**New Plasmasphere Related Terms Derived from IMAGE/EUV**

Dennis Gallagher

A variety of terms intended to describe observed plasmaspheric features
have arisen over the years from in situ observations. With the launch of
IMAGE, the EUV observations have lead to a new and confusing serge of
entirely new terms. In an effort to tame our enthusiasm with being able
to see the plasmasphere in global images and to reduce the growing
confusion, the IMAGE/EUV team has sought to standardize terms for
classes of features found in EUV images. Figure 1 show a summary of
example images and term definitions for various features.

1\. The **plasmapause** in these images is the boundary between
background noise and clearly observable He^+^ content. Analysis has
shown \[Goldstein/Spasojevic?\] that this edge in the observed
plasmasphere corresponds well to the traditional sharp density feature
associated with the plasmapause boundary.

2\. The **plume** is that sunward extended feature associated with
erosion of the plasmasphere during times of enhanced convection. It has
most often been called the plasmaspheric tail in the past, but the
tendency for the public to confuse this tail with the magnetotail, which
after all extends behind the Earth relative to the Sun in a true
tail-like fashion, has lead to a new more general term that also has
some foundation in the literature.

3\. A **channel** is a density depletion that is mostly extended in
azimuth and one likely evolutionary result of a recovering plasmaspheric
convection plume that becomes captures by the corotation, although other
origins may be possible.

4\. **Shoulders** are a new term that is associated with a relatively
smooth outward bump in the plasmapause. The bump-out is often sharp on
the leading (or eastward) corotational boundary, varying smoothly to the
west. However, shoulders are sometimes seen with the opposite morphology
east-to-west.

5\. **Crenulation** is a general term referring to radial irregularities
or undulations with changing MLT in the plasmapause boundary that is
distinct in L-shell. Small and rather large-scale undulations or
crenulations are observed.

6\. **Fingers** are nearly radial, enhanced density features that are
narrow in MLT, but extended in L-shell.

7\. **Notches** are density cavities found in the plasmapause profile.
Notches can be as narrow as fingers, which can be as narrow as 0.1 R~E~,
or can extend an hour or more in MLT. Unlike crenulations, notches are a
single inward excursion of the plasmapause. The interior plasmapause
shape can be varied, however there is sometimes a quite distinct "W"
profile that has yet to be explained. Sometimes notches appear to be
associated with the late evolution of plasmaspheric plumes following a
period of high convection electric field.

**Statistics in the Occurrence of Plasmaspheric Features:**

Table 1 shows the current results from seeking to identify specific
features in EUV images. Features are being sought on a orbit-by-orbit
basis for the IMAGE spacecraft. The greatest number of orbital passes
has been examined for the presence of channel features. Out of 1175
IMAGE orbits starting from "first light" from EUV through most of 2003,
channels have been definitely identified during 158 orbits. Channels
have been tentatively identified during 213, which include definitive
identifications. This corresponds to finding channels in 13% and 18% of
the orbits, respectively. The apparent consequence of this analysis has
been to associate this specific spatial structure with previous in situ
studies that identified what have been called interior plasmaspheric
troughs or cavities. Given the strikingly similar statistical occurrence
of previously reported interior troughs to that now identified as
channels in EUV images, there is good reason to believe we understand
the spatial structure of the features previously observed. Continued
examination of EUV images for the purpose of establishing the
statistical occurrences for the various features can be expected to
further help in the identification of previously observed in situ
plasmaspheric features and help in associating features with possible
drivers for their formation.

References:

Carpenter, D. L., R. R. Anderson, W. Calvert, and M. B. Moldwin, CRRES
observations of density cavities inside the plasmasphere J. Geophys.
Res., 105, 23,323-23,338, 2000.

Horwitz, J. L., R. H. Comfort, and C. R. Chappell, A statistical
characterization of plasmasphere density structure and boundary
locations, J. Geophys. Res., 95, 7937-7947, 1990.

![](media/image1.png){width="5.998611111111111in" height="4.5in"}

Figure 1: The new terms for spatial features found in plasmaspheric
plasmas.