BL106

AU Scale HI Opacity Variations

Abstract

H I absorption measurements toward extragalactic sources and pulsars have shown changes in the H I optical depth T ~ 0.1 on angular scales of order 10-100 mas corresponding to change sin density, spin temperature, or both on linear scales of 5-1000 AU. The current challenge is to move beyond simple cataloging of lines of sight showing these changes to understanding the relevant physics by which these variations are produced and maintained. We have preliminary detections of temporal changes in the optical depth and Zeeman splitting in one velocity component toward 3C 138. However, these detections are only marginally significant. The proposed observations are designed to confirm these measurements of the velocity and magnetic field structure within the small-scale neutral interstellar medium toward 3C 138.

**Investigators**
Name	Other *	Institution	Email	Phone
Joseph Lazio	PI	Naval Research Laboratory	lazio@rsd.nrl.navy.mil	202 404 6329
Miller Goss		NRAO-SOC	mgoss@aoc.nrao.edu	505-835-7300
Crystal Brogan		NRAO New Mexico Facilities	cbrogan@nrao.edu	505 835-7224
Michael Faison		Beloit College		608 363-2258
A. Zauderer		Agnes Scott College
Christopher G. De Pree		Agnes Scott College, GA 30030	cdepree@agnesscott.edu	1-404-471-6266

* PI = Principal Investigator; T = Thesis observations; S = Student

Front Ends

Gregorian L(1.15 to 1.73 GHz)

Back Ends

VLBA recorder and DAR

Type of Observing

Very Long Baseline Interferometry

Switching Type

Allocated time: 12.00 hours.

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