Chromospheric Oscillations in Plages

Abstract

The 5-min oscillations in Hα plages as reported by Bhatnagar and Tanaka (1971, Solar Phys.24, 87) are discussed with respect to spatial – both horizontal and vertical – phase relation. Data for analyses include (a) Hα filtergrams at −0.5 Å (1050 min in total) (b) Hα filtergrams simultaneously taken at ±0.5 Å (60 min) (c) Simultaneous Hα−0.5 Å and FeI 5233 Å videomagnetograms (Doppler mode) (100 min in total). Projection (16 and 35 mm) -photocell method (Bhatnagar and Tanaka, 1971) was used for tracing the data with special attention to the guiding and reproducibility. Also photographic prints were used for comparison. The results are as follows: (1)those showing oscillatory brightness changes are dark dot-like features (size ≲ 1″) in brighter regions of plage and slightly elongated features (1 × 1.5–2″) in less bright regions. The mean number density of these features is one per 2200 ± 500 km square.(2)There are always a few regions with a size 8″ or more in one plage which show very regular oscillations with a period of about 5 min for a long time (at least 6.5 h, Figure 1). The distribution of the intervals between successive intensity peaks is much narrower than the Gaussian distribution and has a skewness towards longer period. The other places in the plage show relatively irregular patterns, but with the mean interval of the fluctuations still being 5 min.(3)In regular 5-min brightness oscillations the phase of oscillations in one wing is complementary to that in the opposite wing, indicating velocity oscillations. When the intensity records are relatively irregular there are many cases (76%) in which the blue wing patterns can be matched well for the whole time sequence with the red wing records by advancing the red wing records by 196 ± 18 s, (see Figure 2) indicating that the observable dark feature first appears in rising mode and then with a constant time lag (close to the free fall time from −0.5 Å to +0.5 Å) appears in the falling mode.(4)The records of brightness variations are very similar when the aperture is changed from 4″ (minimum reliable aperture) to 10″ in most places. Comparison of the oscillation phases in all the measured points in one plage shows that a plage can be divided into several areas (for a certain time) with sizes from 7000 km to 30000 km in each of which the phases are very close to each other for 2 to 10 cycles. Figure 3 shows the distribution of the intensity peaks of oscillations measured at Hα–0.5 Å (the abscissa is time, the ordinate indicates space points arranged geometrically). The boundaries of these areas vary after the coherent cycles. The iso-intensity curves of the intensity-time records (see Figure 3) have slopes corresponding to horizontal phase velocity 100 km-600 km s-1.(5)The phase relation between Hα regular 5 min and FeI 5233 Å 5-min velocity oscillations at the same place is almost constant for more than 5 cycles although the phase lag is different from place to place. (See, for example, the period from 20:00 UT to 21:00 UT in Figure 1. The smoothed-out curve denotes the record of the velocity oscillations in λ 5233 Å taken simultaneously.) The distributin of the phase lags is peaked at about 1 min. Assuming 1000 km for height difference of two lines the vertical phase velocity is equal to 17 km s-1.(6)There is an indication that the flare originating in the plage occurs about 20 min after the oscillations start to be in phase (1-min accuracy) in very large scale (≳20000 km) although the samples (6 flares in three different plages) are not large enough for statistically meaningful discussions.