Experimental study of nonlinear interaction of plasma flow with charged thin current sheets: 1. Boundary structure and motion

We study plasma transport at a thin magnetopause (MP), described hereafter as a thin current sheet (TCS), observed by Cluster at the southern cusp on 13 February 2001 around 20:01 UT. The Cluster observations generally agree with the predictions of the Gas Dynamic Convection Field (GDCF) model in the magnetosheath (MSH) up to the MSH boundary layer, where significant differences are seen. We find for the MP a normal roughly along the GSE xaxis, which implies a clear departure from the local average MP normal, a∼90 km thickness and an outward speed of 35 km/s. Two populations are identified in the MSH boundary layer: the first one roughly perpendicular to the MSH magnetic field, which we interpret as the “incident” MSH plasma, the second one mostly parallel to B. Just after the MP crossing a velocity jet is observed with a peak speed of 240 km/s, perpendicular to B, with MA=3 andβ>10 (peak value 23). The magnetic field clock angle rotates by 70 ◦ across the MP. Ex is the main electric field component on both sides of the MP, displaying a bipolar signature, positive on the MSH side and negative on the opposite side, corresponding to a∼300 V electric potential jump across the TCS. TheE×B velocity generally coincides with the perpendicular velocity measured by CIS; however, in the speed jet a difference between the two is observed, which suggests the need for an extra flow source. We propose that the MP TCS can act locally as an obstacle for low-energy ions ( <350 eV), Correspondence to: E. Amata (emanno.amata@ifsi-roma.inaf.it) being transparent for ions with larger gyroradius. As a result, the penetration of plasma by finite gyroradius is considered as a possible source for the jet. The role of reconnection is briefly discussed. The electrodynamics of the TCS along with mass and momentum transfer across it are further discussed in the companion paper by Savin et al. (2006).

[1]  A. Vaivads,et al.  Structure of the magnetic reconnection diffusion region from four-spacecraft observations. , 2004, Physical review letters.

[2]  P. Louarn,et al.  Alfvén wave interaction with inhomogeneous plasmas: acceleration and energy cascade towards small-scales , 2004 .

[3]  M. Dunlop,et al.  CLUSTER encounters with the high altitude cusp: boundary structure and magnetic field depletions , 2004 .

[4]  M. Dunlop,et al.  Four-spacecraft determination of magnetopause orientation, motion and thickness: comparison with results from single-spacecraft methods , 2004 .

[5]  A. Vaivads,et al.  Thin electron‐scale layers at the magnetopause , 2004 .

[6]  L. Zelenyi,et al.  The penetration of ions into the magnetosphere through the magnetopause turbulent current sheet , 2003 .

[7]  J. Büchner,et al.  Nonlinear instability of thin current sheets in antiparallel and guided magnetic fields , 2003 .

[8]  F. Mozer,et al.  The complex structure of the reconnecting magnetopause , 2003 .

[9]  J. Sauvaud,et al.  On the properties of turbulent boundary layer over polar cusps , 2002 .

[10]  Christopher T. Russell,et al.  Fingerprints of collisionless reconnection at the separator, I, Ambipolar‐Hall signatures , 2002 .

[11]  M. Dunlop,et al.  Cluster observations of the exterior cusp and its surrounding boundaries under northward IMF , 2002 .

[12]  A. Sharma,et al.  “Aging” of the magnetotail thin current sheets , 2002 .

[13]  C. Russell,et al.  Polar‐Interball coordinated observations of plasma and magnetic field characteristics in the regions of the northern and southern distant cusps , 2002 .

[14]  J. Sauvaud,et al.  Turbulent boundary layer at the border of geomagnetic trap , 2001 .

[15]  M. W. Dunlop,et al.  The Cluster Magnetic Field Investigation: overview of in-flight performance and initial results , 2001 .

[16]  E. Guirriec,et al.  Early results from the Whisper instrument on Cluster: an overview , 2001 .

[17]  T. Carozzi,et al.  First results of electric field and density observations by Cluster EFW based on initial months of operation , 2001 .

[18]  M. Dunlop,et al.  The high‐altitude cusps: HEOS 2 , 2000 .

[19]  S. F. Fung,et al.  Configuration of high-latitude and high-altitude boundary layers , 2000 .

[20]  S. A. Boardsen,et al.  An empirical model of the high‐latitude magnetopause , 2000 .

[21]  J. Sauvaud,et al.  Characteristics of the exterior cusp for steady southward interplanetary magnetic field: Interball observations , 2000 .

[22]  C. Russell,et al.  On the processes in the terrestrial magnetosheath. 1. Scheme development , 1999 .

[23]  M. Lockwood,et al.  Plasma transfer processes at the magnetopause , 1999 .

[24]  P. Anderson,et al.  First polar and 1995‐034 observations of the midaltitude cusp during a persistent northward IMF condition , 1997 .

[25]  J. L. Green,et al.  Exterior and interior polar cusps: Observations from Hawkeye , 1997 .

[26]  L. Blomberg,et al.  Dynamic response of the cusp morphology to the solar wind: A case study during passage of the solar wind plasma cloud on February 21, 1994 , 1996 .

[27]  J. L. Green,et al.  Evidence of high‐latitude reconnecting during northward IMF: Hawkeye observations , 1996 .

[28]  K. Stasiewicz.  Finite Larmor radius effects in the magnetosphere , 1993 .

[29]  J. A. V. Allen,et al.  Observed currents on the earth`s high-latitude magnetopause. (Reannouncement with new availability information) , 1992 .

[30]  R. Lundin,et al.  Magnetosheath plasma precipitation in the polar cusp and its control by the interplanetary magnetic field , 1992 .

[31]  G. Paschmann,et al.  The frontside boundary layer of the magnetosphere and the problem of reconnection , 1977 .

[32]  G. Haerendel.  Microscopic plasma processes related to reconnection , 1976 .

[33]  H. Rosenbauer,et al.  Plasma and Magnetic Field Characteristics of the Distant Polar Cusp near Local Noon: The Entry Layer , 1976 .

[34]  L. Frank.  Plasma in the Earth's polar magnetosphere , 1971 .

[35]  E. Amata,et al.  Magnetosheath interaction with high latitude magnetopause: dynamic flow chaotization , 2005 .

[36]  M. Dunlop,et al.  Cluster Observations of the CUSP: Magnetic Structure and Dynamics , 2005 .

[37]  N. Maynard.  Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS , 2005 .

[38]  J. Sauvaud,et al.  Magnetosheath Interaction with the High Latitude Magnetopause , 2005 .

[39]  C. Russell,et al.  Magnetosheath-cusp interface , 2004 .

[40]  G. Siscoe,et al.  Energetic ions, large diamagnetic cavities, and Chapman‐Ferraro cusp , 2003 .

[41]  J. Büchner,et al.  Interball Observations Of Field Aligned Current Signatures Due To Collisionless Reconnection , 1999 .

[42]  J. Šimůnek,et al.  ULF-ELF-VLF-HF plasma wave observations in the polar cusp onboard high and low altitude satellites , 1998 .

[43]  K. Fornacon,et al.  A high-latitude boundary layer crossing- INTERBALL measurements and MHD model results , 1998 .

[44]  Manuel Grande,et al.  PEACE: A PLASMA ELECTRON AND CURRENT EXPERIMENT , 1997 .

[45]  C. Meng,et al.  Ionospheric projections of magnetospheric regions under low and high solar wind pressure conditions , 1994 .

[46]  S. Syrovatskii.  FORMATION OF CURRENT SHEETS IN A PLASMA WITH A FROZEN-IN STRONG MAGNETIC FIELD. , 1971 .