Global dynamo simulation of ionospheric currents and their connection with the equatorial electrojet and counter electrojet : a case study

International audienceThe interrelationship of equatorial and planetary scale ionospheric horizontal currents on quiet days is studied by means of a global ionospheric wind dynamo simulation. This simulation aims at reproducing magnetic and radar data first for a normal quiet day, and then for the strong counterelectrojet event of January 21, 1977, which was previously studied in detail on the basis of coherent backscatter radar data for the Addis-Ababa location. For the reference quiet day (January 27, 1977), the pattern of low and middle latitude currents and electric fields can be roughly reproduced by a combination of the (1,-2) and (2,2) solar tides. Both the Sq current system, and the global electrostatic potential distribution as derived by Richmond et al. [1980] from incoherent scatter data are well simulated. Direct comparison of the computed electric field with the quiet-day averages available for each radar site also show an excellent agreement on the east-west component, but a poorer one on the northsouth component. The counterelectrojet simulation is performed by fitting the H component trace at the magnetic equator and the D trace at midlatitudes. The result appears to give a consistent solution to the problem of the electrical connection between the equatorial counterelectrojet and the planetary dynamo layer. Two horizontal current vortices of opposite directions are found to flow at low latitudes on each side of the noon sector, anticlockwise in the morning and clockwise in the afternoon. They both produce a poleward current flow at low latitudes at noon, a feature that is detected on the magnetic records. The counterelectrojet event is reproduced by a combination of the (2,2) and (2,#) solar tides, assuming that the contribution of the diurnal tide to the altitudeintegrated current flow cancels out. This result is in agreement with a previous simulation study of the counterelectrojet phenomenon

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

Long duration meteor echoes characterized by Doppler spectrum bifurcation

International audienceWe report on a new category of long lasting meteor echoes observed occasionally with HF and VHF radars. These meteoric returns, which have lifetimes from several seconds to a few minutes, are characterized by a distinct Doppler spectral signature showing a pronounced Doppler bifurcation which includes narrow bands of discrete Doppler velocities, often of opposite polarity. The spectral properties imply that Bragg scattering cannot be the generation mechanism, therefore these echoes do not associate with the long living meteor-induced backscatter from the unstable lower E region. A reasonable interpretation needs to explain both the Doppler spectrum bifurcation and the long echo duration. As such, we propose the idea of a structured vertical wind shear in the lower E region which traps different fragments of a meteor trail plasma in the same way that sporadic E layers form. These trail parts inside the shear-related wind profile may act as relatively long-lasting meteoric reflectors moving with different Doppler velocities, also of opposite polarity

Night–side effects on the polar ionospheric convection due to a solar wind pressure impulse

The Sudden Impulse (SI) of solar wind dynamic pressure of 20 february 2000, 21:03 UT, is investigated by making use of data from WIND, GEOTAIL, POLAR and GOES; ground magnetometer chains (Greenland, IMAGE, CANOPUS); SuperDARN HF radars in both Northern and Southern hemispheres. The main effect of the SI described herein is an enhancement of the ionospheric convection around midnight MLT. We suggest that such an enhancement be due to an increase of the dawn–dusk electric field caused by the SI compression of the magnetospheric tail.Published91-932A. Fisica dell'alta atmosferaN/A or not JC

From the Sun to the Earth: impact of the 27-28 May 2003 solar events on the magnetosphere, ionosphere and thermosphere

International audienceDuring 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