A new ULF wave index and its comparison with dynamics of geostationary relativistic electrons

1. A new ULF wave index and its comparison with dynamics of geostationary relativistic electrons O.V.Kozyreva, V.A. Pilipenko Institute of the Physics of the Earth, Moscow M.J. Engebretson Augsburg College, Minneapolis, MN K. Yumoto Kyushu University, Fukuoka

2. The need for a new geomagnetic index Interactions between the solar wind and magnetosphere and processes in the near-Earth space environment have often been viewed using the implicit assumption of quasi-steady and laminar plasma flow. However, the energy transfer processes in the magnetospheric boundary regions have a turbulent character. Existing geomagnetic indices (Kp, Dst, AE, SYMH, PC, etc.) and IMF parameters quantify the energy supply in the solar wind-magnetosphere-ionosphere system. However, these indices characterize the quasi-stationary electrodynamics of the near-Earth environment, whereas convenient tools for the characterization of its variability are lacking. The turbulent character 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 forcing at the boundary layers. A ULF wave index can characterize the turbulent character of the energy transfer from the solar wind into the magnetosphere and the short-scale variability of near-Earth electromagnetic processes. There are many space weather related problems, where a ULF wave index - a rough proxy of the level of low-frequency turbulence, might be of key importance.

3. Impact of the Level of Solar-Wind Turbulence on Auroral Activity A naive expectation is that when the SW is more turbulent, the effective degree of its coupling to the magnetosphere is higher. Auroral response is compared with with similar strength of the SW driver (Bz) for the laminar and turbulent wind flow: • IMF is noisy (var{Bz}>2nT); • IMF is calm (var{Bz}<2nT). The average AE values for the turbulent SW are higher than for the laminar solar wind! This difference is most significant for northward Bz, when one expects the viscous interaction to be dominant over the reconnection. This comparison reveals that the magnetosphere is driven more weakly when the level of SW turbulence is low. In studies of the SW-magnetosphere coupling the SW turbulence is ignored, so ULF turbulence index for the SW must be introduced.

4. Ring current dynamics RC development may result from a sustained enhancement of the convection E driven by the IMF/SW. In this view it is implicitly assumed that there must be some secondary, relatively efficient and continuous, process that scatters particles from open to closed drift paths: fluctuations in the SW(?), ULF waves in the magnetosphere(?). This process, though being of key importance, is not observable in any existing indices. Wave precursors of substorms Enhanced reconnection and viscous interaction in the dayside boundary regions, leading eventually to a substorm, may be accompanied by an elevated level of turbulence. Therefore, substorm break-up may be preceded by an enhanced level of ULF power in the dayside boundary regions? There are events indicating the occurrence of the broadband ULF variations in the nominal dayside cusp region before substorm break-up and sudden suppression of ULF activity after it. Pre-heating of the plasmasheet plasma owing to the resonant absorption of MHD turbulence may provide necessary conditions for the onset of an explosive instability (“thermal catastrophe” by Goertz & Smith [1989]). Application of statistical methods for the search for wave precursors of substorms will benefit from the development of an index quantifying ULF activity.

5. Example of possible dayside wave “precursor” of nighttime substorm

6. ULF noise in seismo-active regions Anomalous ULF noise may occur a few days before strong earthquakes, caused by the crust micro-fracturing at the final stage of the seismic process. Validation of this effect on a large statistical basis will be possible only with the use of a ground ULF wave index, that will provide the seismic community with an effective tool to distinguish local e/m anomalies from global enhancements of ULF wave activity.

7. Geosynchrotron:ULF waves = intermediary between the solar windand “killer” electrons during magnetic storms!? Appearance at GEO of relativistic electrons following storms resists definitive explanation. These electron events are not merely a curiosity for scientists, but they can have disruptive consequences for spacecrafts. While it has been known a general association between storms and electron enhancements, the wide variability of the response and the puzzling time delay (~1-2 days) between storm main phase and the response has frustrated the identification of responsible mechanisms. Some intermediary must more directly provide energy to the electrons?! Rather surprisingly, ULF waves in the Pc5 band (~few mHz) 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]. In a laminar, non-turbulent magnetosphere the “killer” electrons would not appear! Mechanism of the acceleration of ~100 keV electrons supplied by substorms is 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, e.g.

8. Construction of the ULF wave index The wave index as a proxy of global ULF activity is constructed using 1-min data from the following arrays of magnetic stations in Northern hemisphere: ·       INTERMAGNET (filled circles) ·       MACCS (diamonds) ·       CPMN - former “210 Magnetic Meridian Chain” (empty boxes) ·other observatories: Russian Arctic, Iceland (triangles)

9. 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 with Filon’s method in 1-h 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 discriminate 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 in the range 1-8 mHz. 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 powerR=S/T (R=0-1).

10. Global ULF wave index The summation is performed with respect to all N stations where the signal amplitude is above K*Bmax (K = 0.5-1.0; Bmaxis the maximal spectral power in the selected MLF sector) 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 were used; • any LT was considered, so the B-index may be strongly influenced by irregular nightside substorm activity. Additional hourly ULF wave indices • A similar wave index, coined the GEO ULF-index, is calculated from 1-min 3-component magnetic data from the GOES satellite to quantify the ULF geomagnetic variability in the region of geostationary orbit. • To quantify the IMF variability, in the first approximation, we use the variance in an hourly estimate of mean Bz values . Later on, it will be replaced with the interplanetary ULF index calculated from the propagated Wind & ACE 3-component magnetic data.

11. Overall storm activity, solar wind parameters (V, Np), GEO electron (>2 MeV) fluxes, & ULF wave power index (ground and GEO) for the period January-April 1994

12. Comparison of noon-reconstructed relativistic electron (>2 MeV) fluxes with various ULF wave indices for 2 magnetic storms in late March-April 1994

13. “Killer” electrons and satellite anomalies Decline of solar activity, no solar proton events, but numerous anomalies at geostationary satellites (taken from the NOAA database). The menace is from the relativistic electrons! Increases of electrons E=1.8-3.5 MeV detected by LANL produce swarms of malfunctions Comparison with the ULF index, characterizing the global wave activity in the magnetosphere, indicates that ULF wave activity is a possible driver of relativistic electrons?!

14. Role of ULF turbulence in space weather events in 1994 Surprisingly, the sustained intense increase of the GOES-8 noon-reconstructed relativistic electrons fluxes (these fluxes have no diurnal variations compared with raw data) up to ~104 is observed after the weak storm (Dst~-100nT), whereas the increase after the strong storm (Dst~-200nT) is much shorter and less intense (up to ~103 only). The electron behavior matches well the variations of the global ULF-index: after the first weak storm this index increases much more substantially and for a longer period than after the second strong storm! Both the ULF wave index and B-index correspond well to the GEO ULF wave power index calculated from GOES-7 magnetometer data (Hp component only is available). During the March-April 1994 storms geostationary satellites suffered numerous anomalies from the “killer” electrons. Relativistic electron flux has a time delay ~1-2 days with respect to the ULF-index. Thus, this index could be used as a “precursor” of the risk of geostationary satellite anomalies at the declining phase of the solar cycle?!

15. Space Weather Month: September 1999 A strong magnetic storm, caused by a shock followed by a magnetic cloud, occurred 09/22-23 and 3 weak storms occurred on 09/12, 09/16, and 09/26.

16. Comparison of the characteristics of ULF activity with relativistic electron dynamics

17. Space Weather Month: electron radiation and ULF indices Analysis shows, perhaps unexpectedly, that a significant increase of GEO relativistic electron flux (up to 2-3 orders!) is observed not during the main magnetic storm (Dst~-160nT), but during the substorm periods after weak storms (Dst~-50nT). The feature of these intervals is elevated level of ULF index, caused by the occurrence of very intense Pc5 pulsations. The GEO ULF index, characterizing the intensity of ULF activity at the geostationary orbit, also shows 3 enhancements during magnetic storms, similar to the ground global ULF-index. Increases of the ULF-index in general coincide with increases of the IMF Bz variance. This may indicate that there exist an additional factor controlling the ULF activity in the magnetosphere – the level of seeding IMF fluctuations.

18. Provision of the ULF wave index • Scientific consortium comprising • Space Physics Laboratory of Augsburg College, • Space Environment Research Center of Kyushu University, • Institute of the Physics of the Earth • provides the space community with a new hourly ULF wave index, analogous to geomagnetic indices, derived from ground-based and satellite observations in the ULF frequency band. • The database for the interval 1994-2000 will be freely available via anonymous FTP at the following site for testing and validation: • space.augsburg.edu • folder:/pub/MACCS/ULF_Index/ • Comments and data/paper requests are welcomed: • pilipenk@augsburg.edu • engebret@augsburg.edu • yumoto@geo.kyushu-u.ac.jp • We acknowledge: • GOES data from NOAA NSDC; • INTERMAGNET project data; • Ground magnetic data from WDC (DMI, Copenhagen); • OMNI-2 database from NASA NSSDC; • Noon-reconstructed electron fluxes provided by P. O’Brien.