Observations of the mid-Altitude Magnetosheath During a Persistent Northward IMF Condition: Polar CAMMICE Observations
by
M. Grande1, J. Fennell2, S. Livi3*, B. Kellett1, C. H. Perry1, P. Anderson2, J. Roeder2, H. Spence4, T. Fritz4, B. Wilken3
1) Rutherford Appleton Laboratory
2) Aerospace Corporation, Los Angeles, CA, USA.
3) Max Planck Institute for Aeronomy, Katlenburg-Lindau, Germany
4) Boston University, Boston, MA, USA.
* Temporally at Applied Physics Laboratory, MD, USA.
Abstract
On May 29, 1996, the Polar spacecraft appeared to cross into an extended cusp or magnetosheath region. The region was characterized by intense fluxes of ions in the energy range 1-10 keV that had solar wind like ion composition and angular distributions that showed evidence of flows and structure. These ion composition data are combined with energetic proton observations from Polar and plasma observations from the HEO 95-034 and DMSP satellites to examine the spatial extent and plasma characteristics of this region. The ion composition is consistent with expected solar wind composition. The combined spacecraft observations indicate a considerable spatial extent for the observed region, which may be produced by northward IMF interacting with the northern cusp.
Introduction
The dayside magnetosheath plasma is dominated by solar wind ions with clear charge state composition (Gloeckler et al., 1986). While there is plasma leakage into the magnetosheath from closed magnetospheric field regions, it is a generally weak source. The magnetosheath plasma can access magnetic field lines that penetrate or are adjacent to the magnetopause (such as the cusp field lines). The plasma near the magnetopause boundary is classified by the energy flux and density of its components (Newell and Meng, 1988). In the high latitude dayside of the magnetosphere, the different plasma regimes have been identified using both high altitude (see Haerendal and Paschmann, 1982 and references therein) and low altitude data (Newell and Meng, 1988; Newell et al., 1991a and 1991b). The cusp proper is defined by the existence of high density soft electron precipitation combined with intense soft ion precipitation and is found close to local noon. Away from noon the energies and densities of the ion and electrons change (generally hotter and less dense) and are classified variously as boundary layer, mantle, cleft/LLBL, etc. plasma (Newell and Meng, 1988; Smith and Lockwood, 1996). Medium altitude satellites that enter the magnetosheath, observe ions that have the composition ratios and charge states that were frozen in the solar wind stream near the sun (Gloeckler et al., 1986) and sometimes observe them in the outer environs of the magnetosphere (Christon, et al., 1994; Grande, et. al., 1996). Solar wind ions are highly ionized (Gloecker and Geiss, 1989). The high charge state of the higher mass ions makes them easily distinguishable from ionospheric and trapped magnetospheric ions. In the neighborhood of the dayside cusp and cleft regions, the plasma is dominated by these high charge state ions. This fact is used to help identify the plasma regions traversed by Polar.
In this paper we discuss an event in which the magnetospheric satellites Polar and HEO 95-034 traversed near and through the mid-altitude cusp and observed magnetosheath plasma. We use data from the CAMMICE [Charge and Mass Magnetosphere Composition Experiment] MICS [Magnetospheric Ion Composition Sensor] on the Polar satellite. We compare the MICS results with observations by the CEPPAD Imaging Proton Spectrometer (IPS; Blake et al., 1995) on Polar and with simultaneous plasma observations from the high altitude HEO 95-034 satellite (Fennell et al., 1996) and low altitude DMSP F12 and F13 satellites.
MICS measures the ion flux, mass, and charge state for ions with energies in the range of 1 - 400 keV/q (Wilken et al., 1990). During the period of these observations the upper limit of the MICS energy range was set to ~200 keV/q. With the MICS data we are able to detect the different ion charge states that identify the source of the plasma as dominantly solar wind in nature and to estimate its source temperature (Arnaud and Rothenflug, 1985). IPS measures protons with energies ³ 18 keV and provides 4¹ spatial coverage. The HEO 95-034 satellite instrument complement consists of both plasma and energetic particle instruments (Fennell et al., 1996). During this event, the HEO plasma data covered the energy range from about 200 eV to 30 keV and 40 eV to 25 keV for electrons and ions respectively. HEO also makes limited measurements for electrons ³ 130 keV and protons ³ 80 keV. The DMSP plasma instruments measure the flux of electrons and protons with energies in the range 50 eV to ~30 keV (Hardy, et al., 1984).
Observations
On May 29, 1996, the Wind satellite was ~155 Re upstream of the earth while Geotail and IMP-8 were in the near earth solar wind. The Wind magnetic field and plasma observations show that the solar wind velocity was relatively constant. The interplanetary magnetic field (IMF) was anti-sunward and strongly northward for several with BZ approaching the total field intensity at times. Near 0230-0235 UT the IMF rotated sunward at Wind indicating an IMF sector boundary passage and the plasma density increased by ~20% after the rotation. The solar wind velocity was relatively steady near 370 km/sec and the sector boundary arrived near the earth at about 0319 UT. The period that we will report on begins around 0100 UT on this day. The Polar and HEO satellites were in the pre-noon sector heading poleward as shown in Figure 1. The positions of the DMSP satellites, for the early hours on May 29, are also shown in Figure 1. The shadings of the trajectory lines for Polar and HEO mark the extent of the dayside soft particle fluxes observed by these satellites.
HEO was the first of the high altitude satellites to enter into the structured LLBL/cusp like plasma near 0135 UT. The plasma consisted of soft, intense electron fluxes and hot ions with peak energy flux near 500-600 eV, as shown in Figure 2. HEO remained in such a structured plasma regime until ~0317 (coincident with arrival of IMF sector boundary) when the soft electrons disappeared and the average energy of the ions increased to ~900 eV. HEO crossed into the PSBL near 0420 UT and proceeded to lower latitudes, entering the central plasma sheet near 0530 UT and 15 MLT, as shown in Figure 2.
The MICS data show that Polar entered a plasma region characterized by low energy high charge state material near 0300 UT. Polar remained in this population until ~0710 UT when it entered the polar cap, as evidenced by very low fluxes of energetic particles. Figure 3 summarizes the MICS data using multi-panel spectrograms. Figure 3d displays the spin averaged proton intensity which shows that the ring current energy decreased as Polar moved from L ~ 3.2 to higher L-shells (ref. Fig. 1). Figure 3e shows the spin-sectored proton intensities integrated over all energies (note that the proton intensities from 0° to 90° are repeated from 360° to 450°). Clear loss cones were seen between 0100 and 0300 UT, as would be expected in the inner magnetosphere. At ~0300 UT an abrupt change in the angular distribution and spectral character (Fig. 3d) of the protons was observed simultaneous with the loss of low charge state ions (Fig. 3a and 3b). A new low energy proton population appeared which had a complex angular distribution. Examination of Figure 1 (and Table 1) shows that HEO and Polar entered into the structured soft particle region at essentially the same invariant latitude but 1.5 hrs apart in UT and 1 hr apart in MLT. This indicates the low latitude edge of the LLBL and cusp was relatively stable for the 1.5 hour interval. This is also borne out by comparison with the DMSP data, especially the DMSP F12, as shown in Table 1.
The top three panels of Figure 3 show the count rates of O < 3+, He++ and He+. All three ions were present in the inner magnetosphere. However, after 0300 UT only He++ was seen, with intensity peaking around 3 keV. High charge state heavier ions were also seen after 0300 UT, as will be described below. The composition signature is very similar to what would be expected in a cusp crossing. Dispersed energetic ions were not seen immediately south of the cusp, but this is not a universal feature. The new, lower energy H+ population is initially nearly isotropic, but by 0317 UT (approximate time of arrival of the IMF sector boundary) the angular distribution exhibited a predominantly 90° organization interspersed with periods where a clear beam-like distribution was observed. As Polar was leaving the magnetosheath plasma, near 0638 UT, two intervals of beam-like distributions were seen (ref. Fig. 3e). These latter distributions showed an apparent ìflowî direction reversal on either side of 0700 UT. Thus, based on the MICS data, Polar appears to have entered a spatially extended cusp-like region.
The 3D IPS data, shown in Figure 4, exhibited angular distribution features in > 20 keV protons that are very similar to those observed in the MICS data. At ~0359 UT IPS observed a nearly ìpancakeî angular distribution, as does MICS (see Fig. 3e), with maximum flux in anti-sunward direction. (Note, the sector angles in Figs. 3 and 4 are in detector reference frame. The particle reference frame would be a mirror image.) At ~0418 UT IPS detected a unidirectional particle flux. By ~0456 UT the pancake distribution had returned (see Fig. 3 and Fig 4 at 0624 UT) until ~0644 UT, when a beam-like particle distribution, peaked roughly sunward, was observed by both MICS and IPS. By ~0703 the direction of the beam had rapidly shifted anti-sunward and became less pronounced at IPS but still strong at the lower energies measured by MICS. When the definitive magnetometer data become available we will be able to cast these results into proper coordinates.
As noted above, the HEO entry into the magnetosheath-like plasma (see Fig. 2) was evidenced by a rapid decrease in the energy of the plasma electrons and ions and occurred well poleward of the > 130 keV electron trapping boundary (not shown). However, small residual fluxes of > 80 protons were sporadically present (also not shown) in this region. From 0315 to 0418 UT, when HEO 95- 034 was in the post noon sector at L ~ 76.3°-77°, the peak energy of the plasma electron fluxes dropped below the instrument threshold. The ion fluxes remained with the same intensity but their peak energy increased to ~1 keV. This indicated that HEO had entered the cusp proper. At ~ 0420 UT, HEO entered the post noon boundary layer, as evidenced by the approximately one order of magnitude increase in the mean ion energy and a return of the higher energy electrons (see Figure 2).
The DMSP crossings of the dayside soft particle precipitation are summarized in Table 1 along with the entry and exits of the HEO 95-034 and Polar satellites from the region. The DMSP data, on the whole, show good agreement with the HEO and Polar determinations of the position of the low latitude boundary of the cusp. Most of the DMSP F13 observations were taken in the LLBL and cannot not be used to define the cusp boundary. The DMSP F12 data observed a slightly lower latitude for the poleward boundary of the cusp (see F12 at 0105 UT and 0247 UT in Table 2). The difference is probably well within the errors of the simple model field for the IMF conditions.
Composition results
The MICS uses post acceleration of 22.5 kV internally to increase the delectability of the higher mass ions. Thus, high charge state medium-mass ions, such as solar wind O+6, would have its energy increased by ~135 keV during post acceleration and be easily detected. A detailed study was made of the charge state of the high mass components of the LLBL/cusp ion population. The process itself is described in more detail in Grande et al. (1996). The full multi-parameter data (DEís) are recorded for the highest mass ions detected in each data accumulation period. This includes separate measurements of an ionís energy per charge (E/q), velocity (using time of flight (TOF) technique), and total energy (E) deposited in a solid state detector (for details see Wilken et al., 1992). The low energy of the magnetosheath ions complicates their detection because they are heavily scattered and straggle in the TOF foil and, in doing so, provide less than MICS best attainable resolution.
The results of an analysis of the DEís for the May 29 magnetosheath plasma encounter are shown in Figure 5. Figure 5a shows a plot of E vs. E/q for the time period 03:00 to 07:00 UT. Such a plot should show loci that are lines of constant ion charge state. The expected positions of charge states 1 and 2 are shown. For this study, only DEís having higher charge states were selected. These DEís fall within the bounded area towards the top left of the Figure 5a. These selected DEís were used to generate Figure 5b, which shows measured ion energy, E, plotted vs. TOF. Since these are non-relativistic ions, different masses should have different loci. (The expected loci for some of the higher mass ions are shown.) Figure 5b shows that the selected high charge state ions produced a spread of particle masses that centered predominantly on the CNO group. Figure 5c shows a plot of E/q vs. TOF for these same ions. The loci of a few different mass per charge (M/q) ions are also shown on Figure 5c. The majority of the ions grouped where the high charge states of oxygen and carbon are expected to be. (It should be emphasized, however, that it is not possible with this instrument to actually distinguish the loci of the individual species, at these energies and limited DE statistics, as would be possible at higher energies.)
What can be shown from Figure 5, is that there is a fully consistent interpretation for all species in terms of a unified solar wind source. All the M/q combinations in Figure 5c would correspond to a frozen in coronal source temperature, derived from the ionization fractions calculated by Arnaud and Rothenflug (1985), of 1.25 x 106 degrees. This temperature represents a best fit to these data and will form the basis for a future more detailed discussion. Division of the selected DEís into subgroups (not shown) produces ion subgroups with different and relatively well defined charge to mass ratios. This gives confidence that the procedure used is valid. For example, the carbon group showed an M/q centered on 2, corresponding to a charge state for carbon of +6. For oxygen we obtained a broadened distribution centered on M/q of 2.5 to 3, corresponding to a mean charge of +6. For iron (Fe) a mean charge state of 10 was obtained. Magnesium and silicon could not be unequivocally separated the from the oxygen (probably indicates our DEís identified as oxygen contains traces of Si+8 Mg+10 and possibly some C+5). These observations are not unique. There have been several similar cusp encounters by the Polar satellite since May 29, 1966 and these will be the subject of a future extended study.
Summary
We conclude that, during the prolonged northward IMF period early on May 29, 1996, the constellation of Polar, HEO and DMSP spacecraft observed a spatially extended cusp region. The region was characterized by magnetosheath plasma with typical solar wind charge states and mass composition profile. The composition results for this cusp observation are similar to the CCE magnetosheath observations of Gloeckler et al. (1986). The low latitude boundary of the cusp was relatively stationary over hours and its poleward boundary was extended to very high latitudes (L ~ 85°). The angular distribution of the 1 to ~30 keV protons showed a trapped character and flows which persisted for minutes. Detailed analysis of this and similar Polar encounters with the cusp will be done in a future extended study.
Acknowledgments
The authors would like to thank the many individuals at their respective institutions that made the Polar CAMMICE program a success. We especially thank R. Lepping and K. Ogilvie, of Goddard Space Flight Center, for the use of unpublished WIND data for this study. We also thank M. Carter, of Rutherford Appleton Laboratories, for help with the IPS plots. Work at Aerospace and Boston University was supported in part by NASA Contract NAS5-30368. Work at Aerospace was also supported by the Aerospace Sponsored Research program.
Figures and Tables:
Table 1. Boundaries of Soft Plasma Region
Figure 1. Trajectories of Polar and HEO 95-034 satellites (top) and of DMSP F12 and F13 satellites (bottom) in invariant latitude and MLT. The times for the Polar and HEO LLBL/cusp entry are marked (top) and the light shading of their orbit traces indicate the period of time inside this region for these two spacecraft.
Figure 2. Spectrograms of plasma electron (top) and ion (bottom) energy fluxes observed by HEO 95-034 for 0100 - 0700 UT on May 29, 1996.
Figure 3. Spectrograms of plasma ions from the MICS instrument on Polar. The top four panels show the energy-time spectrograms of: (a) the intensities of O+, (b) He++, (c) He+, and (d) H+ respectively. Panel (e) shows the angular distribution of protons with energy > 1 keV measured perpendicular to the Polar satellite spin axis. Each angular sector in panel (e) is ~11.25°wide with MICS view sunward for sector angle ~130°.
Figure 4. IPS EP >18 keV proton angular distributions for times shown. The black rectangle to the right of center are from earth light and the black region near sector 0 and detectors 0 - 3 from sun light.
Figure 5. Spectrograms of MICS ion multi-parameter (DE) data for the 0300 - 0700 UT period. Panel (a) provides ion charge state identification; panel (b) gives ion mass identification and panel (c) shows ion mass per charge (M/q) identification. Panel (d) is energy-time spectrogram the high charge state ions bounded by the enclosed region of panel (a).
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