D3 spicules and the lower chromosphere

Imaging spectroscopic data of the He i 1083 nm limb emission were taken on several dates in October and November 1995 with the NASA/NSO spectromagnetograph at the NSO/Kitt Peak vacuum telescope and on 9 December, 1993 with the Michigan infrared camera at the NSO/Sacramento Peak vacuum tower telescope. Emission line profiles were observed in quiet-Sun and coronal hole locations on the northern and southern solar poles and on the east solar limb. The height of the He I 1083 nm shell above the continuum limb at 1083 nm was measured to be 2.11 -4-0.12 Mm with the Kitt Peak data, and 1.74 -4-0.05 Mm with the Sacramento Peak data. The Kitt Peak data show (1) within the measurement error there is no significant difference in the height or thickness of the emission shell in coronal holes compared with the quiet Sun, (2) the 1083 nm emission intensity drops by 50% in coronal holes, (3) the line width decreases by about 2 km s -~ in coronal holes (suggesting less inclined spicules), (4) the line width of the He l 1083 nm line jumps significantly as the line of sight crosses the solar limb (consistent with a higher temperature upper shell), (5) a quiescent prominence shows a smaller spectral line width (consistent with a cooler temperature or less velocity broadening), and (6) the entire emission shell and the prominence show a He I spectral component ratio of about 8 (suggesting optically thin emission).

Limb observations of He I 1083 nm

Penn¹,

Jones

1996

Activity in the quiet Sun

Labonte

1979

Suspended spicules associated with the enhanced bright network in an active region

Gaizauskas

1984

Abstract. The progressive rotation from the limb onto the disk of a long-lived cluster ofcoaligned He spicules was observed at high spatial resolution on the fringe of a large complex of activity. Although individual spicules were steadily changing, the organized cluster appeared consistently suspended above the photospheric limb when viewed in the wings of He ([A2[ ~ 0.9 ~). The phenomenon is the counterpart near an active region of the dark band discovered in the quiet low chromosphere by Loughhead (1969). But in the present circumstances the effect is perceived as a weakening of emission, i.e. as a gap rather than an obscuration. The initial gap between the off-band spicules and the photospheric limb narrowed and closed in about 4 h. A day later, the cluster of spicules could be identified at the same wavelength with a cluster of elongated dark mottles, similarly coaligned; they were adjacent to, but not in contact with, a foreshortened patch of faculae.The persistence of the gap in this cluster, and its occurrence in isolated spicules reported here in quiet regions, imply that the phenomenon is an inherent property of spicules. It is proposed that the gap results from a spicule-generating process initiated above the temperature minimum.