DYNAMICS OF SOLAR SPICULES
Harold Zirin and Robert Cameron
California Institute of Technology, Solar Astronomy 264-33, Pasadena, CA 91125
phone: +1 626 395-3857, fax: +1 626 395-3814
ABSTRACT
We have studied several hundred images of solar spicules obtained on June 18 and 19 of 1997. The observations were made at Big Bear Solar Observatory with the 65cm telescope feeding a Zeiss 1/4Å filter and a 1536x1024 Kodak CCD. Overexposed observations were made above the limb as well as normal exposures on the limb. The filter was tuned to Ha -0.65Å and a 30sec interval was used. We were limited to a single wavelength because new software was being installed in a new control computer.
The images obtained were processed by high-pass digital filtering of the original FITS images and reregistered by an FFT technique. The image scale is 0.17 arcsec per pixel. The disk was observed on June 18, 1997 to detect the sources of macrospicules and the limb was observed by overexposure on June 19 to determine the height trajectory of the faintest Ha spicules.
We found that:
1. LIMB OBSERVATIONS
About 200 arc seconds of the north limb of the Sun was observed for 5 hours on June 19. Observations were made of the equatorial limb as well for comparison, but this took place after the 5 hours of good seeing and is not strictly comparable. Events on the limb are mainly moving in the plane of the sky and the fact that observations were made at -0.65Å did not really produce important wavelength shift effects, as the typical spicule profile is more than 1Å wide. However, without the other wavelengths we cannot absolutely rule out such effects. Figure 1 shows one of the best images; the dark band along the limb results from the spatial filtering. Rather than the commonly expected up-and-down motion, the spicules tended to fade, sometimes quite abruptly, when they reached the top of their trajectory. This is quite evident when one views the observations in movies running back and forth. In the backward movies, no upward surging is seen, which means that there is no rapid sinking to the surface reversing the upward path.
The lifetimes of most of the events were fairly short, certainly no longer than 10 minutes and some only visible for a few minutes. While it is possible that our use of a single wavelength and a 30sec cadence might permit sudden disappearances, we have found no evidence for such occurrences. Further, other papers given at this conference seem to show similar results. Figure 1 shows a normal background of smaller spicules about 4,000 or 5,000 kilometers high above the limb. The higher events, which extend as high as 20,000 kilometers above the surface, were observed in a whole variety of forms and a wide range of brightness. First there are long spicules, which were observed both vertical and tilted as far as 30 degrees from the radial. While these had the classic shape we tend to think of, very few if any were seen to return to the surface. In general, they would extend and then fade out at the top of their trajectory. In some cases, as we see in Figure1, these are multiple. The second class was eruptions, which fly up to about 15,000 kilometers in an irregular way. These are the same as the phenomena reported by LaBonte (1974). The velocities inferred from the proper motion of these events are quite large, between 50 and 100 km/sec. While spectra of macrospicules do not at present exist, spectra and velocity movies of disk spicules do not show velocities this high. It is possible that the macrospicules have the higher velocity; alternatively we may be seeing a phase velocity.
These data contradict the result of Suematsu et al. (1995), who clearly observed Doppler shifts along the extending part of the trajectory as well as falling Doppler shifts in the later stages of the spicule lifetime. The spicules studied by those authors were highly inclined because of strong closed fields. We viewed their data forward and backward and found a preponderance of upward motion. Further, it is well known that disk spicules are much clearer in the blue wing. It is clearly possible that the spicules in these closed field regions may behave differently than those at the pole. The data is of high quality in both cases.
2. DISK OBSERVATIONS ON JUNE 18
On this day, we were interested in finding the source of macrospicules. Perhaps we can detect some particular characteristic which makes these events so high. So we observed at -0.65Å between the limb and about 30 degrees inward, the field of view being about 180 arc seconds.
When the exposure is correct, one can also study the appearance of limb-crossing spicules. This is shown in Figure 2, where an Ha image has been high-pass filtered. While this procedure mainly confuses the disk spicules, it removes the brightness step at the limb and permits us to examine the limb-crossing spicules. Because of the long line of sight, these are few, but the reader will see some. They are mostly dark, but a few bright spicules also are seen to cross the limb.
The radiative transfer question is somewhat complicated. The limb spicules are only somewhat less bright than the dark elements seen on the disk. So it is a little hard to understand why the foreground ones appear darker. It may be purely a scattering effect, since the foreground spicule is only illuminated from behind. There also is the effect of Doppler brightening (Zirin, 1969) because the moving matter is illuminated by disk continuum, but this is mainly important in line center.
We also took high-sensitivity magnetograms with a cadence of about 210 sec which showed virtually no change in the field in the course of the day and no trace of opposite polarity. We cannot rule out shorter-time transient changes, but there is as yet no evidence for them. At the present solar-minimum period one finds one polarity at the pole.
The appearance of the spicules on the disk is shown in Figure 3, a frame at Ha-0.65Å, which has been somewhat sharpened. The spicules appear slightly different from those at the limb, mostly because the bases are visible. Long spicules, which likely correspond to macrospicules, were easily visible, and arose in the same bushes (Cragg et al., 1963) as normal spicules. These macro events characteristically begin with a structure resembling the Eiffel Tower, one of which is illustrated in Figure 4. Many of these features were seen, at least two or three at any given time. They rise to a height of about 15,000 kilometers and then appear to twin into a double spicule and fade away. We cannot distinctly ascribe any particular characteristic to the bases of these.
Beside the Eiffel Tower form, simple macrospicules appear, unstructured elongated single or multiple forms. These various forms are seen on images which mostly have single spicules, so we do not believe they are artifacts of seeing. In Figure 4, one sees clearly how the Eiffel Tower structure evolves into what may be a rotating or twisting spicule and then into a twin spicule. The twin spicule fades away fairly quickly, not lasting more than one minute. The whole process takes less than 4 minutes, and its scale relative to ordinary spicules may be judged from Figure 3.
It is generally believed that spicules follow the local field lines; thus for the duration of the spicule we have information as to the spatial structure of the magnetic field. We have little evidence of the spatial structure of the field before the spicule begins; however, we can (speculatively) interpret the spicule at 17:16:48 as the beginning of intertwining of the field lines. The field line structure appears twisted at 17:18:50 and simplifies by 17:19:21. The fact that the field lines become twisted simultaneously with the formation of the spicule and become untwisted immediately before it fades is evidence that these Eiffel tower structures may be related to the same disturbance which twists the field.
This work was supported by NASA through grant NAGW-1972.
REFERENCES
Cragg, T., R. Howard, and H. Zirin 1963, Vertical Structures in the Chromosphere, Ap. J. 138, 303
LaBonte, B. 1974, Activity in the Quiet Sun I: Observations of Macrospicules in Ha and D3, Solar Phys. 61, 283
Suematsu, Y, H. Zirin, and H. Wang, H. 1995, High-Resolution Observation of Disk Spicules I. Evolution and Kinematics of Spicules, Ap. J. 450, 411
Zirin, H. 1969, Two Prominence Eruptions and the Problem of Emission, Solar Phys. 7, 243