Evidence of Magnetic Merging from Low-Altitude Spacecraft and Ground-Based Experiments

  1. Edward W. Hones Jr.
  1. Patricia H. Reiff

Published Online: 19 MAR 2013

DOI: 10.1029/GM030p0104

Magnetic Reconnection in Space and Laboratory Plasmas

Magnetic Reconnection in Space and Laboratory Plasmas

How to Cite

Reiff, P. H. (1984) Evidence of Magnetic Merging from Low-Altitude Spacecraft and Ground-Based Experiments, in Magnetic Reconnection in Space and Laboratory Plasmas (ed E. W. Hones), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM030p0104

Author Information

  1. Department of Space Physics and Astronomy, Rice University, Houston, Texas 77251

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1984

ISBN Information

Print ISBN: 9780875900582

Online ISBN: 9781118664223



  • Interplanetary magnetic field (IMF);
  • Low-altitude spacecraft;
  • Magnetic merging;
  • Magnetosphere;
  • Particle distributions


Because dayside merging occurs at the magnetopause (either near the nose or at the cusps), evidence of merging obtained by low altitude spacecraft or ground-based instruments must necessarily be indirect. Nevertheless, the evidence is compelling. Much of the evidence consists of correlations between magnetospheric parameters and the interplanetary magnetic field (IMF). For example, geomagnetic perturbations near the noon cusp are affected by the sign of the east–west component of the IMF, both on the ground (the “Svalgaard-Mansurov effect”) and at spacecraft altitudes; this is most easily explained in terms of the east–west stress transmitted along interconnected field lines. Other lines of evidence include the expansion of the auroral zone and the intensification of the auroral electrojets with increases in the southward component of the IMF, IMF control of the access of energetic solar particles to the polar cap, and the change in the sign of magnetic perturbations as the IMF changes from southward to northward, indicating a region of sunward flow in the polar cap. Using low-altitude spacecraft data, one can also infer the existence of magnetic merging from the significant correlations of the cross-polar-cap potential difference, and of the Birkeland current intensity, with the southward component of the IMF. The observed energy-latitude dispersion of cusp ions, wherein the characteristic energy of precipitating ions decreases with increasing latitude, is most readily explained by magnetic merging. At mid-altitude (3–4 RE), one can even discern the corresponding merging dispersion signature within a single pitch-angle scan. By analyzing these dispersion signatures, one can infer that acceleration of magnetosheath ions by about 1 keV occurs at a geocentric field-aligned distance ∼10 RE, as one expects from magnetic merging. During periods of northward IMF, one can also observe dispersions of the reverse type, which may be caused by merging between northward magnetosheath field lines and geomagnetic field lines tailward of the cusp. Such tail merging may also explain features of the “theta” arc phenomenon.