Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Role of ULF wave activity in solar wind-magnetosphere interactions and the acceleration of magnetospheric electrons N. Romanova 1,2, N. Crosby 2, V. Pilipenko 1, A. Romanov 3 Institute of the Physics of the Earth, Russian Academy of Science, Moscow, Russia Belgian Institute for Space Aeronomy, Brussels, Belgium Moscow State University

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 OUTLINE 1.Introduction: MHD ultra-low frequency waves in Earth’s magnetosphere 2.Construction of ULF wave indices (ground, geostationary and interplanetary) 3.ULF indices: Properties and applications 4.Application of ULF wave index to the problem of relativistic electron acceleration 5.Cumulative ULF index 6.Results of empirical modeling of relativistic electron dynamics using ULF-index 7.Summary

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 MHD waves in Earth's magnetosphere Geomagnetic pulsations were first observed in the ground-based measurements of the Aug. 28, 1859 Great Aurora event [Stewart, 1861]. The almost pure sinusoidal signature of ULF waves when observed on the ground and in space by satellites suggests a resonance phenomenon. This is not unexpected since the size of the magnetospheric cavity is of the same order as the wavelength of the ULF waves. The magnetopause, plasma pause and ionosphere provide convenient boundaries for wave reflection and transmission. Geomagnetic pulsations, i.e ultra-low frequency (ULF) waves in the 1–100 mHz band are found in the magnetosphere. They are a manifestation of hydromagnetic wave activity generated by physical processes resulting from the interaction of the solar wind with the Earth’s magnetosphere. Pulsation frequency is considered to be "ultra" low when it is lower than the natural frequencies of the plasma (e.g. plasma frequency and the ion gyrofrequency)

Two important ULF wave MHD modes in cold plasma: – Compressional (fast) magnetoacoustic. – (Incompressible) Alfven mode. ULF Wave Modes B and perturbation b, the plasma velocity v and wave vector k all lie in a plane. Current density j and electric field E are perpendicular to the plane. B,j, E and k are coplanar. b and v are perpendicular to the plane. Magnetospheric Normal Mode. Alfven Poloidal and Toroidal Resonances: ALFVEN MODE: FAST MODE: Is also referred to as the transverse or guided mode. Propagates isotropically and may establish cavity or waveguide resonances in the magnetosphere and the ionosphere.

Observation of ULF waves Ground based monitoring Space observations INTERMAGNET global network of observatories (over 107 observatories) Belgium Dourbes observatory (RMI) Russian Borok observatory (IPE RAN) Satellites: Cluster (ESA), Double Star, THEMIS, GOES, CRRES, ORBITALS CSA, etc. There are many magnetic arrays in the world: Canadian Geospace Monitoring (CGSM), DMI (Copenhagen), Greenland Coastal Array, MAGIC, MACCS, CPMN, Russian Arctic, CАRISMA, IMAGE, CANMOS, Samnet, etc.

Distribution of ULF Wave Power Jacobs, Kato, Matsushita and Troitskaya [Jacobs et al., JGR, 1964] established a classification system based on frequency bands and knowledge of the phenomena at that time. Continuous Pc 1-2 pulsations are locally generated in the equatorial plan of the magnetosphere by electromagnetic ion cyclotron (EMIC) instability. Compressional Pc 3 from dayside relate to wave- particle interaction in the foreshock and shock. Toroidal Pc 3 or multi-harmonics from dayside are field line resonance harmonics in Pc 3 / Pc 4 range, compressional Pc3 as a driver (coupling with the fundamental toroidal mode). Poloidal Pc 4 from afternoon and evening sides related to injections of energetic plasma and subsequent low activity or convection electric field. Compressional Pc 5 from nightside, dawn and dusk region, related to high beta plasma (ion injections). Toroidal Pc 5 from dawn and dusk flanks are fundamental mode field line resonances; Incoherent noise from everywhere increases with magnetic activity [Anderson, 1993, 1994].

Pc 5 band (2-7mHz) – the range of most intense pulsations. This instability can occur when two distinct layers of a fluid are in relative motion. The interface between the layers develops `wiggles' or rolls which can evolve into vortices. This instability in the interface means that the two layers start to mix, and can lead to fully developed turbulence in either layer. Copyright: K. Nykyri et al. Solar wind impulses or buffeting can excite the dayside of the magnetosphere. The solar wind contains periodic structures, driving a periodic "breathing" space of the magnetosphere and global generation of magnetic waves. Credit animation: Boston University/NOAA ULF waves can be used as diagnostic tools to determine important information on the topology of the dynamic magnetosphere at all latitudes, from both spatial and temporal perspectives. How to estimate the level of ULF wave activity? Equatorial electrojet / ring current activity Dst-index Auroral electrojet activityAU, AL AE-indices Planetary magnetic activityKp-index ULF-activity? Kelvin-Helmholtz “wind-over-water” instability (KHI) on the flanks can excite ULF waves.

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 We derive a ground ULF wave index using the spectral features of ULF power in the Pc5 band averaged over 1 hour from a global array of stations in the Northern hemisphere: INTERMAGNET Greenland Coastal Array MAGIC MACCS CPMN Russian Arctic WDC Construction of the hourly ground ULF wave global index

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007  For any UT, magnetic stations in the MLT sector 03 – 18 (to avoid substorm-related disturbances during night hours), and in the latitudinal range (e.g. 60° - 70° CGM) are selected.  For selected stations, the discrete Fourier transform (DFT) spectra of two horizontal components in a desired frequency band are calculated with the use of Filon’s formula for calculation of integrals of oscillatory functions in a 1 hr time window.  The frequency range for the index definition is the Pc5-band (fL = 2 mHz, fH = 7 mHz) – the range of most intense fluctuations.  Current value of the ULF wave power index is calculated as maximal magnitude of the frequency-integrated wave power in a selected MLT-latitude sector: Algorithm of ULF wave index construction

Discrimination of noise and signal from ULF spectra Noise spectral power in this frequency range is calculated at each station as the area beneath the discrimination level (or background spectrum): Signal spectral power (S) is the area of the bump above the background spectra: The total power index (T) is augmented by a “signal” index (S) to discriminate between broad-band and narrow-band ULF waves. To discriminate between broad-band and narrow-band ULF waves a ratio between signal and total powers is fraction of narrow-band power (R): R=S/T (R=0-1). In a log-linear plot the linear fit LF(f) is applied, which fits the data to a linear model by minimizing the    in the frequency band from f 1 = 1mHz to f 2 = 8 mHz. A discrimination line, separating the background noise and signal spectra, is considered as log F B (f) = LF(f) - . Total spectral power (T): T=S+N

Construction of the hourly space ULF wave indices (geostationary and interplanetary) Ground magnetic fluctuations are not always a perfect image of the ULF fluctuations in the magnetosphere. For example, there is a class of ULF waves, called stormrelated Pc5 pulsations that occur during the recovery phase of magnetic storms in the dusk and noon sectors of the magnetosphere. These ULF waves are generated by ring current protons via various kinds of drift instabilities [Pilipenko, 1990]. Despite their high amplitudes in the magnetosphere, these pulsations are rarely if ever seen on the ground because their small azimuthal scales cause effective screening by the ionosphere. Thus, the ground global index needs to be augmented by a similar index, estimated from data from magnetometers in space. GEO ULF wave index is calculated from 1-min 3-component magnetic data from GOES satellites to quantify the short-term magnetic variability in the region of the geostationary orbit. INTERPLANETARY ULF wave index to quantify the short-term IMF variability is calculated from the 1-min IMF data from the interplanetary satellites IMP8, WIND, ACE. The data are time-shifted to the terrestrial bow shock (~15 RE). In addition to ULF-ground index:

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Finally, the set of the wave power indices from ground, geostationary, and interplanetary monitors provides a researcher with a convenient and easy tool for the statistical study of the role of MHD turbulence in the solar wind-magnetosphere interactions: Ground ULF (T, N, S) GEO ULF (T, N, S) IMF ULF (T, N, S) The final output monthly files contain the hourly values of the following parameters: SW velocity and density IMF components Standard static indices (Dst, AE) Ground global ULF wave indices (T, N, S) Geostationary orbit ULF indices (T, N, S) IMF variability indices (T, N, S) ULF indices data base The database for interval is freely available to space community via mirror anonymous FTP site for testing and validation: ftp://space.augsburg.edu/MACCS/ULF_Index/

Solar wind/magnetosphere coupling Using the introduced IMF ULF index, here we verify the fact that when the SW is more turbulent, the effective degree of its coupling to magnetosphere is higher. The occurrence probability of the log S IMF index. S IMF is signal component part of IMF ULF index. The average AE values for the turbulent SW are higher than for the laminar solar wind. This comparison confirms that the magnetosphere is driven more weakly when the IMF turbulence level is low. The IMF is considered: Noisy when log S IMF > 0 Calm when log S IMF < 0 Auroral response, characterized by the hourly AE index, is compared with the strength of the SW driver, determined by the IMF Bz component, for the laminar and turbulent IMF. So, IMF-ULF index can be used for estimation of turbulence level of IMF along with values of the IMF component dispersion from OMNI database (  B,  Bx,  By,  Bz).

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Properties of the ULF Ground wave index Cross-correlation analysis of ground ULF activity, as characterized by ULF-ground index, and the SW velocity (V SW ) and density (N SW ). The asymmetry of the cross-correlation function indicates that the increase of magnetospheric ULF activity starts earlier statistically than the increase of SW velocity. This may signify that the KHI is not the only mechanism of ULF wave generation, but the irregular SW plasma density enhancements preceding the occurrence of high-speed streams contribute also to ULF wave excitation. Indeed, the SW, V SW and N SW show strong statistical anti- correlation between hourly values, with a shifted peak of cross-correlation function by about 0.5 day indicating that variations of N SW precede those of V SW.

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Is the solar wind velocity the only controlling factor of the magnetospheric ULF wave activity?  The correspondence between the ground wave power and Vsw has somewhat different character for the “slow” ( 450 km/s) solar wind.  “Cut-off” lower & upper boundaries: the intensity of ground fluctuations is within certain limits for any Vsw. The distribution is skewed: for negative Bz the ground wave power is higher than for positive Bz. Thus, reconnection contributes to processes that stimulate the generation of ULF activity. These statistical features should be understood in the frameworks of the theory of ULF wave excitation through the Kelvin-Helmholtz instability.

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 ULF-indices: Potential applications Ring current (RC) dynamics There is a view that RC development results from a sustained enhancement of the convection electric field driven by the IMF/SW. There must be some secondary, relatively efficient and continuous, process that scatters particles from open to closed drift paths: 1.) fluctuations in the SW(?), 2.) ULF waves in the magnetosphere(?). This process is not observable in any existing indices. Wave precursors of substorms The variability of SW and magnetospheric conditions might be an important factor in triggering magnetospheric substorms, but this idea has not been thoroughly examined so far. Substorm break- up may be preceded by an increased level of ULF power in the dayside cusp. High-speed solar-wind streams (HSS), co-rotating interaction regions (CIRs) and their interactions with the magnetosphere CIRs do not produce strong ring currents, they do drive storm-levels of magnetoshere. Their energy is released via explosive reconnection events in the tail and Pc5-wave excitation. Why are Pc5 waves so dominant in HSS and CIR events? [M.Denton, 2006]. Compare geoeffective and ineffective high speed streams Geoeffective high speed streams produce stronger ULF activity [R. McPherron, 2007]. Relativistic electron acceleration is caused by geoeffective high speed streams. ULF noise in seismo-active regions Anomalous ULF noise often occur a few days before strong earthquakes. Problem of radiation belt electron acceleration Are ULF waves intermediary between the solar wind and “killer” electrons during magnetic storms?

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 ULF wave activity and relativistic electron acceleration Relativistic electrons would not appear in the non-turbulent magnetosphere? 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., JGR, 2001; Mathie & Mann, JGR, 2001].

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Are ULF waves intermediary between the solar wind and “killer” electrons during magnetic storms? High solar wind velocity, as well as elevated level of ULF wave activity, precede the growth of relativistic electron flux for ~2 days [Paulikas and Blake, 1979]. Some intermediary must directly provide energy to the electrons. ULF waves are also strongly correlated with V SW. ULF waves in Pc5 band (2-10 mHz) could be a possible intermediary between the SW and electrons. Narrow-band long-lasting ULF waves in the recovery phase might be a driver of the gradual increase of relativistic electron fluxes owing to the drift-resonance acceleration. The acceleration of relativistic electrons is a cumulative effect of the ULF wave turbulence with typical time scale of about several days.

Magnetospheric geosynchrotron Suggested mechanism of the electron acceleration is a revival of the magnetospheric geosynchrotron notion. Pumping of energy into seed electrons is provided by large-scale MHD waves by resonance, when the wave period matches the multiple of the electron drift period. Adiabatic invariants Cyclotron-1st invariant (VLF waves, millisecond) Bounce -2nd invariant ( s) Drift -3rd invariant (Pc5 ULF waves) 1-10 min Poloidal mode has E polarised approximately parallel to drift velocity. Large amplitude (few mV/m) ULF waves can drive rapid transport of MeV electrons over ~10 cycles. Lower amplitude broad-band ULF drive radial diffusion more steadily over 1-2 days. ULF wave-electron drift resonance: Images courtesy of Scot Elkington, LASP.

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Cumulative ULF-index and electron flux variations Correlation between the ULF-index and LANL electron flux increases from ~0.6 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!  = 4 days

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 Ultra-low frequency (ULF) waves in the 1–100 mHz band are general in the magnetosphere. They are a manifestation of hydromagnetic wave activity generated by physical processes resulting from the interaction of the solar wind with the Earth’s magnetosphere. We provide the space community with a new convenient tool for the characterization and monitoring of the turbulent level of the SW- magnetosphere-ionosphere system - ULF wave global power index, derived from ground-based and satellite observations. The wave power index characterizes the ground ULF wave activity on a global scale better than data from selected stations subjected to unavoidable variations of their locations because of Earth’s rotation. Summary This new ULF wave power index is aconvenient and an easy tool for the statistical study of the role of MHD turbulence in the solar wind-magnetosphere interactions and can be applied to various space physics problems. ULF Index provides one with a new parameter to better predict magnetospheric electron flux variations (space radiation models).

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007 The ULF-index database for interval is freely available to space community via mirror anonymous FTP site for testing and validation: FTP: space.augsburg.edu/MACCS/ULF_index/ We have used:  Noon-reconstructed electron fluxes provided by P. O’Brien.  GOES data from NOAA NSDC;  INTERMAGNET project data;  Ground magnetic data from WDC (DMI, Copenhagen);  OMNI-2 database from NASA NSSDC; Comments, suggestions, and requests are welcomed! O. Kozyreva et al. In search of a new ULF wave index: Comparison of Pc5 power with dynamics of geostationary relativistic electrons Planetary and Space Science (2006) N. Romanova et al. ULF wave index and its possible applications in space physics Bulgarian Journal of Physics (2007)

The Cluster mission (ESA) is currently investigating the small-scale structure of the Earth's plasma environment in global magnetotail dynamics, in cross-tail currents, and in the formation and dynamics of the neutral line and of plasmoids. This animation shows the positions of the satellites during the event on 25 November During an opportune alignment that occurred during the recovery phase of a large geomagnetic storm, observations with different instruments from several different points were carried out. A low frequency type of ULF wave, comparable to a beep every 10 minutes, was recorded continuously for many hours by the CARISMA magnetometer chain in Northern Canada. The waves were picked up by more than a dozen scientific satellites including the four ESA Cluster satellites, NASA’s Polar spacecraft and four of the Geostationary Operational Environmental Satellites (GOES). Credit: Andy Kale, University of Alberta

Cyclotron facilities, Louvain-la-Neuve, 27 November 2007

Comparison of electron fluxes with the storm and ULF wave indices (total power T, shown in gray, and narrow-band power S, shown in black) during Space Weather Month (09/10–09/31): Dst, GOES- 10 integral electron (2 MeV) electron fluxes Je, the IMF wave index from propagated ACE data, the GEO ULF wave index derived from 3-component magnetometer data observed by GOES-10, and the global ground ULF index.