maintained from the F2 layer to the equator over long (≥ 1 month) time scales during quiet magnetic conditions. The use of this model enables the longitude dependence of the annual Neq variation to be investigated. ƒ o F2 data from two other pairs of near-conjugate stations at ∼ 50°E and ∼ 180°E suggest that there are probably no other regions where there are such large annual variations in Neq at L = 2.5. Whistler mode group delays from a similar VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E) show an annual variation that is much smaller, and in agreement with the model results at that longitude. At Faraday during solar maximum conditions, the phase of the annual variation is similar to that observed at solar minimum, but the amplitude is smaller, the December–June ratio in Neq being about 2:1.

Solar-wind–magnetosphere coupling, including relativistic electron energization, during high-speed streams

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfven waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfven waves give multi-day periods of intermittent significantly enhanced convection.

Quiet time plasmaspheric electric fields and plasmasphere-ionosphere coupling fluxes at L = 2.5

Abstract Observations of whistler mode signals from the VLF transmitters NAA and NSS in the Northeast U.S.A., made at Faraday, Antarctica (65°S, 64°W), are used to deduce radial plasma drifts and plasmasphere- ionosphere coupling fluxes near L = 2.5. The fluxes measured represent the sum of the field-aligned plasma fluxes through 1000 km altitude in both hemispheres. The method used to obtain the cross- L drifts and fluxes is explained, and then the results from nine consecutive geomagnetically quiet days in July 1986 described. Data from the 9 days were averaged to find the mean diurnal variation in the East-West electric field (which causes the radial plasma drift) and the fluxes. The fluxes were of magnitude 1−3 × 10 12 m −2 s −1 ; the plasmasphere started to fill at sunrise in the Northern (summer) Hemisphere, and to empty again at sunset in the Southern (winter) Hemisphere. The most noticeable features in the cross- L drift were an outward drift from 07:00–12:00 L.T. and an inward drift from 15:00–22:00 L.T. The electric fields in both cases are of magnitude ≈ 0.2 mV m −1 and are thought to be due to the ionospheric dynamo.

Post midnight VLF chorus events, a substorm signature observed at the ground near L = 4

Clouds of energetic electrons, injected sporadically into the nightside magnetosphere during substorm expansion phase onsets, can generate VLF whistler mode noise through the gyroresonance instability, which may then be observed on the ground or in space. Although these substorm-related chorus events (SCEs) have been reported occasionally in the literature, there seems to have been no systematic study, probably because of the lack, until now, of a well-adapted experimental technique. The VLF/ELF Logger Experiment (VELOX) instrument, located at Halley, Antarctica (76°S, 26°W, L = 4.3), is, however, particularly well suited to a systematic study of this aspect of the substorm phenomenon. The data exist almost continuously from January 1992 onward, at 1-s time resolution in eight quasi-logarithmically spaced frequency bands covering the range 0.25–10 kHz. For this paper, 327 days of continuous data from 1992 have been analyzed. The 243 SCEs identified were observed on about 50% of days, almost exclusively in the 2300–0600 MLT local time range, and were characterized by limited duration (typically, ∼10 min at 1.5 kHz) and upward frequency drift of a band of usually weak and relatively unstructured chorus at a rate of ∼200 Hz min−1 between 0.5 and 5 kHz (corresponding to parallel electron energies in the range ∼10–100 keV). This drift is consistent with the combined eastward and inward motion of the resonant electrons due to azimuthal gradient-curvature drift and radial E × B drift under the action of substorm-enhanced westward electric fields of order l mVm−1 near the equatorial plane. The limited MLT viewing window of the station implies an overall detection efficiency for SCEs of ∼20%. The inferred annual mean substorm rate, 1366 ± 188 year−1, and inter substorm interval, 5.5 ± 0.8 hours, are similar to the values derived using other techniques. However, the distribution of intervals between successive SCEs is different from that for substorm-related particle injections at geostationary orbit; in particular, the models around 1 hour rather than 2–3 hours. The SCE as seen by a VELOX-type VLF receiver with a wide field of view is an important alternative ground-observable substorm signature, complementary to those (such as bays and Pi 2 pulsations) indicated by magnetometers.

Ground observations of chorus following geomagnetic storms

[1] It has been suggested that whistler mode chorus waves play a role in acceleration and loss of radiation belt electrons during geomagnetic storms. In this paper we present data from a complete solar cycle ( 1992 - 2002) of nearly continuous (> 95%) VLF/ELF observations from the VLF/ELF Logger Experiment (VELOX) instrument at Halley station, Antarctica (76 degreesS, 27 degreesW, L = 4.3), to determine whether there is statistical evidence for enhanced whistler mode chorus waves during geomagnetic storms. The data comprise 1 s resolution measurements of ELF/VLF wave power in eight frequency bands from 500 Hz to 10 kHz. The variations in chorus activity during several storms, including the well-studied Bastille Day event ( 14 July 2000), show enhanced wave power but are variable from event to event. The average behavior has been found from a superposed epoch analysis using 372 storms with minimum Dst less than - 50 nT, including 82 large storms with minimum Dst less than - 100 nT. Compared with average prestorm levels, the chorus intensity decreases in the storm main phase but is enhanced in the recovery phase, typically maximizing a day after the storm onset. At 1 kHz the enhancement is independent of storm severity, suggesting a saturation effect, whereas larger storms produce larger wave intensities at higher frequencies in the chorus band ( e. g., 3 kHz), which is interpreted as the effect of a chorus source region located on lower L shells than for weaker storms. The storm chorus enhancement maximizes at postdawn local times, leading to a 24 hour recurrence effect. A long-enduring depression in wave intensities, of 10 days or more, is found near the top of the normal chorus band ( similar to 5 kHz). We suggest that this is due to precipitation from enhanced relativistic particle fluxes affecting the subionospheric propagation of spherics from nearby thunderstorm regions across the L = 2 - 4 zone.

VELOX: a new VLF/ELF receiver in Antarctica for the Global Geospace Science mission

VELOX (VLF/ELF Logger Experiment), a new facility for systematically studying the characteristics of magnetospherically generated ELF/VLF radio noise received at a high-latitude ground station (Halley, Antarctica, 76°S, 26°W, L = 4.3), measures continuously at 1 s resolution the absolute power (peak, mean, and minimum), arrival azimuth, and polarisation ellipticity in 8 logarithmically spaced frequency bands ranging from 500 Hz to 9.3 kHz. All filtering etc. is done in real time using Digital Signal Processing (DSP) techniques. Key parameters (1 kHz and 3 kHz power channels only, at 1-minute intervals) for each day are extracted and regularly transferred to the Global Geospace Study Central Data Handling Facility. Data from the first year of operation (1992) show that, whilst the upper channels (6 kHz and 9.3 kHz) are dominated by thunderstorm (spheric) noise, which is strongest at night and repeatable from day to day, magnetospheric chorus and hiss emissions are more important in the 1–4 kHz range of high attenuation in the Earth-ionosphere waveguide. They are highly variable in intensity from below system noise level (15–20 dB above the reference level 10−33 T2 Hz−1) up to a maximum of 60–70 dB. Three classes of event are usually observed during specific local time sectors: substorm-related chorus events in the midnight-dawn sector, dawn chorus, and hiss-like events in the afternoon; all may occasionally be completely absent on quiet days. The substorm chorus events are shorter (typically 10–20 minutes) and more narrow-band than dawn chorus. Both upper and lower cut-off frequencies rise rapidly (∼ 100 Hz/min), consistent with the energy dispersion of resonant electrons as they drift eastward from injection near midnight, and with the inward drift, driven by substorm-enhanced electric fields, of whistler ducts which support propagation to the ground. Afternoon emission events are often punctuated by sudden deep fading, to noise level within 1–2 minutes, usually followed by complete recovery after a few minutes. All frequencies in the emission band are affected simultaneously. The explanation for this effect is unknown, though it could be a cut-off of propagation through the ionosphere to the ground by irregularities or gradients tilting the wave-normals out of the transmission cone. A similar system to VELOX will be deployed on a network of Automatic Geophysical Observatories extending to higher latitudes, south of Halley.

Periodic and quasiperiodic ELF/VLF emissions observed by an array of Antarctic stations

This paper describes amplitude modulations in the frequency range 0–500 mHz of ELF/VLF (0.5–4.0 kHz) radio wave power recorded throughout 1993 and 1995 at Halley and South Pole stations, Antarctica, which lie in approximately the same magnetic meridian and at geomagnetic latitudes (Λ) of 61° and 74°, respectively. Data from the intermediate automatic geophysical observatories P2 and P3 (Λ = 70° and 72°, respectively) were also analyzed where available. In agreement with earlier work, spectrograms have revealed the frequent day-time (typically 0700-1700 MLT) occurrence of modulations lying almost entirely within the two period ranges: 10–60 s and 4–6 s. The first range corresponds to quasiperiodic (QP) emissions, while the latter is typical of the two-hop whistler mode echo period in the plasmatrough, and the events are termed periodic emissions (PEs). QP occurrence rates higher than some earlier studies (335 station-days out of 667 examined) may be attributable to the sensitive spectral analysis technique. The type I QPs (i.e., those correlated with geomagnetic pulsations observed at South Pole and/or P2/P3) were consistent with an upstream wave driver, controlled by the IMF cone angle. Type II QPs (uncorrelated with magnetic pulsations) were always accompanied by PEs, suggesting a link between the two, reinforced by a frequently observed steady increase in period in both phenomena, especially during the morning, possibly associated with increasing densities due to upward flow of photoionized plasma from the ionosphere after dawn. Here we propose that type II QPs are driven by field line resonant ULF waves which in turn are generated by field-aligned currents arising from PE induced electron precipitation.