MULTI-INSTRUMENT STUDY OF THE ENERGY STEP STRUCTURES OF O + AND H + IONS IN THE CUSP AND POLAR CAP REGIONS COSPAR, 2002, Houston, Texas, Paper D

Authors Yulia V. Bogdanova 1,9, Berndt Klecker 1, Goetz Paschmann 1 and CIS TEAM: L. M. Kistler 2, C. Mouikis 2, E. Moebius 2, H. Reme 3, J.M. Bosqued 3, I. Dandouras 3, J.A. Sauvaud 3, A. Korth 4, M.B. Bavassano-Cattaneo 5, C. Carlson 6, G. Parks 6, J.P. McFadden 6, M. McCarthy 7, R. Lundin 8 Collaboration with: Edita Georgescu 1 (FGM team), Matt Taylor 9, Ian Krauklis 9, Andrew Fazakerley 9 (PEACE team), Harri Laakso 10 (EFW team), Nicole Cornilleau-Wehrlin 11 (STAFF team), Patrick Canu 11 (WHISPER team) (1)Max-Planck Institute for Extraterrestrial Physics, Garching, Germany;(2) Space Science Center, University of New Hampshire, Durham, NH, USA; (3) C.E.S.R.., Toulouse, France; (4) Max-Planck Institute for Aeronomie, Kaltenburg-Lindau, Germany; (5) I.F.S.I., Rome, Italy; (6) University of California, Berkeley, CA, USA; (7) University of Washington, Seattle, WA, USA; (8) Swedish Institute of Space Physics, Kiruna, Sweden; (9) MSSL UCL, Dorking, UK; (10) ESA, ESTEC, Noordwijk, Netherlands; (11) C.E.T.P., Velizy, France 2

Abstract We present observations of the cusp crossing by CIS onboard Cluster during May 18, 02:00-04:00 UT, During this event oxygen beams with similar characteristics were observed by three S/C in the cusp and polar cap regions: the narrow O + energy distribution tracks the upper limit of the H + population in the energy-time spectrogram, O + shows an energy dispersion from 1-2 keV in the cusp region to the eV in the polar cap, and energy step structures are observed simultaneously for O + of ionospheric origin and for H + with a mixed population of ionospheric and mirrored magnetosheath particles. We used particle, fields and wave data from CIS, PEACE, FGM, EFW, WHISPER and STAFF instruments onboard the Cluster satellites to investigate the processes which are responsible for these ions energy step structures. Our analysis show that energy steps in both populations are caused by resonant energization by broadband extra low frequency wave fields. From estimation of the time delays with which energy step structures were observed at different S/C we conclude that the plasma region with wave activity is localized in space and moves antisunward in the cusp and polar cap with convection velocity. 3

Cusp event, May , CIS data overview Energy-time spectrograms for H + and O + from the S/C 1, 3 and 4 for May 18, 2001, 02:00-04:00 UT: The H + population shows the typical cusp distribution with energy dispersion. The magnetopause crossing is marked by dashed blue lines. In the oxygen spectrograms we can see a O + beam with the following features: the oxygen beam population ‘tracks’ the upper edge of the H+ population in the energy flux spectrogram. Beam has a very narrow energy range, and shows energy dispersion from 1-2 keV in the cusp region to eV in the polar cap. Simultaneous energy step structures in the H+ and O+ populations. 4

Orbit, configuration and AE index The left figure presents the satellite orbit during 18 May 02:00-04:00 UT. The S/C moved through the northern polar cap and cusp regions from the nightside to the dayside in the X-direction and away from the Earth in the Y- and Z-directions. The right figure shows the AE-index for 18 May. During 02:00-04:00 UT there was high geomagnetic activity with a few subsequent substorms, the Kp-index was 3-. Data from ACE show that during the time period of interest for us ( about 1 hour before the Cluster observations) there were very unstable interplanetary conditions: the Bz-component of the IMF varies from negative to positive values five times. In all coordinates the satellites moved in the following sequence: S/C 1 first, S/C 4 second, S/C 3 third. Separation distances between S/C were rather high: SC1-SC3 – 2,91 Re; SC1-SC4 – 1.11 Re; SC3-SC4 – 1.80 Re. 5

O + 3D-moments comparison from S/C 1, 3 and 4 The figure presents comparisons of the oxygen moments: density, parallel velocity and three components of the perpendicular velocities, from three spacecraft: S/C 1 (black), S/C 3 (green), and S/C 4 (blue). The O + moments were integrated over the whole energy range eV and averaged over 16 seconds. Note, that: the densities measured on three S/C are very similar.  parallel velocities of the outgoing O + are ordered with S/C position: the leading S/C 1 detected ions with the highest energy, S/C 4 was second and measured at the same field lines particles with lower energy, and S/C 3 was last and measured O + ions with the lowest energy. the main component of the velocity perpendicular to the magnetic field was the x- component, the mean value V  x was around –12 km/s whereas V  y and V  z were around zero. This means that the convection in the cusp and polar cap regions is predominantly tailward during this time interval.  V  x measured on S/C 1 and 4 (black and blue lines on the plot) are very similar, especially in the polar cap region - at 02:00-02:30 UT – while the separation distance between S/C 1 and 4 was 1.1 Re. This implies that the convection velocity is similar on this distance scale. 6

O + distribution functions, S/C 1 At 02:25:20 O + distribution function shows beam-like behavior, at 02:35:40, at time of the observation of the “energy step structure” on the spectrogram, distribution function shows high perpendicular heating. 7

O + distribution functions, S/C 1 At 02:53:06 O + distribution function shows beam-like behavior, at 02:56:15 distribution function shows high perpendicular heating 8

O + distribution functions, S/C 1 At 02:57:03 O + distribution function shows high perpendicular heating, at 02:45:27 distribution function shows beam-like behavior. 9

H + pitch-angle spectrogram, S/C 1 Figure shows the H + energy-time spectrogram (first panel) and pitch-angle spectrograms for different energy ranges of H + ions. The maximum of the H + flux is at pitch-angle 90 o, this corresponds to high perpendicular heating. During “energy steps” observation there is a wide pitch angle distribution from 90 to 180 degrees, this corresponds to perpendicular heating processes and H + outgoing flow. 10

3D-distribution of the H + differential flux at 02:44:47 UT, S/C 1 The bottom panel shows 3D-distribution of the H + flux for different energy ranges, star marks parallel to the magnetic field direction, cross – antiparallel direction and line shows perpendicular direction. The globe plot shows that the low energy part of the H + population ( eV) moves upward during “energy step” time and the high-energy part ( eV) undergoes transverse heating. 11

3D-distribution of the H + differential flux at 02:57:03 UT, S/C 1 The globe plot shows that the low energy part of the H + population ( eV) moves upward during “energy step” time and the high-energy part ( eV) undergoes transverse heating. 12

S/C 1, CIS and FGM data The panel shows: H + and O + spectrograms, parallel velocity of the H + (black) O + (blue) ions, and three components of the magnetic field. During time of the “energy steps” there is enhancement of the parallel outflow velocity of the both species. Variations in the By-component of the magnetic field can be probably manifestation of the weak field-aligned currents. H+H+ O+O+ 13

S/C 1, PEACE, EFW and STAFF data During times of energy steps there are enhancements of wave spectral density in a broad frequency range and electron density. 14

S/C 4, CIS and FGM data Panel above shows: H + and O + spectrograms, parallel velocity of the H + (black) O + (blue) ions, and three components of the magnetic field. During time of the “energy steps” there is enhancement of the parallel outflow velocity of the both species. Variations in the By- and Bx-component of the magnetic field can be probably manifestation of the weak field-aligned currents. 15

S/C 4, PEACE, EFW, STAFF and WHISPER data During times of energy steps there are enhancements of wave spectral density in a broad frequency range and electron density. Data from S/C 3 not presenting here show the similar features. 16 Relative intensity

Ions transverse heating mechanisms The heating can be caused by the broadband extra low frequency (BBELF) wave fields, waves near the lower hybrid (LH) frequency, or electromagnetic ion cyclotron (EMIC) waves near 0.5 f H+. The heating can also be correlated with auroral electrons, suprathermal electron burst (STEBs), or precipitating H + ions. Furthermore, types 1 and 2 are often associated with field- aligned currents. In table 1 there is estimation of the different frequencies for the considering event: H + cyclotron frequency f cH+, oxygen cyclotron frequency f cO+, frequency of the EMIC waves f EMIC, electron cyclotron frequency f ce and lower hybrid frequency f LH. 17

Times of O + energy steps at different S/C 1 st energy step in O+ population is observed: S/C 1 – 02:35 UT S/C 4 – 02:41 UT S/C :49 UT. The separation distance between S/C in X- direction: S/C 1 – S/C 4 = 0,45 Re S/C 4 – S/C 3 = 0,65 Re S/C 1 – S/C 3 = 1,1 Re From partial moments: plasma tailward convection velocity: Vx = -11,5 km/s t conv S/C 1 - S/C 4 = 4,2 minutes t conv S/C 4 - S/C 3 = 6 minutes t conv S/C 1 – S/C 3 = 10,1 minutes  t S/C 1 - S/C 4 = 6 minutes  t S/C 4 - S/C 3 = 8 minutes  t S/C 1 – S/C 3 = 14 minutes Plasma convection times between S/C are compatible with the time delays of the energy step observation at different S/C. We conclude that we observe the same plasma region with local wave activity at different S/C with time delay around plasma convection time. 18

Summary We present a study of one cusp crossing event during 18 May 2001, 02:00-04:00 UT. The observation was made during a time with rather high geomagnetic activity and conditions favorable for reconnection at the magnetopause. The O + beam population tracks the upper edge of the H + population in the energy-time spectrogram. It has a very narrow energy range, and shows energy dispersion from 1-2 keV in the cusp region to eV in the polar cap. The O + observation can be explained by a localized source of accelerated ionospheric ions with velocity V, determined by the superposition of outward particle motion V II with tailward convection V  x. V  x as determined at S/C 1 and 4 are very similar in the polar cap region, indicating similar convection on the spatial scale of S/C separation, i.e. 1,1 Re. Energy step structures are observed simultaneously in O + ionospheric population and H + mixed mirrored magnetosheath and ionospheric populations. Our study shows that energy steps in the O + and H + populations are associated with high transverse heating of the both species. Ion energy steps are observed simultaneously with electron density enhancements measured by the PEACE instrument and enhancements of wave activity in a broad frequency range of 0, Hz detected by EFW, STAFF and WHISPER instruments. We conclude that the ion energy step structures observed in both populations are very likely caused by resonant energization by broadband extra low frequency wave fields. A comparison of the time delays between energy step structures of O + population at the different S/C with the plasma convection time between S/C shows, that we observe the same plasma region with the local wave activity at all three S/C with a time delay compatible with the convection time. 19