J. C. Foster

Storm time plasma transport at middle and high latitudes

Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ∼750 m s−1 with a flux of 1014 m−2 s−1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions.

SAPS: A new categorization for sub‐auroral electric fields

Sub-auroral electric fields play critical roles in energizing and transporting ring current ions, as well as convecting thermal plasma in the inner magnetosphere and in the mid- to low-latitude ionosphere. A number of terms have been employed to describe the sub-auroral electric fields and many of these are specifically associated with processes established through their prior use in the literature. The continued use of descriptive terms such as penetration electric fields, polarization jets, and sub-auroral ion drifts could lead to misunderstanding, especially when comparing the broad/narrow, persistent/transient regions of sub-auroral electric field and plasma flow. An inclusive name for these phenomena is sub-auroral polarization streams (SAPS).

Multiradar observations of the polar tongue of ionization

[1] We present a global view of large-scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low-latitude, auroral, and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high-latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high-latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m 3 ) and low electron and ion temperatures (2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high-latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was 50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m 2 s 1 ) were observed in the topside ionosphere

On the importance of E‐field variability for Joule heating in the high‐latitude thermosphere

Joule heating is known to be one of the major energy sources of the upper atmosphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the regions physics. However, Joule heating is currently one of the largest sources of uncertainty in the thermospheres energy budget. In numerical models the distribution of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E-field. That method ignores the important component of Joule heating due to rapid or small-scale fluctuations in E-field or ion drifts. However, E-field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heating in the thermosphere is proportional to the average of the square of the E-field. The computation of the average of the square of the E-field requires knowledge of the statistical characteristics of E-field variability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E-field variability as an upper-atmosphere energy source. We show that the inclusion of E-field variability in the high-latitude convection model can significantly increase the amount of Joule heating for a given pattern.

Prompt midlatitude electric field effects during severe geomagnetic storms

Meridian-plane elevation scans with the Millstone Hill incoherent scatter radar provide evidence of a strong perturbation of the coupled mid-latitude magnetosphere-ionosphere system during the early phases of the November 4, 1993 magnetic storm. A narrow ionospheric trough formed at L=3.5 in the pre-midnight sector, immediately poleward of the Millstone Hill site. The most pronounced radar signature of the developing activity was a brief (20 min) uplifting of the F region plasma equatorward of the trough, such that the peak altitude increased with distance away from the trough. A similar signature had been observed during storm onset on March 20, 1990, and in that event a pronounced topside ionospheric depletion developed in the region far equatorward of the mid-latitude trough and was observed by the radar and the DMSP F9 satellite. During the November 4, 1993 event, the DMSP F10 satellite observed narrow, magnetically conjugate regions of plasma density depletion and strong horizontal and upward plasma velocity (> 1500 m/s) at L=1.5 at the time of the uplifting of the mid-latitude F region observed by the radar. These observations were confined to longitudes near the South Atlantic magnetic anomaly and, in the Nov 1993 case, the perturbation was coincident with the peak of the precipitating particle fluxes associated with innerbelt losses at the anomaly. Both the uplifting of the ionospheric F layer and the triggering of topside density perturbations can be explained in terms of an eastward electric field imposed on the mid and low-latitude ionosphere during the initial stages of the geomagnetic storm. The low-latitude ionospheric perturbations in these events were similar to supersonic equatorial bubbles, triggered by the destabilizing effects of the upward E×B drift associated with the eastward electric field.

An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts

Early observations indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth’s intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave–particle pitch angle scattering deep inside the Earth’s plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations

The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earths radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both “competing” mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.

Electric field variability associated with the Millstone Hill electric field model

Joule heating that is generated at high latitudes in the thermosphere because of the magnetospherically imposed electric potential is proportional to the average of the square of the electric field (E field). Most theoretical Joule heating computations use only average electric fields, resulting in heating that is proportional to the square of the average E field. The computation of the average of the square of the E field requires knowledge about the statistical characteristics of E field variability associated with the average electric field model. In this paper we present the variability associated with the Millstone Hill bin-averaged empirical E field model [Foster et al. 1986] and discuss the implications of variability as an upper atmosphere energy source. We rebinned the radar plasma drift measurements from Millstone Hill, Massachusetts, in magnetic latitude and local time as a function of auroral activity and calculated the average electric fields and the variability associated with them as reflected in the bin standard deviations. We present the E field patterns and the associated variability for both quiet and disturbed geomagnetic conditions for the four seasons. We show that for an electric field model with a Gaussian distribution of small-scale variability around the mean, the average field and the variability have equal contributions to Joule heating generation.

Simultaneous observations of E‐region coherent backscatter and electric field amplitude at F‐region heights with the Millstone Hill UHF Radar

A combined coherent backscatter-incoherent scatter experiment with the Millstone Hill UHF radar provided simultaneous observations of electric field magnitude and coherent backscatter parameters on the same L shell. A carefully-designed geometry used sidelobe coherent contamination from two-stream irregularities at 110 km altitude, appearing at ranges corresponding to F-region altitudes in the main beam, in conjunction with simultaneous uncontaminated F-region observations of the drift velocity in adjacent range gates. Both logarithmic coherent power and the magnitude of the coherent phase velocity Vph vary linearly with . With the assumption that the coherent phase velocity is approximately the perturbed ion sound speed in the heated E region, we find an excellent agreement between the electron temperature inferred from Vph and previous incoherent-scatter results relating E-region Te to E. A maximum value of ∼3100 K has been found for such wave-induced E-region heating.

Observations from Millstone Hill during the geomagnetic disturbances of March and April 1990

The incoherent scatter radars at Millstone Hill operated continuously during the periods March 16–23 and April 6–12, 1990, providing observations of large-scale ionospheric structure and dynamics over a large portion of eastern North America. Major geomagnetic storms occurred during each of these periods, with deep nighttime ionospheric troughs and large magnetospheric convection electric fields observed equatorward of Millstone. The Millstone observations provide a comprehensive data set detailing storm-induced ionospheric effects over a 35° span of latitude during both of these intervals. At the latitude of Millstone the ionospheric peak height hmF2 rose above 600 km in the trough on March 22 and 23 and reached ≈500 km at night on April 11 and 12. Increased recombination, apparently due to the strong electric fields, the temperature dependent recombination rate coefficient, and neutral composition changes, greatly depleted the F2 region over a wide latitude range during the day on April 10, 1990. This resulted in an ionosphere dominated by molecular ions, with ionospheric peak heights below 200 km on this day. A number of frictional heating events during the disturbed periods are seen from comparison of ion temperature and velocity measurements. The most intense event took place near 1200 UT (≈0715 LMT) on April 10, 1990, when Kp reached 8. At 0110 UT on March 21, line of sight ion velocities in excess of 500 m s−1 were observed at the extreme southern limit of the Millstone steerable radars field of view (40° apex magnetic latitude at an altitude of 700 km). These could be due to penetration of magnetospheric electric fields or electric fields associated with ring current shielding in the storm-time outer plasmasphere. About an hour later, ion outflow was observed just equatorward of Millstone. This is most likely due to heating from a latitudinally confined region of intense westward convection. Neutral meridional winds above Millstone were obtained by three different techniques employing radar and Fabry-Perot measurements. The latitude variation of the winds was also estimated from radar measurements of hmF2 and electric fields using the servo model method. Strong equatorward nighttime neutral wind surges were found during both the March and April disturbances, which reached the equatorward limit of the observations at F peak heights.