P. M. Kintner

POLAR observations of coherent electric field structures

The POLAR plasma wave instrument often detects coherent electric field structures in the high altitude polar magnetosphere. The structures appear to be positively charged potentials which are found to move both up and down the ambient magnetic field. Typical estimated velocities and parallel scale sizes are the order of 1000 km/s and 100-1000 meters, respectively. We have observed the structures at radial distances of 2.02 to 8.5 Re and L shells of 6 - 12+, although the they are likely to occur over a broader range of space than suggested by this initial study. The structures are responsible for some of the the spectral features of broadband electrostatic noise, and are similar to recent GEOTAIL and FAST observations of solitary waves.

Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer

A portable civilian GPS spoofer is implemented on a digital signal processor and used to characterize spoofing effects and develop defenses against civilian spoofing. This work is intended to equip GNSS users and receiver manufacturers with authentication methods that are effective against unsophisticated spoofing attacks. The work also serves to refine the civilian spoofing threat assessment by demonstrating the challenges involved in mounting a spoofing attack.

Transverse ion acceleration by localized lower hybrid waves in the topside auroral ionosphere

Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O+ source region below 2000 km. In this paper we demonstrate that intense (100–300 mV/m) lower hybrid waves are responsible for transversely accelerating H+ and O+ ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at altitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse.

Simultaneous Global Positioning System observations of equatorial scintillations and total electron content fluctuations

One aspect of the Global Positioning System (GPS) is the potential to conduct geophysical research, and worldwide networks of GPS receivers have been established to exploit this potential. Several research groups have begun using this global GPS data to study ionospheric total electron content (TEC) variations, also referred to as GPS phase fluctuations, as surrogates for ionospheric scintillations. This paper investigates the relationship between GPS amplitude scintillations and TEC variations for the same line of sight using observations from Ancon, Peru. These observations were taken under equatorial spread F conditions for three nights in April 1997. As expected, only when the spectrum of TEC fluctuations includes significant power at the Fresnel scale do scintillations appear. We also find that when the TEC fluctuation spectrum includes the Fresnel scale, the S4 scintillation index is roughly proportional to measures of TEC fluctuation for the weak scintillations observed. The proportionality constant varies from night to night, however, casting doubt on the ability to predict GPS S4 successfully from TEC fluctuation data alone. We also present a simple theoretical phase screen model and show that if a relationship between TEC fluctuation measures and S4 exists, that relationship depends on the power spectrum of phase variations at the screen. Unfortunately, the available TEC data, at 30 s per sample (with some aliasing apparently permitted), offer limited spectral information. A preliminary comparison of 1 s/sample data with the same data decimated to a 30 s/sample interval suggests, however, that the level of successful S4 prediction, based on TEC fluctuation measures alone, is comparable at either sample rate.

Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves

High-resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB-ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad-energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB-ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so-called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB-ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earths magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower-frequency part (30–400 Hz) of the BB-ELF emissions. The BB-ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfven waves (SKAW), or when large-amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large-scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB-ELF emission causes intense transverse ion heating (TAI) is suggested.

SCIFER‐Transverse ion acceleration and plasma waves

SCIFER encountered the cleft ion fountain within the cleft ionosphere at 1000 MLT and 1400 km altitude where it was possible to investigate the fine structure of transverse ion acceleration (TIA). Latitudinally narrow (30 km) regions of TIA were found to be closely correlated with broadband low frequency electric fields and reduced ionospheric density. The low frequency electric fields extended up to a few kHz with the largest amplitudes of about 10-20 mV/m p-p occurring below 400 Hz. No spectral features ordered by the ion cyclotron frequencies were observed. Outside regions of TIA the ionospheric density was typically 2×10 3 cm -3 while inside regions of TIA the density dropped to 5×10 2 cm -3 . The correlation between TIA, reduced ionospheric density and broadband low frequency electric fields is so exact, sometimes within a few hundred meters, we interpret the broadband low frequency electric fields as current-driven electrostatic waves, perhaps a mixture of ion cyclotron and ion acoustic waves.

Freja observatons of correlated small-scale density depletions and enhanced lower hybrid waves

Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small-scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.

Fading timescales associated with GPS signals and potential consequences

The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of scintillation timescales on the code division multiple access (CDMA) protocol used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelation time (width), with the speed estimated using spaced receivers, and the two independent estimates of speed are shown to possess a linear relationship. Next we show that the scintillation pattern drift speed is given by the difference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds match, the fade timescales lengthen. Additionally, if the fade depth is adequate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of tracking. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving from west to east at speeds of 40–100 m/s (144–360 km/h). This is in the range of typical aircraft speeds.

Interferometric determination of broadband ELF wave phase velocity within a region of transverse auroral ion acceleration

Broadband electric field fluctuations with typical amplitudes of 10–20 mV/m peak-to-peak and frequencies from 0 Hz to 3 kHz (BB-ELF) were observed coincident with a region of ≤200 eV transverse H+ acceleration (TAI) near the poleward edge of the pre-midnight aurora. The coherence and phase velocity of the electric fields were measured using a interferometric antenna array over the frequency range of ≈ 100 Hz to 3 kHz. These electric field fluctuations were found to have the following characteristics: 1) incoherence perpendicular to the geomagnetic field, 2) coherence parallel to the the geomagnetic field, 3) parallel phase velocity (ω/k∥) of 30–35 km/s upwards, 4) 0 < |k∥/k⟂| < 0.22. We show that these properties are compatible with the emission being electrostatic H+ cyclotron (EHC) waves. We also discuss possible generation mechanisms for the waves, and their relationship to the TAI.

The status of observations and theory of high latitude ionospheric and magnetospheric plasma turbulence

A review is given for the current status of both observations and theory of high latitude ionospheric plasma turbulence. The principal purpose of this review is to draw connections between the existing body of experimental evidence of fluid-like plasma turbulence in the ionosphere and the predictions of various fluid plasma models which have been proposed to describe the dynamics of the turbulent ionosphere. To place the ionospheric problem in perspective, a tutorial summary of 3-D and 2-D turbulent cascade theory is included along with citations of its applications in neutral fluid turbulence and the supportive experimental evidence found mainly in atmospheric flows. The high latitude observational evidence for low frequency (ω < Ωi) macroscale (L > ϱi) turbulence is summarized. The evidence includes observations of irregularities which occur over an altitude range of 400 km to 8000 km or possibly higher, and vary over scale sizes of 5 meters (ϱi) to about 2000 km. At the shorter wavelengths the irregularities are known to be nonpropagating hence convective nonlinear processes are important and this suggests that turbulence is the rule. The mathematical description of the turbulent dynamics of much of the ionosphere is suggested to be embodied in a low frequency fluid model, the essential features of which have been discussed in the literature. Application of standard cascade theory to this model leads to testable predictions of the power spectra for the density and electric field. A comparison of the spectral theory and the observations indicates significant agreement in some cases and ambiguity in others, but no apparent contradictions. Suggestions are made for future experimental diagnostics which could resolve ambiguities.