Comparison of temporal fluctuations in the total electron content estimates from EISCAT and GPS along the same line of sight

The impact of space weather events on satellite-based technologies (e.g. satellite navigation and precise positioning) is typically quantified on the basis of the total electron content (TEC) and temporal fluctuations associated with it. GNSS (global navigation satellite systems) TEC measurements are integrated over a long distance and thus may include contributions from different regions of the ionised atmosphere which may prevent the resolution of the mechanisms ultimately responsible for given observations. The purpose of the experiment presented here was to compare TEC estimates from EISCAT and GPS measurements. The EISCAT measurements were obtained along the same line of sight of a given GPS satellite observed from Tromsø. The present analyses focussed on the comparison of temporal fluctuations in the TEC between aligned GPS and EISCAT measurements. A reasonably good agreement was found between temporal fluctuations in TEC observed by EISCAT and those observed by a co-located GPS ionospheric monitor along the same line of sight, indicating a contribution from structures at E and F altitudes mainly to the total TEC in the presence of ionisation enhancements possibly caused by particle precipitation in the nighttime sector. The experiment suggests the great potential in the measurements to be performed by the future EISCAT_3D system, limited only in the localised geographic region to be covered.

STARE velocity at large flow angles: is it related to the ion acoustic speed?

The electron drift and ion-acoustic speed in the E region inferred from EISCAT measurements are compared with concurrent STARE radar velocity data to investigate a recent hypothesis by Bahcivan et al. (2005), that the electrojet irregularity velocity at large flow angles is simply the product of the ion-acoustic speed and the cosine of an angle between the electron flow and the irregularity propagation direction. About 3000 measurements for flow angles of 50°–70° and electron drifts of 400–1500 m/s are considered. It is shown that the correlation coefficient and the slope of the best linear fit line between the predicted STARE velocity (based solely on EISCAT data and the hypothesis of Bahcivan et al. (2005)) and the measured one are both of the order of ~0.4. Velocity predictions are somewhat better if one assumes that the irregularity phase velocity is the line-of-sight component of the E×B drift scaled down by a factor ~0.6 due to off-orthogonality of irregularity propagation (nonzero effective aspect angles of STARE observations).

Comparison of high latitude electron density profiles obtained with the GPS radio occultation technique and EISCAT measurements

To obtain a comprehensive view on high latitude processes by applying different observation techniques, the SIRCUS campaign was initiated in 2001/2002. This paper compares electron density profiles derived from CHAMP radio occultation data and those measured with the EISCAT facility. Since ionospheric profiling with the help of space-based received GPS is a relatively new technique, validations with established independent instruments are of crucial need. We present 28 profiling events for quasi-statistical analyses, which occurred during the SIRCUS campaigns and describe some of them in more detail. We found out that the majority of profile comparisons in electron density peak value and height, as well as in TEC, lie within the error ranges of the two methods. Differences in the ionospheric quantities do not necessarily occur when the locations of the occultation and of the radar site show considerable distances. Differences are more pronounced when the ionosphere is remarkably structured.

Comparison of high-latitude thermospheric meridionalwinds I: optical and radar experimental comparisons

Thermospheric neutral winds at Kiruna, Sweden (67.4°N, 20.4°E) are compared using both direct optical Fabry-Perot Interferometer (FPI) measurements and those derived from European incoherent scatter radar (EISCAT) measurements. This combination of experimental data sets, both covering well over a solar cycle of data, allows for a unique comparison of the thermospheric meridional component of the neutral wind as observed by different experimental techniques. Uniquely in this study the EISCAT measurements are used to provide winds for comparison using two separate techniques: the most popular method based on the work of Salah and Holt (1974) and the Meridional Wind Model (MWM) (Miller et al., 1997) application of servo theory. The balance of forces at this location that produces the observed diurnal pattern are investigated using output from the Coupled Thermosphere and Ionosphere (CTIM) numerical model. Along with detailed comparisons from short periods the climatological behaviour of the winds have been investigated for seasonal and solar cycle dependence using the experimental techniques. While there are features which are consistent between the 3 techniques, such as the evidence of the equinoctial asymmetry, there are also significant differences between the techniques both in terms of trends and absolute values. It is clear from this and previous studies that the high-latitude representation of the thermospheric neutral winds from the empirical Horizontal Wind Model (HWM), though improved from earlier versions, lacks accuracy in many conditions. The relative merits of each technique are discussed and while none of the techniques provides the perfect data set to address model performance at high-latitude, one or more needs to be included in future HWM reformulations. Key words. Meteorology and atmospheric dynamics (thermospheric dynamics), Ionosphere (ionosphere-atmosphere interactions, auroral ionosphere)

Velocities of auroral coherent echoes at 12 and 144 MHz

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.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

EISCAT measurements of solar wind velocity and the associated level of interplanetary scintillation

A relative scintillation index can be derived from EISCAT observations of Interplanetary Scintillation (IPS) usually used to study the solar wind velocity. This provides an ideal opportunity to compare reliable measurements of the solar wind velocity derived for a number of points along the line-of-sight with measurements of the overall level of scintillation. By selecting those occasions where either slow- or fast-stream scattering was dominant, it is shown that at distances from the Sun greater than 30 RS , in both cases the scintillation index fell with increasing distance as a simple power law, typically as R-1.7. The level of scintillation for slow-stream scattering is found to be 2.3 times the level for fast-stream scattering.Key words. Interplanetary physics (solar wind plasma)