J. F. Vickrey

Poynting flux measurements on a satellite: A diagnostic tool for space research

The first satellite observations of the total field-aligned component of the quasi-dc Poynting flux are presented for two passes over the polar region, one in the noon sector and one in the afternoon. The energy input due to electron precipitation is also presented. In the noon pass the downward Poynting flux in the auroral oval was comparable to the kinetic energy input rate. The peak electromagnetic energy input rate of 6 ergs/(cm² s) equaled the peak particle input while the integrated electromagnetic value along the trajectory was 60% that of the particles. In the afternoon pass the peak electromagnetic energy input was also about 6 ergs/(cm² s), but the peak particle energy was 6 times this value. The average electromagnetic input was 10% of the particle input for the pass. In this study we can measure the Poynting flux only over a limited range of scale sizes; thus the contribution to the total energy budget in the polar cap cannot be determined. Both passes show small regions characterized by upward Poynting flux suggesting a neutral wind dynamo. There is also evidence during part of the noontime pass that the external generator acted in opposition to an existing wind field since the Poynting flux was greater than the estimate of Joule heating from the electric field measurement alone (i.e., from ΣpE²). In the course of deriving Poyntings theorem for the geophysical case we also present a proof that ground magnetometer systems respond primarily to the Hall current which does not depend upon geometric cancellation between the field generated by Pedersen and field-aligned currents.

Interpretation and modeling of the high-latitude electromagnetic energy flux

An interpretation of the electromagnetic energy flux at high latitudes under steady state conditions is presented and analyzed through modeling of the large-scale coupling between the high-latitude ionosphere and magnetosphere. In this paper we elucidate the steady state relationship between the electromagnetic energy flux (divergence of the dc Poynting flux), the Joule heating rate, and the mechanical energy transfer rate in the high-latitude ionosphere. We also demonstrate the important role of the neutral wind and its conductivity-weighted distribution with altitude in determining the resultant exchange of electromagnetic energy at high latitudes. Because the Poynting flux approach accounts for the neutral wind implicitly and describes the net electromagnetic energy flux between the magnetosphere and ionosphere, it is a fundamental measure of energy transfer in the system. A significant portion of this energy transfer results in Joule heating; however, the conversion of electromagnetic energy flux into mechanical energy of the neutrals is also considerable and can in some regions exceed the Joule heating rate. We will show that neglect of the neutral dynamics in calculations of the Joule heating rate can be misleading. To evaluate and interpret the electromagnetic energy flux at high latitudes, we employ the vector spherical harmonic model, which is based on the National Center for Atmospheric Research thermosphere-ionosphere general circulation model, to provide the steady state properties of the thermosphere-ionosphere system under moderate to quiet geomagnetic activity. For the specific geophysical conditions modeled we conclude that (1) the electromagnetic energy flux is predominantly directed into the high-latitude ionosphere with greater input in the morning sector than in the evening sector, as supported by DE 2 observations. (2) The Joule heating rate accounts for much of the electromagnetic energy deposited in the ionosphere with the conductivity-weighted neutral wind contributing significantly to the Joule heating rate and thus affecting the net electromagnetic energy flux in the ionosphere. (3) On average, the mechanical energy transfer rate amounts to about 10% to 30% of the net electromagnetic energy flux in the auroral dawn, dusk, and polar cap regions, acting as a sink of electromagnetic energy flux in the dawn and dusk sectors and a source in the polar cap. (4) Weak regions of upward electromagnetic energy flux are found near the convection reversal boundaries where the mechanical energy transfer rate exceeds the Joule heating rate. In general, large upward electromagnetic energy fluxes may be rare, as the always positive Joule heating rate increases irrespective of the source of electromagnetic energy flux; that is, neutral dynamics contribute directly to the Joule heating rate.

On the contribution of the thermospheric neutral wind to high-latitude energetics

Although the neutral winds contribution to ionospheric electrodynamics is well-established at low latitudes, this electrical energy source has been largely ignored at high latitudes, owing to the assumed dominance of the magnetospheric dynamo contribution. Yet, the potential for exchange of electrical energy between the neutral wind dynamo and the magnetospheric dynamo is a direct consequence of the coupling between the two regions by highly conducting magnetic field lines. The integral nature of this coupling precludes the direct separation of the neutral wind and solar wind contributions to the observed electrodynamics. Therefore, to gain some insight into their relative importance, we have performed a simple numerical experiment in which the two dynamos are individually connected to a fixed load and their energetics evaluated separately. To determine the electrical energy flux supplied by the magnetosphere, we treat it as a voltage generator and the ionosphere as a resistive load. The available electrical energy flux generated by the neutral wind dynamo is determined from the mechanical energy stored within an established neutral wind field. This exercise has led to a number of conclusions, including: i) The neutral wind dynamo contributes significantly to high-latitude energetics, particularly in the central polar cap; and ii) In the region near the plasma convection reversal boundary, the amount of energy flux available from the neutral wind dynamo can exceed that provided by the magnetospheric dynamo.

The sondrestrom radar and accompanying ground-based instrumentation

The Sondrestrom radar facility, funded by the NSF Upper Atmospheric Facilities Program, is operated and managed by SRI International. The facility is located on the west coast of Greenland, just north of the Arctic Circle, near 75 deg invariant magnetic latitude. The principal instrument at the facility is the incoherent scatter radar. The incoherent scatter technique allows the direct measurement of ionospheric electron number density, ion velocity, and electron and ion temperature along the radar beam. Because the radar antenna is fully steerable these parameters can be determined as functions of horizontal distance and altitude. Additional ionospheric quantities can be derived using these measured parameters. As part of the ISTP mission, the radar will measure the spatial (horizontal and altitudinal) and temporal variations of ionospheric parameters including electron density, large scale electric field. conductivity, currents, and energy input. Repetitive measurements define variations of parameters with local time, as well.

Coordinated radar and optical measurements of stable auroral arcs at the polar cap boundary

A specialized incoherent scatter radar scanning mode has been developed for use in conjunction with simultaneous real-time all-sky images. These complementary diagnostics are used to examine the aeronomy and electrodynamics of stable auroral arcs that delineate the boundary between the polar cap and the auroral oval. The first arc discussed, observed at 2000 MLT, represents the boundary between antisunward plasma flow in the polar cap and sunward return flow equatorward of the arc. The arc defined an equipotential in the high-latitude convection pattern in that no plasma flowed across the arc. The radar line-of-sight velocity measurements also indicate that this arc is consistent with a convergent electric field and an associated weak upward field-aligned current. The second arc was observed at 2330 MLT and was associated with a nightside gap or magnetic reconnection region. Strong antisunward flow was observed directly across the arc, although a velocity shear was superposed on this steady flow along the poleward edge of the arc. Detailed plasma density, temperature, and line-of-sight velocity measurements from the radar are presented for both arcs to define the electric field, horizontal and field-aligned currents, and thermal plasma parameters associated with these arcs.

Ground‐based signatures for the nightside polar cap boundary

This work describes an application of incoherent scatter (IS) radar and all-sky imaging techniques to the identification of signatures for the nightside auroral zone/polar cap boundary. This identification is made by estimating the characteristic energy of precipitating electrons across all latitudes measured by the IS radar. The primary method used to discern the boundary is based on an algorithm wherein the detailed shape of E region ionization profiles is mapped to the required number flux from a library of monoenergetic ionization profiles. This procedure results in a reconstructed energy distribution for the precipitating electrons from which a characteristic energy is derived. The latitudinal gradient of the characteristic energy is subsequently used to identify the boundary. Monochromatic all-sky imagers are used to establish the time history of the boundary and estimate characteristic energy (at magnetic zenith). Additional plasma signatures such as auroral ionospheric cavities and localized F region Te enhancements are shown to provide contextual clues for the location of the boundary. Three in-depth case study periods are used to qualify the various boundary identification methods. A detailed study of 30 IS radar scans in the geomagnetic meridian is used to conclude that a latitudinal gradient in characteristic energy of −5 to −11 keV per degree, when measured adjacent to an extended region of low-energy precipitation, can locate the boundary with a latitudinal precision of 0.2°.

Coordinated convection measurements in the vicinity of auroral cavities

Meridional radar scans of electron density from the Sondrestrom incoherent scatter radar (Greenland, 66.99°N, 50.95°W) have been used to identify latitudinally narrow, field-aligned depletions of the auroral F region ionosphere. Observations of these so-called auroral cavities have been reported in earlier case studies in close proximity to E layer arcs at the poleward edge of the nightside oval (Doe et al. 1993). These radar data indicated that the cavities and arcs remained as collocated pairs for periods as long as an hour, while coordinated imaging and satellite measurements indicated that the pairs were extended in magnetic local time

Irregularities and Instabilities in the Auroral F Region

The Earth’s F-region ionospheric plasma displays structure perpendicular to the magnetic field on scales from hundreds of kilometers down to centimeters. The physical processes that operate over such a wide range of scale sizes are, of course, very diverse. At the largest scales (λ ≥ 10 km), production, loss, and transport of structured plasma are dominated by aeronomic processes including energy sources of magnetospheric origin. At intermediate (0.1 km < < < 10 km) and small (λ < 100 m) scales, plasma instabilities and cross-field plasma diffusion are often the dominant physical processes controlling the plasma structure. However, because nonlinear plasma processes can couple structures in one scale length regime to other spatial frequencies, the entire spectrum of irregularities must be studied as a whole.

Energy dissipation in structured electrodynamic environments

The coupling of electromagnetic energy into the ionosphere and thermosphere is an essential consideration for understanding the thermal structure and dynamics of the neutral and charged particles at high latitudes. Often the dissipated electromagnetic energy exceeds that deposited by precipitating energetic particles at high latitudes. It is usually assumed that the profile of the ion Pedersen conductivity determines the altitude dependence of the energy dissipation rate. Herein the authors point out the strong altitude dependence of the energy dissipation rate on the spatial scale size of the imposed electric field. To illustrate the importance of such considerations, they show examples of the ubiquity of electric field structure in the high-latitude ionosphere; this is particularly prominent when the interplanetary magnetic field has a northward component. They then show quantitatively how the existence of electric field structure with scale sizes of 10 km or less strongly impacts both the altitude extent over which the electromagnetic energy is dissipated and its partitioning between current systems perpendicular and parallel to the magnetic field.