High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 395

respectively. Similarly, the y component can be directed opposite to the earth's
revolution about the sun or parallel to it, which is referred to as positive or neg-
ative, respectively. Finally, it is worth noting that the tendency of the IMF to
assume a garden hose-like spiral in the solar wind means that, when the IMF
is directed away from the sun, the y component is positive and when the field
line is directed toward the sun, B y is negative. Thus, the terms “toward” and
“away” are sometimes used in the literature to describe the sign of the IMF y
component. One final note concerns the presentation of observational data at
high latitudes. Since the earth's magnetic field lines can be highly stretched on
the nightside and slightly compressed on the dayside, a dipole representation
is not very informative. Thus, it is frequently the practice to describe a point
in the high-latitude region with two parameters. One is the L value, defined as
the equatorial crossing point (in units of R E ) of the magnetic field line passing
through the point. The second is the magnetic local time (MLT), defined using
the angle between the earth-sun line and the plane containing the magnetic axis
of the earth and a line from the center of the earth to the point. Frequently, the
L value is expressed in terms of an invariant latitude

where

cos − 1 1

√ L

= /

60 ◦ . In a centered dipole field, the
latitude at which a magnetic field line passed through the surface of the earth
would be equal to the invariant latitude. The MLT is almost always expressed in
hours. It is also quite common to express the date as a five-digit number, called
the Julian day. In this format the first two digits of the number denote the year
while the last three digits denote which day of the year (assume that January 1
is day 1).
The most radical differences in the convection pattern can be seen by com-
paring signatures when the IMF is southward with signatures when the IMF is
northward. The discussion following is divided into these two categories. How-
ever, it should be emphasized that substantial variability exists in the convection
pattern, and our understanding of the nature of the controlling factors is still
evolving.

An L value of 4, then, corresponds to =

8.2.1 Observations During Southward IMF
Figure 8.7a shows vectors representing the plasma velocity perpendicular to the
magnetic field at points along a satellite track as the spacecraft passes through
the high-latitude convection pattern during a period of southward IMF. The
regions of sunward and antisunward convection are separated by well-defined
reversals near dawn and dusk and the signature of the two-cell convection pat-
tern is indicated by the dashed lines. The auroral zone is coincident with the sun-
ward flow regions as described above. If we arbitrarily assign a zero value for the
electrostatic potential at low latitudes, data of this nature can be integrated along
the satellite track to produce a representative electrostatic potential distribution,