The Trouble with the Local Bubble
  The model of a Local Hot Bubble has been widely accepted as providing a
framework that can explain the ubiquitous presence of the soft X-ray background
diffuse emission. We summarize the current knowledge on this local interstellar
region, paying particular reference to observations that sample emission from
the presumed local million degree K hot plasma. However, we have listed
numerous observations that are seemingly in conflict with the concept of a hot
Local Bubble. In particular, the discovery of solar wind charge exchange that
can generate an appreciable soft X-ray background signal within the
heliosphere, has led to a reassessment of the generally accepted model that
requires a hot local plasma. In order to explain the majority of observations
of the local plasma, we forward two new speculative models that describe the
physical state of the local interstellar gas. One possible scenario is similar
to the present widely accepted model of the Local Hot Bubble, except that it
accounts for only 50% of the soft X-ray emission currently detected in the
galactic plane, has a lower thermal pressure than previously thought, and its
hot plasma is not as hot as previously believed. Although such a model can
solve several difficulties with the traditional hot Local Bubble model, a
heating mechanism for the dimmer and cooler gas remains to be found. The second
possible explanation is that of the Hot Top model, in which the Local Cavity is
an old supernova remnant in which no (or very little) million degree local
plasma is presently required. Instead, the cavity is now thought to be filled
with partially ionized cloudlets of temperature 7000 K that are surrounded by
lower density envelopes of photoionized gas of temperature 20,000 K.
