MAGNETOTAIL LOBE POPULATION AS MEASURED BY INTERBALL-1 SATELLITE Koleva R. 1, Grigorenko E. 2, Sauvaud J.-A. 3 (1) Solar-Terrestrial Influences Laboratory, BAS (1) Solar-Terrestrial Influences Laboratory, BAS (2) Space Research Institute, Russian Academy of Sciences (2) Space Research Institute, Russian Academy of Sciences (3) CESR - CNRS, 9, av. de Colonel-Roche, 31400, Toulouse, France (3) CESR - CNRS, 9, av. de Colonel-Roche, 31400, Toulouse, France UN/ESA/NASA/JAXA Workshop, "First Results from the IHY 2007“, Sozopol, Bulgaria

OUTLINE 1.Motivation and review of current knowledge 2.Example of experimental data 3.Discussion

After Hultqvist, et al. SSR, 1999 WHAT DO WE KNOW ABOUT THE MAGNETOTAIL LOBE WHAT DO WE KNOW ABOUT THE MAGNETOTAIL LOBES two vast regions between the plasma sheet (PSBL) and the magnetotail boundary layers at the magnetopause, which map to the ionospheric polar caps and where the plasma has very low temperature and densities, being for a long time under the threshold of the instruments What is the ion content of the plasma sheet is very important for the development of the magnetospheric disturbances and substorms. One of the routes of ionospheric and solar wind ions to the plasma sheet is via the lobes. Though the plasma is very rarefied, because of their great volume, the lobes contain a considerable quantity of plasma. e.g. Huddlestone et al., JGR 2005: Ionospheric supply to the lobes 1.43 x – 24 x quite 3.6 x – x – active to the plasma sheet 0.55 x – 4.3 x quite 1.0 x – 6.9 x active

Indirect evidence: From low altitude measurements - the polar rain ( Winnigham & Heikkila, 1974 ) unstructured precipitating electrons with energies 100 – 500 eV always exists in the polar caps, but is more intense in the ‘preferred’ hemisphere – northern for IMF away sector, southern for IMF toward sector. - the polar wind (predicted Dessler&Michel, 1966; Bauer, 1966; observed Hoffman, 1970; Brinton et al., 1971) an ambipolar outflow of thermal plasma from the ionosphere at high latitudes to the magnetosphere along geomagnetic field lines Experimental evidence about the lobe population very scarce because: - needed are a spacecraft on a high apogee high inclination orbit and plasma instrumentation capable to measure weak particle fluxes - spacecrafts charge to large positive potential

in situ measurements: in situ measurements: ISEE 1, ISEE 3, GEOTAIL, INTERBALL-1 ISEE 1, ISEE 3, GEOTAIL, INTERBALL-1 encounter of ionospheric ions encounter of ionospheric ions observations of electrons observations of electrons In the near tail - Fairfield & Scudder, 1985, ISEE 1 (based on magnetic field and electron spectra measurements) highly anisotropic electrons – field aligned bi-directionality, identified to be the polar rain electrons, originating in the solar wind (the solar wind heat flux – strahl - electrons) In the distant tail: - Baker et al., 1986; Baker et al., 1987 – ISEE 3, MF and electron spectra - Baker et al., 1997; Shirai et al, 1997, 1998 – Geotail, MF, electron and ion spectra -bidirectional electrons (polar rain) at the magnetopause and near the plasma sheet boundary INTERBALL-1 results - Grigorienko et al., 2001, MF and ion spectra measurements two types of ion transient structures – plasma filaments with PS-like distribution and ion beams with bursty appearance (beamlets), the latter predominantly encountered in the PSBL

AIM: AIM: to study lobe population as observed by INTERBALL-1 DATA: DATA: magnetic field electron spectra ion spectra distance to the NS and MP We surveyed 3 months of measurements – October – December in the ‘central’ near-Earth lobes, -27R E < X GSM, and identified 576 hours of lobe observations, assuring that observations are enough apart from boundary layers

December, 1997 lobes ? lobes PSBL + PS NSPS lobes

Types of plasma In the lobe regions with enhanced electron densities, during all geomagnetic conditions, but with different intensity and occurrence, the following plasma regimes could be observed:  electrons with energies typical for the PS – (keV) the electrons are accompanied by PS ions; diamagnetic effect  electrons with energies up to few hundreds eV (~ 300 – 500) – MSH ions are mixed – PS & MSH like; only in IMF Bz > 0 ions are mixed – PS & MSH like; only in IMF Bz > 0 ions have MSH energies, but are quite isotropic ions have MSH energies, but are quite isotropic – MSH injections or cold PS structures ions have energies up to 200 – 300 eV - the low energy part of mantle plasma no ions, sometimes low energy ions; electrons are anisotropic Types of plasma

TWO MAIN QUESTIONS about the ‘lonely’ electrons  WHAT IS THEIR ORIGIN  WHERE ARE THE IONS WHICH PROVIDE CHARGE NEUTRALITY DECEMBER 22, 1997: AL has been > -50 nT for the previous 12 hours; IMF Bz was -50 nT for the previous 12 hours; IMF Bz was < 0 for long time intervals; Enhanced electron density structures are a ubiquitous Enhanced electron density structures are a ubiquitous for tail lobes.

Are these electrons ‘polar rain’ electrons ? Polar rain: always exists in the polar caps, but is more intense in the ‘preferred’ hemisphere – northern for IMF away sector, southern for IMF toward sector. direct free Explained by the model that the strahl electron flux of the solar wind becomes polar rain by direct free access to the magnetosphere along open field lines (Fairfield & Scudder, 1985; bidirectional distribution at ISEE 1 in the ‘preferred’ lobe, isotropic in the other) In the distant tail (Baker et al., 1987) - the dense thermal electrons, locally entered, are electrostatically bound to the tailward thermal protons; the strahl electrons form the bidirectional population, the most field aligned of them constitute the polar rain. The near tail lobe is relative devoid of locally entered plasma.

Assumptions: -the SW strahl and a small part of the electron hallow within ~ 30 o freely enter the magnetopause and their distribution was adiabatically mapped to the IB-1 location with B/Bo=5. -the strahl was assumed to be 3 o wide in pitch angle (at half width at 250 eV) with temperature 157 eV. -symmetry in Vpar (mirroring without losses) -satisfactory results were obtained only when the temperature of the ‘permitted’ part of the hallo is comparable with that of the strahl -suggesting a gain of 45 eV in the perpendicular direction (consistent with s/c potential) could fit the uprising of the observed distribution around small parallel velocities. -a slight heating of both distributions in perpendicular direction - the measured distribution shows traces of more isotropic population (the outer isocontour is not fitted satisfactory). ATTEMPT TO MODEL ELECTRON DISTRIBUTIONS N SW ~ 3 – 4 cm -3

An appropriate interplanetary magnetic field sector is not the only condition for bi-directional lobe electron fluxes. The less denser and more anisotropic solar wind leads to higher anisotropy of the lobe electrons, irrespectively of the azimuth of the interplanetary magnetic field. The positive spacecraft potential plays a substantial role in modifying the observed electron distributions. Observation of ions is not connected with the electron density away towards N S disturbed conditions unpreferred + preferred; high SW density Nel increases with Bz< 0 – mantle plasma main phase of a storm unpreferred; low SW density bidirectional fluxes Very quiet conditions, Bz > 0 unpreferred; low SW density, heat flux defined; asymmetry in lobe electrons quiet conditions, Bz < 0 unpreferred + preferred; high SW density; no relation except Nlobe with Nsw

Why don’t we observe the ions which keep the charge neutrality? Two possibilities: 1. Charge neutrality is kept by magnetosheath ions, which accompany the entered magnetosheath electrons. We do not observe them, because the density is low – – cm -3 which is under the sensitivity of the ion instruments; 2. Charge neutrality is kept by ionospheric ions, which are not observed due to the positive spacecraft potential and/or the sensitivity threshold of the instruments

CLUSTER Measurements of O+ flowing tailward POLAR: lobal wind

SUMMARY We surveyed 3 months of INTERBALL-1 measurements in the ‘central’ tail lobes at 27 R E < X GSM. Tail lobes are not empty. Regions with enhanced electron densities are very frequently encountered.  Their electrons might have plasma sheet (keV) energies (infrequently) or magnetosheath (~ 300 – 500 eV) energies.  Four different kinds of plasma structures are characterized by electrons with energies up to ~300 – 500 eV. Using only electron (and magnetic field) data can lead to erroneous conclusions. The accompanying ions always should be searched and the location of occurrence should be carefully investigated.  A ubiquitous feature of the examined tail lobes are regions with enhanced electron density and distributions with varying anisotropy, in which low energy (< 100 eV) ions are scarcely registered.

SUMMARY - continue As the lobe field lines are open, the earthward flowing electrons originate in the solar wind. In some cases the tailward electrons are not only mirrored solar wind electrons, but they bear also ionospheric electrons, as the tailward fluxes are stronger than the earthward the electron distributions are complicated and variable; they can be (roughly) modeled assuming the electron population is comprised of strahl and a part of the hallo electrons, within a 30 o cone of pitch angles; the distributions are fitted by adiabatically mapping the initial distribution to the IB-1 location and the electrons gaining energy when passing the positive s/c potential an appropriate interplanetary magnetic field sector is not the only condition for bi-directional lobe electron fluxes, as the width and the temperature of the strahl depend on the density of the solar wind. When the solar wind is dense enough, the strahl electrons are dissolved in the more isotropic distribution of the entered solar wind hallo electrons what keeps the charge neutrality – the accompanying magnetosheath ions or ionospheric ions ? An examination of the lobe ion population, using measurements with more sensitive instruments (CLUSTER) should be made.