L. Eliasson

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.

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.

Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines

Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-V-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy ({approximately} 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction. 16 refs., 4 figs.

Large‐scale auroral plasma density cavities observed by Freja

Freja, the joint Swedish and German scientific satellite, has an orbit inclination that allows it to traverse the auroral oval tangentially and stay for minutes on field lines connected to the auroral energization region. One signature of the auroral energization process is the heating/transverse energization of ionospheric ions. Associated with such transverse heating/energization of ionospheric ions is a depletion of cold plasma in the topside ionosphere. We have studied several Freja passes at ≈1700 km altitude with long time periods of plasma depletion and transverse ion acceleration. Inside these depletion regions the density may decrease by more than two orders of magnitude (from 1000 to ≈10 cm−3). This suggests that transverse ion heating is indeed a very strong mechanism for plasma density depletion in the topside ionosphere.

Transverse ion energization and wave emissions observed by the Freja satellite

Observations by the Freja satellite at altitudes of about 1700 km in the auroral zone sometimes show energization of both light and heavy ions to characteristic energies of up to about 50–100 eV. This ion heating is often associated with lower hybrid waves, and also with wave emissions at lower frequencies. No obvious decrease of electric field wave power below the proton gyrofrequency, reported by other spacecraft at similar altitudes, is observed in the regions of intense ion heating. Furthermore, localized density depletions associated with lower hybrid waves have not been detected in these regions. We present a preliminary model where waves observed during an event in the evening-side auroral zone are used to explain the observed ion energies.

The TESP electron spectrometer and correlator (F7) on Freja

The two-dimensional electron spectrometer on Freja consists of a ‘top-hat’-type electrostatic analyzer with the addition of entrance aperture deflection plates. The field of view of the concentric-hemisphere analyzer is modified from a plane to a cone up to 25° from this plane by applicaiton of bipolar high voltages to the deflection plates. Fast high-voltage sweeps allow full 10 eV–25 KeV, 500-point distribution function measurements in 32 ms. Constant-energy or limited energy-sweep modes allow time resolutions down to 1 ms.

Observations of an upward-directed electron beam with the perpendicular temperature of the cold ionosphere

The Freja TESP electron spectrometer has repeatedly observed similar to 100 eV - 1 keV upward-directed, anti-field-aligned electron beams near 1700 km altitude in the auroral zone. A particularly i ...

Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals

The characteristics of the fine scale electric field associated with the westward traveling surge and large-scale auroral spirals and surges are investigated using high-resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field-aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high-energy electron precipitation but very weak electric fields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in-between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.

Ionospheric signature of the cusp as seen by incoherent scatter radar

Measurements with the Sondre Stromfjord incoherent scatter radar, co-ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May–June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp-cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well-defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other day side precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field-aligned currents.

Freja observations of heating and precipitation of positive ions

The experiments on board Freja are designed to measure auroral particle energization processes with very high temporal and spatial resolution. One main scientific objective is to study ion heating transverse to the magnetic field lines in the auroral region. The Freja orbit with an inclination of 63° allows us to make detailed measurements in the nightside auroral oval during all disturbance levels. We concentrate here on two different observations of transverse ion energization at an altitude of about 1700 km in the northern hemisphere auroral region. The three-dimensional ion mass spectrograph has shown that both heavy and light ions are heated to energies most often in the range from a few eV to some hundred eV. Transversely heated ions are, however, also seen up to the present high energy limit of the hot plasma instrument, 4.5 keV. Ion conics are produced in regions with anisotropic electron fluxes as well as in regions of intense keV proton precipitation. Waves above the lower hybrid frequency are observed in the events presented in this report. These waves may play an important role in the ion heating process. The Freja data indicate that the waves are generated in different ways in these events. Thus, this preliminary investigation confirms that several scenarios are needed to explain the heating of ionospheric plasma and shows some of the possibilities for future studies.