arXiv:0905.3060v2 [astro-ph.SR] 9 Sep 2009
Astronomy & Astrophysics manuscript no. kc
September 9, 2009
c ? ESO 2009
Letter to the Editor
Sunspot seismic halos generated by fast MHD wave refraction
E. Khomenko1,2, and M. Collados1
1Instituto de Astrof´ ısica de Canarias, 38205, C/ V´ ıa L´ actea, s/n, Tenerife, Spain; e-mail: email@example.com
2Main Astronomical Observatory, NAS, 03680, Kyiv, Ukraine.
Received XXX, 2009; accepted xxx, 2009
Aims. We suggest an explanation for the high-frequency power excess surrounding active regions known as seismic halos.
Methods. We use numerical simulations of magneto-acoustic wave propagation in magnetostatic sunspot model.
Results. We propose that seismic halos can be caused by the additional energy injected by high-frequency fast mode
waves refracted in the higher atmosphere due to the rapid increase of the Alfv´ en speed. Our model qualitatively explains
the magnitude of the halo and allows to make some predictions of its behavior that can be checked in future observations.
Key words. Magnetohydrodynamics (MHD) – Sun: magnetic fields – Sun: oscillations – Sun: helioseismology
Almost since the discovery of the 5-min solar oscillations it
is well known that the oscillation power is reduced by some
40–60% in the photospheres of sunspots (Lites et al., 1982;
Abdelatif et al., 1986; Brown et al., 1992; Title et al., 1992;
Hindman & Brown, 1998). Later it was found that the high-
frequency non-trapped wave power shows a suspicious en-
hancement in rings surrounding active regions, both in the
photosphere (Brown et al., 1992) and in the chromosphere
(Braun et al., 1992; Toner & Labonte, 1993). These power
enhancements are known as “halos”. Their observational
properties can be summarized as:
(i) The power enhancement is observed at high frequen-
cies between 5.5 and 7.5 mHz for waves that are usually
non-trapped in the non-magnetic quiet Sun.
larger than in the nearby quite Sun by about 40-
60% (Hindman & Brown, 1998; Braun & Lindsey, 1999;
Donea et al., 2000; Jain & Haber, 2002; Nagashima et al.,
(iii) The halos are observed at intermediate longitudi-
nal magnetic fluxes ?B? = 50 − 300 G, while the acous-
tic power is usually reduced at all frequencies at larger
fluxes (Hindman & Brown, 1998; Thomas & Stanchfield,
2000; Jain & Haber, 2002).
(iv) The radius of the halo increases with height. In
the photosphere the halos are located at the edges of ac-
tive regions, while in the chromosphere they extend to a
large portion of the nearby quiet Sun (Brown et al., 1992;
Braun et al., 1992; Thomas & Stanchfield, 2000).
(v) The power increase in the halo is qualitatively sim-
ilar in sunspots, pores and plages.
(vi) Significant reflection of the upcoming acoustic ra-
diation at 5–6 mHz is detected in active regions, unlike
the behavior of such high-frequency waves in the quiet Sun
(Braun & Lindsey, 2000).
While several plausible mechanisms have been pro-
posed to explain the acoustic power reduction for the
strongest fields in active regions (e.g. MHD mode con-
version; Cally & Bogdan, 1997), no accepted theory ex-
ists to explain the power enhancement in acoustic halos.
The increase of the high-frequency acoustic emission, ini-
tially proposed by Brown et al. (1992) and Braun et al.
(1992), seems not to find observational confirmations,
since the observed continuum intensity does not show the
halo effect (Hindman & Brown, 1998; Jain & Haber, 2002).
Alternatively, the latter authors propose that the velocity in
the surroundings of active regions may become field-aligned
and some type of incompressible waves may be responsible
for halos. This, however, lacks any observational evidence.
Recently, Kuridze et al. (2008) suggested yet another mech-
anism based on acoustic waves trapped in field-free atmo-
spheres lying below small-scale magnetic canopies of net-
work cores and active regions. Interestingly, halos were ob-
served recently in MHD simulations of waves in magnetic
structures by Hanasoge (2008) and Shelyag et al. (2009).
Based on his simulations, Hanasoge (2009) suggests that
the power enhancement in halos is due to magnetic field in-
duced mode mixing resulting in preferential scattering from
low to high wave numbers.
In this Letter we propose a mechanism based on the
fast MHD mode refraction in the vicinity of the transfor-
mation layer (where the Alfv´ en speed vA is equal to the
sound speed cS) that is capable to explain several obser-
vational properties of halos. In addition, we predict some
new properties that can be obtained from observations in
the future to confirm or discard this explanation.
2. Description of methods
We perform 2D numerical experiments that are essen-
tially similar to those by Khomenko et al. (2009), to study
the adiabatic propagation of magneto-acoustic waves ex-
cited by a single source located at sub-photospheric lay-