Do magnetic waves heat the solar atmosphere?

Dr. E.J. Zita (zita@evergreen.edu)The Evergreen State College

Fri.30.May 2003 at Reed CollegeNW Section meeting, American Physical Society (APS)

This work supported by NASA's  Sun-Earth Connection Guest Investigator Program, NRA 00-OSS-01 SEC

Evergreen students and faculty are investigating the unexplained temperature rise from 5770 K at the photosphere to millions of degrees in the corona. We combine theoretical calculations with analysis of observational and numerical data to develop a more complete understanding of the role of magnetohydrodynamic (MHD) waves in heating the chromosphere. One group of students analyzed energy flux in 3D MHD simulations of pressure modes propagating obliquely into a strongly magnetic regions. Another student analyzed UV oscillation data from the SUMER satellite and found that p-modes lose power with altitude, and that the frequency spectrum of oscillations depends on the local field strength. Zita is performing complementary analytic calculations of MHD wave propagation in sheared magnetic field regions. Taken together, these investigations suggest that in regions where magnetic pressure is comparable to plasma pressure (=1), p-modes may mix into Alfvénic and magnetosonic waves. These MHD waves may carry energy to higher altitudes, where it is deposited by Joule heating and field line reconnection. Processes such as these help solve the mystery of the anomalously high coronal temperatures.

ABSTRACT

Magnetic dynamics may heat the solar atmosphere

Magnetic outbursts affect EarthRecent Solar Max:

• More magnetic sunspots• Strong, twisted B fields• Magnetic tearing releases

energy and radiation • Cell phone disruption• Bright, widespread aurorae• Solar flares, prominences,

and coronal mass ejections• Global warming• next solar max around 2011

CME movie

Methods: Simulations

Nordlund’s 3D MHD code models effects of surface acoustic waves near magnetic network regions.

Students wrote programs to analyze supercomputer data from ITAP HAO.

Calculated energy fluxes out of each region.

Pressure (p-)mode oscillates in left half of network region at photosphere. Waves travel up into chromosphere.

Results: Simulations

Magnetic energy fluxes grow; MS and Alfvén out of phase.

Pressure-mode energy flux decreases with height.

Conclusions: Simulations

• Parallel acoustic waves are channeled along field lines

• Oblique acoustic waves can excite magnetic waves and lose energy

• Strong mode mixing near =1 regions• Magnetosonic and Alfvénic waves can carry energy to

high altitudes

Matt Johnson, Sara Petty-Powell, E.J. Zita, 2001, Energy Transport by MHD waves above the photosphere

Methods: Observations

SOHO telescope includes SUMER, which measures solar UV light

UV oscillates in space (brightest in magnetic network regions) and in time (milliHertz frequencies characteristic of photospheric p-modes).

Results: Observations

• Fourier analyze UV oscillations in each wavelength

• Shorter-wavelength UV at higher altitudes, where chromosphere is hotter

• P-mode oscillations weaken with height

Noah S. Heller, E.J. Zita, 2002, Chromospheric UV oscillations depend on altitude and local magnetic field

Conclusions: Observations

• Magnetic waves carry energy to higher altitudes while p-modes weaken.

• Lower frequency oscillations stronger in magnetic regions.

• Higher frequency oscillations stronger in internetwork regions.

Methods: TheoryObservations Schematic Mathematical model

x

• Model sheared field region with a force-free magnetic field:

Bx=0, By = B0 sech(ax), Bz = B0 tanh(ax)

• Write the wave equation in sheared coordinates.

• Solve the wave equation for plasma displacements.

• Find wave characteristics in the sheared field region.

The wave equation describes how forces displace plasma.

= frequency, = displacement, cs = sound speed, vA = Alfvén speedB = total magnetic field, B0 = mean field, b1 = magnetic oscillation

Alfvén waves

Magnetosonic waves

B v

B

v

k || B k ||

2

2 20 1 1 02

0As

vc B b b B

B

Waves transform as they move through a sheared magnetic field region.

Results: Theory

Conclusions: Theory

• Magnetic energy travels along or across magnetic field lines.

• Twisting or shearing increases magnetic energy

• Shearing mode transformation

• Twisting tearing explosive release of magnetic energy.

Summary• Something carries energy from the solar surface to heat the solar atmosphere, …

• … but photospheric pressure modes weaken with altitude.

• P-modes transform into magnetohydrodynamic modes, especially where ~1 or vA ~ cs …

• … then Alfvénic and magnetosonic waves carry energy from the photosphere up into the chromosphere.

• Magnetic waves can heat the chromosphere by tearing, reconnection, and Joule heating.

• Magnetic dynamics are important on the Sun and affect weather and communications on Earth.

AcknowledgementsThanks to Tom Bogdan, Phil Judge, and the staff at the High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR) for hosting our summer visits and teaching us to analyze numerical and satellite data, and to BC Low for suggesting the form of the sheared field.

Thanks to computing staff at Evergreen for setting up Linux boxes with IDL in the Computer Applications Lab.

References•Tom Bogdan, Johnson, Petty-Powell, Zita, et al., 2002, Waves in magnetic flux concentrations, Astronomische Nachrichten, 323 Issues 3/4 p.196•Dick Canfield et al., IEEE Transactions on Plasma Physics, special issue on Space Plasmas, 2000•Noah Heller, E.J. Zita, 2002, Chromospheric UV oscillations: frequency spectra in network and internetwork regions •Matt Johnson, Sara Petty-Powell, E.J. Zita, 2001, Energy Transport by MHD waves above the photosphere•B.C. Low, 1988, Astrophysical Journal 330, 992

E.J. Zita: zita@evergreen.edu: http://www.evergreen.edu/z/zita/research.htm (links to our papers and posters)

The Evergreen State College: http://www.evergreen.edu

HAO = High Altitude Observatory: http://www.hao.ucar.edu

NCAR= National Center for Atmospheric Research: http://www.ncar.ucar.edu/ncar/Montana St. Univ., http://solar.physics.montana.edu/canfield/

SOHO = Solar Heliospheric Observatory: http://sohowww.nascom.nasa.gov/

SUMER = Solar Ultraviolet Measurements of Emitted Radiation: http://www.linmpi.mpg.de/english/projekte/sumer/