THE ROLE OF MAGNETOSPHERIC LOBES IN SOLAR WIND – MAGNETOSPHERE -IONOSPHERE RELATIONS

1. IHY-ISWI Regional Meeting "Heliophysical phenomena and Earth's environment“ Šibenik, Croatia THE ROLE OF MAGNETOSPHERIC LOBES IN SOLAR WIND – MAGNETOSPHERE -IONOSPHERE RELATIONS Koleva R. 1, Grigorenko E. 2 (1) Solar-Terrestrial Influences Laboratory, BAS (2) Space Research Institute, Russian Academy of Sciences

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

3. AfterHultqvist, etal. SSR, 1999 WHY STUDY THE MAGNETOSPHERIC LOBES? two vast regions between the plasma sheet (PSBL) and the magnetotail boundary layers at the magnetopause, where the plasma has very low temperature and densities, being for a long time under the threshold of the instruments The lobes are regions of open field lines , which connect the ionospheric polar caps directly to the solar wind.

4. LET’S HAVE A LOOK AT THE POLAR CAP I. INTO THE POLAR CAP – POLAR RAIN A generally homogeneous electron precipitation – the polar rain – is always present over the entire polar cap A typical spectrogram from DMSP – 830 km altitude, sensor looking at zenith

5. POLAR RAIN ORIGIN Fairfield & Scudder, 1985 The polar rain originates from direct tail entry of the solar wind strahl along the distant tail lobe. The strahl and a higher-energy part of the solar wind hallo electrons freely enter the tail through the distant magnetopause. Their distribution can be mapped adiabatically along the filed lines. Explains the mostremarkable property of the polar rain – the ‘preferred’ hemisphere dependent on IMF Bx

6. II. OUT FROM THE POLAR CAP – POLAR WIND an ambipolar outflow of thermal plasma from the ionosphere at high latitudes to the magnetosphere along geomagnetic field lines CLUSTER measurements of O+ flowing tailward Though the lobe 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 1029 – 24 x 1029 quite (1.23x1029 – 20.6 x1029) 3.6 x 1029 – 47.59 x 1029 – active (2.26x1029 – 37 x1029) to the plasma sheet 0.55 x 1029 – 4.3 x 1029 - quite 1.0 x 1029 – 6.9 x 1029 - active

7. The magnetospheric lobes play the role of a transmitter between: i) the solar wind and the polar ionosphere ii) the polar ionosphere and the magnetotail plasma sheet Direct in situ evidence about the lobe population are 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: • ISEE 1, ISEE 3, GEOTAIL, INTERBALL-1 • encounter of ionospheric ions • observations of electrons

8. We use the unique possibility provided by INTERBALL-1 orbit to study the lobe population DATA: magnetic field electron spectra ion spectra distance to the NS and MP We surveyed3 months of measurements – October – December 1997- in the ‘central’ near-Earth lobes, -27RE< XGSM , and identified 576 hours of lobe observations, assuring that observations are enough apart from boundary layers 1995 - 2000

9. lobes ? lobes PSBL + PS NSPS lobes 8 - 11 December, 1997

10. In the lobe regions during all geomagnetic conditions there exist discrete plasma structures of different origin, but the intrinsic lobe population consists of inhomogeneous anisotropic electrons with energies up to 300 – 500 eV, with no accompanying ions registered. DECEMBER 22, 1997:Very quiet, southward IMF Bz. AL has been > -50 nT for the previous 12 hours; IMF Bz was < 0 for long time intervals; as a rule the Earthward flux exceeds the tailward the field lines are open The electrons are from solar wind origin Only the most field-aligned part of these electrons – within 1o– 2o pitch angle, precipitate as polar rain

11. Our main goal is to estimate the electron concentration in the lobes and understand what parameters control it. WHY? We want to understand why ions are not registered in the lobes. I. The lobe plasma is neutral. Then what ions keep the charge neutrality? From electron density we can estimate the necessary ion flux for magnetosheath energies and test if this flux is below the threshold of the ion spectrometers. If it is above and we do not register magnetosheath ions, this means that charge neutrality is kept by low energy ionospheric ions; we cannot observe them because of the large positive spacecraft potential – hidden population (additional question – where are the polar wind electrons?). II. The lobe plasma is not neutral. Electric fields exist which accelerate the cold ionospheric ions so they could be observed in the distant tail (Geotail results: Hirahara et al., 1996; Seki et al 1996)?

12. Do the lobe electrons exhibit the same properties, controlled by IMF, as the polar rain? • Polar rain properties: • a favoured hemisphere- determined by IMF Bx: Bx<0 favoured is the northern hemisphere; Bx>0 - the southern. The polar rain in the favoured hemisphere is stronger, as the lobe field line point directly to the SW electron flux.Except of rare cases of Bx sign changescannot be checked by IB-1 data as it slowly crosses from one lobe to another. • midnight-noon gradient- polar rain strongest near the dayside cusp. The far downtail a field line reconnects, the weaker is the ion entry, causing weaker polar rain electrons entry. For IB-1 - electron density should decrease with increasing distance to magnetopause. • dawn-dusk asymmetry –(Bz <0)polar rain has higher fluxes on the dawn (dusk)side in the northern(southern)hemisphere for IMF By positive(negative). Polar rain is stronger in the loaded hemisphere. • polar rain is stronger for rapid dayside merging conditions- IMF Bz<0 or |By|>2.5Bz>0. When dayside merging is slow, field lines move correspondingly slowly across the polar cap, resulting inhighly stretched polar cap field lines, which cross MP far downtail (Sotirelis et al, 1997). • polar rain is stronger in the summer hemisphere –effect of the ionosphere, altering the transport of polar rain electrons • no dependence on SW density is observed Fairfield and Scudder model Open/closed far magnetopause for various IMF (after Hasegawa et al., )

13. A. DO WE OBSERVE MIDNIGHT-NOON GRADIENT?We try to find observations at nearly similar IMF and SW conditions (WIND). Northern hemisphere - quite conditions - unpreferred hemisphere - rapid reconnection - no variation with SW density Electron (e-) density increases slightly approaching noon and then – approaching MP The variations of electron density qualitatively follow the polar rain pattern

14. A. DO WE OBSERVE MIDNIGHT-NOON GRADIENT -2 Northern hemisphere -disturbed conditions - rapid reconnection - no variations with Bx sign - no variations with SW changes - further from MP than on Oct 5 - density more than 2 times larger than on Oct 5 e- density does not increase either when approaching noon or MP The variations of electron density do not follow the polar rain pattern

15. B. DO WE OBSERVE A DAWN-DUSK ASSYMETRY IN THE LOBES (night sector of the cap) ? B. DO WE OBSERVE A DAWN-DUSK ASSYMETRY IN THE LOBES (night sector of the cap) ? Southern hemisphere - rapid merging - summer hemisphere - dense SW IMF By changes sign and this could be the reason of e- density decrease/increase. The variations could be more significant, but Bx changes in the opposite directions and probably masks them. Disturbances are not felt. No dependence on distance to MP. Possible dependence on IMF Bx and By, no dependence on distance to MP

16. Southern hemisphere B. DO WE OBSERVE A DAWN-DUSK ASSYMETRY IN THE LOBES -2 • - preferred summer hemisphere • - dense solar wind • - no dependence on merging rate, even inverse relation • inverse dependence on distance to MP • - possible dependence on By The variations of electron density do not follow the polar rain pattern

17. DISSCUSSION • We analyzed several cases of INTERBALL-1 observations of lobe electrons. A small part of these electrons – within about 20 pitch angle cone – constitute the polar rain. • The polar rain has a well pronounced dependence on IMF, explained by the Fairfield and Scudder model, generalized by Newell et al.- the more directly a field line points toward the inflowing solar electron flux, the greater the polar rain intensity in the ionosphere. • We tried to see if similar dependences are peculiar for the lobe electrons. As a base for the analyses we use the footprints of the field lines which bear the lobe electrons, having in mind all uncertainties caused by the model projection (Tsyganenko’96 model was used). • Only in few cases the lobe electron density variations conform to the above model. Besides, the causes for the observed density behavior are not unambiguous. We see several possible reasons for this disparity: • The polar rain dependences on IMF are large scale, and the lobe e-density variations observed are not of the same scale; • As many drivers act simultaneously, a statistical analysis should be applied in order to express the influence of each; • We suggest that the lobe electrons contain much larger part of SW suprathermal halo electrons than does the polar rain. There is an overall tendency of the lobe electron density to increase with the increase of SW density

18. We suggest that the lobe electrons contain much larger part of SW suprathermal halo electrons than does the polar rain - attempt to model electron distributions Ogilvy et al., 2000 NSW ~ 3 – 4 cm-3 • Assumptions: • the SW strahl and a small part of the electron hallow within ~ 30o 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 3o 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).

19. FUTURE RESEARCH • Estimate the influence of the large positive spacecraft potential on electron density; • Perform a statistical study using all 5 years INTERBALL-1 data; • Try to answer the question “what keeps the charge neutrality of the lobe plasma” -accompanying magnetosheath ions or ionospheric ions? • OR in the lobes exist electric fields which accelerate the cold ionospheric ions so they could be observed in the distant tail (Geotail results: Hirahara et al., 1996; Seki et al 1996)?

21. The model trajectories effectively illustrate that ‘‘regions’’ of the magnetosphere are places of temporary residence for ions traveling from one location to another. Ions within the magnetotail, plasma sheet, and ring current are always on their way to somewhere else. An ion may circulate through virtually all regions of the magnetosphere before plummeting back into the ionosphere, exiting to join the solar wind, or being recycled back into the magnetosphere.