Impact of solar wind depression on the dayside magnetosphere under northward interplanetary magnetic field

S. Baraka , L. Ben-Jaffel (1)

1 IAP - Institut d'Astrophysique de Paris

We present a follow up study of the sensitivity of the Earth's magnetosphere to solar wind activity using a particles-in-cell model (Baraka and Ben Jaffel, 2007), but here during northward Interplanetary Magnetic Field (IMF). The formation of the magnetospheric cavity and its elongation around the planet is obtained with the classical structure of a magnetosphere with parallel lobes. An impulsive disturbance is then applied to the system by changing the bulk velocity of the solar wind to simulate a decrease in the solar wind dynamic pressure followed by its recovery. In response to the imposed drop in the solar wind velocity, a gap (abrupt depression) in the incoming solar wind plasma appears moving toward the Earth. The gap's size is a ~15 R_E and is comparable to the sizes previously obtained for both B_z<0 and B_z=0. During the initial phase of the disturbance along the x-axis, the dayside magnetopause (MP) expands slower than the previous cases of IMF orientations as a result of the abrupt depression. The size of the MP expands nonlinearly due to strengthening of its outer boundary by the northward IMF. Also, during the initial 100 Δt, the MP shrank down from 13.3 R_E to ~9.2 R_E before it started expanding, a phenomenon that was also observed for southern IMF conditions but not during the no IMF case. As soon as they felt the solar wind depression, cusps widened at high altitude while dragged in an upright position. For the field's topology, the reconnection between magnetospheric and magnetosheath fields is clearly observed in both the northward and southward cusps areas. Also, the tail region in the northward IMF condition is more confined, in contrast to the fishtail-shape obtained in the southward IMF case. An X-point is formed in the tail at ~110 R_E compared to ~103 R_E and ~80 R_E for B_z=0 and B_z<0, respectively. Our findings are consistent with existing reports from many space observatories (Cluster, Geotail, Themis, etc.) for which predictions are proposed to test furthermore our simulation technique.

Mots clés: Physics - Space Physics; Astrophysics - Earth and Planetary Astrophysics; Physics - Plasma Physics

Domaines: Planète et Univers [physics]

S. Baraka, L. Ben-Jaffel. Impact of solar wind depression on the dayside magnetosphere under northward interplanetary magnetic field. Annales Geophysicae, 2011, 29, pp.31-46. ⟨10.5194/angeo-29-31-2011⟩. ⟨insu-03646096⟩