IMPROVISATION IS the axis around which revolve the various worlds of jazz. It defines the structure of composition, gives vent to the emotional murmuring of musicians and breathes a special sense of a character into music. Without if, jazz would simply be a milder form of pop music.

Pianist Keith Jattett, perhaps more than any other jazz musician, has taken improvisation to its ultimate limit by playing long (they often last more than an hour), solo musical monologues.But rather than simply presenting random thoughts, Jarretts forges his playing into coherent, if not predetermined, compositions. He is like a storyteller who begins his tales without knowing their plots or endings, but manages somehow to construct inner forms for his narratives.

His newest release, Sun Bear Concerts (EC1100), is a whopping 10-record set that documents five complete concerts he gave in Japan in 1976. With a $75 list price and only 1,000 copies available in the U.S., the record is hardly a profitable commercial project. It's more like "coffee table" record that enables Jarrett, now recognized as a "major" jazz musician, to make a statement about his music and himself as an artist.

There are certain stylistic similarities among all the records, and they do provide an exhaustive documentions of Jarrett's skill. An enclosed note states that because of the "special continuity" of the music, the records "allow the listener to perceive the developments that take place... throughtout successive concerts."

Several years ago, Jarrett was one of many young experimental musicians who were defining the creative boundaries of jazz. At that time, ECM's producer, Manfred Eicher, going against all business sense and logic, released Jarrett's Solo Concerts, Bremen Lausanne (EMC3-1035). The three-record set of solo improvisations was a tremensdous success and catapulated Jarrett to international fame.

Now the Sun Bear records display many of Jarrett's strength and weakness and provide a better understanding of his unique approach to jazz and improvisation.

Jarrett's beyond all else, an exceptionally gifted pianist whose techinical skills rival those of many classical masters. He is capable of dramatic and instantaneous leaps of rhythm and tempo. His dynamic range is equally great, going from subtle wisps of sound to thunderous volleys of chords. He has a binding left hand that races along the keyboard while his right hand, seemingly with a mind of its own, hammers out sharp, ostinato figures that contradict, then support his themes. Jarretth's techniques enables him to peruse the musical landscape and then express his visions with such an ease and assurance that they achieve an almost "visible" quality.

These visions-his improvisations-are like a musical stream of consciousness in which an idea is introduced an developed, evolving into series of thoughts that often bear little resemblance to the original theme, giving his work, at times, a disjointed and unrelated quality. At other times, his playing is strictly constructed. Common to each of the pieces, however, is an inner logic that propels them and gives them a sense of form.

There are drawbacks to this approach. Most of the improvisations begin slowly while Jarrett attempts to establish a sense of direction. He tends to rly on two particular devices-a recurring, gospel-oriented accompaniment or a quiet, atmospheric chordal background-when he is stuck for new ideas. He also repeats excessively certain sections, almost as if he is trapped by an idea and is desperately trying to get out of its clutches. Consequently, his playing, particularly over the course of 10 records, often seems redendant and "wordy."

Despite these difficulties (or because of them) the improvisations have an irrestible energy. When Jarett is struggling, there is an electriying nervousness that sparks a note of anticipation for the listener. Jarrett's transitions are also effective. The Osaka recital Features an abstract, fast-paced section that builds until it turns into sweeping arpeggios. A portion of the Kyoto concert is devoted to angular, machine-like images that resolve themselves into an intospective mood.

The means by which these images are projected are as electric as the images themselves. Jarrett is well-versed in various musical styles, and he borrows from these freely. Lisztian romantic flourishes contrasted with strident chords reminiscent of Bartok. Supple, impressionistic layers of sound give way to angular sections that recall the work of Boulez. Jarrett also features many jazz idioms, from a simple blues and the ever presetn tinge of gospel, to the more complex approach of pianists like Art Tatum. At one point (in "Osaka"), Jarrett sounds like a whimsical cross between Schoenberg and Eubie Blake as a sharply dissonant phrase is set against a jumpy, ragtime-like figure.

Sun Bear Concerts is no better and no worse than any of Jarrett's previous solo records. The improvisations have an individual value of their own, but their nature, they do not lend themselves to an easily definable musical course. These records do, however, provide an intriguing picture of Jarrett's work and that it constantly evolves.