J. C. Samson

Proton aurora and substorm intensifications

Ground based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorm intensifications is formed within a region of intense proton precipitation that is well equatorward (approximately four to six degrees) of the nightside open-closed field line boundary. The precipitating protons are from a population that is energized via earthward convection from the magnetotail into the dipolar region of the magnetosphere and may play an important role in the formation of the electron arcs leading to substorm intensifications on dipole-like field lines.

Standing ULF modes of the magnetosphere: A theory

Field line resonances with frequencies in the range 1 to 4 mHz have recently been observed by the JHU/APL HF Doppler radar during quiescent geomagnetic conditions. These structures are observed to have stable frequencies for durations of several hours, leading us to the conclusion that they may be standing waves (in the radial direction, as opposed to standing waves along a field line) of the magnetosphere driven by the solar wind. Using this premise, the propagation of fast mode ULF waves in the magnetosheath and the near earth magnetosphere are examined in an ideal, linearized MHD context. A model is presented in which fast waves propagate in the equatorial plane between the flanks of the bow shock and a turning point deep within the magnetosphere. Due to the magnetic field gradient near the earth, a field line resonance develops between the turning point and the plasmapause. Using a realistic set of magnetospheric parameters, it is possible to reproduce the set of observed frequencies and the respective positions of their field line resonances within the ionosphere (assuming a dipole mapping). However, because the model cavity frequencies are sensitive to magnetosheath parameters, this model does not explain the extreme stability with respect to geomagnetic conditions of the observed frequencies.

Dayside ionospheric convection changes in response to long-period interplanetary magnetic field oscillations - Determination of the ionospheric phase velocity

Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north-south changes of the IMF. During the 6-hour interval in question IMF Bz oscillates between ±7 nT with about a 1-hour period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5° magnetic latitude, but separated by ∼2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (∼10 MLT) and then spread outward toward dawn and dusk with a phase speed of ∼5 km s−1 over the longitude range ∼8 to 12 MLT, slowing to ∼2 km s−1 outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds. We interpret these observations in terms of the ionospheric response to the onset, expansion and decay of magnetic reconnection at the dayside magnetopause.

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Influence of the westward travelling surge

Abstract This paper expands the earlier results of Rostoker and Samson (1981), who noted that there are two latitudinal areas of Pi 2 localization near the high latitude, substorm enhanced electrojets. The detailed study presented here outlines the morphology of the polarizations of the Pi 2s in and near the westward travelling surge. There are two latitudinal areas of Pi 2 localization. A poleward Pi 2 predominates within the surge and to the East, whereas an equatorward Pi 2 predominates equatorward and West of the surge. These Pi 2 localizations appear to correlate with the substorm enhanced westward and eastward electrojets respectively. However, the maximum in the Pi 2 power does not always coincide with the center of the electrojet. The poleward Pi 2 has largest amplitudes to the East of the head of the westward travelling surge. This Pi 2 shows a latitudinal polarization reversal from clockwise on the equatorside (viewed down on H - D plane) to counterclockwise on the poleside of a latitudinal demarcation line, which occurs just poleward of the initial breakup. This demarcation line is usually equatorward of the most poleward expansion of the surge. To the West of the surge front, where the equatorward Pi 2 predominates, there is again a latitudinal polarization reversal but in this case the polarization is counterclockwise equatorward and clockwise poleward of the demarcation line. This demarcation is equatorward of that for the poleward Pi 2, and appears to lie at the latitude of the initial breakup. Consequently, the westward travelling surge appears to mark the longitudinal transition from equatorward to poleward Pi 2. The elliptical polarization of the Pi 2s is most likely caused by azimuthai (longitudinal) expansion of the field-aligned currents in the surge, in association with reflection of the field-aligned current pulses from northern and southern high latitude ionospheres.

The temporal variation of the frequency of high latitude field line resonances

The diurnal variation in the frequencies of the continuum of ULF field line resonances has been calculated by using the cross-spectral phase of the north-south components of data from latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. On most days the continuum is seen only during the local daytime, and only a single harmonic with an inverted U-shaped temporal variation in frequency is seen. At 67° geomagnetic latitude (L = 6.6) the general trend is a resonant frequency around 2 mHz near local dawn, increasing up to ∼5 mHz by 0600-0700 local time, followed by a decrease in frequency to 2 mHz by 1500–1600 local time. Near local noon, the fundamental resonant frequency is ∼3 mHz at 71° (L = 11.3), increasing monotonically to 7 mHz at 65° (L = 6.1). The waves appear to be a part of the resonant Alfven mode continuum as opposed to the single-frequency, driven magnetic field line resonances often seen at high latitudes. The cross-phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magnetometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma MHD theory. With the use of the observed resonant frequencies, the plasma density for June 1, 1990, was 4.2 × 106 H+/m³ at L = 6.6 while the data for June 7, 1990, showed densities up to 100×106 H+/m³. These results suggest that observations of the magnetohydrodynamic continuum in the magnetometer data may give a very effective method for ground-based time-dependent mapping of the equatorial plasma density.

Variation of plasmatrough density derived from magnetospheric field line resonances

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north-south components recorded at seven latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. CANOPUS magnetometers range in latitude from Rankin Inlet (L = 12.4) south to Pinawa (L =4.3). Using cold plasma MHD theory, an R−4 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values are 1–20 H+cm−3 for the plasmatrough and 50–200 H+cm−3 for the plasmasphere. The data recorded on June 7, 1990, shows an afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground-based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989-046 geosynchronous spacecraft. These comparisons show that the ground-based technique should allow a robust procedure for calculating dayside, time-dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.

Auroral disturbances during the January 10, 1997 magnetic storm

It is well known that intense and frequent auroral-zone disturbances, often attributed to substorms, occur during magnetic storms. We examine observations during the January 10, 1997 main phase and find that observed auroral-zone activity was dominated by a combination of global auroral and current enhancements, which are a direct response to solar wind dynamic pressure enhancements, and poleward boundary intensifications, which are localized in longitude and have an auroral signature that moves equatorward from the magnetic separatrix. Poleward and azimuthally expanding regions of auroral activity which accompany substorms are found to contribute significantly less to the observed activity. This suggests that poleward boundary intensifications and dynamic pressure responses may be an important cause of disturbances during periods of enhanced convection such as magnetic storms and convection bays.

Parallel electric fields in dispersive shear Alfvén waves in the dipolar magnetosphere

Existing theories do not explain large parallel electric fields that are associated with keV electron precipitation in auroral arcs. The MHD electron response results in an electrical conductivity in the low altitude magnetosphere that is two orders of magnitude greater than is required. We suggest a new mechanism for forming parallel electric fields based on a nonlocal electron response to standing shear Alfven waves on dipole magnetic fields. Electron trapping is the primary cause of a significant reduction in the collisionless electron conductivity and consequent enhancement of parallel electric fields in the 1–4 mHz frequency range.

Asymmetric multiple auroral arcs and inertial Alfvén waves

High-resolution optical observations by the University of Calgary Portable Auroral Imager show a frequent occurrence of asymmetric multiple small-scale auroral arc structures during auroral substorms. Whereas the classical multiple arc array tends to exhibit a fairly symmetrical configuration, with parallel motions within individual discrete arcs being opposite in direction across the center of the arc array, the multiple arcs to be discussed herein are distinguished by the presence of discrete arcs strictly equatorward of the two bright counter-streaming arcs that would ordinarily define the center of the arc array. The intensity of these parallel equatorward-lying arcs were in most cases found to decrease rapidly in the equatorward direction. By considering the topology of the structures and the spacing between arcs, observations are found to be consistent with recent theories suggesting inertial Alfven waves as a possible cause of fine-scale auroral arcs.

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Location of source regions with respect to the auroral electrojets

Abstract Substorm onsets and intensifications are accompanied on a one-to-one basis by a Pi 2 magnetic pulsation burst. The source region for these pulsations is generally thought to lie in the region of substorm disturbance in the auroral oval. In this paper we outline the characteristics of Pi 2 pulsations in regions near the substorm enhanced electrojet but removed from the locale of the westward travelling surge. We show that a resonance region for the pulsations lies at the equatorwad edge of the westward electrojet, which in the evening sector marks the locus of the Harang discontinuity. Finally we show examples where the maximum amplitude of the Pi 2 is located at or equatorward of the southern border of the eastward electrojet or at the southern border of the westward electrojet. This is clear evidence for the coupling of wave energy into the L -shells far distant from the source of the energy. Mechanisms for Pi 2 generation are discussed in the context of the results presented in this paper.