Incoherent Scatter Measurements of Ion-Neutral Collision Frequencies and Temperatures in the Lower Thermosphere of the Auroral Region

Incoherent scatter observations performed in March and November 1978 at Chatanika have been used for studying the lower thermosphere in the auroral region. Neutral temperatures and densities have been found during periods without Joule heating. We present mean profiles of temperatures and collision frequencies (approximately proportional to neutral densities) for each month and profiles obtained during four specific nights. Between 93 km and 110 km the mean profiles of temperature are in good agreement with the Jacchia (1971) model, and the profiles of collision frequency are similar to those deduced from the model. Consistency checks between these collision frequencies and temperatures were performed, as were various simulations of the data. Neutral temperature profiles between 90 km and 140 km on the four specific nights show variations from one night to another that are not correlated to magnetic activity. However, there are systematic variations during each night that we suggest are due to atmospheric tides

Impact of the 26-30 May 2003 solar events on the earth ionosphere and thermosphere.

During the last week of May 2003, the solar active region AR 10365 produced a large number of flares, several of which were accompanied by Coronal Mass Ejections (CME). Specifically on 27 and 28 May three halo CMEs were observed which had a significant impact on geospace. On 29 May, upon their arrival at the L1 point, in front of the Earth's magnetosphere, two interplanetary shocks and two additional solar wind pressure pulses were recorded by the ACE spacecraft. The interplanetary magnetic field data showed the clear signature of a magnetic cloud passing ACE. In the wake of the successive increases in solar wind pressure, the magnetosphere became strongly compressed and the sub-solar magnetopause moved inside five Earth radii. At low altitudes the increased energy input to the magnetosphere was responsible for a substantial enhancement of Region-1 field-aligned currents. The ionospheric Hall currents also intensified and the entire high-latitude current system moved equatorward by about 10°. Several substorms occurred during this period, some of them - but not all - apparently triggered by the solar wind pressure pulses. The storm's most notable consequences on geospace, including space weather effects, were (1) the expansion of the auroral oval, and aurorae seen at mid latitudes, (2) the significant modification of the total electron content in the sunlight high-latitude ionosphere, (3) the perturbation of radio-wave propagation manifested by HF blackouts and increased GPS signal scintillation, and (4) the heating of the thermosphere, causing increased satellite drag. We discuss the reasons why the May 2003 storm is less intense than the October-November 2003 storms, although several indicators reach similar intensities

Traitement matriciel de signaux a N composantes et étude par couples

Pour des signaux multicomposantes, la méthode matricielle utilisant les éléments propres de la matrice spectrale permet de déterminer le nombre d'excitations décorrélées et, moyennant des hypothèses supplémentaires conformes à la physique du milieu, de déterminer les formes d'ondes des sources au niveau des capteurs. Une approche plus simple est l'analyse des relations entre couples de composantes : elle évalue les coefficients de cohérence et filtre certaines composantes avec des filtres de Wiener construits en prenant un des signaux comme référence. Nous présentons, sur un cas pratique, une analyse comparative des deux méthodes. Les signaux traités sont des ondes naturelles électromagnétiques dûes à des résonnances de la magnétosphère terrestre. Ce sont des signaux à très faible BeT. Les trois premières composantes sont les champs électriques enregistrés en trois points de l'ionosphère par le sondeur européen EISCAT. Les trois autres composantes décrivent le vecteur des variations magnétiques enregistrées à Kevo (Finlande). L'objectif géophysique est de savoir si, entre ces six composantes, il existe une ou plusieurs sources indépendantes et, dans le cas d'une seule source, d'identifier ses paramètres. En particulier, nous montrons que sans l'utilisation de ces méthodes, la mesure de la fonction de transfert ionosphère-sol serait erronée

Meridional thermospheric neutral wind at high latitude over a full solar cycle

EISCAT radar experiments over a full solar cycle between January 1984 and March 1995 have been used to construct meridional neutral wind patterns in the ionospheric F region. For locally geomagnetically quiet periods the neutral winds have been binned according to season, solar activity, and universal time. The diurnal and seasonal behaviors and the effect of the solar flux are described. An empirical model of the meridional neutral wind for the high latitudes at eight altitudes in the ionospheric F region over a full solar cycle is presented. Results are compared with other recent empirical models.Key words. Auroral ionosphere · Thermospheric dynamics · EISCA