Chromospheric UV oscillations depend on altitude and local magnetic field Noah S. Heller and E.J. Zita, The Evergreen State College, Olympia, WA 98505

Chromospheric UV oscillations depend on altitude and local magnetic fieldNoah S. Heller and E.J. Zita, The Evergreen State College, Olympia, WA 98505

Philip Judge, HAO, NCAR, Boulder, CO 80301

ABSTRACT

Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Introduction: UV continuum emissions:

• Are brighter where gas is hotter, that is at higher altitudes• Intensity is greater in strong magnetic regions (network)• Intensity oscillations track photospheric waves traveling

up through chromosphere

Image courtesy of WHO at

http://WHERE

T h


Background: Photospheric oscillations

Supergranules excited by convection at He+ layer: x~ __ Mm, t ~ 1 day, f << 2 mHz

Granules excited by convection at H + layer: x~ __ Mm, t ~ 10 minutes, f ~ 1.5 mHz

p-modes at photosphere: x~ __ Mm, t ~ 5 minutes, f ~ 3 mHz

Image courtesy of WHO at

http://WHERE


SUMER measures chromospheric UV emissions

Image courtesy of Max-Planck-Institut für Aeronomie

http://www.linmpi.mpg.de/english/projekte/sumer/pictures/SUM_SOHO.HTML

Text


UV oscillations vary in space and time

• Sumer slit…

• Greyscale intensity plot for a given wavelegnth

• Timeseries at a give position xi and wavelength


Strong field regions correlate with bright regions

• MDI

• nw

•bright


Fourier transform frequency spectra

I

f

Integrated power

• Power spectrum shows frequency peaks for each xi and

• Integrate power spectrum over all positions, for each

• Sum power over each of three frequency ranges:

LF (0-2 mHz), MF (2-5 mHz), HF (5-10) mHz


2-5 mHz oscillation power increases with wavelength

• do this for LF, MF, HF

•do it for NW and INW

•note trend

Network

+ Internetwork


Oscillation power depends on local field strength

• note trend: power decreases further from photosphere p-modes are source of UV oscillations

•ratios: LF stronger in NW (granulation)

•HF stronger in INW (…)


Magnetic environment can transform waves

• Parallel acoustic waves can propagate freely to field lines

• Oblique acoustic waves can excite magnetic waves and lose energy


DISCUSSION

•


Nonlinearities:

p-modes shocks heating

p-modes transformed to magnetic waves less power to UV oscillations


SUMMARY: chromospheric UV oscillations reveal:

Significance:

•p-modes heat chromosphere and weaken as they rise

•Magnetic field strength and topology affects mode propagation and transformation

Results Interpretation

mHz UV oscillationsweaken with altitude

strong field regions havemore LF oscillations

weak field regions havemore HF oscillations

photospheric p-modes lose energyto heating and mode transformationas they rise into chromosphere

granulation / supergranulation -secular variations

HF modes transform to MHDwaves where field is strong &oblique


Next steps:

Compare to MDI data on local magnetic fields:

• check correspondence between intense UV and strong “network” fields

• investigate magnetic topology: expect p-modes to propagate freely in regions with weak or vertical fields

• expect p-modes to transform to MHD waves in regions with strong and oblique fields, as evident in 2D MHD code data (Johnson, Petty-Powell, Zita)

References and Acknowledgements

B.C. Low, High Altitude Observatory (HAO), NCAR, http://www.hao.ucar.edu1988 Astrophysical Journal 330, 992

E.J. Zita, 2002, The Evergreen State College, http://192.211.16.13/individuals/zita/research.htm

Dick Canfield et al., 1999, Montana St. Univ., http://solar.physics.montana.edu/canfield/

This work is supported by NASA under the Sun-Earth Connection Guest Investigator Program, NRA 00--OSS--01 SEC

Thanks to Dr. B.C.Low (HAO) for suggesting this sheared field for analysis, and to Dr. Tom Bogdan (HAO and NSF) for discussions and solution strategies.


Documents

Chromospheric UV oscillations depend on altitude and local magnetic field Noah S. Heller and E.J. Zita, The Evergreen State College, Olympia, WA 98505