Abstract
Abstract. Multi-diagnostic observations, covering a significant area of northwest Europe, were made during the magnetic storm interval (28–29 April 2001) that occurred during the High Rate SolarMax IGS/GPS-campaign. HF radio observations were made with vertical sounders (St. Petersburg and Sodankyla), oblique incidence sounders (OIS), on paths from Murmansk to St. Petersburg, 1050 km, and Inskip to Leicester, 170 km, Doppler sounders, on paths from Cyprus to St. Petersburg, 2800 km, and Murmansk to St. Petersburg, and a coherent scatter radar (CUTLASS, Hankasalmi, Finland). These, together with total electron content (TEC) measurements made at GPS stations from the Euref network in northwest Europe, are presented in this paper. A broad comparison of radio propagation data with ionospheric data at high and mid latitudes, under quiet and disturbed conditions, was undertaken. This analysis, together with a geophysical interpretation, allow us to better understand the nature of the ionospheric processes which occur during geomagnetic storms. The peculiarity of the storm was that it comprised of three individual substorms, the first of which appears to have been triggered by a compression of the magnetosphere. Besides the storm effects, we have also studied substorm effects in the observations separately, providing an improved understanding of the storm/substorm relationship. The main results of the investigations are the following. A narrow trough is formed some 10h after the storm onset in the TEC which is most likely a result of enhanced ionospheric convection. An enhancement in TEC some 2–3 h after the storm onset is most likely a result of heating and upwelling of the auroral ionosphere caused by enhanced currents. The so-called main effect on ionospheric propagation was observed at mid-latitudes during the first two substorms, but only during the first substorm at high latitudes. Ionospheric irregularities observed by CUTLASS were clearly related to the gradient in TEC associated with the trough. The oblique sounder and Doppler observations also demonstrate differences between the mid-latitude and high-latitude paths during this particular storm. Keywords. Ionosphere (Ionospheric disturbances) – Magnetospheric physics (Storms and substorms) – Radio science (Ionospheric propagation)
Highlights
The High Rate Global Positioning System (GPS)/GLONASS measuring campaign (HIRAC) was initiated by the International GPS Service (IGS) and supported by COST 271 activities with the aim of analysing transionospheric signals received from navigation satellites by the IGS ground station network, in order to study the behaviour of the ionosphere during the recent solar maximum
We note that at the path midpoint (64.5◦ N, 32◦ E) the total electron content (TEC) values increase by ∼50% from 05:00 to 07:00 UT (Fig. 4 top row), at 08:00 UT the TEC values at the path midpoint on 28 April 2001 were ∼25% less than those on 27 April 2001
On the high-latitude path, there is significant absorption which does not allow the HF signal to propagate via the F-region, while at the lower latitudes, the path mid-point is within the TEC trough and no propagation path can be established
Summary
The High Rate GPS/GLONASS measuring campaign (HIRAC) was initiated by the International GPS Service (IGS) and supported by COST 271 activities with the aim of analysing transionospheric signals received from navigation satellites by the IGS ground station network, in order to study the behaviour of the ionosphere during the recent solar maximum. Ticles from the magnetosphere, such that an increase in absorption measured by an auroral zone riometer from 0.0 to 2.5 dB is equivalent to a change in electron density at heights below 90 km of a factor of 100 or more (Lastovicka, 2002). These observations were supported by measurements of solar wind parameters, the interplanetary magnetic field (IMF), the Kp, AE and Dst magnetic indices, riometer absorption data from the Sodankyla observatory, and data from the IMAGE magnetometer network. We conclude the paper with a discussion of the observations with reference to our understanding of magnetic and ionospheric storms and magnetospheric substorms. The enhancement of the lower ionosphere electron density is caused by strongly increased precipitation of energetic par-
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