XRT:
2007-10-24T07:15:45 to 2007-10-24T10:38:54
Science Goal: Coordinated Campaign Observation with THEMIS - c. Measurements of Magnetic Energy and Helicity Fluxes, A search for the roots of Parker's nanoflares using Hinode
Program: TI_just_after_OP_Upload
Target:
xcen=-1000000000 ycen=-1000000000
Instrument: XRT
HOP/JOP: 0
Description:
Tim File-OBS_DEC: AR topology - Al/poly, Ti/poly - long/short exp
Daily Note: none
Request to XRT HOP Number 0011: Successive multi-wavelengths observations
Other Instruments: THEMIS
Scientific Objectives: Daily Note: none
Request to XRT HOP Number 0041:
Other Instruments: Hinode-only
Scientific Objectives: Science justification ===================== In 1972, Parker studied the implications of convective, turbulent motions on the magnetic fields extending into the solar atmosphere.
He proved that for all but highly contrived motions, there is no mechanical equilibrium.
The magnetic fields extending above the photosphere must evolve to dissipate the injected electromagnetic energy on dynamical (Alfvenic) time scales.
He dubbed this state of affairs ""topological dissipation"". It is topological in nature because equilibrium places strict topological constraints on the magnetic field, which are extremely unlikely to be met on the Sun.
Dissipation is required to account for the observation that most coronal and chromospheric structures live longer than the Alfvenic crossing time.
In his 1994 monograph, Parker showed that current sheets are a natural, spontaneous consequence of enforcing the constraint of magnetostatic equilibrium in forced, natural systems.
Current sheets are a natural site for dissipation of magnetic free energy stored in the atmosphere. In 1988, Parker estimated what the release of energy stored in this way might look like, as applied to plage regions of the Sun.
His paper was motivated by space-based observations of variability on short time scales over the chromospheric network, which he ascribed to the ""nanoflare"", a burst of heating of 10^24 erg or so. One of the primary limitations at that time was lack of knowledge of the surface magnetic fields, they being below the seeing limited capability of ground-based telescopes, and limited by the short duration of time series needed to study the evolution of the ""drivers"" of the magnetic fields.
In Parker's 1988 paper, he states:
""It is unfortunate that the motions of the magnetic fibrils are
not presently available from observation, since it is the
jiggling and wandering of those fibrils that provides most if
the energy input to the X-ray corona. We shall assume, for the
sake of discussion, that in keeping with the granule motions of
1-2 km/s, the foot points of the magnetic field are shuffled
about at random with a characteristic velocity v of the order
of 0.5 km/s, and with a correlation length l comparable to a
granule radius.
Hopefully, within the next decade a proper
observational determination of v and l will become available."" In this proposal, we wish to use Hinode to obtain seeing-free time series of duration sufficient to investigate foot point motions in the photosphere.
We wish to study the topological forcing of the overlying atmosphere by tracking magnetic features, the modification of this topology throughout the chromosphere, and any associated dissipation in the corona. It is important for us to include the chromosphere in this study for several reasons.
The plasma beta=1 surfaces occur somewhere in the chromosphere. The twist/ braiding proposed by Parker may not survive to coronal levels, more recent work has indicated that the Maxwell stresses are large only near the ""boundaries"" (photosphere) of the overlying magnetic structure, and that the magnetic fields may be relatively uniform in the bulk of structure, including the corona (e.g., Sakurai and
Levine 1981, Arendt and
Schindler 1988).
LUNDQUIST, L. and GRIGIS, P.
