Importance of Solar Atmospheric Coupling on P-Mode Power within Magnetic Elements

Thursday, 18 December 2014
Andrew Gascoyne, University of Sheffield, Sheffield, S10, United Kingdom and Rekha Jain, University of Sheffield, Sheffield, United Kingdom
It has long been known that the power of p-mode oscillations is reduced
within magnetic plages and sunspots at photospheric level. Recent observations
now suggest that this suppression of power extends into the low chromosphere and is
also present in small magnetic elements far from active regions. We
construct a model to investigate a possible mechanism of this power
loss whereby p modes buffet small magnetic elements and excite MHD
sausage tube waves. These magnetic tube waves propagate along the many
magnetic fibrils which are embedded in the convection zone and expand
into the chromosphere due to the fall in density with height of the
surrounding plasma. We treat the magnetic fibrils as vertically
aligned, thin flux tubes embedded in a two region polytropic-isothermal
atmosphere to study the coupling of p-mode driven sausage waves,
which are excited in the convection zone and propagate into the
overlying chromosphere. The excited tube waves carry energy away from
the p-mode cavity resulting in a deficit of p-mode energy which we
quantify by computing the associated damping rate and absorption
coefficient of the driving p modes. We also compare the vertical
motion within the fibril with the vertical motion of the incident p mode
by constructing the ratio of their powers using HMI data and theory.
In agreement with observational measurements we find that the total
power is suppressed within strong magnetic elements for frequencies
below the acoustic cut-off frequency. We also find that the magnitude
of the power deficit increases with the height above the photosphere
at which the measurement is made. Further, we argue that the area of
the solar disk over which the power suppression extends increases as a
function of height.