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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

We analyze continuum timeseries data from the SUMER instrument aboard the SOHO spacecraft. We investigate how intensity oscillations depend on wavelength and on the magnetic environment. Our 30 data sets range in wavelength from 91-135 nm, which sample heights from 0.5-1.8 Mm above the photosphere. We distinguish between emissions from stronger field “network” regions and weaker field “internetwork” regions . We Fourier transform timeseries and average frequency power spectra over network and internetwork regions. For a frequency range of 2-5 mHz, corresponding to p-mode frequencies, we see a trend of increasing power with increasing wavelength. We discuss the possibility of photospheric p-mode power transfer to chromospheric heating and MHD mode conversion. We find that different frequencies dominate in different magnetic environments.

2Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Introduction: Chromosphere emits UV

• UV continuum emissions are brighter where gas is hotter, that is at higher altitudes• Average height of formation decreases with wavelength

• UV continua are brighter in strong magnetic regions (network) • UV continuum intensity oscillations track photospheric waves traveling up through

chromosphere

T h

3Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

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

The Solar Ultraviolet of Emitted Radiation is a UV spectrograph

Parameters include slit size, cadence and wavelength

Timeseries data track a region of the chromosphere in time

4Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

UV oscillations vary in space and time

• Gray scale plots show intensity variations in space and time

• Variations in space correlate to magnetic field strength

• we call brightest 30% network and dimmenst 40% internetwork

• Variations in time give clues to chromospheric dynamics

5Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Strong field regions correlate with bright regions

• MDI measures line of sight magnetic field strength at 0.2Mm

• mean UV intensity in network regions tends to be brighter than internetwork intensity

• Note correspondence of UV bright strip at x=20 in MDI data

6Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Fourier transform frequency power spectra

I

f

Integrated power

• For each we Fourier transform oscillations in time for each position along the slit

• We average over each network spatial position

• Characteristic noise is where power levels off

• We integrated power for each of three frequency ranges:

(LF) 0-2 mHz, (MF) 2-5 mHz, (HF) 5-10 mHz, normalized to noise level

7Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

2-5 mHz oscillation power increases with wavelength

• Oscillation power decreases with height above photosphere

• Heating due to gas compression

• Transformation to MHD waves (“mode conversion”)

• p-modes are a source of chromospheric UV oscillations

Network

+ Internetwork

8Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Oscillation power depends on local field strength

• stronger LF oscillations in strong-field (network) regions

• stronger HF oscillations in weaker-field regions

network/internetwork power ratios in 5-10 mHz range

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

80 90 100 110 120 130 140

wavelength (nm)

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network/internetwork power ratios in 0-2 mHz range

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

80 90 100 110 120 130 140

wavelength (nm)

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9Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

Magnetic environment can transform waves

• Parallel acoustic waves can propagate freely to field lines

• Oblique acoustic waves can excite magnetic waves and lose energy

10Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

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

11Heller, Zita (TESC) and Judge (HAO), SHINE meeting, Banff CA, 2002 Aug.

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)

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Acknowledgements

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

Thanks to 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 Tom Bogdan for initiating this collaboration between HAO and The Evergreen State College.

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