W. J. Hughes

Theory of hydromagnetic waves in the magnetosphere

Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.

An illustration of modification of geomagnetic pulsation structure by the ionosphere

In this paper we illustrate the results of a companion paper (Hughes and Southwood, 1976) in which we describe how geomagnetic pulsations are screened by the atmosphere and ionosphere. Here, using Fourier synthesis, we map down to the earths surface the fields predicted in the vicinity of a resonating magnetospheric field line by Southwood (1975a). The results demonstrate that the ionosphere-atmosphere system screens out rapid horizontal variation and also provide an illustration of the ionospheric polarization rotation phenomenon. Comparison with recent experimental work is made.

Dynamics of the inner magnetosphere near times of substorm onsets

The electrodynamics of the inner magnetosphere near times of substorm onsets have been investigated using CRRES measurements of magnetic and electric fields, energetic electron fluxes, in conjunction with ground-based observations. Six events were studied in detail, spanning the 2100 to 0000 MLT sector and L values from 5 to 7. In each case the dawn-dusk electric field was enhanced over typical background electric fields, and significant, low-frequency pulsation activity was observed. The amplitudes of the pulsations were larger than the background electric fields. Dusk-dawn excursions of the cross-tail electric field often correlated with changes in currents and particle energies at CRRES and with ULF wave activity observed on the ground. Variations of the electric field and Poynting vectors with periods in the Pi 2 range are consistent with bouncing AlfVen waves that provide electromagnetic communication between the ionosphere and plasma sheet. Magnetic signatures of field-aligned current filaments directed away from the ionosphere, presumably associated with the substorm current wedge, were observed during three orbits. In all cases, ground signatures of substorm expansion were observed at least 5 min before the injection of electrons at CRRES. Field-aligned fluxes of counter-streaming, low-energy electrons were detected after three of the injections. We develop an empirical scenario for substorm onset. The process grows from ripples at the inner edge of the plasma sheet associated with dusk-dawn excursions of the electric field, prior to the beginning of dipolarization. Energy derived from the braking of the inward plasma convection flows into the ionosphere in the form of Poynting flux. Subsequently reflected Poynting flux plays a crucial role in the magnetosphere-ionosphere coupling. Substorms develop when significant energy (positive feedback?) flows in both directions, with the second cycle stronger than the initial. Pseudobreakups occur when energy flow in both directions is weak (negative feedback?). “Explosive-growth-phase” signatures occur after onset, early in the substorm expansion phase. Heated electrons arrive at the spacecraft while convection is earthward, during or at the end of electromagnetic energy flow away from the ionosphere.

CRRES Poynting vector observations of electromagnetic ion cyclotron waves near the plasmapause

Electromagnetic ion cyclotron waves have been observed by the Air Force Geophysics Laboratory triaxial fluxgate magnetometer on the CRRES satellite in the vicinity of the plasmapause. Specific wave events in the 1300–0100 MLT local time sector and over L = 4.8 – 6.7 at magnetic latitudes between ± 20° have been studied with respect to energy propagation. Using the wave magnetic field data and the orthogonal two-component Berkeley electric field experiment data allows the computation of the Poynting vector. Nine events with frequencies below 0.6 Hz have been analyzed. General wave properties showed intensities in the range 4–18 μW m−2. In most events, energy propagation was predominantly away from the equator, supporting the existence of an equatorial source region. There are exceptions and one event, observed 3° from the equator, also showed some propagation back to the equator with equal energy. Four periodic wave packets were seen in another event. This event showed unidirectional propagation away from the equator over all packets with periodicity approximately one-half of the calculated time for a typical bouncing wave packet. Consequently, the periodicity is not the signature of a classical periodic structured Pc 1–2 pulsation event of the type seen on the ground. Generally, the results for these waves with f < fHe+ may be interpreted by assuming an ion cyclotron instability source located near the equator and irregularly emitting wave trains of energy into one or both hemispheres, with minimal reflection at the ionosphere.

Response of the ionosphere to a density pulse in the solar wind: Simulation of traveling convection vortices

We show the response of the Earths magnetosphere and ionosphere to a density pulse in the solar wind using a global 3D MHD simulation model. Flow vortices are generated in the ionosphere and they exhibit many properties similar to those observed during impulsive traveling convection vortices. Two oppositely rotating flow vortices are formed at about 70° magnetic latitude near noon. They separate and move down the morning and evening flanks, greatly weakening as they pass the terminator. Meanwhile a second pair of flow vortices appears at noon at a slightly higher latitude and with opposite flow directions than the first pair. The second pair follows the first and also fades as it reaches the nightside. The results are interpreted as a hydromagnetic wave propagating in the inhomogeneous magnetosphere plasma.

Reconfiguration timescales of ionospheric convection

Changes in ionospheric flow patterns provide direct observations of how the magnetosphere-ionosphere system is coupled to the IMF. The nature and location of reconnection on the magnetopause is communicated to the ionosphere, which responds by reconfiguring global convection patterns. We investigate the dynamics of the changing global ionospheric convection patterns using a large two-dimensional array of ground magnetometers. We have found that for sharp north to south transitions of the IMF, dayside ionospheric convection patterns globally reconfigure themselves in timescales as short as 8 minutes. We find that all local times initially respond within 1-2 minutes of the first detectable response in the ionosphere. The length of time (after the initial onset) required for the ionosphere to transition from its initial state to its new state, which we refer to as the reconfiguration time, is measured. IMF Bz northward to southward transitions have reconfiguration times near local noon of approximately 5 minutes and lengthen approximately 2 minutes for every 3 hours of local time away from noon.

Effects of nightside O+ outflow on magnetospheric dynamics: Results of multifluid MHD modeling

[1] Through spacecraft observations and numerical modeling, it is becoming increasingly well established that ionospheric oxygen is present in the magnetosphere in amounts that can have an effect on magnetospheric dynamics, affecting pressure balance, currents, convection flows, and cross polar cap potential (CPCP). However, an understanding of the various processes through which oxygen may bring about changes in magnetospheric structure and dynamics is still lacking. In this paper, we focus on the role that ionospheric oxygen outflow from the nightside auroral zone plays in determining plasma sheet density, size, and pressure and relate the changes in plasma sheet parameters to changes in CPCP, tail geometry, and magnetospheric convection. We use the Multifluid Lyon-Fedder-Mobarry model as a virtual laboratory to analyze the effects of ionospheric oxygen on magnetospheric parameters. Our simulations use idealized solar wind conditions, constant and uniform ionospheric conductance, and constant nightside oxygen outflow in order to isolate the effects of a change in outflow flux. We concentrate the ionospheric outflow in the nightside auroral region, so as to avoid the possibility of directly loading the distant tail X line with oxygen and hence to avoid a direct modification of the reconnection rate in order to focus on effects within the magnetosphere. We show that the presence of O + outflowing from an isolated patch in the nightside auroral zone increases plasma sheet density and thermal pressure, slows convection, decreases the polar cap potential, and increases the length and width of the nightside magnetosphere. We determine that ionospheric O + mass loading can play a significant role but that strong effects on magnetospheric dynamics require a nightside flux that significantly exceeds statistical observed levels. A more realistic outflow would include both dayside and nightside sources whose total combined flux would be similar to the total fluxes used here.

DMSP F8 observations of the mid‐latitude and low‐latitude topside ionosphere near solar minimum

The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840-km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light-ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field-aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E {times} B drifts qualitatively explain most features of these composition and velocity measurements. 23 refs., 5 figs., 2 tabs.

Characteristics of broadband ULF magnetic pulsations at conjugate cusp latitude stations

Although cusp latitude pulsation studies have for the most part focused on narrowband waves, analysis of magnetometer data from the Arctic has shown that the most common type of dayside long-period ULF wave activity at very high latitudes is broadband noise (Pi1-2), and that its occurrence and intensity is largely controlled by solar wind velocity [Engebretson et al., 1995]. However, the origin of temporal variations in the intensity of these waves is not understood. In order to further investigate these broadband waves and their origins, we present a similar data set from another season, data from a roughly conjugate site, and multi-instrument data. Comparison of conjugate station data revealed that there was a substantial fraction of days during which there was significant temporal disagreement between hemispheres, but the solar wind velocity still appears to control overall daily intensity in broadband power. The coincidence of increased riometer absorption from conjugate locations with strong broadband ULF wave power suggests that precipitating energetic particles are responsible for much of the broadband ULF noise, and further suggests that high solar wind velocity plays a role in precipitation of significant fluxes of energetic particles. Quantitative estimates based on riometer and photometer observations also indicate that modulated electron precipitation is sufficient to drive the broadband pulsations. We review possible source mechanisms for these broadband waves and the precipitating electrons associated with them. Finally, the clear temporal association between these waves and Pc5 waves on closed field lines may suggest a causal connection via modulations of a three-dimensional current system.

Ground observations and possible source regions of two types of Pc 1‐2 micropulsations at very high latitudes

We have used 1-years data from the recently installed Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada and from two stations of the developing “conjugate” array of Automated Geophysical Observatories (AGOs) in Antarctica to study ULF waves in the Pc 1–2 (100–600 mHz) frequency band at cusp and polar cap latitudes (Λ ∼ 74° – 80°). In this paper we focus on the spectral properties and latitudinal and local time distributions of Pc 1–2 events observed during 1994 and use these along with several case studies to infer the source locations of the two major wave types we have observed. We found little variation in center-band frequency of the Pc 1–2 waves we observed, but the average event bandwidth was distinctly wider at stations near 80° MLAT than at stations near 75° MLAT. Broadband waves, with diffuse spectral character, dominated at the higher latitudes, but their occurrence was confined at most stations to within 4 hours of local magnetic noon. Waves with narrower bandwidth were much more common in our data set, and were the statistically dominant wave type at the lower-latitude MACCS stations. Their occurrence was also limited to the dayside but extended both later and more widely in local time than the more broadband waves. These multistation observations, combined with data from the DMSP, IMP 8, and Geotail satellites, suggest the possibility that these two wave types originate in quite different regions near the magnetospheric boundary; the more narrowband waves in the subsolar and postnoon equatorial region, and the more broadband waves in the high-latitude plasma mantle (and possibly at the poleward edge of the cusp). The cusp itself appears to not be a significant source of Pc 1–2 wave activity that can be detected by ground observatories.