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V. Pilipenko , N. Romanova, M. Engebretson, L. Simms

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The Role of ULF wave activity in solar wind-magnetosphere interactions and magnetospheric electron acceleration

V. Pilipenko, N. Romanova, M. Engebretson, L. Simms

Institute of the Physics of the Earth, MoscowAugsburg College, Minneapolis

Construction of the ULF wave indices

- GROUND ULF wave index as a proxy of global Pc5 activity (2-7 mHz) is reconstructed from 1-min data from arrays of magnetic stations in the Northern hemisphere.
- GEO ULF wave index is calculated from 1-min 3-component magnetic data from GOES satellites to quantify the magnetic variability at geostationary orbit.
- INTERPLANETARY ULF wave indices to quantify the IMF & SW variability are calculated from 1-min data of interplanetary satellites. The data are time-shifted to the terrestrial bow shock.

For any UT, we selected stations in the 03 – 18 MLT sector (to avoid substorm-related disturbances), and in the latitudinal range above 60° - 70° CGL. Spectra of two detrended (cut-off 0.5 mHz) horizontal components are calculated in a 1-hour running window

Algorithm of ULF wave index construction

The total power index (T) is augmented by a “signal” index (S) to discriminate between broad-band and narrow-band ULF waves.

the set of the wave power indices from ground, geostationary, and interplanetary monitors

IMF ULF

(T, S)

Ground ULF

(T, S)

GEO ULF

(T, S)

SW ULF

(T, S)

For more details, see:

Kozyreva, O., et al.,

"In search of a new ULF wave index: Comparison of Pc5 power with dynamics of geostationary relativistic electrons"

Planetary and Space Science, 2007.

Signal spectral power (S) is the area of the bump above the background spectrum:

A weak irregular increase of the SW density is observed ~1-2 days before magnetic storm commencements. This effect may be quantified by the ULF wave power index Tn, characterizing the power of density fluctuations: this index starts to grow, prior to storm commencement.

- Statistical distributions of Tn index are compared:
- yearly distribution (white)
- distribution during 12-hour intervals before storm onsets (blue).
- Green color denotes intersection between them.

Low-frequency SW density fluctuations with time scales from ~4 to ~100 min, as estimated by the wavelet power Wn, start to grow, on average, ~1 day prior to storm commencement.

These features of the SW plasma structure prior magnetic storms may be classified as medium-term precursors of severe space weather.

Correlation of the ULF ground index with key parameters of the solar wind

VSW

NSW

PSW

The SW may drive the magnetosphere in a different manner, depending on the upstream turbulence level [Borovsky & Funsten, 2003 ].

The magnetosphere is to be driven more weakly when the level of IMF turbulence is low.

The auroral response is compared with similar strength of the IMF driver (Bz) for laminar and turbulentflow.

The average AE values for the turbulent SW are higher than for laminar solar wind!

This difference is most significant for northward Bz, when one expects the viscous interaction to be dominant over the reconnection.

- The correspondence between the ground wave power and Vsw has a somewhat different character for the “slow” (<450 km/s) & “fast” (>450 km/s) solar wind.
- “Cut-off” lower & upper boundaries: the intensity of ground fluctuations is within certain limits for any V

“Northward & “southward IMF” events have the same dependence on Vsw, but under southward IMF the ground ULF wave response is higher.

The distribution is skewed: for negative Bz the ground wave power is higher than for positive Bz.

Correspondence between the IMF turbulence and V is similar:

lower and upper cutoffs, change of dependence around 450 km/s.

High-speed solar wind cannot be a laminar flow!

In contrast to the ground ULF activity, the distributions of IMF wave turbulence and V are practically the same for Bz>0 & Bz<0.

The appearance of relativistic electrons (E > 2 MeV) following storms resists definitive explanation. These electron events are not merely a curiosity for scientists, but they have disruptive consequences for spacecraft.

While a general association between storms and electron enhancements is well known, the wide variability of the response and its puzzling time delay (~1-2 days) from the storm main phase has frustrated identification of responsible mechanisms.

High solar wind velocity, as well as elevated level of ULF wave activity, precede the growth of relativistic electron flux by ~2 days.

Geosynchrotron:Are ULF waves an intermediary between the solar windand “killer” electrons during magnetic storms!?

Some intermediary must directly provide energy to the electrons!

The mechanism of acceleration of ~100 keV electrons supplied by substorms is a revival of the idea of the magnetospheric geosynchrotron. Pumping of energy into seed electrons is provided by large-scale MHD waves in a resonant way, when the wave period matches a multiple of the electron drift period.

Rather surprisingly, ULF waves in the Pc5 band have emerged as a possible energy reservoir: the presence of Pc5 wave power after minimum Dst is a good indicator of relativistic electron response [O’Brien et al., 2001].

Surprisingly, a sustained increase of the relativistic electrons (E>2 MeV) fluxes up to 2-3 orders is observed after weak storms (Dst~-50-100nT), whereas the increase after strong storms (Dst~-200nT) is much shorter and less intense. Moreover, there are events when electron bursts occur without storms.

Relativistic electrons would not appear in the non-turbulent magnetosphere!?

Cumulative ULF-index and electron flux variations

Correlation between the ULF-index and the LANL electron flux increases from ~0.5 to ~0.8 for ULF index values time-integrated over their pre-history :

Increase of correlation implies the occurrence of a cumulative effect, that is, long-lasting ULF wave activity is more important for the electron flux increase than just instantaneous values!

Correlation of e-fluxes with the cumulative ULF index is even higher that with V!

Path analysis is an extension of multiple regression - a diagram showing possible causal relationships between the variables. The relative strengths of the path coefficients (standardized regression coefficients) are used to determine which paths have the most influence on the dependent variable.

Solid lines represent positive associations, dashed lines represent negative ones. The strength of the association is shown by the line thickness.

The effect of each independent variable on ground ULF activity, characterized by the Tgr index, is considered to be a combination of both direct and indirect paths.

V has the greatest overall effect, but the contribution from indirect paths is greater for Bz than for V or N.

V is still a dominating factor. As might be expected, the direct influence of Bz decreased, but the direct influence of N increased.

The influence of V and N through Timf also increases.

- Conclusions
- The wave power index characterizes the ground ULF wave activity on a global scale better than data from selected stations subjected to unavoidable variations because of Earth’s rotation.
- The interplanetary ULF index reveals elevated variability of the SW plasma prior to magnetic storms, which may be classified as medium-term precursors of severe space weather.
- ULF wave power is a good predictor for MeV electron flux variations. The cumulative ULF-index describes the relativistic electron dynamics better than the instantaneous ULF wave power level.
- The ULF wave power index, characterizing the level of ULF turbulence in near-Earth space, should be taken into account by any adequate space radiation model.
- The set of ground and interplanetary indices is a convenient tool for statistic analysis of the solar wind – magnetosphere interaction with account for turbulence aspects.

Acknowledgements:

- Noon-reconstructed electron fluxes provided by P. O’Brien.
- GOES and LANL data;
- INTERMAGNET project data;
- Ground magnetic data from WDC (DMI, Copenhagen);
- OMNI-2 and OMNI-1min database from NASA NSSDC;

Comments, suggestions, and requests

of the index database (1994-2004) are welcomed!

Anonynous FTP: space.augsburg.edu/MACCS/ULF_index/