S. C. Buchert

Simultaneous EISCAT Svalbard and VHF radar observations of ion upflows at different aspect angles

A simultaneous EISCAT Svalbard and VHF radar experiment has shown that field-aligned (FA) ion upflows observed at an altitude of 665 km in the dayside cusp are associated with significant anisotropy of ion temperature, isotropic increases of electron temperature and enhancements of electron density. There is no clear correspondence between the enhancements of the electric field strength and the occurrence of the ion upflows. This suggests that the upflow is driven primarily by precipitation. The data support that in addition to “direct” precipitation effects, namely enhanced ambipolar diffusion and heat flux, also wave-particle interaction, like wave-induced transverse ion heating, which causes a hydrodynamic mirror force, may play a role.

Statistical characteristics of electromagnetic energy transfer between the magnetosphere, the ionosphere, and the thermosphere

We have determined, based on 28 days of European Incoherent Scatter Common Program 1 mode I data obtained between 1989 and 1991, statistical characteristics of the energy-coupling processes between the lower thermosphere, ionosphere, and magnetosphere through an analysis of the electromagnetic energy transfer rate J·E, the Joule heating rate J·E′, and the mechanical energy transfer rate U·(J×B) at altitudes of 125, 117, 109, and 101 km. At all altitudes the input electromagnetic energy is distributed to both Joule heating and mechanical energy. The energy distributed to Joule heating is larger than that to mechanical energy, but the latter is generally not negligible. All three rates respectively have two maxima, not in the midnight region but in the dawn and dusk. The enhancements of these rates have positive correlations with the increase of geomagnetic activity represented by the Kp index. The electromagnetic energy transfer rate is greatest at 117 km, becoming smaller with decreasing altitude. It is mostly positive but can be negative. At 117 km the mechanical energy transfer rate is considerably smaller than the electromagnetic energy transfer rate, suggesting that most of the electromagnetic energy at this altitude is converted to Joule heating and a small portion of the electromagnetic energy goes to mechanical energy. At 125 km the mechanical energy transfer rate is larger than that at 117 km. On average, 65% of the input electromagnetic energy is converted to Joule heating and 35% is converted to neutral mechanical energy. At 109 and 101 km altitude the mechanical energy transfer rate becomes negative, hence the Joule heating rate is greater than the electromagnetic energy transfer rate, suggesting that not only electromagnetic energy but also mechanical energy contribute to Joule heating.

Naturally enhanced ion-acoustic lines seen with the EISCAT Svalbard Radar

Abstract With the EISCAT radars on the Scandinavian mainland it has been observed, that events of high electron temperatures and upward ion flows in the upper F region are often well correlated. During such events one can also frequently witness the development of so-called naturally enhanced ionacoustic shoulders in the radar spectra. The origin of the possibly superthermal plasma fluctuations, which cause the unusual echoes, has not yet been clearly identified. It is shown in this work, that very similar events, namely enhanced radar power after a period of increasing electron heating in the F region, can also be seen with the new EISCAT Svalbard Radar (ESR) at 74° magnetic latitude on the dayside. IS radars actually sense plasma fluctuation at slightly different wavelengths by varying cyclically the transmitter frequency. At ESR, during the superthermal enhancements, the received power seems to change significantly over the applied frequencies. This new observation favours a recently suggested model where ion-acoustic fluctuation are caused by the parametric decay of beam induced Langmuir waves. Other models of ion-acoustic “turbulence” would predict a smooth, Kolmogoroff-type k-spectrum, but in the parametric decay model the power of waves produced by this mechanism would indeed depend strongly in their wavelength.

Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study

The Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in ...

Generation of atmospheric gravity waves associated with auroral activity in the polar F region

Relations between auroral activities and the generation of neutral-wind oscillations in the polar F region (150–300 km) were investigated using data from the European Incoherent Scatter (EISCAT) radar, the all-sky auroral camera, and the IMAGE (International Monitor for Auroral Geomagnetic Effects) magnetograms. We dealt with two cases: observations on March 1, 1995 (case 1), and on March 29, 1995 (case 2). For both cases the field-aligned component of the neutral-wind velocity estimated from EISCAT radar data had dominant oscillation periods of 20–30 min, which are longer than the typical Brunt-Vaisala period in the polar F region (≃13 min). The observed oscillations showed the downward propagation of the phase with time. These properties on the oscillation period and the phase are general ones of atmospheric gravity waves (AGWs). For case 1 the all-sky auroral images obtained at Kilpisjarvi showed the auroral arc extending in an almost zonal direction near a distance estimated using wave parameters derived from the equation of the dispersion relation for AGWs applicable to the observed oscillations. This suggested that the auroral arc appeared to be the effective generator of the observed oscillations. The comparison of observed phase lines with predicted ones using models by Francis [1974] and Kato et al. [1977] showed agreements between the two for both cases. The comparison suggests that effective parameters of the wave source in characterizing neutral-wind oscillations would be the horizontal distribution of the wave source and the distance between the observing point and the source region. It was concluded that geomagnetic activities on March 1 and 29, 1995, in northern Scandinavia significantly related to the generation of the observed oscillations. The conclusion implies that geomagnetic activities at high latitudes are an important source to generate AGWs, as indicated by previous theoretical studies.

The motion of ions in the auroral ionosphere

The ion motion in the E and F regions relative to the local magnetic field line and the horizontal plane has been determined from the European incoherent scatter (EISCAT) common program 1 version I (CP-1-I) data. The ion motion highly depends on altitude and magnetospheric electric field strength. Under the condition of a strong electric field (|E| > 25 mV/m), ions move perpendicularly to the local magnetic field line both in the F region and at the upper part of the E region (≥117 km). This suggests the ions moving by the E × B drift, which is expected when the ion-neutral collision frequency is much smaller than the ion gyrofrequency. At lower altitudes, however, the ions no longer move perpendicularly to the magnetic field line but move horizontally, as expected from a stronger interaction between the ions and neutrals. Under the condition of a weak electric field (5 mV/m < |E| < 10 mV/m) the neutral winds tend to move the ions horizontally everywhere in the E region and even in the F region. The main cause of these ion motions and their dependence on altitude and electric field strength comes from the relative importance of the magnetospheric electric field and the large-scale neutral wind drag to the ionospheric ions.

On the source of the polar wind in the polar topside ionosphere: First results from the EISCAT Svalbard radar

[1]xa0We present quantitative radar observations of both hydrogen ion (H+) and oxygen ion (O+) upflow in the topside polar ionosphere using measurements that were recently carried out with the EISCAT Svalbard Radar and the Reimei satellite. H+ upflow was clearly observed equatorward of the cusp above 500 km altitude. Within the cusp the H+ density was very low, and the upflow was dominated by O+ ions, but on closed field lines the H+ became the larger contributor to the upward flux above about 550 km. The total flux seemed to be conserved, and so below 550 km altitude O+ (with a small upward velocity of ∼50 m s−1) appeared to determine the upward flux which was then maintained by H+ in the topside ionosphere. We also found that the H+ density in the topside polar ionosphere was several times higher than current predictions of ionospheric models like IRI2001.

Ionospheric conductivity modulation in ULF pulsations

Using incoherent scatter radar and magnetometer measurements, we report that during terrestrial magnetic Pc5 pulsations in the afternoon sector, a modulation of particle precipitation and ionospheric conductivities by a factor of 2 occurs in addition to high-amplitude variations of electric and magnetic fields. The event thus seems to be considerably more complicated than previously studied ones where information about conductivities was mostly not available. Our ground-based data set gives us several clues about magnetospheric processes. The origin of the conductivity variations seems to be periodically modulated diffusion of hot electrons into the loss cone that is in turn caused by a ring current instability. The direction of the phase propagation of the observed disturbances is also consistent with the hypothesis of a ring current source. From the ionospheric electron densities we can roughly estimate the equatorial phase space diffusion rate which seems relatively high. In addition, strong electric field and Poynting flux variations suggest that intense coupling to shear Alfven modes happens in the magnetosphere. The latitudinal variation of power and wave polarization shows features of a field line resonance. Furthermore, power spectral analysis of conductivities, electric and magnetic fields, reveals that there is a turbulent-like background in all three parameters, which is of magnetospheric origin but modified by the ionosphere. The power law slope of the conductivity spectra is comparable to that of the electric field, while the ground magnetic field shows a steeper decrease with frequency because of the shielding of small-scale current structures. A clear anticorrelation between conductivities and the eastward electric field is interpreted as an ionospheric polarization effect, which transmits Alfven waves from the ionosphere upward. Finally, we show that due to the time-varying conductivities only the handedness (ratio of left- and right-handed components) of the Hall current is very close to that of the magnetic field, while the electric field has a significantly different polarization.

Thermospheric and ionospheric dynamics in the auroral region

Abstract Behavior of the thermosphere and the ionosphere in the auroral region, especially near an auroral arc, is quite complicated. There have been a number of reports on vertical neutral winds in the auroral region, suggesting that heat sources associated with local auroral activities could cause extremely large upwelling and downwelling of the thermosphere. A two-dimensional nonhydrostatic thermosphere-ionosphere model is used to study variations in the thermosphere and the ionosphere associated with a moving auroral arc. The results are compared with data obtained by the EISCAT (European Incoherent Scatter) radar measured in the vicinity of auroral arcs. The overall behavior of the observed ion motion parallel to magnetic field lines is interpreted as perturbation caused by neutral wind driven by auroral arcs. The model calculation suggests that temporal and spatial variations in the heating region significantly influence the structure of neutral and ion motion near auroral arcs.

Field‐aligned ion motions in the E and F regions

[1]xa0Characteristics of field-aligned (FA) ion motions in the E and F region (below F peak) ionosphere have been determined based on an analysis of European incoherent scatter (EISCAT) Svalbard and Kiruna-Sodankyla-Tromso (KST) data. Sporadic/burst upward FA ion motions are observed in southward electric field enhancement regions or in the postmidnight region, while relatively stable downward FA ion motions are seen in northward electric field enhancement regions or in other regions. Long-term (diurnal) variations of these upward and downward FA ions are likely driven by the large-scale from day-to-night thermospheric wind. However, each short-lived upward FA flow has neither one-to-one correspondence to an enhancement (or depletion) of the electric field nor that of electron density. The driving mechanism of the short-lived upward flow cannot thus be understood yet, although atmospheric gravity waves can be one of the possible mechanisms to create the short-term upward FA ion motions in the southward electric field region or in the postmidnight region.