Abstract. Several factors are known to control the HF echo occurrence rate, including electron density distribution in the ionosphere (affecting the propagation path of the radar wave), D-region radio wave absorption, and ionospheric irregularity intensity. In this study, we consider 4 days of CUTLASS Finland radar observations over an area where the EISCAT incoherent scatter radar has continuously monitored ionospheric parameters. We illustrate that for the event under consideration, the D-region absorption was not the major factor affecting the echo appearance. We show that the electron density distribution and the radar frequency selection were much more significant factors. The electron density magnitude affects the echo occurrence in two different ways. For small F-region densities, a minimum value of 1 × 10 11 m −3 is required to have sufficient radio wave refraction so that the orthogonality (with the magnetic field lines) condition is met. For too large densities, radio wave strong "over-refraction" leads to the ionospheric echo disappearance. We estimate that the over-refraction is important for densities greater than 4×10 11 m −3 . We also investigated the backscatter power and the electric field magnitude relationship and found no obvious relationship contrary to the expectation that the gradientdrift plasma instability would lead to stronger irregularity intensity/echo power for larger electric fields.

Abstract. Two Doppler coherent radar systems are currently working at Hankasalmi, Finland, the STARE and CUTLASS radars operating at ∼ 144 MHz and ∼ 12 MHz, respectively. The STARE beam 3 is nearly co-located with the CUTLASS beam 5, providing an opportunity for echo velocity comparison along the same direction but at significantly different radar frequencies. In this study we consider an event when STARE radar echoes are detected at the same ranges as CUT-LASS radar echoes. The observations are complemented by EISCAT measurements of the ionospheric electric field and electron density behaviour at one range of 900 km. Two separate situations are studied; for the first one, CUTLASS observed F-region echoes (including the range of the EIS-CAT measurements), while for the second one CUTLASS observed E-region echoes. In both cases STARE E-region measurements were available. We show that F-region CUT-LASS velocities agree well with the convection component along the CUTLASS radar beam, while STARE velocities are typically smaller by a factor of 2-3. For the second case, STARE velocities are found to be either smaller or larger than CUTLASS velocities, depending on the range. Plasma physics of E-and F-region irregularities is discussed in attempt to explain the inferred relationship between various velocities. Special attention is paid to ionospheric refraction that is important for the detection of 12-MHz echoes.

During the nights of 8-9 and 9-10 February 1997, Fabry-Perot interferometers were operated from the EISCAT radar site at Ramfjord (69.59 o N, 19.23 o E) and Skibotn (69.35 o N, 20.36 o E). From Ramfjord, horizontal neutral winds were measured in the lower and upper thermosphere using the auroral/airglow emissions at 557.7 and 630 nm, respectively. From Skibotn, thermospheric neutral temperatures were measured using the same wavelengths. The EISCAT radar measured ion temperatures up the local magnetic field line in the height range 90-580 km during the first night. Neutral winds are compared to the HWM-90 and CTIP-200 models with poor agreement. Neutral temperatures are compared to the MSISE-90 and CTIP-200 models as well as EISCAT ion temperatures with good agreement.

[1] To study the spatial structure of midlatitude sporadic E (E s ) layers, the ultraviolet resonant scattering by magnesium ions (Mg + ) in an E s layer was observed during the evening twilight with the Magnesium Ion Imager (MII) on the sounding rocket launched from the Uchinoura Space Center in Kagoshima, Japan. The in situ electron density measured by an onboard impedance probe showed that the E s layer was located at an altitude of 100 km during both the ascent and descent of the flight. Simultaneous observation with a ground-based ionosonde at Yamagawa identified the signature of horizontally "patchy" structures in the E s layer. The MII successfully scanned the horizontal Mg + density perturbations in the E s layer and found that they had patchy and frontal structures. The horizontal scale and alignment of the observed frontal structure is generally consistent with a proposed theory. To our knowledge, this is the first observation of the two-dimensional horizontal structure of Mg + in an E s layer.Citation: Kurihara, J., et al. (2010), Horizontal structure of sporadic E layer observed with a rocket-borne magnesium ion imager,

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Abstract. In usual incoherent scatter data analysis, the plasma distribution function is assumed to be Maxwellian. In space plasmas, however, distribution functions with a high energy tail which can be well modeled by a generalized Lorentzian distribution function with spectral index kappa (kappa distribution) have been observed. We have theoretically calculated incoherent scatter spectra for a plasma that consists of electrons with kappa distribution function and ions with Maxwellian neglecting the eects of the magnetic ®eld and collisions. The ion line spectra have a double-humped shape similar to those from a Maxwellian plasma. The electron temperatures are underestimated, however, by up to 40% when interpreted assuming Maxwellian distribution. Ion temperatures and electron densities are aected little. Accordingly, actual electron temperatures might be underestimated when an energy input maintaining a high energy tail exists. We have also calculated plasma lines with the kappa distribution function. They are enhanced in total strength, and the peak frequencies appear to be slightly shifted to the transmitter frequency compared to the peak frequencies for a Maxwellian distribution. The damping rate depends on the electron temperature. For lower electron temperatures, plasma lines for electrons with a j distribution function are more strongly damped than for a Maxwellian distribution. For higher electron temperatures, however, they have a relatively sharp peak.

[1] Vertical ion velocities in excess of a few tens m s À1 have been found in the lower ionosphere (95-105 km) at high latitudes. Statistics using data from the European Incoherent Scatter (EISCAT) Tromsø UHF radar are performed to understand general character of vertical motions in the lower ionosphere for geomagnetically quiet conditions. We estimate the probability distribution function for the real vertical ion velocity, taking into account the measurement uncertainty. This calculation suggests that while most vertical ion velocities have amplitudes smaller than 20 m s À1 at 100 km level, the magnitudes larger than 20 m s À1 are observed as statistical exception. To understand if these large vertical motions in the lower ionosphere are associated with thermospheric motions, differences between ion and neutral wind velocities are calculated using data from experiments with the Sondrestrom incoherent scatter radar on 5 and 12 September 2003. The vertical ion velocities show magnitudes larger than 20 m s À1 below 103 km where the ion velocity is considered to be equal to the neutral wind velocity during the experiments according to calculations of the velocity difference.

No abstract