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2000ApJS..127..323G - Astrophys. J., Suppl. Ser., 127, 323-323 (2000/April-0)

Using Balmer line shapes to diagnose physical conditions in collisionless shock waves.


Abstract (from CDS):

When a collisionless shock propagates through a partially ionized, tenuous medium (n≤1 cm–3), the neutral atoms overrun by the shock produce Balmer line emission as they are excited by collisions in the hot postshock gas. The collisional excitation of neutral atoms and their charge exchange with hot postshock protons produces both narrow- and broad-velocity components in each Balmer line. The shape, width, and relative Doppler shift between these two components indicate the temperature and viewing angle of gas close to the shock front. Although such analyses have been applied to a number of young supernova remnants, such as Tycho, SN 1006, and RCW 86 (shock velocities between 800 and 2500 km.s–1), the lack of knowledge about the degree of electron-proton and proton-proton equilibration in these shocks has prevented a precise determination of shock parameters from the data.

Using long slit spectroscopic observations (resolution 0.75-1 Å) at several positions along Tycho's supernova remnant, we have obtained high signal-to-noise measurements of the broad Hα line profile, and have examined its shape for departures from a Maxwellian distribution. Our observations include a nearly edge-on filament and a nearly face-on filament. In the latter case, the Doppler shift between broad and narrow components is large enough to produce a clear separation between the two lines; this enables us to examine the broad component shape in greater detail. We find that the profile of the broad Hα line is well fit by a Maxwellian shape, indicating that the postshock heating has established a Maxwellian proton distribution. Comparison with proton distributions measured by spacecraft in the Earth's bow shock allows us to determine in principle whether the same plasma heating processes operate at very high magnetosonic Mach numbers (inaccessible in terrestrial laboratories).

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