1. as an indicator of the turbulent level of the magnetosphere and IMF:
ULF wave index
as an indicator of the turbulent level of the magnetosphere and IMF:
Application for 1994 electron events
Pilipenko, V., Kozyreva, O.
Institute of the Physics of the Earth, Moscow
Engebretson, M.J.
Augsburg College, Minneapolis, MN
Yumoto, K.
Kyushu University, Fukuoka
Watermann, J.
Danish Meteorological Institute, Copenhagen

2. A necessity of a new “wave” index
The interaction between the solar wind (SW) and magnetosphere is the primary driver of many processes in the near-Earth space environment. Standard geomagnetic indices, which quantify the energy supply in certain regions of the magnetosphere-ionosphere system, implicitly assume quasi- steady and laminar plasma flow in the near-Earth environment.
However, the energy transfer processes in the magnetosphere have a turbulent character. The turbulent nature of SW drivers and the existence of natural MHD waveguides and resonators in the ULF frequency range (~1-10 mHz) ensures a quasi-periodic response to any external forcing. Therefore, much of the turbulent nature of SW-magnetosphere-ionosphere interactions can be monitored with ULF observations.
A wide range of space physics studies, such as substorm physics, particle diffusion and acceleration, SW-magnetosphere-ionosphere coupling, etc. will benefit from the introduction of an ULF wave index - a rough proxy of the level and character of low-frequency turbulence.
Here we outline an attempt to introduce such an index and give an example of its application to the electron energization problem.

3. Construction of the ULF wave index
The wave index as a proxy of global ULF activity is reconstructed from one- min data from arrays of magnetic stations in the Northern hemisphere:
INTERMAGNET (filled circles)
Greenland Coastal Array+MAGIC (crosses)
MACCS (diamonds)
CPMN (210 Magnetic Meridian Chain) (empty boxes)
Others (Russian Arctic, selected WDC) (triangles)

4. Algorithm of the ULF wave index construction
For any UT, magnetic stations in the MLT sector 05 – 15, and in the latitudinal range 60° - 75° CGM are selected.
Spectra of two detrended (cut-off 0.5 mHz) horizontal components are calculated using Filon’s method in an 1-hour time window.
The frequency range for the index definition is the Pc5 band (fL=3 mHz, fH=7mHz) – the range of the most intense fluctuations.
In order to distinguish broad-band and narrow-band variations we applied an algorithm based on the determination of “bump” above the linear fit to background “colored-noise” spectra.
Global ULF wave index
The summation is performed with respect to all N stations where the signal amplitude is above K*Bmax (K = ; Bmax is the maximal spectral power in the selected MLF sector)
As a result one obtains:
Noise spectral power (N) - the band-integrated area beneath the background spectra;
Signal spectral power (S) - the area of the bump above the background spectra;
Total spectral power (T) - T=S+N
Measure of the fraction of narrow-band power R=S/T (R=0-1).

5. Final product: the zoo of hourly ULF wave indices
Advances of the new ULF-index
Drawbacks of the wave index used by O’Brien et al. [2001] (named for brevity the B-index)
Usage of all 3 ULF magnetic components to calculate the power, whereas the vertical Z component is very sensitive to local geoelectric inhomogeneties;
only 11 INTERMAGNET stations with large uneven spatial gaps between them;
No MLT selection was made, so the B-index may be strongly influenced by irregular nightside substorm activity.
This may produce a time offset between indices: B-index is behind ULF-index by ~1 day ?
Final product: the zoo of hourly ULF wave indices
A global ground ULF index as determined by world-wide array of magnetometers;
A ULF GEO wave index is calculated from 3-component magnetic data from GOES satellites to quantify the magnetic variability in the region of geostationary orbit;
To quantify the IMF variability, an interplanetary ULF index is calculated using IMF data (time-shifted to the terrestrial bow shock ~15 RE) from the interplanetary satellites IMP8, WIND, or ACE.

6. “Killer” electrons and satellite anomalies During the March-April 1994 period - decline of solar activity, no solar proton events, but geostationary satellites suffered numerous anomalies (according to the NOAA database). The menace comes from relativistic electrons: Increases of electrons E= MeV detected by LANL produce swarms of malfunctions.
The relativistic electron events are not merely a curiosity for scientists, but they can have disruptive consequences for spacecraft!

7. for the period January-April 1994
Storm activity (Dst), solar wind parameters (V, Np), GOES-7 raw & noon-proxy electron (>2 MeV) fluxes, and ULF wave power indices (ground and GEO)
for the period January-April 1994
* noon-reconstructed fluxes have no diurnal variations compared with raw data

8. Relativistic electron (>2 MeV) response & various ULF wave indices for 3 magnetic storms in March-April 1994

9. Electron events in 1994
A sustained intense increase of relativistic electron fluxes up to Je~104 was observed after weak storms (Dst~-100nT), whereas the increase after the strong storm (Dst~-200nT) was much shorter and less intense (up to ~103 only).
The electron behavior matches well the variations of the global ULF index: after weak storms this index increases much more substantially and for a longer period than after the strong storm!

10. Statistical relationships between relativistic electron flux and magnetic & ULF indices
Geomagnetic disturbances, as well as an elevated level of ULF wave activity, precede the growth of relativistic electron flux for ~2 days.
(Daily averaged values)
Sometimes the correlation of electron flux with the ULF index is even higher than with other indices (Dst, AE) or SW velocity.
The ULF index should be taken into account by any adequate space radiation model!

11. Cross-correlation between the electron flux variations, ULF-index, and SW velocity
The cross-correlation coefficient shows that the electron flux increases ~2 days after enhancements of ULF wave activity and solar wind velocity
ULF index somewhat better characterizes the electron response than the B-index
Correlation between the ULF-index and electron flux increases for ULF index values time-integrated over their pre-history :
Increase of correlation, probably, implies the occurrence of a cumulative effect, that is, long-lasting (with characteristic time ) ULF wave activity is more important for the electron flux increase than just instantaneous values!

12. Example of Possible Usefulness of ULF index: Study of the relativistic electron response to magnetic storms
ULF waves in Pc5 band (1-10 mHz) could be a possible intermediary between the SW and electrons [O’Brien et al., JGR, 2001; Mathie & Mann, JGR, 2001]. In a laminar non-turbulent magnetosphere the “killer” electrons would not appear!
Broad-band and short-lived ULF oscillations during the main storm phase are caused by other mechanisms (particle injection?) than typical Pc5, and their small transverse spatial scale [Pilipenko et al., JASTP, 2001] does not match the conditions necessary for the electron resonant acceleration by ULF waves.
Narrow-band and long-lasting ULF waves in the recovery phase might be related to the gradual increase of relativistic electron fluxes owing to drift-resonance acceleration.
The acceleration of relativistic electrons is a cumulative effect of the ULF wave turbulence with typical time scale ~2 days.
A mechanism of the 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 the multiple of the electron drift period.

13. folder: /MACCS/ULF_Index/
Scientific consortium comprising
Space Research Institute (Moscow), Institute of the Physics of the Earth (Moscow), Space Center of Augsburg College (Minneapolis), Space Environment Research Center of Kyushu University (Fukuoka), and Danish Meteorological Institute (Copenhagen)
will provide the space community with a new convenient tool for the characterization and monitoring of turbulent level of the SW-magnetosphere-ionosphere system - ULF wave power index, derived from ground-based and satellite observations.
The final output monthly files contain the hourly values of the following parameters:
SW velocity & density, IMF components
standard static indices (Dst, AE)
ground global ULF wave indices (T, N, S)
GEO ULF indices (T, N, S)
IMF variability indices (T, N, S)
The database for interval is freely available to space community via mirror anonymous FTP site for testing and validation:
space.augsburg.edu
folder: /MACCS/ULF_Index/
A CD with ULF index database may be requested!
Comments, suggestions, and requests are welcomed!
We acknowledge the provision of
Noon-reconstructed electron fluxes & B-index by P. O’Brien.
GOES data from NOAA NSDC;
Data from INTERMAGNET, CPMN, MACCS, and Greenland arrays;
OMNI-2 database from NASA NSSDC;