Issue |
Astron. Astrophys. Suppl. Ser.
Volume 125, Number 2, October II 1997
|
|
---|---|---|
Page(s) | 381 - 390 | |
DOI | https://doi.org/10.1051/aas:1997228 | |
Published online | 15 October 1997 |
Active region effects on solar irradiance at Na I D lines
1
Dipartimento di Scienze Fisiche dell' Università "Federico II", Mostra d' Oltremare pad. 19, I-80125 Napoli, Italy
2
Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy
Send offprint request to: G. Severino
Received:
9
July
1996
Accepted:
3
January
1997
The possibility to detect solar oscillations in the low
frequency domain depends crucially on the power contrast
among the oscillation signal
and other time dependent signals in the same frequency range.
The signal to noise ratio
is increased by our ability to understand and remove solar sources of
noise.
In measurements of the mean Doppler velocity shift of the
integrated solar disk, the solar noise has a line component
spectrum with a major peak at 13.1 days, and
a second less prominent peak at 27.2 days.
Active region modulation is believed almost
completely responsible for this signal.
We develop simulations of
the flux and velocity fluctuations
produced by different solar active region distributions,
based on an analytical description of their action.
From a grid of models of active regions and from their spatial
distribution over the disk,
we calculate the synthetic flux profile in the Na I line
and determine the velocity measure
of a resonance spectrometer.
Our velocity results are compared with the offset velocities
from the IRIS network.
There is a rather good agreement between the observed and computed velocities,
and the plage contribution to the noise appears to be dominant.
The simulation allows to test calibration procedures and to
study the effect on the spurious velocities
of different parameters, such as
the intensity thresholds used to
determine the areas of spots and plages,
and the contrast of the active regions.
In particular, we find
that the inclusion of intrinsic line shifts in plages can change strongly
both the amplitude and the shape of the simulated signal,
and then may be an important source of uncertainty
for the simulation.
Key words: solar activity / solar oscillations / solar rotation / methods: data analysis
© European Southern Observatory (ESO), 1997