Asgeir Brekke

Two‐step development of geomagnetic storms

Using the Dst index, more than 1200 geomagnetic storms, from weak to intense, spanning over three solar cycles have been examined statistically. Interplanetary magnetic field (IMF) and solar wind data have also been used in the study. It is found that for more than 50% of intense magnetic storms, the main phase undergoes a two-step growth in the ring current. That is, before the ring current has decayed significantly to the prestorm level, anew major particle injection occurs, leading to a further development of the ring current, and making Dst decrease a second time. Thus intense magnetic storms may often be the result of two closely spaced moderate storms. The corresponding signature in the interplanetary medium is the arrival of double-structured southward IMF at the magnetosphere.

Ionospheric modification experiments in northern Scandinavia

Abstract The heating facility at Ramfjordmoen near Tromso, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F -region cross modulation.

The solar flux influence on quiet time conductances in the auroral ionosphere

Hall and Pedersen conductances derived from EISCAT Common Program 1 (CP-1) data during half a solar cycle are presented. The data set consists of 9 quiet summer days with 10.7-cm solar flux (Sa) ranging from solar minimum (Sa=74) to solar maximum (Sa=248) conditions. The solar flux and the solar zenith angle (χ) dependence of Hall (∑H) and Pedersen (∑P) conductances are well represented by respectively. The present results are in good agreement with the theoretical model calculations by Rasmussen et al. [1988] for solar minimum conditions. For higher solar activity levels, however, our model gives consistently higher conductance values than older models.

Ionospheric modification experiments with the Tromsø heating facility

Abstract The experiments performed up to mid 1984 with the heating facility at Ramfjordmoen near Tromso, Norway, are summarized. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF frequencies, excitation of E -region small-scale irregularities and of F -region small- and large-scale irregularities, anomalous absorption of HF wave on long and short time scales, excitation of incoherent backscatter plasma and ion lines, stimulated radio wave emission and F -region in situ measurements.

Neutral air turbulence in the upper atmosphere observed during the Energy Budget Campaign

A number of different experimental techniques employed in the campaign provided measurements on the fine scale structure of the upper atmosphere, from which information about turbulent intensity, eddy transport and eddy dissipation rates may be extracted. The turbulent state of the mesosphere was shown to be highly variable and significant differences were found between observations obtained during the four salvoes launched during different degrees of geomagnetic disturbance.

Studies of the E region neutral wind in the disturbed auroral ionosphere

Mean neutral wind velocity vectors observed at auroral E region altitudes by European incoherent scatter radar (EISCAT) for disturbed days are found to be rather similar to the mean wind velocity vectors observed on quiet days. It is demonstrated that the main difference in the E field between quiet and disturbed days is an enhancement in the diurnal component by a factor of 3. The semidiurnal, 6-, and 8-hour components display only minor differences. The amplitudes of the tidal components of the neutral wind increase in general above 109 km during ionospheric disturbances in particular for the diurnal tide of the zonal wind. The disturbance pattern of the neutral wind has a stronger equatorward component above ∼109 km than the corresponding quiet time pattern. In general, on disturbed days there is an additional eastward motion from the late evening until prenoon. During the daytime the northward wind is enhanced, while in the afternoon stronger westward velocities are seen above 109 km. In the afternoon sector until late evening during disturbed conditions there is also a southward component above ∼109 km. Below 109 km the semidiurnal tide appears to be enhanced during disturbed conditions. It is found that only part of the disturbance in the E region neutral wind can be explained by ion drag while Joule heating and particle precipitation may play an additional role. Satisfying agreement is found between the variations of the diurnal components of the E region neutral wind observed in the present work and the recent model predictions by Fesen et al. [1991].

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.

Mesosphere and lower thermosphere (80–120 km) winds and tides from near Troms0 (70°N, 19°E) : comparisons between radars (MF, EISCAT, VHF) and rockets

Abstract Winds and tides from the MF radar near Tromso are first compared with data from the SOUSY VHF radar near Andenes (69°N, 16°E) and from rocket chaff and falling sphere systems, both obtained during SINE (Summer in Northern Europe; 7 June–19 July 1987). Wind directions and tidal phases are in excellent agreement, but magnitudes and amplitudes consistently demonstrate that the MF winds are reduced by 35%. This is associated with reduced signal to noise for the MF radar scatter due to minimal systems (loops) used for the spaced antennas. The adjusted winds and tides are then compared with data obtained from the co-located EISCAT and the Saskatoon MF radars, for a total of six campaigns (1987–1989) covering each season of the year. The data almost overlap near 100 km for three campaigns and do overlap from 90 to 100 km for the other three. There is, in general, excellent agreement between the incoherent scatter and MF radars, and clear seasonal variations are revealed. Differences are attributed to low signal to noise conditions for either radar, lack of complete data overlap, and strong wind gradients in the lower thermosphere.

Studies of the E region neutral wind in the quiet auroral ionosphere

Quiet time auroral E region neutral wind data obtained by European Incoherent Scatter Radar (EISCAT) have been analyzed in order to establish the background mean neutral velocities as well as tidal influence on the wind. There is a rather strong and persistent mean eastward wind present in all seasons in the E region below 120 km. It is strongest in summer (∼60 m s−1), weak in fall and spring, and weaker in winter (∼20 m s−1). The zonal wind is westward above 120 km, in general stronger in winter (∼70 m s−1) and weaker in the other seasons (∼30 m s−1). The northward mean wind is rather small (∼10 m s−1). It is shifted gradually from being mainly southward in winter to becoming northward in summer. The mean vertical velocity is of the order of 5 m s−1 in the auroral E region. The amplitude of the diurnal tides of the vertical component have broad minima in the height region between 90 and 120 km. The horizontal diurnal tide is dominant in the upper E region, while the semidiurnal tide has a maximum at 110 km, in particular in the eastward component. The 8- and 6-hour tides are rather constant in amplitude by height. The results obtained are in good agreement with other results published on the basis of incoherent scatter radar measurements. The comparison with theoretical models underlines the need for more modeling of the auroral E region neutral tides including the ionized plasma as well as electric fields and mean background winds. Theoretical studies of the vertical motion with respect to the influence of tides are especially lacking in this important region.

Study on neutral wind contribution to the electrodynamics in the polar ionosphere using EISCAT CP‐1 data

Energy coupling between the thermosphere, ionosphere and magnetosphere is studied quantitatively through an analysis using the European Incoherent Scatter (EISCAT) Common Program (CP) −1 version H data obtained on May 3, 1988. A negative excursion of the H component in the Tromso magnetogram occurred during the experiment period, which involved the following two features: (1) the electric potential across the polar cap was expected to be reduced abruptly in association with a sudden change of the interplanetary magnetic field (IMF) Bz polarity from southward to around null and (2) the negative excursion had a relatively long duration of development (about 4 hours), which may drive neutrals to move significantly through ion drag. In order to investigate the energy coupling between the thermosphere and ionosphere, we evaluate quantitatively the electromagnetic energy flux J·E, the Joule heating rate J·E′ (E′ = E + U × B), and the mechanical energy transfer rate U·(J × B), where U is the neutral wind velocity. The CP-I-H experiment provides directly or indirectly all quantities above at altitudes of 101 km, 109 km, 119 km, and 132 km. The results are summarized as follows. (1) The amplitude of the neutral wind related electric field U × B varied greatly with altitude, i.e., at altitudes above 119 km it often became larger than 50% of the amplitude of the observed electric field; (2) during the late recovery phase of the negative excursion of the H component of the Tromso magnetic field, the neutral wind related electric field tended to be canceled with the observed electric field; (3) in the E region the neutral wind mechanical energy transfer rate U·(J × B) is not negligible but is comparable to the Joule heating rate J·E′; and (4) in particular, at higher altitudes (132 km high) the conversion from the neutral wind mechanical energy to the electromagnetic energy occasionally may occur.