We have investigated characteristics of ion upflow and naturally enhanced ion‐acoustic lines (NEIALs) based on the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) data continuously obtained ...between March 2007 and February 2008. For the ion upflow study we have used approximately 78,000 field‐aligned profiles obtained with the ESR. For the NEIAL study we have identified approximately 1500 NEIALs in the ESR data at altitudes between 100 and 500 km. The occurrence frequency of ion upflow shows two peaks, at about 0800 and 1300 magnetic local time (MLT), while only one strong peak is seen around 0900 MLT for NEIALs. The upward ion flux also has only one peak around 1100–1300 MLT. The occurrence frequency of ion upflow varies strongly over season. It is higher in winter than in summer, whereas NEIALs are more frequent in summer than in winter. NEIALs frequently occur under high geomagnetic activity and also high solar activity conditions. Approximately 10% of NEIALs in the F region ionosphere were accompanied by NEIALs in the E region (occurred at altitudes below 200 km). About half of the E region enhanced echoes did not have an F region counterpart. Upshifted NEIALs dominate in the E region whereas downshifted NEIALs are usually stronger above an altitude of 300 km. The high occurrence frequency of NEIALs in the prenoon region (0800–1000 MLT) might be associated with acceleration of thermal ions to suprathermal ones. At the same MLT and geomagnetic latitude suprathermal ions and broadband extremely low frequency (BBELF) wave activity have been observed, according to previous studies.
We present observations from three Cassini flybys of Titan using data from the radio and plasma wave science, magnetometer and plasma spectrometer instruments. We combine magnetic field and cold ...plasma measurements with calculated conductivities and conclude that there are currents of the order of 10 to 100 nA m−2 flowing in the ionosphere of Titan. The currents below the exobase (∼1400 km) are principally field parallel and Hall in nature, while the Pedersen current is negligible in comparison. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 μV m−1.
Traditionally, due to observational constraints, ionospheric modelling and data analysis techniques have been devised either in one dimension (e.g. along a single radar beam), or in two dimensions ...(e.g. over a network of magnetometers). With new upcoming missions like the Swarm ionospheric multi-satellite project, or the EISCAT 3-D project, the time has come to take into account variations in all three dimensions simultaneously, as they occur in the real ionosphere. The link between ionospheric electrodynamics and the neutral atmosphere circulation which has gained increasing interest in the recent years also intrinsically requires a truly 3-dimensional (3-D) description. In this paper, we identify five major science questions that need to be addressed by 3-D ionospheric modelling and data analysis. We briefly review what proceedings in the young field of 3-D ionospheric electrodynamics have been made in the past to address these selected question, and we outline how these issues can be addressed in the future with additional observations and/or improved data analysis and simulation techniques. Throughout the paper, we limit the discussion to high-latitude and mesoscale ionospheric electrodynamics, and to directly data-driven (not statistical) data analysis.
The relationship between bulk ion upflows and suprathermal ions was investigated using data simultaneously obtained from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and the Reimei ...satellite. Simultaneous observations were conducted in November 2005 and August 2006, and 14 conjunction data sets have been obtained at approximately 630 km in the dayside ionosphere. Suprathermal ions with energies of a few eV were present in the dayside cusp region, and the ion velocity distribution changed from an isotropic Maxwellian near the cusp region to tail heating at energies above a few eV in the cusp region. The velocity distribution of the suprathermal ions has a peak perpendicular or oblique to the geomagnetic field, and the temperature of the suprathermal ions was 0.9–1.4 eV. An increase in the phase space density (PSD) of the suprathermal ions, measured with the Reimei, was correlated with bulk ion upflow observed at the same altitude using EISCAT, and with the energy flux of precipitating electrons with energies of 50–500 eV. The PSD also has a good correlation with the electron temperature, which was increased by precipitation, but not with the ion temperature (0.1–0.3 eV) at the same altitude measured with EISCAT. These results suggest that plasma waves such as broadband extremely low frequency (BBELF) wavefields associated with precipitation are connected to the bulk ion upflows in the cusp and effectively cause the heating of suprathermal ions. The heating of suprathermal ions disagrees with anisotropic heating due to O+−O resonant charge exchange.
Phased arrays provide new possibilities for remote sensing with radars. By imposing an azimuthal phase variation, electromagnetic beams that carry orbital angular momentum can be formed. Such beams ...have a phase structure that appears twisted and as a result an intensity null in the center of the beam cross section. Here we numerically investigate twisted beams for incoherent scatter radars that are used to study the ionosphere. We discuss the possibility of utilizing such radar beams to probe twisted beams of plasma waves and flows transverse to the beam axis, such as associated with auroral arcs. Transverse plasma flows may give rise to a rotational frequency shift of the scatter from a twisted beam and Doppler broadening due to the beam divergence, the latter also occurring with regular beams. Although the angular momentum effects of the considered large scale flows are generally small, sheared and vortical flows transverse to the beam axis can in principle be discriminated from unidirectional flows with beams carrying orbital angular momentum.
Key Points
Phased array antennas can be used to twist radar beams
Twisted radar beams can be used to detect angular momentum effects in plasma
Beam divergence causes a Doppler broadening of the incoherent scatter spectrum
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
A study is presented of the effect of plasma density irregularities in the auroral and equatorial electrojets on the DC currents giving rise to these irregularities. It is shown that the theory ...predicts extra dissipation and deviation in the direction of the mean current flow from that of E0 × B0/B2, in general agreement with observations. The average power input to the turbulent plasma, equal to the product of the externally applied electric field and the mean current, can be equated to the power input with that resulting in a plasma devoid of irregularities but with an increased collision frequency. Similarly one can compute the deviation in direction of the current flow due to the irregularities and compare this with the deviation caused in a plasma without irregularities, but with an increased collision frequency, and from this comparison assign an anomalous dissipation in the plasma caused by the irregularities. Our study applies elementary principles and deals with the mean current driven by an external field, including effects of the mean values of second order terms in the perturbed quantities. We do not discuss the elementary processes giving rise to the irregularities in the plasma; only their effect on the mean current flow. The anomalous dissipation or the increased apparent collision frequency, is related directly to the angular power spectrum of plasma density fluctuations. It is shown that the power dissipated by the DC current in these anomalous collisions is exactly the same as the power dissipated through normal collisions by the wave modes making up the power spectrum leading to the anomalous DC current dissipation. The study also suggests that the saturation of the instabilities, giving rise to the electrojet turbulence, is likely to be caused by the control of the current by the anomalous resistivity rather than by local nonlinear effects.
We present a detailed study of waves with frequencies near the proton gyrofrequency in the high-altitude cusp for northward IMF as observed by the Cluster spacecraft. Waves in this regime can be ...important for energization of ions and electrons and for energy transfer between different plasma populations. These waves are present in the entire cusp with the highest amplitudes being associated with localized regions of downward precipitating ions, most probably originating from the reconnection site at the magnetopause. The Poynting flux carried by these waves is downward/upward at frequencies below/above the proton gyrofrequency, which is consistent with the waves being generated near the local proton gyrofrequency in an extended region along the flux tube. We suggest that the waves can be generated by the precipitating ions that show shell-like distributions. There is no clear polarization of the perpendicular wave components with respect to the background magnetic field, while the waves are polarized in a parallel-perpendicular plane. The coherence length is of the order of one ion-gyroradius in the direction perpendicular to the ambient magnetic field and a few times larger or more in the parallel direction. The perpendicular phase velocity was found to be of the order of 100km/s, an order of magnitude lower than the local Alfvén speed. The perpendicular wavelength is of the order of a few proton gyroradius or less. Based on our multi-spacecraft observations we conclude that the waves cannot be ion-whistlers, while we suggest that the waves can belong to the kinetic Alfvén branch below the proton gyrofrequency fcp and be described as non-potential ion-cyclotron waves (electromagnetic ion-Bernstein waves) above. Linear wave growth calculations using kinetic code show considerable wave growth of non-potential ion cyclotron waves at wavelengths agreeing with observations. Inhomogeneities in the plasma on the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear effects or both of these should be taken into account.
Observations by the EISCAT Svalbard radar show that electron temperatures Te in the cusp electrojet reach up to about 4000 K. The heat is tapped and converted from plasma convection in the near Earth ...space by a Pedersen current that is carried by electrons due to the presence of irregularities and their demagnetising effect. The heat is transfered to the neutral gas by collisions. In order to enhance Te to such high temperatures the maximally possible dissipation at 50% demagnetisation must nearly be reached. The effective Pedersen conductances are found to be enhanced by up to 60% compared to classical values. Conductivities and conductances respond significantly to variations of the electric field strength E, and "Ohm's law" for the ionosphere becomes non-linear for large E.
Simultaneous European Incoherent Scatter (EISCAT) radar observations using two EISCAT radars, the Tromsø (69.6°N, 19.2°E) UHF radar and the EISCAT Svalbard radar (Longyearbyen, 78.2°N, 16.0°E), were ...conducted for 8 consecutive days, from 11 to 19 November 2003, to study the lower thermospheric wind dynamics in the polar region. Altitude profiles of the amplitudes of the diurnal and semidiurnal components at Tromsø and Longyearbyen are similar. The semidiurnal amplitudes in the meridional and zonal components exhibit maxima at 105–107 km, with values of ∼70–90 m s−1. The semidiurnal phases vary with roughly a 30 km vertical wavelength between 98 and 110 km. The quasi‐2 day wave (Q2DW) was not detected in the lower thermosphere at either Tromsø or Longyearbyen during the period, while it was found between 70 and 82 km in the mesosphere with the colocated Tromsø MF radar. This observational result suggests that in general, the Q2DW attenuates in the mesosphere and cannot penetrate into the lower thermosphere in winter. Ion drag acceleration of the wind is generally negligible below 107 km at Tromsø and below 118 km at Longyearbyen, but significant ion drag acceleration is found above these heights. A comparison of mean wind and tidal amplitudes and phases with National Center for Atmospheric Research Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model predictions for the period shows some agreement.
Naturally enhanced ion-acoustic lines (NEIALs) between 1200 and 1900 km altitude are investigated. The NEIALs were found in the background gates of data from the European Incoherent Scatter (EISCAT) ...Svalbard radar (ESR) at 78° N looking field-aligned. Only strongly enhanced lines are detected at such high altitudes. The estimated enhancement above incoherent scattering integrated over the antenna beam and preintegration time of 10 s reaches about 10 000. Both lines are always enhanced above 1000 km altitude, and the downshifted line, corresponding to upward propagating ion-acoustic waves, is always stronger than the upshifted line, for downgoing waves. The ratio of the downshifted and upshifted peaks is often remarkably constant along a profile. Using the line positions as indicators of the ion-acoustic speeds and the bulk drift velocity, we find that the bulk drift does not exceed the ion-acoustic (sound) speed, but extrapolation of the profiles suggests that the sound barrier is reached around 2000 km in one event. The highest ion-acoustic speed is seen near 600 km, above the density peak, indicating that electrons are heated not only by ionizing precipitation but significantly also by upgoing waves. Upflow continues to speed up above the estimated temperature maximum. A certain qualitative similarity to the solar corona seems to be the case.
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