942 research outputs found
Generation of Alfven Waves by Magnetic Reconnection
In this paper, results of 2.5-dimensional magnetohydrodynamical simulations
are reported for the magnetic reconnection of non-perfectly antiparallel
magnetic fields. The magnetic field has a component perpendicular to the
computational plane, that is, guide field. The angle theta between magnetic
field lines in two half regions is a key parameter in our simulations whereas
the initial distribution of the plasma is assumed to be simple; density and
pressure are uniform except for the current sheet region. Alfven waves are
generated at the reconnection point and propagate along the reconnected field
line. The energy fluxes of the Alfven waves and magneto-acoustic waves (slow
mode and fast mode) generated by the magnetic reconnection are measured. Each
flux shows the similar time evolution independent of theta. The percentage of
the energies (time integral of energy fluxes) carried by the Alfven waves and
magneto-acoustic waves to the released magnetic energy are calculated. The
Alfven waves carry 38.9%, 36.0%, and 29.5% of the released magnetic energy at
the maximum (theta=80^\circ) in the case of beta=0.1, 1, and 20 respectively,
where beta is the plasma beta (the ratio of gas pressure to magnetic pressure).
The magneto-acoustic waves carry 16.2% (theta=70^\circ), 25.9%
(theta=60^\circ), and 75.0% (theta=180^\circ) of the energy at the maximum.
Implications of these results for solar coronal heating and acceleration of
high-speed solar wind are discussed.Comment: Accepted for publication in PASJ. 24 pages, 11 figure
Observations of Chromospheric Anemone Jets with Hinode SOT and Hida Ca II Spectroheliogram
We present the first simultaneous observations of chromospheric "anemone"
jets in solar active regions with Hinode SOT Ca II H broadband filetergram and
Ca II K spetroheliogram on the Domeless Solar Telescope (DST) at Hida
Observatory. During the coordinated observation, 9 chromospheric anemone jets
were simultaneously observed with the two instruments. These observations
revealed three important features, i.e.: (1) the jets are generated in the
lower chromosphere, (2) the length and lifetime of the jets are 0.4-5 Mm and
40-320 sec, (3) the apparent velocity of the jets with Hinode SOT are 3-24
km/s, while Ca II K3 component at the jets show blueshifts (in 5 events) in the
range of 2- 6 km/s. The chromospheric anemone jets are associated with mixed
polarity regions which are either small emerging flux regions or moving
magnetic features. It is found that the Ca II K line often show red or blue
asymmetry in K2/K1 component: the footpoint of the jets associated with
emerging flux regions often show redshift (2-16 km/s), while the one with
moving magnetic features show blueshift (around 5 km/s). Detailed analysis of
magnetic evolution of the jet foaming regions revealed that the reconnection
rate (or canceling rate) of the total magnetic flux at the footpoint of the
jets are of order of 10^{16} Mx/s, and the resulting magnetic energy release
rate (1.1-10) x 10^{24} erg/s, with the total energy release (1-13) x 10^{26}
erg for the duration of the magnetic cancellations, 130s. These are comparable
to the estimated total energy, 10^{26} erg, in a single chromospheric anemone
jet. An observation-based physical model of the jet is presented. The relation
between chromospheric anemone jets and Ellerman bombs is discussed.Comment: 22 pages, 27 figures, accepted for Publications of the Astronomical
Society of Japa
Statistical properties of superflares on solar-type stars based on 1-min cadence data
We searched for superflares on solar-type stars using Kepler data with 1 min
sampling in order to detect superflares with short duration. We found 187
superflares on 23 solar-type stars whose bolometric energy ranges from the
order of erg to erg. Some superflares show multiple peaks
with the peak separation of the order of - seconds which is
comparable to the periods of quasi-periodic pulsations in solar and stellar
flares. Using these new data combined with the results from the data with 30
min sampling, we found the occurrence frequency () of superflares as a
function of flare energy () shows the power-law distribution () with for erg which is
consistent with the previous results. The average occurrence rate of
superflares with the energy of erg which is equivalent to X100 solar
flares is about once in 500-600 years. The upper limit of energy released by
superflares is basically comparable to a fraction of the magnetic energy stored
near starspots which is estimated from the photometry. We also found that the
duration of superflares () increases with the flare energy () as . This can be explained if we assume the time-scale of
flares is determined by the Alfvn time.Comment: Accepted for for publication in Earth, Planets and Spac
- …