NBP7 -Table 5.2

Table 5.2: Summary of frequencies (mHz) and correlations for NBP7. See Section 5.3.1 for details.

	From:	1.00	1.33	1.67	2.00	2.33	2.67	3.00	3.33	3.67	4.00	4.33
λ	To:	1.22	1.56	1.89	2.22	2.56	2.89	3.22	3.56	3.89	4.22	4.55
Ca II K3		99	95	99	99							95,95
Hα		99,99	99,95		95,95				99
Mg I b2		99	99,95	99,95			95		95	95		99
Mg I b1-0.4 Å		95,99	95,95,95	99		95		99

Wavelengths		Correlation Coefficient	Frequency (mHz)	Timelag (Units of 45 s)	Speed (km s-1
λ1	λ2
Ca II K3	Mg I b2	0.66 - 0.72	1.5 -1.7	8-11	2-4
Ca II K3	Mg I b1-0.4 Å	0.81 - 0.91	1.3 - 1.5	0 - 3	12-40+
Hα	Mg I b2	0.69 - 0.81	1.8 - 2.0	0 - -3	3-22+
		0.75 - 0.79	2.8 - 3.0	-1 - -3	3-22
		0.83 - 0.84	3.55 - 3.75	4 - 5	2-6
Hα	Mg I b1-0.4 Å	0.72 - 0.81	1.8 - 2.0	1 - 2	11-33
Mg I b2	Mg I b1-0.4 Å	0.96 - 0.98	1.8 - 2.0	1 - 5	2-12
		0.69 - 0.70	3.3 - 3.5	2	5-6

Correlations across Hα, Mg I b₂ and Mg I b₁-0.4 Å show a possible travelling wave at ~1.9 mHz at a speed around c_s. The FFTs also reveal a strong peak around this frequency in the lower chromospheric lines, with a weaker peak in Hα, and the latter also show the appearance of a strong peak around 3.6 mHz. There is also a correlation around 3.6 mHz (Hα to Mg I b₂) and at 3.4 mHz (Mg I b₂ to Mg I b₁-0.4 Å). This is consistent with Tests 1-4, where a transverse wave at 1.9 mHz travels up through the atmosphere and couples to a longitudinal wave at around twice the original frequency when it reaches the height of Hα (core) formation temperature. This longitudinal wave then continues to travel upward and eventually shocks, explaining the lack of power at ~3.5 mHz in Ca II K₃. The strong peaks at ~2.0 mHz in Ca II K₃ are unexpected, but do not appear to be correlated with oscillations at the same frequency in the lower chromosphere.

In the 1.33-1.56 mHz range the oscillations are initially weak but increase in power in Mg I b₂ and Hα, before decreasing again in Ca II K₃. However, there is no power at twice this frequency in Ca II K₃, and the only correlation is between Ca II K₃ and Mg I b₁-0.4 Å. There is a downward- moving wave between Hα and Mg I b₂ (2.9 mHz) and a slow upward- moving wave between Ca II K₃ and Mg I b₂ (1.6 mHz). Below 1.3 mHz there is significant power in all wavelengths, but no indication of any correlation. Several frequencies seem to be localised in height, specifically the peaks around 2.5 mHz in the two lower lines, and the peaks at 4.4 mHz in Mg I b₂ and Ca II K₃. These may be driven by events spatially removed from the NBP at each height, e.g. granular overshoot (Lites et al. 1993).