IMF-dependent large-scale structure of plasma in the Earth's magnetotail - a study based on Prognoz-8 data Barbara Popielawska Space Research Center, PAS, Warsaw, Poland

Introduction Subject: Large-scale structure of plasma in the tail (R<30 R E ) under quasi-steady solar wind conditions and a time-scale for its response to changes in the solar wind. History: Prognoz-8 data analysis started in the era post IMP-6,8 and ISEE-1,2: the real world was rather different than it was expected from the pattern established before (first noted by Jimenez et al.,1984); highly inclined orbit allowed to see more. Now: ISTP results not at odds with Prognoz-8 data (e.g., Geotail - cold dense PS advancing from the tail flank under BZ IMF >0)

Experiment Description Russian Prognoz-8 satellite, managed by Space Research Institute (IKI RAN), launch 25 Dec Elliptical orbit with inclination angle 65 o, perigee of 550 km, apogee ~30 R E (initially in the evening sector, in July at 04h LT, to 24h LT in Sept.). Orbital period ~4 days. Spin axis was Sun-stabilized, spin period s. Tail crossings from July to Sept. 1981, data from 12 orbits available.

Prognoz-8 orbit and T89 footprints o Footprints in CorMLAT vs MLT Xgsm Ygsm dZ ns Flank orbits  12  12 Flank orbits 

Prognoz-8 Magnetospheric Ion Composition Spectrometer PROMICS-2 - IRF, Kiruna, Rickard Lundin P.-I.  

What were (are) the expectations ? Two sources of plasma - the solar wind and the ionosphere. Three main routes to populate the plasma sheet by solar wind ions (always mixed with some ionospheric component) - convection paths in X-Z, X-Y and Y-Z GSM planes, respectively.

(d) (a) LLBL source (from Lundin et al., 1991) (b)Mantle source (Ashour-Abdalla et al. 1993) (c)Ionospheric contribution (Chappell et al., 1987) (d)Lennartsson concection cell (Lennartsson, 1992)

As a result of multiple sources and an extended entrance region, a variety of plasma domains is found in the nightside tail : mantle, LLBL, cold ion beams in the lobe, plasma sheet (PS) formed from the mantle - PSBL and CPS (as defined from ISEE-1,2 data) PS formed from LLBL source some sash-originated plasma on open and closed magnetic field lines (?) cont. -What were (are) the expectations ?

Tool: magnetic and electric field models N.A. Tsyganenko, December 1998 (here I use T96_01 model) Weimer 1996 model Assumptions: equipotential magnetic field lines E ind is not important ? Lennartsson’s cell

Plasma at the dawn flank (DTP ) Contrasting examples under northward vs southward or BY-dominated IMF How models fit the reality? Time scale of the response to IMF changes

The same region of the tail - why such a different plasma encountered? 1.5 nPa vs 2 nPa ~400 km/s vs ~400 km/s but very different IMF

T96_01 magnetic field model, parametrized by the solar wind data from IMP-8 and/or ISEE-3 yellow oval - average diffuse aurora at IMF BZ>0, Viking data (Elphinstone et al., 1990) red oval - Feldstein-Starkov (1967) statistical oval of discrete auroras, math- fitted by Holzworth an Meng(1975)

persistently dominating dawnward IMF

 PS under dominating dawnward BY IMF, active convection 03:00-07:00 UT - quasi-stationary convection under BZ IMF = ~ -4 nT

long interval of the lobe due to a dominating duskward BY IMF plasma at ~07:00 UT ? - IMF data gap, but AE pattern suggest continuing southward IMF

Plasma in the tail center (DTP ) Contrasting examples under northward vs southward and BY-dominated IMF How models fit the reality? Time scale of the response to IMF changes

Near-Earth tail during strong substorms Post-onset plasma sheet thinnings in the morning sector Mantle access to the near-Earth plasma sheet and cold plasma accommodation to the plasma sheet in PSBL Magnetic model without the current wedge completely fails here

steady IMF BZ = - 5 nTsteady IMF BZ =-10 nT

Summary Preliminary analysis of PROMIC-2 data (as well as earlier analysis of the data from SKS plasma instrument by Jimenez et al., 1984) have shown that in the magnetotail at R<30 R E what is encountered is not only the lobe plasma (mantle and cold ion beams) on open magnetic field lines and PSBL/CPS on closed magnetic field lines, with the dynamics prescribed by thinning/expansion scenario. „strange”So, PROMICS-2 data from 12 tail crossings by Prognoz-8 were analysed with the aim to check whether the contemporary empirical models of the magnetic (Tsyganenko, 1996) and electric (Weimer, 1996) fields in the Earth’s magnetosphere are consistent with Prognoz-8 „strange” observations of a large–scale structure of plasma in the Earth’s magnetotail. It seems that the solar-wind driven reconfiguration of magnetic and electric fields in the tail, and the following changes of plasma convection, produce a more rich plasma structure in the tail than it was prescribed in classical synthetic pictures. The response to IMF changes is fast (5 -10 min) under southward IMF, and longer (up to 40 min in our data) under northward IMF. The most interesting results were obtained for the quiet plasma sheet under northward directed interplanetary magnetic field (IMF) and for the situation of a dominating BY (dawn-dusk ) component of IMF. It was found that a kind of „warm envelope” is formed around a classical hot plasma sheet under these conditions. In general, the Weimer electric field model was found to be quite successful in predicting qualitatively the large-scale structure of plasma in the tail directly driven by the solar wind. Also, signatures of the Lennartsson convection cell were found in plasma data under moderately driven conditions.

cont. Summary Under a dominating northward BZ of IMF the plasma sheet seems to be built up from the LLBL slowly advancing from the flank toward the tail center. Now, Geotail data document this very well. In the central part of the tail, plasma signatures are seen related to the NBZ convection cells at high latitudes. Under a dominating IMF BY during moderately disturbed conditions, the magnetic flux tubes just above the hot regular plasma sheet are seen to be loaded with a relatively dense, warm, mantle-unlike plasma.This soft plasma may advance from the flanks along the Lennartsson’s convection cell. Additionally, Prognoz-8 provided interesting observations in the near-Earth tail during 2 strong substorms with multiple intensifications. Post-onset plasma sheet thinnings/ expansions of a short time scale were accompanied by local accommodation of cold plasma into the plasma sheet. This process took place at the plasma sheet boundary, in the presence of intense electric field fluctuations (1-100 Hz). Similar events were reported by Geotail.